J . Phys. Chem. 1990, 94, 6659-6663
6659
Site of One-Electron Reduction of Ni( I I ) Porphyrins. Formation of Ni( I ) Porphyrin or Ni( I I ) Porphyrin ?r-Radical Anion G. S. Nahor,t P. Neta,*Vt P. Hambright,$L. R. Robinson,t and A. Harrimani Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899; Department of Chemistry, Howard University, Washington, D.C. 20059; and Center for Fast Kinetics Research, University of Texas at Austin, Austin, Texas 78712 (Received: January 22, 1990; In Final Form: April 5. 1990)
The products of one-electron reduction of a series of nickel(l1) porphyrins (Ni"P) have been examined by radiolytic reduction in protic media to determine whether Ni'P or the a-radical anion Ni"P'- is formed. Kinetic spectrophotometric detection was utilized to record absorption spectra of the product immediately after one-electron reduction, and y-radiolysis was used for recording spectra of the stable reduction products. In the submillisecond time scale, certain Ni"-porphyrins were found to be reduced to Ni'P, whereas others gave Ni"P'-. The initial reduction product was dependent on the nature of substituents on the porphyrin but not on the redox potential; a meso-4-pyridyl group seems to direct the reduction toward the porphyrin ring, whereas in most other cases reduction occurred on the metal. On longer time scales (minutes) all porphyrins produced the two-electron reduced products, chlorin or phlorin anion. This suggests that both Ni'P and N i l P disproportionate to give Ni"P and Ni"P2-, such that for Ni'P an intramolecular electron transfer takes place from Nil to its porphyrin ligand.
negatively charged derivatives, the porphyrin was dissolved in water Introduction and reflxued with a 3-fold excess of NiCI,.6H20, until the 645-nm Complexes of nickel(I1) with tetrapyrroles are of interest as absorption peak due to the free-base porphyrin had disappeared. models for factor F430 of the methyl coenzyme M reductase, an The solution was filtered and adjusted to pH 3 with HCI, small enzyme that catalyzes the final step of methane production in portions of solid o-phenanthrolineIs were added, and the pH was biological Recently, Nil1 complexes with tetraazamaintained at this value, until the metalloporphyrin precipitated. macrocycles showed catalytic activity in the reduction of alkyl The solid was collected on a 0.45-pm Millipore filter, washed with halides to alkanes or alkenes4 This reduction was suggested to distilled water, and stirred in an aqueous slurry of ion-exchange occur by an inner-sphere mechanism involving an intermediate beads in the sodium form until the porphyrin dissolved. After Ni' species.4a Nil is crucial for this activity since this is the only this solution was filtered, it was slowly passed through a column oxidation state of nickel capable of forming a Ni-C bond upon of the same resin, and the product isolated by lyophylization. reaction with the organic m ~ l e c u l e . ~ * ~ Literature methods were used to prepare the positively charged Nil species have been observed and characterized following nickel(I1) porphyrin^.^^,'^ The absorption spectra of the mesol' one-electron reduction of several Nil1 tetrapyrrole complexes, including factor F4305 and chlorins and b a c t e r i o ~ h l o r i n s . ~ ~ ~ ( I ) Ellefson, W. L.; Whitman, W. B.; Wolfe, R. S. Proc. Nail. Acad. Sci. Recently, Ni'--xtaethylisobacteriochlorin was isolated.6b It was U.S.A. 1982, 79, 3707. suggested that Nil species can be obtained only with such partially (2) Hausinger, R. P.; Orme-Johnson, W. H.; Walsh, C. Biochemistry 1984, saturated macrocycles as the i s o b a c t e r i ~ c h l o r i nand ~ * ~that re23, 801. duction of Ni"-porphyrins (Nil*P) will lead to formation of a (3) Daniels, L.; Sparling, R.; Sprott, G. D. Biocfiim.Biopfiys. Acta 1984, 768, 113. porphyrin *radical anion (Ni"P'-). These authors argued that (4) (a) Lexa, D.; SavEant, J.