the tetraazamacrocyclic 
 architecture of the tropocoronand ligand provides a flexible scaffold 
 that can accommodate varied coordination environments and stabilize 
 unusual or otherwise unachievable geometries at transition - metal centers 
 ( chart 1 ) . in the absence of entatic tuning , cobalt(ii )
metal centers 
 display a proclivity toward high - spin octahedral or tetrahedral configurations , 
 and cobalt(iii ) centers prefer low - spin octahedral geometries .
square - planar coordination environments are also 
 readily observed for four - coordinate cobalt(iii ) complexes , but small 
 molecules with cobalt(iii ) centers in tetrahedral environments are 
 significantly scarce .
the cobalt(iii ) center in [ cow12o40 ] exists in a tetrahedral environment , as does cobalt(iii ) in [ co(nor)4 ] , where nor is the norbornyl anion . 
 a small number of mononuclear cobalt(iii ) imides
have been prepared 
 and structurally characterized , and in these species , the multiple - bond 
 character of the metal 
the reason for the rarity of cobalt(iii ) in tetrahedral environments 
 becomes clear upon comparison of orbital splitting diagrams for a 
 d metal center in square - planar versus tetrahedral geometries . 

the orbital filling diagram for a square - planar d center 
 is shown in figure 1 for an s = 1 electronic configuration .
the allocation 
 of electrons corresponds to minimization of electron repulsion interactions 
 and is the observed electronic distribution in [ co(tc-4,4 ) ] complexes .
in contrast , the orbital 
 distribution in a d tetrahedral orbital splitting diagram 
 results in the partial occupancy of two destabilized t2 molecular orbitals , with no contribution to minimization of electron 
 repulsion interactions .
that is , the transition from square - planar 
 to tetrahedral geometry results in partial filling of orbitals that 
 are destabilized in the tetrahedral configuration relative to the 
 square - planar one , without minimizing electron repulsion .
the square - planar 
 configuration also minimizes the electron  ligand repulsion 
 from the dx  y orbital .
orbital correlation diagram for the transition 
 from square - planar to tetrahedral geometry , shown for the electron 
 occupancy corresponding to a d metal center . modified 
 from albright , burdett , and whangbo . having the ability to control 
 the environment at the metal center simply by changing the tropocoronand 
 linker chain length places us in a unique situation to examine unusual 
 coordination environments .
the previously established series of four - coordinate 
 cobalt(iii ) tropocoronand complexes comprises [ co(tc-3,3)](x ) ( x = 
 bph4 , bar4 ) and [ co(tc-4,4)](bph4 ) .
these species exist in square - planar 
 or distorted square - planar geometries , with twist angles , defined 
 as the angle between planes formed by the metal and the two sets of 
 aminotroponeiminate nitrogen atoms , of 8 and 41 for the 
 four - coordinate complexes . in our pursuit 
 to examine the size dependence of cobalt(ii ) and cobalt(iii ) tropocoronands 
 on their ability to tune reactivity with nitric oxide , we prepared and structurally characterized two pseudotetrahedral 
 cobalt(iii ) complexes , [ co(tc-5,5)](bf4 ) and [ co(tc-6,6)](bph4 ) .
handling of air- and moisture - sensitive materials was conducted 
 in an mbraun glovebox under a nitrogen atmosphere .
methylene chloride and 
 tetrahydrofuran ( thf ) solvents were purified by passage through activated 
 alumina and stored over 4  molecular sieves under a nitrogen 
 atmosphere prior to use .
deuterated nmr solvents were obtained from 
 cambridge isotope laboratories , stored under an inert nitrogen atmosphere , 
 and used without further purification .
the syntheses of [ co(tc-5,5 ) ] , [ co(tc-6,6 ] , [ zn(tc-5,5 ) ] , and [ zn(tc-6,6 ) ] are described elsewhere .
fc(bph4 ) was prepared according 
 to previously published procedures . to a solution of [ co(tc-5,5 ) ] 

