Structural, conformational, and spectroscopic studies of primary amine complexes of iron(II) porphyrins

Article abstract of DOI:10.1021/ic990178q

Three novel bis(primary amine)iron(II) porphyrins [Fe(TPP)(RNH₂)₂], where RNH₂ = 1-butylamine, benzylamine, and phenethylamine, have been synthesized and characterized by X-ray crystallography and IR, electronic, and M?ssbauer spectroscopy. The compounds provide unprecedented structural data for the coordination of primary amines by iron(II) porphyrins. The Fe-N_ax distances of [Fe(TPP)(1-BuNH₂)₂], [Fe(TPP)(BzNH₂)₂], and [Fe-(TPP)(PhCH₂CH₂NH₂)₂] are 2.039(3), 2.043(3), and 2.028(2) ?, respectively. The Fe-N_p distances of the three complexes average 1.990(2) ?. The zero-field M?ssbauer spectra (5-300 K) show comparable isomer shifts (0.393(1)-0.493(1) mm/s) and quadrupole splittings (1.144(6)-1.204(3) mm/s) that are consistent with an S = 0 iron(II) assignment in each case. The bis(primary amine) complexes are structurally and spectroscopically similar to [Fe(TPP)(Py)₂] derivatives, where Py = an unsubstituted pyridine. Molecular mechanics (MM) calculations with a force field parametrized for primary and secondary amine complexes of iron(II) porphyrins show that stable conformations arise when the α-CH₂ and NH₂ protons of the coordinated ligands are staggered relative to the Fe-N_p bonds of the porphyrin core. The lowest energy conformations of the three [Fe(TPP)(RNH₂)₂] complexes therefore have the ligand α-carbons positioned directly over the Fe-N_p bonds of the porphyrin core. The X-ray structure of [Fe(TPP)(PhCH₂CH₂NH₂)₂] lies close to the global minimum (Φ₁, Φ₂ fa = 0, 180°) on the potential surface, while [Fe(TPP)(BzNH₂)₂] and [Fe(TPP)(1-BuNH₂)₂] show deviations that may be attributed to packing interactions in the solid and intrinsically low barriers to axial ligand rotation (<0.5 kcal/mol). Three types of minimum energy conformation are accessible for [Fe(TPP)(Pip)₂]. The lowest energy conformation has an S₄-ruffled porphyrin core. The conformation which matches the X-ray structure (Radonovich, L. J.; Bloom, A.; Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 2073-2078) is a local minimum (1.6 kcal/mol higher in energy than the global minimum) with exact inversion symmetry. Higher in vacuo strain energy barriers (～2.2 kcal/mol) separate the potential minima of [Fe(TPP)(Pip)₂], consistent with the increased bulk of the secondary amine axial ligands.

Full text of DOI:10.1021/ic990178q

4724
Inorg. Chem. 1999, 38, 4724-4736
Structural, Conformational, and Spectroscopic Studies of Primary Amine Complexes of
Iron(II) Porphyrins
Orde Q. Munro,*,† P. Sizwe Madlala,† Richard A. F. Warby,† Takele B. Seda,‡ and
Giovanni Hearne‡
School of Chemical and Physical Sciences, University of Natal, Private Bag X01,
Scottsville, Pietermaritzburg 3209, South Africa, and Department of Physics, University of the
Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa
ReceiVed February 25, 1999
Three novel bis(primary amine)iron(II) porphyrins [Fe(TPP)(RNH2)2], where RNH2 ) 1-butylamine, benzylamine,
and phenethylamine, have been synthesized and characterized by X-ray crystallography and IR, electronic, and
Mo¨ssbauer spectroscopy. The compounds provide unprecedented structural data for the coordination of primary
amines by iron(II) porphyrins. The Fe-Nax distances of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe-
(TPP)(PhCH2CH2NH2)2] are 2.039(3), 2.043(3), and 2.028(2) Å, respectively. The Fe-Np distances of the three
complexes average 1.990(2) Å. The zero-field Mo¨ssbauer spectra (5-300 K) show comparable isomer shifts
(0.393(1)-0.493(1) mm/s) and quadrupole splittings (1.144(6)-1.204(3) mm/s) that are consistent with an S )
0 iron(II) assignment in each case. The bis(primary amine) complexes are structurally and spectroscopically similar
to [Fe(TPP)(Py)2] derivatives, where Py ) an unsubstituted pyridine. Molecular mechanics (MM) calculations
with a force field parametrized for primary and secondary amine complexes of iron(II) porphyrins show that
stable conformations arise when the R-CH2 and NH2 protons of the coordinated ligands are staggered relative to
the Fe-Np bonds of the porphyrin core. The lowest energy conformations of the three [Fe(TPP)(RNH2)2] complexes
therefore have the ligand R-carbons positioned directly over the Fe-Np bonds of the porphyrin core. The X-ray
structure of [Fe(TPP)(PhCH2CH2NH2)2] lies close to the global minimum (φ1, φ2 ) 0, 180°) on the potential
surface, while [Fe(TPP)(BzNH2)2] and [Fe(TPP)(1-BuNH2)2] show deviations that may be attributed to packing
interactions in the solid and intrinsically low barriers to axial ligand rotation (<0.5 kcal/mol). Three types of
minimum energy conformation are accessible for [Fe(TPP)(Pip)2]. The lowest energy conformation has an S4-
ruffled porphyrin core. The conformation which matches the X-ray structure (Radonovich, L. J.; Bloom, A.;
Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 2073-2078) is a local minimum (1.6 kcal/mol higher in energy than
the global minimum) with exact inversion symmetry. Higher in vacuo strain energy barriers (∼2.2 kcal/mol)
separate the potential minima of [Fe(TPP)(Pip)2], consistent with the increased bulk of the secondary amine axial
ligands.
Introduction
decades. These efforts largely reflect an attempt to understand
the functional role of the axial ligands in the bis(histidine)
cytochromes b10-16 and c17-19 which cycle between the Fe(II)
and Fe(III) oxidation states in vivo. In contrast, the coordination
of alkylamine ligands by both Fe(II) and Fe(III) porphyrins
remains relatively unexplored. However, the recent 1.96-Å
resolution X-ray structure of turnip cytochrome f has shown
Six-coordinate imidazole and pyridine complexes of Fe(II)
porphyrins have been amply characterized by crystallographic,1-5
spectroscopic,6-8 and computational methods8,9 over the last two
* To whom correspondence should be addressed. Electronic mail:
MunroO@chem.unp.ac.za.
† University of Natal, Pietermaritzburg.
‡ University of the Witwatersrand.
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10.1021/ic990178q CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/29/1999

