Intramolecular NH???Pt interactions of platinum(II) diimine complexes with phenyl ligands

Article abstract of DOI:10.1021/ic1007582

Pt(pipNC)₂(phen) [pipNC^- = 1-(piperidylmethyl)phenyl anion; phen = 1,10-phenanthroline] was prepared by the reaction of cis-Pt(pipNC)₂ with phen. Crystallographic and ¹H NMR data establish that the phen ligand is bidentate, whereas each piperidyl ligand is monodentate and bonded to the platinum at the ortho position of the phenyl group. Acidic conditions allowed for isolation of the salts of diprotonated Pt(pipNHC)₂(diimine)²⁺ adducts (diimine = phen, 2,2′-bipyridine, or 5,5′-ditrifluoromethyl-2,2′-bipyridine). Crystallographic and spectroscopic data for the diprotonated complexes are consistent with H???Pt interactions (2.32-2.51 A) involving the piperidinium groups, suggesting that the metal center behaves as a Bronsted base. Metal-to-ligand (diimine) charge-transfer states of Pt(pipNHC)₂(phen)²⁺ in solution are strongly destabilized (>2500 cm^-1) relative to Pt(pipNC)₂(phen), in keeping with the notion that NH???Pt interactions effectively reduce the electron density at the metal center. Though N???Pt interactions in Pt(pipNC)₂(phen) appear to be weaker than those found for outer-sphere two-electron reagents, such as Pt(pip₂NCN)(tpy) ⁺ [pip₂NCN^- = 1,3-bis(piperidylmethylphenyl anion; tpy = 2,2′:6′,2′-terpyridine], each of the Pt(pipNC)₂(diimine) complexes undergoes diimine ligand dissociation to give back cis-Pt(pipNC)₂ and free diimine ligand. Electrochemical measurements on the deprotonated complexes suggest that the piperidyl groups help to stabilize higher oxidation states of the metal center, whereas protonation of the piperidyl groups has a destabilizing influence.

Full text of DOI:10.1021/ic1007582

9798 Inorg. Chem. 2010, 49, 9798–9808
DOI: 10.1021/ic1007582
Intramolecular NH Pt Interactions of Platinum(II) Diimine Complexes with
3 3 3
Phenyl Ligands
Sayandev Chatterjee,† Jeanette A. Krause,† Allen G. Oliver,‡ and William B. Connick*,†
†Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172,
United States, and ‡Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame,
Indiana 46556, United States
Received April 20, 2010
Pt(pipNC)2(phen) [pipNC- = 1-(piperidylmethyl)phenyl anion; phen = 1,10-phenanthroline] was prepared by the reaction of
cis-Pt(pipNC)2 with phen. Crystallographic and H NMR data establish that the phen ligand is bidentate, whereas each
piperidyl ligand is monodentate and bonded to the platinum at the ortho position of the phenyl group. Acidic conditions allowed
1
for isolation of the salts of diprotonated Pt(pipNHC)2(diimine)2þ adducts (diimine = phen, 2,20-bipyridine, or 5,50-
ditrifluoromethyl-2,20-bipyridine). Crystallographic and spectroscopic data for the diprotonated complexes are consistent with
˚
H
Pt interactions (2.32-2.51 A) involving the piperidinium groups, suggesting that the metal center behaves as
3 3 3
a Brønsted base. Metal-to-ligand (diimine) charge-transfer states of Pt(pipNHC)2(phen)2þ in solution are strongly destabilized
(>2500 cm-1) relative to Pt(pipNC)2(phen), in keeping with the notion that NH Pt interactions effectively reduce the
3 3 3
electron density at the metal center. Though N Pt interactions in Pt(pipNC)2(phen) appear to be weaker than those found
3 3 3
for outer-sphere two-electron reagents, such as Pt(pip2NCN)(tpy)þ [pip2NCN-= 1,3-bis(piperidylmethylphenyl anion; tpy =
2,20:60,20-terpyridine], each of the Pt(pipNC)2(diimine) complexes undergoes diimine ligand dissociation to give back
cis-Pt(pipNC)2 and free diimine ligand. Electrochemical measurements on the deprotonated complexes suggest that the
piperidyl groups help to stabilize higher oxidation states of the metal center, whereas protonation of the piperidyl groups has a
destabilizing influence.
Scheme 1. Pt(tpy)(pip2NCN)þ and Pt(pipNC)2(phen)
Introduction
The availability of dangling nucleophilic groups near the
axial sites of square-planar platinum(II) complexes can have
a profound influence on the reactivity and electronic struc-
ture.1-3 For example, Pt(tpy)(pip2NCN)þ (Scheme 1) under-
goes nearly reversible cooperative two-electron transfer at
0.4 V vs Ag/AgCl (ΔEp = 43 mV, 0.01 V/s), whereas the com-
*To whom correspondence should be addressed. E-mail: bill.connick@
uc.edu. Fax: (þ1) 513-556-9239.
(1) For example, see:(a) Dixon, K. R. Inorg. Chem. 1977, 16, 2618–2624.
(b) Sarneski, J. E.; McPhail, A. T.; Onan, K. D.; Erickson, L. E.; Reilley, C. N.
J. Am. Chem. Soc. 1977, 99, 7376–7378. (c) Wieghardt, K.; Koppen, M.;
Swiridoff, W.; Weiss, J. J. Chem. Soc., Dalton Trans. 1983, 1869–1872.
(d) Nikol, H.; B€urgi, H.-B.; Hardcastle, K. I.; Gray, H. B. Inorg. Chem. 1995,
34, 6319–6322. (e) Grant, G. J.; Spangler, N. J.; Setzer, W. N.; Van Derveer, D. G.;
Mehne, L. F. Inorg. Chim. Acta 1996, 246, 31–40. (f) Davies, M. S.; Hambley,
T. W. Inorg. Chem. 1998, 37, 5408–5409. (g) Wong, K.-H.; Chan, M. C.-W.; Che,
plex lacking piperidyl groups [i.e., Pt(tpy)(2,6-dmph)þ;
2,6-dmph- = 2,6-dimethylphenyl anion] is not oxidized at
<1.3 V.4,5 The observed behavior is consistent with inver-
sion of the one-electron redox potentials [i.e., E1°0(d6/d7) <
E2°0(d7/d8)] and coordination of the piperidyl groups to the
d6-electron metal center to give a six-coordinate complex.
The remarkable reversibility of this reaction, as well as spectros-
copic and crystallographic data, have led to the suggestion
C.-M. Chem. Eur. J. 1999, 5, 2845–2849. (h) Prokopchuk, E. M.; Jenkins,
;
H. A.; Puddephatt, R. J. Organometallics 1999, 18, 2861–2866. (i) Yip, J. H. K.;
Suwarno; Vittal, J. J. Inorg. Chem. 2000, 39, 3537–3543. (j) Shi, J.-C.; Chao,
H.-Y.; Fu, W.-F.; Peng, S.-M.; Che, C.-M. Dalton Trans. 2000, 3128–3132.
(k) Green, T. W.; Lieberman, R.; Mitchell, N.; Krause, J. A.; Connick, W. B.
Inorg. Chem. 2005, 44, 1955–1965.
(2) Blake, A. J.; Gould, R. O.; Holder, A. J.; Hyde, T. I.; Lavery, A. J.;
Odulate, M. O.; Schroder, M. J. Chem. Soc., Chem. Commun. 1987, 118–120.
(3) Vedernikov, A. N.; Pink, M.; Caulton, K. G. Inorg. Chem. 2004, 43,
3642–3646.
(4) Jude, H.; Carroll, G. T.; Connick, W. B. Chemtracts 2003, 16, 13–22.
(5) Jude, H.; Krause Bauer, J. A.; Connick, W. B. J. Am. Chem. Soc. 2003,
125, 3446–3447.
r
pubs.acs.org/IC
Published on Web 10/08/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9799
that the dangling piperidyl groups interact with the PtII center
prior to or during anodic electron transfer, effectively pre-
organizing the molecule for electron transfer.4,5 Interestingly,
although there are a number of platinum complexes with
ligand architectures capable of stabilizing the d8-electron
square-planar and d6-electron octahedral coordination geo-
metries, those systems are typically characterized by chemically
irreversible redox reactions or selective stabilization of the
d7-electron adduct, consistent with E2°0(d7/d8) < E1°0(d6/d7).6
To better understand the properties of Pt(tpy)(pip2NCN)þ,
we have prepared structural and electronic analogues. Here we
report the synthesis and properties of Pt(pipNC)2(diimine)
complexes, such as Pt(pipNC)2(phen) (Scheme 1), and their
protonated adducts, Pt(pipNHC)2(diimine)2þ. Structural, spec-
troscopic, and electrochemical results suggest that the metal in
Pt(pipNC)2(diimine) complexes has significant Brønsted base
measurements, the working electrode was polished with 0.05 μm
alumina, rinsed with distilled water, and wiped dry using a Kimwipe.
All reported potentials are referenced vs Ag/AgCl (3.0 M NaCl).
