Unexpected photoisomerization of a pincer-type amido ligand leads to facial coordination at R(IV)

Article abstract of DOI:10.1021/ic052014h

The divalent complex (BQA)PtMe undergoes oxidative addition with MeI to afford the octahedral complex cis-(mer-BQA)PtMe₂I {(BQA)^- = bis(8-quinolinyl)amide}. When this molecule is irradiated with visible light, it isomerizes to (fac-BQA)PtMe₂I, where the BQA ligand adopts an unexpected facial coordination mode. The amide nitrogen in this molecule is sp³ hybridized and can be easily quartemized with HBF₄, resulting in [H(fac-BQA)PtMe₂I][BF₄], with only minor perturbation to the coordination sphere.

Full text of DOI:10.1021/ic052014h

Inorg. Chem. 2006, 45, 4316−4318
Unexpected Photoisomerization of a Pincer-type Amido Ligand Leads to
Facial Coordination at Pt(IV)
Seth B. Harkins and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of
Chemical Synthesis, California Institute of Technology, Pasadena, California 91125
Received November 22, 2005
Scheme 1
The divalent complex (BQA)PtMe undergoes oxidative addition with
MeI to afford the octahedral complex cis-(mer-BQA)PtMe2I
bis(8-quinolinyl)amide . When this molecule is irradiated with
{
(BQA)-
)
}
visible light, it isomerizes to (fac-BQA)PtMe2I, where the BQA ligand
adopts an unexpected facial coordination mode. The amide nitrogen
in this molecule is sp3 hybridized and can be easily quarternized
with HBF4, resulting in [H(fac-BQA)PtMe2I][BF4], with only minor
perturbation to the coordination sphere.
Scheme 2
Pincer-type amido ligands constructed using diarylamido
backbones are considerably more flexible than one might
expect.1-3 For example, the tridentate diarylamides [PNP]-
and [SNS]- can exhibit the expected mer configuration when
bound to a transition metal (e.g., square planar [SNS]CuCl)
or can conversely adopt a highly twisted structure in
bimetallic systems (e.g., D2 {(SNS)Cu}2.)2 The inherent
stability of these D2 twisted structure types was first noted
from the dimeric structure of {(BQA)Li}2 (BQA ) bis(8-
quinolinyl)amido).3 We now report the surprising discovery
that the pincer-type BQA ligand can accommodate, and may
even thermodynamically prefer, a facially coordinated ge-
ometry for a mononuclear, octahedral Pt(IV) center. This
geometry gives rise to an atypical amido substituent that is
sp3 rather than sp2 hybridized.4 The possibility for such
geometric flexibility is important to note with respect to the
growing family of diarylamido pincer-type ligands.5,6
(BQA)PtOTf reacts with benzene in a base-promoted process
(NiPr2Et) to generate (BQA)PtPh and a stoichiometric
equivalent of [HNiPr2Et][OTf].7 Whereas this reaction re-
quired elevated temperatures (ca. 150 °C) and extensive
reaction times (∼36 h), the addition of B(C6F5)3 to
(BQA)PtMe (1) in benzene facilitates a similar transforma-
tion at 23 °C within seconds (Scheme 1, see Supporting
Information). Because Pt(IV) intermediates are likely opera-
tive (e.g., [(BQA)Pt(H)(Ph)(L)]+)8,9 in the transformation,
we sought to examine a well-defined (BQA)Pt(II/IV) redox
couple.10
The addition of excess MeI to 1 in acetone solution suits
this purpose to afford cis-(mer-BQA)PtMe2I (2) as a purple
solid in very good yield (Scheme 2). The reaction proceeds
sluggishly at 25 °C and needs to be heated at 70 °C for 18
h to reach completion. Complex 2 exhibits an intense
absorption at 534 nm (ꢀ 15 500 M-1 cm-1) that is slightly
red-shifted relative to 1 (λmax ) 527 nm (ꢀ 12 000 M-1
The system of interest was elucidated while probing the
reaction chemistry of square planar (BQA)Pt-X systems (X
) OTf, Me, Ph). We had previously established that
* To whom correspondence should be addressed.