Primary Amine Complexes of Iron(II) Porphyrins
Inorganic Chemistry, Vol. 38, No. 21, 1999 4725
that the R-amino group of Tyr-120 coordinates trans to His-25
in this c-type heme protein.21,22 Thus, not only are amines of
interest from the standpoint of delineating their coordination
chemistry with iron porphyrins, but this class of ligand appears
to be functionally significant in at least one hemoprotein.
Despite these spectroscopic studies, there have been no
systematic structural studies on bis(amine) complexes of iron
porphyrins; the only X-ray structure to date is that of [Fe(TPP)-
(Pip)2].37 In this paper, we present a general method for the
synthesis of bis(primary amine) complexes of low-spin iron(II)
porphyrins. Three complexes, [Fe(TPP)(1-BuNH2)2], [Fe(TPP)-
(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2], have been char-
acterized by X-ray crystallography and electronic, IR, and
Mo¨ssbauer spectroscopy. The X-ray data have been used to
parametrize a molecular mechanics (MM) force field for bis-
(amine) low-spin iron(II) porphyrins. Primary and secondary
amine complexes of the type [Fe(TPP)L2], where L ) 1-butyl-
amine and piperidine, have been used for conformational
analysis by MM methods to determine the optimum axial ligand
orientations.
The reaction of primary and secondary amines with Fe(III)
porphyrins in nonaqueous solvents results in base-catalyzed one-
electron reduction and concomitant dissociation of the depro-
tonated amine radical.23 In the presence of excess amine,
bis(amine)iron(II) complexes are obtained.7,23-26 The mechanism
of this reaction has been studied for piperidine23 and a range of
primary amines27 in nonaqueous solvents. Only sterically
unhindered alkylamines with a C-H proton adjacent to the NH
or NH2 group are capable of reducing ferric porphyrins.27,28
Interestingly, spectroscopic studies in aqueous solution show
that base-catalyzed reduction to the ferrous state is quenched,
affording stable low-spin iron(III) complexes in the presence
of excess amine.29,30 Mo¨ssbauer data have been reported for a
number of Fe(II) systems, including [Fe(TPP)(Pip)2],31,32 [Fe-
(OEP)(NH3)2],7 and several [FeII(PPIX)L2] derivatives, where
L is a primary or secondary amine.33 In the latter study, frozen
aqueous solutions were used and reduction was effected with
sodium dithionite. Mo¨ssbauer and EPR data have also been
reported for [FeIII(TPP)(NH3)2](CF3SO3)34 and a range of [FeIII-
(PPIX)L2] complexes, where L is a primary, secondary, or
tertiary amine.35,36
Experimental Section
General Information. All manipulations were carried out under
nitrogen using a double manifold vacuum line, Schlenkware, and
cannula techniques. THF and hexane were distilled over sodium/
benzophenone and dichloromethane over CaH2. Benzylamine (UniLab)
was distilled over CaH2 and stored under nitrogen over 4 Å molecular
sieves. 1-Butylamine and phenethylamine (Aldrich) were distilled over
CaH2 and used immediately. H2TPP was synthesized using published
procedures.38 [Fe(TPP)Cl] was prepared by metalation of H2TPP with
anhydrous ferrous chloride in refluxing DMF.39 Silver triflate (Fluka)
was used as received.
Electronic spectra were recorded with a Shimadzu UV-2101PC UV-
vis scanning spectrophotometer using dry, degassed methylene chloride
solutions in 1.0 and 0.1 cm path length cuvettes under nitrogen. IR
spectra were recorded with a Shimadzu FTIR-4300 spectrometer as
KBr pellets. Mo¨ssbauer spectra of polycrystalline samples of [Fe(TPP)-
(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2]
(∼30-40 mg/cm2) in Teflon cups were recorded at selected temper-
atures from 5 to 300 K in a flow cryostat using a 57Co(Rh) source
(∼40 mCi). Doppler velocities, effected with a triangular reference
waveform, were calibrated against an iron foil standard. Each spectrum
was recorded in 512 channels and folded with its mirror image to give
the spectral data in 256 channels.40 The data were fitted either to the
sum of two Lorentzian components on a shallow parabolic background
or to the sum of two inequiValent quadrupole doublets to take into
account a minor (∼2% in fresh samples) iron(III) impurity, the signal
from which gradually increased over a period of several months. In
the latter case, the line widths of each resonance comprising a given
doublet were constrained to be equal. Elemental analyses (between 3
and 6 measurements per sample) were obtained with a Perkin-Elmer
CHN 2400 elemental analyzer on polycrystalline samples (∼2-3 mg).
Synthesis of [Fe(TPP)(BzNH2)2]. To [Fe(TPP)Cl] (200 mg, 0.284
mmol) and silver triflate (87.6 mg, 0.341 mmol) in a two-neck 100-
mL round-bottom flask under nitrogen was added 20 mL of freshly
distilled THF. The solution was allowed to stir for ∼12 h at room
temperature. A discrete transformation of the electronic spectrum of
[Fe(TPP)Cl] was observed upon substitution of chloride ion by
CF3SO3-; the Soret, Qv, and Qo bands shifted from 417, 509, and 575
nm to 406, 515, and 673 nm, respectively. The solvent was removed
in vacuo and the red-brown solid redissolved in dichloromethane (20
mL). The solution was filtered to remove precipitated silver chloride
and transferred in four 5-mL aliquots (∼71 µmol) to four Schlenk tubes
containing 600 µL (8.28 mmol) of benzylamine. In each case the
(20) Abbreviations: 1-BuNH2, 1-butylamine; 1-BzlIm, 1-benzylimidazole;
BzNH2, benzylamine; 4-CNPy, 4-cyanopyridine; 3-CNPy, 3-cyan-
opyridine; Ca, Cb, Cm, porphyrin R-, â-, and meso-carbons; Cp and
CL, phenyl and ligand carbons; 1,2-Me2Im, 1,2-dimethylimidazole;
DMF, N,N-dimethylformamide; EFG, electric field gradient; HIm;
imidazole; His, histidine; i-PrNH2, isopropylamine; L, ligand in
general; R-MeBzNH2, R-methylbenzylamine; 1-MeIm, 1-methylimi-
dazole; 4-MeIm-, 4-methylimidazole anion; 4-MePy, 4-methylpyri-
dine; MM, molecular mechanics; Np, porphinato nitrogen; Nax, axial
ligand nitrogen; OEP, 2,3,7,8,12,13,17,18-octaethylporphyrin dianion;
PhCH2CH2NH2, phenethylamine; Pip, piperidine; Py, pyridine; PPIX,
di- or trianion of protoporphyrin IX; TMP, 5,10,15,20-tetramesitylpor-
phyrin dianion; THF, tetrahydrofuran; TPP, 5,10,15,20-tetraphenylpor-
phyrin dianion; Tyr, tyrosine; 1-VinIm, 1-vinylimidazole.
(21) Martinez, S. E.; Huang, D.; Szczepaniak, A.; Cramer, W. A.; Smith,
J. L. Structure 1994, 2, 95-105.
(22) Martinez, S. E.; Huang, D.; Ponomarev, M.; Cramer, W. A.; Smith,
J. L. Protein Sci. 1996, 5, 1081-1092.
(23) Del Gaudio, J.; La Mar, G. N. J. Am. Chem. Soc. 1978, 100, 1112-
1119.
(24) Del Gaudio, J.; La Mar, G. N. J. Am. Chem. Soc. 1976, 98, 3014-
3015.
(25) Dixon, D. W.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1985, 107,
808-813.
(26) The redox chemistry of several bis(amino ester) complexes of a chiral
Ru(II) porphyrin in dichloromethane solution has recently been
described. As with iron, the Ru(II) complexes are the most stable
(Morice, C.; Lemaux, P.; Moinet, C.; Simonneaux, G. Inorg. Chim.
Acta 1998, 273, 142-150).
(27) Castro, C. E.; Jamin, M.; Yokoyama, W.; Wade, R. J. Am. Chem.
Soc. 1986, 108, 4179-4187.
(28) Morice, C.; Le Maux, P.; Simonneaux, G. Inorg. Chem. 1998, 37,
6100-6103.
(29) Byfield, M. P.; Pratt, J. M. J. Chem. Soc., Chem. Commun. 1992,
214-215.
(30) Byfield, M. P.; Hamza, M. S. A.; Pratt, J. M. J. Chem. Soc., Dalton
Trans. 1993, 1641-1654.
(31) Collman, J. P.; Hoard, J. L.; Kim, N.; Lang, G.; Reed, C. A. J. Am.
Chem. Soc. 1975, 97, 2676-2681.
(37) Radonovich, L. J.; Bloom, A.; Hoard, J. L. J. Am. Chem. Soc. 1972,
94, 2073-2078.
(32) Epstein, L. M.; Straub, D. K.; Maricondi, C. Inorg. Chem. 1967, 6,
1720-1724.
(38) Barnett, G. H.; Hudson, M. F.; Smith, K. M. J. Chem. Soc., Perkin
Trans. 1 1975, 1401-1403.
(33) Ahmet, M. T.; Al-Jaff, G.; Silver, J.; Wilson, M. T. Inorg. Chim. Acta
1991, 183, 43-49.
(39) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443.
(34) Kim, Y. O.; Goff, H. M. Inorg. Chem. 1990, 29, 3907-3908.
(35) Marsh, P. J.; Silver, J.; Symons, M. C. R.; Taiwo, F. A. J. Chem.
Soc., Dalton Trans. 1996, 2361-2369.
(40) The data folding procedure eliminates geometrical effects and gives
a flat baseline. The transmission integral routine of the program
NORMOS (Wissenschafteliche Elektronik GmbH (WISSEL), Starn-
berg, Germany) was used to effect all data transforms.
(36) Brautigan, D. L.; Feinberg, B. A.; Hoffman, B. M.; Margoliash, E.;
Peisach, J.; Blumberg, W. E. J. Biol. Chem. 1977, 252, 574-582.

4726 Inorganic Chemistry, Vol. 38, No. 21, 1999
Munro et al.
solution changed color from brown to deep red on swirling, consistent
with reduction of the metal to the ferrous state.27 The solutions were
layered with hexane; X-ray-quality crystals were observed after 4 days.
The deep red crystals of [Fe(TPP)(BzNH2)2] were collected by filtration
and washed with hexane to remove colorless crystals of benzylamine.
Isolated yield: 55 mg, 22%. Anal. Calcd for C58H46N6Fe: C, 78.91;
H, 5.25; N, 9.52. Found: C, 77.25; H, 4.56; N, 9.21. IR (KBr pellet):
1535 cm-1 (m, δ(NH2)), 874 cm-1 (w, Fw(NH2)). UV-vis (CH2Cl2)
[λmax, nm (ꢀ, M-1 cm-1)]: 425 (267 × 103), 494 (4.42 × 103), 531
(21.3 × 103), 562 (5.25 × 103).
) 0.374 mm-1) using a semiempirical absorption correction based on
ψ scans (360°) of 9 reflections with ø > 75°.41 A total of 4840, 6878,
and 3782 observed reflections (Fo g 2.0σ(Fo)) were collected and
averaged to 3786, 6159, and 3210 unique data for [Fe(TPP)(1-BuNH2)2],
[Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2], respectively.
The structures of the three low-spin iron(II) porphyrins were solved
in the triclinic space group P1h with the Patterson vector superposition
procedure of SHELXS-9342 as implemented in the SHELX-9743a suite
of programs. The iron atom of [Fe(TPP)(BzNH2)2] was located at a
general position; the iron atoms of [Fe(TPP)(1-BuNH2)2] and [Fe(TPP)-
(PhCH2CH2NH2)2] were located at a center of inversion (the unit cell
origin). Difference Fourier syntheses were used to locate the remaining
non-hydrogen atoms. The structures were refined anisotropically against
F2 with SHELXL-97.43a In each case, a final difference Fourier synthesis
led to location of all hydrogens atoms, including those of the
coordinated amine nitrogens. All were included as idealized contributors
in the least-squares process with standard SHELXL-97 idealization
parameters. The final refinements of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)-
(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] converged to the discrep-
ancy indices listed below. The maximum (and minimum) electron
densities on the final difference Fourier maps of [Fe(TPP)(1-BuNH2)2],
[Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] were 0.292
(-0.298), 0.379 (-0.312), and 0.216 (-0.253) e/Å3, respectively.
Complete crystallographic details, fractional atomic coordinates for
all non-hydrogen atoms, anisotropic thermal parameters, fixed hydrogen
atom coordinates, bond lengths, bond angles, and dihedral angles for
[Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2-
NH2)2] are given in the Supporting Information (Tables S1-S21).
[Fe(TPP)(1-BuNH2)2]: C52H50FeN6, fw ) 814.83 amu, a ) 10.118-
(10) Å, b ) 11.086(14) Å, c ) 11.205(3) Å, R ) 94.15(4)°, â ) 105.62-
(5)°, γ ) 113.88(6)°, V ) 1083.1(18) Å3, triclinic, P1h, Z ) 1, Dc )
1.249 g cm-3, µ ) 0.391 mm-1, T ) 293(2) K, R1 (wR2)44 ) 0.0401
(0.0985) for 3193 unique data with I > 2σ (I), R1 (wR2) ) 0.0541
(0.1108) for all 3786 data (Rint ) 0.0176).
Synthesis of [Fe(TPP)(1-BuNH2)2]. To [Fe(TPP)Cl] (150 mg, 0.213
mmol) and silver triflate (65.7 mg, 0.256 mmol) in a two-neck 100-
mL round bottom flask under nitrogen was added 20 mL of freshly
distilled THF. The solution was stirred for ∼15 h at room temperature
prior to removing the solvent in vacuo. The red-brown solid was
redissolved in dichloromethane (20 mL) and cannula-filtered in four
aliquots (∼5 mL, 53 µmol of [Fe(TPP)(OSO2CF3)]) into four Schlenk
tubes, each containing 600 µL (6.07 mmol) of 1-butylamine. The
solutions changed color from red-brown to deep red on swirling; each
was layered with hexane and set aside for crystallization. X-ray-quality
crystals were isolated after 4 days by filtration and washed with hexane.
Isolated yield: 84.5 mg, 49%. Anal. Calcd for C52H50N6Fe: C, 76.65;
H, 6.19; N, 10.32. Found: C, 76.37; H, 5.98; N, 10.51. IR (KBr
pellet): 3381 cm-1 (m, ν(N-H)), 1537 cm-1 (m, δ(NH2)). UV-vis
(CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 426 (235 × 103), 496 (4.41 × 103),
532 (19.8 × 103), 563 (5.28 × 103).
Synthesis of [Fe(TPP)(PhCH2CH2NH2)2]. To [Fe(TPP)Cl] (147 mg,
0.209 mmol) and silver triflate (64.9 mg, 0.253 mmol) in a two-neck
100-mL round-bottom flask under nitrogen was added 40 mL of freshly
distilled THF. The solution was stirred for 12 h at room temperature
prior to removing the solvent in vacuo. The red-brown solid was
redissolved in dichloromethane (∼16 mL) and cannula-filtered into a
50-mL two-neck round bottom flask to which 1.3 mL (10.5 mmol) of
freshly-distilled PhCH2CH2NH2 was added under nitrogen. Caution!
Phenethylamine is a toxic (possible nerVous system sensitizer), corrosiVe
compound and should be handled under strictly anaerobic conditions
in a fume hood. The solution turned from red-brown to deep red on
swirling. The reaction mixture was transferred to five 25-mL Schlenk
tubes (∼3 mL aliquots) and layered with hexane. X-ray-quality crystals
were isolated after 4 days by filtration and washed with 96% ethanol
to remove colorless crystals of PhCH2CH2NH2. Isolated yield: 109
mg, 57%. Anal. Calcd for C60H50N6Fe: C, 79.11; H, 5.53; N, 9.27.
Found: C, 78.79; H, 5.45; N, 9.66. IR (KBr pellet): 1537 cm-1 (m,
δ(NH2)). UV-vis (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 426 (274 × 103),
531 (23.3 × 103), 562 (6.35 × 103).
[Fe(TPP)(BzNH2)2]: C58H46FeN6, fw ) 882.86 amu, a ) 11.742-
(5) Å, b ) 12.348(6) Å, c ) 17.404(4) Å, R ) 97.69(3)°, â ) 101.97-
(3)°, γ ) 112.16(4)°, V ) 2222.7(15) Å3, triclinic, P1h, Z ) 2, Dc )
1.319 g cm-3, µ ) 0.387 mm-1, T ) 293(2) K, R1 (wR2)44 0.0436
(0.1004) for 4702 unique data with I > 2σ (I), R1 (wR2) ) 0.0710
(0.1226) for all 6159 data (Rint ) 0.0194).
[Fe(TPP)(PhCH2CH2NH2)2]: C60H50FeN6, fw ) 910.91 amu, a )
10.9625(16) Å, b ) 11.203(3) Å, c ) 11.299(4) Å, R ) 75.23(3)°, â
) 89.12(2)°, γ ) 60.419(17)°, V ) 1156.8(5) Å3, triclinic, P1h, Z ) 1,
Dc ) 1.308 g cm-3, µ ) 0.374 mm-1, T ) 293(2) K, R1 (wR2)44
)
Synthesis and Attempted Crystallization of [Fe(TPP)(R-[+]-r-
MeBzNH2)2]. The reaction was carried out as above with [Fe(TPP)Cl]
(150 mg, 0.213 mmol), silver triflate (62 mg, 0.24 mmol), and excess
R-[+]-R-methylbenzylamine (∼2 mL). The solution turned deep red
following the addition of the amine. The visible spectrum in CH2Cl2
showed bands at 530 and 564 nm, consistent with reduction of the
metal to the ferrous state. Attempts to grow single crystals of [Fe-
(TPP)(R-[+]-R-MeBzNH2)2] from several solvents were unsuccessful;
crystals of [Fe(TPP)]2O were obtained after prolonged periods (>7
days).
Synthesis and Attempted Crystallization of [Fe(TPP)(i-PrNH2)2].
The reaction was carried out as before with [Fe(TPP)Cl] (142 mg, 0.202
mmol), silver triflate (68.5 mg, 0.267 mmol), and excess isopropylamine
(∼4 mL). Reduction of the metal to the ferrous state was observed on
swirling the reaction mixture. Attempts to grow single crystals of [Fe-
(TPP)(i-PrNH2)2] from CH2Cl2/hexane were unsuccessful.
0.0319 (0.0806) for 2859 unique data with I > 2σ(I), R1 (wR2) ) 0.0405
(0.0890) for all 3210 data (Rint ) 0.0122).
Molecular Mechanics Calculations. These were performed on an
IBM-compatible computer with HyperChem 5.02 (MM+ force field).45
Porphyrin force field parameters were taken from our published set
(41) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, A24, 351.
(42) Sheldrick, G. M.; Dauter, Z.; Wilson, K. S.; Hope, H.; Sieker, L. C.
Acta Crystallogr., Sect. D 1993, D49, 18-23.
(43) (a) SHELX-97: Sheldrick, G. M. J. Appl. Crystallogr., manuscript in
preparation. (b) Oscail and ORTEX V7e 1999: P. McArdle, Crystal-
lography Centre, Chemistry Department, NUI Galway, Ireland (McAr-
dle, P. J. Appl. Crystallogr. 1995, 28, 65). (c) Ortep-3 for Windows
V1.01â: Louis J. Farrugia, Department of Chemistry, University of
Glasgow, Glasgow G12 8QQ, Scotland, 1998. (d) ORTEP III: Burnett,
M. N.; Johnson, C. K. Oak Ridge National Laboratory Report ORNL-
6895, 1996.
X-ray Structure Determinations. Crystals of [Fe(TPP)(1-BuNH2)2],
[Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] were purple-black
six-sided (0.43 × 0.35 × 0.18 mm), dark red eight-sided (0.35 × 0.30
× 0.15 mm), and dark red seven-sided (0.62 × 0.27 × 0.27 mm)
rhombs, respectively. X-ray diffraction data were collected on an Enraf-
Nonius CAD4 diffractometer at 293(2) K with graphite-monochromated
Mo KR radiation (λh ) 0.717 03 Å). Intensities of all reflections were
reduced using Lorentz and polarization correction factors; the data were
(44) R1 ) ∑|Fo| - |Fc|/∑|Fo| and wR2 ) {∑[w(Fo2 - Fc )2]/∑[wFo ]}1/2
.
2
4
R factors R1 are based on F, with F set to zero for negative F2. The
criterion of F2 > 2σ(F2) was used only for calculating R1. R factors
based on F2 (wR2) are statistically about twice as large as those based
on F.
(45) HyperChem 5.02: Hypercube, Inc., 1115 NW 4th St., Gainsville, FL
32601-4256. Other programs used in this study: (a) AXUM, Technical
Graphics and Data Analysis, V. 3.0. TriMetrix Inc., 444 NE Ravenna
Boulevard, Suite 210, Seattle, WA 98115. (b) Corel Draw 8. Corel
Corp., 1600 Carling Ave., Ottawa, Ontario, Canada K1Z 8R7.
also corrected for absorption ([Fe(TPP)(1-BuNH2)2], µ ) 0.390 mm-1
[Fe(TPP)(BzNH2)2], µ ) 0.387 mm-1; [Fe(TPP)(PhCH2CH2NH2)2], µ
;

