Electronic structure of four-coordinate iron(I) complex supported by a bis(phosphaethenyl)pyridine ligand

Article abstract of DOI:10.1021/ja102009n

A 15-electron iron complex with a formal Fe(I) center, [FeBr(BPEP)] (BPEP) 2,6-bis(1-phenyl-2-phosphaethenyl)pyridine), was prepared by one-electron reduction of the dibromide precursor [FeBr₂(BPEP)]. The single-crystal diffraction analysis revealed a distorted trigonal monopyramidal arrangement around the iron center, and SQUID magnetometry established the S) 3/2 ground state. The Moessbauer isomer shift value (δ) 0.59 mm/s) was consistent with a high-spin Fe(I) center of [FeBr(BPEP)]. DFT calculations for a model complex revealed two highly delocalized molecular orbitals formed by bonding and antibonding interactions between the d_Z² (Fe) and π^* (BPEP) orbitals. Orbital occupancy analysis demonstrated the electronic structure with a high-spin Fe(I) center. The effective dπ-pπ interaction between iron and BPEP was concluded to be responsible for the highly distorted structure of [FeBr(BPEP)], with its rather uncommon trigonal monopyramidal configuration.

Full text of DOI:10.1021/ja102009n

Published on Web 07/01/2010
Electronic Structure of Four-Coordinate Iron(I) Complex Supported by a
Bis(phosphaethenyl)pyridine Ligand
Yumiko Nakajima,*,† Yoshihide Nakao,‡ Shigeyoshi Sakaki,‡,§ Yoshinori Tamada,† Teruo Ono,† and
Fumiyuki Ozawa*,†
Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan, Department of Molecular
Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and
Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, Sakyo-ku, Kyoto 610-8103, Japan
Received March 9, 2010; E-mail: ozawa@scl.kyoto-u.ac.jp
Scheme 1
Abstract: A 15-electron iron complex with a formal Fe(I) center,
[FeBr(BPEP)] (BPEP ) 2,6-bis(1-phenyl-2-phosphaethenyl)py-
ridine), was prepared by one-electron reduction of the dibro-
mide precursor [FeBr2(BPEP)]. The single-crystal diffraction
analysis revealed a distorted trigonal monopyramidal arrange-
ment around the iron center, and SQUID magnetometry
established the S ) 3/2 ground state. The Mo¨ssbauer isomer
shift value (δ ) 0.59 mm/s) was consistent with a high-spin
Fe(I) center of [FeBr(BPEP)]. DFT calculations for a model
complex revealed two highly delocalized molecular orbitals
formed by bonding and antibonding interactions between the
2
dz (Fe) and π* (BPEP) orbitals. Orbital occupancy analysis
demonstrated the electronic structure with a high-spin Fe(I)
center. The effective dπ-pπ interaction between iron and
BPEP was concluded to be responsible for the highly distorted
structure of [FeBr(BPEP)], with its rather uncommon trigonal
The BPEP-coordinated iron dibromide [FeBr2(BPEP)] (1) was
synthesized in 31% yield by the reaction of FeBr2 with (E,E)-BPEP
in toluene at 80 °C. SQUID magnetization data established a ground
state of S ) 2, which is consistent with a high-spin Fe(II) center.
The Mo¨ssbauer spectroscopic parameters (δ ) 0.78 mm/s, ∆EQ )
2.96 mm/s) were also in the range of high-spin Fe(II) complexes.7
The geometric structure was determined by single-crystal X-ray
monopyramidal configuration.
Coordinatively unsaturated complexes with low-valent iron
analysis, showing a pseudo square pyramidal configuration around
centers have been recognized as key intermediates in catalytic
the iron atom.
transformations and metalloenzymatic activation systems.1,2
Complex 1 underwent one-electron reduction with KC8 at
ambient temperature to give [FeBr(BPEP)] (2) in 95% yield. The
magnetic moment of 2 ranged from 3.5 to 4.2 µB at 5-300 K,
demonstrating an S ) 3/2 ground state. The Mo¨ssbauer isomer shift
of δ ) 0.59 mm/s (∆EQ ) 2.68 mm/s) was clearly smaller than
that of 1 and relatively closer to the values hitherto reported for
four-coordinate Fe(I) complexes with high-spin states (0.41-0.46
Low-coordinate Fe(I) complexes have attracted particular interest
recently because of their capabilities in small-molecule activa-
tion.3 However, there are a limited number of well-defined Fe(I)
complexes, and their highly changeable spin states, which are
thought to be responsible for the complexity of iron-catalyzed
reactions, are not well understood.3i,4 For example, a recent
report by Chirik et al. demonstrated the profundity of iron
mm/s).3i,8 Actually, the observed isomer shift was consistent with
chemistry.5 They found that [FeCl(iPrPDI)] (3; PrPDI ) 2,6-
i
the calculated value for a model compound of 2 with a high-spin
Fe(I) center (0.64 mm/s, vide infra).
