4737
Inorg. Chem. 1993,32, 4737-4744
Cobalt Complexes of Octaethyloxophlorin. Metal-Centered Redox Chemistry in the Presence of a Redox-Active Ligand Alan L. Balch,' Marinella Mazzanti, and Marilyn M. Olmstead Department of Chemistry, University of California, Davis, California 95616
Received May 7, 1993@ Two cobalt complexes of octaethyloxophlorin(HzOEPOH), which are related by one-electron oxidation and proton transfer, have been characterized by X-ray diffraction. Deep orange ((py)ColI(OEPOH-py)} crystallizes in the triclinic space group P1 with a = 9.267(3) A, b = 14.007(4) A, c = 15.816(6) A, a = 101.70(2)O, B = 92.41(3)O, and y = 101.51(2)o at 130(2) K with Z = 2. Refinement of 4615 reflections with F > 4.0u(F) and 490 parameters yielded R = 0.053, R, = 0.055. The structure involves a five-coordinate cobalt ion with the axial Co-N distance (2.204(3) A) longer than the in-plane Co-N distances (average 1.988 A). This elongation is consistent with the presence of a low-spin (5' = l/2) d7 ion with the d2 orbital filled. The macrocycle has an ordered structure with a second pyridine participating in a hydrogen bond to the meso hydroxyl group. Lime-green solutions of ((py)2C0111(OEPO)} produce blue crystals that form in the monoclinic space group P21 with a = 19.177(4) A, b = 20.039(4) A, c = 10.547(2) A, and 0 = 100.36O at 130(2) K with Z = 4. Refinement of 4874 reflections with Z > 2 4 I ) and 973 parameters yielded R = 0.054. The structure was refined as a racemic twin. There are two independent molecules in the unit cell. Each contains a six-coordinate cobalt with short in- and out-of-plane Co-N distances, which average 1.958 A for the Co-N(pyrro1e) and 1.961 A for the Co-N(pyridine) distances, and a severely saddle-shaped macrocycle. In both molecules the meso keto groups show some disorder, and in the second molecule three of the ethyl groups are disordered. The electronicspectrumof ((py)CoIl(OEPoH-py)) shows a typical porphyrinlike spectrum (A, nm: 404 (Soret ), 530) while the spectrum of ((py)2C0111(OEPO)}shows characteristic lowenergy absorption (A, nm: 442 (Soret), 532,622,676) that distinguishes this trianionicmacrocycle fromoxophlorin radical complexes like ((py)ZnII(OEPO)).
Scheme I
Introduction Meso-hydroxylation of metalloporphyrins is a key step in the degradation of this macrocycle both in model systems1.2and in heme catabolism where heme oxygenase is the enzyme which initiates the proce~s.~-~ The resulting meso hydroxyporphyrins or oxophlorins, 1,6and their metal complexes are considerably
m
m -m
l a
!Y
m
l b
more easily oxidized than are their porphyrin counterpart^.^^^ The oxidation of octaethyloxophlorin (HzOEPOH, 1; a 4 = Et; m, m' = H) to form a free radical was demonstrated some time ago,'s9 and recently this laboratory showed that oxidation of the zinc(I1) and nickel(I1) complexes of H2OEPOH produce airAbstract published in Advance ACS Abstracts, October 1, 1993. (1) Warburg, 0.;Negelein, E. Chem. Ber. 1930, 63, 1816. (2) Bonnett, R.;Dimsdale, M. J. J. Chem. Soc., Perkin Trans. 1 1972,2540. (3) OCarra, P. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: New York, 1975; p 123. (4) Schmid,.R.; McDonagh, A. F. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. 6, p 257. (5) Bissell, D. M. In Liver: Normal Function and Disease. Vol. 4; Bile Pigmentsand Jaundice; Ostrow, J. D., Ed.;Marcel Decker: New York, @
--.
1986: n 133. -r
(6) Clezy, P. S . In The Porphyrins; Dolphin,D., Ed.;Academic Press: New York, 1978; Vol. 11, p 103. (7) Fuhrhop, J.-H.; Besecke, S.; Subramanian, J. J. Chem. Soc., Chem. Commun. 1973, 1 . (8) Fuhrhop, J.-H.; Besecke, S.; Subramanian, J.; Mengersen, C.; Riesner, D. J . Am. Chem. Sac. 1915.97.7141. (9) Bonnett,R.;Dimsdale,M.J.;Sales,K. D. J. Chem.Soc.,Chem. Commun.