Primary Amine Complexes of Iron(II) Porphyrins
Inorganic Chemistry, Vol. 38, No. 21, 1999 4727
for low-spin iron(III) porphyrins;45-48 these were used in conjunction
with new bond stretching, angle bending, and dihedral angle parameters
for bis(amine) low-spin iron(II) derivatives.49 Input structures were
either the X-ray structures of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2],
[Fe(TPP)(PhCH2CH2NH2)2], and [Fe(TPP)(Pip)2]37 (orthogonalized
coordinates) or idealized structures with planar core conformations. A
root mean square gradient termination cutoff of 0.004 kcal/(Å mol)
was used for geometry optimization with the Polak-Ribiere conjugate
gradient algorithm. A dielectric constant of 1.5 D was employed for
all calculations. The vacuum dielectric constant (1.0 D) was not used
because even in the gas phase some screening of intramolecular dipole-
dipole interactions occurs.50,51 Partial atomic charges were not included
in the calculations.48,52,53 Comparison of the energy-minimized and
X-ray structures of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], [Fe-
(TPP)(PhCH2CH2NH2)2], and [Fe(TPP)(Pip)2] gave acceptable rmsd’s
(bond lengths, bond angles, and dihedral angles).
mol) was used for geometry optimization with the Polak-Ribiere
conjugate gradient algorithm.
Results
Crystal Structures. The molecular structures and numbering
schemes for the crystallographically unique atoms of [Fe(TPP)-
(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2-
NH2)2] are shown in the ORTEP plots of Figure 1. [Fe(TPP)(1-
BuNH2)2] and [Fe(TPP)(PhCH2CH2NH2)2] have crystallo-
graphically required inversion symmetry, as evidenced by the
anti arrangement of the axial ligands in each case. The R-CH2
groups of the 1-butylamine ligands of [Fe(TPP)(1-BuNH2)2] are
positioned approximately over the closest Fe-Np bonds; the
dihedral angle N(1)-Fe-N(3)-C(31) measures 15.2(3)°. The
phenethylamine ligands of [Fe(TPP)(PhCH2CH2NH2)2] are
positioned directly over the closest Fe-Np bonds with a dihedral
angle to the R-CH2 groups, N(1)-Fe-N(3)-C(31), of 0.24-
(18)°. In contrast, [Fe(TPP)(BzNH2)2] lacks inversion symmetry;
the R-CH2 groups of the two axial benzylamine ligands are
positioned one approximately over the nearest Fe-Np bond and
the other approximately over the nearest bisector of a cis-Np-
Fe-Np angle. The dihedral angles defining the ligand orienta-
tions relative to the porphyrin core, N(3)-Fe-N(5)-C(51) and
N(4)-Fe-N(6)-C(61), measure 18.2(4) and 30.1(4)°, respec-
tively. The meso-phenyl groups of [Fe(TPP)(1-BuNH2)2], [Fe-
(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] are slightly
to moderately tilted from the heme normal; individual Ca-Cm-
Cp-Cp angles range from 70.8 to 89.8° (Table 1).
Formal diagrams of the porphinato cores of [Fe(TPP)(1-
BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2]
are shown in Figure 2; the perpendicular displacement of each
crystallographically unique atom from the 24-atom porphyrin
mean plane and the averaged values of the chemically unique
bond distances and angles are displayed in each case. The
individual Fe-Np bond distances and orientations of the axial
ligands relative to the Fe-Np bonds are also shown. The
porphyrin core of each derivative is approximately planar; the
individual atomic displacements are all <0.13 Å. With the
exception of the required inversion symmetry for [Fe(TPP)(1-
BuNH2)2] and [Fe(TPP)(PhCH2CH2NH2)2], no systematic pat-
tern of atomic displacements leading to a well-defined symmetry
is evident.
The average Fe-Np bond lengths for [Fe(TPP)(1-BuNH2)2],
[Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] are 1.989-
(1), 1.992(4), and 1.989(4) Å, respectively. These are somewhat
shorter than the Fe-Np distances of other bis(N-donor) low-
spin iron(II) porphyrins.1-3,5,55 The average Fe-Nax bond length
for [Fe(TPP)(BzNH2)2] is 2.043(3) Å; the unique Fe-Nax bond
lengths of [Fe(TPP)(1-BuNH2)2] and [Fe(TPP)(PhCH2CH2-
NH2)2] are 2.039(3) and 2.028(2) Å, respectively. The Nax-
Fe-Nax angles are 180.0° for [Fe(TPP)(1-BuNH2)2] and [Fe-
(TPP)(PhCH2CH2NH2)2], consistent with the crystallographically
required inversion symmetry. In contrast, the Nax-Fe-Nax angle
for [Fe(TPP)(BzNH2)2] is 176.2(1)°. The Np-Fe-Nax angles
span the range 88.1(1)-91.9(1)° for [Fe(TPP)(1-BuNH2)2], 87.3-
(1)-92.0(1)° for [Fe(TPP)(BzNH2)2], and 86.3(7)-93.8(7)° for
[Fe(TPP)(PhCH2CH2NH2)2], consistent with a modest off-axis
tilt for each alkylamine ligand. Selected bond lengths, bond
angles, and dihedral angles for [Fe(TPP)(1-BuNH2)2], [Fe(TPP)-
(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] are given in Table
1; complete listings of structural data are given in the Supporting
Information.
A crystal packing calculation for [Fe(TPP)(1-BuNH2)2] was used to
evaluate the role of intermolecular nonbonded interactions in perturbing
the orientations of the porphyrin phenyl groups and axial ligands in
this class of compounds. Specifically, a lattice subset comprising 15
molecules (30 asymmetric units) was generated from the fractional
coordinates of the X-ray structure. The centermost molecule, surrounded
by 14 invariant neighboring molecules, was chosen for geometry
optimization with fixed Fe(II) coordinates to maintain the metal ion at
its special position within the lattice. No other restraints were necessary.
Conformational surfaces for [Fe(TPP)(1-BuNH2)2] and [Fe(TPP)-
(Pip)2] were calculated by counter-rotating the axial ligands from 0 to
360° (for both φ1 and φ2) in 10° increments, producing a total of 372
starting conformations for refinement. Dihedral angles involving a
porphyrin nitrogen atom, the Fe(II) ion, a coordinated axial nitrogen,
and a ligand R-carbon (Np-Fe-Nax-CL, φ) were used to define the
axial ligand orientations.54 A maximum of 2000 least-squares cycles
with a root mean square gradient termination cutoff of 0.01 kcal/(Å
(46) Munro, O. Q.; Bradley, J. C.; Hancock, R. D.; Marques, H. M.;
Marsicano, F.; Wade, P. W. J. Am. Chem. Soc. 1992, 114, 7218-
7230.
(47) Marques, H. M.; Munro, O. Q.; Grimmer, N. E.; Levendis, D. C.;
Marsicano, F.; Pattrick, G.; Markoulides, T. J. Chem. Soc., Faraday
Trans. 1995, 91, 1741-1749.
(48) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935-954.
(49) The following parameters were developed for bis(amine) low-spin Fe-
(II) porphyrins using the X-ray structures of [Fe(TPP)(1-BuNH2)2],
[Fe(TPP)(BzNH2)2], [Fe(TPP)(PhCH2CH2NH2)2], and [Fe(TPP)-
(Pip)2]37 for parametrization. Bond deformation: bond, ks (mdyn Å-1),
l0 (Å); Np-Fe(II), 1.850, 1.922; Nax-Fe(II), 1.900, 2.002. Bond angle
deformation: angle, kθ (mdyn Å rad-2), θ0 (deg); trans-Np-Fe(II)-
Np, 0.005, 180.0; cis-Np-Fe(II)-Np, 0.200, 90.0; Np-Fe(II)-Nax,
0.300, 90.0; Nax-Fe(II)-Nax, 1.000, 180.0; Ca-Np-Fe(II), 0.700,
126.8; C(sp3)-Nax-Fe(II), 0.400, 120.0; H-Nax-Fe(II), 0.400,
109.47. Dihedral angle deformation: dihedral angle, V1, V2, V3 (kcal
mol-1); Ca-Np-Fe(II)-Np (Np-Fe(II)-Np trans), 0.000, 0.000, 0.000;
Ca-Np-Fe(II)-Np (Np-Fe(II)-Np cis), 0.000, 0.100, 0.000; Fe(II)-
Nax-C(sp3)-H, 0.000, 0.000, 0.520; Fe(II)-Nax-C(sp3)-C(sp3),
-0.200, 0.730, 0.800; Fe(II)-Nax-C(sp3)-C(sp2), 0.000, 0.000, 0.000;
Np-Fe(II)-Nax-C(sp3), 0.000, 0.000, 0.000; Np-Fe(II)-Nax-H,
0.000, 0.000, 0.000; Nax-Fe(II)-Nax-H, 0.000, 0.000, 0.000; C(sp2)-
C(sp2)-C(sp3)-Nax, 0.000, 0.000, 0.000.
(50) (a) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127. (b) Allinger, N.
L.; Yuh, Y. MM2(87). Distributed to academic users by QCPE, under
special agreement with Molecular Design Ltd., San Leandro, CA. (c)
Sprague, J. T.; Tai, J. C.; Young, Y.; Allinger, N. L. J. Comput. Chem.
1987, 8, 581.
(51) Jensen, F. Introduction to Computational Chemistry; Wiley: New
York, 1999; pp 23-25.
(52) Shelnutt, J. A.; Medforth, C. J.; Berber, M. D.; Barkigia, K. M.; Smith,
K. M. J. Am. Chem. Soc. 1991, 113, 4077-4087.
(53) The force field includes the standard MM250 bond dipoles for the C-C
and C-N bonds. All M-L bond dipoles have an assigned value of
zero.
(54) HyperChem uses stiff restraining force constants (V2 ) 250 kcal/mol)
to fix the selected dihedral angles during geometry optimization. These
are removed after convergence, and the total steric energy is determined
by a single point calculation with the normal force constants for all
dihedral angles in the molecule.
(55) The structures of [Fe(TMP)(4-CNPy)2], [Fe(TMP)(3-CNPy)2], and Fe-
(TMP)(4-MePy)2] have mean Fe-Np distances of 1.992(1), 1.988(0),
and 1.996(0) Å, respectively.5

