Influence of the trans substituent on N2 bonding in iron(II) - Phosphane complexes: Structure, synthesis, and properties of the monomeric adducts trans-[FeXN2(depe)2]BPh4, X = Cl, Br

Article abstract of DOI:10.1002/(SICI)1521-3773(19980403)37:6<815::AID-ANIE815>3.0.CO;2-6

Not a dimer but a monomer was found in the X-ray structure analysis of the complex 'trans-[{FeCl-(depe)₂}₂(μ-N₂)](BPh₄)₂' (depe = Et₂PCH₂CH_2- PEt₂). The complexes [FeXN₂(depe)₂]BPh₄ (X = Cl, Br; structure of the cation for X = Cl shown on the right) are much less stable than the analogous hydride compounds and undergo N₂ exchange at room temperature even in the solid state.

Full text of DOI:10.1002/(SICI)1521-3773(19980403)37:6<815::AID-ANIE815>3.0.CO;2-6

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513 cm 1 (1) and 509 cm 1 (2). We succeeded in characterizing
complexes 1 and 2 by Raman spectroscopy (30 K, 30 mW,
excitation wavelength l ˆ 514.5 nm). The band for nNN was
Influence of the trans Substituent on N2
Bonding in Iron(ii) ± Phosphane Complexes:
Structure, Synthesis, and Properties of the
Monomeric Adducts
1
1
found at 2088 cm for 1 and at 2090 cm for 2. Correspond-
ing to the decrease in the intensity of the NN peak, rapid
decay of the samples upon laser irradiation was observed.
Both 1 (Figure 1a, b) and 2 were studied by Mössbauer
spectroscopy under nitrogen at various temperatures (Ta-
ble 1). The spectrum of 1 at 300 K (Figure 1b) shows two
trans-[FeXN2(depe)2]BPh4, X ˆ Cl, Br**
Beatrix E. Wiesler, Nicolai Lehnert, Felix Tuczek,*
Jörg Neuhausen, and Wolfgang Tremel
In our investigations on the bonding and activation of
dinitrogen in transition metal complexes, we prepared the
compound ªtrans-[{FeCl(depe)2}2(m-N2)](BPh4)2º (depe ˆ
Et2PCH2CH2PEt2) according to a method described by
Bellerby et al.[1] This compound is the only known example
of an N2-bridged iron(ii) dimer with octahedral coordination
of donor ligands, and thus is of interest as a potential
phosphane-analogous N2 precursor of diazene-bridged FeII
dimers with an octahedral thiolate/thioether coordination
environment.[2] We recently characterized the electronic and
vibrational properties of these compounds.[3] The known
dinuclear m-N2 ± Fe complexes include Fe0 dimers with
trigonal-bipyramidal coordination by P-donor and CO ligands
besides N2, as well as tetracoordinate FeII complexes with
diphosphane and cyclopentadienyl co-ligands.[4] Otherwise,
N2-bridged octahedral complexes are only known for the
higher homologues ruthenium(ii) and osmium(ii).
Table 1. Mössbauer parameters of 1 and 2 under N2 in mms 1 (d vs. a-iron)
T [K]
d (I)
DEQ (I)
d (II)
DEQ (II)
1
2
100
150
250
300
0.256(3)
0.244(4)
0.23(2)
0.21(1)
1.398(5)
1.371(7)
1.31(3)
1.30(2)
0.43(2)
0.38(3)
0.36(12)
0.41(3)
2.64(3)
2.64(5)
2.65(24)
2.66(7)
150
250
300
0.268(8)
0.23(1)
0.23(2)
1.42(2)
1.39(3)
1.34(4)
0.39(1)
0.34(1)
0.33(1)
2.70(1)
2.61(3)
2.61(3)
Doubts about the dimeric structure of the above-mentioned
Fe ± N2 ± depe complex were raised in 1993 by Hughes et al. on
the basis of a comparison with the analogous dmpe system
(dmpe ˆ Me2PCH2CH2PMe2), for which a monomeric struc-
ture was inferred from spectroscopic data.[5] Indeed, our X-ray
crystallograpic characterization of the chloro and bromo depe
complexes [FeClN2(depe)2]BPh4 (1) and [FeBrN2(depe)2]-
BPh4 (2) revealed a monomeric octahedral structure with end-
on terminally bound dinitrogen trans to the halide ligand. This
‡
is the first X-ray structure of an [FeXN2P4] complex with X ˆ
halide. Corresponding structures with X ˆ H are known.[6]
The reaction of [FeX2(depe)2] (X ˆ Cl, Br)[7] with N2 in
methanol at room temperature yields an orange solution.
