2227
Inorg. Chem. 1994, 33, 2227-2235
Bistability and Low-Energy Electron Transfer in Cobalt Complexes Containing Catecholate and Semiquinone Ligands Ok-Sang Jungt and Cortlandt G. Pierpont' Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 Received October 6,1993'
Studies have been carried out on intramolecular cobalt-quinone electron transfer for a series of complexes of general form CoIII(N-N)(SQ) (Cat), where N-N is 2,2'-bipyridine (bpy), N,N,N',N'-tetramethylethylenediamine (tmeda), 1,lO-phenanthroline (phen), bis(2-pyridyl) ketone (py)2CO), 5-nitro- 1,lo-phenanthroline (NOlphen), dipyrido[3,2-a:2',3'-c]phenazine (dppz), and 4,5-diazafluoren-9-one (dafl) and S Q and Cat are the 3 3 - or 3,6-di-tertbutylcatechol and semiquinone ligands. Optical absorptions that appear in the 2500-nm (4000-cm-1) region of the infrared for the Co(II1) complexes are assigned as Cat Co(II1) charge transfer transitions. Spectral changes observed for the complexes in toluene solution result from an equilibrium between ColI1(N-N)(SQ)(Cat) and CoII(N-N)(SQ)2 redox isomers. Magnetic measurements on solid samples of Co(bpy)(3,5-DBSQ)(3,5-DBCat) and Co(phen)(3,6-DBSQ)(3,6-DBCat) show that the equilibrium exists in the solid state. The temperature range of the equilibrium is dependent upon the donation effect of the nitrogen coligand; values for the Co(III)/Co(II) transition temperature have been determined that follow coligand donor strength. Crystallographic characterization of Co(tmeda)(3,6-DBSQ)(3,6-DBCat) (Co(tmeda)(3,6-DBSQ)(3,6-DBCat): monoclinic, P21/n, a = 1 1.379(2) A, b = 34.510(5) A, c = 17.583(3) A, B = 91.73(1)', V = 6902(2) A3, Z = 8) has provided bond lengths showing that the metal is low-spin Co(II1) at room temperature. Structural characterization of Co(N02~hen)(3,6-DBSQ)~ (Co(NO2phen)(3,6-DBSQ)r2toluene:monoclinic, Pc, a = 12.626(2) A, b = 11.297(2) A, c = 17.974(3) A, = 105.55( l)", V = 2470( 1) A3, Z = 2) and Co(dafl)(3,6-DBSQ)2 (Co(dafl)(3,6-DBSQ)z*2toluene: monoclinic, Pc, u = 12.385(5) A, b = 11.230(5) A, c = 17.892(6) A, 0 = 105.41(2)', V = 2400(2) A3, 2 = 2) has shown that the complex molecules are trigonal prismatic in structure at room temperature, with features that are consistent with the high-spin Co(I1) charge distribution. Magnetic measurements on Co(N02phen)(3,6-DBSQ)2 indicate that it remains in the Co(I1) form a t low temperature.
-
Introduction Several years ago, we reported that a complex of cobaltcontaining quinone ligands exhibited facile metal-quinone electron transfer in so1ution.l The complex was found to exist in a Co(111) form, Co111(bpy)(3,5-DBSQ)(3,5-DBCat), in thesolid state at room temperature. In toluene solution an equilibrium between Co(II1) and Co(I1) species (eq 1)was observed from changes in
magnetism and magnetic resonance spectra. Tautomeric forms of the complex are related by the transfer of an electron between the metal and a chelated quinone ligand. Recent studies on the magnetic properties of related cobalt complexes have shown that the electron transfer process may be observed on samples in the solid state.2.3 It has since been reported for complexes of other metals containing quinone ligands? and it appears responsible t Permanent address: Korea Institute of Science and Technology, Cheongryang. Seoul, Korea. Abstract published in Aduance ACS Abstracts, April 1, 1994. (1) Buchanan, R.M.;Pierpont, C. G. J. Am. Chem. SOC.1980,102,4951. (2) Abakumov, G.A,; Cherkasov, V. K.; Bubnov, M. P.;Ellert, 0.G.; Dobrokhotova, Z. B.; Zakharov, L. N.; Struchkov, Y . T. Dokl. Akad. Nauk 1993,328, 12. (3) (a) Adams, D. M.; Dei, A.; Rheingold,A. L.; Hendrickson,D. N. Angew. Chem., Inr. Ed. Engl. 1993, 32, 880. (b) Adams, D. M.;Dei, A.; Rheingold, A. L.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 8221. (4) (a) Lynch, M.W.; Hendrickson, D. N.; Fitzgerald, B. J.; Pierpont, C. G.J.Am. Chem.Soc. 1984,106,2041. (b)Abakumov,G.A.;Razuvaev, G.A.; Nevodchikov, V. I.;Cherkasov, V. K.J . Organomet. Chem. 1988, 341,485. (c) Rakhimov, R.R.;Solozhenkin, P. M.; Kopitaya, N. N.; Pupkov, V. S.;ProkoTev, A. I. Dokl. Akad. Nauk SSSR 1988, 300, 1177. (d) Abakumov, G.A.; Garnov, V. A.; Nevodchikov, V. I.; Cherkasov, V. K. Dokl. Akad. Nauk SSSR 1989, 304, 107.
