Synthesis, spectroscopic and structural characterization of the pentacoordinate high-spin Fe (III) isothiocyanate picket fence porphyrin complex

Article abstract of DOI:10.1016/S0277-5387(98)00156-9

The preparation and the characterization of the (isothiocyanato) (α,α,α,α-tetrakis (o-pivalamidophenyl) porphinato) iron (III) complex is described. The synthesis procedure utilizes the crown ether 18-C-6 to solubilize potassium thiocyanate. ¹H NMR and EPR data of the isolated product are in accordance with a high-spin (S=52) ferric porphyrin species. The X-ray molecular structure confirms the above deductions. The iron atom is pentacoordinated by the four nitrogen atoms of the pyrrole rings of the porphyrin and the nitrogen atom of the NCS^- group. It lies at 0.504 A out of the porphinato plane and 0.462 A out of the four nitrogen plane. The Fe-N (NCS) bond length is 1.956 (5) A and the Fe-N-C (NCS) bond angle is 176.1 (5) °.

Full text of DOI:10.1016/S0277-5387(98)00156-9

Polyhedron Vol[ 06\ No[ 19\ pp[ 2596Ð2501\ 0887
Þ 0887 Elsevier Science Ltd
Pergamon
All rights reserved[ Printed in Great Britain
9166Ð4276:87 ,08[99¦9[99
\
PII] S9166Ð4276"87#99045Ð8
Synthesis\ spectroscopic and structural
characterization of the pentacoordinate high!spin
Fe"III# isothiocyanate {{picket fence|| porphyrin
complex
Habib Nasria\ꢀ and Mongi Debbabib
a Faculte des Sciences de Monastir\ 4999 Monastir\ Tunisia
ꢁ
b Ecole Nationale d|Ingenieurs de Monastir "E[N[I[M[#\ 4999 Monastir\ Tunisia
ꢁ
"Received 00 February 0887^ accepted 1 April 0887#
Abstract*The preparation and the characterization of the "isothiocyanato#"a\a\a\a!tetrakis"o!pivalamido!
phenyl#porphinato#iron"III# complex is described[ The synthesis procedure utilizes the crown ether 07!C!5 to
0
solubilize potassium thiocyanate[ H NMR and EPR data of the isolated product are in accordance with a
high!spin "Sꢂ4:1# ferric porphyrin species[ The X!ray molecular structure con_rms the above deductions[ The
iron atom is pentacoordinated by the four nitrogen atoms of the pyrrole rings of the porphyrin and the nitrogen
atom of the NCS− group[ It lies at 9[493 A out of the porphinato plane and 9[351 A out of the four nitrogen
Ä
Ä
Ä
plane[ The FeÐN"NCS# bond length is 0[845"4# A and the FeÐNÐC"NCS# bond angle is 065[0"4#>[ Þ 0887
Elsevier Science Ltd[ All rights reserved
Keywords] porphyrin^ iron"III# complex^ isothiocyanate complex^ high!spin Fe"III#^ molecular structure[
———————————————————————————————————————————————
INTRODUCTION
Korszun and Mo}at ð2Ł have determined the structure
of the isothiocyanato!liganded hemoglobin which
shows a spin equilibrium between the high!spin NCS−
pentacoordinate heme and the {{"NCS#MetHb proxi!
mal histidine|| low spin six coordinate methem!
oglobine[
The isothiocyanate ferric porphyrinate was men!
tioned in the literature for decades ð3Ł as a high!spin
"Sꢂ4:1# pentacoordinate derivative[ However\ a
complete characterization of such species has not been
previously reported[
It is the aim of this work to synthesize and spectro!
scopically and structurally characterize the isothio!
cyanate Fe"III# picket fence porphyrin complex[
We also report in this paper the use of crown ethers
to solubilize the inorganic potassium thiocyanate salt
in organic solvents typically used with iron por!
phyrinates[
The use of the picket fence porphyrin for the charac!
terization of this ferric porphyrinate was directly
related to the fact that ionic not very bulky ligands as
it is shown by X!ray molecular structures ð4Ł\ are in
The interest in iron porphyrins stems not only from
the diverse biological roles of the iron porphyrin moi!
ety found in proteins\ cytochromes\ peroxidases\ etc[
but also from the array of physical properties observed
for the coordinated Fe"II# and Fe"III#[
In most cases\ when weak crystal _eld ligands such
as halides or pseudohalides are bound to an iron"III#
of a metalloporphyrin or a hemoprotein\ the resulting
porphyrinate is high spin and _ve coordinate[ Never!
theless\ many examples of six coordinate high!spin
heme!like species exist\ i[e[ "1!MeHIm#ðFe"F#1"TPP#Ł
ð0Ł[ In some special circumstances the ferric ion may
be in the spin {{crossover|| region and show thermal
spin!equilibrium[ Thus\ in the case of the azide ligand\
Scheidt et al[ ð1Ł\ con_rmed the high spin "Sꢂ
4:1#\low spin "Sꢂ0:1# equilibrium of the bisazide
Fe"III# porphyrin species^ ðFe"N2#1"TpivPP#Ł−[
ꢀ Author to whom correspondence should be addressed[
2596

