Five-coordinate complexes [FeX(depe)2]BPh4, X = Cl, Br: Electronic structure and spin-forbidden reaction with N2

Article abstract of DOI:10.1021/ic0111987

The bonding of N₂ to the five-coordinate complexes [FeX(depe)₂]⁺, X = Cl (1a) and Br (1b), has been investigated with the help of X-ray crystallography, spectroscopy, and quantum-chemical calculations. Complexes 1a and 1b are found to have an XP₄ coordination that is intermediate between square-pyramidal and trigonal-bipyramidal. Moessbauer and optical absorption spectroscopy coupled with angular overlap model (AOM) calculations reveal that 1a and 1b have ³B₁ ground states deriving from a (xz)¹(Z²)¹ configuration. The zero-field splitting for this state is found to be 30-35 cm^-1. In contrast, the analogous dinitrogen complexes [FeX(N₂)(depe)₂]⁺, X = Cl (2a) and Br (2b), characterized earlier are low-spin (S = 0; Wiesler, B. E.; Lehnert, N.; Tuczek, F.; Neuhausen, J.; Tremel, W. Angew. Chem, Int. Ed. 1998, 37, 815-817). N₂ bonding and release in these systems are thus spin-forbidden. It is shown by density functional theory (DFT) calculations of the chloro complex that the crossing from the singlet state (ground state of 2a) to the triplet state (ground state of la) along the Fe-N coordinate occurs at r_c = 2.4 A. Importantly, this intersystem crossing lowers the enthalpy calculated for N₂ release by 10-18 kcal/mol. The free reaction enthalpy ΔG° for this process is calculated to be 4.7 kcal/mol, which explains the thermal instability of N₂ complex 2a with respect to the loss of N₂. The differences in reactivity of analogous trans hydrido systems are discussed.

Full text of DOI:10.1021/ic0111987

Inorg. Chem. 2002, 41, 3491−3499
Five-Coordinate Complexes [FeX(depe)2]BPh4, X ) Cl, Br: Electronic
Structure and Spin-Forbidden Reaction with N †
2
Oliver Franke,‡ Beatrix E. Wiesler,§ Nicolai Lehnert,‡ Christian Na1ther,‡ Vadim Ksenofontov,§
,‡
Jo1rg Neuhausen,| and Felix Tuczek*
Christian Albrechts UniVersita¨t Kiel, Otto Hahn Platz 6/7, D-24098 Kiel, Germany, Johannes
Gutenberg UniVersita¨t Mainz, Staudingerweg 9, D-55099 Mainz, Germany, and Johannes
Gutenberg UniVersita¨t Mainz, Becherweg 25, D-55099 Mainz, Germany
Received November 26, 2001
The bonding of N to the five-coordinate complexes [FeX(depe)2]+, X) Cl (1a) and Br (1b), has been investigated
2
with the help of X-ray crystallography, spectroscopy, and quantum-chemical calculations. Complexes 1a and 1b
are found to have an XP coordination that is intermediate between square-pyramidal and trigonal-bipyramidal.
4
Mo¨ssbauer and optical absorption spectroscopy coupled with angular overlap model (AOM) calculations reveal that
1a and 1b have 3B ground states deriving from a (xz)1(z2)1 configuration. The zero-field splitting for this state is
1
found to be 30−35 cm-1. In contrast, the analogous dinitrogen complexes [FeX(N )(depe)2]+, X ) Cl (2a) and Br
2
(2b), characterized earlier are low-spin (S ) 0; Wiesler, B. E.; Lehnert, N.; Tuczek, F.; Neuhausen, J.; Tremel, W.
Angew. Chem, Int. Ed. 1998, 37, 815−817). N bonding and release in these systems are thus spin-forbidden. It
2
is shown by density functional theory (DFT) calculations of the chloro complex that the crossing from the singlet
state (ground state of 2a) to the triplet state (ground state of 1a) along the Fe−N coordinate occurs at rC ) 2.4
Å. Importantly, this intersystem crossing lowers the enthalpy calculated for N release by 10−18 kcal/mol. The free
2
reaction enthalpy ∆G° for this process is calculated to be 4.7 kcal/mol, which explains the thermal instability of N
2
complex 2a with respect to the loss of N . The differences in reactivity of analogous trans hydrido systems are
2
discussed.
Introduction
Because of the generally low affinity of Fe(II) toward N2,5
iron dinitrogen systems are ideal candidates to study this
problem, particularly with respect to the requirements
concerning the structure and geometry of the N2 binding
fragment, the influence of the coligands, and the electronic
structure of the central atom. Moreover, iron plays a major
role in the industrial as well as biological processes of
nitrogen fixation2,6 and, like other d6 metal centers, is of
interest as a potential candidate for the reproduction of the
enzymatic reaction in vitro.7,8
Nitrogen fixation continues to present a fascinating and
challenging problem to many scientific disciplines ranging
from inorganic chemistry to molecular biology.1,2 As a part
of our ongoing efforts to investigate the conditions for the
metal-centered reduction and protonation of dinitrogen,3,4 we
also became interested in the process of N2 binding which
represents the initial step along this reaction pathway.
* To whom correspondence should be addressed. E-mail: ftuczek@ac.uni-
kiel.de.
† Dedicated to Prof. Dr. Dieter Sellmann on the occasion of his 60th
birthday.
In an earlier communication, we reported on the synthesis
as well as the structural and spectroscopic characterization
‡ Christian Albrechts Universita¨t Kiel.
§ Johannes Gutenberg Universita¨t Mainz, Staudingerweg 9.
| Johannes Gutenberg Universita¨t Mainz, Becherweg 25.
(1) Fryzuk, M.; Johnson, S. A. Coord. Chem. ReV. 2000, 200, 379-419.
