Structural consequences of hydroxamate and tropolonate binding to iron porphyrins

Article abstract of DOI:10.1039/a905485e

The solid-state structures of iron porphyrin hydroxamate complexes reveal an unusual monodentate η¹-O binding mode; the bidentate η²-O,O binding of the tropolonate anion results in an apical displacement of iron of 0.80 A from the 24-atom mean porphyrin plane.

Full text of DOI:10.1039/a905485e

Structural consequences of hydroxamate and tropolonate binding to iron
porphyrins
Lin Cheng,a Masood A. Khan,a Douglas R. Powell,b Richard W. Taylora and George B. Richter-Addo*a
a Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, OK 73019,
USA. E-mail: grichteraddo@ou.edu
b X-ray Structural Laboratory, Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison,
WI 53706, USA
Received (in Bloomington, IN, USA) 7th July 1999, Accepted 13th August 1999
The solid-state structures of iron porphyrin hydroxamate
hydroxamate ligand, which displays moderate hydrogen-bond-
ing10 [N(5A)–O(2C) 2.750(5) Å, O(1A)–O(1C) 2.673(5) Å] to
a second hydroxamic acid molecule.§ The average Fe–N(por)
bond length is 2.052 Å and is in the range typical of five-
coordinate iron(iii) high-spin porphyrins,11 and the Fe atom is
pulled out by 0.48 Å from the 24-atom porphyrin plane.
Unexpectedly, the reaction of a stirred suspension of [Fe-
(tpivpp)Cl] in THF (in the presence of a small amount of
pyridine) with a 1+1 mixture of excess benzohydroxamic acid
and NaH at room temperature for 1.5 h results, after crystalliza-
tion in air, in the formation of the previously reported
[Fe(tpivpp)(NO2)(py)] in 25% isolated yield which we identi-
fied by X-ray crystallography. This known compound was
previously obtained from the reaction of [Fe(tpivpp)(NO2)2]2
with pyridine.12 The net formation of [Fe(tpivpp)(NO2)(py)]
1
complexes reveal an unusual monodentate h -O binding
2
mode; the bidentate h -O,O binding of the tropolonate anion
results in an apical displacement of iron of 0.80 Å from the
24-atom mean porphyrin plane.
Hydroxamic acids are ubiquitous in biology.1 Hydroxamate
groups are present in a large number of siderophores, and the
hydroxamic acid functional group is a constituent of certain
antibiotics, antifungal agents, tumor inhibitors, and even some
food additives.1,2 The troponoid functional group is present in a
number of alkaloids and antibiotics.3 The tropolone group has
been used as a model for the antimitotics colchicine (Col) and
trimethylcolchicine (Tmca), and it has been shown that some Ru
and Pt complexes of Col and Tmca exhibit antitumor activity
comparable to that of cisplatin.4 The coordination chemistry of
hydroxamic acids and their derivatives has been reviewed,1,5
and the hydroxamate functional group behaves as a typical
2
chelating (bidentate, h ) ligand towards metals in their com-
plexes. Hydroxamic acids can be viewed as valence iso-
electronic analogs of NONOates (diazeniumdiolates),6 in which
the C–H(X) group has been replaced with the valence
isoelectronic N atom.
We recently reported that the Cupferron anion (Ph[N2O2]2,
an NONOate) binds in a bidentate mode to iron(iii) porphyrins.7
Surprisingly, there have been very few reports on the inter-
actions of hydroxamic acids (or hydroxamates) with synthetic
metalloporphyrins8 or natural heme biomolecules,9 and there
has been no report on the interaction of tropolone with
metalloporphyrins. Thus, we sought to determine the nature of
binding of typically bidentate hydroxamates (with non-equiva-
lent O-donor atoms, NO vs. CO) and the tropolonate anion (with
equivalent O-donor atoms) with the metal center in metallo-
porphyrins. The ligands were selected to include an acyclic
hydroxamic acid where the O-donor atoms could exist in cis or
trans conformations, a cyclic hydroxamic acid with the O-donor
atoms locked in the cis conformation, and tropolone where the
O-donor atoms are locked in the cis form. We are now pleased
to report our preliminary results on (i) the unambiguous
Fig. 1 (a) Molecular structure of one of the two independent molecules of
1·HONHCOPh. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (°) (the data for the second independent
molecule are shown in square brackets): Fe(1A)–N(por) 2.049(4)–2.057(4)
[2.043(4)–2.056(4)], Fe(1A)–O(1A) 1.927(4) [1.923(3)], O(1A)–N(5A)
1.377(5) [1.363(5)], N(5A)–C(37A) 1.341(6) [1.333(6)], C(37A)–O(2A)
1.235(6) [1.237(6)], O(1C)–O(1A) 2.673(5) [2.876(5)], O(2C)–N(5A)
2.750(5) [2.739(6)], O(1C)–N(1C) 1.402(5) [1.389(5)], N(1C)–C(1C)
1.309(6) [1.321(7)], C(1C)–O(2C) 1.251(6) [1.242(6)]; Fe(1A)–O(1A)–
N(5A) 111.0(3) [108.3(3)], O(1A)–N(5A)–C(37A) 118.8(4) [121.3(5)]. (b)
Molecular structure of 2. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (°): Fe(1)–N(por)
2.055(2)–2.065(2), Fe(1)–O(1) 1.9163(14), O(1)–N(5) 1.367(2); Fe(1)–
O(1)–N(5) 115.32(11).
1
determination of an unusual h interaction of hydroxamates
with iron(iii) porphyrins, and (ii) the remarkable structural
2
consequence of h binding of the tropolone anion with an
iron(iii) porphyrin.
Benzohydroxamic acid (4 equiv.) was reacted with
[Fe(oep)]2(m-O) (0.050 g, 0.042 mmol) in toluene at 70 °C for
1.5 h. Upon cooling, black microcrystals precipitated which
were collected by filtration and dried to give [Fe(oep)(ONH-
COPh)]·HONHCOPh (1·HONHCOPh) in 52% isolated
yield.†‡ The molecular structure of 1·HONHCOPh [Fig. 1(a)]
1
reveals an unusual h -O binding mode for this acyclic
Chem. Commun., 1999, 1941–1942
This journal is © The Royal Society of Chemistry 1999
1941