2597
H[ Nasri and M[ Debbabi
Fig[ 0[ Visible spectrum of ðFe"NCS#"TpivPP#Ł in chlorobenzene solution "room temperature#[
all cases bound to the iron by the pocket side of the are axially symmetric^ `_ꢂ4[7 and `>ꢂ0[87 for the
protected porphyrin[ Consequently\ these complexes frozen chlorobenzene solution and `_ꢂ4[6 for the
are remarkably stable[
powder "crushed single crystals#[ These results provide
unequivocal evidence that either in solid state or in
solution the ðFe"NCS#"TpivPP#Ł complex is high!spin
"Sꢂ4:1#[
RESULTS AND DISCUSSION
A proton NMR spectrum of ðFe"NCS#"TpivPP#Ł
in CDCl2\ recorded at room temperature\ supports the
latter conclusion[ Thus\ the pyrrole proton resonance
at 64[6 ppm "line with ¼254 Hz# is in the range 099Ð
69 ppm\ a down_eld region characteristic of high!spin
iron"III# tetraarylporphyrin complexes ð00Ł[ The
ortho!phenyl protons of the macrocycle are not
observed[ Strong dipolar interactions should be the
cause of the important broadening[ Meta!phenyl pro!
tons are highly anisotropic giving rise to two singlets
at 00[4 and 03[3 ppm[
The reported synthesis for halide and pseudohalide
Fe"III# porphyrin species make use of organic!soluble
iron porphyrinates and typically water!soluble inor!
ganic salts ð5Ł "KCl\ KBr\ NaN2\ etc[# or aqueous
acids "CH2COOH ð6Ł\ HNO2 ð7Ł\ etc[#[ In our case\
the "isothiocyanato#"a\a\a\a!tetrakis"o!pivalamido!
phenyl#porphinato#iron"III# complex has been pre!
pared by using only one organic phase[ The 07!C!
5 crown ether was used to solubilize the potassium
thiocyanate in chlorobenzene solution0[ Our synthetic
strategy has the advantage of avoiding the use of a
mixture of an organic and an aqueous solution and
obtaining the anionic reagent in high concentration[
Figure 0 represents the electronic spectrum of the
ðFe"NCS#"TpivPP#Ł species recorded in chloro!
benzene[ The spectrum belongs to the hyper class ð8Ł[
The Soret and the b bands values "respectively\ 306
and 402 nm# are quite identical to those of other penta!
coordinate Fe"III# high!spin meso!porphyrin species
ð09Ł[
The IR spectrum of ðFe"NCS#"TpivPP#Ł shows that
the value of the stretching frequency n"CN# which is
1904 cm−0\ is a clear evidence that NCS− is N!bonded
to the iron "an isothiocyanate# ð01Ł[
The molecular structure of the ðFe"NCS#"TpivPP#Ł
complex is illustrated in Fig[ 1[ The NCS− ligand lies\
as expected\ in the axial position inside the cavity
formed by the pivalamide arms of the picket fence
porphyrin[ The iron atom is _ve coordinated and the
resulting polyhedron is a slightly deformed square
pyramid with the nitrogen atom of the NCS− group
in the axial position[ Selected bond distances and
angles for this complex are given in Table 0[ Other
distances and angles\ fractional coordinates and ther!
mal parameters have been deposited as supplementary
data with the editor[
The EPR spectra carried out at 66 K show similar
features in solution and in solid state[ Both spectra
0 The Krypto_x 111 can also be used[
1 The process is based on an adoption of the DIFABS ð08Ł
logic to area detector geometry ð07Ł[
2 R!factors based on F1 "wR1# are statistically about twice
as large as those based on F[
The bond parameters for the coordination environ!
ment of the Fe"III# in this compound and for a selec!