(2) Prokaryotic Nitrogen Fixation: A Model System for the Analysis of a
Biological Process; Triplett, E. W., Ed.; Horizon Scientific Press:
Wymondham, U.K., 2000.
(5) Tuczek, F.; Lehnert, N. Angew. Chem., Int. Ed. 1998, 37, 2636-2638.
(6) (a) Catalytic Ammonia Synthesis: Fundamentals and Practice (Fun-
damental and Applied Catalysis); Jennings, J. R., Ed.; Plenum
Publishing: New York, 1991. (b) Appl, M. Ammonia: Principles and
Industrial Practice; Wiley-VCH: New York, 1999.
(3) (a) Lehnert, N.; Wiesler, E. B.; Tuczek, F.; Hennige, A.; Sellmann,
D. J. Am. Chem. Soc. 1997, 119, 8869-8888. (b) Lehnert, N.; Tuczek,
F. Inorg. Chem. 1999, 38, 1659-1682.
(4) Wiesler, E. B.; Lehnert, N.; Tuczek, F.; Neuhausen, J.; Tremel, W.
Angew. Chem., Int. Ed. 1998, 37, 815-817.
(7) (a) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115-1133. (b)
Henderson, R. A.; Leigh, G. J.; Pickett, C. AdV. Inorg. Chem.
Radiochem. 1983, 27, 197-292.
(8) Sellmann, D.; Utz, J.; Blum, N.; Heinemann, F. W. Coord. Chem.
ReV. 1999, 190-192(0), 607-627.
10.1021/ic0111987 CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/05/2002
Inorganic Chemistry, Vol. 41, No. 13, 2002 3491

Franke et al.
spectrometer. Mo¨ssbauer spectra were recorded with a WISSEL
setup equipped with a He flow-through cryostat (Oxford CF506);
isomer shifts are quoted versus R-Fe. Magnetic susceptibility was
measured with a Physical Instruments (PI) vibrating sample (Foner)
magnetometer. Susceptibility data were corrected for diamagnetic
contributions using Pascal’s constants.
of the octahedral dinitrogen complex [FeCl(N2)(depe)2]BPh4
(2a; depe ) 1,2-bis(diethylphosphino)ethane).4 Using Mo¨ss-
bauer spectroscopy, we found that this compound has an Fe-
N2 bond that is thermally labile at 300 K. Because 2a itself
exhibits a quadrupole doublet with δIS ) 0.26 and ∆EQ )
1.40 mm/s at 100 K, we attributed a second doublet with a
much larger quadrupole splitting present in the Mo¨ssbauer
spectra of 2a to the corresponding dinitrogen-free species
generated by thermal decay of 2a. In addition, complex 2a
was found to be photosensitive releasing N2 under UV-vis
irradiation. Now we have directly isolated the compound
[FeCl(depe)2]BPh4 (1a) from a methanolic solution of [FeCl2-
(depe)2]. Its Mo¨ssbauer parameters are almost identical with
those of the dinitrogen-free species in the matrix of 2a and
thus support our earlier assignment. In a similar manner, we
have also prepared the corresponding bromo complex [FeBr-
(depe)2]BPh4 (1b). This is related to the dinitrogen complex
[FeBr(N2)(depe)2]BPh4 (2b) which also exhibits the tendency
to lose dinitrogen at room temperature.4 In contrast, analo-
gous hydride containing dinitrogen complexes [FeH(N2)-
(diphos)2]+ (diphos ) depe,9a,b dppe,9c and dmpe9d) appear
to be thermally stable.10a Moreover, these systems and their
five-coordinate precursors exhibit a remarkable stability
toward O2. Thus, [FeH(dppe)2]+ in a THF solution or in the
solid-state picks up N2 from air, leading to [FeH(N2)-
(dppe)2]+.10a,b A methanolic solution of [FeCl(depe)2]+, on
the other hand, is extremely air sensitive.10a,11 To understand
these drastic differences in reactivity between otherwise
identical hydrido and halogeno complexes, we investigated
in detail the geometric and electronic structures of the five-
coordinate fragments [FeX(depe)2]+, X ) Cl (1a) and Br
(1b), by X-ray crystallography and spectroscopy (UV-vis
absorption, Mo¨ssbauer). The optical absorption spectra are
evaluated with the angular overlap model (AOM) leading
to an unambiguous assignment of the electronic ground state.
The bonding of N2 in these systems is studied by density
functional theory (DFT) calculations. A forthcoming paper
will describe analogous investigations on the corresponding
five-coordinate hydrido systems and their six-coordinate
dinitrogen adducts.12
Crystal Structure Determinations. Intensity data of compound
1a were collected using a Siemens P4 four-circle diffractometer,13
and those of compound 1b, using a STOE imaging plate diffraction
system using Mo KR radiation. All structures were solved with
direct methods using SHELXS-86 for compound 1a and SHELXS-
97 for compound 1b. Refinement was done against F2 using
SHELXL-93. In 1a, all non-H atoms were refined anisotropically.
In the subsequent refinement, the H atoms were positioned with
idealized geometry and isotropic temperature parameters. In one
of the crystallographically independent complex cations of com-
pound 1b, one phosphorus atom and five carbon atoms are
disordered and were refined using a split model. All non-hydrogen
atoms except P5, C2′, C21′, C22′, C23′, and C24′ in compound 1b
were refined using anisotropic displacement parameters. The
hydrogen atoms were positioned with idealized geometry and
refined using a riding model. Further information is contained in
Tables 1 and 2 and in the Supporting Information.