collected on Bruker (Siemens) P4 diffractometers with Mo-Ka radiation (l
= 0.71073 Å). The structures were solved using the SHELXTL system and
refined by full-matrix least squares on F2 using all reflections. The X-ray
structural study of 1·HONHCOPh was performed at the University of
Wisconsin using a SMART/CCD system, and the studies for 2·C6H5Me and
3·C6H5Me were performed at the University of Oklahoma.
‡ Data for 1.HONHCOPh: IR (KBr, cm21): nCO = 1632s br; also 2965m,
2929w, 2870w, 1575s, 1515w, 1469s, 1451s, 1373m, 1312m, 1269m,
1216w, 1147s, 1111w, 1056m, 1016s, 982m, 958s, 916w, 894m, 841m,
795w, 748w, 731w, 697s, 677w. UV–VIS [l/nm (e/mM21 cm21), 1.10 3
1025
M
in benzene]: 351(40), 394(74), 488(8). Crystal data:
C
50H57FeN6O4, M = 861.87, monoclinic, P21/c, a = 26.165(5), b =
14.2716(18), c = 24.375(4) Å, b = 99.390(2)°, V = 8980(3) Å3, Z = 8, Dc
= 1.275 g cm23, T = 138(2) K. Final R1 = 0.0746 (GOF = 0.956) for
7645 ‘observed’ reflections with I > 2s(I). The structure contains two
independent molecules which have similar geometry [Fig. 1(a)]. CCDC
182/1388.
§ The related [Fe{t(p-Me)pp}(L)] (L = benzohydroxamate, Cupferrate)
complexes were proposed on the basis of IR spectroscopy to contain
monodentate axial ligands5 although we have shown by X-ray crystallog-
raphy that the [Fe(por)(Cupferrate)] [por = tpp, t(p-OMe)pp] complexes
Fig. 2 Molecular structure of 3·C6H5Me. Hydrogen atoms have been
omitted for clarity. Selected bond distances (Å) and angles (°): Fe–N(por)
2.088(7)–2.161(9), Fe(1)–O(1) 2.064(6), Fe(1)–O(2) 2.067(6), O(1)–C(37)
1.256(9), O(2)–C(43) 1.261(11), C(37)–C(43) 1.486(12), C(39)–C(40)
1.37(2), C(40)–C(41) 1.39(2); Fe(1)–O(1)–C(37) 120.5(6), O(1)–C(37)–
C(43) 113.2(8), C(37)–C(43)–O(2) 112.3(8), C(43)–O(2)–Fe(1) 120.7(6).
2
display bidentate h -O,O coordination of the axial cupferrate ligands.7
¶ The acid-catalyzed hydrolysis of hydroxamic acids to give hydrox-
ylamines and the parent carboxylic acids has been reviewed.13
∑ Data for 2: IR (KBr, cm21): nCO = 1703s; also 2964s, 2933s, 2871m,
1469m, 1456m, 1436w, 1373m, 1315w, 1268m, 1212s, 1147m, 1110w,
1055m, 1015m, 982w, 958s, 916w, 840m, 748w, 729m, 698w, 656w, 475w.
UV–VIS [l/nm (e/mM21 cm21), 1.02 3 1025 M in benzene]: 372(74),
397(92), 500(10), 525(10), 620(7). Crystal data: C40H48FeN5O3·C6H5CH3,
from benzohydroxamic acid is remarkable and, to the best of our
knowledge, is the first report of a hydroxamate effectively
donating its ‘NO’ group to a metal center.¶
¯
M = 794.82, triclinic, P1, a = 13.2595(10), b = 13.2893(11), c =
The related reaction of N-hydroxysuccinimide HONC4H4O2
(4 equiv.) with [Fe(oep)]2(m-O) (0.060 g, 0.050 mmol) in hot
13.9116(11) Å, a
= 74.034(6), b = 69.406(5), g = 65.593°, V =
2065.3(3) Å3, Z = 2, Dc = 1.278 g cm23, T = 173(2) K. Final R1 = 0.0461
(GOF = 1.045) for 5742 ‘observed’ reflections with I > 2s(I). The
1
toluene gave, after work-up, [Fe(oep)(h -ONC4H4O2)] 2 in
78% isolated yield.†∑ The molecular structure is shown in
structure contains
182/1388.
a disordered toluene solvent molecule. CCDC
1
Fig. 1(b), and the h -O binding mode of the hydroxamate ligand
is also revealed, this time with no additional hydrogen-bonding
interactions that stabilize this binding mode. The carbonyl O