-M.; Su, K. B.; Wang, D. L. J. Am. Cfiem. the partially saturated ligands allow bending of the ring to increase Soc. 1987,109,6464. (b) Bakac, A.; Espenson, J. H. J . Am. Cfiem.Soc. 1986, the space between the four nitrogen atoms to accommodate the 108, 713. (c) Espenson, J. H.; Ram, M. S.; Bakac, A. J. Am. Cfiem.SOC. larger Nil ion. Such bending is less likely with porphyrins since 1987, 109, 6892. ( 5 ) (a) Albracht, S. P. J.; Ankel-Fuchs, D.; Van der Zwaan, J. W.; Fontijn it would cause severe distortion of the aromatic system. As a R. D.; Thauer, R. K. Biocfiim. Biopfiys. Acta 1986, 870, 50. (b) Jaun, B.; result, Nil may be forced out of the plane and thus create an Pflatz, A. J. Cfiem. SOC.,Cfiem. Commun. 1986, 1327. unstable species. Therefore, Ni"P will tend to be reduced to the (6) Stolzenberg, A. M.; Stershic, M. T. (a) Inorg. Cfiem. 1987, 26, 1970. a-radical anion (reaction l).6c,7 In fact, reduction of several (b) J . Am. Cfiem. SOC.1988, 110, 5397. (c) J . Am. Cfiem.SOC.1988, 110,
Ni"-porphyrins was suggested to occur on the ligand.* In contrast, recent results by Lexa et aL9 indicated the reduction of certain Ni"P to the Ni'P state (reaction 2). Since the two modes Ni"P
+ e-
-
Ni'P
(2)
of reduction lead to products with a large difference in the potential catalytic activity, it is important to elucidate the nature of these products. We have utilized pulse radiolysis and steady-state y-radiolysis techniques to reduce a series of nickel( 11) porphyrins and to examine the one- and two-electron reduction products.
Experimental Sectionlo The metal-free porphyrins were synthesized by conventional methods"J2 and have been used in previous studies in these l a b ~ r a t o r i e s . ' ~ -To ~ ~prepare the nickel(I1) complexes of the 'National Institute of Standards and Technology. *Howard University. University of Texas at Austin.
0022-3654/90/2094-6659$02.50/0
6391. (7) Renner, M. W.; Forman, J.; Fajer, J.; Simpson, D.; Smith, K. M.; Barkigia, M. Biopfiys. J. 1988, 53, 277a. Renner, M. W.; Forman, A,; Wu, W.; Chang, C. K.; Fajer, J. J . Am. Cfiem. SOC.1989, 1 1 1 , 8618. Furenlid, L. R.; Renner, M. W.; Smith, K. M.; Fajer, J. J . Am. Cfiem.SOC.1990,112, 1634.
(8) (a) Kadish, K. M.; Sazou, D.; Liu, Y. M.; Saoiabi, A,; Ferhat, M.; Guilard, R. Inorg. Cfiem. 1988,27,686. (b) Ibid. 1988,27, 1198. (c) Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Cfiem. 1984, 23, 817. (9) Lexa, D.; Momenteau, M.; Mispelter, J.; Saveant J.-M. Inorg. Cfiem. 1989, 28, 30.
(IO) The identification of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified are necessarily the best available for the purpose. ( 1 1 ) Hambright, P. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; Chapter 7. (12) Shamim, A.; Worthington, P.; Hambright, P. J. Cfiem.SOC.Pakistan 1981, 3, 1. (13) Baral, S.; Neta, P.; Hambright, P. Radial. Pfiys. Cfiem. 1984, 24, 245. (14) Nahor, G. S.; Neta, P.; Hambright, P.; Thompson, A. N., Jr.; Harriman, A. J . Pfiys. Cfiem. 1989, 93, 6181. ( ! 5 ) We thank Professor D. Lavallee, Hunter College, for suggestions on this isolation procedure. (16) Pasternack, R. F.; Spiro, E. G.; Teach, M. J . Inorg. Nucl. Cfiem. 1974, 36, 599. (17) Hambright, P.; Fleischer, E. B. Inorg. Cfiem. 1970, 9, 1757.