1970,962.
stable, isolablefree radicals as shown in Scheme I.IoJ1When the axial pyridine ligands are removed, the nickel radical readily ~~
(10) Balch, A. L.; Noll, B. C.; Zovinka, E. P. J. Am. Chem. Soc. 1992,114, 3380.
0020-1669/93/1332-4737%04.00/0 Q 1993 American Chemical Society
Balch et al.
4738 Inorganic Chemistry, Vol. 32, No.22, 1993
C
li
g, -
1 8
1 6
4 0
700
Wevolmgth (nm)
Figure 1. Electronic absorption spectra, pyridine solutions A,[, nm (e, M - ~ C X ~ A, - ~((py)@ll(OEPO)) )]: [430sh(1.6 X 10s),442 (2.3 X lo5), 532 (1.2 X lo'), 622 sh (1.6 X lo'), 676 (4.6 X lo')];B, ((py)COn(OEPOH-py)} [404 (1.6 X lo4), 530 (1.4 X lo3)]; C, ((py)Znn(OEPOH-py)) [420 (2.8 X lOS), 546 (1.6 X lo'), 568 (1.0 X lo')]; D,
{(py)Zn~I(OEPO')) [404 (5.1 X lo4), 438 (1.2 X lo5), 489 (9.8 X lo3), 547 (7.5 X lo3), 637 (3.6 X lO3), 709 (3.2 X lo3), 743 (3.3 X lo3), 819 (1.4 X lo4)], Traces C and D were adapted from ref 10.
(and reversibly) dimerizes through the formation of a new C-C bond.12 As part of the systematic examination of the properties of metal complexes of H20EPOH, the behavior of the cobalt complexes has been examined. Bonnett and Dimsdale reported that treatment of CoI1(OEP) (OEP is the dianion of octaethylporphyrin) with hydrogen peroxide in pyridine produces a green solution of ( ( P ~ ) ~ C O ~ ~ ~ ( Owhich E P O )can ) be isolated as deep blue crystals.2 Here we report the structure of this complex, which contains the ligand in the keto tautomeric form, lb, and its reduction to form the previously unknown cobalt(I1) complex. ReSUltS
Solution Characterization of ((py)zCfl(OEPO)Jand ((py)C@(OEPOH-py)]. Blue crystals of ((py)2C0111(OEPO)),which dissolvein pyridineto produce a lime-greensolution,were obtained by oxidation of Corl(OEP) with hydrogen peroxide in pyridine. The procedure of Bonnett and Dimsdale was followed,2but we found that the chromatography step was unnecessary and that it reduced the yield. Treatment of a green pyridine solution of {(PY)~CO~~~(OEPO)) with aqueous sodium dithionite gives a redorange solution of ((py)CoII(OEPOH--py)), from which red crystals of the complex can be isolated by the addition of water. This cobalt(I1) complex is very air sensitive. Treatment with dioxygen or iodine in pyridine results in its oxidation back to form ((py)2Cor1I(0EPO)),as shown in eq 1. Thus, the redox
( 1 1 ) Balch, A. L.; Noll, B. C.; Phillips, S . L.; Reid, S . M.;Zovinka, E. P. Inorg. Chem., preceding paper in this issue. (1 2) Balch, A. L.; Noll, B. C.;Reid, S . M.; Zovinka, E. P. J . Am. Chem. SOC. 1993, 115, 2531.
