numerous transition - metal 
 porphyrins containing iron , cobalt , nickel , 
 or copper central metal ions have been investigated over the last 
 50 years as to their electrochemical properties under a variety of 
 solution conditions . studies of  simple  
 metalloporphyrins containing substituted 5,10,15,20-tetraphenylporphyrin 
 ( tpp ) or 2,3,7,8,12,13,17,18-octaethylporphyrin ( oep ) macrocycles 
 led to a fairly good understanding of the expected redox potentials , 
 separations between redox processes and sites of electron transfer 
 as a function of the porphyrin structure , metal oxidation state , number 
 and type of bound axial ligands , and specific solvent / supporting electrolyte 
 system . simple electrochemical criteria 
 were formulated and utilized many 
 times to suggest the site of electron transfer during studies in the 
 1960s , 1970s , and 1980s . for example , 
 in the case of oep complexes , the absolute potential difference between 
 the first porphyrin ring - centered oxidation yielding a -cation 
 radical and the first porphyrin ring - centered reduction yielding a 
 -anion radical [ the highest occupied molecular orbital ( homo)lowest 
 unoccupied molecular orbital ( lumo ) gap ] was said to be 2.25  
 0.15 v independent of the central metal ion oxidation state , with the only exception being derivatives of molybdenum and manganese . a similar homo the expected potential difference between the 
 first and second ring oxidations of octaethylporphyrins or tetraphenylporphyrins 
 was generally 0.29  0.05 v , while that between the first and 
 second ring reductions was often 0.42  0.05 v. these three potential differences were then used as key 
 diagnostic criteria for assigning the site of electron transfer in 
 early studies of oep- and tpp - type derivatives , and the same criteria continue to be used today in many 
 publications reporting the electrochemistry of metalloporphyrins in 
 nonaqueous media . however , in the 1990s , a large number of nonplanar 
 porphyrins containing 
 copper , iron , cobalt , or nickel central metal ions were synthesized 
 by smith and co - workers , and the electrochemical properties of these compounds were then 
 investigated in nonaqueous media . these electrochemical studies showed that 
 previously utilized electrochemical diagnostic criteria for assigning 
 the sites of electron transfer might no longer be applicable . for 
 example , the potential separation between the two ring - centered oxidations 
 of many nonplanar nickel(ii ) porphyrins was often equal to zero in 
 dichloromethane ( ch2cl2 ) containing 0.1 m tetra - n - butylammonium perchlorate as the supporting electrolyte ; 
 i.e. , the two one - electron oxidations were overlapped to give an overall 
 two - electron - transfer process in a single step . the number of redox processes a transition - metal porphyrin will 
 undergo , as well as the site of oxidation and reduction in these compounds , 
 will often vary with the solution conditions used to carry out the 
 electrochemical measurements . for example , in solvents such as ch2cl2 , benzonitrile ( phcn ) , or tetrahydrofuran , most 
 cobalt(ii ) porphyrins can be oxidized in three successive one - electron - transfer 
 steps , the first of which unambiguously involves a co / co redox process . however , ( tpp)co 
 and related cobalt(ii ) porphyrins can also undergo a ring - centered 
 redox process in the first electron abstraction . in coordinating solvents 
 or solvents containing trace water , the first electron is abstracted 
 from the co center , but an initial oxidation 
 at the porphyrin -ring system was shown to occur for ( tpp)co 
 in a very dry dichloromethane solvent , and this was followed by a co / co process , 
 either in the second or third of the three observed oxidation processes . such changes in the site of electron transfer for oxidation of 
 ( tpp)ni and other nickel(ii ) porphyrins may also be accomplished 
 by changes in the solvent . for example , the first oxidation is metal - centered 
 in pyridine and macrocycle - centered in nonbonding 
