The linear tetraphos ligand prP4 (1,1,4,8,11,1-hexaphenyl-1,4,8, 11-tetraphosphaundecane, Ph2PCH2CH2P(Ph)CH 2CH2CH2P(Ph)CH2CH 2PPh2): Synthesis and characterization of cis- and trans-[Fe(NCS)2(prP4)]

Article abstract of DOI:10.1021/om030579k

The stereochemistry of prP₄ (l,l,4,8,11,11-hexaphenyl-l,4,8,11- tetraphosphaundecane) and its coordination properties in mononuclear complexes are explored. The free ligand is obtained as a mixture of meso and rac diastereomers from the reaction of dilithium 1,3-bis(phenylphosphido)propane- tetrakis(tetrahydrofuran) ( Lippp ) with 2 equiv of l-chloro-2(diphenylphosphino)ethane. Reaction of prP₄ with FeCl ₂ and KSCN leads to cis- and trans-[Fe(NCS)₂(prP ₄)] complexes of the meso and the rac ligands; the composition of this mixture has been fully analyzed. The iron isothiocyanato complexes cis-α- and trans-[Fe(NCS)₂(rac-prP₄)] have been characterized by X-ray structure determinations. The ³¹P NMR spectrum of the free tetraphos ligand shows an AA'XX' coupling scheme with three groups of signals centered at -20.6 and -20.7 ppm and at -12.4 ppm, which belong to the rac and meso isomers, respectively. ³¹P NMR spectra of the cis and trans iron isothiocyanato complexes of rac-prP₄ exhibit AA'XX' patterns as well, which can be reproduced by simulation, trans[Fe(NCS) ₂(meso-prP₄)] is identified by its ³¹P NMR spectrum, which is similar to that of its rac counterpart. On the basis of NMR spectroscopy it is found that the cis-α complex of the rac ligand converts into the trans-rac complex with a half-life of approximately 2 days. Infrared and Raman spectra of the cis-α and trans isomers of [Fe(NCS) ₂(rac-prP₄)] reveal characteristic differences in the region of the C-N stretching vibrations.

Full text of DOI:10.1021/om030579k

3252
Organometallics 2004, 23, 3252-3258
Th e Lin ea r Tetr a p h os Liga n d p r P 4
(1,1,4,8,11,11-Hexa p h en yl-1,4,8,11-tetr a p h osp h a u n d eca n e,
P h 2P CH2CH2P (P h )CH2CH2CH2P (P h )CH2CH2P P h 2):
Syn th esis a n d Ch a r a cter iza tion of cis- a n d
tr a n s-[F e(NCS)2(p r P 4)]†
Carsten M. Habeck, Christian Hoberg, Gerhard Peters, Christian Na¨ther, and
Felix Tuczek*
Institut fu¨r Anorganische Chemie, Christian-Albrechts-Universita¨t Kiel,
D-24098 Kiel, Germany
Received August 13, 2003
The stereochemistry of prP4 (1,1,4,8,11,11-hexaphenyl-1,4,8,11-tetraphosphaundecane) and
its coordination properties in mononuclear complexes are explored. The free ligand is obtained
as a mixture of meso and rac diastereomers from the reaction of dilithium 1,3-bis-
(phenylphosphido)propane-tetrakis(tetrahydrofuran) (“Lippp”) with 2 equiv of 1-chloro-2-
(diphenylphosphino)ethane. Reaction of prP4 with FeCl2 and KSCN leads to cis- and trans-
[Fe(NCS)2(prP4)] complexes of the meso and the rac ligands; the composition of this mixture
has been fully analyzed. The iron isothiocyanato complexes cis-R- and trans-[Fe(NCS)2(rac-
prP4)] have been characterized by X-ray structure determinations. The 31P NMR spectrum
of the free tetraphos ligand shows an AA′XX′ coupling scheme with three groups of signals
centered at -20.6 and -20.7 ppm and at -12.4 ppm, which belong to the rac and meso
isomers, respectively. 31P NMR spectra of the cis and trans iron isothiocyanato complexes of
rac-prP4 exhibit AA′XX′ patterns as well, which can be reproduced by simulation. trans-
[Fe(NCS)2(meso-prP4)] is identified by its 31P NMR spectrum, which is similar to that of its
rac counterpart. On the basis of NMR spectroscopy it is found that the cis-R complex of the
rac ligand converts into the trans-rac complex with a half-life of approximately 2 days.
