Energy & Fuels 1990,4,661-664 an isotopically heavier source. In the Condor sediment, 11 has an isotopic value 3.3% heavier than 8 and 0.9% lighter than the average of 1 and 2. It would appear from what has been outlined above that chlorophyll c derived petroporphyrins are isotopically heavy in these two sediments and that chlorophyll c3, which contains a 7carboxymethyl group,24may be inferred as an alternative source for 11. Previously it had been suggested that the two 13(1)methyl-substituted porphyrins, 3 and 7, might be related through a common precursor of unknown origin.8 Since (24) Fookes, C. J. R.; Jeffries, S. W. J. Chem. Soc., Chem. Commun. 1989, 1827-1828.
661
6 is most likely the hydrogenated derivative of 7, it too should be related. Although 3 and 6 have not been isolated together from the same sediment in this study, their relative carbon isotopic values (3 from Condor oil shale and 6 from Julia Creek oil shale; Table I) have the same relationship; they lie almost at the midpoint between 8 and 1 or 2. Thus the isotopic evidence also supports the notion of a common precursor, possibly chlorophyll c. Acknowledgment. We thank CSR Limited and Southern Pacific Petroleum/Central Pacific Minerals for providing the Julia Creek oil shale and the Condor oil shale, respectively. Mr. A. Juodvalkis is gratefully acknowledged for his technical assistance with the HPLC porphyrin isolations.
Diastereoisomers in Sedimentary Vanadyl Porphyrins Christopher J. Boreham* Bureau of Mineral Resources, Geology and Geophysics, P.O. Box 378, Canberra, 2601, Australia
Peter S. Clezy Department of Organic Chemistry, University of New South Wales, P.O. Box 1, Kensington, NSW,2033, Australia
Glen B. Robertson Research School of Chemistry, Australian National University, P.O. Box 4, Canberra, 2601, Australia Received April 27, 1990. Revised Manuscript Received August 6, 1990
The observation that the product of vanadyl insertion into the free-base porphyrin Id-methyl15,17-propanoporphyrin separated into two near equal area peaks on HPLC analysis precipitated an investigation into the structure of both vanadyl porphyrins. Semipreparative HPLC and TLC were used to isolate pure components, and an X-ray structure determination on the more polar compound revealed a structure wherein the out-of-plane vanadyl and 15I-methyl groups were opposed, referred to as up-axial, relative to the plane of the porphyrin ring. The other complex is tentatively assigned to the diastereoisomer in which both groups are juxtaposed, down-axial. Metalloporphyrins in which the metal is coplanar with the ligand plane cannot have this diastereoisomerism. Sedimentary vanadyl porphyrins from the immature Serpiano oil shale contain both diastereoisomers but in a different ratio compared to the synthetic mixture. Artificial maturation experiments on the diastereoisomers suggest isomer interconversion can occur and the up-axial configuration is the more stable.
Introduction During the past few decades our understanding of the processes that lead to the formation of petroporphyrins has increased dramatically. The early hypothesis of a direct pathway from chlorophyll or heme to porphyrin' has had to be drastically modified as investigations continue.2$ ~
~~~
~~
~
(1) Treibs, A. Ann. Chem. 1934,509, 103-114. (2) Baker, E. W.; Louda, J. W. Biological Markers in the Sedimentary Record; Johns, R. B., Ed.; Elaevier: New York, 1986; pp 124-225. (3) Filby, R.H.; Van Berkel, G. J. Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 2-39.
Recognition of a much greater complexity of the interconversion has resulted, principally, from increased ability to characterize individual petroporphyrins through the use of new or improved instrumentation. Thus, methods used to characterise petroporphyrins have evolved from UV-vis and mass spectral analysis4of bulk porphyrins or of simple chromatographic concentrates, to structural identification of single porphyrins by high-field NMR,"12 by X-ray (4) Baker, E. W.;Palmer, S. E. The Porphyrins, Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. lA, pp 485-552. (5) Fookes, C. J. R. J. Chem. SOC.,Chem. Commun.1983,1472-1473. (6) Fookes, C.J. R. J. Chem. Soc., Chem. Commun.1983,1474-1476.
0887-0624/90/2504-0661$02.50/0 Published 1990 by the American Chemical Society
Boreham et al.