 or weakly coordinating solvents such as ch2cl2 or phcn . the potentials for the first 
 two oxidations of ( tpp)ni are very close to each other 
 in some solvent / supporting electrolyte systems , and the site of the first electron transfer can be easily 
 shifted from the conjugated -ring system to the metal or from 
 the metal to the  system by changes in the temperature , phenyl ring substituents , and/or planarity of the macrocycle . a third oxidation to give the ni dication was 
 expected 
 to occur after formation of the ni dication radical in 
 noncoordinating solvents , but no more than two oxidation processes 
 were ever reported for any nickel(ii ) porphyrin until 1993 , when it 
 was shown that three successive one - electron oxidations were exhibited 
 by nickel(ii ) derivatives containing a nonplanar macrocycle such as 
 2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin ( omtpp ) 
 or 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin ( oetpp ) , the third of which involves the ni / ni process at relatively positive potentials . in a 
 detailed electrochemical study of different nickel(ii ) porphyrins , the potential separation between the first two , 
 ring - centered , oxidations was shown to vary between 0.0 and 400 mv 
 depending on a variety of factors , including the nonplanarity of the 
 macrocycle , the type of -cation radical , a1u versus 
 a2u , and the ability of anions from the supporting electrolyte 
 to complex with the oxidized species . the principal goal of 
 the present work is to probe the limits of 
 nonplanar porphyrins in facilitating the detection of m / m processes and to investigate the possibility of 
 observing a copper(ii)/copper(iii ) porphyrin dication redox process , 
 which is expected to occur at more positive potentials than even the 
 ni / ni couple . the cu / cu process has never been experimentally observed but should 
 occur in porphyrins , as it does in the structurely related corroles , 
 which exist in a stable cu oxidation state . using the nonplanar copper(ii ) porphyrins , ( dpp)cu where dpp = 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin 
 and ( oetpp)cu , we show that nonplanar 
 porphyrin macrocycles can be oxidized in three one - electron - transfer 
 steps , the last of which does indeed involve formation of a copper(iii ) 
 porphyrin dication at extremely positive potentials . sections i and ii describe studies of the electrochemistry in noncoordinating 
 or coordinating solvents , respectively , of a series of nickel tetraalkyporphyrins 
 ( see chart 1 ) with varying degrees of nonplanarity . the studies reported in section i were required 
 because formation of the nickel(iii ) porphyrin dication has only been 
 reported for a handful of very nonplanar porphyrins , and it was important 
 to confirm that this process can be observed for other easily oxidizable 
 and nonplanar nickel(ii ) porphyrins . in section ii , the effect of a coordinating solvent on the oxidation processes 
 of nonplanar nickel porphyrins is examined for the first time . it 
 is demonstrated that nonplanar nickel porphyrins in coordinating solvents 
 show a switch to oxidation at the metal center for the first oxidation , 
 as noted previously for nominally planar porphyrins . with the presence 
 of the ni dication confirmed for a range of other nickel 
 porphyrins and the effect of coordinating solvents on the ni / ni oxidation clarified , section iii describes investigations of the cu / cu redox processes in ( dpp)cu and ( oetpp)cu . benzonitrile ( phcn ) , obtained from fluka 
 chemika or aldrich co. , was distilled over phosphorus pentoxide ( p2o5 ) under vacuum prior to use . absolute dichloromethane 
 ( ch2cl2 ) and pyridine ( py ) were received from 
 aldrich co. and used as received . high - purity dinitrogen from trigas 
 was used to deoxygenate the solution before each electrochemical experiment . tetra - n - butylammonium perchlorate ( tbap ) was purchased 
 from fluka chemika co. and used without further purification . ( dpp)ni , ( dpp)cu , ( oetpp)ni , ( oetpp)cu , and nickel(ii ) tetraalkylporphyrins were synthesized as described in the literature . cyclic voltammetry measurements were 