4728 Inorganic Chemistry, Vol. 38, No. 21, 1999
Munro et al.
(TPP)(PhCH2CH2NH2)2] are similar (Figure 3); the Soret, Qv,
and Qo bands occur at ∼426, ∼532, and ∼562 nm, respectively.
The wavelengths and intensity pattern of the Q-bands (Qo less
intense than Qv, both sharp) are consistent with other low-spin
iron(II) porphyrins with meso-aryl substituents and two axial
N-donor ligands.1 The visible and UV bands show a systematic
increase in intensity from [Fe(TPP)(1-BuNH2)2] to [Fe(TPP)-
(PhCH2CH2NH2)2]; this is particularly evident for the Qv band
at 532 nm. Because of the possible lability of the six-coordinate
iron(II) species in solution, excess alkylamine ligand was used
in each case to ensure that the spectrum of the six-coordinate
derivative was obtained. Importantly, the IR spectra of all three
crystalline samples used to prepare solutions for UV-visible
spectroscopy showed no bands at 892 and 878 cm-1 due to
possible contamination by the stable iron(III) hydrolysis product
[Fe(TPP)]2O.56 Bands in the visible spectrum due to this species
(572 and 612 nm)56 are also absent.
Zero-field Mo¨ssbauer spectra recorded as a function of
temperature for [Fe(TPP)(BzNH2)2] are shown in Figure 4; the
spectra for [Fe(TPP)(1-BuNH2)2] and [Fe(TPP)(PhCH2CH2-
NH2)2] (not shown) are similar. Quadrupole splittings, isomer
shifts, and line widths at different temperatures for the three
primary amine complexes are given in Table 2 along with data
for related systems.7,32,33 The isomer shifts, δ, of the primary
amine complexes range from 0.393(1) to 0.493(1) mm/s and
show a weak temperature dependence due to the second-order
Doppler shift.57 The quadrupole splittings, ∆EQ, for the three
primary amine complexes range from 1.144(6) to 1.204(3) mm/s
and also increase marginally (∼0-0.04 mm/s) with temperature.
The spectra were adequately fitted with equivalent quadrupole
doublet component line widths, consistent with normal relax-
ation behavior for this class of low-spin iron(II) porphyrinates.57
However, the polycrystalline samples were not indefinitely stable
even when stored in a desiccator. Slow air-oxidation was
observed over a period of several months, particularly for [Fe-
(TPP)(BzNH2)2], the Mo¨ssbauer spectrum of which showed an
increase in signal intensity (2-21%) from a high-spin iron(III)
oxidation product (δ ) 0.353(6) mm/s, ∆EQ ) 0.64(1) mm/s).
Molecular Mechanics Calculations. MM-calculated and
crystallographically observed bond distances, bond angles, and
dihedral angles for [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2],
[Fe(TPP)(PhCH2CH2NH2)2], and [Fe(TPP)(Pip)2]37 are com-
pared in Tables S22-S28 of the Supporting Information. The
average difference between the calculated and observed struc-
tures is 0.011(11) Å (bond distances), 0.7(1.0)° (bond angles),
and 1.7(1.7)° (dihedral angles). This level of agreement exceeds
that obtained previously with our force field for S4-ruffled and
planar bis(imidazole)iron(III) porphyrins.48
Figure 5 compares the calculated (gas phase) and crystallo-
graphically observed structures of [Fe(TPP)(1-BuNH2)2], [Fe-
(TPP)(BzNH2)2], [Fe(TPP)(PhCH2CH2NH2)2], and [Fe(TPP)-
(Pip)2].58 The porphyrin core conformations and coordination
sphere geometries of the X-ray structures are well modeled in
the calculated structures (rmsd’s < 0.08 Å). The largest
deviations are for the axial ligand and porphyrin phenyl groups.
The meso-phenyl group orientations (Ca-Cm-Cp-Cp) average
90.0° for both the X-ray and calculated structures. However,
Figure 1. Labeled ORTEP plots (ORTEP-3)43c of the X-ray structures
of (a) [Fe(TPP)(1-BuNH2)2], (b) [Fe(TPP)(BzNH2)2], and (c) [Fe(TPP)-
(PhCH2CH2NH2)2]. Thermal ellipsoids are drawn at the 30% probability
level. Hydrogen atoms have been omitted for clarity.
(56) Fleischer, E. B.; Srivastava, T. S. J. Am. Chem. Soc. 1969, 91, 2403-
2405.
(57) Debrunner, P. G. In Iron Porphyrins; Physical Bioinorganic Chemistry
Series; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading,
MA, 1989; Part 3, pp 139-234.
(58) Each calculated structure has been fitted to the X-ray structure by
least-squares minimization of the positional differences between the
iron, porphyrin core, and axial nitrogen atoms.
Electronic and Mo1ssbauer Spectroscopy. The electronic
spectra of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe-