Peak currents (ip) were estimated with respect to the extrapolated
baseline current, as described by Kissinger and Heineman.11
cis-Pt(pipNC)2. All glassware and reagents were rigorously
dried prior to use. Under an argon gas atmosphere, N-butyl-
lithium (3.42 mL of a 1.6 M solution in hexanes, 5.5 mmol) was
added to a stirred solution of pipNCBr (1.52 g, 6.0 mmol) in
20 mL of THF at -70 °C. After 30 min, the solution of lithiated
ligand was cannula-transferred to a mixture of Pt(COD)Cl2
(0.93 g, 2.5 mmol) in 75 mL of THF at -70 °C. The mixture was
stirred for 1 h at -70 °C, warmed to room temperature, and then
stirred for an additional 12 h. The filtrate was rotary-evaporated
to dryness. Water (50 mL) was added to the solid, and the prod-
uct was extracted with CH2Cl2 (3 ꢀ 50 mL). The organic layers
were dried over anhydrous MgSO4 and rotary-evaporated to
dryness. After the addition of hexanes to the residue, the mixture
was sonicated, and the white solid was collected. Yield: 0.82 g,
61%. Anal. Calcd for C24H32N2Pt: C, 53.03 H, 5.93; N, 5.12.
Found: C, 53.17; H, 5.85; N, 5.12. 1H NMR (CDCl3, δ): 1.35-
1.6 (4H, m, CH2), 1.85-2.0 (8H, m, CH2), 3.21 (4H, m, CH2),
3.36 (4H, m, CH2), 4.23 (4H, s with Pt satellites, JH-Pt = 21 Hz,
benzylic CH2), 6.9-7.0 (5H, m, CH), 7.31 (2H, t, CH), 7.41 (1H,
d with Pt satellites, JH-Pt = 50 Hz, CH).
Pt(pipNC)2(phen). A mixture of cis-Pt(pipNC)2 (54 mg,
0.1 mmol) and 1,10-phenanthroline (18 mg, 0.1 mmol) in dichloro-
methane (30 mL) was refluxed for 3 h. The resulting yellow-orange
solution was rotary-evaporated to dryness to give an orange-
yellow solid. The product was contaminated with starting materi-
als and is unstable in solution (see the text). Using a chloroform
or dichloroethane solvent produces similar results. The complex
also is readily prepared in situ by deprotonation of Pt(pipNHC)2-
(phen)2þ with 2 equiv of TBAOH. 1H NMR (CD2Cl2, δ): 0.89-
3.59 (overlapping with TBAOH resonances), 3.93 (1.6H, dd,), 4.09
(2.4H, dd), 6.87 (2H, dd, JH-H = 11.2 and 5.6 Hz), 6.92 (2H, dd,
character. In the case of the protonated adducts, NH Pt
3 3 3
interactions have a profound influence on the electronic
structures.
Experimental Section
General Considerations. K2PtCl4 was obtained from Pressure
Chemical Co. All other reagents were purchased from Acros.
Tetrahydrofuran (THF) was distilled from Na(s) and benzo-
phenone. Pt(COD)Cl2,7 Pt(phen)(Ph)2 (Ph = phenyl),8 2-(bromo-
methyl)bromobenzene,
2-(CH2(C5H10N))C6H3Br(pipNCBr),9
and 5,50-ditrifluoromethyl-2,20-bipyridine (dtfmbpy)10 were pre-
pared according to literature procedures. Syntheses involving
amines were carried out in an inert argon atmosphere using stan-
dard Schlenk techniques. Argon was predried using activated
sieves, and trace impurities of oxygen were removed with activated
R3-11 catalyst from Schweizerhall.
1H NMR spectra were recorded at room temperature using a
Bruker AC 400 MHz instrument. Deuterated solvents CDCl3
[0.03% tetramethylsilane (TMS) (v/v)], CD3CN, and CD2Cl2
were purchased from Cambridge Isotope Laboratories. UV-
visible absorption spectra were recorded using a HP8453 UV-
visible spectrophotometer. Mass spectra were obtained by ele-
ctrospray ionization of acetonitrile solutions using a Micromass
Q-TOF-2 instrument.
Cyclic voltammetry measurements were carried out using a
standard three-electrode cell and a 100 B/W electrochemical work-
station from Bioanalytical Systems. Scans were recorded of solu-
tions containing 0.1 M TBAPF6. All scans were recorded using a
platinum wire auxiliary electrode and gold or platinum working
electrodes. The use of glassy carbon working electrodes resulted in
electrode passivation and, consequently, was avoided. Between
J
H-H = 11.2 and 7.2 Hz), 7.32 (2H, d, JH-H = 8 Hz), 7.61 (2H, dd,
H-H = 6.4 and 3.2 Hz), 7.66 (2H, d, JH-H = 4 Hz), 7.99 (2H, s),
J
8.52 (2H, d, JH-H = 8 Hz), 8.60 (2H, d, JH-H = 8 Hz).
[Pt(pipNHC)2(phen)](PF6)2. Trifluoroacetic acid (14.7 μL,
0.2 mmol) was added to a suspension of cis-Pt(pipNC)2 (54 mg,
0.1 mmol) in acetonitrile (30 mL). The mixture was stirred and
sonicated, and an equimolar amount of 1,10-phenanthroline
(18 mg, 0.1 mmol) was added to the resulting solution. The mix-
ture was refluxed for 3 h, and the yellow solution was allowed to
cool to room temperature. NH4PF6 (41 mg, 0.25 mmol) was
added, and the solution was rotary-evaporated to dryness. The
product was dissolved in a minimum amount of acetonitrile, and
diethyl ether was added to induce precipitation. Yield: 0.072 g,
71%. Anal. Calcd for C36H42F12N4P2Pt: C, 42.57; H, 4.17; N,
5.52. Found: C, 42.35; H, 4.17; N, 5.51. 1H NMR (CD3CN, δ):
0.87 (2H, m), 1.276 (2H, m), 1.46 (6H, m), 1.74 (2H, m), 2.50
(2H, dd, R-H, JH-H =12.0 and 8.6 Hz), 2.91 (2H, dd, R-H,
(6) For example, see:(a) Boucher, H. A.; Lawrance, G. A.; Lay, P. A.;
Sargeson, A. M.; Bond, A. M.; Sangster, D. F.; Sullivan, J. C. J. Am. Chem.
Soc. 1983, 105, 4652–4661. (b) Blake, A. J.; Gould, R. O.; Holder, A. J.; Hyde,
J
H-H =16.0 and 8.0 Hz), 3.10 (2H, d, R-H, JH-H = 6.4 Hz), 3.39
€
T. I.; Lavery, A. J.; Odulate, M. O.; Scrhoder, M. J. Chem. Soc., Dalton Trans.
(2H, d, R-H, JH-H=10.0 Hz), 3.76 (2H, dd, benzylic CH2, JH-H
=
1987, 118–120. (c) McAuley, A.; Whitcombe, T. W. Inorg. Chem. 1988, 27,
3090–3099. (d) Blake, A. J.; Holder, A. J.; Roberts, Y. V.; Schroder, M. J. Chem.
Soc., Chem. Commun. 1993, 260–262. (e) Grant, G. J.; Spangler, N. J.; Setzer,
W. N.; Van Derveer, D. G.; Mehne, L. F. Inorg. Chim. Acta 1996, 246, 31–40.
(f) Brown, K. N.; Hockless, D. C. R.; Sargeson, A. M. J. Chem. Soc., Dalton
Trans. 1999, 2171–2176. (g) Matsumoto, M.; Funahashi, S.; Takagi, H. D. Z.
Naturforsch. 1999, 54B, 1138–1146. (h) Matsumoto, M.; Itoh, M.; Funahashi, S.;
Takagi, H. D. Can. J. Chem. 1999, 77, 1638–1647. (i) Haines, R. I.; Hutchings,
D. R.; McCormack, T. M. J. Inorg. Biochem. 2001, 85, 1–7. (j) Brown, K. N.;
Geue, R. J.; Hambley, T. W.; Hockless, D. C. R.; Rae, A. D.; Sargeson, A. M. Org.
Biomol. Chem. 2003, 1, 1598–1608.
12.8 and 2.4 Hz), 3.88 (2H, dd, benzylic CH2, JH-H=12.8 and
2.4 Hz), 7.11 (2H, d, JH-H=8.0 Hz), 7.20 (2H, dd, JH-H = 9.6
and 7.2 Hz), 7.36 (2H, dd, JH-H = 9.6 and 5.6 Hz), 7.93 (2H, dd,
JH-H = 15.2 and 7.6 Hz), 7.95 (2H, dd, JH-H=16.8 and 8.4),
8.02 (2H, s, N-H), 8.25 (2H, s), 8.57 (2H, d, JH-H = 4.4 Hz),
8.86 (2H, d, JH-H = 8.0 Hz). 1H NMR (CD2Cl2, δ): 0.99 (2H, m),
1.43-1.53 (8H, m), 1.87 (2H, m), 2.51 (2H, d, R-H, JH-H
7.6 Hz), 2.98 (2H, d, R-H, JH-H = 8.4 Hz), 3.16 (2H, d, R-H,
H-H = 10.8 Hz), 3.47 (2H, d, R-H, JH-H = 10.8 Hz), 3.71 (2H,
=
J
dd, benzylic CH2, JH-H = 13.2 and 2.4 Hz), 3.94 (2H, dd, ben-
zylic CH2, JH-H = 7.2 and 2.4 Hz), 7.20 (2H, d, JH-H = 13.2 Hz),
7.29 (2H, dd, JH-H = 6.8 and 6.4 Hz), 7.43 (2H, dd, JH-H = 8.8
(7) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem. Soc.
