A New Class of Luminescent Polypyridine Complexes of Rhenium(I) Containing cis-Carbonyl Ligands

Full text of DOI:10.1021/ic971199s

2618
Inorg. Chem. 1998, 37, 2618-2619
A New Class of Luminescent Polypyridine Complexes of Rhenium(I) Containing cis-Carbonyl Ligands
Erick Schutte, Jeffrey B. Helms, Stephen M. Woessner, John Bowen, and B. Patrick Sullivan*
Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071-3838
ReceiVed September 19, 1997
The preparation and characterization of new luminescent metal
complexes, especially those exhibiting metal-to-ligand charge
transfer excited states,1 has contributed to the understanding of
the fundamental principles of excited state decay. As this field
has matured, applications of luminescent MLCT excited states,
in particular Re(I)-polypyridine complexes containing carbonyl
ligands,2 have been found in such diverse areas as solar energy
conversion,3 biological labeling,4 and sensor development.5 Of
specific interest to us is the discovery of new excited states which
are capable of probing their immediate physical environment, for
example by reporting on changes in pressure, temperature, or
effective dielectric properties.6 In this Communication we report
several different, high-yield routes to a new class of long-lived
luminescent complexes of the type cis-Re(CO)2(N-N)(P-P)+
(where N-N is a chelate polypyridine ligand and P-P is a chelate
phosphine). Also, the synthetic procedures reported here provide
a new, but general, entry route to the known cis-trans-Re(CO)2-
(P)2(N-N)+ complexes, several of which have been prepared by
other methods.7 All the new Re excited states are of extraordinary
stability, exhibit spectral responses well to the red of similar
tricarbonyl complexes of Re(I), and apparently do not conform
to the same energy gap law as fac-Re(bpy)(CO)3L(+/0) complexes
(L is a variety of neutral and anionic ligands).8
OTf with stoichiometric P-P in ODB.11 Route 4 in Scheme 1 is
the direct preparation of complexes of the type cis-trans-Re(CO)2-
(P)2(N-N)+ where P can be PPh3, or even bidentate phosphines
like trans-Ph(H)CdC(H)Ph that can serve as the basis for linear
oligomeric chromophores.12 In all cases, reaction times for ODB
heated at reflux were between 5 and 18 h, and yields after
purification ranged from 60 to 80%. For all the routes, the
intermediate fac-Re(bpy)(CO)3(phosphine)+ appears early in the
reaction, which implies that the success of the chemistry relies
on the ability of phosphine ligands to labilize a carbonyl ligand
in the coordination sphere under the extreme temperature condi-
tions. Characterization of the complexes was achieved by a
combination of elemental analysis and IR and 31P NMR spec-
troscopies (see Table 1). As is shown in Table 1, two intense
(9) All complexes of the type fac-Re(CO)3(P-P)Cl were prepared by a
modification of the basic procedure found in the following: Carriedo,
G. A.; Luz Rodriguez, M.; Garcia-Grande, S.; Aguirre, A. Inorg. Chim.
Acta 1990, 178, 101. See also: Edwards, D. A.; Marshalsea, J. J.
Organomet. Chem. 1977, 131, 73. Preparation of fac-Re(c-dppene)-
(CO)3Cl: [Re(CO)5Cl] (1.0 g) and 1.15 g of c-dppene were combined
in a 100 mL round bottom flask containing ca. 50 mL of deoxygenated
toluene. The solution was then placed under an N2 blanket and refluxed
for 6 h. After this time the white precipitate was filtered from the hot
mixture. The precipitate was washed with another 50 mL of hot toluene
and discarded. The filtrate was reduced to ca. 20 mL by rotary
evaporation and poured slowly into 100 mL of stirred ether. The white
precipitate that formed was washed with 50 mL (3×) of ether and then
dried. Preparation of cis-[Re(CO)2(c-dppene)(phen)]PF6: fac-Re(c-
dppene)(CO)3Cl (100 mg), 53 mg of TlPF6 (7% excess), and 28 mg of
1,10-phenanthroline (10% excess) were combined in a 50 mL round
bottom flask which was covered with aluminum foil. Approximately 5
mL of o-dichlorobenzene was added, and solution was refluxed for 7 h,
resulting in a yellow-orange color. The solution was allowed to cool,
followed by filtration over diatomaceous earth to remove the chalky-
white TlCl precipitate. This was washed with ca. 100 mL of CH2Cl2,
and the filtrate was allowed to evaporate over a 48 h period. Ethyl ether
was added to cause precipitation of the yellow product, which was
subsequently filtered off and washed with 50 mL of ethyl ether. Anal.