Primary Amine Complexes of Iron(II) Porphyrins
Inorganic Chemistry, Vol. 38, No. 21, 1999 4729
Table 1. Selected Bond Lengths, Bond Angles, and Dihedral Angles for [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and
[Fe(TPP)(PhCH2CH2NH2)2]a
(A) Bond Lengths
[Fe(TPP)(1-BuNH2)2]/
[Fe(TPP)(BzNH2)2]
[Fe(TPP)(PhCH2CH2NH2)2]
bond
length (Å)
bond
length (Å)
Fe-N(1)
Fe-N(2)
Fe-N(3)
Fe-N(4)
Fe-N(5)
Fe-N(6)
1.993(3)
1.995(3)
1.986(3)
1.994(3)
2.045(3)
2.041(3)
Fe-N(1)
Fe-N(2)
Fe-N(3)
Fe-N(1)
Fe-N(2)
Fe-N(3)
1.988(3)b
1.989(2)b
2.039(3)b
1.9920(18)c
1.9858(18)c
2.0278(18)c
(B) Bond Angles
[Fe(TPP)(1-BuNH2)2]/
[Fe(TPP)(PhCH2CH2NH2)2]
[Fe(TPP)(BzNH2)2]
angle
deg
angle
deg
N(1)-Fe-N(2)
N(1)-Fe-N(3)
N(1)-Fe-N(4)
N(1)-Fe-N(5)
N(1)-Fe-N(6)
N(3)-Fe-N(2)
N(3)-Fe-N(4)
N(3)-Fe-N(5)
N(3)-Fe-N(6)
N(4)-Fe-N(2)
N(4)-Fe-N(5)
N(4)-Fe-N(6)
N(5)-Fe-N(2)
N(6)-Fe-N(2)
N(6)-Fe-N(5)
89.73(12)
N(1)d-Fe-N(2)d
N(1)-Fe-N(2)d
N(1)d-Fe-N(3)
N(1)-Fe-N(3)
N(2)d-Fe-N(3)
N(2)-Fe-N(3)
N(1)d-Fe-N(1)
N(2)d-Fe-N(2)
N(3)d-Fe-N(3)
N(2)d-Fe-N(1)
N(2)-Fe-N(1)
N(2)d-Fe-N(3)
N(2)-Fe-N(3)
N(1)-Fe-N(3)
N(1)d-Fe-N(3)
90.22(10)b
89.78(10)b
88.06(12)b
91.94(12)b
90.75(10)
89.25(10)b
180.0b,c
179.32(11)
90.05(12)
89.33(13)
87.34(13)
90.07(12)
90.15(12)
91.32(12)
92.01(13)
179.68(12)
90.08(12)
91.71(12)
89.69(12)
88.51(12)
176.22(13)
180.0b,c
180.0b,c
90.09(7)c
89.91(7)c
89.84(7)c
90.16(7)c
93.75(7)c
86.25(7)c
(C) Dihedral Angles
[Fe(TPP)(1-BuNH2)2]/
[Fe(TPP)(PhCH2CH2NH2)2]
[Fe(TPP)(BzNH2)2]
angle
deg
angle
deg
N(1)-Fe-N(5)-C(51)
162.0(4)
N(1)d-Fe-N(3)-C(31)
-164.8(3)b
N(2)-Fe-N(5)-C(51)
-108.2(4)
-18.2(4)
72.0(4)
-120.1(4)
150.1(4)
60.1(4)
-30.1(4)
-92.0(4)
90.2(4)
N(1)-Fe-N(3)-C(31)
15.2(3)b
N(3)-Fe-N(5)-C(51)
N(2)d-Fe-N(3)-C(31)
-74.6(3)b
105.4(3)b
-89.84(18)c
90.16(18)c
0.24(18)c
-179.76(18)c
78.6(3)b
N(4)-Fe-N(5)-C(51)
N(2)-Fe-N(3)-C(31)
N(1)-Fe-N(6)-C(61)
N(2)d-Fe-N(3)-C(31)
N(2)-Fe-N(6)-C(61)
N(2)-Fe-N(3)-C(31)
N(3)-Fe-N(6)-C(61)
N(1)-Fe-N(3)-C(31)
N(4)-Fe-N(6)-C(61)
N(1)d-Fe-N(3)-C(31)
C(a2)-C(m1)-C(11)-C(12)
C(a3)-C(m1)-C(11)-C(12)
C(a4)-C(m2)-C(21)-C(22)
C(a5)-C(m2)-C(21)-C(22)
C(a6)-C(m3)-C(31)-C(32)
C(a7)-C(m3)-C(31)-C(32)
C(a1)-C(m4)-C(41)-C(42)
C(a8)-C(m4)-C(41)-C(42)
C(a2)-C(m1)-C(11)-C(12)
C(a3)-C(m1)-C(11)-C(12)
C(a1)d-C(m2)-C(21)-C(22)
C(a4)-C(m2)-C(21)-C(22)
C(a2)-C(m1)-C(11)-C(12)
C(a3)-C(m1)-C(11)-C(12)
C(a4)-C(m2)-C(21)-C(22)
C(a1)d-C(m2)-C(21)-C(22)
-99.4(3)b
-76.6(3)b
105.7(3)b
93.2(3)c
83.5(5)
-93.2(5)
-92.2(4)
92.3(5)
-72.6(5)
109.2(4)
-85.2(3)c
86.7(3)c
-94.2(3)c
a The estimated standard deviations of the least significant digits are given in parentheses. b [Fe(TPP)(1-BuNH2)2]. c [Fe(TPP)(PhCH2CH2NH2)2].
d Symmetry equivalent (-x, -y, -z).
each experimental mean has a large associated esd due to
individual orientations which deviate from 90° by as much as
26° in the case of [Fe(TPP)(Pip)2] (Table S22).59
The X-ray orientations of the axial ligands of [Fe(TPP)-
(PhCH2CH2NH2)2] (defined by the dihedral angle φ, Np-Fe-
Nax-CL) are reproduced exactly in the calculated in vacuo
structure (φ1, φ2 ) 0°, 180°). In the case of [Fe(TPP)(1-
BuNH2)2], the calculated axial ligand orientations (0 and 180°)
differed by 15.2° from the crystallographically observed orienta-
tions. The axial ligand dihedral angles were therefore restrained
at the X-ray values during geometry optimization to obtain the
closest conformational fit to the crystal structure (Figure 5). A
similarly restrained conformation of [Fe(TPP)(BzNH2)2] was
used for comparison with the X-ray structure since the calculated
minimum energy orientations of the axial ligands (φ1, φ2 ) 0,
90°; isoenergetic with 0, 180°) differed significantly from the
crystallographically observed orientations (18.2 and 30.1°). No
restraints were required to fit the X-ray conformation of [Fe-
(59) The calculated (gas phase) mean phenyl group orientations of the axial
benzylamine and phenethylamine ligands of [Fe(TPP)(BzNH2)2] and
[Fe(TPP)(PhCH2CH2NH2)2] differ from the X-ray orientations by 8.9-
(21) and 12.8(9)°, respectively. These differences are consistent with
crystal packing effects on the X-ray conformations.
(60) Quinn, R.; Strouse, C. E.; Valentine, J. S. Inorg. Chem. 1983, 22,
2934-3940.
(61) Little, R. G.; Dymock, K. R.; Ibers, J. A. J. Am. Chem. Soc. 1975,
97, 4532-4539.

4730 Inorganic Chemistry, Vol. 38, No. 21, 1999
Munro et al.
Figure 3. Electronic spectra of [Fe(TPP)(1-BuNH2)2] (52.6 µM, a),
[Fe(TPP)(BzNH2)2] (60.6 µM, b), and [Fe(TPP)(PhCH2CH2NH2)2] (66.1
µM, c) recorded in dry CH2Cl2 solution at 25(1) °C under nitrogen.
The free ligand (RNH2) concentrations are 0.81 M for [Fe(TPP)(1-
BuNH2)2], 0.29 M for [Fe(TPP)(BzNH2)2], and 0.38 M for [Fe(TPP)-
(PhCH2CH2NH2)2].
Figure 2. Formal diagrams of the porphyrin cores of (a) [Fe(TPP)-
(1-BuNH2)2], (b) [Fe(TPP)(BzNH2)2], and (c) [Fe(TPP)(PhCH2CH2-
NH2)2]. Averaged values (and their esd’s) of the chemically unique
bond distances (in Å) and angles (in deg) are shown. The perpendicular
displacements (in units of 0.01 Å) of the iron and 24 porphyrin core
atoms from the porphyrin mean plane are also displayed. The dihedral
angle (deg) of the above-plane axial ligand (Np-Fe-Nax-CL) is
indicated by the solid line in each diagram. The dashed line for [Fe-
(TPP)(BzNH2)2] gives the unique dihedral angle of the below-plane
ligand.
(TPP)(Pip)2]; the calculated minimum energy conformation (φ1,
φ2 ) 70, 110°) matches the X-ray structure and has exact
inversion symmetry.
[Fe(TPP)(1-BuNH2)2] was used to evaluate crystal packing
effects for this class of compounds. The differences between
the phenyl group and axial ligand orientations of the calculated
(in vacuo) and X-ray structures were significantly minimized
when the geometry optimization was performed on a single
molecule within its crystal lattice environment. Specifically, a
markedly improved fit (rmsd ) 0.026 Å) of the calculated and
observed conformations was obtained if a complete set of
neighboring molecules was included in the calculation. Figure
6 shows the 14 invariant structures and the geometry optimized
conformation of [Fe(TPP)(1-BuNH2)2] calculated by this method.
The “lattice” conformation was 1.6 kcal/mol higher in energy
than the gas phase conformation and had similar Fe-Nax and
Fe-Np bond distances.
Figure 4. Zero-field Mo¨ssbauer spectra of [Fe(TPP)(BzNH2)2] taken
at 297, 50, and 5 K. The solid lines are least-squares fits of the data to
the sum of two inequiValent quadrupole doublets.
Figure 7 compares conformational energy surfaces for [Fe-
(TPP)(1-BuNH2)2] and [Fe(TPP)(Pip)2] as plots of the change