1976, 98, 6521–6528.
(8) Klein, A.; McInnes, E. J. L.; Kaim, W. J. Chem. Soc., Dalton Trans.
2002, 2371–2378.
(9) Mehta, N. B.; Strelitz, J. Z. J. Org. Chem. 1962, 27, 4412–4418.
(10) Chan, K. S.; Tse, A. K.-S. Synth. Commun. 1993, 23, 1929–1934.
(11) Kissinger, P. T.; Heineman, W. R. J. Chem. Educ. 1983, 60, 702–706.

9800 Inorganic Chemistry, Vol. 49, No. 21, 2010
Chatterjee et al.
˚
and 8.0 Hz), 7.73 (2H, s, N-H), 7.92 (2H, dd, JH-H=8.4 and
5.2 Hz), 7.97 (2H, d, JPt-H =60.8 Hz, JH-H =7.6 Hz), 8.19
(2H, s), 8.49 (2H, d, JH-H =4.8Hz), 8.77 (2H, d, JH-H =8.0Hz).
MS-ESI (m/z): 870.78, [Pt(pipNHC)2(phen)](PF6)þ; 724.82,
[Pt(pipNHC)(pipNC)(phen)]þ.
radiation tuned to λ = 0.775 A. Intensity data for [Pt(pipNHC)2-
(phen)](PF6)2, [Pt(pipNHC)2(phen)](PF6)2 2CH3CN, and [Pt-
3
(pipNHC)2(bpy)](PF6)2 were collected using a Bruker SMART6000
CCD diffractometer with graphite-monochromated Cu KR ra-
diation, λ=1.54178 A. Data were integrated using SAINT and
˚
corrected for decay and for Lorentz and polarization effects;
absorption and beam corrections were applied based on a multi-
scan technique (SADABS).
[Pt(pipNHC)2(bpy)](PF6)2. Trifluoroacetic acid (14.7 μL,
0.2 mmol) was added to a suspension of cis-Pt(pipNC)2 (54 mg,
0.1 mmol) in acetonitrile (30 mL). The mixture was stirred and
sonicated, and an equimolar amount of 2,20-bipyridine (15.6 mg,
0.1 mmol) was added to the resulting solution. The mixture was
refluxed for 3 h, and the yellow solution was allowed to cool to
room temperature. NH4PF6 (41 mg, 0.25 mmol) was added, and
the resultant solution was rotary-evaporated to dryness. The
product was dissolved in a minimum amount of acetonitrile, and
diethyl ether was added to induce precipitation. Yield: 0.062 g,
63%. Anal. Calcd for C34H42F12N4P2Pt: C, 41.18; H, 4.27; N,
5.65. Found: C, 40.63; H, 4.26; N, 5.67. 1H NMR (CD3CN, δ):
0.95 (2H, m), 1.32 (2H, m), 1.55 (6H, m), 1.81 (2H, m), 2.50 (2H,
Each structure was solved by a combination of direct methods
(SHELXTL or SIR) and the difference Fourier technique and
refined by full-matrix least squares on F2 (SHELXTL). Non-H
atoms were refined with anisotropic displacement parameters.
[Pt(pipNHC)2(bpy)](PF6)2 crystallizes with two independent
molecules in the asymmetric unit (designated as A and B). The
crystal used in the determination of Pt(pipNC)2(phen) appeared
to be a split crystal with a rotation of approximately 2° relating
the two domains (CELL_NOW). The data were refined using a
suitable twin law.
The N-H H atoms for both the solvated and nonsolvated
forms of [Pt(pipNHC)2(phen)](PF6)2 were located directly
from the difference map, and the positions were refined; for
dd, R-H, JH-H = 18.0 and 10.0 Hz), 2.91 (2H, dd, R-H, JH-H
18.0 and 10.0 Hz), 3.07 (2H, d, R-H, JH-H = 13.2 Hz), 3.40 (2H,
d, R-H, JH-H = 12.0 Hz), 3.73 (2H, dd, benzylic CH2, JH-H
13.2 and 8.0 Hz), 3.86 (2H, dd, benzylic CH2, JH-H = 13.2 and
2.4 Hz), 7.07 (2H, d, JH-H = 7.2 Hz), 7.16 (2H, dd, JH-H = 9.2
and 7.6 Hz), 7.31 (2H, dd, JH-H = 6.8 and 8.8 Hz), 7.59 (2H, dd,
=
=
[Pt(pipNHC)2(bpy)](PF6)2 and [Pt(pipNHC)2(dtfmbpy)](PF6)2
3
H2O, they were located directly and the positions were held fixed at
that location. Likewise, the H atoms on the water solvent were
located directly and the positions held fixed in subsequent refine-
ments. All remaining H atoms in the structures were either located
or calculated based on geometric criteria and treated with a riding
model. The H-atom isotropic displacement parameters were de-
fined as 1.2Ueq of the adjacent atom for the complex and 1.5Ueq of
the adjacent atom for the solvate. Pt(pipNC)2(phen) crystallizes
witha disordered toluene molecule(∼25% occupancy). A suitable
disorder model was not forthcoming; thus, the solvate contri-
bution was subtracted from the reflection data (PLATON).
[Pt(pipNHC)2(phen)](PF6)2 crystallizes with two molecules of
CH3CN in the lattice when grown from CH3CN-Et2O and as
the nonsolvated form when grown from CH2Cl2-Et2O, whereas
[Pt(pipNHC)2(dtfmbpy)](PF6)2 crystallizes as a hydrate from
J
H-H = 6.4 and 8.4 Hz), 7.89 (2H, s, N-H), 7.89 (2H, d with Pt
satellites, JH-Pt = 60 Hz, JH-H = 6.8 Hz), 8.22 (2H, d, JH-H
=
5.6 Hz), 8.29 (2H, dd, JH-H = 8.0 and 9.6 Hz), 8.47 (2H, d, JH-H
= 8.0 Hz). MS-ESI (m/z): 846.27, [Pt(pipNHC)2(bpy)](PF6)þ.
[Pt(pipNHC)2(dtfmbpy)](PF6)2. The product was isolated as a
light-yellow solid by following the procedure for [Pt(pipNHC)2-
(bpy)](PF6)2 and substituting 2,20-bipyridine with dtfmbpy
(29 mg, 0.1 mmol; cis-Pt(pipNC)2, 54 mg, 0.1 mmol). Yield:
0.070 g, 62%. 1H NMR (CD3CN, δ): 1.15 (2H, m), 1.29 (2H, m),
1.57 (6H, m), 1.76 (2H, m), 2.39 (2H, dd, R-H, JH-H=13.6 and
6.0 Hz), 2.86 (2H, dd, R-H, JH-H=12.0 and 3.2 Hz), 3.02 (2H,
dd, R-H, JH-H=12.0 and 3.0 Hz), 3.29 (2H, m, R-H), 3.87 (2H,
dd, benzylic CH2, JH-H=12.8 and 2.4 Hz), 4.06 (2H, dd, ben-
zylic CH2, JH-H = 12.8 and 2.4 Hz), 7.16 (2H, dd, JH-H=8.0 and
3.2 Hz), 7.20 (2H, dd, JH-H = 9.6 and 5.6 Hz), 7.36 (2H, dd, JH-H
= 8.0 and 2.4 Hz), 8.50 (2H, bs, N-H), 7.90 (2H, d with Pt
CH3CN-Et2O. For [Pt(pipNHC)2(dtfmbpy)](PF6)2 H2O, several
3
F atoms of the -CF3 groups are disordered; however, a suitable
multicomponent disorder model was not obtained. Disordered
-
PF6 counterions were refined with multicomponent disorder
models. Crystallographic data are summarized in Tables 1 and 2.
Selected bond distances and angles are given in Table 3.
satellites, JH-Pt = 60 Hz, JH-H = 13.2 Hz), 8.43 (2H, d, JH-H
=
9.6 Hz), 8.68 (2H, d, JH-H = 5.6 Hz), 8.71 (2H, dd, JH-H =8.0and
3.2 Hz). MS-ESI (m/z): 982.76, [Pt(pipNHC)2(dtfmbpy)](PF6)þ.
X-ray Crystallography.12 Small orange-yellow blocks of Pt-
(pipNC)2(phen) (1) were obtained by slow evaporation from a
CH2Cl2-benzene-toluene solution. Yellow blocks of [Pt(pipNHC)2-
(phen)](PF6)2.2CH3CN (2), [Pt(pipNHC)2(phen)](PF6)2 (3), and
[Pt(pipNHC)2(bpy)](PF6)2 (4), as well as yellow plates of
[Pt(pipNHC)2(dtfmbpy)](PF6)2.H2O (5), were obtained by vapor
diffusion of Et2O vapors into acetonitrile or methylene chloride
solutions of the respective salts. Colorless needles of cis-Pt-
(pipNC)2 (6) were grown from N,N-dimethylformamide (DMF).