atoms do not interact with the metal center, with (carbonyl)O-
to-Fe distances of 3.85 Å [O(3)] and 3.79 Å [O(2)]. The Fe atom
is apically displaced by 0.44 Å from the 24-atom porphyrin
plane towards the axial ligand.
** Data for 3·C6H5Me: IR (KBr, cm21): nCO 1591s; also 2965s, 2929m,
2871m, 1520s, 1495w, 1469m, 1437s, 1406m, 1366s, 1314w, 1268m,
1226s, 1218m, 1143m, 1110m, 1055s, 1013s, 980m, 954s, 914m, 877m,
842m, 748m, 738m, 729s, 714w, 695s, 543m. UV–VIS [l/nm (e/mM21
cm21), 1.01 3 1025 M in benzene]: 332(42), 393(76), 556(12). Crystal
data: C50H57FeN4O2, M = 801.85, monoclinic, P21, a = 10.409(3), b =
14.473(3), c = 14.803(3) Å, b = 109.16(2)°, V = 2106.6(8) Å3, Z = 2, Dc
= 1.264 g cm23, T = 173(2) K. Final R1 = 0.0706 (GOF = 1.093) for
3546 ‘observed’ reflections with I > 2s(I). The esds for the displacements
of the 24 individual C and N atoms from the calculated least-squares plane
lie in the range 0.006–0.009 Å, with the Fe atom displaced 0.802(2) Å from
this plane. CCDC 182/1388.
We then employed tropolone in these reactions in order to
2
obtain an h -O,O binding mode of the axial ligand. The reaction
of tropolone with [Fe(oep)]2(m-O) in hot toluene gave, after
2
work-up, dark purple microcrystals of [Fe(oep)(h -OC7H5-
O)].toluene (3·C6H5Me) in 86% isolated yield.†** The molec-
ular structure of 3·C6H5Me is shown in Fig. 2. The most
noticeable features of the structure are (i) the tropolonate ligand
is bound to the iron center in a bidentate mode, with essentially
equivalent Fe–O distances and an O–Fe–O bite angle of
73.1(2)°,†† (ii) the tropolone plane is nearly coincident with a
(por)N–Fe–N(por) plane containing diagonal porphyrin ni-
trogens, (iii) the toluene molecule is coplanar with the tropolone
ligand and is in close proximity (ca. 3.5 Å) to it, and most
importantly (iv) the Fe atom is apically displaced by 0.80 Å
from the 24-atom plane** of the porphyrin towards the
tropolone ligand! To the best of our knowledge, this is the
largest reported mean displacement of an Fe atom from a
porphyrin plane. In comparison, related displacements of 0.69,
†† The D3 symmetric [Fe(tropolonate)3] complex exhibits a related O–Fe–
O bite angle of 77.8°.14
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3904.
2
0.60 and 0.61 Å have been observed for [Fe{t(p-OMe)pp}{h -
2
2
ONN(Ph)O}],7 [Fe(tpp)(h -O2NO)],15 and [Fe(tpivpp)(h -
O2NO)],16 respectively.
The research results discussed in this publication were made
possible by the OCAST award for project number HN97-088
from the Oklahoma Center for the Advancement of Science and
Technology (USA). M. A. K. and D. R. P. thank the National
Science Foundation (CHE-9310384 and CHE-9310428) and the
respective universities for funds for the purchase of the X-ray
instrumentation. We thank Dr Scheidt for providing us with a
preprint of ref. 11 prior to publication.
Notes and references
† All three compounds give satisfactory elemental analyses (±0.4%) for C,
H and N: tpivpp = picket fence porphyrinato dianion. Crystal data were
16 O. Q. Munro and W. R. Scheidt, Inorg. Chem., 1998, 37, 2308.
Communication 9/05485E
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Chem. Commun., 1999, 1941–1942