Generation of an Unprecedented Excited State Oxidant in a Coordinately Unsaturated Platinum Complex

Full text of DOI:10.1021/ic971400z

1432
Inorg. Chem. 1998, 37, 1432-1433
Generation of an Unprecedented Excited State Oxidant in a Coordinately Unsaturated Platinum Complex
Karl Base and Mark W. Grinstaff*
Department of Chemistry, P. M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27708
ReceiVed NoVember 6, 1997
Alteration of the ligand environment surrounding a metal center
to afford an electronic structure with a specific photochemical
activity is an area of widespread interest. The manipulation of
metal diimine excited states1,2 in the design of artificial photo-
synthetic receptors, molecular photonic devices, photocatalysis,
and novel electron-transfer systems is actively pursued.3-5
Coordinately saturated polypyridyl d6 metal complexes [e.g., Ru-
(bpy)3] have been extensively studied but are limited to outer-
sphere electron-transfer reactions. Luminescent square-planar
platinum diimine (i.e., phenanthroline and bipyridine) complexes,
however, are coordinately unsaturated, and potentially can be used
for additional applications including bimolecular reactions. Work
on platinum diimine chromophores over the last 20 years has
primarily focused on halide and conjugated thiol ligands (e.g.,
toluene 1,2-dithiolate and 1,2-dicyanoethylene-1,2-dithiolate).6-11
Current research goals in this area are directed toward (i)
increasing the solubility properties of the Pt chromophores, (ii)
improving the photochemical and thermal stability of these Pt
Figure 1. ORTEPII drawing of 2 with 50% probability ellipsoids showing
the atomic numbering system. H atoms are not shown.
chromophores, and, importantly, (iii) tuning the excited-state
reduction and oxidation potentials for enhanced and more diverse
reactivity. Herein, we report the synthesis and characterization
of novel diimine dithiolate o-carborane platinum(II) complexes
that are photochemically stable, soluble in solvents of varying
polarities, and potent oxidizing agents in their excited state.
4,7-Diphenyl-1,10-phenanthroline[1,2-dithiolato-1,2-dicarba-
closo-dodecaborane(12)]platinum(II) [1] and 4,7-diphenyl-1,10-
phenanthroline-bis[1-thiolato-1,2-dicarba-closo-dodecaborane-
(12)]platinum(II) [2] were synthesized as shown in Scheme 1.
The complexes were purified by column chromatography to yield
analytically pure crystalline solids.12,13
Scheme 1. Synthesis of Complexes 1 and 2
The ORTEP drawing of 2 is shown in Figure 1.14 The Pt-S
carborane and Pt-N diimine bond lengths are 2.313(3), 2.2864(22),
2.082(8), and 2.078(7) Å, respectively, and are in agreement with
other Pt diimine dithiolate structures.15,16 The C-S bond lengths
(1) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 6, 759-833.
(1.773(9) and 1.806(10) Å) are consistent with a C-S single bond
in a metal-thiolato-carborane complex.17,18 Of particular note
is the lack of molecular stacking observed in the crystal structure,
since platinum(II) complexes are notorious for intermolecular
stacking with short Pt-Pt or π-π (3.24 and 3.51 Å, respectively)
distances.19,20 The three-dimensional space occupied by the
o-carborane cage hinders close molecular interactions, and as a
result these complexes are soluble in solvents of varying polarities.
A solution of 1 exhibits an intense absorption at 290 nm for
the π-π* transition of diphenylphenanthroline, and a second
absorption at longer wavelength that is strongly solvent dependent
(2) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret,
C.; Balzani, V. Chem. ReV. 1994, 94, 993-1019.
(3) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-170.
(4) Supramolecular Photochemistry; Balzani, V., Ed.; D. Reidel Pub. Co.:
Dordrecht, Holland, 1987.
(5) Gray, H. B.; Winkler, J. R. Annu. ReV. Biochem. 1996, 65, 537-561.
(6) Chan, C. W.; Cheng, L. K.; Che, C. M. Coord. Chem. ReV. 1994, 132,
87-97.
(7) Vogler, C.; Schwederski, B.; Klein, A.; Kaim, W. J. Organomet. Chem.
1992, 436, 367-378.
(8) Barigelletti, F.; Sandrini, D.; Maestri, M.; Balzani, V.; von Zelewsky,
A.; Chassot, L.; Jolliet, P.; Maeder, U. Inorg. Chem. 1988, 27, 3644-
3647.