( 150 mg , 0.35 mmol ) in methylene chloride was added fc(bf4 ) ( 94.4 mg , 0.346 mmol ) .
the solution was evaporated to dryness 
 and the resultant solid was washed with diethyl ether ( et2o ) to remove ferrocene .
recrystallization 
 from dichloromethane ( dcm)/et2o at 30 c yielded x - ray - quality 
 crystals ( 174 mg , 97% yield ) .
uv  vis  nir [ cdcl3 ;  , nm (  , m cm ) ] : 349 ( 13650 ) , 412 ( 15670 ) , 757 ( 11420 ) , 1166 ( 9183 ) .
calcd for c24h30bcof4n40.32ch2cl2 : c , 53.35 ; h , 5.64 ; n , 10.23 . found : c , 53.33 ; h , 5.89 ; n , 10.21 . 

evidence for ch2cl2 appeared in the nmr spectrum 
 taken in cd2cl2 . to a solution of [ co(tc-6,6 ) ]
( 200 mg , 0.4 mmol ) 
 in methylene chloride was added fc(bph4 ) ( 218 mg , 0.434 mmol ) in the dark . 

the solution was evaporated 
 to dryness , and the resultant solid was washed with et2o to remove ferrocene .
recrystallization 
 from dcm / et2o at 30 c yielded x - ray - quality 
 crystals ( 330 mg , 97% yield ) .
note : we found methylene 
 chloride solutions of fc(bph4 ) to be unstable and attribute 
 variations in product yield to this property .
nir 
 [ cdcl3 ;  , nm (  , m cm ) ] : 271 ( 60900 ) , 349 ( 21470 ) , 424 ( 34560 ) , 702 ( 2362 ) , 
 981 ( 561 ) , 1240 ( 475 ) .
calcd 
 for c50h54bcon4 : c , 76.92 ; h , 6.97 ; 
 n , 7.18 . found : c , 76.68 ; h , 6.71 ; n , 7.33 . to a solution of h2tc-5,5 ( 300 mg , 0.8 
 mmol ) in thf was added nahmds ( 292 mg , 1.59 mmol ) , and the reaction was allowed 
 to stir for 10 min .
gacl3 ( 448 mg , 1.59 mmol ) was added 
 to the solution as a solid , and the reaction was left to stir overnight . 

the solution was evaporated to dryness , suspended in ch2cl2 , and filtered through celite .
recrystallization 
 from ch2cl2/et2o at 30 c 
 yielded x - ray - quality crystals ( 370 mg , 70% yield ) .
h 
 nmr ( cd2cl2 ) :  1.68 ( m , 8h , ch2 ) , 1.91 ( m , 4h , ch2 ) , 3.74 , ( m , 4h , ch2 ) , 3.85 ( m , 4h , ch2 ) , 7.10 ( t , j = 10 hz , 2h , 
 arh ) , 7.26 ( d , j = 8 hz , 2h , arh ) , 7.64 ( t , j = 12 hz , 2h , arh ) . 

esi - ms ( [ m  gacl4 ] ) : m / z 433.1 ( calcd m / z 433.12 ) .
uv  vis [ cdcl3 ;  , nm (  , 
 m cm ) ] : 274 ( sh , 63390 ) , 280 
 ( 70810 ) , 365 ( 40920 ) , 416 ( sh , 19480 ) , 433 ( 27090 ) .
calcd for c24h30n4ga2cl4 : c , 43.96 ; h , 4.61 ; n , 
 8.54 . found : c , 43.88 ; h , 4.44 ; n , 8.42 . to a solution of h2tc-6,6 ( 54 mg , 0.13 mmol ) in thf was added 
 nahmds ( 45 mg , 0.27 mmol ) , and the reaction was allowed 
 to stir for 15 min .
gacl3 ( 70 mg , 0.27 mmol ) was added 
 to the solution as a solid , and the reaction was left to stir overnight . 