--T-i-=f
10
8
6
0
PPM
Figure 2. 300-MHz 'H NMR spectrum of ((py)zCo'"(OEPO)] in pyridine4 at 23 OC. Resonances denoted by asterisksresult from residual undeuterated pyridine and from water.
to the previously described couples, ((py)ZnI1(OEPOH-py))/((py)Znll(OEPO))loand (Ni1'(OEPOH))/((py)zNi1I(OE~)).l In each case, the pair of complexes is related by the loss of one electron and one proton. However, the site of redox activity is different for the cobalt complexes where it is the metal, not the ligand, that is involved. Figure 1 compares the electronic absorption spectra of ((py)2CdII(OEPO))(trace A) and ((py)CoII(OEPOH-py)] (trace B) with those of {(py)ZnII(OEPOH-py)] (trace C) and ((py)ZnII(OEP0) (trace D).lo The spectra of ( ( ~ ~ ) C O ~ ~ ( O E P O H - ~ ~ ) ) and ((py)ZnII(OEPOH-py)) are similar and follow the pattern that is typical for porphyrins. The spectra of {(py)2Cd1I(OEPO)) and {(py)ZnlI(OEPO)), however, are distinctly different from one another and from their reduced counterparts. Thelow-energy absorption at 8 19 nm in trace D is indicative of the formation of an oxophlorin radical.'O A similar spectrum (with ,A, at 825 nm) is seen for ( ( P Y ) z N ~ ~ ~ ( O E P O For ) ) .{(py)~Co~~~(0EP0)) ~~ the spectrum is quite distinct with an intense, low-energy absorption at 675 nm. This spectrum agrees well with that reported by Bonnett and Dimsdale for this complex in pyridine solution. The lH NMR spectrum of ((P~)ZCO~~'(OEPO)) in pyridine-& solution is shown in Figure 2. In contrast to previous reports that describe the lH NMR spectra of oxophlorinsand their complexes as broad or poorly resolved,this spectrum consistsof narrow lines with good resolution. The two meso protons are observed at 8.4 and 8.95 ppm. The methylene resonances appear as two, clearly resolved quartets and one overlapping pair of quartets. One distinct methyl triplet probably results from the ethyl groups (a) adjacent to the meso keto group while the other three methyl triplets overlapone another. This spectrum establishes the nature of the cobalt spin state as low-spin (S = 0) d6. Careful examination of the region from 14 to 9 ppm shows that no resonances are present. This is the region where the resonance of the meso hydroxyl proton of {(py)Znr1(OEPOH-py))is observed, but since {(py)~Co~~~(OEPO)) lacks this functional group, no resonance is found. Crystal and Molecular Structure of ((py)C@(OEPOH-py)). The structure of this complex has been examined by X-ray crystallography. A drawing of the molecule, which has no crystallographically imposed symmetry, is shown in Figure 3. Atomic coordinates are given in Table I, and Table I1 contains selected bond distances and angles. The cobalt ion is five-coordinate. The four Co-N(pyrro1e) distances have nearly equal lengths (average 1.988 A) while the axial Co-N(py) distance (2.203(3) A) is longer. This situation
Cobalt Complexes of Octaethyloxophlorin
Inorganic Chemistry, Vol. 32, No. 22, 1993 4739 Table I. Atomic Coordinates ( X l e ) and Equivalent Isotropic Displacement Coefficients (Az X 10') for ((py)Corl(OEPOH-py))
c43 C38
C42
aC25
& C24
Figure 3. Perspectiveview of ((py)Co"(OEPOH-py)) with 50%thermal