662 Energy & Fuels, Vol. 4 , No. 6,1990
la
R=Et
2
l b R=Me IC R = H
Fi re 2. Crystal structure of the up-axial isomer, 2U,of vanadyl 15 -methyl-15,17-propanoporphyrin.
P
V023-26),by GC-MS of derivatized porphyrin^,^' and by tandem mass spectrometry (MS-MS).28 This paper reports on the identification of a pair of diastereoisomers of vanadyl porphyrins that can exist when there is a methyl group attached to a chiral carbon within an exocyclic ring on the porphyrin. Known alkyl porphyrin structures 2,11 3,S 4,22 5,12and 622are candidates for this coordination isomerism when they occur as their vanadyl analogues. When both diastereoisomers are present in sedimentary extracts of vanadyl porphyrins, a further degree of complexity is introduced into the interpretation of the HPLC trace.
3
R'
Experimental Section 5
6 R'*2,3.4= ZxMe, 2xEt
Figure 1. Structure of vanadyl porphyrins.
diffraction,13-15by high-resolution HPLC of free-base porphyrinsle21 and metalloporphyrins (mainly Ni22*23 and (7) Fookes, C. J. R. J. Chem. Soc., Chem. Commun. 1985, 706-708. (8) Ocampo,R.; Callot, H. J.; Albrecht, P. Tetrahedron Lett. 1984,25, 2589-2592. (9) Ocampo, R.; Callot, H. J.; Albrecht, P. J. Chem. Soc., Chem. Commun. 1985, 198-201. (10) Verne-Mmer, J.; Ocampo, R.; Callot, H. J.; Albrecht, P. J. Chem. Soc., Chem. Commun. 1987, 1581-1583. (11) Chicarelli, M. I.; Wolff, G. A,; Maxwell, J. R. Tetrahedron 1984, 40,4033-4039. (12) Chicarelli, M. I.; Kaur, S.; Maxwell, J. R. Metal Complexes in
Fossil Fuels; Filby, R. H., Branthaver, J. F., E&.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 40-67. (13) Ekstrom, A.; Fookes, C. J. R.; Hambley, T.; Loeh, H. J.; Miller, S. A.; Taylor, J. C. Nature 1983,206, 173-174. (14) Storm, C. B.; Krane, J.; Skjtene, T.; Telnaes, N.; Branthaver, J. F.; Baker, E. W. Science 1984,223, 1075-1076. (15) Quirke, J. M. E.; Maxwell, J. R. Tetrahedron 1980,36,3453-3456. (16) Hajibrahim, S. K.; Tibbetts, P. J. C.; Watts, C. D.; Watts, J. R.; Eglinton, G.; Colin, H.; Guiochon, G. Anal. Chem. 1978, 50, 549-553. (17) MacKenzie, A. S.; Quirke, J. M. E.; Maxwell, J. R. Advances in Organic Geochemistry, 1979; Douglas, A. G., Maxwell, J. R., Eds.; Pergamon Press: Oxford, 1980; pp 239-248. (18) Barwise, A. J. G.; Park, P. J. D. Advances in Organic Geochemistry, 1981; Bjoroy, M., Ed.; Wiley: London, 1983; pp 668-674. (19) Barwise, A. J. G.; Evershed, G. P.; Wolff, G. A.; Eglinton, G.; Maxwell, J. R. J. Chromatogr. 1986, 368, 1-9. (20) Chicarelli, M. I.; Wolff, G. A.; Maxwell, J. R. J.Chromatogr. 1986, 368, 11-19.