Infrared and Raman spectra of the cis-R and trans isomers of [Fe(NCS)2(rac-prP4)] reveal
characteristic differences in the region of the C-N stretching vibrations.
In tr od u ction
ethane) and depe (bis(diethylphosphino)ethane), which
have been employed in the field of synthetic nitrogen
fixation as coligands of molybdenum and tungsten
dinitrogen complexes.3 A problem with these systems
and their counterparts possessing monodentate phos-
phine ligands is the breaking of phosphine-metal bonds
at the NNH2 and NNH3 stages of the conversion of
coordinated N2 to NH3.3a,d With respect to the construc-
tion of a catalytic cycle based upon the “P4 platform”,4
however, maximum stability of the equatorial phosphine
ligation is desirable. Employing a tetradentate ligand
such as prP4 in Mo/W dinitrogen systems might be a
promising strategy toward this goal. So far, however,
only binuclear complexes of prP4 have been prepared.2a
In the present paper, the stereochemistry of prP4 and
its coordination properties in mononuclear complexes
are explored. To this end, iron thiocyanato complexes
of prP4 have been synthesized and characterized both
structurally and spectroscopically. Iron complexes of
tetraphos-I have been investigated intensively.1a,c,e In
analogy to this ligand, prP4 is expected to have three
Since King and Kapoor’s pioneering study on the
synthesis and ligation properties of the tetradentate
ligand tetraphos-I (1,1,4,7,10,10-hexaphenyl-1,4,7,10-
tetraphosphadecane), many investigations of this ligand
and its transition-metal complexes have been per-
formed.1 In contrast, only a handful of studies on the
analogous tetradentate phosphine ligand 1,1,4,8,11,11-
hexaphenyl-1,4,8,11-tetraphosphaundecane (“prP4”), pos-
sessing a central C3 group, have appeared.2 We are
interested in this ligand as a potential alternative to
the bidentate ligands dppe (bis(diphenylphosphino)-
† Dedicated to Prof. Dr. G.-V. Ro¨schenthaler on the occasion of his
60th anniversary.
* To whom correspondence should be addressed. E-mail: ftuczek@
ac.uni-kiel.de. Fax: +49 431 880 1520.
(1) (a) King, R. B.; Kapoor, R. N.; Saran, M. S.; Kapoor, P. N. Inorg.
Chem. 1971, 10(9), 1851. (b) Bautista, M. T.; Earl, K. A.; Maltby, P.
A.; Morris, R. H. Can. J . Chem. 1994, 72, 547. (c) Ghilardi, C. A.;
Midollini, S.; Sacconi, L.; Stoppioni, P. J . Organomet. Chem. 1981, 205,
193. (d) Goller, H.; Bru¨ggeller, P. Inorg. Chim. Acta 1992, 197, 75. (e)
Bacci, M.; Midollini, S.; Stoppioni, P.; Sacconi, L. Inorg. Chem. 1973,
12(8), 1801. (f) Oberhauser, W.; Bachmann, C.; Bru¨ggeller, P. Polyhe-
dron 1995, 14(6), 787. (g) Bru¨ggeller, P. Inorg. Chim. Acta 1989, 155,
45. (h) Chen, J -D.; Cotton, F. A.; Hong, B. Inorg. Chem. 1993, 32,
2343. (i) King, R. B.; Kapoor, P. N. J . Am. Chem. Soc. 1971, 93(17),
4158.
(3) (a) Henderson, R. A.; Leigh, G. J .; Pickett, C. J . Adv. Inorg.
Radiochem. 1983, 27, 197. (b) Fryzuk, M.; J ohnson, S. A. Coord. Chem.
Rev. 2000, 200, 379. (c) Hidai, M.; Mizobe, Y. Chem. Rev. 1995, 95,
1115. (d) Horn, K. H.; Lehnert, N.; Tuczek, F. Inorg. Chem. 2003, 42,
1076.
(4) Alias, Y.; Ibrahim, S. K.; Queiros, M. A.; Fonseca, A.; Talarmin,
J .; Volant, F.; Pickett, C. J . J . Chem. Soc., Dalton Trans. 1997, 4807.
(2) (a) Brown, J . M.; Canning, L. R. J . Organomet. Chem. 1984, 267,
179. (b) DuBois, D. L.; Myers, W. H.; Meek, D. W. J . Chem. Soc., Dalton
Trans. 1975, 1011.