the solution was evaporated to dryness , suspended in ch2cl2 , and filtered through celite .
recrystallization 
 from ch2cl2/et2o at 30 c 
 yielded x - ray - quality crystals ( 52 mg , 57% yield ) .
h nmr 
 ( cd2cl2 ) :  1.131.26 ( m , 8h , ch2 ) , 1.44 ( m , 4h , ch2 ) , 2.192.26 ( m , 4h , ch2 ) , 3.643.71 
 ( m , 4h , ch2 ) , 3.974.02 ( m , 4h , 
 ch2 ) , 7.12 ( t , j = 8 
 hz , 2h , arh ) , 7.35 ( d , j = 8 hz , 2h , arh ) , 7.60 
 ( t , j = 6 hz , 2h , arh ) .
esi - ms ( [ m  gacl4 ] ) : m / z 471.1 ( calcd m / z 471.20 ) .
vis [ cdcl3 ;  , nm (  , 
 m cm ) ] : 272 ( sh , 73750 ) , 280 
 ( 83830 ) , 363 ( 45720 ) , 410 ( sh , 22980 ) , 422 ( sh , 28930 ) , 429 ( 37980 ) . 
 ir ( kbr ; cm ) : c = n 1514 , 

calcd for c26h34n4ga2cl4 : c , 45.67 ; h , 5.01 ; 
 n , 8.19 .
evan s method measurements 
 were made in cd2cl2 on a 500 mhz varian inova 
 spectrometer , and the temperature was measured by the residual peak 
 separation of the h nmr of neat cd3od .
optical spectra were recorded on a varian cary 
 5000 uv  vis  nir spectrophotometer in 6sq starna cells . 

fourier transform 
 infrared spectra were recorded on a thermo nicolet avatar 360 spectrometer 
 running the omnic software package .
electrospray 
 ionization mass spectrometry ( esi - ms ) analyses were performed on the 
 agilent 1100 series lc / msd trap spectrometer .
cyclic voltammograms 
 were recorded under nitrogen using the versastat3 potentiostat ( princeton 
 applied research ) and v3 studio software . a glassy 
 carbon working electrode ,
samples were prepared 
 as 35 mm solutions in methylene chloride with 0.1 m ( n - bu4n)(pf6 ) as the supporting electrolyte . 
 reported spectra
the reversible 
 fc / fc couple appeared at 0.57 v vs ag / ag .
crystals were mounted in paratone 
 n oil and frozen at 100 k under a cold nitrogen stream controlled 
 by a cryopad low - temperature apparatus .
data were collected on a bruker 
 apex ccd x - ray diffractometer with graphite - monochromated mo k 
 radiation (  = 0.71073  ) controlled by the apex2 software package .
the structure was solved by direct methods using shelxs-97 and refined by full - matrix least squares on f using the shelxl-97 program 
 incorporated into the shelxtl software package .
hydrogen 
 atoms were assigned idealized positions and given thermal parameters 
 1.2 times the thermal parameters of the atoms to which they are attached . 