After the addition of NaBPh4, crystals of [FeXN2(depe)2]-
BPh4 (X ˆ Cl 1[8], X ˆ Br 2[9]) precipitate. In the IR spectrum
the N ± N stretching mode is observed in a region typical for
complexes in which N2 is coordinated end-on (1: nÄNN ˆ 2088, 2:
1
nÄNN ˆ 2091 cm 1; cf. nÄNN ˆ 2090 cm in [FeHN2(depe)2]BPh4
(3)[7]) but at lower wavenumber than in the analogous, but not
structurally characterized, compounds [FeXN2(dmpe)2]BPh4
(X ˆ Cl: nÄNN ˆ 2105, X ˆ Br: nÄNN ˆ 2107cm 1).[10] A further
band associated with the Fe(ii) ± N2 unit is observed at
Figure 1. Mössbauer spectra of 1 under N2 and He atmospheres. Trel
relative transmission.
ˆ
doublets with quadrupole splittings DEQ of 1.30 (I) and
[*] Dr. F. Tuczek, Dipl.-Chem. B. E. Wiesler, Dipl.-Chem. N. Lehnert
Insitut für Anorganische und Analytische Chemie der Universität
Staudingerweg 9, D-55099 Mainz (Germany)
1
2.66 mms (II) and an isomer shift of d ˆ 0.21 (I) and
0.41 mms 1 (II). Doublet (I) is assigned to complex 1, which is
in agreement with Mössbauer parameters given in the
literature and a prediction by Silver.[11, 12] Correspondingly,
DEQ and d were determined for complex 2 to be 1.34 and
Fax: (‡49)6131-39-2990
E-mail: tuczek@iacgu7.chemie.uni-mainz.de
Dr. J. Neuhausen, Prof. Dr. W. Tremel
Insitut für Anorganische und Analytische Chemie der Universität
Becherweg 24, D-55099 Mainz (Germany)
1
0.23 mms , respectively (Table 1). Doublet (II) is attributed
to the analogous N2-free complex, because its intensity
increases in a He atmosphere at room temperature (Fig-
ure 1c). Interestingly, this transformation is reversible: Re-
peating the measurement in an N2 atmosphere again yields
[**] We thank S. Stauf for single-crystal X-ray measurements, Prof. Dr. P.
Gütlich for providing the Mössbauer instruments, and the Deutsche
Forschungsgemeinschaft (Tu 58-04/2) and Fonds der Chemischen
Industrie for financial support.
Angew. Chem. Int. Ed. 1998, 37, No. 6
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the original spectrum (Figure 1d). Hence, the complex under-
goes N2 exchange at room temperature in the solid state, and
the N2-free product reversibly binds N2 in the solid state.
These observations demonstrate the lability of the Fe ± N
bond in these complexes.
The monomeric structure of 1 is shown in Figure 2. In the
distorted octahedral environment of the iron atom, the end-
on-bound N2 is trans to the chloride ligand. One of the two
strongest s donor and therefore has the greatest influence in
this respect. This is in accordance with the observation that
the bis-hydrido unit has as strong a bonding capability for N2
as the hydride-free ruthenium and osmium complexes.[20] In
comparison, halides are in general weaker donors and also
have p-donor character. The latter is an additional disadvan-
tage as the p-donor ligand mixes with the t2g orbitals which are
responsible for backbonding, and hence the Fe ± N overlap is
reduced. Molecular orbital calculations seem to confirm this
assumption,[21] which is of general significance for the stability
of transition metal ± N2 adducts.