10.1021/om030579k CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/27/2004

Fe Complexes of a Linear Tetraphos Ligand
Organometallics, Vol. 23, No. 13, 2004 3253
Ch a r t 1. m eso a n d r a c Isom er s of th e p r P 4 Liga n d
Sch em e 1. Syn th etic Rou te for th e p r P 4 Liga n d
Ch a r t 2. P ossible Isom er s for Octa h ed r a l p r P 4
Com p lexes
identity of this isomer, however, was not specified.2 To
conserve the original isomeric composition of the prP4
ligand, we refrained from subsequent pyrolysis of the
reaction product as performed by various authors.1d,f,2a
Spectroscopic and structural characterization of the free
ligand and its iron thiocyanato complexes are presented
in the following sections.
NMR Sp ectr oscop y a n d Sp ectr a l Sim u la tion of
F r ee p r P 4. The 31P NMR spectrum of the free prP4
ligand shows three groups of signals corresponding to
two isomers (Figure 1). Each isomer gives a four-line
spectrum consisting of two doublets. Because of the
higher shielding of the alkyl as compared to the phenyl
groups, the signals at ∼ -20 ppm are assigned to the
spins of the asymmetric phosphorus atoms and the
signals at -12.4 ppm to the terminal ones. This assign-
ment is supported by the fact that the signals of the
latter atoms have identical shifts in both isomers and
therefore are superposed, whereas the doublets of the
asymmetric P atoms exhibit slightly different chemical
shifts (-20.6 and -20.7 ppm), resulting in a four-line
group. On the basis of the relative NMR intensities of
the various [Fe(NCS)2(prP4)] complexes prepared from
the diastereomeric ligand mixture of Figure 1 (see
below), the pair of signals at lower field (high intensity)
is assigned to the meso isomer of prP4. As shown in
Figure 1, the spectrum of each isomer can be simulated
on the basis of an AA′XX′ system with the coupling
constants for the AX and A′X′ interactions being 29 Hz,
respectively, and all other constants being 0.2b
P r im a r y Rea ction P r od u ct w ith [F e(NCS)2]. To
further characterize the prP4 ligand and define its
stereochemistry, complexes with iron(II) and thiocyan-
ate were prepared. The 31P NMR spectrum of the
primary reaction product of FeCl2, KSCN, and prP4
indicates the presence of three species, each of which
gives rise to two multiplets (Figure 2). The various
subspectra could be assigned to specific isomers of [Fe-
(NCS)2(prP4)] on the basis of fractional crystallization
of the three trans and cis bis(isothiocyanato)iron com-
plexes trans-[Fe(NCS)2(rac-prP4)] (1), trans-[Fe(NCS)2-
(rac-prP4)]‚CH2Cl2 (2), and cis-R-[Fe(NCS)2(rac-prP4)]‚
CH2Cl2 (3) and measurement of their individual NMR
spectra. Here the crystal structures of 1-3 are reported
and their 31P NMR spectra are discussed.
isomers, one meso and one rac diastereomer, the latter
existing as a racemic mixture of two enantiomers (Chart
1). The two asymmetric phosphorus atoms in the the
meso compound exhibit R,S and S,R configurations,
respectively, whereas the stereoisomers of the rac
compound have R,R and S,S configurations at the
asymmetric P atoms, respectively. In further analogy
to tetraphos-I, the rac isomer of the ligand can form
octahedral complexes in three different ways: cis-R, cis-
â, and trans (Chart 2).5 Owing to steric reasons, the
meso ligand has only two possibilities to form octahedral
complexes: trans- and cis-â. Reacting prP4 with FeCl2
and 2 equiv of KSCN afforded the complexes trans-[Fe-
(NCS)2(rac-prP4)] (1), trans-[Fe(NCS)2(rac-prP4)]‚CH2-
Cl2 (2), and cis-R-[Fe(NCS)2(rac-prP4)]‚CH2Cl2 (3), as
well as trans-[Fe(NCS)2(meso-prP4)] (4). 1-3 were char-
acterized by single-crystal X-ray structure analyses and
by 31P NMR as well as optical and vibrational spectros-
copy. Evidence for the formation of complex 4 was
obtained by 31P NMR. Here the structural and spectro-
scopic properties of 1-4 are presented and the conse-
quences with respect to the coordination properties of
prP4 are discussed.
Resu lts a n d Discu ssion
Syn th esis a n d Sp ectr oscop ic P r op er ties of p r P 4.