the structure of [ ga(tc-6,6)](gacl4 ) contained 
 voids filled with heavily disordered solvent molecules .
the program squeeze(28 ) was used to remove 
 the contributions of the disordered solvent to the structure factors . 
 the electron density attributed to disordered solvent molecules created 
 a channel along the 65 screw axis and corresponded to seven 
 molecules of methylene chloride or et2o .
the crystal of 
 [ ga(tc-6,6)](gacl4 ) was an inversion twin , and the percentage 
 of the main twin component was refined to 58.2% .
[ co(tc-5,5 ) ] 
 was successfully oxidized to [ co(tc-5,5)](bf4 ) by reaction 
 with ferrocenium tetrafluoroborate in dcm ( figure 2 , left , and tables s1 and s2 in the supporting 
 information , si )
. structural characterization of [ co(tc-5,5)](bf4 ) revealed the twist angle at cobalt to be 65. the average bond distance between the metal center 
 and coordinating nitrogen atoms , co1nave , is 1.85 
  , comparable to the 1.86 and 1.87  values in [ co(tc-3,3)](bph4 ) and [ co(tc-4,4)](bar4 ) , respectively . thermal 
 ellipsoid plots for [ co(tc-5,5)](bf4 ) ( left ) and [ co(tc-6,6)](bph4 ) ( right ) , shown at 50% probability .
we also oxidized [ co(tc-6,6 ) ] to [ co(tc-6,6)](bph4 ) with ferrocenium tetraphenylborate and structurally characterized 
 the resulting cobalt(iii ) product ( figure 2 , right , and figure s2 and tables s1 and s3 in the si ) . the twist angle in [ co(tc-6,6 ) ] is 74 , 
 the largest value observed for cobalt(iii ) tropocoronands to date .
the co1nave bond distance 
 is 1.82  , somewhat shorter than that in previously reported 
 cobalt(iii ) tropocoronand coordination compounds .
a comparison of bond lengths and twist angles for four - coordinate cobalt(ii ) and -(iii ) tropocoronand 
 complexes is provided in table 1 .
tetrahedral geometry is rare 
 for cobalt(iii ) and , to our knowledge , has been observed in mononuclear 
 small molecules only for [ co(nor)4 ] and 
 a handful of cobalt(iii ) imido complexes to date .
the paucity of cobalt(iii ) in tetrahedral environments and the unusual 
 geometries of the metal centers in [ co(tc-5,5 ) ] and [ co(tc-6,6 ) ] raised the possibility that oxidation of the parent cobalt(ii ) 
 compounds occurred at the ligand rather than the metal center .
we 
 therefore prepared and characterized [ ga(tc-5,5)](gacl4 ) and [ ga(tc-6,6)](gacl4 ) and compared their structural 
 and electrochemical properties with those of the analogous cobalt 
 complexes ( figure 3 and tables s1 , s4 , and 
 s5 in the si ) .
thermal ellipsoid plots 
 for [ ga(tc-5,5)](gacl4 ) and [ ga(tc-6,6)](gacl4 ) , depicted at 50% probability .
the [ ga(tc-5,5)](gacl4 ) complex crystallizes in p21/c and exhibits crystallographic 
 disorder over the entirety of the tropocoronand ligand ( figure s1 
 in the si ) .
the n4-coordinated 
 gallium(iii ) center has a twist angle of 81 , displaying distorted 
 tetrahedral geometry . the average gallium 
[ ga(tc-6,6)](gacl4 ) crystallizes in p65 and exhibits 
 nearly perfect tetrahedral coordination at the gallium(iii ) center . 
 the twist angle in [ ga(tc-6,6 ) ]
comparison of analogous bond distances 
 between the [ co(tc - n , n ) ] and [ ga(tc - n , n ) ] complexes provided insight into the electron 
 distribution in the former species ( figures 4 and 5 ) .
nitrogen bonds in 
 both cobalt complexes are shorter than those in the analogous gallium(iii ) 
 tropocoronands .
n bond is probably a consequence of the greater covalent 
 character of the cobalt tropocoronand compared to the gallium tropocoronand 
 complex .
comparison of structural parameters for 
 [ co(tc-5,5)](bf4 ) ( left ) and [ ga(tc-5,5)](gacl4 ) ( right ) .
distances (  ) shown are representative of bond distances 
 within the tropocoronand ligand .
comparison of structural parameters for [ co(tc-6,6)](bph4 ) ( left ) and [ ga(tc-6,6)](gacl4 ) ( right ) .
distances (  ) 
 shown are representative of bond distances within the tropocoronand 
 ligand . in a review of the experimental 
 and theoretical properties of transition - metal complexes bound to 
 redox noninnocent ligands , ray et al .
note difficulties in using x - ray 
 structural parameters to draw conclusions regarding ligand- versus 
 metal - based oxidation in delocalized systems , where the electron of 
 a ligand radical is shared between two different ligands .
the tropocoronand complexes provide such a delocalized 
 system , for if a ligand radical were to form , the electron would be 
 able to travel between the two aminotroponeiminate rings via the metal 
 center .
we are also aware of reports describing noninnocent ligands 
 that undergo negligible structural rearrangement upon changes in redox 
 state .
we therefore turned to electrochemical methods for further evidence 
 that oxidation of [ co(tc-5,5 ) ] and [ co(tc-6,6 ) ] results in cobalt(iii ) 
 species .
previously published cyclic voltammetry studies of [ co(tc-5,5 ) ] and [ co(tc-6,6 ) ] were repeated and 
 compared to the results of analogous studies of [ ga(tc-5,5)](gacl4 ) and [ ga(tc-6,6)](gacl4 ) ( figure 6 ) .
we observed reversible couples at 0.344 v vs fc / fc for [ co(tc-5,5 ) ] and 0.367 v vs fc / fc for [ co(tc-6,6 ) ] .
we assign these processes to metal - based redox 
 reactions by comparison with electrochemical studies of [ zn(tc-5,5 ) ] 
 and [ zn(tc-6,6 ) ] .
the absence of similar 
 reversible processes in the voltammograms of the zinc complexes is 
 consistent with these redox events being metal - based .
additionally , 
 we assign the irreversible features at 0.467 v in the [ co(tc-5,5 ) ] 
 voltammogram and at 0.646 v in the [ co(tc-6,6 ) ] voltammogram to ligand - based 
 oxidations .
the cobalt(ii)/cobalt(iii ) couples observed here appeared 
 at 100 mv more negative than those previously published .
the reason for this discrepancy is unknown , 
 and the current values are considered to be correct .
cyclic voltammetry 
 of [ ga(tc-5,5)](gacl4 ) and [ ga(tc-6,6)](gacl4 ) revealed ligand - based oxidations at 0.846 and 0.920 v vs fc / fc , respectively .
ligand oxidation occurred at more positive potentials in the gallium(iii ) 
 complexes than in the analogous zinc(ii ) and cobalt(iii ) compounds .
the shift to more positive potentials in the 
 gallium(iii ) tropocoronands relative to the zinc(ii ) analogues can 
 be attributed to the higher oxidation state of the gallium center . 