0 1990 American Chemical Society
6660 The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 and @-pyrrolei8-substituted complexes have been reported. The abbreviations used for the various porphyrins are given in Table I . Note that the overall electronic charges have been omitted for clarity of presentation and Nil-porphyrins are referred to as Ni'P without consideration of the additional negative charge. ,Y,N-Dimethylformamide (DMF, Aldrich HPLC grade) was purified and dried by distillation from CaH, and stored over molecular sieves. Tetra-n-butyl-ammonium perchlorate was also obtained from Aldrich. Water was purified with a Millipore Super-Q system (radiolytic studies) or by deionizing and then triple distillation from quartz vessels (electrochemical studies). Other materials used were of analytical grade. Solutions were prepared freshly before use and were purged with Ar or N 2 (Matheson, UHP). Pulse radiolysis experiments were carried out with a Febetron Model 705 accelerator, producing 2-MeV electron pulses of 50-ns duration. The radiation dose ranged from 5 to 20 Gy/pulse, yielding 3-1 2 WMof radicals (for G = 6). Dosimetry was carried out with N20-saturated 0.01 M KSCN aqueous solutions. The detection system consisted of a 300-W Xe lamp, a Kratos monochromator, an RCA 4840 photomultiplier, and associated optical components. The signals were transferred through a Tektronix 7612 digitizer to a personal computer for analysis. Steady-state irradiation was carried out in a Gammacell 220 @Co source with a dose rate of 135 Gy/min. Absorption spectra were recorded on a Cary 219 spectrophotometer. Reduction potentials of the various nickel(I1) porphyrins were measured in DMF, with 0.2 M tetra-n-butylammonium perchlorate as electrolyte, or in water, with 0.2 M KCI as electrolyte, by cyclic voltammetry and pulse differential voltammetry under an Ar atmosphere. The working electrode was a hanging mercury drop (HMDE), and the counterelectrode was a Pt microelectrode, with SCE or Ag/AgCI as reference electrodes. All potentials are reported vs SCE and were reproducible to within *20 mV. Results and Discussion Electrochemistry ofthe Nickel( I I ) Porphyrins. The reduction peak potentials (E,) or half-wave potentials (E,,,) of the various Nil'-porphyrins were studied in aqueous media and/or in DMF, depending on their solubility. Measurements in aqueous solutions were made, whenever possible, to relate them to the radiolytic experiments. Aqueous solutions containing IO4 M porphyrin and 0.2 M KCI at pFJ 7 under Ar were studied by cyclic voltammetry as well as by differential pulse voltammetry. A hanging mercury drop electrode (HMDE) was used, with a pt counterelectrode and an Ag/AgCI reference. All the peak potentials E, (Table I ) corresponded to diffusion-controlled electrode processes involving addition of a single electron, except for the case of TM4PyP, which underwent a two-electron reduction process. Reduction was irreversible except where noted otherwise. Half-wave potentials in D M F solutions, containing 2 X M porphyrin and 0.2 M tetra(n-buty1)ammonium perchlorate, under Ar, were measured as above but with SCE as reference. All reported peaks (Table I) corresponded to diffusion-controlled electrode processes involving addition of a single electron. The only exception was TM4PyP, which underwent a two-electron reduction. Mostly, reduction was quasi-reversible provided the reduction scan was stopped soon after the first peak. The reduction potentials for the Nil1-porphyrins are very similar to those reported for the corresponding free-base porphyrins (H,P) in DMFi9 (Table I), indicating that Ni" has little inductive effect. Figure l a shows a plot of the reduction potentials measured for the Ni"P vs the values reported for H2P. The results of Lexa et aL9 are also included in the figure (but the result for TM4PyP is excluded because it was for a two-electron reduction). The straight line fits the equation E , ,(Ni"P) = 0.93EiI2(H2P)- 0.13. A similar plot of E, for N i d in water vs E , , , for H2P in D M F is more (18) Caughey, W. S.; Fujimoto. W.; Johnson, B. Biochemisrry 1966, 5, 3830. (19) Worthington, P.; Hambright, P.;Williams, R. F. X.;Reid, J.; Burnham, C . ; Shamim, A.; Turay, J.; Bell, D. M.; Kirkland, R.; Little, R. G.; Datta-Gupta, N.: Eisner. U. J . Inorg. Biochem. 1980. 12, 281
Nahor et al.