High spin Fe"III# isothiocyanate porphyrin complex
2598
Fig[ 1[ ORTEP diagram of the ðFe"NCS#"TpivPP#Ł molecule displaying the atom!labeling scheme used throughout the
paper[ Ellipsoids are drawn at the 29) probability level[
a
Ä
Table 0[ Selected bond distances "A# and angles "># for ðFe"NCS#"TpivPP#Ł
Iron coordination polyhedron
FeÐN"8#
FeÐN"0#
FeÐN"1#
FeÐN"2#
FeÐN"3#
0[845"4#
1[946"3#
1[943"3#
1[945"3#
1[943"3#
N"0#ÐFeÐN"8#
N"1#ÐFeÐN"8#
N"2#ÐFeÐN"8#
N"3#ÐFeÐN"8#
093[1"1#
099[5"1#
093[4"1#
091[7"1#
Thiocyanate li`and
N"8#ÐN"34#
C"34#ÐS
N"0#ÐFeÐN"1#
N"0#ÐFeÐN"2#
N"0#ÐFeÐN"3#
N"1#ÐFeÐN"2#
N"1#ÐFeÐN"3#
75[6"1#
040[2"1#
76[4"1#
76[2"1#
045[5"1#
0[038"7#
0[591"6#
FeÐN"8#ÐC"34#
SÐC"34#ÐN"8#
065[0"4#
067[5"5#
aThe numbers in parentheses are the estimated standard deviations[
Ä
tion of some other Fe"III# metalloporphyrins are 1[954 A[ These values are a clear indication that these
given in Table 1[ A brief description of the molecular species are high!spin ð03Ł[ In order to accommodate
structure of the otherwise analogous ðFe"NCS#"TPP#Ł the high!spin iron"III# atom\ the porphinato cores
complex was mentioned by Bloom and Hoard ð02Ł[ undergo a signi_cant radial expansion[ This is very
The values of the FeÐNp distances of the TpivPP well illustrated by the long FeÐPC and FeÐPN distances
and TPP derivatives are\ respectively\ 1[944"1# and shown by ðFe"NCS#"TpivPP#Ł and ðFe"NCS#"TPP#Ł[

2509
H[ Nasri and M[ Debbabi
Ä
Table 1[ Porphinato core parameters "A# for selected Fe"III# porphyrin complexes
Complex
FeÐNp
FeÐXL
FeÐPC
FeÐPN
da
Spin "S#
Refs[
ðFe"NCS#"PivPP#Ł
ðFe"NCS#"TPP#Ł
ðFe"NCS#"Py#"OEP#Ł
ðFe"NCS#"Py#"TPP#Ł
1[944 "1#
1[954
1[937 "3#
0[877 "8#
FeÐNLb] 0[845 "4#
FeÐNLb] 0[846
9[493
9[449
9[139c
9[949c
9[351
9[374
9[079c
9[949c
9[931
9[954
−
4:1
4:1
4:1
0:1
this work
ð02Ł
ð04Ł
FeÐNLb] 1[920 "1#
FeÐNLb] 0[831 "3#
−
ð04Ł
adꢂthe doming[
bFeÐNLꢂFeÐN"NCS# distance[
cDisplacement toward the isothiocyanate ligand[
The corresponding values are\ respectively\ 9[493 and case#[ Both isothiocyanate TpivPP and OEP deriva!
Ä
Ä
9[351 A for the former complex and 9[44 and 9[374 A tives exhibit rather identical values for the FeÐNÐ
for the latter[ For both ðFe"NCS#"Porph#Ł derivatives\ C"NCS# angle ðrespectively\ 065[0 "4#> and 065[9 "1#>Ł[
the FeÐN"NCS# distances have the same values^ These two derivatives present very close NÐCÐS angle
Ä
0[845"4# A for the picket fence porphyrin product and values with 067[5> "5# for the picket fence complex
Ä
0[846 A for the tetraphenylporphyrin moiety[
and 068[5> "1# for the OEP six coordinate species[
The porphyrinato core in ðFe"NCS#"TpivPP# is
As expected for an N!bonded thiocyanate to an
iron atom\ the FeÐNCS group\ is found to be essen! decidedly non!planar as can be seen from an inspec!
tially linear for all NCS− derivatives shown in Table tion of Fig[ 2[ The pattern of the atom displacements
1 "except the ambiguous ðFe"NCS#"Py#"TPP#Ł ð04Ł in the porphinato core correspond closely to those
Fig[ 2[ Formal diagram of the porphinato core in the ðFe"NCS#"TpivPP#Ł molecule displaying the displacement\ in units of
Ä
9[90 A\ from mean plane of the porphinato core[ Also displayed on the diagram is the average value of each type of bond
distance and angle in the porphinato core[

