Inorg. Chem. 1993, 32, 2562-2569
2562
Symmetrical and Unsymmetrical Quadruply Aza-Bridged Closely Interspaced Cofacial Bis( 5,10,15,20-Tetraphenylporphyrins). 4. Structure and Conformational Effects on Electrochemistry and the Catalysis of Electrochemical Reduction of Dioxygen by Doubly, Triply, and Quadruply N,N-Dimethylene Sulfonamide Bridged Dimeric Bis( cobalt tetraphenylporphyrins) Seungwon Jeon, Orn Almarsson, Rafik Karaman, Andrei Blask6, and Thomas C. Bruice' Department of Chemistry, University of California at Santa Barbara, Santa Barbara, California 93 106 Received November 25, 1992
Investigations of the structural and electrochemical properties of free base, bis(zinc), and bis(coba1t) derivatives
of a number of quadruply, as well as doubly and triply, N,N-dimethylenesulfonamidebridged cofacial bis(5,10,15,20tetraphenylporphyrins) (Chart 11) were conducted. The solution structure of [3-pyridylI4,determined by 2D NMR spectroscopyand distance geometry refinement, differs little from the gas-phase structure calculated by molecular mechanics. The 'H NMR chemical shifts of the N-H pyrrolic protons (in CDCl3 at 25 "C) of the quadruply bridged dimers [ l-naphthylI4, [3-pyridyl]4, [isopropyl]4, and [methyl14 are linearly related to Taft's Es steric parameters for the R substituent (=naphthyl, phenyl, isopropyl, and methyl) of the N,N-dimethylene sulfonamide bridges [ (-CH2)2NS02R]. The results show that the center to center distancesof the bis(porphyrins) decreasewith increasing stericdemandsof R. For the bis(prphyrin) with thelargest R substituent ([ l-napthyl]4),potentialsfor electrochemical oxidations of both the free base and bis(zinc) derivative establish a strong u-u interaction between the cofacial porphyrin rings. In the catalysis of the electrochemical reduction of 02 the cobalt(I1)-ligated bis(porphyrins) may be divided into two groups, with theneutral dimers Co2-[methyl]d, Co~-[isopropyl]4,and COT[ I-naphthyll4exhibiting modest catalytic efficiencies in the 4e- reduction of dioxygen to water (between 44 and 51% HzO), while the tetracationic porphyrin dimers (C02-[N-methyl-3-pyridyl+]4, Co2-[N-isopropyl-3-pyridyl+]4, Coz-[N-benzhydryl3-pyridyl+]4,and tetraprotonated Co2-[3-pyridyl.H+I4J are more effective (between 66 and 71% 4e-reduction). For the positively charged N-substituted pyridine dimers, the bulkof the N-substituents [-CH3,-CH(CH3)z,-CH(C6H&, -HI does not affect the catalytic properties nor Ct-Ct distances. With employment of the NJV-dimethylenesulfonamide bridging structures [ (-CH2)2NS02R], the quadruply bridged tetracationic bis(coba1t) porphyrin dimer Co2-[3pyridyl.H+I4is more effective in catalyzing the 4e- reduction of 02 than are the two isomeric triply bridged and tricationic dimers Co2-[3-pyridyl.H+] 3(A) and Co2-[ 3-pyridy1.H+l3(B) while the doubly bridged, dicationic Co2[3-pyridyl.H+I2 is the least active. Structural flexibility ([3-pyridyl]~> [3-pyridylI3(B) > [3-pyridyl]3(A) > [3-pyridyl]4) and the number of positive charges on the dimers account for the order of catalytic efficiency.