Synthesis of [FeX2(depe)2] (X ) Cl, Br) (3a, 3b). These
complexes were prepared following modified literature procedures.14
A solution of depe (6 mmol) in 10 mL of toluene was added to a
suspension of the appropriate anhydrous ferrous salt (3 mmol) in
30 mL of toluene. The reaction mixture was refluxed for 1 h
resulting in a green (X ) Cl) and yellow-green (X ) Br) solution,
respectively. After cooling to room temperature, the solution was
filtered, and the volume of the filtrate was reduced to 20 mL.
Cooling the filtrate to -40 °C for 2 days yielded a crystalline
product which was separated by filtration and dried under vacuum.
Purity was checked by elemental analysis.
Synthesis of [FeX(depe)2]BPh4 (X ) Cl, Br) (1a, 1b). A
mixture of [FeX2(depe)2] (0.5 mmol) in 50 mL of methanol was
allowed to stir overnight at room temperature under 1 atm of Ar,
resulting in an orange solution. Addition of a solution of NaBPh4
(1.1 mmol) in 10 mL of methanol slowly gave a crystalline product
which was separated by filtration and dried under vacuum. Purity
was checked by elemental analysis.
Synthesis of [FeHCl(depe)2]BPh4 (4). This complex was
prepared following literature procedures.15
Experimental Section
Crystallographic Characterization
The structure of 1a consists of [FeCl(depe)2]+ cations and
BPh4- anions (Table 1). The coordination geometry around
the metal center is best described as square-pyramidal with
a distortion toward trigonal-bipyramidal (Figure 1). A view
of the first coordination sphere around the Fe ion (Figure 2)
reveals that the chloro ligand is in apical position, perpen-
dicular (within (2°) to two of the four phosphorus atoms,
P(2) and P(3), which form an almost linear P-Fe-P axis.
The other two phosphorus atoms, P(1) and P(4), are bent
Synthetic Procedures and Physical Methods. Synthesis and
handling of all compounds were performed in inert gas atmosphere
by use of Schlenk techniques and gloveboxes. All solvents were
dried following literature procedures. Reagents and the phosphine
ligand 1,2-bis(diethylphosphino)ethane (depe) were obtained from
commercial sources and used without further purification. UV-
vis spectra were measured with a Varian CARY 5 UV-vis-NIR
(9) (a) Bancroft, G. M.; Mays, J. J.; Prater, B. E. J. Chem. Soc., Chem.
Commun. 1969, 585. (b) Buys, I. E.; Field, L. D.; Hambley, T. W.;
McQueen, A. E. D. Acta Crystallogr. 1993, C49, 1056-1059. (c)
Azizian, H.; Morris, R. H. Inorg. Chem. 1983, 22, 6-9. (d) Hills, A.;
Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. J. J. Organomet. Chem.
1990, 391, C41-C44.
(13) Details on this crystal structure determination have been deposited at
Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopold-
shafen, under the number CSD-410052.
(14) (a) Mays, M. J.; Prater, B. E. Inorg. Synth. 1974, 15, 21-29. (b) Evans,
D. J.; Henderson, R. A.; Hills, A.; Hughes, D. L.; Oglieve, K. E. J.
Chem. Soc., Dalton Trans. 1992, 1259-1265.
(15) Wiesler, B. E.; Tuczek, F.; Na¨ther, C.; Bensch, W. Acta Crystallogr.
1998, C54, 44-46.
(10) (a) Wiesler, B. E. PhD dissertation, University of Mainz, 1999. (b)
Aresta, M.; Giannocaro, P.; Rossi, M.; Sacco, A. Inorg. Chim. Acta
1971, 5, 115-118.
(11) Bellerby, J. M.; Mays, M. J.; Sears, P. L. J. Chem. Soc., Dalton Trans.
1976, 1232-1236.
(12) Franke, O.; Wiesler, B. E.; Tuczek, F. To be published.
3492 Inorganic Chemistry, Vol. 41, No. 13, 2002

FiWe-Coordinate Complexes [FeX(depe)2]BPh4
Figure 2. View of the first coordination sphere of 1a showing the square-
pyramidal/trigonal-bipyramidal geometry around the central Fe(II) ion.
Table 2. Selected Bond Lengths (Å) and Angles (deg) of 1a and 1b
[FeCl(depe)2]BPh4 (1a)
[FeBr(depe)2]BPh4 (1b)
Fe-P(1)
Fe-P(2)
Fe-P(3)
Fe-P(4)
Fe-X
2.3340 (8)
2.2791 (9)
2.2740 (9)
2.3198 (9)
2.3633 (9)
2.3091 (7)
2.2800 (7)
2.2831 (7)
2.3184 (7)
2.4892 (5)
Figure 1. Perspective view of [FeCl(depe)2]+ (1a) (vibrational ellipsoids
with 30% probability).
[FeCl(depe)2]BPh4 (1a)
[FeBr(depe)2]BPh4 (1b)
P(1)-Fe-P(2)
P(1)-Fe-P(3)
P(1)-Fe-P(4)
P(2)-Fe-P(3)
P(2)-Fe-P(4)
P(3)-Fe-P(4)
P(1)-Fe-X
84.35 (3)
95.40 (3)
152.52 (4)
178.51 (4)
95.80 (3)
84.45 (3)
106.37 (3)
88.44 (3)
90.22 (3)
101.11 (3)
83.76 (3)
96.59 (3)
157.06 (3)
178.77 (3)
95.52 (3)
83.70 (3)
101.62 (2)
91.68 (2)
89.41 (2)
101.32 (2)
Table 1. Crystal and Refinement Data for Compounds 1a and 1b
1a
1b
formula
MW/g‚mol-1
cryst color
cryst syst
space group
a, Å
C44H68BClFeP4
822.97
C44H68BBrFeP4
867.43
orange
orange
orthorhombic
P212121
10.935 (2)
15.883 (1)
25.520 (3)
90.00
triclinic
P1h
P(2)-Fe-X
P(3)-Fe-X
P(4)-Fe-X
13.209 (1)
16.538 (1)
23.213 (2)
107.46 (1)
96.05 (1)
107.38 (1)
4509.4 (5)
150
b, Å
c, Å
the following structural differences: the Fe-P bond lengths
in 2a range between 2.28 and 2.298 Å whereas the “axial”
Fe-P bonds in 1a are somewhat shortened (Fe-P(2) 2.279
Å, Fe-P(3) 2.274 Å) and the “equatorial” Fe-P bonds are
significantly elongated (Fe-P(1) 2.334 Å, Fe-P(4) 2.320
Å). In addition, the Fe-Cl distance is larger by 0.052 Å in
1a than in 2a. Upon removal of N2 from 2a leading to 1a,
two of the Cl-Fe-P angles remain constant within (2°
while the other two are enlarged to 101° and 106°,
respectively (vide supra). Similar results have been recorded
for the analogous five-coordinate bromo system 1b and its
structural relationship with the corresponding octahedral
bromo-N2 complex 2b.