ligand oxidation in the gallium complexes may occur at more positive 
 potentials than in the corresponding cobalt species because the cobalt 
 center may be better able to stabilize the additional charge through 
 covalent metal  ligand interactions than gallium .
cyclic voltammograms 
 of ( a ) [ co(tc-5,5 ) ] , ( b ) [ co(tc-6,6 ) ] , ( c ) [ ga(tc-5,5)](gacl4 ) , and ( d ) [ ga(tc-6,6)](gacl4 ) , referenced to fc / fc . we attempted to characterize [ co(tc-5,5)](bf4 ) by x - band electron paramagnetic resonance ( epr ) spectroscopy 
 but were unable to observe any signal at 77 k. helium temperature 
 epr spectroscopic studies of [ co(tc-6,6)](bph4 ) were equally 
 unrevealing .
the h nmr spectrum of [ co(tc-5,5)](bf4 ) shows broad , low - intensity peaks in the diamagnetic region 
 of the spectrum , which may correspond to a tropocoronand - containing 
 species ( figure s3 in the si ) .
peaks 
 corresponding to protons of the tropocoronand ligand are absent in 
 the h nmr spectrum of [ co(tc-6,6)](bph4 ) , but 
 peaks for the tetraphenylborate counteranion are readily observed 
 ( figure s4 in the si ) .
previous studies 
 revealed that , as the total length of the linker chains ( n + m ) of a ( tc - m , n ) complex increases , both the geometry of the 
 metal center and spin state change .
the spin state of the distorted 
 square - planar [ ni(tc-4,5 ) ] is s = 0 , and that of 
 the distorted tetrahedral [ ni(tc-5,5 ) ] is s = 1 . in the cobalt(ii )
system , a spin - state change 
 is observed between [ co(tc-4,4 ) ] ( s = /2 ) and [ co(tc-4,5 ) ] ( s = /2 ) .
we previously reported 
 that [ co(tc-3,3)](bph4 ) ( twist angle = 8 ) and [ co(tc-4,4)](bph4 ) ( twist angle = 41 ) have spin state s = 1 at room temperature and magnetic moments of eff = 3.1 and 3.6 b , respectively . to determine whether a spin - state change occurs in the 
 [ co(tc - m , n ) ] series , 