Analytical HPLC. HPLC analysis was performed by using Waters equipment employing three pumps, a U6K injector, and a 490 multiwavelength detector.22The vanadyl porphyrin elution profile was visualized by monitoring the adsorbance at X = 413 nm as well as by simultaneously absorbance-ratio recording at 415 nm/570 nm.z3 High-resolution reversed-phase HPLC employed a three-column combination (two 250 X 4.6 mm i.d., 150 X 4.6 m m i.d., 3-rm C18Hypersil) under isocratic flow at 0.8 mL/min in methanol/ acetonitrile/water.% Sedimentary vanadyl porphyrins extracted from the Serpiano oil shale (Triassic, Monte S.Georgia, Switzerland) were analyzed according to this analytical method (2 fig of vanadyl porphyrin per injection). Isolation of Two Diastereoisomers of 2. The free-base porphyrin, 15*-methyl-15,17-propanoporphyrin was synthesizedB and vanadyl inserted to give 2 as described previouslyz3(3 and (21) Barwise, A. J. G. Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 100-109. (22) Boreham, C. J.; Fookes, C. J. R. J. Chromatogr. 1989, 467, 195-208. (23) Boreham, C. J. Biological Markers in Sediments and Petroleum,
a Tribute t o W .K . Seijert; Moldowan, J. M., Ed.; Prentice Hall, in press. (24) Sundararaman, P. Anal. Chem. 1985,57,2204-2206. (25) Sundararaman, P.; Bigge, W. R.; Reynolds, J. C.; Petzer, J. C. Geochim. Cosmochim. Acta 1988,52,2337-2341. (26) Sundararaman, P. Biological Markers in Sediments and Petroleum, a Tribute to W. K . Seijert; Moldowan, J. M., Ed.: Prentice Hall, in press. (27) Marriott, P. J.; Gill, J. P.; Eglinton, G. J. Chromatogr. 1982,249, 291-310. (28) Johnson, J. V.; Britton, E. D.; Yost, R. A,; Quirke, J. M. E. Anal. Chem. 1986,58, 2204-2206. (29) Clezy, P. S.; Fookes, C. J. R.: Prashar, J. K. J. Chem. SOC.,Chem. Commun. 1988, 83-84.
Diastereoisomers in Sedimentary Vanadyl Porphyrins 6 were also prepared by VO insertion into the freebase porphyrin). From 2, two separate peaks were collected under the following conditions: vanadyl porphyrin (100pg in CHC1,) was injected onto two semipreparative columns in series (300 X 7.8 mm; Waters 10-pm C18 Bondapak) under isocratic flow at 3 mL/min (4545:lO; acetonitri1e:methanol:water). Final purification of each HPLC peak by TLC (250pg of vanadyl porphyrin on Merck 200 x 200 X 0.25 mm silica 60 plates; petroleum ether:dichloromethane as 30:70) gave individual isomers in >99% purity with respect to the other isomer, as determined by analytical HPLC. Crystal Structure Analysis. Dark red crystals of the most polar isomer, suitable for X-ray diffraction analysis, were grown by vapor diffusion (n-pentaneinto CHzClzsolution). Crystals were monoclinic: space group E l l a with a, 15.551 (2)A; b, 13.946 (2)A; c, 12.594(2)A; 9, = 105.45 (1)O;and 2 4 (2' = 24 O C ) . Reflection intensities were recorded on a Philips PW1100/20 diffractometer operating in 8-28 scan mode (scan velocity Som i d 28;2 X 7 s backgrounds at extremes; Mo K a radiation; graphite crystal monochromator; 3 < 0 < 25'; *h + k + 1; 5097 measured reflections excluding standards). The specimen crystal dimensions were 0.175X 0.12X 0.047mm parallel to the reciprocal vectors 110,l(-l)O,and 001,respectively. Reflection intensities were corrected for Lorentz and polarization effects but not for absorption (transmission factor range 0.96-0.98),for crystal degradation ( 3u);w = { u , l ~ ~ 0.00o~l~~fj-11. l+ H atoms on methyl carbons 21,26,27and 29 could not be located unambiguously by difference Fourier synthesis and were not included in the scattering model. Other H atoms were located by calculation (C-H = 1.08 A, Y = 0.05 AZ). Difference syntheses showed unambiguous anisotropic effects in the vicinity of the vanadium and oxygen atoms, and in subsequent refinement these atoms (V and 0)were allowed anisotropic thermal parameters. At convergence (164parameters, 1252 reflections) R = 0.112,w R = 0.117, GOF = 2.55,A/u < 0.11,and -0.66 < Ap CO.99 e A". Tables Sl-S4 (on microfiche; see the paragraph at the end of the paper regarding supplementary material) contains the basic data on (SI) crystal structure,(52)atomic coordinates and thermal parameters, (S3)bond lengths, and (S4)selected bond angles.