In view of the weakness of the Fe ± N bond, it is not
surprising that 1 has a monomeric stucture, as the bridging
coordination of N2 requires a certain stability of the Fe ± N
bond. Therefore, the existence of bridged, octahedrally
coordinated iron(ii) ± N2 systems at room temperature appears
very unlikely.
Received: October 7, 1997 [Z11010IE]
German version: Angew. Chem. 1998, 110, 856 ± 858
Figure 2. Structure of the cation of 1 (thermal ellipsoids are drawn at the
30% probability level; H atoms omitted for clarity). Selected bond
lengths [Š] and angles [8]: Fe(2) ± P(1) 2.293(3), Fe(2) ± P(3) 2.294(4),
Fe(2) ± P(6) 2.280(3), Fe(2) ± P(8) 2.298(4), Fe(2) ± Cl(1) 2.311(3), Fe(2) ±
N(4) 1.784(9), N(3) ± N(4) 1.073(11), P(3)-Fe(2)-P(6) 96.58(12), P(3)-Fe(2)-
P(1) 84.63(12), P(6)-Fe(2)-P(8) 84.18(12), P(1)-Fe(2)-P(8) 94.54(12), P(3)-
Fe(2)-N(4) 89.3(3), P(3)-Fe(2)-Cl(1) 89.71(12), P(1)-Fe(2)-N(4) 92.5(3),
P(6)-Fe(2)-Cl(1) 86.64(12), Fe(2)-N(4)-N(3) 177.5(10).
Keywords: iron ´ Moessbauer spectroscopy ´ N2 complexes ´
nitrogen fixation ´ P ligands
[1] J. M. Bellerby, M. J. Mays, P. L. Sears, J. Chem. Soc. Dalton Trans.
1976, 1232 ± 1236.
[2] D. Sellmann, A. Hennige, Angew. Chem. 1997, 109, 270 ± 271; Angew.
Chem. Int. Ed. Engl. 1997, 36, 276 ± 278.
[3] a) N. Lehnert, B. Wiesler, F. Tuczek, A. Hennige, D. Sellmann, J. Am.
Chem. Soc. 1997, 119, 8869 ± 8878; b) N. Lehnert, B. Wiesler, F. Tuczek,
A. Hennige, D. Sellmann, ibid. 1997, 119, 8879 ± 8888
independent cations in the unit cell exhibits a partial disorder
between the chloride and dinitrogen ligands.[13] In the follow-
ing discussion of bond lengths we exclusively consider the
geometry of the undisordered cation (Figure 2). The N ± N
bond length is comparable to that in the related complex
[FeH(N2)(depe)2]BPh4 (3),[14] the Fe ± N bond is slightly
shorter, and the Fe ± P bonds are considerably longer (3:
N ± N 1.070(12), Fe ± N 1.825(7), Fe ± P (av) 2.240 Š). Fur-
thermore, the Fe ± Cl bond is slightly shorter than those in
[FeHCl(depe)2] (4)[15] and [FeCl2(depe)2] (5)[16] (4: Fe ± Cl
2.404(2), Fe ± P (av) 2.208; 5: Fe-Cl 2.344(2), Fe ± P (av)
2.260 Š). We also obtained single crystals of [FeBr(N2)-
(depe)2]BPh4 (2) that were suitable for an X-ray structure
analysis. Initial results confirm that the cations in 2 are
structurally analogous to those of 1. However, due to the
strong disorder of the ligands in the two symmetry independ-
ent cations, a precise determination of the geometry was not
possible up to now.[17]
In general iron(ii) ± dinitrogen complexes are stable if they
contain tertiary phosphane and hydride ions as co-ligands.[18]
This is demonstrated by the ability of [FeH4(PEtPh2)3] and
[FeH(Ph2PCH2CH2PPh2)2]BPh4 to fix dinitrogen from air.[19]
In contrast, the dinitrogen ligands in 1 and 2 are only weakly
bound. A possible explanation for this difference is that the
bond between the transition metal and dinitrogen is domi-
nated by p backbonding into the p* orbitals of N2. This
interaction can be enhanced by co-ligands that lower the
effective nuclear charge Zeff of the metal and thus raise the
energy of the backbonding d orbitals. The hydride ion is the
[4] a) H. Kandler, C. Gauss, W. Bidell, S. Rosenberger, T. Bürgi, I. L.