Calcd: C, 49.74; N, 2.90; H, 3.34. Found: C, 49.93; N, 2.83; H, 3.15.
(10) Preparation of [cis-Re(CO)2(dppm)(bpy)]PF6 from fac-Re(bpy)(CO)3-
Cl: fac-Re(bpy)(CO)3Cl (100 mg), 76 mg of TlPF6, and 91 mg (10%
excess) of dppm were combined in a 50 mL round bottom flask which
was covered with aluminum foil. Approximately 7.5 mL of o-dichlo-
robenzene was added, and N2 was bubbled through the solution for 5
min, which was then placed under a N2 blanket and refluxed for 5 h.
During this time the solution changed from white to orange-red. The
reaction workup was exactly the same as that for cis-[Re(CO)2(c-dppene)-
(phen)]PF6. Anal. Calcd: C, 47.90; N, 3.02; H, 3.26 Found: C, 48.02;
N, 3.09; H, 3.31.
As shown in Scheme 1, three methods were used to prepare
the complexes. These are route 1, reaction of fac-Re(P-P)(CO)3Cl
with stoichiometric N-N and TlPF6 in o-dichlorobenzene (ODB);9
route 2, reaction of fac-Re(N-N)(CO)3Cl with stoichiometric P-P
and TlPF6 in ODB;10 and route 3, reaction of fac-Re(N-N)(CO)3-
(1) (a) Kalyanasundaram, K. Photochemistry of polypyridine and porphyrin
complexes; Academic Press: London, 1992. (b) Lees, J. Chem. ReV.
1987, 87, 711-743. (c) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna,
F.; Belser, P.; Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (d)
Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (e) Crosby, G. A.;
Highland, K. A.; Truesdell, K. A. Coord. Chem. ReV. 1985, 64, 41. (f)
DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. ReV.
1985, 65, 65. (g) Watts, R. J. J. Chem. Educ. 1983, 60, 834.
(2) Some leading references: (a) Wallace, L.; Rillema, D. P. Inorg. Chem.
1993, 32, 3836. (b) Worl, L. A.; Duesing, R.; Chen, P.; Della Ciana, L.;
Meyer, T. J. J. Chem. Soc., Dalton Trans. 1991, 843. (c) Sacksteder, L.;
Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; Degraff, B. A.
Inorg. Chem. 1990, 29, 4335. (d) Hino, J. K.; Della Ciana, L.; Dressick,
W. J.; Sullivan, B. P. Inorg. Chem. 1992, 31, 1072. (e) Paulson, S.;
Morris, K.; Sullivan, B. P. J. Chem. Soc., Chem. Commun. 1992, 1615.
(f) Shaver, R. J.; Rillema, D. P. Inorg. Chem. 1992, 31, 4101. (g) Striplin,
D. R.; Crosby, G. A. Chem. Phys. Lett. 1994, 221, 426.
(3) Meyer, G. J., Ed. Molecular Level Artificial Photosynthetic Materials.
Prog. Inorg. Chem. (Karlin, K. D., Ed.) 1996, 44.
(4) (a) Oriskovich, T. A.; White, P. S.; Thorp, H. H. Inorg. Chem. 1995,
34, 1629. (b) Connick, W. B.; Di Bilio, A. J.; Hill, M. G.; Winkler, J.
R.; Gray, H. B. Inorg. Chim. Acta 1995, 240, 169.
(5) See, for example: (a) MacQueen, D. B.; Schanze, K. S. J. Am. Chem.
Soc. 1991, 113, 6108-6110. (b) Shen, Y.; Sullivan, B. P. Inorg. Chem.
1995, 34, 6235. (c) Shen, Y.; Sullivan, B. P. J. Chem. Educ. 1997, 74, 685.