R, °
â, °
90.00
90.00
γ, °
V, Å3
4432.3 (1)
168
temp, K
Z
4
4
Dcalcd, g‚cm-3
F(000)
1.233
1.278
1760
1832
2θ range
h/k/l ranges
abs corr
µ, mm-1
max/min transm
measured reflns
Rint
4°-54°
3°-54°
-1/13, -1/20, -1/32 -16/16, -21/19, -29/29
empirical
0.574
0.836/0.735
6587
0.0123
6328
face-indexed
1.39
0.761/0.841
35419
0.0372
17838
13801
943
0.0380
0.1027
1.049
independent reflns
reflns with I > 2σ(I) 5338
refined params
R1 [I > 2σ(I)]
wR2 [all data]
GOF
455
0.0309
0.0716
0.940
Mo1ssbauer Spectroscopy and Magnetic Susceptibility
min/max res, e‚Å-3
Flack x-parameter
0.34/-0.24
0.008 (15)
0.92/-0.68
Using Mo¨ssbauer spectroscopy, we showed earlier that the
octahedral dinitrogen complex [FeCl(N2)(depe)2]BPh4 (2a)
loses dinitrogen at room temperature and that this process
is reversible to some extent.4 A second doublet in the
Mo¨ssbauer spectrum of 2a exhibiting a large quadrupole
splitting of 2.64 mm/s (100 K) was assigned to the dinitro-
gen-free complex (δIS ) 0.43(2) mm/s). In fact, the Mo¨ss-
bauer parameters of the five-coordinate complex 1a prepared
independently (δIS ) 0.440(5) mm/s; ∆EQ ) 2.735(9) mm/s
at T ) 100 K) are almost identical to those of the presumably
dinitrogen-free species in the matrix of 2a and thus confirm
our earlier assignment. Likewise, a second doublet was found
in the Mo¨ssbauer spectrum of dinitrogen complex [FeBr-
(N2)(depe)2]BPh4, 2b, with parameters (δIS ) 0.39(1) mm/s
and ∆EQ ) 2.70 mm/s at 150 K) similar to those of the five-
away from the Fe-Cl bond. In a trigonal bipyramid, the
angles Cl-Fe-P(1), Cl-Fe-P(4), and P(1)-Fe-P(4) would
be close to 120°, while the corresponding values in 1a are
106°, 101°, and 152°, respectively. The resulting “open” face
in this coordination sphere represents a potential site for
attack of an additional ligand, for example, dinitrogen. Fe-
Cl and Fe-P distances are within the range characteristic
of related Fe(II) low-spin systems (2.18-2.36 Å); the
P-Fe-P bite angle is about 84°, a typical value for the depe
ligand.9 Relevant bond distances and angles are listed in
Table 2.
A comparison of 1a with the related octahedral dinitrogen
complex 2a which has been characterized earlier4 reveals
Inorganic Chemistry, Vol. 41, No. 13, 2002 3493

Franke et al.
Figure 3. Effective magnetic moment µeff in µB (1) and inverse molar
susceptibility 1/øm (+, diamagnetic contributions subtracted, along with a
fit according to eq 1) of 1a vs temperature (K).
coordinate bromo complex 1b (δIS ) 0.433(3) and ∆EQ )
2.710(5) mm/s at 150 K).
To further characterize the five-coordinate species 1a and
1b, magnetic susceptibility data were recorded. Figure 3
shows the inverse molar susceptibility as well as the effective
magnetic moment µeff of 1a as a function of temperature.
With a µeff of 3.1 µB at room temperature that is only slightly
above the spin-only value for a triplet, the ground-state spin
of 1a must be S ) 1 representing the “intermediate” spin
state for Fe(II). Below 50 K, the magnetic moment sharply
decreases (µeff ) 1.5 µB at T ) 7 K). Likewise, the inverse
molar susceptibility follows Curie-Weiss law and becomes
temperature-independent below 20 K. A similar behavior is
observed for 1b. From Mo¨ssbauer spectroscopy, antiferro-
magnetic ordering of the triplet molecules at low tempera-
tures can be excluded. Rather, zero-field splitting of the S
) 1 ground state with the MS ) 0 state lying at lowest energy
is found to be the origin for the deviation from Curie’s law.
Consequently, the magnetic data of 1a and 1b are fitted with
the spin-Hamiltonian16,17
Figure 4. Structures of the model systems used for AOM calculations
along with coordinate systems. (a) Model system [FeX(P)4] for the
compounds 1a (X ) Cl, Θ ) 104°) and 1b (X ) Br, Θ ) 101°). (b) Model
system [Fe(X1)(X2)(P)4] for the compounds 3a (X1 ) X2 ) Cl), 3b (X1
) X2 ) Br), and 4 (X1 ) H, X2 ) Cl).