Eremenko, D. Veghini, O. Orama, P. Burger, H. Berke, Chem. Eur. J.
1995, 1, 541 ± 548; b) H. Berke, W. Bankhardt, G. Huttner, J.
von Seyerl, L. Zsolnai, Chem. Ber. 1981, 114, 2754 ± 2768; c) W. E.
Silverthorn, J. Chem. Soc. Chem. Commun. 1971, 1310 ± 1311; d) D.
Sellmann, E. Kleinschmidt, J. Organomet. Chem. 1977, 140, 211 ± 219;
e) D. Sellmann, E. Kleinschmidt, Angew. Chem. 1975, 87, 595 ± 596;
Angew. Chem. Int. Ed. Engl. 1975, 14, 571.
[5] D. L. Hughes, G. J. Leigh, M. Jimenez-Tenorio, A. T. Rowley, J. Chem.
Soc. Dalton Trans. 1993, 75 ± 82.
[6] a) I. E. Buys, L. D. Field, T. W. Hambley, A. E. D McQueen, Acta
Cryst. 1993, C49, 1056 ± 1059; b) A. Hills, D. L. Hughes, M. Jimenez-
Tenorio, G. J. Leigh, J. Organomet. Chem. 1990, 391, C41 ± C44; c) see
ref. [5].
[7] M. J. Mays, B. E. Prater, Inorg. Synth. 1974, 15, 21 ± 28.
[8] All reactions were carried out under dry N2 in a glovebox or by
standard Schlenk techniques. All solvents were distilled under Ar
from an appropriate drying agent prior to use. The spectra were
measured with the following instruments: IR spectra (CsI pellets):
Matson Instruments 2030 Galaxy FTIR spectrometer, UV/Vis spectra
(methanol, N2 atmosphere): Bruins Omega 10 spectrophotometer,
NMR spectra: Bruker DRX 400.
A solution of [FeCl2(depe)2] (0.4 g, 0.74 mmol) in MeOH (50 mL) was
stirred at room temperature under N2 over night. After the addition of
NaBPh4 (0.37 g, 1.1 mmol) in MeOH (10 mL), orange cubes of 1
precipitated slowly and were collected by filtration under N2. Yield:
1
0.6 g (95%); H NMR (400 MHz, CDCl3, 233 K): d ˆ 1.19 (m, CH3),
1.88 (m, CH2), 7.13 (m, Ph); 31P NMR (200 MHz, solid state, 303 K):
d ˆ 61.5 (quin); 31P NMR(400 MHz, CDCl3, 233 K): d ˆ 65.7 (s); IR
(CsI): nÄNN ˆ 2088 cm 1; UV/Vis: lmax(e) ˆ 280 (7575), 333 (sh; 2272),
382 (2020), 420 nm (sh; 1818 Lmol 1 cm 1); elemental analysis calcd
for C44H68BClN2P4Fe (850.99): C 62.1, H 8.1, N 3.3; found: C 62.0, H
8.2, N 2.0. Small amounts of oxygen caused an immediate color change
of the orange solutions of 1 and 2 to red (1) or green (2). After the
816
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addition of NaBPh4, correpondingly colored needles precipitated,
which regained an orange color under an N2 atmosphere.