 performed at 298 k on an eg&g model 173 potentiostat coupled with 
 an eg&g model 175 universal programmer in a deaerated phcn solution 
 containing 0.1 m tbap as the supporting electrolyte . a three - electrode 
 system composed of a glassy carbon working electrode , a platinum wire 
 counter electrode , and a saturated calomel reference electrode ( sce ) 
 was utilized . the reference electrode was separated from the bulk 
 of the solution by a fritted - glass bridge filled with a solvent / supporting 
 electrolyte mixture . cyclic voltammograms of the four 
 investigated nickel(ii ) tetraalkylporphyrins(tt - bup)ni ( 1 ) , ( ti - prp)ni ( 2 ) , ( tetprp)ni ( 3 ) , and ( ti - bup)ni ( 4)are shown in figure 1 . in 
 the solid state , these porphyrins adopt progressively more nonplanar 
 structures because the meso substituents become bulkier 
 on going from primary alkyl groups ( 4 ) to secondary alkyl 
 groups ( 3 and 2 ) and finally to tertiary 
 alkyl groups ( 1 ) . compounds 13 undergo two one - electron 
 reductions and three one - electron oxidations within the potential 
 range of the solvent ( + 2.0 to 2.0 v vs sce ) . compound 4 undergoes three oxidations and one reduction , with the second 
 reduction not being observed in phcn . the first reduction of all four 
 compounds occurs at similar e1/2 values 
 of 1.44 to 1.46 v vs sce , as seen in figure 1 . the first oxidations of the porphyrins with less 
 bulky primary or secondary alkane substituents ( 24 ) are also similar to each other ( e1/2 = 0.890.92 v ) . this is not the case for the second 
 oxidations of these three derivatives , which follow the order of 4 ( 1.06 v ) < 3 ( 1.10 v ) < 2 ( 1.18 v ) . compound 1 is the most distorted of the four 
 investigated tetraalkylporphyrins , and the first two oxidations are 
 shifted negatively compared to the reactions of compounds 24 ( by 160190 mv for the first oxidation 
 and 110230 mv for the second oxidation ) . the third ( metal - centered ) 
 oxidations of compounds 14 also 
 vary significantly as a function of the peripheral substituents , with e1/2 values ranging from 1.82 v for 4 to 1.58 v for 1 ( see table 1 ) . data obtained in 
 ch2cl2 containing 0.1 m tbap . predicted e1/2 value ( see the text and figure 5 ) . the overall oxidative behavior of 
 the strongly ruffled porphyrin 1 closely resembles that 
 which was previously described for 
 the strongly saddled porphyrins , ( omtpp)ni and ( tc6pp)ni ( where omtpp = dianion 
 of 2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin 
 and tc6pp = 2,3,7,8,12,13,17,18-tetracyclohexenyl-5,10,15,20-tetraphenylporphyrin ) 
 in the same solvent ( phcn ) . all three porphyrins are oxidized at experimentally 
 identical potentials of 0.730.74 v , and all three porphyrins 
 also have very similar e1/2 values for 
 the second and third redox processes . this is true despite differences 
 in both the alkyl and aryl meso substituents on the macrocycles of 1 , ( omtpp)ni and ( tc6tpp)ni , and in the type of 
 nonplanar structure ( ruffled for 1(35 ) versus saddled for the other two compounds ) . previous studies of porphyrin substituent effects have shown 
 that 
 changes in e1/2 are influenced by the 
 electronic effect of the substituents on the meso- and -pyrrole positions of the macrocycle as well as by conformational distortion of the macrocycle 
 induced by crowding at the porphyrin periphery through peri interactions . for instance , ( oetpp)ni is substantially easier 
 to oxidize than ( tpp)ni ( see table 1 ) because 
 of its saddle conformation and yields a complex postulated as a high - spin 
 ni -cation radical . the more facile macrocycle oxidation of 1 compared 
 to 2 , 3 , or 4 is consistent 
 with the strongly ruffled macrocycle of 1 that results 
 from steric clashes between the substituents ( t - bu ) 
 and the adjacent pyrrole rings . the potential 
 difference ( e1/2 ) between the first 
 two oxidations of the tetraaryl - substituted porphyrins 
 ranges from 140 to 290 mv and follows the order : 4 ( 140 
 mv ) < 3 ( 180 mv ) < 1 ( 220 mv ) < 2 ( 290 mv ) . this suggests that other factors contribute 