Table 3. AOM Parameters (B ) 650, C ) 4B, All Parameters in
cm-1
)
Cl
Br
P
H
eσ
eπ
4000
1400
3300
1800
7200
300
12000
0
H ) SDS + âHgS
(1)
4a). As the absorption spectra of 1a and 1b alone do not
provide sufficient information to obtain the AOM eσ and eπ
ligand-field parameters of X and P,18 1a and 1b were treated
simultaneously with the corresponding octahedral complexes
[FeCl2(depe)2] (3a), [FeBr2(depe)2] (3b), and [FeHCl(depe)2]
(4) using one common set of parameters (Table 3). Restric-
tions to the parameter space were further imposed by the
experimentally observed spin of the ground states, that is, S
) 0 for the six- and S ) 1 for the five-coordinate complexes,
respectively.
(S ) spin-operator, â ) Bohr magneton, g ) g-tensor, H )
magnetic field, D ) zero-field splitting tensor). This leads
to zero-field splitting parameter D of +35.6 (+30.7) cm-1
and g-values of g| ) 2.516 (2.223) and g ) 2.093 (2.114)
for 1a (1b).
Thus, the X-ray, Mo¨ssbauer, and magnetic susceptibility
results indicate a five-coordinate structure for 1a and 1b
exhibiting an S ) 1 ground state with a large zero-field
splitting. The following section describes the full character-
ization of this ground state based on optical absorption
spectroscopy coupled to an angular overlap model (AOM)
analysis.
The spectra of 3a, 3b, and 4 were fitted on the basis of
model systems of D4h (3a, 3b) and C4V symmetry (4),
respectively (Figure 4b), and the results are listed in Table
4. The UV-vis spectra of dichloro and dibromo complexes
3a and 3b are closely related (Figure 5). Absorption bands
at 14100 cm-1 (709 nm; ꢀ ) 70 M-1 cm-1) (3a) and 13320
cm-1 (751 nm; ꢀ ) 60 M-1 cm-1) (3b) can be assigned to
Optical Absorption Spectroscopy and AOM
Parametrization
The electronic structure of compounds 1a and 1b was
determined on the basis of AOM calculations on a [FeX-
(P)4] (X ) Cl, Br) model system of C2V symmetry (Figure
a
transitions from the 1A1g ground state to the 1Eg sublevel of
1T1g split in D4h. The low-intensity transitions at 563 and
(16) Kahn, O. Molecular Magnetism, VCH Publishers: New York, 1993.
(17) Boyd, P. D.; Buckingham, D. A.; McMeeking, R. F.; Mitra, S. Inorg.
Chem. 1979, 18, 3585-3591.
(18) (a) Larsen, E.; La Mar, G. N. J. Chem. Educ. 1974, 51, 633-640. (b)
Scho¨nherr, T. Top. Curr. Chem. 1998, 191, 88-111.
3494 Inorganic Chemistry, Vol. 41, No. 13, 2002

FiWe-Coordinate Complexes [FeX(depe)2]BPh4
Table 4. Calculated and Observed Ligand Field Energies (kK) of
Complexes 3a, 3b, and 4
1Eg
method z2 r xz,yz x2-y2 r xy x2-y2 r xz,yz z2 r xy
1A2g
1Eg
1B2g
a
b
complex
[FeCl2(depe)2] exptla
14.1
11.3
13.3
9.3
24.6
23.4
24.3
22.2
(3a)
AOMb
[FeBr2(depe)2] exptla
(3b)
AOMb
18.8
18.8
20.5
18.8
1B2
1Ea
1A2
1Eb
[FeHCl(depe)2]
(4)
exptla
AOMb
21.5
19.1
26.0
27.0
18.7
28.6
a Measured in dichloromethane at room temperature. b Applying the
parameters listed in Table 1.
609 nm, respectively, are attributed to an oxidized species
forming via a five-coordinate intermediate because these
transitions also appear in the spectra of 1a and 1b (see later).
Obviously, the 1A2g r 1A1g transition is missing in the spectra
of 3a and 3b, in agreement with the ligand field spectrum
of the d6 low-spin system [W(N2)2(depe)2] which also has
effective D4h symmetry.3b In analogy, the absorption bands
at 24630 cm-1 (406 nm; ꢀ ) 200 M-1 cm-1) (3a) and 24330
cm-1 (411 nm; ꢀ ) 220 M-1 cm-1) (3b) are assigned to the
b
1Eg component of 1T2g, 1Eg . The shift of the 1Ega transitions
to lower energy by ∼40 nm in the spectrum of 3b as
compared to 3a is caused by the smaller σ and higher π
donating effect of the bromo compared to the chloro ligand
(see also AOM parameters listed in Table 3); a much smaller
1
b
1
shift to lower energy is observed for the Eg r A1g band.
The spectrum of 4 exhibits an intense band at 21500 cm-1
(466 nm; ꢀ ) 950 M-1 cm-1) which is assigned to the
a
transition 1Eg r 1A1g. This transition is significantly shifted
to higher energy with respect to 3a and 3b because the
hydrido ligand has a high σ-donor strength and lacks π-donor
capacity. In addition, the intensity of this band is now much
higher because the center of inversion is lost in the C4V
symmetry of this molecule, and thus, z2-pz mixing becomes
1
1
effective. The corresponding Eb r A1 transition is calcu-
lated to be at 27000 cm-1 and may be related to the shoulder
appearing in the spectrum at 26000 cm-1. The intensity of
1
1
this band is lower than that of the Ea r A1 transition, in
contrast to the bis-halide systems 3a and 3b which have more
1
b
1
intense Eg r A1g transitions.