0020-1669/94/ 1333-2227%04.50/0
for unique thermo- and photophysical properties of certain concatenated metal-quinone complexes in the solid state.295 Intramolecular electron transfer is a consequence of charge localization within the molecule and the close energy separation between localized quinone and metal electronic levels. It is reasonable toexpect that molecules that show this property should have associated optical transitions, but a t unusually low energies. We now report the results of studies on tautomeric equilibria for a series of cobalt complexes prepared with nitrogen-donor coligands of varying strength as a-donors. Ligands used in this investigation include 2,2'-bipyridine (bpy), N,N,N',N'-tetramethylethylenediamine (tmeda), 1,lO-phenanthroline (phen), bis(Zpyridyl) ketone ((py)2CO), 5-nitro- 1,IO-phenanthroline (NO*phen), dipyrido[ 3,2-~:2',3'-c]phenazine (dppz), and 4,Sdiazafluoren-9-one (dafl). The results of this investigation show that
daf 1
-
the CoIII(N-N)(SQ)(Cat) forms of the complexes have intense Cat Co(II1) charge transfer transitions in the 2500-nm region of the infrared. Tautomeric equilibria have been studied in solution and in the solid state. Transition temperatures (T,)for the Co(III)/Co(II) equilibria in both media have been found to be dependent upon the coligand. (5) Lange, C. W.; Foldeaki,
M.;Nevodchikov, V. I.; Cherkasov, V. K.; Abakumov, G.A.; Pierpont, C. G.J. Am. Chem. Soc. 1992,114,4220.
0 1994 American Chemical Society
Jung and Pierpont
2228 inorganic Chemistry, Vol. 33, No. 10, 1994
Table 1. Crystallographic Data for Co(tmeda)(3,6-DBSQ)(3,6-DBCat), CO(NO~~~~~)(~,~-DBSQ)Z.~C~H~, and Co(dafl)(3,6-DBSQ)r2C7Hsa Co(tmeda)( 3,6-DBSQ)(3,6-DBCat) Co(N02phen)( 3,6-DBSQ)r2C,Hs Co(dafl)(3,6-DBSQ)r2C.rHs mol wt color crystal system space group a, A
b, A C, A
69 d?, vol,
Z Dale, g ~ m - ~
p, mm-1 R, Rw GOF a
615.8 blue-green monoclinic P21In 11.379(2) 34.5 10(5) 17.583(3) 91.73( 1) 6902(2) 8 1.185 0.530 0.058,0.062 1.14
Radiation, Mo Ka (0.710 73 A); temp, 293-298 K. R =
892.9 black-brown monoclinic Pc 12.626(2) 11.297(2) 17.974(3) 105.55(1) 2470(1) 2 1.201 0.393 0.054, 0.064 1.30
EIIFd - IFJl/zlFd. R,
Experimental Section Materials. 3,5-Di-tert-butyl-l,2-benzoquinone(3,5-DBBQ), 2,2‘bipyridine (bpy), N,N,N’,N’-tetramethylethylenediamine (tmeda), 1,10-phenanthroline (phen), 5-nitro-1 ,lo-phenanthroline (NOzphen), and bis(2-pyridyl) ketone ((py)zCO) were purchased from Aldrich. Dicobaltoctacarbonyl was purchased from Strem Chemical Co. 3.6-Di-tertbutyl- 1,2-benzoquinone(3,6-DBBQ)> dipyrido[3,2-a:2’,3’-c]phenazine (dppz),6b4,5-diazafluoren-9-one (dafl)k and Co(bpy)(3,5-DBSQ) (3,5DBCat)’ were prepared using literature procedures. An initial sample of Co(bpy)(3,6-DBSQ)(3,6-DBCat) was provided by Professor G. A. Abakumovaz Complex Syntheses. Co(tmeda)(3,6DBSQ)(3,6DBCat). Co2(CO)s (86 mg, 0.25 mmol) and tmeda (60 mg, 0.52 mmol) were combined in 30 mL of toluene. The mixture was stirred for 5 min, and 3,6-DBBQ (220 mg, 1.0 mmol) in 30 mL of toluene was added. The solution was stirred under Ar for 2 h a t room temperature. Evaporation of the solvent produced a dark blue residue of the complex. Crystals of Co(tmeda)(3,6-DBSQ)(3,6-DBCat) suitable for crystallographic characterization were grown from acetone solution. Yield: 234 mg (76%). Synthetic procedures used to form complexes containing other nitrogendonor ligands followed the procedure above. Details are included with the supplementary material. PhysicalMeasurements. Electronic spectra were recorded on a PerkinElmer Lambda 9 spectrophotometer equipped with a RMC-Cryosystems cryostat. Magnetic measurements were made using a Quantum Design SQUID Magnetometer at a field strength of 5 kG. Infraredspectra were recorded on a Perkin-Elmer 1600 FTIR with samples prepared as KBr pellets. EPR spectra were recordedon a Bruker ESP-300E and referenced to DPPH as the g-value standard. Crystallographic Structure Determinations. Co(tm&) (3,GDBSQ)(3,6-DBCat). Dark blue crystals of thecomplex were grown from acetone. Axial photographs indicated monoclinic symmetry and the centered settings of 25 intense reflections with 28values between 20° and 25O gave the unit cell dimensions listed in Table 1. Data were collected by 8-28 scans within the angular range 3.(1-45O.Density measurements together with the unit cell volume indicated that there were two independent complex moleculesper asymmetric region of theunit cell. Both Coatoms were located on a sharpened Patterson map and phases generated from the locations of these atoms gave the positions of other atoms of the structure. Final cycles of refinement converged with discrepancy indices of R = 0.058 and R, = 0.062. Selected atom positions are listed in Table 2; tables containing a full listingof atom positions, anisotropicdisplacement parameters, and hydrogen atom locations are available as supplementary material. Co(NO&en)(3,GDBsQ)~.Black-browncrystalsof thecomplex were grown from toluene. Axial photographs indicated monoclinic symmetry and the centered settings of 25 intense reflections with 28 values between 21 and 35’ gave the unit cell dimensions listed in Table 1. Data were collected by 8-20 scans within the angular range 3.0-50O. Density measurements together with the unit cell volume indicated that there were two complex molecules per unit cell. The coordinates of the coblt atom were determined using a sharpened Patterson map, and phases (6) (a) Belostotskaya. I. S.;Komissarova, N. L.; Dzhuaryan, E. V.; Ershov, V.V. Izr.Akad. Nauk,SSSR1972,1594. (b)Dickeson, J.E.;Summers, L. A. Ausr. J. Chem. 1970,23,1023. (c) Henderson, L. J., Jr.; Fronczek, F. R.; Cherry, W. R.J. Am. Chem. Soc. 1984, 106, 5876.