(9) Webb, D. L.; Rossiello, D. L. Inorg. Chem. 1971, 10, 2213-2218.
(10) Camassei, F. D.; Ancarani-Rossiello, F. J. Lumin. 1973, 8, 71-81.
(11) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 103, 1949-
1960 and references therein.
(12) The synthetic procedures for 1 and 2 are found in the Supporting
Information section. Data for 1: calcd, C, 45.07; H, 3.78; N, 3.62; found,
C, 44.94; H, 3.93; N, 3.84. Data for 2: calcd, C, 38.30; H, 4.36; N,
3.19; found, C, 38.51; H, 4.49; N, 3.08.
(13) For a review of boron chemistry, see: Chem. ReV. 1992, 92, 177-362.
(14) This is the first crystal structure of a bis(monothiolate) diimine platinum-
(II) complex as well as the first crystal structure of a thiolate-carborane-
platinum complex. The complete crystallographic data for 2 are found
in the Supporting Information.
(15) Zuo, J. L.; Xiong, R. G.; You, X. Z.; Huang, X. Y. Inorg. Chim. Acta
1995, 237, 177-180.
(16) Bevilacqua, J.; Eisenberg, R. Inorg. Chem. 1994, 33, 2913-2923.
(17) Teixidor, F.; Ayllon, J. A.; Vinas, C.; Sillanpaa, R.; Kivekas, R.; Casabo,
J. Inorg. Chem. 1994, 33, 4815-4818.
(18) Crespo, O.; Gimeno, C.; Jones, G. P.; Ahrens, B.; Laguna, A. Inorg.
Chem. 1997, 36, 6, 495-500.
(19) Osborn, R. S.; Rogers, D. J. Chem. Soc., Dalton. Trans. 1974, 1002-
1004.
(20) Connick, W. B.; Marsh, R. E.; Schaefer, W. P.; Gray, H. B. Inorg. Chem.
1997, 36, 6, 913-922.
S0020-1669(97)01400-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/17/1998

Communications
Inorganic Chemistry, Vol. 37, No. 7, 1998 1433
Figure 2. Electronic absorption and emission spectra of 1 (s) and 2
(- - -) in tetrahydro-2-methylfuran. Emission spectra recorded at 77
K (10-6 M chromophore concentration; excitation of 1 and 2 at 436 and
380 nm, respectively). No emission was observed for 2 at longer
wavelength excitation.
Figure 3. Plot of Em (77 K) vs the difference between the ground-state
oxidation and reduction potentials for the previous diimine dithiolate
platinum(II) complexes11 (squares) and complex 1 (triangle).
of complex 1 and diphenylphenanthroline dichloride platinum-
(II) contain strong vibrations at 1425 and 1400 cm-1 correspond-
ing to a CdC stretching mode. The 1,2-dithiol-1,2-dicarba-closo-
dodecaborane(12) has stretching frequencies at 2600 and 1000
cm-1 for the B-H and C-S, respectively. This evidence supports
an emissive state that originates from a metal (d) dithiolate to
diimine charge-transfer excited state. At room temperature,
emission from complex 1 is blue-shifted, of less intensity, and
has a lifetime of 7 ns. As shown in Figure 3, a plot of the energy
of emission vs the difference between the ground state oxidation
and reduction potentials is well correlated for Pt(diimine)(1,2-
dithiolate) complexes, and, importantly, complex 1 lies upon the
line.25
Two important conclusions can be drawn from these data, given
that there is a strong correlation between the ground state redox
potentials and the emission energy. First, the emissive state and
the absorbing state are of the same origin. Second, the excited
state energy of complex 1 is dramatically different from previous
Pt(diimine)(1,2-dithiolate) complexes. The excited state reduction
and oxidation potentials of complex 1 were estimated to be 1.09
and -0.63 V, respectively.26 Comparison of these values to the
previous ten Pt(diimine)(1,2-dithiolate) excited state potentials
(reduction 0.56 ( 0.09 V; oxidation -1.41 ( 0.17 V)11 reveals
the excited state of complex 1 to be a stronger oxidant.
These novel platinum diimine thiolate o-carborane complexes
possess several favorable properties for study, including an intense
visible charge-transfer band, increased solubility, and an energetic
excited state. On the basis of the evidence provided above, we
are currently pursuing electron-transfer quenching experiments
to determine the utility of these complexes, as well as exploring
additional diimine and dithiolate molecular structures designed
to afford more favorable properties such as longer excited state
lifetimes in fluid solution.