Optical absorption spectra of 1a and 1b are displayed in
Figure 6. The absorption bands at 17790 cm-1 (562 nm) (1a)
and 16470 cm-1 (607 nm) (1b) are caused by the reaction
with O2 because the intensity of these absorption bands
increases when the solution is exposed to air. Removal of
one halogen ligand in 3a and 3b or the hydrido ligand in 4
generates the d-orbital level scheme of 1a and 1b, respec-
tively, which is given in Figure 7. Importantly, the z2 orbital
is significantly lowered in energy and now becomes popu-
lated, leading to an S ) 1 ground state. On the other hand,
the π donor action of the remaining halide ligand in
conjunction with the bending of two phosphine ligands in
the xz plane (cf. Figure 4a) puts the xz orbital at highest
energy within the former t2g orbital manifold. Shifting one
electron from this orbital to z2 leads to a 3B1 [(xz)1(z2)1] state
Figure 5. Optical absorption spectra of the octahedral complexes [FeCl2-
(depe)2] (3a), [FeBr2(depe)2] (3b), and [FeHCl(depe)2] (4) in dichlo-
romethane at room temperature.
which represents the triplet of lowest energy within the level
scheme of Figure 7 and therefore the ground state of 1a and
1b.
Using the AOM parameters of Table 3, the lowest-energy
3
ligand-field transitions of 1a and 1b (3A2 r B1; orbital
transition xz f z2) are predicted to be at 8000 cm-1 (1a)
and 7300 cm-1 (1b), respectively. Optical absorption spectra
obtained on solid mulls show this band at 7500 cm-1 (1333
nm) (1a) and 7120 cm-1 (1404 nm) (1b), respectively (Figure
8). The other spin- and electric-dipole-allowed ligand field
transitions corresponding to the d-orbital scheme of Figure
Inorganic Chemistry, Vol. 41, No. 13, 2002 3495

Franke et al.
Figure 6. Optical absorption spectra of compounds 1a and 1b in
dichloromethane at room temperature.
Figure 8. Optical absorption spectra of compounds 1a and 1b obtained
on solid mulls between sapphire windows at 10 K.
Table 5. Calculated and Observed Transition Energies (kK) of
Complexes 1a and 1b
3A2
3A1
3A2
complex
[FeCl(depe)2]BPh4 exptla
(1a)
AOMb
[FeBr(depe)2]BPh4 exptla
(1b)
AOMb
method z2 r yz x2-y2 r xz
x2-y2 r yz
7.5c
8.0
18.2
18.5
17.5, 18.3, 22.4d
17.3, 18.1, 22.2d
7.1c
7.3
a Measured in dichloromethane at room temperature. b Applying the
parameters listed in Table 1. c Obtained on solid mulls between sapphire
windows. d This orbital transition generates three 3A2 states which split in
energy because of spin coupling.
in the UV-vis region at 393 nm (1a; ꢀ ) 1150 M-1 cm-1)
and 395 nm (1b; ꢀ ) 1000 M-1 cm-1) which correspond to
CT transitions from halide and phosphine into empty or half-
filled orbitals of the Fe(II) ion (Figure 6).
Discussion: Geometric/Electronic Structure of the
Five-Coordinate Complexes and DFT Investigation of
Their Spin-Forbidden Reaction with N2
The spectroscopic and magnetic studies described in the
preceding sections have lead to a definite assignment of the
electronic ground state of five-coordinate systems [FeX-
(depe)2]BPh4, X ) Cl, Br (1a,1b). While the magnetic
susceptibility data indicated an S ) 1 ground state exhibiting
a zero-field splitting of 35 and 30 cm-1 for 1a and 1b,
Figure 7. d-Orbital level scheme of compounds 1a and 1b obtained from
the parameters of Table 3.
7 are listed in Table 5. However, the corresponding absorp-
tion bands are masked by features of much higher intensity
3496 Inorganic Chemistry, Vol. 41, No. 13, 2002

FiWe-Coordinate Complexes [FeX(depe)2]BPh4
respectively, optical absorption spectroscopy along with
AOM calculations allowed us to fully characterize this
3
electronic ground state as B1 deriving from a (xz)1(z2)1
configuration. In contrast, the corresponding six-coordinate
1
dinitrogen complexes exhibit A1g ground states with (t2g)6
configuration.4 The shift of one electron from xz to z2 in the
five-coordinate fragment has been found to be due to three
factors: (i) the energetic lowering of the z2 orbital caused
by removal of one axial ligand (N2); (ii) the concomitant
bending of two phosphine ligands away from the Fe-Cl
bond raising the energy of the xz orbital through σ donation;
and (iii) the further energy increase of this orbital by
π-donation from the halide ligand. The combined effects of
i-iii lead to a small separation between xz and z2 (5700 cm-1,
Figure 7), thus favoring a triplet ground state.
The zero-field splitting calculated by the AOM was found
to be very sensitive to the value of the P-Fe-P angle Θ
(Figure 4). On the basis of a Θ value of 104° (average of
101° and 106° measured experimentally, cf. Figure 2) and a
spin-orbit coupling constant ú of 360 cm-1,19 the 3B1 ground
state of 1a was predicted to have a zero-field splitting of
200 cm-1. The calculations showed the ground state to be
further split by a rhombic splitting E, but this was found to
be small (5 cm-1). Increasing Θ to 106° lowers the zero-
field splitting predicted for 1a to 100 cm-1. The difference
to the experimentally determined value (36 cm-1 for 1a, vide
supra) is due to covalent mixing between metal d and ligand
orbitals. On the basis of a Θ value of 101°, a zero-field
splitting of 150 cm-1 is calculated for 1b.