contours. is typical for low-spin (S = l/*) cobalt(I1) porphyrins13J4(e.g., {( l-methylimida~ole)Co(OEP)}~~ ) in which occupancy of the dz2 orbital by a single electron results in elongation of the bond along the z axis. The hydroxyporphyrin itself is nearly planar. Figure 4 shows a drawing that gives the out-of-plane distances for each atom within the core of the macrocycle. Crystals of ((py)Coll(OEPOH--py))are isostructuralwith those of {(py)Znll(OEPOH-.py)).lo However, the Co-N(pyrro1e) distances are shorter than the Zn-N (pyrrole) distances (average 2.079 A), but the Co-N(py) distance is slightly longer than the Zn-N(py) distance (2.183(8) A). This lengthening of the CoN(py) bond may be the result of steric interaction of the axial ligand with the macrocycle. The cobalt ion is 0.13 A out of the N4 plane, whereas the zinc ion is 0.31 A out of that plane. Thus, in the cobalt structure, the axial ligand is drawn in toward the macrocycle relative to the zinc complex. This is compensated by elongation of the Co-N(py) bond. In general the structural parameters for {(py)Co1I(OEPOH-py)]are consistent with the presence of low-spin (S = I / * ) Co". This is the common spin state observed for Coil in porphyrin comp1e~es.l~In both structures there is a second pyridine molecule which is hydrogen bonded to the meso hydroxy group. In the cobalt complex, the hydrogen atom involved in this was located on a difference map. The refined 0-H distance is 0.92(6) 8, and the N(6)-H distance is 1.75(6) A. The presence of this hydrogen-bonded pyridine is an important factor in the production of an ordered structure for both the cobalt and zinc oxophlorin complexes. Many porphyrin derivatives like the oxophlorins, which have sterically small perturbations to the core of the macrocycle, show varying degrees of disorder in the solid state (vide infra), but that is clearly not the case for these cobalt and zinc complexes. The axial pyridine ligand is oriented so that it is nearly eclipsed with regard to the N(1)-Co and N ( 3 ) Z o bonds. The dihedral angles between the plane of the pyridine ligand and the Co-N( 1) and Co-N(3) bonds are 8 and go, respectively. This situation, in which the plane of the axial ligand nearly eclipses two of the Co-N bonds within the macrocycle, is characteristic of fivecoordinate cobalt(I1) porphyrin c ~ m p l e x e s . ~ ~ J ~ Crystal and Molecular Structure of { (py)2Conr(OEPO)j. Atomic coordinatesare given in Table 111,and selected interatomic distances and angles are presented in Table IV. There are two independent molecules within the unit cell. Both suffer from some degree of disorder which involves the location of the meso keto substituent. Figure 5 shows a drawing of one of the molecules of thecomplex. The cobalt ion has an octahedral structure, and the oxophlorin ligand is ruffled.14J6 In this molecule there are two sites for the (13) Scheidt, W. R. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. 111, p 463. (14) Scheidt, W. R.; Lee,Y.J. Srrucr. Bonding 1987, 64, 1. (15) Little, R. G.; Ibers, J. A. J. Am. Chem. SOC.,1974, 96, 4452.
X
Y
z
2260(1) 1447(3) 2166(4) 2608(4) 2677(4) 2157(4) -154(3) 1697(4) 1924(4) 1909(5) 2157(5) 2292(5) 2536(5) 2674(5) 2897(5) 2932(5) 2776(4) 2839(4) 2815(4) 3004(4) 2997(4) 2780(4) 2613(4) 2297(4) 2030(4) 1696(4) 1804(4) 1775(5) 3193(6) 2350(5) 3973(6) 3007(5) 1496(5) 3043(5) 1553(6) 3087(5) 1590(5) 3083(5) 1562(5) 2137(5) 3726(5) 1352(5) 2689(5) -787(5) -2295(5) -3221(5) -2597(5) -1082(5) -1468(4) -2120(6) -3610(6) 4501(6) -3 85 8(6) -2349(6)