[λmax nm (solvent; M-1 cm-1); 482 (C6H6; 6.1 × 103); 472 (C6H5-
Cl; 1.3 × 104); 460 (CH3Cl; 1.1 × 104); 454 (CH2Cl2; 1.2 ×
104); 446 ((CH3)2CO; 1.4 × 104); 437 (CH3CN; 8.7 × 103)]. The
absorption spectrum of 2, however, is blue-shifted approximately
100 nm and partly obscured by the π-π* transition of the
diphenylphenanthroline [λmax nm (solvent); 380 (C6H6); 375 (CH2-
Cl2); 370 ((CH3)2CO); 360 (CH3CN)]. Solutions of 1 and 2 in
DMF, CHCl3, CH2Cl2, and C6H6 are stable to irradiation into the
long-wavelength absorption band compared to previous Pt-
(diimine)(dithiolate) complexes such as Pt(bpy)(tdt).21,22 Cyclic
voltammograms of 1 and 2 in CH2Cl2 (0.1 M TBA+PF6- vs Ag/
AgCl) reveal a quasi-reversible reduction at -1.35 and -1.2 V,
respectively, as well as a quasi-reversible oxidation at 1.4 V for
complex 1 only. The oxidation of complex 2 is not observable
within the solvent limit of 1.8 V.
The charge-transfer transition in Pt(diimine)(dithiolate) com-
plexes was originally characterized as a ligand-to-ligand charge-
transfer transition (LLCT) by Vogler and Kunkely,22,23 and this
assignment has been recently modified by Eisenberg to be a metal
dithiolate to diimine charge-transfer transition.24 A charge-transfer
transition assignment from a mixed platinum thiolate HOMO to
a π* diimine LUMO for the o-carboranethiolate complexes is
consistent with the following observations. The visible transition
is only observable for the metal complex, and neither independent
ligand absorbs in this region, substantiating the role of the metal
in the transition. This transition is solvatochromic, suggesting
that the HOMO has both metal (d) and dithiolate ligand character,
and there is a dipole difference between the ground and excited
state. The solvatochromism of the o-carborane dithiolate complex
(∆E 2100 cm-1; C6H6 - CH3NO2) is smaller compared to
previous aromatic dithiolate Pt diimine complexes, indicating that
the dipole difference between the ground and excited state is of
lesser magnitude. The LUMO is primarily of diimine character
as evidenced by the quasi-reversible reduction for complexes 1
and 2 and its similarity to previous Pt(diimine)(dithiolate)
complexes.
Acknowledgment. This work was supported by the Lord
Foundation, Duke University, and Johnson Matthey Co. Inc. We
thank T. J. Meyer and his group for the use of an emission
instrument.
Photoluminescence from 1 and 2 is observed at 298 and 77 K,
and the emission spectra at 77 K are shown in Figure 2. The
emission is highly structured with well-resolved peaks at 542 and
593 nm for 1 and at 501 and 529 nm for 2. For complex 1, the
vibrational spacing is about 1500 cm-1. This spacing most closely
corresponds to the CdC stretch of the diimine ligand. IR spectra
Supporting Information Available: Tables of X-ray experimental
details and crystallography data for complex 2 are available (37 pages).
Ordering information is given on any current masthead page.
IC971400Z
(25) The emissive state of 2 is still under investigation. The oxidation of 2 is
not observed within the 1.8 V solvent window, but the complex should
be to the right of complex 1 in Figure 3. The emission energy is calculated
to be 2.48 V.
(26) The excited reduction and oxidation state potentials were estimated using
E ) E(Pt0/-) + E00 and E ) E(Pt+/0) - E00, respectively. The absolute
values estimated may be in error since we used ground state potentials
that were not completely reversible. However, the relative excited state
energy of complex 1 and the correlation to other Pt complexes are correct.
E00 for 1 and 2 was determined to be 2.25 and 2.48 V, respectively.
(21) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 119, 11620-
11627.
(22) Vogler, A.; Kunkely, H. J. Am. Chem. Soc. 1981, 103, 1559-1560.
(23) Miller, T. R.; Dance, I. G. J. Am. Chem. Soc. 1973, 95, 6970-6978.
(24) Cummings, S. D.; Eisenberg, R. Inorg. Chem. 1995, 34, 2007-2014.
This assignment is based on a localized MO configuration and is an
approximation.