After the electronic and geometric structures of [FeX(N2)-
(depe)2]+ and [FeX(depe)2]+, X ) Cl (2a,1a) and Br (2b,1b),
were established, it appeared of interest to investigate the
energetics of N2 binding in these systems by DFT calcula-
tions. As in earlier investigations, we chose the B3LYP
functional combined with the LANL2DZ core potentials for
the DFT calculations.20 The geometry of the octahedral
dinitrogen model complex [FeCl(N2)(PH2CH2CH2PH2)2]+
(2a′) shown in Figure 9, top, was obtained by optimizing
the corresponding structure obtained from the crystal struc-
ture of 2a,4 simplified by replacement of the terminal ethyl
groups of the depe ligands through hydrogen atoms. On the
other hand, the ethylene bridges of these bidentate ligands
were retained to obtain a realistic geometry. Five-coordinate
model system [FeCl(PH2CH2CH2PH2)2]+ (1a′) shown in
Figure 9, bottom, was obtained by removing the N2 ligand
Figure 9. Optimized structures for model complexes 2a′ (top) and 1a′
(bottom).
Table 6. Selected Bond Lengths (Å) and Angles (deg) for the
Optimized Structures of 2a′ and 1a′
[FeCl(N2)-
(PH2CH2CH2PH2)2]+
(2a′)
[FeCl-
(PH2CH2CH2PH2)2]+
(1a′)
Fe-P(a)a
2.360
2.366
2.367
2.360
2.365
84.76
94.49
84.76
171.49
169.39
94.49
84.69
85.97
85.97
84.69
1.850
1.142
180
2.472
2.367
2.367
2.472
2.370
83.36
97.80
83.36
173.90
158.27
97.80
100.87
86.95
86.95
100.87
Fe-P(b)
Fe-P(c)
Fe-P(d)
Fe-Cl
P(a)-Fe-P(b)
P(b)-Fe-P(d)
P(c)-Fe-P(d)
P(b)-Fe-P(c)
P(a)-Fe-P(d)
P(a)-Fe-P(c)
P(a)-Fe-Cl
P(b)-Fe-Cl
P(c)-Fe-Cl
P(d)-Fe-Cl
Fe-N
N-N
N-Fe-Cl
a The phosphine designations refer to Figure 9.
from the optimized structure of 2a′ and optimizing the
resulting five-coordinate fragment. Selected bond lengths and
angles for both optimized structures shown in Figure 9 are
given in Table 6.
(19) (a) Sugano, S.; Tanabe, Y.; Kamimura, H. Multiplets of Transition
Metal Ions; Academic Press: New York, 1970. (b) Griffith, J. S. The
Theory of Transition-Metal Ions; Cambridge University Press: New
York, 1971.
FesN and FesP distances in the octahedral, optimized
structure 2a′ are slightly larger (about 0.07 Å) than experi-
mental values, and the FesCl distance is about 0.05 Å larger
than that determined experimentally. The bite angle of the
bidentate phosphine ligands is well reproduced (84.76° calcd
vs 84.63° obsd). The decrease of the PsFesCl angles from
90° observed experimentally, that is, the slight bending of
the four phosphine groups toward the axial Cl ligand, is by
3°-4° higher than that observed. For five-coordinate frag-
ment 1a′, the calculation reproduces the bending of two
phosphines away from FesCl (P(a)sFesP(d) ) 158.27°
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
Inorganic Chemistry, Vol. 41, No. 13, 2002 3497

Franke et al.
tively similar results are obtained from a nonrelaxed PES
for a [FeCl(PH3)4]+ + N2 model system based on the
equilibrium geometry of 1a.21 In contrast to this simpler
treatment, however, the triplet potential surface in the rPES
of Figure 10 appears almost flat between 2.0 and 3.5 Å with
an extremely shallow minima around 2.8 Å. The almost
dissociative character of the triplet surface provides an
explanation for the marked photochemical lability of 2a and
2b.22
The spin-crossover along the Fe-N coordinate also affects
the thermodynamic stability of Fe-N2 complex 2a with
respect to loss of N2. Retaining a singlet state s, the reaction
enthalpy for release of N2 from [FeCl(N2)(depe)2]+ would
be ∆E°s ) ∆H°s ) 28.21 kcal/mol (zero-point vibrational
energy corrected by 1/2hcν˜ ) 0.7190 kcal/mol for ν˜ (Fe-N)
) 503 cm-1 10a). However, the crossing of the triplet state t
becoming ground state of the system for Fe-N distances
larger than rC ) 2.45 Å lowers the reaction enthalpy for
release of N2 by Es(Fe-N ) ∞) - Et(Fe-N ) ∞) ) 10.09
kcal/mol, resulting in an effective ∆E° value for N2 release
of 18.27 kcal/mol. These values have been calculated for
the gas phase, but on the basis of the conservation of charge
during the reaction, no major contributions of the solvation
energy are expected in condensed phase. Because of the
liberation of N2, the entropy change S° for this reaction at
300 K is estimated to approximately correspond to the
entropy of free N2 at 300 K, S(N2; 300 K) ) 192 J/mol‚K,23
corrected for the loss of entropy stored in the Fe-N2 bond
(about 3 J/mol‚K for ν˜ (Fe-N) ) 503 cm-1 23); the
contributions to the entropy due to all other modes are
neglected. This leads to an effective entropy change of 189
J/mol‚K for release of N2, so that a free reaction enthalpy
∆G° ) ∆H° - T∆S° of 4.7 kcal/mol for N2 release (or -4.7
kcal/mol for N2 bonding) results at 300 K. This value is in
agreement with a thermally allowed reaction confirming the
observed thermal instability of [FeCl(N2)(depe)2] regarding
the loss of N2.