High spin Fe"III# isothiocyanate porphyrin complex
2500
required by exact D1d symmetry\ often simply called
A total of 09919 unique re~ections was collected
S3!ru/ing[ Least squares planes calculations indicate and 5680 having I×1s"I# ðmerging R"int#ꢂ9[021Ł[
Ä
a small doming of the porphyrin "9[931 A#[
Structure analysis and re_nement[ The structure was
solved in the centrosymmetric space group P10:n using
the direct methods program SHELXS!75 ð19Ł[ All
non!hydrogen atoms were re_ned anisotropically and
all hydrogen atoms were idealized with the standard
SHELXL!82 ð10Ł idealization method[ The _nal cycles
of re_nement based on F1 of 622 variables converged
to the _nal R0ꢂ9[968 and wR1ꢂ9[0562 for re~ections
EXPERIMENTAL
The complex ðFe"SO2CF2#"H1O#"TpivPP#Ł was pre!
pared as described previously ð05Ł[ The 07!C!5 crown
ether was recrystallized in toluene\ dried under vac!
uum overnight and stored under argon[ Toluene and
hexanes were distilled from their sodiumÐbenzo!
phenone solution under a nitrogen atmosphere[ Chlo!
robenzene was puri_ed by washing with sulfuric acid
and then distilled over P1O4[
with I×1s"I#[ The _nal di}erence Fourier map had
−2
Ä
the largest peak at 9[65 eA
[
Acknowled`ements*We gratefully acknowledge professor
W[ R[ Scheidt of the University of Notre Dame "Indiana\
U[S[A[# for the use of the Enraf Nonius area detector FAST
di}ractometer[ We also acknowledge M[ K[ Ellison from the
University of Notre Dame "Indiana\ U[S[A[# for her kind
help in recording the EPR spectra[
UV!VIS spectra were recorded on a Perkin!Elmer
Lambda 08 spectrometer\ Fourier!transform IR data
on a Bio!Rad FT S3999 spectrometer and EPR spec!
tra were obtained at 66 K on a Varian E!line spec!
trometer operating at X!band frequency[
0H NMR data were obtained on a Nicolet NT299
spectrometer operating at 299[952 MHz[
REFERENCES
0[ Scheidt\ W[ R[\ Lee\ Y[ J[\ Tamai\ S[ and Hatano\
K[\ J[ Am[ Chem[ Soc[\ 0872\ 094\ 667[
1[ Ellison\ M[ K[\ Nasri\ H[\ Xia\ Y[!M[\ Marchan\
J[ C[\ Schultz\ C[ E[\ Debrunner\ P[ G[ and
Scheidt\ W[ R[\ Inor`[ Chem[\ 0886\ 25\ 3793[
2[ Korszun\ Z[ R[ and Mo}at\ K[\ J[ Mol[ Biol[\
0870\ 034\ 704[
3[ S[ Mitra\ Iron Porphyrins\ Part II\ eds[ A[ B[ P[
Lever and H[ B[ Gray[ Addison!Wesley Pub!
lishing Company\ Massachusetts\ 0872\ Chap 0[
Preparation of the ðFe"NCS#"TpivPP#Ł compound
The "isothiocyanato#"a\a\a\a!tetrakis"o!pivalamido!
phenyl#porphinato#iron"III# complex was prepared
by reacting ðFe"SO2CF2#"H1O#"TpivPP#Ł "9[905
mmol# with KSCN "9[21 mmol# and 07!crown!5
"9[05 mmol# in 19 ml of C5H4Cl[ The resulting red
brownish solution was then _ltered and an X!ray qual!
ity crystals were prepared by slow di}usion of hexanes