864.0 black monoclinic Pc 12.38 5( 5) 11.230(5) 17.892(6) 105.41(2) 2400(2) 2 1.196 0.401 0.061, 0.061 1.29
= [Cw(lFd - IFcl)z/zw(Fo)2]1/2
Table 2. Selected Atom Coordinates (Xl@) and Equivalent Isotropic Displacement Parameters (A2) for Co(tmeda)( 3,6-DBSQ)(3,6-DBCat) c o1 01 02 c1 c2 c3 c4 c5 C6 03 04 C15 C16 C17 C18 C19 c20 N1 N2 c02 05 06 c35 C36 c37 C38 c39 C40 07 08 C50 C5 1 C52 c53 c54 N3 N4
1621(1) 415(4) 2638(4) 834(6) 109(6) 718(7) 1943(7) 2675(6) 2085(6) 1810(4) 506(4) 866(6) 652(6) -379(6) -1 121(6) -893(6) 136(7) 147l(6) 2940(5) 2801(1) 1443(4) 2866(4) 1066(6) -25(6) -253(7) 531(7) 1599(6) 1863(7) 1824(4) 2432(6) 1884(6) 2643(7) 3836(7) 4405(6) 3654(6) 2732(6) 3849(6)
23 12(1) 2673( 1) 2748( 1) 2998(2) 3300(2) 36 18(2) 3653(2) 3378(2) 3041(2) 2292(1) 1916( 1) 2116(2) 2 138(2) 1956(2) 1767(2) 1742(2) 1919(2) 2319(2) 1928(2) 5310(1) 5623(1) 5298(1) 5594(2) 5732(2) 5686(2) 5509(2) 5371(2) 5412(2) 4880(1) 4572(2) 4230(2) 3948(2) 3998(2) 4334(2) 4626(2) 5333(2) 5786(2)
4312(1) 4406(2) 4333(3) 4678(3) 4961(4) 52 19(4) 5198(4) 4908(4) 4637(3) 5360(2) 4406(3) 5647(4) 6427(4) 6644(4) 6 129(4) 5346(4) 5 123(4) 3158(3) 4 197(3) 8113(1) 7959(3) 7056(2) 7256(4) 6980(4) 6215(5) 57 32(4) 5967(4) 6753(4) 8083(2) 7850(4) 7607(4) 7357(4) 7337(4) 7583(3) 7857(3) 9262(3) 8 18l(4)
generated from the location of this atom gave the positions of other atoms of the structure including the locations of two toluene solvent molecules of crystallization. Final cycles of refinement converged with discrepancy indices of R = 0.054 and R, = 0.064. Selected atom locations are listed in Table 3; tables containing a full listing of atom positions, anisotropic displacement parameters, and hydrogen atom locations were deposited with an earlier communication of this s t r ~ c t u r e . ~ Co(dafl)(J,CDBSQ)z. Blackcrystalsof the complexwere grown from toluene. Axial photographs indicated monoclinic symmetry and the centered settings of 25 intense reflections with 28 values between 19 and 34’ gave the unit cell dimensions listed in Table 1. Data were collected by 8-28 scans within the angular range 3.0-50O. Density measurements together with the unit cell volume indicated that there were two complex molecules per unit cell. The coordinates of the cobalt atom were determined using a sharpened Patterson map and phases generated from (7) Jung, 0 . - S . ; Pierpont, C. B. J. Am. Chem. SOC. 1994, 116, 127.
Inorganic Chemistry, Vol. 33, NO.10, 1994 2229
C d a t e c h o l a t e and C d e m i q u i n o n e Complexes
Table 4. Selected Atom Coordinates ( X l v ) and Equivalent Isotrouic DisDlacement Parameters (A2) for Co(dafl)(3,6-DBSC%
Table 3. Selected Atom Coordinates ( X l v ) and Equivalent Isotropic Displacement Parameters (A2) for Co(NOmhenM3.6-DBS0)~ co 01 02
c1
c2 c3 c4 c5 C6 03 04 C15 C16 C17 C18 C19 c20 N1 N2 C29 C30 C3 1 C32 c33 c34 c35 C36 c37 C38 c39 C40 N3 05 06
0 1222(6) 1200(6) 2098(9) 2045( 10) 2963(9) 3894(9) 3879(9) 3046(9) -1212(6) -1 126(6) -2069(9) -1990(9) -2874( 10) -3778( 10) -3858( 10) -3031(9) -15(8) -24(6) -7w 21(9) 7( 10) 24(10) 103) 1(9) 21(10) -13(13) -9(12) -33(8) 43(11) 74(10) 86( 13) 224( 15) -222( 17)
756(1) 1458(6) -554(7) 834(11) -312(9) -1 108(10) -704( 1 1) 4 18( 12) 1205(10) 1435(6) -605(7) 806( 1 1) -374(9) -1219( 10) -816( 13) 334(12) 1 155( 10) 2523(7) 503(8) 2604( 11) 3746( 13) 4735(12) 4601(12) 3480(9) 1480(12) 1469(17) 345(28) -619( 16) -507( 10) 36 1O(20) 2583(22) 4786(22) 47 13( 13) 5672( 16)
co 01 02
0 893(4) 219(5) 1120(6) 748(7) 964(7) 1529(7) 1877(6) 1699(6) 450(4) -172(5) 347(6) 30(6) -81(7) 108(7) 423(8) 559(7) 475(7) -1 196(5) -1236(7) -1 583( 10) -1 122(11) -364( 10) -60(9) -1 620(9) -2369( 10) -2729(11) -2334(13) -1536(9) -2387( 10) -2765( 9) -2806( 11) -3440(9) -2606( 12)
c1 c2 c3 c4
c5
C6 03 04 C15 C16 C17 C18 C19 c20 N1 N2 C29 C30 C3 1 C32 c33 c34 C38 c37 C36 c35 c39 05
RWult.9
Synthetic procedures have been developed for the formation of a series of bis(quinone) complexes of cobalt that contain a variety of chelating nitrogen-donor coligands. Differences in coligand donation may be used to modulate the orbital energy of the central cobalt atom permiting investigation of the dependence of charge distribution on subtle changes in coligand bonding effects. This would appear as a change in the Co(III)/ Co(I1) transition temperature ( Tc)for the reaction shown in eq 2. Since Co(II1) and Co(I1) forms of the complexes have F!