(1) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 2030-
2031.
1
cm-1)).7 There are three diagnostic resonances in the H
(2) Harkins, S. B.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 2885-
NMR spectrum that exhibit strong 195Pt-H coupling: the
2893.
3
(3) Peters, J. C.; Harkins, S. B.; Brown, S. D.; Day, M. W. Inorg. Chem.
2001, 40, 5083-5091.
2-position of the quinoline ring (8.71 ppm, JPtH ) 41 Hz),
(4) Fryzuk, M. D.; Joshi, K.; Chadha, R. K. J. Am. Chem. Soc. 1989,
111, 4498-4500.
(7) Harkins, S. B.; Peters, J. C. Organometallics 2002, 21, 1753-1755.
(8) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879-2932.
(9) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed.
1998, 37, 2181-2192.
(5) Ozerov, O. V.; Guo, C. Y.; Papkov, V. A.; Foxman, B. M. J. Am.
Chem. Soc. 2004, 126, 4792-4793.
(6) Liang, L. C.; Lee, W. Y.; Yin, C. C. Organometallics 2004, 23, 3538-
3547.
(10) Rendina, L. M.; Puddephatt, R. J. Chem. ReV. 1997, 97, 1735-1754.
4316 Inorganic Chemistry, Vol. 45, No. 11, 2006
10.1021/ic052014h CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/25/2006

COMMUNICATION
Figure 1. Solid-state molecular structures (50% ellipsoids) for 2, 3, and 4 (BF4 anion omitted).
Table 1. Bond Lengths and Angles for (mer-BQA)PtMe2I (2), (fac-BQA)PtMe2I (3), and [H(fac-BQA)PtMe2I][BF4] (4)
2
3
4
2
3
4
Pt-N1
Pt-N2
Pt-N3
Pt-I
2.008(4) Å
2.033(3) Å
2.017(4) Å
2.783(1) Å
2.088(5) Å
2.102(5) Å
2.137(3) Å
2.036(3) Å
2.146(3) Å
2.630(2) Å
2.052(3) Å
2.045(3) Å
2.152(4) Å
N2-Pt-I
90.16(9)°
81.47(16)°
81.82(16)°
163.22(17)°
87.15(17)°
131.93(42)°
177.73(7)°
81.06(10)°
80.53(10)°
97.50(10)°
90.55(12)°
114.82(25)°
177.95(15)°
80.62(11)°
-
94.67(12)°
87.08(16)°
114.02(17)°
2.135(6) Å
N2-Pt-N1
b
b
-
N2-Pt-N3
2.598(1) Å
N1-Pt-N3
Pt-C19a
Pt-C20
2.152(4) Å
C19{10}-Pt-C20{10}a
C8-N2-C17{8′}c
b
-
a C17, C19, and C20 corresponds to 2 and 3, C8′ and C10 corresponds to 4. b Values are omitted because they are generated by the symmetry transformation
{x, -y + 1/2, z}. c C8′ generated by a symmetry transformation {x, -y + 1/2, z}.
the equatorial methyl group trans to the amido N atom (1.67
ppm, 2JPtH ) 60 Hz), and the axial methyl group (1.43 ppm,
2JPtH ) 70 Hz). These assignments are supported by a 1D
NOE coupling experiment, performed by saturation (0.5 s
mixing time) of the 2-position proton of the quinoline ring.
As expected, NOE is observed to the equatorial methyl
protons, but no NOE is observed to the axial methyl protons.
The solid-state structure of 2 was determined by X-ray
diffraction (Figure 1) and confirms the cis relationship
between the methyl groups and axial placement of the iodo
ligand.