Introduction
chart I
A number of symmetrical and unsymmetrical quadruply bridged closely interspaced cofacial bis(5,10,15,20-tetraphenylporphyrins) have been synthesized in our laboratory and examined as catalysts for the 4e- reduction of dioxygen to water.] Geometrical parameters for the dimeric porphyrins (Chart I), such as theinterplanardistance (P-P), thecenter tocenter distance (Ct-Ct), and the lateral shift (LS), as well as the shape of the cavity, appear to be determined by the conformation of the three atom N,N-dimethylene amine bridges.' r,2 In the present study, the Ct-Ct distance in the representative [3-pyridylI4 (see Chart 11) has been made available by determination of its solution structure from two-dimensional IH NMR (ROESY) and subsequent distance geometry refinement. The ( I ) (a) Abbreviations used: Fora given bis(5,10,15,20-tetraphenylporphyrin) linked by x N,N-dimethylenesulfonamide linkages through the m-position of each meso-phenyl ring and carrying a group R on the sulfur atom,
the suggested shorthand is [R] ,. Thus, the quadruply methanesulfonamide-linked dimer is referred to as [methyl],. Where more than one isomer exists. suffixes in parentheses, i.e. (A) and (B), are used for differentiation. (b) Bookser, B. C.; Bruice, T.C. J . Am. Chem. SOC. 1991, 113.4208. (c) Karaman, R.; Bruice, T.C. J . , O r g . Chem. 1991, 56.3470. (d) Karaman, R.; Blask6, A.; Almarsson, 0.;Arasasingham. R. D.;Bruice, T. C. J . Am. Chem. SOC.1992,114,4889. (e) Karaman, R.: Jeon,S.;Almarsson,O.; Bruice,T. C. J . Am. Chem.Soc. 1992,114, 4899. ( 0 Karaman, R.; Almarsson, 0.:Bruice. T. C. J O r g . Chem. 1992, 57. 1 5 5 5 . (2) (a) Chang, C. K.; Liu, H. Y.;Abdalmuhdi. 1. J . Am. Chem. SOC.1984, 106,2725. (b) Collman. J. P.; Hendricks. N. H.; Leider.C. R.; Ngameni. E.; L'Her, .M. Inorg. Chem. 1988,27,387. (c) Sawaguchi. T.; Matsue. T.; Itaya, K.; Uchida, I. Elecrrochim. Aria 1991, 36. 703. (d) Chang. C. K. J . Hererocyl. Chem. 1988, 12. 1287.
0020-1669/93/1332-2562$04.00/0
Porphyrin through the centers, perpendicular to the plane of the macrocycles LS
solution structure is compared with the CHARM, calculated structure. Also described are the effect of the number of the bridges and the size and charge of their substituents on the electrochemical properties of both free base, bis(zinc), and bis(coba1t) derivativesas well as the catalysis of electrochemical reduction of oxygen by the latter.
Results and Discussion The structures of thequadruply bridgedcofacialbis(5,10,15,20tetraphenylporphyrins) of this study are shown in Chart 11. For the N,N-dimethylenesulfonamidelinker unit employed [(-CH& NSOZR],the variation of Ct-Ct distance is dependent upon the identity of the R-group. The means by which substituent size determines interplanar distances" is shown on inspection of the two structures of Chart 111. The four legs which secure the two porphyrin rings can be considered to have a secondary nitrogen as a "knee". The knee is maximally extended (maximal Ct-Ct distance; a in Chart 111) when the sulfonamide substituent is small, but as the bulk of the substituent becomes greater, van der 0 1993 American Chemical Society
Bis(coba1t tetraphenylporphyrins)
Inorganic Chemistry, Vol. 32, No. 11, 1993 2563
chart I1
Table I. Pyrrolic N-IH Chemical Shifts (ppm vs TMS, at 500 MHz in CDC13 at 25 "C) and CHARM, calculated Ct-CtO Distances (A) for Quadruply N,N-Dimethylene Sulfonamide Bridged Free-Base Bis(5,10, 15,20-tetraphenylporphyrins)
fi
compd [ 1-naphthyl14 t 3-pyrid yll 4
[isopropyl14 [methyl14
a(N-H)' -4.68 -4.34 -4.28 -4.03
Ct-CtQ 4.88 5.03 5.75 5.80
Esb
-1.64 -0.9oc -0.47
0.00
See Chart I for definition of Ct-Ct and ref 1f for molecular dynamics protocol. See footnote 3. Parameter for phenyl rings.
*
-3.951
4
CHa
[N-methyl 3-pyridyIl4
R=
[3-pyridyIl4 R = [methyl],
R=
-CH3
CH(CHdz
[N-isopropyl 3-pyridylI4 R =
-+J
[isopropyl]4
R = -CH(CH&
-1.8
-1.4
-1.0
-0.6
-0.2
0.2
E, Figure 1. 'HNMR pyrrolic N-H chemical shifts (at 500 MHz in CDC13 at 25 "C) vs E s ~for . ~symmetrical quadruply N,N-dimethylene sulfonamide bridged free-base 5,10,15,20-tetraphenylporphyrindimers [ 1-naphthyl14, [3-pyridyl]4, [isopropyl]4, and [methylI4. The Es parameter for phenyl was used for the pyridine ring.