Figure 1 shows the X-ray structure of 2. The iron atom adopts
neither a tetrahedral nor square planar configuration but rather
(iPr2C6H3N)CMe)2C5H3N), a formal Fe(I) complex with a
ground state of S ) 3/2, is in reality an Fe(II) complex, with
the Fe(II) center (S ) 2) coupled with a ligand-centered radical
(S ) 1/2) to form the S ) 3/2 state.
Herein we report a four-coordinate complex, [FeBr(BPEP)] (2),
a distorted trigonal monopyramidal configuration, which is
uncommon for four-coordinate iron complexes.9 The basal plane
supported by a novel 2,6-bis(1-phenyl-2-phosphaethenyl)pyridine
is composed of the two phosphorus and the bromine and iron
ligand (BPEP, Scheme 1). BPEP is a phosphaalkene analogue of
atoms, while the nitrogen atom occupies the apex.10,11 The sum
2,6-bis(imino)pyridines such as iPrPDI (in 3 above). Unlike imines,
of the angles in the basal plane is nearly 360°. The PdC bond
however, phosphaalkenes, containing a PdC bond, have an
lengths (1.719(6), 1.713(6) Å) are comparable to those of 1
extremely low-lying π* orbital as well as high-lying lone pair
(1.712(11), 1.681(11) Å); however, the FesP (2.2716(17),
2.2883(17) Å) and FesN (2.035(5) Å) bond lengths are
significantly shorter than those of 1 (2.501(4), 2.507(4), and
2.216(9) Å, respectively) and in the range of formal Fe(I)
electrons and effectively stabilize low-valent transition-metal com-
plexes.6 We found that this unique ligand property allowed
successful stabilization of 2, which contains a coordinatively
unsaturated 15-electron system with a high-spin Fe(I) center.
complexes with high-spin states.3d,5
A remarkable feature of 2 is the highly distorted structure,
which appears to have been formed by elimination of one of
the equatorial Br atoms from 1. Another point that we noted
† Institute for Chemical Research.
‡ Graduate School of Engineering.
§ Fukui Institute for Fundamental Chemistry.
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C O M M U N I C A T I O N S
The electronic structure of the square planar complex 2asp was
examined in a similar fashion. The optimized structure of 2asp was
15.9 kcal/mol higher in energy than that of 2adis. Interestingly, 2asp
had a sextet ground state (S ) 5/2), which would be regarded as
consisting of a high-spin Fe(II) center (S ) 2) and a ligand-localized
radical (S ) 1/2). The Fe-P bonds were elongated by 0.11 Å, and
the Mayer’s bond order decreased accordingly, compared with 2adis.
Thus the weaker bonding interaction between Fe and P in 2asp than
in 2adis was evidenced.
In conclusion, we succeeded in synthesizing a coordinatively
unsaturated complex with a formal Fe(I) center (2), using BPEP
as a PNP pincer-type phosphaalkene ligand. The complex adopts
a distorted trigonal monopyramidal configuration, which enables
Figure 1. ORTEP drawing of [FeBr(BPEP)] (2) with 50% probability
ellipsoids. Hydrogen atoms, a disordered tBu group, and an E2O molecule
are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe-P1,
2.2716(17); Fe-P2, 2.2883(17); Fe-N, 2.035(5); Fe-Br, 2.3304(10);
P1-C6, 1.719(6); P2-C13, 1.713(6); P1-Fe-P2, 132.27(7); P1-Fe-Br,
112.27(5); P2-Fe-Br, 115.32(5); N-Fe-Br, 119.19(13); N-Fe-P1,
80.04(14); N-Fe-P2, 80.35(14).