Oxidative addition using d3-MeI under the same conditions
affords a 1:1 mixture of axial and equatorial CD3 products
(1H and 2H NMR). This result suggests a labile iodide ligand
in acetone solution that facilitates methyl group isomeriza-
tion, in accord with observations made by Puddephatt and
co-workers concerning the {cis-1-(NdCHC6H4)-2-(Nd
CHC6H5)C6H10}PtMe2I system.11
photoisomerized to trans-(mer-BQA)PtMe2I, an isomer of
2 in which the iodide ligand is coordinated in the equatorial
position, trans to the amido N atom.
Single crystals of the new product were subjected to XRD
analysis, which firmly established the identity of the pho-
toproduct as cis-(fac-BQA)PtMe2I, 3 (Figure 1). The BQA
ligand in 3 is coordinated facially, and the two methyl groups
are cis disposed to one another (90.5(1)°) and, therefore, trans
to the quinolinyl N-donor nitrogen ligands. The most
interesting structural aspect of 3 is that the amide donor is
highly pyramidalized and is best described as sp3 hybridized,
reflected by the contraction of the C8-N2-C17 angle from
131.93(42)° in 2 to 114.82(25)° in 3. This geometry at
nitrogen had not to our knowledge been previously observed
for transition metal diarylamide complexes. Also distinctive
in the structure of 3 is that the iodide ligand now resides at
the position trans to the amido N donor and that the Pt-I
bond distance exhibits a significant contraction (ca. 0.15 Å)
upon isomerization from 2 to 3 (see Table 1 for all key
structural parameters). This contraction presumably reflects
the greater trans influence of a methyl versus the amido
substituent. The approximately tetrahedral geometry at the
amide nitrogen in 3 causes a dramatic decrease in its lone
pair donation to Pt, indicated by a large shift in an amide-
to-Pt LMCT band (534 nm in 2; 422 nm in 3). The charge-
transfer assignment is supported by the empirical observation
that addition of HBF4 etherate to 3 in CH2Cl2 completely
bleaches the absorption band at 422 nm but does not affect
the low-energy band at 510 nm (Figure 2).
We noted that 2 decays in solution in the presence of
ambient room light but is stable when stored in the dark. It
was subsequently found that photolysis of 2 using a 100 W
incandescent light bulb over 48 h effects a gradual color
1
change from purple to red. The H NMR spectrum of the
solution establishes quantitative conversion to a single new
complex with one methyl group resonance at 1.25 ppm (2JPtH
) 71 Hz) that integrates to 6H and exhibits no NOE to the
2-position protons of the quinoline rings. In a control
experiment, complex 2 proved to be completely stable in
acetone solution when incubated at 70 °C in the absence of
light over a period of 24 h. Also, irradiation of an acetone-
d6 solution of 2 with a 300 W medium-pressure Hg light
source filtered with a 560 nm long pass filter and a 2 mm
path of water (to screen infrared radiation) converted 2 to
its new isomer in ca. 3 h as monitored by UV-vis. These
data are suggestive of a photoisomerization process and led
us to the initial but incorrect assumption that 2 had
HBF4 addition to 3 produces the protonated complex 4
(Figure 1), which is isolated in near quantitative yield (97%)
1
as a rose-colored solid. Its H NMR spectrum shows a
195
signature downfield resonance (10.33 ppm), exhibiting
-
Pt satellites (2JPtH ) 21 Hz) for the N-H proton, and a new
and broad absorption in the IR spectrum at 3127 cm-1
attributable to the N-H vibration. 195Pt coupling to the two
methyl groups (2JPtH ) 70 Hz) is virtually unchanged. A 1D-
NOE (1.0 s mixing time) experiment with saturation of the
(11) Baar, C. R.; Jenkins, H. A.; Vittal, J. J.; Yap, G. P. A.; Puddephatt,
R. J. Organometallics 1998, 17, 2805-2818.
Inorganic Chemistry, Vol. 45, No. 11, 2006 4317

COMMUNICATION
a related mer to fac photoisomerization.17 However, (BQA)-
PtMe2I and L′Ru(CO)Cl2 are dissimilar because the latter
system involves an isomerization resulting from the pho-
tolytic dissociation of CO and does not feature a highly
pyramidalized geometry about the central nitrogen.