2095( 1) 5 17l(2) 3421(2) 1598(2) 841(2) 2647(3) 1538(2) 4296(3) 4257(3) 5076(3) 47 18(3) 3700(3) 3092(3) 2118(3) 1490(3) 582(3) 662(3) -97 (3) -14(3) -798(3) -396(3) 602(3) 1228(3) 2166(3) 2758(3) 361 l(3) 3537(3) 6123(3) 6773(3) 5278(3) 5729(4) 1804(3) 1733(3) -358(3) -1071 (3) -1 831(3) -2465(3) -9 14(3) -1399(4) 2466(3) 2592(4) 4442(3) 5280(3) 595(3) 237(4) 894(4) 1875(3) 2152(3) 4873(3) 3912(4) 3571(4) 4251(5) 5249(4) 5520(4)
2901(1) 2291(2) 3606(2) 3968(2) 2205(2) 1843(2) 2742(2) 2468(3) 3332(3) 4064(3) 4789(3) 4501(3) 505 9(3) 48 19(3) 54 19(3) 493 l(3) 4031(3) 3342(3) 2485(3) 1764(3) 1042(3) 1317(3) 773(3) 1014( 3) 398(3) 854(3) 1756(3) 4064(3) 3861(3) 57 15(3) 60 16(3) 6386(3) 6755(3) 5221(3) 5156(3) 1836(3) 1927(3) 121(3) -344(3) -556(3) -806(3) 466(3) 483(3) 2358(3) 2194(3) 2449(3) 2837(3) 2972(3) 1958(3) 1655(3) 1480(3) 1627(3) 1937(3) 2095(3)
U(W)"
"Equivalent isotropic U defined as one-third of the trace of the orthogonalized Uij tensor. meso keto substituent. In the major form, that group is present atsiteO(1a) with0.69(l)occupancy, whiletheminorsite,O(lb), has a fractional occupancy of 0.31( 1). The other, independent molecule has a six-coordinate cobalt and a ruffled oxophlorin core. However, for the second molecule, the meso keto substituent is disordered over three sites. In all cases, however, the C-O bond distances are 1.25 A, a value which is appropriate for a keto, rather than a hydroxy, substituent. Molecule 2 also suffers from disorder in the positions of two entire ethyl groups and the methyl portion of another ethyl group. Despite the differences in the disorder, the two molecules have similar geometries. In both cases, the in-plane and out-of-plane Co-N distances are nearly equal and fall in a narrow range: 1.948(7)-1.969(7) A for molecule 1, 1.948(7)-1.972(7) A for
-
(16) Munro, 0. Q.;Bradley, J. C.; Hancock, R. D.; Marques, H. M.; Marsicano, F.; Wade, P. W. J. Am. Chem. SOC.1992, 114, 7218.
Balch et al.
4740 Inorganic Chemistry, Vol. 32, No. 22, 1993 Table II. Bond Lengths (A) and Angles (deg) for
I(PY )CO"(OEPOH-PY 11 Bond Lengths 1.983(3) Co-N(2) 1.993(3) Co-N(4) 2.203(3) O-C(1) 1.383(6) N(l)-C(5) 1.387(5) N(2)-C(10) 1.384(6) N(3)-C(15) 1.37l(5) N(4)-C(20) 1.340(5) N(5)-C(41) 1.387(6) C(l)-C(20) 1.458(6) C(3)-C(4) 1.373(6) C(4)-C(5) 1.354(6) C(5)-C(6) 1.454(6) C(7)-C(8) 1.371(6) C(11)-C(12) 1.457(6) C(13)-C(14) 1.385(6) C(14)-C(15) 1.363(6) C( 15)-C( 16) 1.451(6) C( 17)-C(18)
1.990(4) 1.987(4) 1.372(6) 1.383(5) 1.373(6) 1.388(5) 1.381(6) 1.343(6) 1.405(5) 1.370(7) 1.438(6) 1.385(7) 1.454(7) 1.385(6) 1.372(6) 1.435(6) 1.372(7) 1.445(6)
Bond Angles N( 1)-Co-N(2) N(2)-Co-N( 3) N(2)-Co-N(4) N( l)-Co-N(5) N(3)-Co-N(5) Co-N( 1)-C(2) ~(2)-~(1)-~(5) Co-N(Z)-C( 10) Co-N(3)-C(12) C( 12)-N(3)-C( 15) Co-N(4)-C(20) Co-N(5)-C(37) C(37)-N(5)-C(41) 1)-C(20) N(l)-C(2)4(1) c(1)-~(2)-~(3) C(3)-C(4)-C(5) N(l)-C(5)-C(6) C(5)-C(6)-C(7) ~(2)-~:(7)-~(8) C(7)-C(8)-C(9) " w ) - ~ ( 9 ) C(9)-C(lO)-C(ll) N(3)4(12)-C(ll) C(ll)-C(l2)-C(13) C( 13)-C( 14)-C(15) N(3)-C(15)-C(16) C(15)-C(16)-C(17) N(4)-C( 17)-C( 18) C( 17)-C(18)-C(19) ~(4~-~~20)-~ C( 1)-C(20)-C( 19)
o-c(
90.8(1) 88.7(1) 173.6(1) 94.5(1) 94.0(1) 128.9(3) 104.5(3) 128.1(3) 127.8(3) 104.6(3) 128.8(3) 122.6(3) 116.0(3) 117.0(4) 123.8(4) 124.8(4) 107.2(4) 125.1(4) 125.9(4) 110.7(4) 106.7(4) 111.2i3j 123.6(4) 124.4(4) 124.4(4) 107.1(4) 123.8(4) 125.9(4) 111.0(4) 107.2(4) (123.0(4) 1) 125.4(4)