Primary Amine Complexes of Iron(II) Porphyrins
Inorganic Chemistry, Vol. 38, No. 21, 1999 4731
Table 2. Mo¨ssbauer Data for Bis(amine)iron(II) Porphyrin Complexesa
complex
[Fe(TPP)(BzNH2)2]d
T (K)
δ (mm/s)b
∆EQ (mm/s)
Γ (mm/s)c
0.16
0.16
0.17
0.10
0.13
0.18
0.13
5
50
80
297
80
297
80
297
78
78
78
78
78
78
78
78
78
78
0.493(1)
0.478(2)
0.480(1)
0.400(2)
0.436(2)
0.401(2)
0.439(2)
0.393(1)
0.47(1)
0.47(3)
0.45(2)
0.47(1)
0.52(1)
0.49(1)
0.48(1)
0.48(1)
0.48(2)
0.52(1)
0.49(1)
0.51(1)
0.41(1)
0.51(1)
0.50(1)
0.47(1)
0.42(1)
1.162(2)
1.169(2)
1.175(2)
1.204(3)
1.144(6)
1.151(3)
1.147(4)
1.154(4)
1.08(1)
1.09(1)
1.07(1)
1.09(1)
1.15(1)
1.09(1)
1.03(1)
1.09(1)
1.03(1)
1.40(2)
1.11(1)
1.10(1)
1.18(1)
1.44(1)
1.44(1)
1.49(1)
1.52(1)
[Fe(TPP)(1-BuNH2)2]d
[Fe(TPP)(PhCH2CH2NH2)2]d
0.15
[Fe(PPIX)(CH3NH2)2]e
[Fe(PPIX)(EtNH2)2]e
0.17(1)
0.17(1)
0.18(1)
0.18(1)
0.15(1)
0.22(1)
0.17(1)
0.13(1)
0.13(2)
0.16(1)
0.31(1), 0.32(1)g
0.30(1), 0.30(1)g
0.23(1), 0.28(1)g
[Fe(PPIX)(Et2NH)2]e
[Fe(PPIX)(HO(CH2)2NH2)2]e
[Fe(PPIX)(H2N(CH2)2NH2)2]e
[Fe(PPIX)(n-PrNH2)2]e
[Fe(PPIX)(n-BuNH2)2]e
[Fe(PPIX)(sec-BuNH2)2]e
[Fe(PPIX)(n-octylamine)2]e
[Fe(PPIX)(Pip)2]e
[Fe(OEP)(NH3)2]f
4.2
115
295
4.2
77
195
300
[Fe(TPP)(Pip)2]h
a The estimated errors of the least significant digits are given in parentheses. b Isomer shifts are relative to metallic iron. c Half-width at half-
maximum. d This work. Source line width ) 0.16 mm/s. The sample line widths were fixed at the tabulated values. e Reference 33. f Reference 7.
g Full width at half-maximum. h Reference 31.
in total steric energy with axial ligand orientation. (Alternative
plots of -∆UT with ligand dihedral angle are given in Figure
S1 to emphasize the energy minima.) The surface for [Fe(TPP)-
(1-BuNH2)2] exhibits numerous isoenergetic minima and maxima.
Two distinct types of high-energy conformation are evident:
(1) conformations in which the axial ligands are eclipsed and
oriented over the bisector of a cis-Np-Fe-Np angle, e.g., φ1,
φ2 ) 45, 315° (∆UT ) 0.80 kcal/mol) and (2) those with
staggered axial ligands (relative orientations, ∆φ, of 90 or 180°)
positioned over the bisector of a cis-Np-Fe-Np angle, e.g., φ1,
φ2 ) 45, 225° (∆UT ) 0.69 kcal/mol). In both cases the R-CH2
protons of the butylamine ligands point directly at adjacent
pyrrole nitrogens (H‚‚‚Np ) 2.70 Å). Two distinct classes of
low-energy conformation for [Fe(TPP)(1-BuNH2)2] are also
evident in Figure 7. The lowest energy conformers (∆UT ) 0
kcal/mol) have staggered axial ligands (∆φ ) 90 or 180°)
positioned directly over a cis or trans pair of Fe-Np bonds,
e.g., φ1, φ2 ) 0, 180°. Local minima (∆UT ) 0.14 kcal/mol)
occur when the axial ligands are exactly eclipsed (∆φ ) 0°)
and lie directly over a single Fe-Np bond, e.g., φ1, φ2 ) 0, 0°.
These conformations stagger the R-CH2 protons of the ligands
relative to the pyrrole nitrogens (H‚‚‚Np ) 2.84 Å).
The surface for [Fe(TPP)(Pip)2] (Figure 7) shows two types
of high-energy conformation depending on the relative orienta-
tions of the axial piperidine ligands. The changes in total steric
energy with axial ligand orientation are also considerably larger
(∆UTmax ∼3.8 kcal/mol) than for [Fe(TPP)(1-BuNH2)2], con-
sistent with the increased steric bulk of the axial ligands. The
highest energy conformations (∆UT ∼ 3.8 kcal/mol, e.g., φ1,
φ2 ) 110, 70°) have exact inversion symmetry; the N-H bonds
of the axial piperidine ligands eclipse a pair of trans-Fe-Np
bonds leading to Np-Fe-Nax-CL dihedral angles of ∼20°
relative to the closest Fe-Np vectors. Local energy maxima
(∆UT ∼3.0 kcal/mol) occur when the N-H bonds of the axial
piperidine ligands eclipse a pair of cis-Fe-Np bonds, e.g., φ1,
φ2 ) 20, 70°. For both types of maximum, the equatorial pairs
of R-CH2 protons of the ligands directly eclipse a pair of trans
porphyrin nitrogens, leading to short nonbonded contacts (H‚‚
‚Np ) 2.43 Å) and a high steric energy.
Three distinct types of low-energy conformation are evident
for [Fe(TPP)(Pip)2]. In the lowest energy conformations (∆UT
) 0 kcal/mol, e.g., φ1, φ2 ) 70, 20°) the N-H bonds of the
axial ligands are staggered (90° apart) and are positioned over
the bisectors of adjacent cis-Np-Fe-Np angles. The first type
of local minimum (∆UT ∼1.6 kcal/mol, e.g., φ1, φ2 ) 160, 20°)
has exact inversion symmetry; the N-H bonds of the axial
ligands are staggered (180° apart) and eclipse the bisector of a
cis-Np-Fe-Np angle. In the second type of local minimum
(∆UT ∼1.8 kcal/mol, e.g., φ1, φ2 ) 340, 20°) the N-H bonds
of the axial ligands are exactly eclipsed and are positioned over
the bisector of a cis-Np-Fe-Np angle. For all minimum energy
conformations, the axial pairs of R-CH2 protons of the ligands
point toward a pair of cis porphyrin nitrogens, favoring longer
nonbonded contacts (H‚‚‚Np 2.72 Å) than for the higher energy
conformations.
Discussion
Molecular Structures. The structures of [Fe(TPP)(1-
BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2]
are unique in several respects. Most importantly, they are the
first X-ray structures of primary amine complexes of iron(II)
porphyrins and provide unprecedented stereochemical data for
the coordination of RNH2 ligands by simple iron porphyrins.
They also serve as a useful starting point from which to explore
biologically relevant amine complexes of iron porphyrins. The
unusual heme axial ligand combination of the plant cytochromes
f (His-Fe-NH2R, where NH2R is the R-NH2 group of Tyr-1)
is of particular interest in this context since the recently reported
X-ray structure of turnip cytochrome f21,22 has established a
definitive structural role for the alkylamine ligand. Specifically,
heme incorporation and coordination of the R-NH2 group of
Tyr-1 is thought to occur after (1) translocation of the protein

4732 Inorganic Chemistry, Vol. 38, No. 21, 1999
Munro et al.
Figure 6. (a) Perspective view of the calculated structure of [Fe(TPP)-
(1-BuNH2)2] (solid lines) within its lattice environment. The coordinates
of the Fe(II) ion and all atoms of the 14 neighboring complexes were
fixed during geometry optimization of the highlighted molecule. (b)
Least-squares fit of the calculated (solid lines) and X-ray structure
(broken lines) of [Fe(TPP)(1-BuNH2)2] for the calculation involving
14 lattice neighbors. The rmsd (Fe, 24 porphyrin core atoms, axial
nitrogens) is 0.026 Å. Hydrogen atoms have been omitted for clarity
in both diagrams.
the folding process which culminates in the redox-active tertiary
structure of the principal domain of the functional cytochrome.21
The Fe-Nax bonds for [Fe(TPP)(1-BuNH2)2] (2.043 Å), [Fe-
(TPP)(BzNH2)2] (2.039 Å), and [Fe(TPP)(PhCH2CH2NH2)2]
(2.028 Å) are identical (within four standard deviations) and
average 2.037(8) Å. There are no other structurally characterized
primary amine complexes in the literature for comparison.
However, it is noteworthy that the mean Fe-NH2R distance is
significantly shorter than the Fe-Nax distance of [Fe(TPP)(Pip)2]
(2.127 Å), consistent with the fact that R-unsubstituted primary
amines are sterically less bulky than secondary amines. The
mean Fe-Nax distance for the three [Fe(TPP)(RNH2)2] com-
plexes of this study is equivalent to the axial distances observed
for the low-spin bis(pyridine) derivatives [Fe(TPP)(Py)2] (2.037
Å)3 and [Fe(TPP)(Py)2]‚2Py (2.039 Å).2 However, this agree-
ment is probably coincidental since coordinated pyridines and
imidazoles with a range of electronic structures show a
substantial variation in their Fe-Nax distances: 1.996 Å ([Fe-
(TMP)(4-CNPy)2]),5 2.004 Å ([Fe(TPP)(1-VinIm)2]),1 2.010 Å
([Fe(TMP)(4-MePy)2]),5 and 2.026 Å ([Fe(TMP)(3-CNPy)2]).5
Interestingly, the Fe-Nax distances of [Fe(TPP)(1-BuNH2)2],
[Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] are all
Figure 5. Comparison of MM-calculated (gas phase, solid lines) and
crystallographically observed (broken lines) structures of low-spin bis-
(amine)iron(II) porphyrins. The X-ray structures to which the calculated
structures have been fitted (and rmsd’s) are (a) [Fe(TPP)(1-BuNH2)2]
(0.042 Å), (b) [Fe(TPP)(BzNH2)2] (0.058 Å), (c) [Fe(TPP)(PhCH2CH2-
NH2)2] (0.032 Å), and (d) [Fe(TPP)(Pip)2] (0.073 Å). Hydrogen atoms
have been omitted for clarity.
from the chloroplast stroma across the thylakoid membrane and
(2) cleavage of the leader segment of the polypeptide chain.
This event (heme incorporation) therefore marks the onset of

Primary Amine Complexes of Iron(II) Porphyrins
Inorganic Chemistry, Vol. 38, No. 21, 1999 4733
significantly longer than the Fe-NH2R distance (1.94 Å)
reported for the 1.96-Å resolution X-ray structure of turnip
ferrocytochrome f.22 This is unexpected since the axial ligand
in cytochrome f is an R-substituted primary amine and is
therefore sterically more hindered than those of this study.
Moreover, our calculations have indicated a somewhat weaker
Fe-Nax interaction in the gas phase structure of [Fe(TPP)(R-
[+]-R-MeBzNH2)2] (Figure S2), which shows equivalent Fe-
Nax distances (2.052 Å) that are some 0.013 Å longer than those
calculated for [Fe(TPP)(BzNH2)2].
The Fe-NHis distance (1.93 Å) of turnip cytochrome f is also
unusually short for an iron(II) porphyrin and lies closer to that
found for imidazolate complexes of iron(III) porphyrins such
as [Fe(TPP)(5-MeIm)2]- (mean Fe-Nax ) 1.943(21) Å60). A
reasonable suggestion is that the tertiary structure of the protein
coupled with the S4-ruffled porphyrin core probably enforces a
strong coordination interaction for the axial ligands in cyto-
chrome f. The functional significance of this enhanced interac-
tion is unclear. However, it is worth noting that even if the
coordination distances for ferrocytochrome f are considered to
be accurate to (0.05 Å at 1.96-Å resolution, the Fe-Nax
distances are still significantly shorter than those found for the
[Fe(TPP)(RNH2)2] complexes of this study and other bis-
(imidazole)iron(II) porphyrins.1
The axial ligand orientations of alkylamine complexes of iron
porphyrins may be defined by the dihedral angles Np-Fe-Nax-
CL, where CL is an R-CH2 carbon. For [Fe(TPP)(1-BuNH2)2],
[Fe(TPP)(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2], the axial
ligand dihedral angles range from ∼0 to 30.1°. The conformation
of [Fe(TPP)(PhCH2CH2NH2)2] is “unusual” since the axial
ligands exactly eclipse a pair of trans-Fe-Np bonds (φ1, φ2 ≈
0, 180°). Such a conformation is rarely observed60,61 in bis-
(pyridine) and bis(imidazole) complexes of iron(II/III) porphy-
rinates. This reflects the unfavorable steric interactions that arise
when the ligand ortho protons point directly at the pyrrole
nitrogens. However, in the case of R-unsubstituted primary
amines, the R-CH2 protons point away from the pyrrole
nitrogens when the R-carbon and pyrrole nitrogens are eclipsed.
As illustrated in Figure 8, this conformation is clearly favored
on steric grounds, a finding that is supported by the MM
calculations of Figures 5 and 7 (vide supra).
The fact that neither [Fe(TPP)(1-BuNH2)2] nor [Fe(TPP)-
(BzNH2)2] exhibits the same axial ligand orientations as [Fe-
(TPP)(PhCH2CH2NH2)2] suggests an intrinsically low barrier
to axial ligand rotation for R-unsubstituted primary amines.
Clearly, packing interactions could easily affect the axial ligand
(and porphyrin phenyl group) orientations in the solid state. The
role of packing interactions is, in fact, strikingly demonstrated
by the conformation of [Fe(TPP)(1-BuNH2)2] calculated in the
presence of 14 lattice neighbors (Figure 6). First, there is good
agreement between the calculated and observed dihedral angles
for the meso-phenyl groups (none deviate from the experimental
values by more than 3°). This confirms the well-known62 role
of intermolecular nonbonded interactions in perturbing the
phenyl group orientations of TPP derivatives. Second, in contrast
to the gas phase calculation, the axial ligands no longer have a
minimum energy orientation of 0° (φ ) 7° in Figure 6).
Although the calculated ligand orientations do not exactly match
those of the X-ray structure (15.2°), the results clearly show
that both the axial ligands and the meso-phenyl groups require
some adjustments (rotations of up to ∼15°) from their in vacuo
orientations before optimal packing is achieved.
Figure 7. Plot of the change in steric energy (∆UT) as a function of
axial amine orientation for [Fe(TPP)(1-BuNH2)2] and [Fe(TPP)(Pip)2].
A contour map of the three-dimensional surface is shown in each case;
broken lines indicate regions encompassing minima on each surface.
φ1 and φ2 correspond to the dihedral angles Np-Fe(II)-Nax-CL for
the top and bottom ligands, respectively.
(62) Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1-70.