For X-ray examination and data collection, suitable crystals
were mounted in a cryo-loop with paratone-N and transferred
immediately to the goniostat bathed in a cold stream. Intensity
Results and Discussion
Synthesis. The synthesis of complexes with dangling
nucleophilic groups at the axial sites of a square-planar
metal center poses a significant challenge because of diffi-
culties in forcing the metal to bond to one donor ligand over
another. An efficient synthetic route to diarylplatinum(II)
complexes with chelating diimine ligands and dangling
amine groups is illustrated in Scheme 2. The products were
1
characterized using H NMR spectroscopy and mass
spectrometry. Elemental analyses were in good agree-
ment with the product formulations, with the exception
of [Pt(pipNHC)2(dtfmbpy)](PF6)2, which was consistently
slightly high in C and H and low in N, possibly because
of trace amounts of triphenylphosphine from the synthesis
of dtfmbpy. cis-Pt(pipNC)2 was prepared in ∼60% yield
following modification of the procedure for the synthesis of
cis-Pt(2-Me2NCH2C6H4)2.13 The reaction yielded exclu-
sively the cis isomer, as confirmed by the magnitude14
data for cis-Pt(pipNC)2 and [Pt(pipNHC)2(dtfmbpy)](PF6)2 H2O
3
were collected using a Bruker APEX2 CCD detector, whereas data
for Pt(pipNC)2(phen) were collected using a Bruker Platinum200
CCD detector at Beamline 11.3.1 at the Advanced Light Source
(Lawrence Berkeley National Laboratory) with synchrotron
(12) (a) APEX2 v1.0-27, 2.1-4, v2.0-2, SMART v5.631, and SAINT v6.45A,
v7.06A, v7.34A; Bruker AXS, Inc.: Madison, WI. (b) Sheldrick, G. M. SADABS
€
€
v2.10; University of Gottingen: Gottingen, Germany; 2008. (c) Sheldrick, G. M.
€
€
CELL_NOW, University of Gottingen: Gottingen, Germany, 2005. (d) Sheldrick,
G. M. SHELXTL v6.14; Bruker AXS, Inc.: Madison, WI. (e) PLATON v210103:
Spek, A. L. Acta Crystallogr. 1990, A46, C34. (f) Brandenburg, K. Diamond v3.2c;
Crystal Impact: Bonn, Germany. (g) SIR2004 v1.0: Burla, M. C.; Camalli, M.;
Carrozzini, B.; Cascarano, G. L.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl.
Crystallogr. 2003, 36, 1103.
(13) van der Ploeg, A. F. M. J.; van Koten, G.; Schmitz, J. E. J.; van der
Linden, J. G. M. Inorg. Chim. Acta 1982, 58, 53–58.
(14) Longoni, G.; Fantucci, P.; Chini, P.; Canziani, F. J. Organomet.
Chem. 1972, 39, 413–425.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9801
Table 1. Crystal and Structure Refinement Data for 1-3
1
2
3
formula
C36H40N4Pt
723.81
monoclinic
P21/n
11.202(2)
17.076(3)
18.359(4)
90
103.796(6)
90
3410.6(12)/4
1.410
5.132
173(2)
27 892
5772/0.0650
0.0486/0.0979
0.0575/0.1011
[C36H42N4Pt](PF6)2 2CH3CN
1097.87
triclinic
P1
11.3849(3)
12.3342(3)
17.2114(4)
93.346(1)
101.915(1)
112.898(1)
2152.52(9)/2
1.694
7.601
150(2)
18 401
7421/0.0235
0.0229/0.0566
0.0236/0.0570
[C36H42N4Pt](PF6)2
1015.77
triclinic
3
fw, g/mol
cryst syst
space group
P1
˚
a, A
10.5423(2)
12.2067(2)
15.9387(3)
86.019(1)
71.343(1)
72.672(1)
1871.95(6)/2
1.802
8.664
150(2)
15 685
6415/0.0227
0.0313/0.0786
0.0342/0.0803
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A /Z
F
calcd (g/cm3)
μ, mm-1
T, K
reflns collected
indep reflns, Rint
R1/wR2 [I > 2σ(I)]a
R1/wR2 (all data)a
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo||, wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2
.
Table 2. Crystal and Structure Refinement Data for 4-6
4
5
6
formula
[C34H42N4Pt](PF6)2
991.75
monoclinic
Cc
26.0414(5)
11.3056(2)
25.2438(5)
90
93.313(1)
90
7419.7(2)/8
1.776
8.724
150(2)
30 383
12 271/0.0364
0.0238/0.0589
0.0241/0.0590
[C36H40N4F6Pt](PF6)2 H2O
3
C24H32N2Pt
543.61
orthorhombic
Pbca
10.751(2)
19.283(4)
19.935(4)
90
90
90
4132.7(14)/8
1.747
8.426
150(2)
39 678
3778/0.1048
0.0284/0.0661
0.0438/0.0737
fw, g/mol
cryst syst
space group
1145.77
monoclinic
Cc
˚
a, A
21.3160(10)
17.1154(10)
13.3214(6)
90
120.459(1)
90
4189.3(4)/4
1.817
4.391
200(2)
23 246
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A /Z
F
calcd (g/cm3)
μ, mm-1
T, K
reflns collected
indep reflns, Rint
R1/wR2 [I > 2σ(I)]a
R1/wR2 (all data)a
9155/0.0714
0.0413/0.0954
0.0460/0.0983
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo||, wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2
.
Table 3. Selected Distances and Angles for 1-5
1
2
3
4Aa
4Ba
5
Pt-C13
Pt-N1
Pt-N2
Pt-C25
2.012(8)
2.110(6)
2.094(6)
2.017(7)
4.726(7)
4.668(7)
2.024(3)
2.114(2)
2.120(2)
2.024(3)
4.634(2)
3.192(2)
2.40(4)
2.030(4)
2.123(3)
2.116(3)
2.016(4)
3.258(3)
3.214(3)
2.33
2.035(4)
2.111(3)
2.108(3)
2.027(4)
3.187(3)
3.345(4)
2.32
2.040(4)
2.117(3)
2.105(4)
2.028(4)
3.197(3)
3.327(3)
2.37
2.024(5)
2.097(9)
2.097(5)
2.036(13)
3.290(5)
4.519(6)
2.51
Pt N3
3 3 3
Pt N4
Pt H3
Pt H4
3 3 3
3 3 3
5.22(3)
2.34
2.47
2.49
5.17
3 3 3
N1-Pt-N2
N1-Pt-C13
C13-Pt-C25
N2-Pt-C25
N2-Pt-Cl3
N1-Pt-C25
N3-H3-Pt
N4-H4-Pt
79.7(2)
93.5(2)
93.1(3)
94.0(2)
171.9(3)
172.7(3)
79.12(9)
79.19(13)
96.80(14)
89.57(15)
94.42(14)
174.14(13)
173.61(13)
155
78.33(13)
95.53(14)
90.26(15)
95.69(14)
173.74(13)
171.38(15)
148
77.99(13)
96.27(14)
89.25(15)
96.46(14)
174.26(13)
173.81(14)
141
77.6(2)
96.3(3)
90.7(3)
95.2(3)
172.6(2)
172.04(19)
153
94.58(10)
91.72(10)
94.90(9)
171.83(9)
172.85(10)
150(3)
153
152
146
a 4 crystallizes as two independent molecules (A and B) in the asymmetric unit.
of the J(195Pt-1H) coupling constant and X-ray crystal-
lography. The nearly colorless compound gives rise to
intense yellow luminescence in the solid state and is very
soluble in chloroform and methylene chloride but much
less soluble in DMF and only sparingly soluble in
acetonitrile.

9802 Inorganic Chemistry, Vol. 49, No. 21, 2010
Scheme 2. Synthetic Schemea
Chatterjee et al.
a Conditions: (i) piperidine/benzene, reflux; (ii) n-BuLi, Pt(COD)Cl2/THF, -70 °C; (iii) 1,10-phenanthroline/CH2Cl2, reflux; (iv) TFA, 1,10-
phenanthroline/CH3CN, reflux; (v) (a) TFA, 2,20-bipyridine/CH3CN, reflux, (b) TFA, 5,50-ditrifluoromethyl-2,20-bipyridine/CH3CN, reflux;
(vi) TBAOH.
Refluxing cis-Pt(pipNC)2 with phen in dichloromethane
gives an apparent equilibrium mixture of Pt(pipNC)2-
(phen) and starting materials. Because dissociation of the
phen ligand is slow (vide infra), the Pt(pipNC)2(phen)
complex can be isolated from this mixture. The dissociation
reactions of the bpy and dtfmbpy analogues are more rapid.
For example, attempts to prepare the bpy adduct by the
same procedure resulted in the expected change in solution
color from colorless to yellow-orange, but efforts to isolate
the product yielded only starting materials. On the other
hand, hexafluorophosphate salts of the protonated adducts
of each of the three diimine complexes, Pt(pipNHC)2-
(diimine)2þ, were readily prepared in 60-70% yield from
the reaction of the diimine ligand with an acidic solution
of cis-Pt(pipNC)2 (Scheme 2). Best results were obtained
when cis-Pt(pipNC)2 was completely dissolved in an aceto-
nitrile solution with 2 equiv of trifluoroacetic acid prior to
the addition of the diimine. The resulting protonated ad-
ducts are stable in an acetonitrile and methylene chloride
solution and can be treated with 2 equiv of base to give
transiently stable solutions of the deprotonated complexes.