,
-.6
-
1
-.9
F1: Z
v, W
5.
-1.2
-1.5-
n
a z
W
Wn
-
L -.9 -5
"j1.3
EI,P(HPP) Figure 1. Correlation of the reduction potentials measured for Ni"P in DMF (a) and in water (b) with the values for the corresponding free-base porphyrins in DMF reported in ref 19: (0)values obtained in this work; (0) values taken from ref 9.
limited in scope but does not give a good straight line (Figure lb). This poor correlation might arise from the irreversible electrochemical behavior found in water since E, is not a thermodynamic Overall, the electrochemical data property, in contrast to give no indication of the site of reduction. No conclusion can be reached, on the basis of these results, as to whether reduction gives NilP or Nil1P'-. Pulse Radiolytic Reduction. Pulse radiolysis experiments were carried out to characterize the initial products of one-electron reduction of the various Nil1-porphyrins by absorption spectroscopy. Ni"P was pulse irradiated under reducing conditions, in most cases in deoxygenated aqueous/2-propanol solutions. Under these experimental conditions the porphyrin is reduced by the solvated electron and by the 2-hydroxy-2-propyl radical derived from 2-PrOH: H20
--
OH, H, ea;,
-
H', H2, H 2 0 2
+ O H / H (CH3),COH + H 2 0 / H 2 eaq + Ni"P (NIP)(CH,),COH + Ni"P (Nip)- + (CH,),CO + H+
(CH3)&HOH
-
-
-
(3)
(4) (5) (6)
The overall yield in this system is G 6 reduction equivalents per 100 eV of absorbed energy. In other media the yields may be considerably lower. Transient differential absorption spectra were monitored in the range 460-800 nmZ0immediately after the reduction reactions were complete (50-100 p s ) . With all porphyrins studied, the differential spectra indicate bleaching of the Q-band of the Ni'IP, located around 550 nm, and formation of new peaks (Figure 2). (20) Since porphyrin concentrations had to be > 5 X IO-' M to ensure efficient reaction with the radical from 2-PrOH. we were unable to monitor lower wavelengths due to light absorption by the solution.
One-Electron Reduction of Nickel(I1) Porphyrins
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6661
.06I
1
I
0
I
TSPP w
0
Z a m LT
0
vr
m
a
4 550 650 750
I 550
650
A
750
950
nm
h
nm
,
.04
I
TM4PyP
.15 .02
w
0
0 Z
E 0
m
z a
vr m a
o
Id
i
-.02
-.04
4
E
.'
a
.05
a
2m
I
I
550
I
650
I
750
nm
h .04 I
I?
I
'
w 0 Z a m LT
0
vr
I
m Q
0 I
I
I
550
650
750
h
z
.02
Q
550
650
h
TAP 0
'
-"%50
nm
.04
w
I
750
nm
i 1
m
E
0
vr
m
a
o
The positions and intensities of the new peaks were strongly dependent on the porphyrin structure. These species decayed, generally in two successive processes on the millisecond time scale, to give phlorin anions or chlorins as stable products (see below). The spectra monitored with Ni11TM4PyP and NiIITSPMPyP (Figure 2) as well as with NiIITSPPyP and NiIIDPDMPyP (not shown) exhibit strong absorptions with broad peaks around 700 nm. Such peaks are characteristic of porphyrin *-radical anions, as observed under similar conditions for zinc porphyrins and for
other metalloporphyrins that are capable of ring reduction only.*I Therefore, we conclude that these four NiILporphyrins are reduced on the porphyrin ligand to give the corresponding *-radical anions, (21) See, for example: Neta, P.; Scherz, A.; Levanon, H. J. Am. Chem.