One possible mechanism for the conversion of 2 to 3
proceeds through an intermediate in which the iodide ligand
has dissociated prior to photoisomerization. Absorption of a
photon by this intermediate would destabilize the metal-
amide bond, allowing for relaxation to a structure that
accommodates an sp3-hybridized geometry about the amide
nitrogen. This isomer could then be trapped by coordination
of iodide to generate the observed product 3. This scenario
is hypothetical, but consistent with the observation that the
iodide ligand appears to be quite labile in acetone, allowing
rapid scrambling of the two methyl ligands. Also, the
photoisomerization process is severely attenuated when the
acetone solution is saturated with NaI. The observed shorten-
ing of the Pt-I bond distance in 3 compared to 2, without
an appreciable change in the Pt-N2 bond distance, supports
the idea of a strengthened Pt-I interaction in 3.
To conclude, the photoisomerization process described
herein underscores the geometric flexibility accessible to the
[BQA]- ligand. Despite its typical preference to bind in a
planar tridentate fashion, it can adopt quite distorted struc-
tures, as in the fac platinum(IV) complexes 3 and 4. Given
this and related findings1-3 caution should be exercised when
generally referring to ligands such as these as pincer-type.
Figure 2. Absorption spectra of 2, 3, and 4 in CH2Cl2.
N-H proton resonance indicates coupling to both the
8-position proton of the aryl ring (8.78 ppm) and the methyl
ligands (1.54 ppm).
The solid-state structure of 4 was determined by X-ray
diffraction (Figure 1) and is consistent with the solution data.
Most notably, the Pt-N2 (the amido substituent in 3) and
the Pt-Me bond distances each elongate by ca. 0.1 Å. H11,
the proton on N2, is hydrogen bonded to the BF4- counter-
anion (N2-F1 ) 2.821(7) Å). Neutral diarylamine adducts
of transition metals are very uncommon, and the Pt-N2 bond
of 4 is amongst the shortest of its type to be reported. For
12
comparison, unchelated diphenylamine adducts of AlCl3
and GaMe2Cl13 have been reported and exhibit Caryl-N-
Caryl bond angles of 110.5(4)° and 105.2(2)°, respectively.
These angles are contracted by comparison to the C8-N2-
C8′ angles of 3 (114.82(25)°) and 4 (114.02(17)°).
Anionic tridentate ligands that are able to isomerize from
mer to fac in octahedral or pseudooctahedral complexes are
very unusual.4,14-16 The (BQA)PtIV system is particularly
intriguing because the reaction is photolytically driven. Shaw
and co-workers have reported the octahedral complex L′Ru-
(CO)Cl2 {L′ ) ((Ph2P)CH2(tBu)CdN-)2} which undergoes
Acknowledgment. This work was supported with funds
provided by the DOE (PECASE) and the MC2 program in
collaboration with BP. S.B.H. thanks the International
Precious Metals Institute. Larry Henling provided crystal-
lographic assistance.
(12) Burford, N.; Hicks, R. G.; Royan, B. W.; Borecka, B.; Cameron, T.
S. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1991, 47, 1066-
1067.
Supporting Information Available: X-ray crystallographic files
(CIF) and complete synthetic details (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) Niemeyer, M.; Goodwin, T. J.; Risbud, S. H.; Power, P. P. Chem.
Mater. 1996, 8, 2745-2750.
(14) Fryzuk, M. D.; Gao, X. L.; Rettig, S. J. Organometallics 1995, 14,
4236-4241.
IC052014H
(15) Canty, A. J.; Patel, J.; Skelton, B. W.; White, A. H. J. Organomet.
Chem. 2000, 599, 195-199.
(17) Shaw, B. L.; Ike, U. U.; Perera, S. D.; Thornton-Pett, M. Inorg. Chim.
Acta 1998, 279, 95-103.
(16) Hartshorn, C. M.; Steel, P. J. Organometallics 1998, 17, 3487-3496.
4318 Inorganic Chemistry, Vol. 45, No. 11, 2006