Crystal Structures. The structures of cis-Pt(pipNC)2
and Pt(pipNC)2(phen), as well as salts of the protonated
adducts, Pt(pipNHC)2(phen)2þ, Pt(pipNHC)2(bpy)2þ, and
Pt(pipNHC)2(dtfmbpy)2þ, were confirmed by X-ray crys-
tallography (Figure 1). The pip2NC- ligands of Pt(pipNC)2
adopt a cis configuration with 80.53(19)° (C1-Pt-N1) and
80.11(19)° (C13-Pt-N2) bite angles. To alleviate steric
repulsion between the aryl and piperidyl groups, the com-
plex is tetrahedrally distorted, resulting in a dihedral angle
between the C1-Pt-N1 and C13-Pt-N2 planes of 13.1(2)°
and nonzero ligand backbone torsion angles [C1-C6-
C7-N1, 36.7(6)°; C13-C18-C19-N2, 34.8(6)°]. On the
other hand, for the four compounds with diimine ligands,
the coordination geometry is closer to square-planar. The
pipNC- and pipNHC groups accommodate the chelating
diimine ligand by rotation of the planar aryl groups out of
the coordination plane defined by the Pt atom and the four
nearest-neighbor atoms, resulting in dihedral angles of
86.8(2)-88.0(3)° for Pt(pipNC)2(phen) and 55.0(1)-78.3-
(2)° for the three protonated adducts. The resulting Pt-N-
˚
(diimine) distances [2.094(6)-2.123(3) A] fall within the
range observed for related complexes with aryl groups
trans to monodentate pyridyl or bidentate diimine ligands
15-25
˚
[2.08(1)-2.201(2) A]. These distances are substan-
tially longer than those observed for monodentate pyridyl
(15) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2004, 43,
725–733.
(16) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2005, 44,
1211–1220.
(17) Kuehl, C. J.; Arif, A. M.; Stang, P. J. Org. Lett. 2000, 2, 3727–3729.
(18) Albinati, A.; Lianza, F.; Pregosin, P. S.; Mueller, B. Inorg. Chem.
1994, 33, 2522–2526.
(19) Chase, P. A.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van Koten,
G. J. Mol. Catal. A: Chem. 2006, 254, 2–19.
(20) Romeo, R.; Arena, G.; Scolaro, L. M.; Plutino, M. R.; Bruno, G.;
ꢀ
Nicolo, F. Inorg. Chem. 1994, 33, 4029–4037.
(21) Plutino, M. R.; Scolaro, L. M.; Albinati, A.; Romeo, R. J. Am. Chem.
Soc. 2004, 126, 6470–6484.
(22) Debaerdemaeker, T.; Hohenadel, R.; Brune, H.-A. J. Organomet.
Chem. 1988, 350, 109–114.
(23) Casas, J. M.; Fornies, J.; Martin, A. J. Chem. Soc., Dalton Trans.
1997, 9, 1559–1563.
(24) Sun, Y.; Ross, N.; Zhao, S.-B.; Huszarik, K.; Jia, W.-L.; Wang,
R.-Y.; Macartney, D.; Wang, S. J. Am. Chem. Soc. 2007, 129, 7510–7511.
(25) Ong, C. M.; Jennings, M. C.; Puddephatt, R. J. Can. J. Chem. 2003,
81, 1196–1205.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9803
Figure 1. ORTEP diagram of (a) cis-Pt(pipNC)2 and(b) Pt(pipNC)2(phen) and cationic parts of (c) Pt(pipNHC)2(phen)(PF6)2 2CH3CN, (d) Pt(pipNHC)2-
3
(phen)(PF6)2, (e) Pt(pipNHC)2(bpy)(PF6)2 (only one of the two independent molecules is shown), and (f) Pt(pipNHC)2(dtfmbpy)(PF6)2 H2O. H atoms
3
[with the exception of those bonded to N(piperidyl)] are omitted for clarity. 50% probability thermal ellipsoids.
groups situated trans to ligands having a weaker trans
observed for compounds with pyridyl groups positioned
4,5,15,22,28
influence26 [e.g., Pt(phen)(en)2þ, 2.02(1) A].27 The Pt-C-
trans to aryl ligands [1.910(3)-2.038(2) A].
˚
˚
(aryl) distances [2.012(8)-2.040(4) A] are significantly
For each diimine complex, the pipNC- and pipNHC
groups adopt the more sterically favored head-to-tail con-
formation in which the piperidyl groups are directed toward
opposite faces of the coordination plane. Interestingly,
in crystals of Pt(pipNC)2(phen), the piperidyl N atoms
˚
longer than those found for cis-Pt(pipNC)2 [Pt-C1,
˚
˚
1.980(5) A; Pt-C(13), 1.985(5) A] but within the range
(26) Connick, W. B.; Marsh, R. E.; Schaefer, W. P.; Gray, H. B. Inorg.
Chem. 1997, 36, 913–922.
(27) Kato, M.; Takahashi, J. Acta Crystallogr., Sect. C.: Cryst. Struct.
Commun. 1999, 55, 1809–1812.
(28) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2002, 41,
2275–2281.

9804 Inorganic Chemistry, Vol. 49, No. 21, 2010
Chatterjee et al.
˚
are directed away from the Pt atom [Pt N4, 4.668(7) A;
Thus, hydrogen bonding to solvate can effectively compete
with the formation of a second intramolecular NH Pt
interaction.
3 3 3
˚
Pt N3, 4.726(7) A], forming short intramolecular
3 3 3
3 3 3
˚
H29, 2.54 A;
contacts with phenyl groups [N4
3 3 3
1H NMR Spectroscopy. The room-temperature 1H
NMR spectra of the protonated complexes show the
expected pattern of resonances, and the spectra are con-
centration-independent over the investigated range (up to
0.01 M). Notably, in each case, one set of diimine reso-
nances and four phenyl proton resonances are observed
in the aromatic region. The benzylic proton resonances
appear in an AB pattern near 3.8-3.9 ppm in CD3CN
solution for the phen and bpy complexes [between 4.1 and
3.8 ppm for Pt(pipNHC)2(dtfmbpy)2þ], establishing that
these protons are diastereotopic and that the coupling
to the amine proton is weak. [In contrast, the benzylic
proton resonances for cis-Pt(pipNC)2 appear as a sharp
singlet with 195Pt satellites, indicating that relaxation of
the tetrahedral distortion observed in the solid state is rapid
on the NMR time scale.] For the protonated adducts, the
R-piperidyl protons give rise to four distinct resonances with
equal intensities from 2.5 to 3.2 ppm, and a similar pattern
is observed upfield for the β-proton resonances. Taken
together with the observations for the benzylic proton reso-
nances, these data confirm that the combination of inver-
sion about the piperidyl N and inversion of the piperidyl
ring is slow on the NMR time scale, as was expected for
a protonated piperidyl group. In addition, asynchronous
rotation about the Pt-C bond must also be slow, as was
expected from steric considerations.36-39 Overall, the data
are consistent with the presence of a single rotamer, which
we hypothesize to be the head-to-tail isomer, as observed in
the crystal structures and in keeping with the steric demands
of the piperidyl groups. This configuration would allow for
˚
N3 H17, 2.47 A]. This arrangement is distinctly dif-
3 3 3
ferent from our observations for other platinum(II)
complexes with dangling piperidyl groups, which typi-
cally show at least one short contact between a piperidyl
N atom and the PtII center; for example, the crystal struc-
ture of [Pt(phtpy)(pip2NCN)](BF4) (phtpy = 4-phenyl-
2,20:60,200-terpyridine) reveals that each of the complexes
in the asymmetric unit has a short Pt N(piperidyl)
3 3 3
4
contact [3.512(5) and 3.195(5) A]. Each of those pre-
˚
viously studied complexes is positively charged, and
therefore the metal centers are expected to be more
electrophilic than those for Pt(pip2NC)(diimine). Interest-
ingly, each of the four protonated complexes has at least
one protonated piperidyl N atom directed toward the Pt
˚
atom, resulting in short NH Pt distances (2.32-2.51 A),
3 3 3
which are significantly shorter than the sum of the
van der Waals radii. In order to accommodate this inter-
action, the dihedral angles between the phenyl ring and
the coordination plane fall in a relatively narrow range
(55-65°). These results suggest that the Pt center has
partial Brønsted base character, as is expected for
proton-sponge-type behavior in which the protonated
amine is stabilized by interaction with the nucleophilic
metal center. Similar interactions have been noted by van
Koten et al.29,30 and others18,23,31-33 for PtII centers with
nearby protonated amine groups; in other cases, such
interactions have not been observed.34 In the present case,
the geometric constraints of the pipNHC ligand result in
bent NH Pt (141-155°) angles and prevent a more
3 3 3
linear arrangement, which would be optimal for a dipole-
monopole interaction. Nevertheless, it is apparent that the
geometries of the NH Ptinteractions are consistent with
the possibility of two NH Pt interactions.