SOC.1979, 101, 3624. Neta, P. J. Phys. Chem. 1981, 85,3618. Baral, S.; Neta, P.; Hambright, P. Radior. Phys. Chem. 1984, 24, 245. Baral, S.; Hambright, P.; Neta, P. J. Phys. Chem. 1984, 88, 1595. Richoux, M.-C.; Neta, P.;Harriman, A.; Baral, S.; Hambright, P. J. Phys. Chem. 1986, 90,
2462.
The Journal of Physical Chemistry, Vol. 94, No. 17, I990
6662
Nahor et al.
TABLE I: Reduction Potentials and One-Electron-Reduction Products of the Nickel PorDhvrins electrochemistry E, for Ni"P" Et12 for E for porphyrin in H20(pH 7) Ni"P" in D M F H2Pa,b'in D M F ~~~~
DP TCP TTP TSPP TSPPyP TFSPP T2NPP T4NPP TCSPP TSPMPyP DPDMPyP TMZPyP TM3PyP TAP TM4PyP
c
- I .3?
-1.30
c
-1.1
c
-1.21 - I .20 -1.21 - I .09 -0.99 - I .02 -0.90
-1.16f -1.05
-0.87
C
-1.38 -1.22 -1.19 c
-0.96 -0.91
-1.08 -0.66 DP TCP TTP TSPP TSPPyP TFSPP TZNPP T4NPP TCSPP TSPMPyP DPDMPy P TM2PyP TM3PyP TM4PyP TAP DSPDNPP
-0.805 -0.77 -0.73 -0.5 15
pulse radiolysis medium (pH) 2-PrOH/aq ( I 2)
species obsd Ni'P
Id
-1.10 -1.09 -1.04 -0.95 -0.93 -0.87 -0.90 -0.80 -0.79 -0.74d -0.68 -0.67 -0.54
Abbreviations of the Porphyrins deuteroporphyrin tetrakis(4-isopropylpheny1)porphyrin (tetracumylporphyrin) tetrakis(3-methy1phenyl)porphyrin (tetratolylporphyrin) tetrakis(4-sulfonatopheny1)porphyrin tris(4-sulfonatophenyl)-4-pyridylporphyrin tetrakis( 4-fluoro-3-su1fonatophenyl)porphyrin tetrakis( 2-nitropheny1)porphyrin
tetrakis(4-nitropheny1)porphyrin tetrakis( 2,6-dichloro-3-sulfonatophenyl)porphyrin tris(4-sulfonatophenyl)(lv-methyl-4-pyridyl)porphyrin diphenylbis(N-methyl-4-pyridy1)porphyrin tetrakis(N-methyl-2-pyridy1)porphyrin tetrakis(N-methyl-3-pyridy1)porphyrin tetrakis(N-methyl-4-pyridy1)porphyrin tetrakis(N,N,N-trimethyl-4-anilino)porphyrin bis(4-sulfonatophenyI)bis(4-nitrophenyl)porphyrin
"Determined as described in the text, given in V vs SCE, accurate to f 2 0 mV. bFrom ref 19, except where noted. cCompound insufficiently soluble for this measurement. Determined in this work in DMF. 'Same result obtained in alkaline 2-propanol/acetone. /Reversible wave.
in agreement with a previous suggestion.*a However, in the earlier studysa the reduction of Ni'ITMPyP in DMF gave a product with an absorption band at 840 nm, which was assigned to the a-radical anion. It is unclear whether the peak of the a-radical anion shifts from 740 to 840 nm upon changing from water to DMF as solvent or whether the 840-nm peak in DMF is ascribable to the a-dianion, produced by rapid disproportionation of the radical anion (see below). All other Nil1-porphyrins studied produced transients that lacked this strong absorption around 700 nm. Instead, these primary reduction products exhibited weaker absorptions (Figure 2) in the 590-700-nm region, in some cases with a minor peak around 600 nm. In addition to the bleaching of the Q-band of NiI'P, new peaks around 500 nm were observed, possibly due to a blue shift of the Q-band. This spectral change is assigned to formation of Ni'P, based on the previous interpretation of such spectra, which was supported by EPR and other measurement^.^ It is clear from the above results that Ni"-porphyrins bearing alkyl or substituted phenyl groups are reduced at the metal center, whereas those bearing meso-4-pyridyl groups are reduced on the ligand. The effect of the 4-pyridyl group is the predominant factor, as the presence of even a single 4-pyridyl is sufficient to direct reduction toward the ligand. The strong effect of TM4PyP in directing the first reduction step toward the ligand has been demonstrated also with cobalt porphyrins, where the dicyano complex of Co"'TM4PyP was foundZZto be reduced to the aradical anion Co1"TM4PyP*-, the only known a-radical anion of a Co"'-porphyrin. It appears that this effect is unique to the 4-pyridyl group. Experiments with Nil'-porphyrins bearing 2-pyridyl or 3-pyridyl groups gave transient spectra (see Figure 2 for Ni"TM2PyP) that (22) Mosseri, S.; Neta, P.; Harriman, A.; Hambright, P. J . Inorg. Bio&em. 1990. 39. 93.