Results and Discussion Analytical HPLC examination of the product from VO insertion into the uncomplexed porphyrin of 2 revealed two peaks of near equal proportions. The vanadyl porphyrins 3 and 6 also showed two peaks (the corresponding product from nickel insertion into each of the above free-base porphyrins22did not show this phenomenon and gave only one peak). For 2 the earlier eluting peak, the more polar, was slightly more abundant (52%). Isolation of each peak in bulk quantities (milligram) was readily achieved by repeated semipreparative HPLC. Mass spectral analysis showed both to have an identical molecular weight. Demetalation of an individual peak followed by remetalation returned a product identical with the original insertion reaction (i.e., reappearance of two peaks in relative proportions as before). This strongly suggested that the two peaks were due to isomers formed during vanadium insertion. Although 2 has a chiral center a t C15', the analytical conditions employed would not be expected to separate enantiomers. In principle, the preparative reaction can yield four distinct isomers, viz., the up-axial/down-axial and the up-equatorial/down-equatorialdiastereomeric pairs (with "up" denoting the vanadyl group on the same face of the porphyrin as the methine-H on the C15' chiral (30)SHELXS-88; Sheldrick, G. M. Crystallographic Computing 3; Sheldrick, G. M., Kfiger, C., Goggard, R., as. Oxford ; University Press: Oxford, England, 1985; pp 175-189. (31) Sheldrick, G.M. SHELX-76, Program for Crystal Structure Determination; University of Cambridge: Cambridge, England, 1976.
Energy & Fuels, Vol. 4, No. 6, 1990 663
center and the axial/equatorial denoting the methyl group disposition at that center. Each isomer would in turn be present as enantiomers. In order to assign unambiguous structures to both isomers, we attempted to grow single crystals of each. However, this was successful only for the more polar isomer. Crystal Structure. In the convention outlined above the crystallized isomer (Figure 2) is up-axial (2U)and is present as the enantiomeric mixture. The second, less polar isomer is tentatively assigned as the enantiomeric Persuasive support for the mixture of down-axial (2D). latter assignment follows from strain-energy calculations (MM2)32for the uncomplexed porphyrin of 2, which indicate that the axial conformation (of the C15I-methyl group) is favored over the equatorial conformation by ca. 3 kcal/mol. Further support arises from the chromatographic behavior related to the polarity difference. In the up-axial configuration the polar V=O group and the 15I-methyl would have no interaction. However, when juxtaposed in the down-axial configuration, the methyl group can provide a barrier to interaction of the vanadyl group with solid supports, effectively reducing the overall molecular polarity. Bond lengths and bond angles agree well with those reported for other vanadyl complexes (VO-DPEP, VOOEP, VO-ETIO, VO-TPP).14,33-36V-N distances [2.02 (2b2.10 (2) A] are equal to within experimental error (average 2.06 A). Two separate studies of VO-DPEP14s33W have reported (internally) inequivalent V-N distances due, apparently, to the less than 4-fold symmetry of the porphyrin ligand. Similar inequivalences in the present complex are possibly masked by the relatively high associated bond length standard errors. The V-0 distance [1.545 (12) A] is smaller than previously found in vanadyl p ~ r p h y r i n s ' ~ Jand ~ - ~is~ not much altered by a riding correction (0 on V, A = 0.008 A). The vanadium is displaced 0.50 (1)8, above the ligand (nitrogen) plane in the direction of the V 0 vector (metals that are positioned within the ligand plane, e.g., Ni2+, will not form these diastereoisomers). The out-of-plane displacement is simand ilar to previously reported values (0.48-0.54 A),14*33-38 the V=O bond is not significantly tilted from the plane perpendicular. Although the crystal is solvent free, the crystal packing arrangement is very similar to that in VO-DPEP-1,2-dichloroethanesolvate. The interplane separation between inversion related molecular pairs in 2U (i.e., faces not carrying vanadyl groups) is 3.62 A and that between the screw axis related pairs is 3.54 A. Occurrence of Both Isomers in Sedimentary Vanadyl Porphyrins. The occurrence of 2 was first reported in the Serpiano oil shale." The metalloporphyrin was initially demetalated, and the structure was determined by 'H NMR as the Zn complex. Since this sediment is a known source of 2, the total vanadyl alkylporphyrin fraction was examined for the presence of the diastereoisomers, 2U and 2D. Figure 3 shows the high-resolution reversed-phase HPLC traces under the two most favorable mobile-phase conditions for isomer identification; no single solvent program was found that could resolve both isomers from other coeluting vanadyl porphyrins. Peak identification was by coinjection with the synthetic standard and
-
(32) Burkert, U.; Allinger, N. L. Molecular Mechanics; American Chemical Society: Washington, DC, 1982. (33) Pettersen, R. C.; Alexander, L. E. J . Am. Chem. SOC.1968, 90, 3873.