The prP4 ligand was prepared in analogy to a published
synthesis of tetraphos-I (Scheme 1).1b Like this ligand,
prP4 exists in two isomeric forms, meso and rac (vide
supra). In analogy to the synthesis of tetraphos-I giving
70% meso and 30% rac, the preparation of prP4 leads
to a mixture of both isomers, however, with varying
proportions. This result is in contrast to previous reports
on the synthesis of prP4 (however, by different routes),
where formation of only one isomer was claimed. The
Str u ctu r es of tr a n s-[F e(NCS)2(r a c-p r P 4)] (1) a n d
tr a n s-[F e(NCS)2(r a c-p r P 4)]‚CH2Cl2 (2). The first spe-
cies could be identified as the trans-rac isomer found
in the two pseudopolymorphic forms 1 and 2 which
cocrystallize from concentrated solutions of the primary
reaction product. Complex 1 has the rhombohedral
(5) (a) Bosnich, B.; J ackson, W. G.; Wild, S. B. J . Am. Chem. Soc.
1973, 95, 8269. (b) Gavrilova, A. L.; Bosnich, B. Chem. Rev. 2004, 104,
349.

3254 Organometallics, Vol. 23, No. 13, 2004
Habeck et al.
F igu r e 1. Experimental (bottom) and simulated (top) 31P NMR spectra of prP4 (asterisks mark the signals of the rac
isomer).
F igu r e 2. 31P NMR spectra of the primary reaction product of FeCl2 with prP4 and 2 equiv of KSCN, containing three
different isomers.
space group R3h, whereas crystals of 2 have the mono-
clinic space group P21/c (Table 1). In both complexes the
thiocyanate ligands are bound through the nitrogen
atoms (Figure 3). Important bond lengths and angles
for all complexes are collected in Table 2. The average
Fe-N bond lengths are ∼1.93 Å in both complexes, and
the P-Fe bond lengths are between 2.238(1) and 2.310-
(1) Å. The N-Fe-N angle is 178.56(6)° for 1 and 174.58-
(6)° for 2. The two thiocyanate groups are almost linear;
i.e., the S-C-N angles are 179.0° for 1 and 178.4(2)°
for 2. The N-C bonds have an average length of about
1.16 Å, and the C-S bond lengths are ∼1.63 Å. Fe-
N-C angles have values of 176.0(2) and 171.9(2)° for 1
and 175.5(2) and 167.4(2)° for 2.
NMR Sp ectr a a n d Sp ectr a l Sim u la tion of tr a n s-
[F e(NCS)2(p r P 4)]. The spectrum of the pure complex
trans-[Fe(NCS)2(rac-prP4)] is shown in Figure 4. As in
case of the 31P NMR spectrum of free prP4, the spectrum
can be interpreted on the basis of an AA′XX′ coupling
scheme. There are two groups of signals, both multip-
lets, centered at 77.5 and 62.6 ppm. The peaks at 62.6
ppm are caused by the spins of the asymmetric (inner)
phosphorus atoms, whereas the multiplet at 77.5 ppm
is derived from the terminal P atoms. Simulation with
the coupling constants given in Table 3 shows very good
agreement with the measured spectrum (Figure 4). The
cis coupling constant between the A and X spins has a
value of -26 Hz. With 112 Hz (A/X′ and A′/X) the

Fe Complexes of a Linear Tetraphos Ligand
Organometallics, Vol. 23, No. 13, 2004 3255
F igu r e 4. Experimental (bottom) and simulated (top) 31
NMR spectra of trans-[Fe(NCS)2(rac-prP4)] (1).
P
F igu r e 3. Single-crystal structure of trans-[Fe(NCS)2(rac-
prP4)] (1) with the labeling scheme.
Ta ble 1. Cr ysta llogr a p h ic Da ta for
tr a n s-[F e(NCS)2(r a c-p r P 4)] (1),
tr a n s-[F e(NCS)2(r a c-p r P 4)]·CH2Cl2 (2), a n d
cis-r-[F e(NCS)2(r a c-p r P 4)]·CH2Cl2 (3)
1
2
3
chem formula
C
45H44N2-
FeS2
C
46H46N2-
FeS2Cl2
C
46H46N2-
FeS2Cl2
mol wt
cryst color
cryst syst
space group
a/Å
856.67
red
rhombohedral
R3h
21.107(1)
21.107(1)
49.945(3)
90
90
120
19 271(2)
18
941.6
red
941.6
red
triclinic
P1h
9.4592(8)
13.488(1)
18.659(2)
102.40(1)
99.726(1)
105.89(1)
2169.6(2)
2
monoclinic
P21/c
13.360(1)
17.393(1)
20.196(2)
90
108.46(1)
90
4451.5(6)
4
b/Å
c/Å
R/deg
â/deg
F igu r e 5. Single-crystal structure of cis-R-[Fe(NCS)2(rac-
prP4)] (3) with the labeling scheme.