3 3 3
The piperidyl NH protons give rise to a broadened
resonance, which occurs near ∼8 ppm in CD3CN for the
phen and bpy complexes [8.5 ppm for Pt(pipNHC)2-
(dtfmbpy)2þ]. It is shifted by ∼1.0 ppm downfield from
that of Pt(pip2NCNH2)(tpy)3þ.5 The Pt center in the latter
complex is comparatively electron-poor, and there is no
3 3 3
hydrogen bonding to the metal rather than three-center/
two-electron agostic interactions.18,32,35 Interestingly, in
crystals containing Pt(pipNHC)2(phen)2þ and Pt(pipNHC)2-
(dtfmbpy)2þ with solvate molecules, the NH group of
one of the piperidyl groups is directed toward the Pt
center [Pt(pipNHC)2(phen)2þ, NH Pt, 2.40(4) A; Pt-
˚
evidence of NH Pt interactions. A downfield chemical
shift of proton resonances from free ligand values is a
3 3 3
3 3 3
(pipNHC)2(dtfmbpy)2þ, NH Pt, 2.51 A], whereas
˚
3 3 3
the NH group of the other is directed toward a solvate
signature of NH Pt hydrogen bonding,33,40 and our
3 3 3
molecule [Pt(pipNHC)2(phen)2þ, NH N(CH3CN),
observations are consistent with this possibility. In DMF-
d7, the NH resonance shifts linearly upfield with tempera-
ture (9.10-8.83 ppm, 295-370 K; Figure S2 in the Sup-
porting Information), whereas the other resonances are
essentially unaffected; the water resonance undergoes a
downfield shift of 0.61 ppm (from 3.49 to 2.88 ppm) over
the same temperature range. However, we have not observed
195Pt-1H coupling in any solvent (CD3CN, DMF-d7, and
CD2Cl2), and Casas et al. have cautioned that the mere
observation of a downfield shift from the free ligand is not
proof of a Pt-H bonding interaction.23 For example,
shielding due to paramagnetic anisotropy of the PtII ion
3 3 3
2.08(4) A; Pt(pipNHC)2(dtfmbpy)2þ, NH N(H2O),
˚
3 3 3
˚
1.90A; Figure S1 in the Supporting Information). For crys-
tals containing Pt(pipNHC)2(phen)2þ and Pt(pipNHC)2-
(bpy)2þ without solvate, both piperidyl groups are directed
toward the Pt center [Pt(pipNHC)2(phen)2þ, NH Pt,
3 3 3
2.33 and 2.34 A; Pt(pipNHC)2(bpy)2þ, NH Pt, 2.32 and
˚
3 3 3
˚
2.47 A for molecule A and 2.37 and 2.49 A for molecule B).
˚
(29) Wehman-Ooyevaar, I. C. M.; Grove, D. M.; Kooijman, H.; van der
Sluis, P.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1992, 114, 9916–9924.
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T. W. Aust. J. Chem. 2000, 53, 451–456.
(32) Yao, W.; Eisenstein, O.; Crabtree, R. H. Inorg. Chim. Acta 1997, 254,
105–111.
(33) Brammer, L.; Charnock, J. M.; Goggin, P. L.; Goodfellow, R. J.;
Orpen, A. G.; Koetzle, T. F. J. Chem. Soc., Dalton Trans. 1991, 1789–1798.
(34) For example, see:Hinman, J. G.; Baar, C. R.; Jennings, M. C.;
Puddephatt, R. J. Organometallics 2000, 19, 563–570.
(35) Brammer, L.; McCann, M. C.; Bullock, R. M.; McMullan, R. K.;
Sherwood, P. Organometallics 1992, 11, 2339–2341.
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M. L.; Cusumano, M. Eur. J. Inorg. Chem. 2003, 2612–2618.
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Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9805
also is expected to induce a significant downfield shift.41
Therefore, the NMR evidence for the persistence of a
NH Pt interaction in solution for the protonated com-
3 3 3
plexes can only be regarded as circumstantial.
In the 1H NMR spectrum of Pt(pipNC)2(phen) in
CD2Cl2, one set of diimine resonances and four phenyl
proton resonances are observed in the aromatic region.
The benzylic protons give rise to two characteristic AB
patterns centered at 4.09 ppm (J, 13.9 Hz; Δν/J, 3.7) and
3.93 ppm (J, 14.6 Hz; Δν/J, 7.1), respectively, in a 3:2
intensity ratio (Δν is the chemical shift difference between
the furthest upfield resonance and the furthest downfield
resonance). These observations suggest that rotation of
the phenyl group about the Pt-C bond remains slow on
the NMR time scale. The two isomers present in unequal
amounts are tentatively attributed to the head-to-tail and
head-to-head conformers.
In CD3CN, DMF-d7, and CD2Cl2, the 1H NMR spec-
trum of the deprotonated complex is time-dependent. For
example, in DMF-d7 and CD2Cl2, resonances associa-
ted with Pt(pipNC)2(phen) gradually lose intensity, while
resonances associated with cis-Pt(pipNC)2 and the free
phen ligand gain intensity, indicating the occurrence of
ligand dissociation:
Figure 2. Electronic absorption spectra of Pt(phen)(Ph) (CH Cl ,
2
;
;
2
2
CH3CN, ---), Pt(pipNHC)2(phen)2þ (CH Cl , blue ; CH CN, blue ---),
;
;
and Pt(pipNC)2(phen) (CH2Cl2, red ; CH CN, red ---) in freshly
2
2
3
3
prepared solutions. Longest wavelength absorption maximum (νmax) vs
ET(30) of a series of solvents (left-to-right: chloroform, methylene chlo-
ride, acetone, DMSO, and acetonitrile) for Pt(phen)(Ph)2 (O) and Pt-
(pipNC)2(phen) (redb); the lines showthe linearbestfit for the purposeof
showing trends in the data.
PtðpipNCÞ2ðphenÞhcis-PtðpipNCÞ2 þ phen
solid, which dissolves in CH2Cl2 to give two intense
bands, maximizing at 266 and 310 nm, respectively.
The ligand precursor pipNCBr (ε<350 M-1 cm-1) and
The reaction was monitored by a gradual decrease in the
intensity of the coordinated phen ligand resonances rel-
ative to a TMS standard immediately following the addi-
tion of 2 equiv of TBAOH to a solution of Pt(pipNHC)2-
(phen)2þ (Figure S3 in the Supporting Information). The
fraction of Pt(pipNC)2(phen) remaining and the fraction
of product formed as a function of time are approximately
modeled by single-exponential functions. Under typical
conditions (∼10-2 M), the resulting decay constants were
10-5-10-6 s-1. After several days in CD2Cl2, the spectra
ceasedto changebutshowedthe presence of both reactants
and products, suggesting that an equilibrium had been
established and, therefore, the back reaction of cis-Pt-
(pipNC)2 and phen contributes to the observed dissocia-
tion kinetics. In a CD3CN solution, the disappearance of
Pt(pipNC)2(phen) resonances is accompanied by precipi-
N-isopropylidenemethylamine (ε < 100 M-1 cm-1 42,43
)
only absorb weakly at these wavelengths, indicating
the involvement of the Pt center in the electronic transi-
tions. The spectrum is very similar to that of cis-Pt-
(2-Me2NCH2C6H4)2, and by analogy, the transitions are
assigned as having significant metal-to-ligand charge-
transfer (MLCT) character.13
For the diimine complexes, we have focused our atten-
tion on the phen derivatives and the Pt(phen)(phenyl)2
model complex because of the greater stability of Pt-
(pipNC)2(phen) compared to the other deprotonated
adducts. The yellow protonated adducts, [Pt(pipNHC)2-
(diimine)](PF6)2, dissolve in CH2Cl2, CH3CN, and DMF
to give yellow solutions, whereas Pt(pipNC)2(phen) gives
yellow-orange solutions. In keeping with the absorption
spectra of related platinum(II) diimine compounds,44-46
Pt(phen)(Ph)2, Pt(pipNHC)2(phen)2þ, and Pt(pipNC)2-
(phen) each exhibit an intense ligand-centered transition,
1
tation. The H NMR spectrum of the precipitate in a
CDCl3 solution confirms the presence of cis-Pt(pipNC)2
(which is insolublein CD3CN) and free phen ligand, aswell
as a small fraction of an insoluble precipitate that remains
unidentified. The disappearance of Pt(pipNC)2(phen) res-
onances in a CD3CN solution is considerably more rapid
(∼10-3 s-1), as was expected for precipitation of products
effectively driving the ligand dissociation reaction to com-
pletion. It is not presently known if other solvent effects,
such as specific interactions between the solvent and com-
plex, also play a significant role in the observed reactivity.
1H NMR spectra of Pt(pipNC)2(bpy) and Pt(pipNHC)2-
(dtfmbpy) confirm that these complexes also undergo
diimine ligand dissociation, albeit much more rapidly.
Electronic Spectroscopy. In order to assess the influence
of the dangling piperidyl and piperidinium groups on the
electronic structures of these complexes, electronic ab-
sorption spectra were recorded. cis-Pt(pipNC)2 is a colorless
maximizing near 270-280 nm (20-30 000 M-1 cm-1
;
Figure 2). Measurements in different solvents (methanol,
acetonitrile, DMSO, acetone, methylene chloride, and
chloroform) indicate that the absorption maxima are
relatively insensitive to variations in Reichardt’s ET(30)
solvent parameter,47 with sensitivities comparable to those
observed for ligand-centered transitions of Pt(bpy)Cl2.48
(42) Brinton, R. K.; Chang, S. Ber. Bunsenges. Phys. Chem. 1968, 72, 217–
221.