4734 Inorganic Chemistry, Vol. 38, No. 21, 1999
Munro et al.
Figure 8. ORTEP diagram (ORTEX 7e)43b of [Fe(TPP)(PhCH2CH2-
NH2)2] viewed down the N(3)-Fe-N(3)′ axis (approximately perpen-
dicular to the heme plane). Only the NH2 and R-CH2 groups of each
ligand are shown for clarity. Thermal ellipsoids are drawn at the 30%
probability level. Selected nonbonded contacts are indicated by broken
lines.
The nonbonded interactions which perturb the axial ligand
orientations of [Fe(TPP)(1-BuNH2)2] have been quantified in
Figure 9 which shows a stereoview of the unit cell and all atoms
within a 5.0-Å radius of C(32). The relevant nonbonded contacts
between phenyl group 2 of a neighboring porphyrin and the
axial ligand are the following: H(32a)‚‚‚C(24), 3.248 Å; H(32a)‚
‚‚C(25), 3.545 Å; H(31a)‚‚‚C(25), 3.439 Å; H(3a)‚‚‚C(25), 3.745
Å. The close proximity of this neighboring phenyl group to one
side of the axial ligand clearly offsets the orientation of the
ligand from its gas phase minimum (0°). A similar analysis of
the packing interactions which affect the axial ligand orientations
of [Fe(TPP)(BzNH2)2] is given in Figures S3-S5.
The porphyrin cores of the three [Fe(TPP)(RNH2)2] deriva-
tives are roughly planar. Although the individual atomic
displacements are small (<0.13 Å), there is evidence for slight,
local distortions that reflect accommodation of specific axial
ligand-porphyrin core nonbonded interactions. For example,
C(a6) and C(b6) of [Fe(TPP)(BzNH2)2] (Figure 2b) are displaced
below the porphyrin mean plane, while N(4), C(a7), and C(b7)
are displaced above the porphyrin mean plane, consistent with
the orientations of the above-plane ligand (30.1° relative to the
Fe-N(3) bond) and the below-plane ligand (18.2° relative to
the Fe-N(4) bond), respectively. Local distortions of this type
have been noted previously for [Fe(TMP)(1,2-Me2Im)2)]-
(ClO4).48
Figure 9. (a) Stereoview of the unit cell of [Fe(TPP)(1-BuNH2)2]. The
axial butylamine ligands come into close contact with a meso-phenyl
group of a neighboring porphyrin. (b) ORTEP diagram (30% probability
surfaces for non-hydrogen atoms) showing all atoms within a 5.0-Å
radius of C(32). Selected atoms have been labeled. The close nonbonded
contacts which offset the ligand dihedral angle (N(1)-Fe-N(3)-C(31))
from 0° (gas-phase orientation) to 15.2° mainly involve C(24) and C(25)
of the neighboring phenyl group and H(32a) of the ligand.
reflects the fact that many are S4-ruffled.62 In contrast, all known
[FeII(porphyrin)L2] complexes, where L ) an amine, imidazole,
or pyridine derivative, are planar. Since the average Fe-Np
distance for [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe-
(TPP)(PhCH2CH2NH2)2] is 1.990(2) Å, the Fe-Np distances
for bis(primary amine)iron(II) porphyrinates appear to be
intrinsically shorter than those of comparable bis(imidazole) and
bis(pyridine) ferrous complexes. However, this may be due to
the small sample size for the three classes of compound,
particularly since the mean Fe-Np distance of [Fe(TPP)(Pip)2]
is 2.004(4) Å.37
Electronic and Mo1ssbauer Spectroscopy. The electronic
spectra of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe-
(TPP)(PhCH2CH2NH2)2] confirm the crystallographically ob-
The X-ray structures of several bis(imidazole)- and bis-
(pyridine)iron(II) porphyrins exhibit an average Fe-Np distance
(2.002 Å)1-3 that is only 0.023 Å longer than the average in-
plane distance for the low-spin iron(III) analogs (1.979
Å).48,60,61,63-71 This reflects the similar iron antibonding orbital
populations (<0.6 e)8,9 for the two oxidation states, even though
iron(II) has a slightly larger radius than iron(III).72 However, it
is noteworthy that, in addition to the smaller radius for iron-
(III), the shorter mean Fe-Np distance for the ferric complexes
(65) Scheidt, W. R.; Osvath, S. R.; Lee, Y. J. J. Am. Chem. Soc. 1987,
109, 1958-1963.
(66) Higgins, T.; Safo, M. K.; Scheidt, W. R. Inorg. Chim. Acta 1990,
178, 261-267.
(67) Scheidt, W. R.; Kirner, J. F.; Hoard, J. L.; Reed, C. A. J. Am. Chem.
Soc. 1987, 109, 1963-1968.
(68) Inniss, D.; Soltis, S. M.; Strouse, C. E. J. Am. Chem. Soc. 1988, 110,
5644-5650.
(69) Quinn, R.; Valentine, J. S.; Byrn, M. P.; Strouse, C. E. J. Am. Chem.
Soc. 1987, 109, 3301-3308.
(70) Collins, D. M.; Countryman, R.; Hoard, J. L. J. Am. Chem. Soc. 1972,
94, 2066-2072.
(63) Safo, M. K.; Gupta, G. P.; Walker, F. A.; Scheidt, W. R. J. Am. Chem.
Soc. 1991, 113, 5497-5510.
(64) Safo, M. K.; Gupta, G. P.; Walker, F. A.; Watson, C. T.; Simonis,
U.; Scheidt, W. R. J. Am. Chem. Soc. 1992, 114, 7066-7075.
(71) Safo, M. K.; Walker, F. A.; Raitsimring, A. M.; Walters, W. P.; Dolata,
D. P.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem. Soc. 1994, 116,
7760-7770.
(72) Scheidt, W. R.; Reed, C. A. Chem. ReV. 1981, 81, 543-555.

Primary Amine Complexes of Iron(II) Porphyrins
Inorganic Chemistry, Vol. 38, No. 21, 1999 4735
served low-spin Fe(II) oxidation state. Although the spectra of
the three derivatives show Q and B band maxima at the same
wavelengths (562, 532, and 426 nm), the molar absorptivities
increase in the order [Fe(TPP)(1-BuNH2)2] < [Fe(TPP)-
(BzNH2)2] < [Fe(TPP)(PhCH2CH2NH2)2] at 532 and 426 nm.
The enhancement of oscillator strength for the Q and B bands
may reflect an exciton interaction73 between the transition
dipoles of the phenyl groups of the axial ligands and the xy-
polarized transition dipoles of the porphyrin ring.74 Coupling
of the transition dipoles is expected to be largest in the PhCH2-
CH2NH2 complex since the phenyl substituents of the axial
ligands are closer to being parallel with the heme group, at least
in the solid state (Figure 1). The wavelengths of the Q and B
band maxima for the three primary amine complexes compare
favorably with those reported for [Fe(TPP)(1-VinIm)2] and [Fe-
(TPP)(1-BzlIm)2] (420-427 nm, B(0,0); 532-537 nm, Qv;
562-566 nm Qo)1 and the bis(pyridine) derivatives [Fe(TMP)-
(4-CNPy)2], [Fe(TMP)(3-CNPy)2], and [Fe(TMP)(4-MePy)2].5
orientations and conformations for low-spin iron(II) complexes
of the type [Fe(TPP)L2], where L ) a primary or secondary
amine, and to use this information to delineate the factors which
control the axial ligand orientations of [Fe(TPP)(1-BuNH2)2],
[Fe(TPP)(BzNH2)2], [Fe(TPP)(PhCH2CH2NH2)2], and [Fe(TPP)-
(Pip)2].37
The conformational energy surface for [Fe(TPP)(1-BuNH2)2]
shown in Figure 7 is representative of the three [Fe(TPP)-
(RNH2)2] complexes of this study. The axial ligands of the
lowest energy conformations (∆UT ) 0 kcal/mol) exactly eclipse
a pair of cis or trans Fe-Np bonds, leading to staggered
arrangements with ∆φ ) 90 or 180°, respectively (Figure S6).
From Figure 8, such a conformation clearly leads to optimal
N-H‚‚‚Np and R-C-H‚‚‚Np nonbonded distances. Local minima
(∆UT ) 0.14 kcal/mol) are observed when the axial ligands
eclipse the same Fe-Np bond in the porphyrin core. Since the
calculated in vacuo barriers to conformational interconversion
(0.34 and 0.46 kcal/mol)76 are low for [Fe(TPP)(1-BuNH2)2],
considerable rotational freedom should exist both in the gas
phase and in solution (for which slightly higher barriers are
expected77). However, close nonbonded contacts in the solid
state (Figure 9) probably restrict rotation of the axial ligands in
the lattice. This is reflected by the absence of disorder for the
alkylamine ligands of this study, a situation which is not
mirrored in the analogous [Co(TPP)(RNH2)2]SbF6 compounds
which crystallize in different space groups.78
Comparison of the calculated minima in Figure 7 and the
X-ray data suggests that crystal packing effects, which perturb
the orientations of the axial ligands and meso-phenyl groups,
are manifest to a lesser or greater degree depending on the axial
ligands. Thus, the X-ray structure of [Fe(TPP)(PhCH2CH2NH2)2]
(φ1, φ2 ≈ 0, 180°) is largely unperturbed and is located virtually
at the strain energy minimum on the potential surface.79 The
axial ligands of the X-ray conformation of [Fe(TPP)(1-BuNH2)2]
(φ1, φ2 ) 15.2, 164.8°) are moderately perturbed (Figure 9).
The structure lies close to the calculated minimum (φ1, φ2 0,
180°) and is therefore only slightly higher in energy (∼0.13
kcal/mol). However, when packing-induced rotations of the
meso-phenyl groups are also taken into account (Figure 6), the
calculated conformation more closely matches the X-ray
structure and has an even higher relative energy (∼1.6 kcal/
mol). The noncentrosymmetric X-ray structure of [Fe(TPP)-
(BzNH2)2] lies furthest from a calculated energy minimum (φ1,
φ2 ) 30.1, 288.2° in Figure 7), consistent with significant
packing effects on the orientations of the axial ligands. Inspec-
tion of the unit cell for [Fe(TPP)(BzNH2)2] (Figure S3) and the
lattice environment within a 5.1-Å radius of C(52) and C(63)
of the axial ligands (Figures S4 and S5) indicates that the X-ray
conformation is stabilized by a π-π interaction between a ligand
phenyl group (C(52)-C(57)) and a phenyl ring of a neighboring
porphyrin. Moreover, short nonbonded contacts with C(63) of
the second phenyl group clearly cant the C(61)-C(62) bond
relative to the heme normal. These intermolecular interactions
The Mo¨ssbauer spectra of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)-
(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] show similar, weakly
temperature-dependent isomer shifts (δ) and quadrupole split-
tings (∆EQ) that are consistent with a low-spin Fe(II) oxidation
state57 for the heme iron (Table 2). The δ-values of the three
primary amine complexes at 80 K are, on average (0.452(2)
mm/s), equivalent to those typical of bis(imidazole)- and bis-
(pyridine)(porphinato)iron(II) derivatives (0.44(3) mm/s)1,7 and,
more expectedly, those reported for the bis(amine) complexes
of [Fe(OEP)], [Fe(TPP)], and [FeII(PPIX)].7,32,33 The total metal
s-electron density in Fe(II) porphyrins therefore appears to be
relatively insensitive to the type of axial N-donor ligand and
porphyrin ligand coordinated to the metal.
The quadrupole splittings of [Fe(TPP)(1-BuNH2)2], [Fe(TPP)-
(BzNH2)2], and [Fe(TPP)(PhCH2CH2NH2)2] average 1.16(2)
mm/s at 80 K and are within the range of ∆EQ values observed
for [Fe(TPP)(NH3)2] (1.10-1.18 mm/s)7 and the ferrous pro-
toporphyrin IX complexes (1.07-1.15 mm/s)33 in Table 2.
However, the ∆EQ values of [Fe(TPP)(Pip)2] are significantly
larger (1.44-1.52 mm/s) than those of the RNH2 complexes,
consistent with a larger difference between the Fe-Np (2.004
Å) and Fe-Nax (2.127 Å) bonds37 and, consequently, a larger
EFG at the nucleus. The slightly larger ∆EQ values for [Fe-
(TPP)(BzNH2)2] relative to [Fe(TPP)(1-BuNH2)2] and [Fe(TPP)-
(PhCH2CH2NH2)2] (Table 2) reflect the noncentrosymmetric
coordination geometry of the benzylamine derivative.75 Interest-
ingly, the ∆EQ values of several bis(pyridine)iron(II) porphy-
rinates (1.18(5) mm/s)7 are comparable to those of the [Fe-
(TPP)(RNH2)2] complexes. This is consistent with the similar
axial and equatorial coordination distances for these two classes
of (porphinato)iron(II) complex (vide supra). The ∆EQ values
of the RNH2 complexes (Table 2) are, however, larger than those
reported for several centrosymmetric bis(imidazole)iron(II)
porphyrins (1.02(3) mm/s).1 A likely explanation is that the latter
complexes show a smaller Fe-Nax/Fe-Np structural anisotropy.
Molecular Mechanics Calculations. The objectives of this
study were to determine the optimum (gas phase) axial ligand
(76) There are two types of rotational barrier for the primary amine ligands
of [Fe(TPP)(RNH2)2] derivatives. These are exemplified by the saddle
points with coordinates φ1, φ2 ) 90, 45° (0.34 kcal/mol) and φ1, φ2 )
180, 135° (0.46 kcal/mol).
(77) Whitenell, R. M.; Wilson, K. R. In ReViews in Computational
Chemistry; Lipkowitz, K. B., Boyd, D. B., Eds.; VCH: New York,
1993; Vol. IV, pp 67-148.
(73) Munro, O. Q.; Marques, H. M. Inorg. Chem. 1996, 35, 3768-3779.
(74) Eaton, W. A.; Hofrichter, J. In Methods in Enzymology, Antonini, E.,
Rossi-Barnardi, L., Chiancone, E., Eds.; Academic: New York, 1981;
Vol. 76, pp 175-261.
(75) The following structurally characterized low-spin iron(II) porphyrins
are all centrosymmetric: [Fe(TPP)(1-VinIm)2],1 [Fe(TPP)(1-BzlIm)2],1
[Fe(TPP)(Py)2],3 [Fe(TPP)(Py)2]‚2Py,2 [Fe(TMP)(4-CNPy)2],5 [Fe-
(TMP)(3-CNPy)2],5 and [Fe(TMP)(4-MePy)2].5 [Fe(TPP)(BzNH2)2] is
the first example of a noncentrosymmetric bis(N-donor)(porphinato)-
iron(II) complex.
(78) Munro, O. Q.; Shabalala, S. C.; Brown, N. J. Unpublished work.
(79) The crystal structure is not the true global minimum since the Ca-
Cm-Cp-Cp dihedral angles (Table 1) deviate from the calculated
minimum energy value of 90° by up to 4.8°. This tilting of the meso-
aryl groups in the crystal structure reflects modest intermolecular
nonbonded (packing) interactions.62