 to the e1/2 differences between 
 the first two oxidations , such as the type of dication formed ( a1u vs a2u ) or the anion binding affinity of the 
 dication . interestingly , however , the 
 reversible metal - centered ni / ni reactions 
 of 14 can be seen to shift to more 
 positive potentials with increased nonplanar deformation ( 1.82 v for 4 vs 1.58 v for 1 ) . given the similar electron - donating / withdrawing 
 effects of the substituents ( as shown by the identical reduction potentials 
 for 14 ) , this may suggest an effect 
 of nonplanarity on the ni / ni reactions . as discussed above , the ni / ni process 
 is observed only after the two one - electron ring - centered abstractions 
 in phcn . the first one - electron oxidation leads to a ni -cation radical whose uv  visible spectrum exhibits 
 a decreased intensity soret band and a broad band in the visible region 
 of the spectrum . the same types of spectral changes are seen for all 
 four tetraalkylporphyrins in phcn , an example of which is shown in 
 figure 2a for 3 during controlled - potential 
 oxidation at 1.0 v in a thin - layer cell . visible spectral 
 changes of 3 upon the ( a ) 
 first oxidation in phcn , ( b ) first oxidation on py , ( c ) first reduction 
 in phcn , and ( d ) first reduction in py containing 0.1 m tbap . as earlier demonstrated for other 
 nickel(ii ) porphyrins , the site for the 
 first oxidation of 3 is quite different when the reaction 
 is carried out in py because 
 this solvent coordinates to the singly oxidized form of the compound . 
 under these conditions , the soret band of neutral 3 at 
 424 nm decreases in intensity , while a new well - defined soret band 
 grows in at 443 nm for the singly oxidized species ( see figure 2b ) . at the same time , the q band of ni at 545 nm disappears , and two well - defined new q bands grow in at 
 563 and 600 nm . there is no broad band between 600 and 700 nm , indicating 
 the lack of a -cation radical . this type of spectral change 
 suggests that oxidation in py has occurred at the central metal ion 
 rather than at the porphyrin macrocycle . singly oxidized 1 , 2 , and 4 exhibit spectral changes similar 
 to those of 3 in py . visible 
 bands for the neutral and singly oxidized tetraalkylporphyrins in 
 these two solvents is given in table 2 . the 
 shift in the site of electron transfer upon a change of the solvent 
 from phcn to py has been well documented in the literature and is due to coordination of py to the singly 
 oxidized form of the porphyrin . examples of the uv  visible spectral changes 
 for the reduction 
 of 3 in the two solvents are provided in figure 2c , d . visible spectral changes are seen upon reduction of 
 all four tetraalkylporphyrins whether the solvent is phcn or py . the electrochemistry 
 of ( tpp)cu , ( dpp)cu , and ( oetpp)cu was also investigated , and cyclic 
 voltammograms of these three compounds in phcn containing 0.1 m tbap 
 are shown in figure 3 . two reductions are observed 
 for each porphyrin , as expected , and two oxidations are also seen 
 for ( tpp)cu . surprisingly , three oxidation processes are seen for 
 ( dpp)cu and ( oetpp)cu , the latter of which has never before been reported . cyclic 
 voltammograms of ( tpp)cu , ( dpp)cu , ( oetpp)cu , and solvent 
 background in phcn containing 0.1 m tbap . all four redox processes of ( tpp)cu 
 are assigned as macrocycle - centered 
 electron transfers to give a porphyrin -anion radical and dianion 
 upon reduction and a porphyrin -cation radical and dication 
 upon oxidation . the first two reductions 
 and first two oxidations of ( dpp)cu and ( oetpp)cu are also centered at the 
 conjugated -ring system of the porphyrin , 
 with half - wave potentials for oxidation being shifted negatively by 
 about 500 mv compared to ( tpp)cu due to the nonplanarity of these 
 two macrocycles ( see the exact e1/2 values 
 in table 1 ) . the third oxidation of ( dpp)cu 
 and ( oetpp)cu might at first be 
 rationalized in terms of a solvent impurity or perhaps by formation 
 of an isoporphyrin . clean  