(43) Nelson, D. A.; Worman, J. J. Tetrahedron Lett. 1966, 507–509.
(44) Fleeman, W. D.; Connick, W. B. Comments Inorg. Chem. 2002, 23,
205–230.
(45) Jin, V. X.; Ranford, J. D. Inorg. Chim. Acta 2000, 304, 38–44.
(46) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447–457.
(47) Reichardt, C. Chem. Rev. 1994, 94, 2319–2358.
(48) Gidney, P. M.; Gillard, R. D.; Heaton, B. T. J. Chem. Soc., Dalton
Trans. 1973, 132–134.
(41) Buckingham, A. D.; Stephens, P. J. J. Chem. Soc. 1964, 4583–4587.

9806 Inorganic Chemistry, Vol. 49, No. 21, 2010
Chatterjee et al.
Each of the complexes also exhibits several absorption fea-
tures in the 350-380 nm range. The shoulders and maxima
in this region are somewhat solvent-sensitive but do not
track with polarity in an obvious pattern. The situation is
likely complicated by overlapping bands with ligand-
centered and MLCT character in this region.45,49
Platinum(II) phenanthroline complexes with phenyl
donor ligands are reasonably expected to exhibit one or
more MLCT transitions in the 400-500 nm region.8 The
stabilization of MLCT states under these circumstances is
consistent with the strongly electron-donating ancillary
ligands destabilizing the filled 5d(Pt) levels relative to the
diimine π levels.44,50 Therefore, it is not surprising that
both the model complex [i.e., Pt(phen)(Ph)2] and Pt-
(pipNC)2(phen) give rise to a long-wavelength absorption
band in the 420-450 nm range (∼2000 M-1 cm-1
;
Figure 2). The longest-wavelength absorption profiles are
qualitatively similar. Interestingly, in methylene chloride, the
maximum for Pt(pipNC)2(phen) is blue-shifted by 870 cm-1
from that of Pt(phen)(Ph)2, whereas in acetonitrile, the max-
imum for Pt(pipNC)2(phen) is red-shifted by 850 cm-1 from
that of Pt(phen)(Ph)2. In this respect, the situation is
different from that found for Pt(tpy)(pip2NCN)þ.51 The
electronic spectrum of the terpyridyl complex at wave-
lengths <500 nm is similar to that of the Pt(tpy)(2,6-dmph)þ
model complex. However, whereas the model complex is
essentially nonabsorbing at >500 nm, Pt(tpy)(pip2NCN)þ
shows a new absorption band, appearing as a weak shoulder
near 550 nm (∼300 M-1 cm-1), which has been attributed
to interaction of the dangling piperidyl group with the metal
Figure 3. UV-visible absorption spectra recorded during the addition
of 100 μM TBAOH to a 50 μM solution of [Pt(pipNHC)2(phen)](PF6)2 in
CH2Cl2. The addition of (a) 1 equiv and (b) a second 1 equiv in 0.1 M
increments.
markable influence of the protonated piperidyl groups on
the MLCT energies exceeds expectations for a through-
bond inductive effect because the piperidinium N atoms
are four and five covalent bonds removed from the metal
center and diimine ligands, respectively. We attribute this
center. The absence of similar evidence of Pt N-
3 3 3
(piperidyl) interactions in Pt(pipNC)2(phen) is consis-
tent with the lower electrophilicity of the metal center of
this complex compared to Pt(tpy)(pip2NCN)þ. On the
other hand, there are surprising differences in the solvent
sensitivities of the longest-wavelength MLCT bands of
Pt(phen)(Ph)2 and Pt(pipNC)2(phen), pointing to the
subtle influence of the dangling piperidyl groups on the
electronic structure. As expected,8 the band for Pt(phen)-
(Ph)2 exhibits negative solvatochromism, shifting to longer
wavelength with decreasing solvent polarity; the depen-
dence on ET is comparable to that noted for MLCT transi-
tions of Pt(bpy)Cl2.48 Interestingly, although the maximum
for Pt(pipNC)2(phen) typically lies within 1000 cm-1 of
that for Pt(phen)(Ph)2 in most solvents, the band exhibits a
distinctly positive solvatochromic behavior (inset, Figure 2).
In other words, the solvent sensitivities of the long-
wavelength band are approximately reversed for Pt-
(pipNC)2(phen) (CH3CN, 441 nm; CH2Cl2, 421 nm) and
Pt(phen)(Ph)2 (CH3CN, 425 nm; CH2Cl2, 437 nm). The
reason for these differences is not certain, and the possibility
that positive solvatochromism is a characteristic of apical
behavior to axial NH Pt interactions, which effectively
3 3 3
reduce the electron density at the metal center and thereby
destabilize MLCT states. Consistent with intramolecular
interactions rather than intermolecular interactions, we
note that, over the concentration range of these measure-
ments (up to 0.2 mM), the spectra obey Beer’s law.
Although the proton-acceptor properties of PtII centers
are well documented,18,23,29-33 little attention has been
given to the influence of these interactions on the electronic
structures of the complexes. The gradual addition of 1
equiv of base to a solution of Pt(pipNHC)2(phen)2þ in
CH2Cl2 causes the longest-wavelength tail of the absorp-
tion spectrum to broaden and shift slightly (∼800 cm-1) to
the red (Figure 3a). The gradual addition of a second 1
equiv shifts the longest-wavelength absorption maximum
by an additional 2800 cm-1 to the red (Figure 3b). Both
titration steps exhibit sharp isosbestic points characteristic
of an A f B f C process. These observations are con-
sistent with significantly different pKa values for the mono-
and diprotonated adducts, as would be expected if the
protons interact with the metal center on opposite faces of
the coordination plane.
PtII N interactions is currently under investigation.
3 3 3
Protonation of the piperidylgroupshas a dramatic effect
on the longest-wavelength band, causing it to disappear
entirely (or blue shift by >2500 cm-1) or become obscured
by other transitions at wavelengths <400 nm. This re-
Electrochemistry. To assess the influence of the dan-
gling piperidinium and piperidyl groups on the redox
properties of Pt(pipNHC)2(phen)2þ and Pt(pipNC)2-
(phen), respectively, cyclic voltammograms (CVs) were
recorded in a 0.1 M TBAPF6-methylene chloride solu-
tion (Figure 4). The CV of Pt(pipNHC)2(phen)2þ (0.75 V/s,
gold working electrode) shows no oxidation at potentials
(49) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529–1533.
(50) Connick, W. B.; Miskowski, V. M.; Houlding, V. H.; Gray, H. B.
Inorg. Chem. 2000, 39, 2585–2592.
(51) The absorption spectrum of the latter complex in CH2Cl2 shows a
distinct shoulder near 550 nm which is absent from the spectrum of the
Pt(tpy)(Ph)þ model compound.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 9807
Figure 4. CVsof1.0mM[Pt(pipNHC)2(phen)](PF6)2 (0.1M TBAPF6 in CH2Cl2) uponthe additionofTBAOHata sweep rateof0.100V/s(gold working
electrode): (a) no added base; (b) 0.5 mM TBAOH; (c) 1.0 mM TBAOH; (d) 1.5 mM TBAOH; (e) 2.0 mM TBAOH; (f) 2.5 mM TBAOH.
<1.5 V vs Ag/AgCl and two broad irreversible reduction
waves at -0.6 and -0.75 V, respectively. The addition
of 0.5 mol equiv of TBAOH resulted in the appearance
of an irreversible wave characterized by an anodic peak
(Epa) maximizing near 0.8 V and a very weak feature near
0.45 V. The addition of 1 equiv of base caused a signifi-
cant increase in the current near 0.8 V with little change
at 0.45 V. On the other hand, the further addition of base
(up to 2 equiv in total) caused a significant increase in the
current associated with the 0.45 V feature at the expense of
the current associated with the process at higher poten-
tial. The 0.45 V wave exhibited a slight splitting, suggest-
ing two closely overlapping processes. The appearance
of the 0.45 V wave was accompanied by a back-reduction
process near -0.3 V. The latter reduction was only
observed with sweeping initially in the anodic direction
to a reversal potential J0.45 V, indicating that the electro-
active species at -0.3 V ultimately originates from the
oxidation process at 0.45 V. Even at scan rates up to 5 V/s,
no reduction was observed at more positive potentials
(i.e., >-0.3 V), suggesting that the electrochemical pro-
duct generated at 0.45 V reacts rapidly to produce a
chemical product (i.e., an EC mechanism). The overall
chemical reversibility, as reported by the relative peak
currents of the anodic 0.45 V wave and the cathodic
-0.3 V wave, increases with an increase in the concentra-
tion of the base. The ratio of the peak currents also
is scan-rate-dependent. At low scan rates (0.025 V/s),
there was virtually no current associated with the back-
reduction at 0.3 V, indicating that the 0.45 V electro-
chemical product is somewhat unstable. The addition of
more than 2.5 equiv of base resulted in decomposition,
which was demonstrated by the disappearance of the
0.45 and -0.3 V processes and a significant broadening
of Pt(pipNC)2(phen) in acetonitrile prevented an exhaustive
study (Epa, 0.35 V;Epc, -0.2 V). The electrochemistry also is
electrode-dependent. Notably, the return wave near -0.3 V
is absent when using a platinum working electrode.