clearly lack the strong a-radical anion absorption peak at 700 nm, although they exhibit a peak at 620 nm that is more intense than those observed for the other Ni' porphyrins. Figure 2 shows that there is a crucial difference, both in intensity and in peak position, between this type of spectrum and that of a typical a-radical anion observed with the corresponding Zn-porphyrin under the same conditions. The finding that a 4-pyridyl group directs one-electron reduction toward the ligand implies that the a-radical anion is stabilized by resonance forms such as
CH3
CH3
CH3
The other pyridylporphyrins, TM3PyP and TM2PyP, do not yield the a-radical anions of the Nil1 state because such resonance stabilization is unlikely. This is due to the position of the nitrogen in TM3PyP and to steric hindrance in TM2PyP. In separate studies, we attempted to follow reactions of Ni'TSPP with several compounds that are known to react with other Nil-tetraazamacrocyclic comple~es,2~ such as CHJ, CH,CI, COz, and N,O. We were not able to detect any appreciable shortening of the lifetime of Ni'TSPP when these compounds were added (23) (a) Tait, A. M.; Hoffman, M. Z.; Hayon, E. Inorg. Chem. 1 9 7 6 , I 5 , 934. (b) Jubran, N.;Ginzburg, G.; Cohen, H.; Koresh, Y . ; Meyerstein, D. Inorg. Cfiem. 1985, 24, 251, (c) Bakac, A.; Espenson, J . H. J . Am. Chem. SOC.1986,108, 713.
J . Phys. Chem. 1990, 94, 6663-6666
in large excess, with the exception of N20. The effect of saturation with N,O was small and did not permit evaluation of the rate constant. y- Radiolysis Experiments. Stable products of the radiolytic reduction of Ni''-porphyrins were examined by monitoring the absorption spectra after y-irradiation as a function of the radiation dose. In none of these experiments could we detect Ni'P as a stable product, even with Ni"-porphyrins that produced Ni'P on short time scales (as monitored by pulse radiolysis). It seems that both Ni'P and Ni"P'-, referred to collectively as (Ni"P)- in eqs 7 and 8, are converted to stable products resulting from two-electron 2(NiIiP)-
+ 2H+
-
+ Ni"PH2 + Ni"PH- + OH-
Ni"P
6663
the chlorin resulted in destruction of the chromophore. y-Radiolysis of Ni'ITSPP was carried out in the presence of CH,CI, CO,, or N20, all of which are known to react with related Nil-tetraazamacrocyclic complexes.*' Only N z O inhibited the degradation of the Ni"P to any measurable extent, whereas the other compounds did not affect the rate of destruction of the porphyrin, in agreement with the above-mentioned pulse radiolysis results. These findings indicate that Ni'TSPP is a much poorer reductant than other Nil macrocyclic species and suggest that NiTSPP would be a less efficient reduction catalyst as compared with the other Ni complexes studied.