γ/deg
V/Å3
Z
D
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for tr a n s-[F e(NCS)2(r a c-p r P 4)] (1),
tr a n s-[F e(NCS)2(r a c-p r P 4)]·CH2Cl2 (2), a n d
cis-r-[F e(NCS)2(r a c-p r P 4)]·CH2Cl2 (3)
calcd/g cm-3
1.329
3-52
0.63
1.405
3-56
0.73
1.441
3-56
0.75
2θ range/deg
µ/mm-1
T/K
150
150
150
no. of measd rflns
Rint
47 627
0.0312
9253
41 427
0.0370
10 645
9061
21 170
0.0273
10 252
8220
1
2
3
Fe-N1
Fe-N2
S1-C1
S2-C2
C-N1
1.932(2)
1.933(2)
1.629(2)
1.634(2)
1.164(2)
1.167(2)
2.300(1)
2.238(1)
2.261(1)
2.265(1)
1.933(2)
1.935(2)
1.629(2)
1.636(2)
1.166(2)
1.169(2)
2.302(1)
2.243(1)
2.249(1)
2.310(1)
Fe-N1
Fe-N2
S1-C1
S2-C2
C-N1
1.973(2)
1.964(2)
1.641(2)
1.634(2)
1.167(2)
1.167(2)
2.288(1)
2.248(6)
2.238(5)
2.300(1)
no. of indep rflns
no. of rflns with
Fo > 4σ(Fo)
no. of params
R1 (Fo > 4σ(Fo))
wR2 (all data)
GOF
8058
488
533
519
0.0369
0.1023
1.040
0.0346
0.0934
1.0136
1.21/-1.12
0.0329
0.0864
1.014
C-N2
C-N2
Fe-P1
Fe-P2
Fe-P3
Fe-P4
Fe-P1
Fe-P2
Fe-P3
Fe-P4
resid electron
0.42/-0.36
0.36/-0.84
dens/e/Å3
constant for the trans coupling is greater than for the
cis coupling, and the remaining cis couplings between
the X/X′ and A/A′ atoms correspond to (32 and (62 Hz,
respectively.
N1-Fe-N2 178.56(6) 174.58(6) N1-Fe-N2 88.99(6)
Fe-N1-C1
Fe-N2-C2
P1-Fe-P3
P2-Fe-P4
N2-Fe-P3
176.0(2)
171.9(2)
167.4(2)
175.5(2)
Fe-N1-C
Fe-N2-C
176.5(2)
170.9(2)
174.36(5)
168.83(5)
174.22(2) 172.37(2) N1-Fe-P2
172.04(2) 174.07(2) N2-Fe-P3
Str u ctu r e of cis-r-[F e(NCS)2(r a c-p r P 4)] (3). The
cis complex 3 crystallizes in the triclinic space group
P1h with two formula units in the unit cell (Table 2).
Fe-P bond lengths in this complex are comparable to
those in the two trans-pseudopolymorphs 1 and 2 (Table
2). The Fe-N bond lengths of 1.964(2) and 1.973(2) Å
in 3 are somewhat longer than the corresponding bonds
in the trans complexes 1 and 2. The two C-N bond
lengths are 1.167(2) Å, and the C-S bond lengths are
1.634(2) and 1.641(2) Å, respectively. The angle between
the two cis-coordinated nitrogen atoms of 88.99(6)°
deviates only slightly from 90° (Figure 5). The angles
between the trans standing atoms N2 and P3 and
83.44(4)
84.45(4)
P1-Fe- P4 176.34(2)
Ta ble 3. Cou p lin g Con sta n ts for 31P Sp ectr a of
tr a n s-[F e(NCS)2(r a c-p r P 4)]
coupling constant
value (Hz)
J AX ) J A′X′
J AX′ ) J A′X
J XX′
-26
112
(62
(32
J AA′
between N1 and P2 are 168.83(5) and 174.36(4)°,
respectively.