 until beyond 2.00 v vs sce ( see figure 3 ) , 
 and there is no evidence for coupled chemical reactions and formation 
 of an isoporphyrin , as indicated by variable scan rate measurements , 
 low - temperature measurements , and multiple measurements on the same 
 compounds taken with different batches of solvent . thus , a more likely 
 interpretation would be a metal - centered oxidation , as observed for 
 the nickel porphyrins described in detail above . the conversion 
 of cu to cu in the third 
 oxidation of ( dpp)cu and ( oetpp)cu is also strongly suggested by a 
 comparison of the measured e1/2 values 
 for this process with redox potentials for the m reaction 
 of other transition - metal porphyrins that have the same macrocycles , 
 namely , ( dpp)m and ( oetpp)m , where m = fe , 
 co , and ni . one might expect to see a linear relationship between 
 the third ionization potential of the central metal ion and e1/2 for the m / m processes 
 of the ( dpp)m and ( oetpp)m complexes , and this is exactly what is 
 observed . examples of cyclic voltammograms are shown in figure 4 for the ( dpp)m derivatives containing 
 fe , co , 
 ni , and cu , while plots of the measured e1/2 values for the m / m reaction of the four 
 porphyrins versus the third ionization potential of the central metal 
 are shown in figure 5a for ( dpp)m and in figure 5b 
 for ( oetpp)m . linear relationships are observed for both 
 series of compounds using the third ionization potential of the central metal ion ( in ev ) and newly measured e1/2 values of the earlier characterized fe , 
 co , ni , and cu derivatives of ( dpp)m and ( oetpp)m in phcn . a third 
 oxidation is not observed for ( tpp)cu under the same solution conditions , 
 but extrapolation of the linear relationship in figure 5c for ( tpp)m , where m = au , fe , co , and ni , to 
 the third ionization potential of cu gives a predicted 
 half - wave potential of 2.47 v for the cu / cu process of ( tpp)cu in phcn . this third oxidation can not be observed 
 experimentally because of the positive potential limit of the solvent . cyclic 
 voltammograms of ( dpp)m in phcn containing 0.1 m tbap where 
 m = fe , co , ni , and cu . correlation between gas - phase ionization energies for 
 m / m and m / m redox 
 processes 
 of ( a ) ( dpp)m , ( b ) ( oetpp)m , and ( c ) ( tpp)m in phcn containing 0.1 
 m tbap ( see table 1 for potential ) . the cu / cu process of ( tpp)cu 
 is predicted to occur at 2.47 v based on the correlation in part c. the data in figures 3 and 5 suggest that a cu / cu process should 
 be observed for other copper porphyrins under solution conditions 
 where more positive potentials might be accessible . this possibility 
 will be investigated in future studies with different solvent / supporting 
 electrolyte combinations .
electrochemical studies of the oxidation 
 of dodecasubstituted and 
 highly nonplanar nickel porphyrins in a noncoordinating solvent have 
 previously revealed the first nickel(iii ) porphyrin dication . 
 herein , 
 we investigate if these nonplanar porphyrins can also be used to detect 
 the so far unobserved copper(iii ) porphyrin dication . 
 electrochemical 
 studies of the oxidation of ( dpp)cu and ( oetpp)cu show three processes , 
 the first two of which are macrocycle - centered to give the porphyrin 
 dication followed by a cuii / cuiii process at 
 more positive potential . 
 support for the assignment of the cuii / cuiii process comes from the linear relationships 
 observed between e1/2 and the third ionization 
 potential of the central metal ions for iron , cobalt , nickel , and 
 copper complexes of ( dpp)m and ( oetpp)m . 
 in addition , the oxidation 
 behavior of additional nonplanar nickel porphyrins is investigated 
 in a noncoordinating solvent , with nickel meso - tetraalkylporphyrins 
 also being found to form nickel(iii ) porphyrin dications . 
 finally , 
 examination of the nickel meso - tetraalkylporphyrins 
 in a coordinating solvent ( pyridine ) reveals that the first oxidation 
 becomes metal - centered under these conditions , as was previously noted 
 for a range of nominally planar porphyrins .