A Kinetically Stabilized [1.1]Paracyclophane:
Isolation and X-Ray Structural Analysis**
[9] A solution of [FeBr2(depe)2] (0.35 g, 0.56 mmol) in MeOH (40 mL)
was stirred at room temperature under N2 overnight. After the
addition of NaBPh4 (0.28 g, 0.84 mmol) in MeOH (10 mL), orange
prisms of 2 precipitated slowly and were collected by filtration under
nitrogen. Yield: 0.45g (90%); 1H NMR (200 MHz, CD3OD, 303 K):
d ˆ 1.19 (m, CH3), 1.98 (m, CH2), 7.15 (m, Ph); 31P NMR (400 MHz,
CD3OD, 233 K): d ˆ 63.8 (s); IR (CsI): nÄNN ˆ 2091 cm 1; UV/Vis:
lmax(e) ˆ 284 (8666), 328 (sh; 4000), 394 nm (1818 Lmol 1 cm 1);
elemental analysis calcd for C44H68BBrN2P4Fe (895.45): C 59.0, H
7.7, N 3.1; found: C 56.3, H 7.4, N 1.6.
Hidetoshi Kawai, Takanori Suzuki, Masakazu Ohkita,
and Takashi Tsuji*
In 1993 we reported preparation of the first [1.1]para-
cyclophane, the bis(methoxycarbonyl) derivative 2b.[1] The
low thermal stability of 2b and of the subsequently synthe-
sized 2a[2]Ðboth persistent only below
208C in dilute
solutionÐvirtually precluded exploration of their physical
and chemical properties. The lability of [1.1]paracyclophanes
seems to arise from susceptibility of the bridgehead carbon
atoms toward addition of numerous reagents, upon which the
steric strain inherent in the system is largely relieved. In other
words, it seems plausible that the [1.1]paracyclophane skel-
[10] a) A. Hills, D. L. Hughes, M. Jimenez-Tenorio, G. J. Leigh, J.
Organomet. Chem. 1990, 391, C41 ± C44; b) see ref. [5].
[11] d ˆ 0.26 mms 1 and DEQ ˆ 1.42 mms 1 at 77 K; see ref. [5].
[12] J. Silver, Inorg. Chim. Acta 1991, 184, 235 ± 242.
[13] X-ray structure analysis of 1: An orange cube was mounted on a glass
fiber by standard inert-gas techniques and cooled immediately. Crystal
Å
data for 1: triclinic, P1 ( ꢀ no. 2), a ˆ 13.425(3), b ˆ 17.399(5), c ˆ
19.937(5) Š, a ˆ 102.02(2), b ˆ 91.16(1), g ˆ 95.7(3)8, Vˆ 4528(2) Š3,
1
1
Z ˆ 4, 1calcd ˆ 1.248 gcm
,
m(MoKa) ˆ 0.566 mm
. The data were
collected on
a
Siemens P4 diffractometer (MoKa radiation, l ˆ
0.71073 Š, graphite monochromator, Tˆ 203 K) in the range 4 <
2q < 508; 16528 (15785 symmetry independent) reflections were
measured. The phase problem was solved by direct methods (SIR92),
from which the heavy atoms were located. The other non-hydrogen
atoms were found in subsequent cycles of refinements (SHELXL93)
and difference Fourier maps. The hydrogen atoms were taken into
account at idealized sites and with isotropic thermal parameters in the
final cycle of refinement; 494 parameters, semiempirical absorption
correction (y scan) R1 ˆ 0.077 (3108 reflections with Fo > 4s(Fo)),
wR2 ˆ 0.202 (all data). The unit cell contains two symmetrically
independent cations. One of these cations shows some disorder of the
chlorine and dinitrogen ligands. This disorder over the two trans
positions (N2, Cl) was modeled by partial occupation of these
positions with Cl and N2 with a fixed N ± N bond length of 1.10 Š.