Co"(N-N)(SQ),
977(1) 1703(7) -328(8) 1044(11) -1 19(10) -9 15( 11) -538( 11) 616(11) 1380(12) 1688(7) -3 35(9) 1042(12) -152(11) -971 (1 3) -6OO( 13) 552(13) 1325(12) 2832(14) 666(12) 2839( 18) 3805( 13) 4901(1 5) 4917( 17) 3884(22) 1756(13) -205( 13) -42(20) 1107(14) 2032( 15) 3335(15) 393 1(9)
0 900(4) 217(5) 1173(6) 788(7) 1065(7) 1656(7) 1995(7) 1781(8) 461(5) -175(5) 441(6) 78(6) 64(6) 325(8) 662(7) 719(7) -5 14(9) -1391(6) -1259(9) -1 7 17( 10) -1382( 12) -593( 13) -2 13( 10) -1681(9) -1904( 13) -2650( 10) -2940(9) -241 l(9) -2482( 10) -3040( 6)
Table 5. Electronic Spectra and Solution Co(III)/Co(II) Transition Temperature for the Co(N-N)(3,6-DBQ)2 Series
the location of this atom gave the positions of other atoms of the structure includingthe locationsof two toluene solvent moleculesof crystallization. Final cycles of refinement converged with discrepancy indices of R = 0.061 and R, = 0.061. Selected atom positions are listed in Table 4; tables containing a full listing of atom positions, anisotropicdisplacement parameters, and hydrogen atom locations are available as supplementary material.
Co"'(N-N) (SQ)(Cat)
0 1193(7) 1200(8) 2060(11) 2052( 11) 2947( 11) 3807( 10) 3809(13) 2957(12) -1 199(8) -1 137(8) -2039( 10) -1 990( 1 1) -2858( 11) -3738( 11) -3797(13) -2957( 13) 8(12) 12(10) 17(13) 46( 13) 15(14) lO(14) -16( 19) 21(11) 20( 13) 72( 15) 73(13) 49( 12) 67( 13) 72(10)
(2)
characteristic electronic spectra, optical spectroscopy has been used to monitor equilibria in toluene solutionsover an appropriate temperature range. This procedure has been used to estimate solution Tctemperatures. The possibility for electron transfer in solid samples of the complexes has also been investigated. The temperature dependenceof opticalspectra has been used to provide information on the Co(III)/Co(II) transition in the solid state, and the temperature dependence of magnetic properties has also been investigated for selected members of the series. Crystallographic structure determinations have been carried out for three complexes that either haveanunusually high Tcvalue(Co(tmeda)(3,6-DBSQ)(3,6-DBCat)) or remain exclusively in the Co(I1) form at all temperatures measured (Co(N02phen)(3,6-DBSQ)2, Co(dafl) (3,6-DBSQ)2)).
Co1"(N-N)(3,6-DBSQ)Co"(N-N)(3.6-DBCat) (3.6-DBSOh .. .. .,Cat Co(II1) CT,b nm Amu,nm Tc,cK N-N , ,A nm" >320 2200 (PY)2CO 590 850 310 650,730 (sh) 2400 tmeda 820 275 600,710 (sh) 2650 bPY 850 265 600,700 (sh) 2550 phen 850 265 590 2500 dPPZ 820