Figure 10. Relaxed potential energy surface (rPES) along the Fe-N
coordinate for the reaction 2a′ T 1a′ + N2.
in 1a′ vs P(1)sFesP(4) ) 152° in 1a). However, the two
other P-ligands are still slightly bent toward FesCl in 1a′
(P(b)sFesCldP(c)sFesCl ) 86.95°) whereas these ligands
are almost perpendicular to FesCl in the real structure of
1a. While the calculated FesP bond lengths are generally
too large in 1a′ (by up to 0.15 Å), the calculation correctly
reproduces the alternation of the FesP distances in this five-
coordinate molecule; that is, the “bent” (or “equatorial”)
P-ligands have larger FesP distances than the “linear” (or
“axial”) ones (calcd, 2.472 vs 2.367 Å; expt, 2.32-2.33 vs
2.27-2.28 Å).
After determination of the optimized structures of the six-
coordinate N2 complex and the five-coordinate N2-free
fragment, a relaxed potential energy surface scan (rPES)
along the Fe-N coordinate was generated. This means that
for each point of the surface all structural parameters were
optimized apart from the value of the Fe-N distance which
was scanned from 1.4 to 3.5 Å. Figure 10 gives the rPES
for the singlet state and the lowest triplet state of the system
[FeCl(depe)2]+ + N2 obtained in this manner. The figure
also contains the energies for the fully dissociated system at
r(Fe-N) ) ∞. The triplet energy at this point corresponds
to the ground-state energy of 1a′ plus the energy of free N2
obtained at the same level of calculation (B3LYP/LANL2DZ).
In agreement with the spectroscopic/AOM results, the triplet
state is calculated to be the ground state at this fully
Analogous PES calculations on N2 dissociation from the
hydride-containing model complex [FeH(PH3)4(N2)]+ indi-
cate that for all Fe-N distances the singlet state is ground
state and no spin-crossover along the Fe-N coordinate
occurs.21 In the hydrido complexes, the z2 orbital is raised
by a very strong σ-antibonding interaction so much in energy
that its involvement in a triplet ground state becomes
impossible. Moreover, the energy of the xz and yz orbitals
involved in the back-bonding of N2 is not increased by π
donation from the trans ligand which in the case of the
halogeno systems 2a and 2b has been found to be one of
the factors that contribute to the lability of the Fe-N2 bond.
Therefore, trans hydrido N2 systems should have an intrinsi-
cally higher termodynamic stability than their trans halide
counterparts, in agreement with the observation. More
1
dissociated configuration, and the A1 state represents an
excited state. In contrast, the 1A1 state is the ground state at
the Fe-N equilibrium distance, and the triplet state is an
excited state at this geometry. Consequently, the singlet and
the triplet state have to cross at some intermediate distance,
and the rPES shows this crossover to occur at r(Fe-N) )
2.45 Å ≡ rC. The transition between the two states |...xzRz2R
(triplet) and |...xzRxzâ (singlet) is enabled by spin-orbit
coupling ú Σili‚si through the matrix element z2R|ly‚sy|xzâ
) -i31/2‚i/2. As a consequence, the two surfaces repel each
other in the manner of an avoided crossing (not shown in
the figure), and the resulting lower hypersurface (corre-
sponding to E ) E(singlet) for r < rC and E ) E(triplet) for
r > rC) defines the surface for bonding/dissociation of the
N2 ligand in the system [FeCl(N2)(depe)2]+ + N2. Qualita-
(21) Lehnert, N. PhD. dissertation, University of Mainz, 1999.
(22) There exists considerable difficulty to converge the triplet state around
2 Å, probably because of orbital level crossing within the lowest triplet
states at about this distance.
(23) Moore, W. J.; Hummel, D. O. Physikalische Chemie (Physical
Chemistry); Walter de Gruyter: Berlin, 1976.
3498 Inorganic Chemistry, Vol. 41, No. 13, 2002

FiWe-Coordinate Complexes [FeX(depe)2]BPh4
unoccupied in the corresponding hydrido systems (vide
supra), making this reaction path with O2 impossible. This
explains the stability with respect to O2 of [FeH(dppe)2]+
fixing dinitrogen from air.10b Thus, the complementary
reactivity between otherwise related hydrido and halogeno
phosphine complexes with respect to N2 and O2 (hydrido,
inertness toward O2, favorable reaction with N2; halogeno,
favorable reaction with O2, low affinity toward N2) can be
traced back to the characteristic differences in their electronic
structure.
Acknowledgment. F.T. thanks P. Gu¨tlich (University of
Mainz) for measuring time on his Mo¨ssbauer and magnetic
susceptibility setups as well as W. Tremel (University of
Mainz) and W. Bensch (University of Kiel) for measuring
time on their X-ray diffractometers. Funding of this research
by Deutsche Forschungsgemeinschaft (DFG Tu58/07) and
Fonds der Chemischen Industrie (FCI) is gratefully acknowl-
edged.
Figure 11. Reaction of 1a′ and 1b′ with dioxygen (schematically).
detailed investigations on the electronic structure of Fe(II)
hydrido-N2 complexes will be presented in a forthcoming
paper.12
Electronic-structural differences may also explain the
observed reactivity differences toward dioxygen between
analogous hydrido and halogeno phosphine complexes: the
five-coordinate halogeno systems having an unpaired electron
in the z2 orbital react with O2 in the usual manner by
transferring this electron into the O2 π* orbital and binding
Supporting Information Available: Lists with atomic coor-
dinates and isotropic and anisotropic displacement parameters as
well as bond lengths and angles for 1a and 1b. Crystallographic
information in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
-
to O2 as Fe(III) (Figure 11). Therefore, solutions of these
ions are extremely air sensitive. In contrast, the z2 orbital is
IC0111987
Inorganic Chemistry, Vol. 41, No. 13, 2002 3499