Because the observed electrochemical processes are
irreversible, the peak potentials represent limiting values
of the thermodynamic potentials. Therefore, the accumu-
lated electrochemical data, in conjunction with the UV-
visible titration results, establish that the monoproton-
ated adduct is oxidized at e0.8 V, whereas Pt(pipNC)-
(phen) is oxidized at e0.45 V vs Ag/AgCl. The irreversible
oxidation of Pt(pipNC)(pipNHC)(phen)þ near 0.8 V is
reasonably attributed to either a pipNC-centered or
metal-centered oxidation. For example, we note that the
free ligand precursor, pipNCBr, exhibits an adsorption
process at almost identical potentials (Epa, 0.82 V, 100 mV/s).
On the other hand, an argument for metal-centered
oxidation could be made based on the observation
that, because of the steric constraints of the mesityl
ligands, Pt(phen)(mesityl)2 exhibits a nearly reversible
PtIII/II couple at ∼0.80 V vs Ag/AgCl (0.45 V vs Fcþ/Fc;
0.1 V/s; 0.1 M TBAPF6 in THF).8 Although the CVs
of cis-Pt(pipNC)2 (Epa, 0.97 V) and Pt(phen)(Ph)2 (Epa,
1.17 V) show irreversible processes at slightly more
positive potentials, the availability of a dangling piperidyl
group at the axial site of Pt(pipNC)(pipNHC)(phen)þ can
be reasonably expected to decrease the PtIII/II redox
potential.3-5
The oxidation of Pt(pipNC)2(phen) occurs at signifi-
cantly more negative potentials than the oxidation of
pipNCBr, Pt(phen)(Ph)2, or Pt(phen)(mesityl)2. There-
fore, the wave at 0.45 V is tentatively attributed to oxida-
tion of the metal center, which because of the instability of
platinum(III) is expected to directly (or following chemi-
cal steps) yield platinum(IV) products. Consistent with
this assignment, we note that the cathodic shift with re-
spect to the PtIII/II couple of Pt(phen)(mesityl)2 (0.80 V) is
1
of resonances in the H NMR spectrum. The electro-
chemistry is qualitatively similar in DMF with similar
values of Epa (0.5 V) and Epc (-0.2 V). The instability

9808 Inorganic Chemistry, Vol. 49, No. 21, 2010
Chatterjee et al.
in accordance with stabilization of the electrochemical prod-
ucts by interaction of the dangling piperidyl groups with the
oxidized metal center. Attempts to assess electron stoichi-
ometry by bulk electrolysis or spectroelectrochemistry were
hampered by the instability of the electrochemical products.
Nevertheless, accompanying UV-visible absorption mea-
surements show a pronounced decrease in absorbance at
wavelengths >325 nm. This behavior is qualitatively con-
sistent with a metal-centered oxidation because bands with
MLCT character are expected to blue-shift or to disappear
altogether. The cathodic process near -0.3 V is tentatively
assigned to the reduction of a platinum(III) or platinum(IV)
product whose identity is not yet certain.
Conclusions
An effective synthetic route to diarylplatinum(II) diimine
complexes with two dangling piperidyl groups involves prepara-
tion of the diprotonated piperidinium adducts, Pt(pipNHC)2-
(diimine)2þ. Deprotonation yields Pt(pipNC)2(diimine) prod-
ucts. Crystallographic and spectroscopic data for the
protonated complexes are consistent with NH Pt interac-
tions involving the piperidinium groups, suggesting that the
metal center behaves as a Brønsted base. The geometries of
3 3 3
the NH Pt interactions in the solid state are consistent
3 3 3
with hydrogen bonding to the metal rather than three-center/
two-electron agostic interactions. NH Ptinteractions have
3 3 3
a significant impact on the electronic structures of these com-
plexes, strongly destabilizing MLCT states and inducing a
substantial anodic shift in metal-centered redox processes. By
Although the chemical irreversibility of the oxidation
process near 0.45 V prevents assessment of the electro-
chemical reversibility or extraction of precise thermo-
dynamic information, it is noteworthy that the peak potential
of the anodic process shows a characteristic dependence on
the sweep rate, with Epa undergoing a substantial anodic
shift with an increase in the scan rate.4,5,52,53 For example,
Epa increases from 0.42 to 0.77 V as the scan rate is varied
from 0.05 to 4.0 V/s. Over the same scan rate range, Epc
of the cathodic process near -0.3 V decreases by only
0.06 mV (Figure S6 in the Supporting Information). In the
case of the PtIV/II couple of Pt(tpy)(pip2NCN)þ, this pat-
tern of behavior has been attributed to the formation of a
preorganized structure, such as a five- or six-coordinate
complex, prior to or during anodic electron transfer.4,5
The conversion of six-coordinate platinum(IV) to four-
coordinate platinum(II) and axial bond cleavage during
the reverse (cathodic) process are likely to require less
preorganization, making Epc less sensitive to the scan rate.
The anodic peak potential suggests that the formal PtIV/II
couple occurs at e0.45 V. To better gauge where this
couple may reasonably lie, we have noted that investiga-
tions of six-coordinate ruthenium complexes show that
substitution of a pyridyl group for a phenyl group [e.g., Ru-
contrast, N Pt interactions in the deprotonated adducts,
3 3 3
Pt(pipNC)2(diimine), must be weak relative to outer-sphere
two-electron reagents, such as Pt(pip2NCN)(tpy)þ. Despite
the low electrophilicity of the metal center, the deprotonated
complexes undergo diimine ligand dissociation to give cis-
Pt(pipNC)2. It may be inferred that these reactions are
initiated by coordination of a dangling piperdidyl group at
the axial site of the square-planar complex.57 In the case of
Pt(pipNC)2(phen), the presence of piperidyl groups also sta-
bilizes higher oxidation states, in accordance with the notion
that the amine groups are available to bind at the axial sites of
the oxidized metal center. Nevertheless, the chemical and elec-
trochemical reversibility is substantially lower than that of
Pt(pip2NCNH2)(tpy)þ. We believe that a contributing factor
is the lower electrophilicity of the metal center in Pt(pipNC)2-
(phen), which is anticipated to decrease the tendency of the
dangling piperidyl groups to interact at the open coordina-
tion sites compared to the tpy adduct. These interactions are
expected to preorganize the complex for electron transfer,
thereby increasing the rate of heterogeneous electron trans-
fer. In addition, it would appear that the ligand denticity and
conformational flexibility of Pt(pipNC)2(phen) favor chemi-
cal steps following electron transfer that are comparatively
slow in the case of a reagent with two potentially meridional-
coordinating ligands (e.g., Pt(pip2NCN)(tpy)þ).
(bpy)33þ/2þ, 1.29 V; Ru(bpy)2(2-phenylpyridine)2þ/þ
,
0.66 V vs SCE;54 Ru(tpy)23þ/2þ, 1.27 V; Ru(tpy)(6-phenyl-
2,20-bipyridine)2þ/þ, 0.43 V vs SSCE]55,56 causes a 0.6-0.8
V cathodic shift in the RuIII/II redox couple. Therefore, by
comparison to Pt(tpy)(pip2NCN)þ (0.4 V vs Ag/AgCl; 0.1
M TBAPF6 in CH3CN), and as expected for the increased
σ/π-donor properties of phenyl versus a pyridyl group, the
PtIV/II couple of Pt(pipNC)2(phen) is reasonably predicted
to occur in the vicinity of -0.4 V for the analogous reac-
tion. Thus, the appearance of Epa for Pt(pipNC)2(phen)
near 0.45 V is suggestive of substantially decreased electro-
chemical reversibility in this system or even an entirely
different mechanism. In this regard, it has occurred to us
that the reduced electrophilicity of the Pt center of Pt-
(pipNC)2(phen) compared to Pt(tpy)(pip2NCN)þ is likely
to reduce the tendency to form the preorganized structure
necessary to initiate metal-centered oxidation at relatively
negative potentials.
Acknowledgment. Funding for the SMART6000 CCD
diffractometer was through NSF-MRI Grant CHE-
0215950. W.B.C. thanks the National Science Foundation
(Grant CHE-0134975) for their generous support and the
Arnold and Mabel Beckman Foundation for a Young
Investigator Award. S.C. thanks the University of Cincinnati
University Research Council for a summer fellowship, and
the University of Cincinnati Industrial Affiliates Program
for their generous support. We also thank Dr. Hershel Jude
and Bonnie M. Young for helpful discussions. Synchrotron
data were collected through the SCrALS (Service Crystal-
lography at Advanced Light Source) project at Beamline
11.3.1 at the Advanced Light Source (ALS), Lawrence
Berkeley National Laboratory. The ALS is supported by
the U.S. Department of Energy, Office of Energy Sciences,
under Contract DE-AC02-05CH11231.
(52) Fernandes, J. B.; Zhang, L. Q.; Schultz, F. A. J. Electroanal. Chem.
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Supporting Information Available: Figures S1-S6, Table S1, and
crystallographic data in CIF format for all compounds. This mate-
rial is available free of charge via the Internet at http://pubs.acs.org.
(57) Langford, C. H.; Gray, H. B. Ligand Substitution Processes; W. A.
Benjamin, Inc.: Reading, MA, 1966.