(6) See, for example: (a) Vining, W. J.; Caspar, J. V.; Meyer, T. J. J. Phys.
Chem. 1985, 89, 1095. (b) Lang, J. M.; Dreger, Z. A.; Drickamer, H. G.
Chem. Phys. Lett. 1992, 192, 299. (c) Sullivan, B. P. J. Phys. Chem.
1989, 93, 24.
(7) (a) Caspar, J. V. Ph.D. Dissertation, University of North Carolina, Chapel
Hill, 1983. It was noted here that dicarbonyl Re(I) complexes had longer
relative lifetimes than tricarbonyls. (b) Caspar, J. V.; Sullivan, B. P.;
Meyer, T. J. Inorg. Chem. 1984, 23, 2104. (c) Luong, J. C. Ph.D.
Dissertation, Massachsetts Institutue of Technology, 1981.
(11) Preparation of [cis-Re(CO)2(dppm)(bpy)](OTf) from fac-Re(bpy)(CO)3-
OTf: Re(bpy)(CO)3(OTf) (155 mg, 0.27 mmol) and dppm (104 mg, 0.27
mmol) were placed in a 50 mL round bottom flask containing 10 mL of
o-dichlorobenzene. The reaction mixture was purged with nitrogen for
30 min and refluxed for 5 h, after which time 40 mL of diethyl ether
was added, following cooling. The yellow-orange solid which precipitated
was collected and washed with ether, resulting in an orange oil. This oil
was dissolved in dichloromethane and dropped into stirring ether, giving
the yellow-orange product, which was collected, washed with ether, and
air-dried (188 mg, 73% yield).
(12) Woessner, S.; Sullivan, B. P. Work in progress.
(13) (a) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.
Chem. 1986, 90, 3722. (b) Henry, B, R.; Siebrand, W. In Organic
Molecular Photophysics; Birks, J. B., Ed.; Wiley: London, 1973; Vol.
1, Chapter 4. (c) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem.
ReV. 1977, 77, 793. (d) Freed, K. F. Acc. Chem. Res. 1978, 11, 74. (e)
Lin, S. H. Radiationless Transitions; Academic Press: New York, 1980.
(f) Heller, E. J.; Brown, R. C. J. Chem. Phys. 1983, 79, 3336.
(8) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
S0020-1669(97)01199-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/07/1998

Communications
Inorganic Chemistry, Vol. 37, No. 11, 1998 2619
Scheme 1. New Preparative Reactions (Routes 1-4; See Text) for the cis-Carbonyl Complexes
Table 1. Spectral and Photophysical Data for the Complexesa,b
no.
complex
31P NMR shifts (ppm)c
IR freq (cm-1
)
Eop (nm)d
Eem (nm)d
τ (ns)e
φ f
1
2
3
4
5
6
cis-Re(CO)2(dppm)(bpy)+
-30.7, -18.7
+40.0, +49.9
+41.1, +51.3
+16.6, -5.7
-28.6, +14.5
+24.6
1950, 1884
1956, 1886
1960, 1890
1942, 1873
1940, 1869
1938, 1868
439 (3.53)
389 (3.56)
390 (3.70)
420 (3.38)
428 (3.45)
425 (3.54)
642
612
603
637
634
620
768
378
0.020
0.033
0.123
0.014
0.014
0.039g
cis-Re(CO)2(c-dppene)(bpy)+
cis-Re(CO)2(c-dppene)(phen)+
cis-trans-Re(CO)2(t-dppene)2(bpy)+
cis-trans-Re(CO)2(dppm)2(bpy)+
cis-trans-Re(CO)2(PPh3)2(bpy)+
3840
191
199
775g
a All complexes are triflate salts in CH2Cl2 except complex 3. b Abbreviations: dppm is bis(diphenylphosphino)methane, c-dppene is
cis-(bis(diphenylphosphino))ethylene, t-dppene is trans-(bis(diphenylphosphino))ethylene, bpy is 2,2′-bipyridine, and phen is 1,10-phenanthroline.
d
c In CH3CN with an 85% phosphoric acid external standard. Eop and Eem are respectively the absorption and emission spectral maxima corresponding
to the lowest energy MLCT excited state (log ꢀmax is reported). e Measured by the phase shift demodulation method with a glycogen scattering
standard. Errors are (5%. f Measured against an air-saturated sample of fac-[Re(bpy)(CO)3py]OTf in water (φ ) 0.012). g Reported value of 801
-
ns as a PF6 salt in CH2Cl2 (Caspar, J. V. Ph.D. Dissertation, 1982).