2
an effective bonding interaction between the dz orbital of iron
and the π* orbital of BPEP. Mo¨ssbauer spectroscopy and
theoretical calculations revealed the electronic structure that is
assignable to a high-spin Fe(I) complex. These features are
was the Mo¨ssbauer isomer shift consistent with a high-spin Fe(I)
center. To study these points in detail, we carried out DFT
calculations for the model compound [FeBr(bpep)] (2a), in which
the 2,4,6-tri-tert-butylphenyl (Mes*) groups on the BPEP ligand
are replaced by 3,5-dimethylphenyl (Xyl) groups (Figure 2).
The distorted trigonal monopyramidal geometry of 2 was
reproduced in the optimized structure of 2adis. Figure 2 illustrates
important natural orbitals of the distorted complex 2adis. The orbital
occupancy values given in parentheses were evaluated by broken
symmetry DFT calculations. Five of the seven d electrons are found
in metal-localized orbitals as unpaired [B (dyz, 1e), C (dxz, 1e), D
i
remarkably different from those of 3, which contains PrPDI, a
nitrogen analogue of BPEP: 3 adopts a square planar configu-
ration and has an Fe(II) center.5 It is likely that the presence of
an extremely low-lying π* orbital,12 which is characteristic of
phosphaalkene ligands, is responsible for the unique geometry
and electronic structure of 2. The reactivity of this novel 15-
electron iron system is currently under investigation.
Acknowledgment. This work was supported by Grants-in-
Aid for Scientific Research on Priority Areas, “Synergy of
Elements” and “Molecular Theory for Real Systems”, from
MEXT, Japan. We are grateful to Dr. Y. Ohki, Mr. S. Ohta,
and Prof. K. Tatsumi (Nagoya Univ.) for SQUID measurement;
to Dr. T. Sasamori and Prof. N. Tokitoh (Kyoto Univ.) for
crystallographic assistance; and to Dr. G. Juhasz and Prof. K.
Yoshizawa (Kyusyu Univ.) for isomer shift calculation, through
activities on the MEXT Joint Project of Chemical Synthesis Core
Research Institutions.
2
2
(dx -y , 1e)] and paired electrons [F (dxy, 2e)], respectively. The other
two electrons are accommodated in the highly delocalized orbitals
A and E, formed by antibonding and bonding interactions between
2
the π* (bpep) and dz (Fe) orbitals, with orbital occupancies of 0.24
and 1.76, respectively. Although the orbital symmetry between π*
2
and dz may be considered inconsistent, the distorted trigonal
monopyramidal geometry, in which the iron atom is lifted signifi-
cantly off the PNP plane (∠P-Fe-P ) 132.5°), enables effective
interaction between these orbitals. The total spin density of the iron
was estimated as 3.33, which is in accordance with the Fe(I) center
of 2. The Mo¨ssbauer isomer shift for 2adis was estimated as δ )
0.64 mm/s.
Supporting Information Available: Experimental procedures and
crystallographic data of 1 and 2; computational methods and data for
DFT calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(1) (a) Sherry, B. D.; Fu¨rstner, A. Acc. Chem. Res. 2008, 41, 1500. (b) Bolm,
C.; Legros, J.; Paih, J. L.; Zani, L. Chem. ReV. 2004, 104, 6217.
(2) (a) Tard, C.; Pickett, C. J. Chem. ReV. 2009, 109, 2245. (b) Hoffman, B. M.;
Dean, D. R.; Seefeldt, L. C. Acc. Chem. Res. 2009, 42, 609. (c) Fontecilla-
Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. ReV. 2007,
107, 4273. (d) Rees, D. C. Annu. ReV. Biochem. 2002, 71, 221. (e)
Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. ReV. 2000,
206-207, 533. (f) Eady, R. R. Chem. ReV. 1996, 96, 3013.
(3) (a) Lu, C. C.; Saouma, C. T.; Day, M. W.; Peters, J. C. J. Am. Chem. Soc.
2007, 129, 4. (b) Brown, S. D.; Mehn, M. P.; Peters, J. C. J. Am. Chem.
Soc. 2005, 127, 13146. (c) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am.
Chem. Soc. 2003, 125, 322. (d) Betley, T. A.; Peters, J. C. J. Am. Chem.
Soc. 2003, 125, 10782. (e) Bai, G.; Wei, P.; Das, A. K.; Stephan, D. W.
Dalton Trans. 2006, 1141. (f) Kisko, J. L.; Hascall, T.; Parkin, G. J. Am.