( 1 1)
Conclusion The results obtained in this study indicate clearly that certain Nil'-porphyrins are reduced at the central metal whereas others are reduced on the ligand, as formulated in reactions 1 and 2. In fact, most of the porphyrins studied gave the Ni' species rather than the Nil' x-radical anion. Whether Nil1-porphyrins are reduced to the Nil state or to the x-radical anion seems to depend on the structure of the porphyrin, specifically on the presence of a 4-pyridyl group, and not on its electrochemical reduction potential. Our findings are in agreement with recent results of Lexa et al.,9 both studies indicating that formation of Ni'P is the more common route for reduction of Ni'lP. These findings are in apparent contrast with previous s t ~ d i e s ,examining ~.~ long-lived products, which concluded that Nil-porphyrins are unstable species and that Ni"P is reduced on the ring and not at the metal. It should be pointed out, however, that the one-electron-reduced species observed here in protic solvents are unstable intermediates that were observed only under pulse radiolytic conditions and which reacted within I1 s to form other products. Their ultimate fate was confirmed by y-radiolysis studies, where only twoelectron-reduced porphyrinic products could be observed.
The phlorin produced in these reactions is reoxidized to Ni"P by exposing the solution to air, but the chlorins were not oxidizable under these conditions. We found no evidence for demetalation (as reportedsb for NiI'TPyP) under our conditions. Irradiation of the phlorin anion yielded a chlorin, and further reduction of
Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy and by the Howard University Faculty Research Support Program. We thank Drs. D. Lexa, J. Fajer, and B. G. Maiya for helpful discussions.
2(Ni1'P)-
+ H20
-
Ni"P
(7)
(8)
reduction of the porphyrin ligand. In acidic and neutral solutions the chlorin (NiIlPH,) was formed (characterized by a sharp peak at 600-620 nm), and in alkaline solutions the phlorin anion (NiI'PH-) was the main product (characterized by a broad peak near 800 nm). This is quite expected in protic media, which favors disproportionation products, and similar behavior has been reported previously.6*s These findings indicate that even when the initial site of reduction of Ni"P is at the metal center, the stable twoelectron reduction product is formed by reduction of the porphyrin ring. The mechanism may involve an intramolecular electron transfer from Ni' to its ligand, possibly promoted by disproportionation and protonation:
-
Ni'P s Ni"P'Ni'P
or 2NiiP
+ Ni"P'- + H+
-
N?'P
Ni"P
H+ + N i ' P - -Ni"P
(9)
+ Ni'lPH+ NiI'PH-
(IO)
Proton Chemical Shift Anisotropies Measured from Transient Muitispin Order L. Werbelow Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 (Received: January 24, 1990)
Due to the small chemical shift range and the large dipole moment of protonium, accurate determination of proton chemical shift anisotropies in isotropic fluids is difficult. However, since shift anisotropy and dipolar interactions are temporally correlated, one can probe the electronic shielding tensor with a sensitivity commonly associated with the magnetic dipole-dipole interaction. The unique signature of temporal correlation is transient multispin order. This signature is apparent when one-spin order is sequestered. Pertinent equations and potential methodologies are derived/described.
Introduction The nuclear magnetic spin relaxation characteristics of liquid water have been studied by numerous workers dating back to the inception of magnetic resonance more than 40 years ag0.I Scattered throughout the published scientific record are thousands of NMR relaxation studies that depict the role water plays as both a solute and a solvent. Relaxation studies of the dipolar relaxed IH isotope or the quadrupolar relaxed 2H and I7O isotopes provide ( I ) Bloembergen, N.; Purcell, E. M.;Pound, R. v. phys. Reo, 1948, 73, 679.
0022-3654/90/2094-6663$02.50/0
a multitude of isotopomers for investigation. The ubiquity of water and aqueous solutions is undeniable. Paradoxically, there exist many ill-resolved questions regarding proton spin relaxation in this simple A, spin grouping. The characterization and exploitation of the chemical shift anisotropy component of relaxation is one such example. From a practical viewpoint, any ambiguity associated with various relaxation parameters impacts directly upon the credibility of many current and proiected applications of NMR such as ultra-high-field NMR, muliisph (2D)'NMR, the study of heterogeneity (e.g., pore size distributions), and the development of numerous magnetic resonance imaging techniques. From a theoretical viewpoint, the A,
0 1990 American Chemical Society