4736 Inorganic Chemistry, Vol. 38, No. 21, 1999
Munro et al.
collectively favor the noncentrosymmetric conformation even
though it has a higher energy (∼0.5 kcal/mol) than the in vacuo
minimum.
Conclusion
The structures of three novel bis(primary amine)(porphinato)-
iron(II) complexes, [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2],
and [Fe(TPP)(PhCH2CH2NH2)2], have been determined. Of
particular interest is the mean Fe-Nax distance (2.037 Å) which
is considerably longer than the Fe(II)-amine distance of turnip
cytochrome f (1.94 Å) but shorter than that of [Fe(TPP)(Pip)2]
(2.127 Å). The Mo¨ssbauer spectra of the three [Fe(TPP)-
(RNH2)2] derivatives confirm the low-spin Fe(II) oxidation state
and show comparable quadrupole splittings and isomer shifts
to several bis(pyridine)- and bis(alkylamine)iron(II) porphyrins.
MM calculations (in vacuo) indicate that the preferred
orientations of the axial ligands of [Fe(TPP)(RNH2)2] derivatives
position the ligand R-carbons directly over the pyrrole nitrogens
of the porphyrin core, thereby minimizing the axial ligand R-C-
H‚‚‚Np nonbonded interactions. Although the lowest energy
conformations of [Fe(TPP)(Pip)2] are S4-ruffled and have the
axial ligand R-carbons oriented at 20° relative to the nearest
Fe-Np bonds, this conformation also minimizes R-C-H‚‚‚Np
nonbonded contacts. MM calculations on a single [Fe(TPP)(1-
BuNH2)2] complex within its crystal lattice environment have
been used to show that intermolecular nonbonded interactions
significantly influence the observed orientations of the axial
ligands and the meso-phenyl groups in these TPP derivatives.
The surface for [Fe(TPP)(Pip)2] (Figure 7) shows more
distinct local minima and maxima than the surface for [Fe(TPP)-
(1-BuNH2)2]. This is mainly due to the increase in axial ligand
steric bulk in the bis(piperidine) derivative which leads to larger
rotational barriers for the axial ligands (e.g., ∼2.2 kcal/mol at
φ1, φ2 ) 70, 70°). The saddle point conformations of [Fe(TPP)-
(Pip)2] are characterized by a partly staggered axial ligand
arrangement in which the N-H group of one ligand eclipses
an Fe-Np bond while the N-H group of the trans ligand
eclipses the bisector of a cis-Np-Fe-Np angle in the porphyrin
core. The latter ligand orientation places the axial pair of R-CH2
protons directly over adjacent pyrrole nitrogens, leading to much
of the increase in steric energy relative to the minimum (vide
supra).
The lowest energy conformations of [Fe(TPP)(Pip)2] (e.g.,
φ1, φ2 ) 70, 20°) have S4-ruffled porphyrin cores (Figure S6)
with mean Fe-Nax and Fe-Np distances of 2.110(0) and 1.991-
(3) Å, respectively. In contrast, the calculated conformation that
best models the X-ray structure of [Fe(TPP)(Pip)2]37 (φ1, φ2 )
70, 110°) is a local minimum (∆UT ≈ 1.6 kcal/mol) with exact
inversion symmetry and a planar porphyrin core. The calculated
coordination group distances (Fe-Nax ) 2.128 Å, Fe-Np )
2.000 Å) are also considerably longer than those of the S4-ruffled
conformations. Thus, as for planar and S4-ruffled bis(imidazole)
complexes of iron(III) porphyrins,48,62 a measurable contraction
of the Fe-Np bonds is predicted for S4-ruffled iron(II) porphy-
rins.80 As noted earlier, the calculated energy minima for [Fe-
(TPP)(Pip)2] in Figure 7 reflect the fact that stable conformations
arise when the equatorial pairs of R-CH2 protons eclipse cis
porphyrin nitrogens since this leads to the least repulsive set of
nonbonded contacts. This conclusion is consistent with the MM
data recently reported for [Ni(TPP)(Pip)2] by Shelnutt and co-
workers.81 Thus, as for the primary amine derivatives (Figure
8), optimization of the R-C-H‚‚‚Np and N-H‚‚‚Np nonbonded
interactions dictates the preferred axial ligand orientations.82
Acknowledgment. We thank (1) the URF, University of
Natal, for financial support, (2) Niyum S. Ramesar for her
assistance with X-ray data collections, and (3) Professors John
S. Field and Raymond J. Haines for their helpful suggestions
(O.Q.M.). G.H. thanks the University of the Witwatersrand and
the National Research Foundation, Pretoria, for financial support.
Supporting Information Available: Complete crystallographic
details, atomic coordinates, anisotropic thermal parameters, fixed
hydrogen atom coordinates, bond lengths, bond angles, and dihedral
angles for [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2], and [Fe(TPP)-
(PhCH2CH2NH2)2] (Tables S1-S21), X-ray crystallographic files in
CIF format, comparisons of crystallographic and MM-calculated
geometric parameters for [Fe(TPP)(1-BuNH2)2], [Fe(TPP)(BzNH2)2],
[Fe(TPP)(PhCH2CH2NH2)2], and [Fe(TPP)(Pip)2] (Tables S22-S28),
and plots of -∆UT with axial ligand orientation for [Fe(TPP)(1-
BuNH2)2] and [Fe(TPP)(Pip)2], an MM-calculated structure of [Fe(TPP)-
(R-[+]-R-MeBzNH2)2], a unit cell diagram for [Fe(TPP)(BzNH2)2],
ORTEP diagrams of all atoms within a 5.1-Å radius of C(63) and C(52)
of [Fe(TPP)(BzNH2)2], and stereoscopic views of selected calculated
minima for [Fe(TPP)(1-BuNH2)2] and [Fe(TPP)(Pip)2] taken from the
surfaces in Figure 7 (Figures S1-S6). This material is available free
of charge via the Internet at http://pubs.acs.org.
(80) This prediction awaits experimental confirmation since there are
currently no X-ray structures of S4-ruffled low-spin (porphinato)iron-
(II) derivatives with N-donor axial ligands.
(81) Jia, S. L.; Jentzen, W.; Shang, M.; Song, X. Z.; Ma, J. G.; Scheidt,
W. R.; Shelnutt, J. A. Inorg. Chem. 1998, 37, 4402-4412.
(82) The crystallographically required inversion symmetry of [Fe(TPP)-
(Pip)2]37 leads to selection of a local minimum energy structure rather
than the global minimum. A more favorable lattice enthalpy presum-
ably outweighs the increase in strain energy (∼1.6 kcal/mol) of the
more symmetrical conformation.
IC990178Q