Chem. Soc. 1998, 120, 10561. (g) Stokes, S. L.; Davis, W. M.; Odom,
A. L.; Cummins, C. C. Organometallics 1996, 15, 4521. (h) Smith, J. M.;
Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.; Lukat-Rodgers, G.;
Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.; Holland, P. L. J. Am. Chem.
Soc. 2006, 128, 756. (i) Stoian, S. A.; Yu, Y.; Smith, J. M.; Holland, P. L.;
Bominaar, E. L.; Mu¨nck, E. Inorg. Chem. 2005, 44, 4915.
(4) (a) Ling, Y.; Zhang, Y. J. Am. Chem. Soc. 2009, 131, 6386. (b) Conradie,
J.; Ghosh, A. J. Chem. Theory Comput. 2008, 4, 1576.
(5) (a) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E. J.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2005, 3406. (b) Bart, S. C.;
Chłopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.;
Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13901.
(6) (a) Le Floch, P. Coord. Chem. ReV. 2006, 250, 627. (b) Mathey, F. Angew.
Chem., Int. Ed. 2003, 42, 1578. (c) Weber, L. Coord. Chem. ReV. 2005,
249, 741. (d) Mu¨ller, C.; Vogt, D. Dalton. Trans. 2007, 5505. (e) Ozawa,
F.; Yoshifuji, M. Dalton Trans. 2006, 4987.
Figure 2. (a) Natural orbital diagram for a model compound with distorted
trigonal monopyramidal geometry (2adis). The values in parentheses represent
orbital occupancy. (b) A schematic view of the orbital interaction between
bpep and iron in molecular orbital E.
(7) (a) Reger, D. L.; Gardinier, J. R.; Gemmill, W. R.; Smith, M. D.; Shahin,
A. M.; Long, G. J.; Rebbouh, L.; Grandjean, F. J. Am. Chem. Soc. 2005,
9
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C O M M U N I C A T I O N S
127, 2303. (b) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Mu¨nck, E.; Holland,
P. L. J. Am. Chem. Soc. 2004, 126, 4522. (c) Eckert, N. A.; Stoian, S.;
Smith, J. M.; Bominaar, E. L.; Mu¨nck, E.; Holland, P. L. J. Am. Chem.
Soc. 2005, 127, 9344.
(10) Note that the structure of 2 is significantly distorted from the ideal trigonal
monopyramidal geometry, owing to the rigid pincer-type ligand structure
of BPEP.
(11) The possibility of the hydrido structure [FeH(Br)(BPEP)] was excluded
for the following reasons. (a) No notable absorption was observed in the
metal hydride region of the IR spectrum (1700-2200 cm-1). (b) Complex
2 readily combined with PhCN to form the five-coordinate complex
[FeBr(PhCN)(BPEP)] (see Supporting Information).
(8) Yu, Y.; Sadique, A. R.; Smith, J. M.; Dugan, T. R.; Cowley, R. E.;
Brennessel, W. W.; Flaschenriem, C. J.; Bill, E.; Cundari, T. R.; Holland,
P. L. J. Am. Chem. Soc. 2008, 130, 6624.
(9) (a) Weintrob, E. C.; Tofan, D.; Bercaw, J. E. Inorg. Chem. 2009, 48, 3808.
(b) Buschhorn, D. T.; Pink, M.; Fan, H.; Caulton, K. G. Inorg. Chem. 2008,
47, 5129. (c) Ingleson, M. J.; Fullmer, B. C.; Buschhorn, D. T.; Fan, H.;
Pink, M.; Huffman, J. C.; Caulton, K. G. Inorg. Chem. 2008, 47, 407. (d)
Eckert, N. A.; Vaddadi, S.; Stoian, S.; Lachicotte, R. J.; Cundari, T. R.;
Holland, P. L. Angew. Chem., Int. Ed. 2006, 45, 6868. (e) Hung, C.-H.;
Chang, F.-C.; Lin, C.-Y.; Rachlewicz, K.; Ste¸pien´, M.; Latos-Graz˙n´ski,
L.; Lee, G.-H.; Peng, S.-M. Inorg. Chem. 2004, 43, 4118.
(12) The π* orbital of a truncated model of bpep (3,5-(HP)CH)2C5H3N) was
located 0.78 eV lower than that of pdi (3,5-(HN)CH)2C5H3N) by DFT
calculations.
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