the magnetic susceptibility of [ co(tc-6,6)](bph4 ) was measured 
 by evan s method at room temperature .
the magnetic moment was 
 determined to be 5.38 b , as expected for s = 2 , confirming that a spin - state change does occur .
calculations were performed with the use of orca(40 ) to supplement our understanding of 
 the electronic structure of the [ co(tc - n , n ) ] series ( n = 36 ) . 
 in all cases ,
the optimized geometries [ bp / svp(tzvp on co ) ] of the s = 2 state had twist angles 1321 larger than 
 that those of the s = 1 state ( table s6 in the si ) .
this result supports the connection between 
 the spin - state change and the square - planar to tetrahedral change 
 in geometry .
subsequent energy computations at the b3lyp / def2-tzvp 
 level predicted that the s = 1 state is more stable 
 than the s = 2 state in all four species , with triplet 
quintet 
 gaps of 24.9 , 18.9 , 8.0 , and 7.2 kcal / mol for n = 
 36 , respectively ( figure s6 in the si ) .
our experimental results show that a spin - state change to s = 2 occurs for n = 5 or 6 .
the computations 
 suggest that the ground - state electronic structure for n = 5 and 6 is s = 1 , but there is a low energy barrier 
 to accessing the s = 2 state ( 78 kcal / mol ) . 

even though this level of theory incorrectly predicts the ground state 
 spin for n = 5 and 6 , the trend in the tripletquintet 
 energy gap across the series n = 36 is correct . 

quintet energy gap correlates almost linearly with 
 the experimental twist angle ( r = 0.936 ; 
 figure s6 in the si ) .
electronic transitions 
 predicted for the series support the spin - state change but are less 
 conclusive ( figures s7s10 in the si ) .
molecular orbitals involved in the electronic 
 transitions for all complexes contained significant mixing between 
 the cobalt and tropocoronand orbitals .
thus , assignments for the transitions 
 in the electronic spectra could not be made without further work .
comparison of structural and electrochemical 
 properties of the cobalt and gallium species [ co(tc-5,5 ) ] , [ co(tc-6,6 ) ] , [ ga(tc-5,5 ) ] , and [ ga(tc-6,6 ) ] confirmed the cobalt(iii ) character of the metal centers 
 in [ co(tc-5,5)](bf4 ) and [ co(tc-6,6)](bph4 ) . 

the synthesis of [ co(tc-5,5)](bf4 ) and [ co(tc-6,6)](bph4 ) augments the number of cobalt(iii ) species having the rare 
 pseudotetrahedral geometry .
together with previous results from our 
 laboratory , the present study reveals that a spin - state change from s = 1 to s = 2 occurs as the ligands tune 
 the geometry of the complexes from pseudo - square - planar for [ co(tc - n , n)](bph4 ) ( n = 3 , 4 ) to pseudotetrahedral for [ co(tc-6,6)](bph4 ) .
 the 
 preparation and characterization of two mononuclear cobalt(iii ) tropocoronand 
 complexes , [ co(tc-5,5)](bf4 ) and [ co(tc-6,6)](bph4 ) , are reported . 
 the cobalt(iii ) centers exist in rare pseudotetrahedral 
 conformations , with twist angles of 65 and 74 for the 
 [ co(tc-5,5]+ and [ co(tc-6,6)]+ species , respectively . 
 
 structural and electrochemical characteristics are compared with those 
 of newly synthesized [ ga(tc-5,5)](gacl4 ) and [ ga(tc-6,6)](gacl4 ) analogues . 
 the spin state of the pseudotetrahedral [ co(tc-6,6)](bph4 ) compound was determined to be s = 2 , a 
 change in spin state from the value of s = 1 that 
 occurs in the square - planar and distorted square - planar complexes , 
 [ co(tc-3,3)](x ) ( x = bph4 , bar4 ) and 
 [ co(tc-4,4)](bph4 ) , respectively . 
Introduction
Experimental Methods
Results and Discussion
Summary
and Conclusions