The occupation ratio Cl :N2 of the two positions was refined to 56:44
and 44:56, respectively. Anisotropic thermal parameters were refined
for the heavy atoms (Fe, P, Cl); C and N atoms were refined
isotropically. In the discussion of the molecular geomety, we
exclusively describe the cation which is not disordered. Further
details of the crystal structure investigations may be obtained from the
Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopold-
shafen, Germany (fax: (‡49)7247-808-666 (Frau S. Höhler-Schlimm);
e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository number
CSD-407743.
eton might be kinetically stabilized by the introduction of
substituents that sterically shield all four bridgehead sites.
Although fulfillment of such a requirement appears formida-
ble at first, examination of molecular models suggests that
kinetic stabilization of the [1.1]paracyclophane system might
be achieved through 2c, for which the appropriate precursor 1
should also be accessible. Here we report the isolation and X-
ray structural analysis of 2c as well as its interconversion with
the corresponding transannular addition product 3c.
The synthesis of precursor 1c is outlined in Scheme 1.
Although addition of 5[3] to 6 was significantly slower than
that of 5 to 4,[2] anti-bis-adduct 7 was obtained after prolonged
irradiation of 6 in a saturated solution of 5 in CH2Cl2. The
conversion of 7 into 10 via 8[4] and 9 proceeded uneventfully,
and 10 was transformed into diamide 13 by addition of
LiNMe2 followed by hydrolysis. However, the carbon atoms
adjacent to the amide groups in 13 are already so sterically
hindered that initial attempts to prepare 11 (e.g. by reaction of
12[5] with PhSeBr, and of 13 with KH/PhSe2[6]) proved
fruitless. Taking a hint from the reaction of (phenylselenyl)-
amines with unsaturated carbonyl compounds,[7] we obtained
11 by the reaction of 10 with PhSeNMe2.[8] Oxidation of 11 to
the selenoxide and subsequent elimination of PhSeOH
proceeded smoothly to afford 1c.
[14] I. E. Buys, L. D. Field, T. W. Hambley, A. E. D. McQueen, Acta
Crystallogr. Sect. C 1993, C49, 1056 ± 1059.
[15] B. E. Wiesler, F. Tuczek, C. Näther, W. Bensch, Acta Crystallogr. Sect.
C 1998, 54, 44 ± 46.
[16] M. V. Baker, L. D. Field, T. W. Hambley, Inorg. Chem. 1988, 27, 2872 ±
2876.
[17] Preliminary crystal data of 2: triclinic, P1 (no. 1), a ˆ 11.5727(3), b ˆ
13.3353(4), c ˆ 16.6220(3) Š, a ˆ 108.23(1), b ˆ 101.235(2), g ˆ
1
104.545(2)8, Vˆ 2251(1) Š3, Z ˆ 2, 1calcd ˆ 1.321 gcm
,
m(MoKa) ˆ
1
1.40 mm
.
[18] C. A. Ghilardi, S. Midollini, L. Sacconi, P. Stoppioni, J. Organomet.
Chem. 1981, 205, 193 ± 202.
[19] P. Giannoccaro, M. Rossi, A. Sacco, Coord. Chem. Rev. 1972, 8, 77 ±
79; M. Aresta, P. Giannoccaro, M. Rossi, A. Sacco, Inorg. Chim. Acta
1971, 5, 203 ± 206.
Irradiation of 1c in degassed n-decane with a low-pressure
mercury lamp at 208C led to formation of the corresponding
[20] M. Aresta, P. Giannoccaro, M. Rossi, A. Sacco, Inorg. Chim. Acta
1971, 5, 203 ± 206.
[21] N. Lehnert, F. Tuczek, J. Am. Chem. Soc., submitted.
[*] Prof. Dr. T. Tsuji, H. Kawai, Prof. Dr. T. Suzuki, Dr. M. Ohkita
Division of Chemistry, Graduate School of Science
Hokkaido University, Sapporo 060 (Japan)
Fax: (‡81)11-746-2557
E-mail: tsuji@science.hokudai.ac.jp
[**] This research was supported by a Grant-in-Aid for Scientific Research
(08454192) from the Japanese Ministry of Education, Science, Sport,
and Culture.
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1433-7851/98/3706-0817 $ 17.50+.50/0
817