NMR Sp ectr a a n d Sp ectr a l Sim u la tion of cis-r-
[F e(NCS)2(r a c-p r P 4)] (3). The second species in the

3256 Organometallics, Vol. 23, No. 13, 2004
Habeck et al.
F igu r e 6. Simulated (top) and experimental (bottom) 31
NMR spectra of cis-R-[Fe(NCS)2(rac-prP4)] (3).
P
31P NMR spectrum of the primary reaction product has
been identified by comparison with the spectrum of pure
cis-R-[Fe(NCS)2(rac-prP4)]. Time-dependent 31P NMR
spectroscopy of the pure cis-R compound indicates that
the cis compound slowly (with a half-life of 2 days)
converts to the corresponding trans complex in solution
(see Figure S1 in the Supporting Information), demon-
strating that the trans geometry of 1 and 2 is slightly
favored over the cis-R geometry found in 3. This result
is in qualitative agreement with DFT calculations on
trans- and cis-[Fe(NCS)2(H2P(CH2)2P(CH2)3P(CH2)2-
PH2)] being models for 1/2 and 3, respectively. These
calculations indicate that the trans complex is by 5.5
kcal/mol lower in energy than the cis complex (see the
Supporting Information).
As is evident from Figure 6, the 31P NMR spectrum
of cis-R-[Fe(NCS)2(rac-prP4)] exhibits two signals of
triplet appearance located at 60.0 and 64.4 ppm, re-
spectively. In analogy to the 31P spectrum of, for
example, cis-[Rh(H)2(rac-tetraphos-I)]+,2a this spectral
pattern can be simulated on the basis of an AA′XX′
scheme with one cis P-P coupling constant for AX, A′X′,
AX′, and A′X (29 Hz; Figure 6).
NMR Sp ectr a a n d Sp ectr a l Sim u la tion of tr a n s-
[F e(NCS)2(m eso-p r P 4)] (4). The third species in the
primary reaction product exhibits a spectral pattern
similar to that of the trans-rac compound: however,
with different chemical shifts (center shifts 70 and 62
ppm vs 62.8 and 77.5 ppm in trans-rac; Figure 7). This
spectrum, which corresponds to the major species in
Figure 1, is therefore assigned to the trans-meso isomer
4. As recrystallization of the primary reaction product
primarily gives the trans-rac compounds 1 and 2 (vide
supra), the solubility of the trans-meso isomer must be
higher than that of its trans-rac counterpart. Notably,
there is no indication in the primary reaction product
for the presence of the cis-â isomers, which in principle
could be formed by both meso- and rac-prP4. The 31P
NMR spectra of these isomers of [Fe(NCS)2(prP4)]
should be qualitatively different from either one of the
trans and cis-â complexes. As the yield of the primary
reaction product is almost quantitative, it is concluded
that the meso- and rac-cis-â isomers are not formed to
a significant degree in the reaction of FeCl2, KSCN, and
prP4.
F igu r e 7. 31P NMR spectra of trans-[Fe(NCS)2(rac-prP4)]
(top) and trans-[Fe(NCS)2(meso-prP4)] (bottom).
pound 1 shows bands typical for an iron(II) low-spin
1
complex. The 1A1 f T1 transition is observed at 533
1
nm, and A1 f 1T2 is found at 431 nm (see Figure S2 in
the Supporting Information). In addition to these bands
a charge-transfer band is found at 350 nm. Moreover,
1 and 3 were studied by vibrational spectroscopy,
making use of 15NCS isotopomers. The IR spectrum of
trans-[Fe(NCS)2(rac-prP4)] (1) shows three isotope-
sensitive bands, assigned to the antisymmetric CN
stretch νas(CN) at 2099 cm-1, the antisymmetric CS
stretch νas(CS) at 818 cm-1, and the antisymmetric Fe-
N-C bend δas(FeNC) at 433 cm-1 (see Figure S3 and
Table S1 in the Supporting Information). Upon isotope
substitution, the νas(CN) signal shifts by about 28 cm-1
to 2071 cm-1, the νas(CS) signal by 9 cm-1 to 807 cm-1
,
and νas(FeN) by 7 cm-1 to 426 cm-1. In the FT-Raman
spectra the analogous symmetric vibrations could be
identified (see Figure S4 and Table S1 in the Supporting
Information): i.e., at 2105 cm-1 the symmetric CN
stretch νs(CN), shifting by about 30 cm-1 on isotope
substitution, as well as the symmetric CS stretch νs-
(CS) at 819 in the 14N compound and 806 cm-1 in the
15N complex. On the basis of these data a normal
coordinate analysis was performed, which also is in-
cluded in the Supporting Information.