into C5H4Cl[ The crystalline material was washed with
several portions of water and then with hexanes[
UV!VIS ðC5H4Cl\ lmax in nm\ "log o#Ł] 306 "3[88#^
402 "3[10#^ 475 sh "2[52#^ 544 "2[43# IR "KBr\ cm−0#]
n"CN#] 1904[ EPR "C5H4Cl\ 66 K#] `_ꢂ4[7^
`>ꢂ0[87[ 0H NMR "299 MHz\ CDCl2#] H"pyrr#]
64[6 ppm^ H"meta!Ph#] 00[4\ 03[3^ H"para!Ph#]
7[9 ppm[
4[ M[ Schappacher\ Ph[D[ Thesis\ Universite Louis
ꢁ
Pasteur\ Strasbourg\ France\ 0871[
5[ J[ W[ Buchler\ Porphyrins and Metalloporphyrins\
ed[ K[ M[ Smith[ Elsener Scienti_c Publishing
Company\ Amsterdam\ 0864\ Chap[ 4[
6[ Oumous\ H[\ Lecomte\ C[\ Protas\ J[\ Cocolios\
P[ and Guillard\ R[\ Polyhedron\ 0873\ 2\ 540[
7[ Phillipi\ M[ A[\ Shimomura\ E[ T[ and Go}\ H[
M[\ Inor`[ Chem[\ 0870\ 19\ 2893[
8[ M[ Gouterman\ The Porphyrins\ Vol[ III\ ed[ D[
Dolphin[ Academic Press\ New York\ 0867\
Chap[ I[
09[ H[ Nasri\ Ph[D[ Thesis\ Universite Louis Pasteur\
ꢁ
Determination of crystal and molecular structure
Strasbourg\ France\ 0876[
00[ G[ N[ La Mar and F[ A[ Walker\ The Porphyrins\
Vol[ IV\ ed[ D[ Dolphin[ Academic Press\ New!
York\ 0867[
01[ K[ Nakamoto\ Infrared and Raman Spectra of
Inor`anic and Coordination Complexes\ 3th edn[
J[ Wiley and Sons\ New York\ 0875[
02[ A[ Bloom and J[ L[ Hoard\ unpublished results[
03[ Scheidt\ W[ R[ and Reed\ C[\ Chem[ Rev[\ 0870\
70\ 432[
04[ Scheidt\ W[ R[\ Lee\ Y[ J[\ Geiger\ D[ K[\ Taylor\
K[ and Hatano\ K[\ J[ Am[ Chem[ Soc[\ 0871\ 093\
2256[
05[ Gismelseed\ A[\ Bominaar\ E[ L[\ Bill\ E[\ Traut!
wein\ A[ X[\ Nasri\ H[\ Doppelt\ P[\ Mandon\ D[\
Fischer\ J[ and Weiss\ R[\ Inor`[ Chem[\ 0889\ 18\
1630[
Crystal data[
monoclinic\ space group P10:n\ aꢂ02[126"2# A\
C54H53N8O3SFe\ Mꢂ0012[1\
Ä
Ä
Ä
bꢂ07[161"3# A\
cꢂ12[567 A\
bꢂ83[809"09#>\
2
Vꢂ4695[9 A \ Dcꢂ0[20 g=cm−2\ Zꢂ3\ F"999#ꢂ1613\
Ä
−0
Ä
lꢂ9[60962 A\ m"MoÐKa#ꢂ24[2 cm
[
Data collection and processin`[ A dark single crystal
of approximate size 9[5×9[2×9[2 mm2 was examined
on an Enraf!Nonius FAST area detector di}ract!
ometer at 016 K[ All measurements were performed
with graphite!monochromated MoÐKa radiation[
Data collection and reduction were de_ned as before
ð06Ł[ Intensity data were corrected for absorption
e}ects using a modi_ed version of the absorption cor!
rection program DIFABS1[

2501
H[ Nasri and M[ Debbabi
06[ Scheidt\ W[ R[ and Tyrk\ I[ T[\ Inor`[ Chem[\ 08[ Walker\ N[ P[ and Stuart\ D[\ Acta Crystallo`r[
0883\ 22\ 0203[ A\ 0872\ 28\ 047[
07[ Karaulov\ A[ I[\ School of Chemistry and Applied 19[ Scheldrick\ G[ M[\ Acta Crystallo`r[ A\ 0889\ 35\
Chemistry\ University of Wales\ College of Card! 356[
i}\ Cardi} CF0 2TB\ U[K[\ personal communi! 10[ G[ M[ Scheldrick\ J[ Appl[ Crystallo`r[ "in prep!
cation[ aration#[