(34) Pettersen, R. C. Acta Crystallogr. 1969, B25, 2527. (35) Molinaro, F. S.; Ibers, J. A. Inorg. Chem. 1976, 9, 2278. (36) Drew, M. G. B.; Mitchell, P. C. H.; Scott, C. E. Inorg. Chim. Acta 1984,82, 63.
Boreham et al.
664 Energy & Fuels, Vol. 4, No. 6, 1990 la I
Absorbance 413 nm
lc
40
BO
40
BO
80
100
40
BO
80
100
40
BO
80
100
Ab a oibI nce 413 nm
Retention time (min)
80 Retention time (min)
100 11-2118
Figure 3. HPLC traces of the total vanadyl alkylporpyhyrinsfrom the Serpiano oil shale. (A) Mobile phase 76195; MeOHCH3CN:H20. (B) Same as (A) but spiked with 2. (C) Mobile phase 33:62:5; MeOH:CH,CN:H,O. (D)Same as (C) but spiked with 2.
maintainance of the ratio-absorbance trace between pristine and spiked runs (Figure 3A and 3B; Figure 3C and 3D). The distribution of the isomers was found to be different from that observed in the synthetic mixture. In the natural system, 2U was the most dominant by far, being about twice as abundant as 2D (68%:32%). Three possibilities may account for this: (i) the up-axial isomer is thermodynamically the most stable and the ratio observed in the geological sample is at or approaching equilibrium. In the synthetic mixture metalation of the free-base porphyrin is under kinetic control and metal insertion on the side of the porphyrin plane opposed to the axial 15'-methyl group is favored slightly; (ii) the up-axial isomer is more stable than the down-axial isomer which subsequently decomposes faster; (iii) an environmental control operates (the uncomplexed porphyrin can be either bound to or unassociated with the kerogen structure) which limits the accessibility of the coordinating metal to one side of plane of the free-base porphyrin. Explanation iii does not easily lend itself to experimentation. The validity of the other two explanations was addressed by attempting to artificially equilibrate each individual isomer at high temperature. The pure isomer was heated at 270 O C in an evacuated quartz tube (10 Pg of VanadYl Porphyrin: 10 P L of water) for 1 and 4 weeks and the product was examined by analytical HPLC. After 1 week, ca. 50% of the porphyrin had
decomposed and the up-axial isomer had equilibrated to a lesser extent (2U:2D as 67%:33%) compared with the down-axial (2U:2D as 37%:63%). After 4 weeks ca. 90% had decomposed and the product isomer distribution obtained from 2U and 2D was 2U:2D as 90%:10% and 2U:2D as 32%-68%, respectively. Therefore, under these conditions it appears 2U is the more stable isomer and that both decomposition and equilibration are competing reactions. In the simulated system, the former reaction has the faster rate and the diastereoisomers never reached a common equilibrium value. During early diagenesis of the sedimentary organic matter, vanadyl porphyrins are first produced from their demetalated precursors2 (in the past at a lower maturity than the present Serpiano sediment). What was the isomer distribution in the Serpiano sediment at the time of metalation remains unresolved.
Acknowledgment. Mr. Algis Juodvalkis is thanked for the semipreparative HPLC separation of the diastereoisomers. The Serpiano oil shale was kindly donated by Dr. James Maxwell. C.J.B. publishes with the permission of the director, Bureau of Mineral Resources, Geology and Geophysics. Supplementary Material Available: Tables SI-SIV listing crystal data, atomic coordinates and thermal parameters, bond lengths, and selected bond angles for vanadyl 15'-methyl-15,17propanoporphyrin (5 pages). Ordering information is given on any current masthead page.