CO vibrational modes in the region of ca. 1870-1890 and 1940-
1960 cm-1 and the appropriate number of 31P NMR spectral peaks
are consistent with the structural assignments of the complexes.
All complexes exhibit a yellow to red-orange coloration in the
solid and in solution that is due to MLCT transitions in the 390-
440 nm region. The corresponding luminescence occurs in the
ca. 600-640 nm range (see Table 1). Of special interest from a
photochemical perspective are the complexes cis-[Re(CO)2-
(dppm)(bpy)]OTf (1) and cis-[Re(CO)2(c-dppene)(phen)]PF6 (3)
(dppm is bis(diphenylphosphino)methane and c-dppene is cis-
1,2-(bis(diphenylphosphino))ethylene). Complex 1 possesses
spectral and photophysical parameters similar to those of Ru-
(bpy)32+; for example, compare the energy maxima for absorption
and emission and the lifetimes and quantum yields of the lowest
MLCT excited state for 1 (440 nm, 642 nm, 768 ns, 0.029; CH2-
Cl2) and Ru(bpy)32+ (449 nm, 607 nm, 490 ns, 0.024; CH2Cl2).7a
Complex 3 exhibits a lifetime of 3.84 µs and a quantum efficiency
of emission of 12.3% (CH2Cl2), both of which are extraordinary
in the photochemistry of MLCT excited state chromophores.
The new complexes exhibit surprisingly long lifetimes relative
to the fac-Re(bpy)(CO)3Ln+ series (L is Cl- (n ) 0) or L is a
neutral nitrogen or phosphorus donor (n ) 1)). For example,
complex 1 and cis-[Re(CO)2(c-dppene)(bpy)]OTf (2) exhibit
lifetimes of 768 and 378 ns, respectively. This difference is
primarily due to a smaller than expected nonradiative rate con-
stant, knr.13 In the energy gap law analysis published by Caspar
and Meyer for fac-Re(bpy)(CO)3Ln+ the linear relationship ln knr
) 40.155 - 1.458Eem was found.8 On the basis of this finding,
the calculated knr values for 1 and 2 are 1.13 × 107 and 3.80 ×
107 s-1 versus the observed values of 1.27 × 106 and 2.56 × 106
s-1
. )
Given the relatively constant value of kr (ca. 105 s-1
characteristic of the fac-Re(bpy)(CO)3Ln+ series, the calculated
knr values predict lifetimes of 26 and 85 ns for 1 and 2. Several
explanations are possible for this behavior. One is the presence
of higher-lying states that can be thermally populated, leading to
either longer lifetimes for the dicarbonyls or shorter ones for the
tricarbonyls depending on the identity of the state (e.g., ligand-
localized versus d-d).14 Another is an inherent difference in
Franck-Condon factors. Since some studies have shown that
medium-frequency bpy modes, in addition to higher CO modes,
act as energy acceptors,2b one possible cause of the dramatically
increased relative lifetimes is the removal of a critical high-energy
mode that participates in energy disposal and its replacement with
fewer modes of smaller quantum spacing. Our future work will
be aimed at resolving this issue. An important point to remember,
however, is that regardless of origin, the room temperature energy
gap law behavior is an empirical yardstick by which MLCT exci-
ted states may be judged for their utility in numerous applications.
Acknowledgment. B.P.S. thanks the DOD (AFOSR) and NSF
EPSCoR programs and Dr. John Endicott for helpful discussions.
IC971199S
(14) A meaningful comparison of knr values can be made only if the potential
temperature dependence of this rate constant is taken into account. Studies
of the temperature variation of knr for both the tricabonyls and dicarbonyls
must be done before any conclusion concerning the origin of the lifetime
differences can be drawn.