N(l)-Co-N(3) N(l)-Co-N(4) N(3)-Co-N(4) N(2)-Co-N(5) N(4)-Co-N(5) Co-N(l)-C(S) Co-N(2)-C(7) C(7)-N(Z)-C(lO) Co-N(3)-C(15) Co-N(4)-C(17) C( 17)-N(4)-C(20) Co-N(5)-C(41) O-C(l)-C(Z) C(2)-C(l)-C(20) N(l)-C(2)4(3) C(2)-C(3)-C(4) N( l)-C(S)-C(4) C(4)-C(5)-C(6) N(2)-C(7)-C(6) C(6)4(7)-C(8) C/8)-C(9)-C(lO) N(2)-CiiO)k( 1'1) C( lo) 241)) 0.056b
I (PY)2CJ1'(OEPO)) C&&ON6O 764.9 PZI, monoclinic 19.177(4) 20.039(4) 10.547(2) 100.36(1) 3987.0(14) 4 120(2) 1.541 78 (Cu Ka) 3.703 1.274 0.55-0.72 0.054 (I> 2 4 ) ) 0.1 44c
the oxophlorin trianion in the simple, nonbridging form. The characteristicfeature is the low-energy absorption at ca. 700 nm with a prominent shoulder at lower wavelengths. Thus for ( ( ~ Y ) ~ C O ~ ~ ~ ( these O E Poccur O ) J at 676 and 622 nm, respectively, while for ( ( P Y ) ~ M ~ ~ ~ ~ ( Othey E P appear O ) } at 743 and 678 nm (see trace i of Figure 3 in ref 2) and for ((py)2Ga111(OEPO)} they are found at 692 and 636 nm.23 In contrast, complexes that contain the oxophlorin radical have a single absorption that occurs above 800 nm.IoJ1 Experimental Section
Preparation of Compounds. ((py)&G(OEPO)). This was obtained by a modification of the procedure of Bonnett and Dimsdale.2 Under a dinitrogen atmosphere, a 0.3-mL portion (0.086 mmol) of 1% hydrogen peroxide in pyridine was added to a red solution of 30 mg (0.05 mmol) of Cotl(OEP) in 15 mL of pyridine. The solution was heated to 60 OC for 20 min. Water ( 5 mL) was added to the resulting lime-green solution. The microcrystallineblue precipitate was collected by filtration and washed with water. The solid was dried under vacuum; yield 35 mg, 92%. { (py)C#(OEPOH-py)). Under a dinitrogen atmosphere, a 5-mL portion of a saturated solution of sodium dithionite in water was added to a green solution of 10 mg (0.014 mmol) of { ( ~ ~ ) ~ C O ~ ~ ~ (inO10 EPO)) mL of pyridine. The reaction mixture, which immediately turned deep orange, was stirred overnight. Upon addition of 10 mL of water to the orange solution, a red-orange solid formed. This was collected by fiitration, washed with water, and dried under vacuum for 12 h to give 9 mg (92%) of the product. The product is very air sensitive, and prolonged vacuum drying produces a product whose solubilityin dichloromethane is reduced, Figure 6. Diagrams of the core of the two molecules of {(py)2C0~~~- presumably due to the loss of pyridine. (OEPO)) with the displacement of each atom (in units of 0.01 A) from X-ray Data Collection. { (py)C#(OEPOH-py)). Dark orange plates the plane of the macrocycle: A, molecule 1; B, molecule 2. were obtained by addition of water to a pyridine solution of the complex N(pyrro1e) bonds is characteristic of a number of complexes such and then allowing the mixture to stand for several days. A suitable crystal as [(py)2Fe111(TPP)]+ 27 and [(L)FelI1(TMP)]+ (TMP is the was coated with a light hydrocarbon oil and mounted in the 130 K dinitrogen stream of a Siemens R 3 m/V diffractometer that was equipped dianion of tetramesitylporphyrin; L is 4-(dimethy1amino)pyriwith a locally modified low-temperature apparatus. Two check reflections dine,28 3-eth~lpyridine,2~3 - ~ h l o r o p y r i d i n e , or ~ ~ 4-cyanopyrishowed only random (