Waals repulsions exclude the substituent from the interplanar space and the knee begins to bend (providing decrease in Ct-Ct distance by a screwing down motion; b in Chart 111). This conformational change becomes quite clear when viewed via computer graphics. Effect of the Steric Bulk of the Sulfonamide R Substituent of the Quadruply Bridged Cofacial Bis(5,10,15,20-tetraphenylporphyfins) on the 'H NMR Chemical Shifts of the Pyrrolic N-H Protons. In a previous investigation, using quadruply bridged cofacialporphyrins with varying "leg structures", we foundIfthat there exists a positive relationship between the size of the substituent on the secondary nitrogen "knee" (-CN, -CONH2, -S02R) and the ' H NMR chemical shift of the pyrrolic N-H protons. The upfield chemical shift of the pyrrolic N-H resonances for the dimers of this study (Chart 11) are provided in Table I. With increased steric demands by substituent R the aforementioned 'H NMR resonances shift upfield from corresponding signals for monomeric porphyrins due to the increased proximity of one porphyrin ring to the anisotropic ring current of the other porphyrin. This, again, speaks for decreased Ct-Ct distances in the free-base dimers with increased substituent size. Indeed, there is a good linear relationship between Taft's steric constants3 ( E s ) for the substituents R and the pyrrole proton NMR shifts (Figure l).4 Calculated CHARM,,, interplanar
distances have been shown to have a reasonable linear correlation with 'H NMR shiftsIeJfsuch that the& values of R-substituents on (-CH2)2NS02R should, therefore, be linearly related to CtCt. The use of CHARM, to calculate Ct-Ct distances appears justified by the observation that CHARM,,, calculated distances agree closely with those determined by 2D IH NMR ROESY measurements, as shown in what follows. Determination of a Solution Structure for [3-pyridyl]4by 2D NMR Spectroscopy and Distance Geometry Refinement. An expanded contour plot of the ROESY spectrum of [3-pyridylI4 in CDCl3 solution is shown in Figure 2. With the usual assumption5that all proton-proton interactions can be described by the same correlation time, the ROESY peaks can be related to the sixth power of the distance between the protons involved in the interactions (see eq 4 in the Experimental Section). Using the fixed H5'-H6/ interatomic distance (2.46 A) in [3-pyridylI4 as standard, the average distances for six types of interactions were calculated (Chart IV). A dihedral angle of approximately 20° between the phenyl rings and the porphyrin plane is required (3) Taft, R. W. In Steric Eflects in Organic Chemistry; Newman, M.S., Ed.; John Wiley: New York, 1976;p598. &for I-naphthylwasobtained from: Packer, J.; Vaughan, J.; Wong, E. J . Org. Chem. 1958,23, 1373. The -0.40 value for acid-catalyzed ester hydrolysis was made relative to methyl by subtracting 1.24 to give -1.64. (4) A similar relationship was observed between thepyrrolic N-H resonances and ES of four unsymmetrical quadruply bridged dimers, where one of the sulfonamide linkers is replaced by a carbonate linkage.Id' This unsymmetrical dimer series, isolated as byproducts in the synthesis of the symmetrical dimers, displays N-H resonances slightly downfield shifted compared with the symmetrical counterparts. The plot of N-H resonances vs Es gives a modest linear correlation with a slope slightly smaller than that for the symmetrical series. ( 5 ) (a) Kessler, H.; Bats, J. W.; Griesinger, C.; Koll, S.; Will, M.; Wagner, K. J . Am. Chem. SOC.1988,110, 1033. (b) Kessler, H.; Griesinger, C.; Kersebaum, R.; Wagner, E.: Ernst, R. J . Am. Chem. SOC.1987, 109, 607.
2564 Inorganic Chemistry, Vol. 32, No. 11, 1993
Jeon et al.
I =
h
E
40
-
, in 3.0 a”
6.0
5.5
6.5
Center-center distance
7.0
(A)
Figure 3. Dependence of the total CHARM,,,energy on the center to center (Ct-Ct)distances between the porphyrin rings in distancegeometry refined ROESY structures for [3-pyridyl]4. Table 11. ROESY (Exptl) and Most Stable Structure (Calcd) Interproton Distances (in A) for [3-pyridyl]4 -.
’,
,
9
I
I
,
8
1
1
a
I
t
7
#