Vibr a tion a l P r op er ties of cis-r-[F e(NCS)2(r a c-
p r P 4)]. In agreement with the loss of inversion sym-
metry, the vibrational selection rules valid for the trans
complexes (symmetric T Raman and antisymmetric T
IR) do not apply any more; i.e., both components of the
Fe-NCS vibrations now can appear in the IR and in
the Raman spectrum. The IR spectrum shows a small
splitting of the ν(CN) stretches at 2112 and 2105 cm-1
,
and the CS stretches correspond to a broad feature at
805 cm-1 (see Figure S5 in the Supporting Information).
Other stretches could not be assigned, due to lack of
isotope substitution data (Figure S5). In the Raman
spectrum, the features at 2107 and 2103 cm-1 can be
identified as in-phase and out-of phase CN stretches and
the feature at 809 cm-1 as the split ν(CS) stretch (Table
S1).
Vibr a tion a l a n d Electr on ic P r op er ties of tr a n s-
[F e(NCS)2(r a c-p r P 4)]. Complexes 1 and 3 were further
characterized by vibrational and UV/vis absorption
spectroscopy. The UV/vis spectrum of the trans com-
Su m m a r y
The tetraphos ligand prP4 has been prepared and
characterized by spectroscopy as well as X-ray structure

Fe Complexes of a Linear Tetraphos Ligand
Organometallics, Vol. 23, No. 13, 2004 3257
with 40 mL of degassed water, the organic layer was dried
over Na2SO4, and the solvents were removed with a rotary
evaporator at 100 °C (water vacuum). Addition of 100 mL of
hot absolute ethanol and subsequent slow cooling gave a white
precipitate, which was recrystallized from ethanol. Yield: 41.6
g (67%). 31P NMR (C2D5OD, 400 MHz): δ -17.85 ppm (s).
Dilith iu m 1,3-Bis(ph en ylph osph ido)pr opan e-Tetr akis-
(t et r a h yd r ofu r a n ), P h (Li)P CH 2CH 2CH 2P (Li)P h ‚4TH F
(“Lip p p ”). A 2.29 g portion of lithium metal (0.33 mol) was
added to 60 mL of thf and the mixture cooled to 0 °C. Then a
solution of 13.98 g of 1,3-bis(diphenylphosphino)propane (dppp;
0.0339 mol) in 80 mL of thf was added dropwise with stirring.
The solution became orange-yellow and was refluxed by the
use of an ultrasonic bath for 45 min, resulting in a dark red
mixture. The warm mixture was separated from the precipi-
tate by an injection needle to remove unreacted lithium metal.
Then the volume of the filtrate was reduced to 20 mL and
cooled to -78 °C. The resulting bright yellow powder was
filtered and washed twice with 5 mL of cold diethyl ether.
Recrystallization from thf gave yellow crystals. Anal. Calcd:
C, 66.4; H, 8.6. Found: C, 66.1; H, 8.5. Yield: 12.9 g (68%).
analysis of three isomeric iron thiocyanato complexes.
In analogy to the synthesis of the linear tetraphos-I
ligand, generating a mixture of meso and rac, prP4 forms
as a mixture of both diastereomers. The ratio between
the two isomers varies from preparation to preparation;
the factors influencing this composition could not be
elucidated. Interestingly, two alternative routes of
synthesizing prP4 were found to occur stereoselectively.2a,b
The identity of the resulting isomer of prP4 and the
origin of the observed stereoselectivities, however, are
unclear in both cases.
Free prP4 shows a 31P NMR spectrum which is
interpreted on the basis of an AA′XX′ spin system. To
further characterize this ligand, three iron(II) bis-
(thiocyanate) complexes were crystallized and investi-
gated by single-crystal X-ray structure determination
and 31P NMR spectroscopy. The compounds were found
to be associated with two isomers, trans- and cis-R-[Fe-
(NCS)2(rac-prP4)]. The 31P spectra of these complexes
exhibit AA′XX‘ patterns as well. Both isomers can also
be distinguished on the basis of their vibrational
spectra, showing typical differences in the region of the
CN stretching vibrations. No evidence for formation of
the cis-â isomer could be found, but the trans-meso
compound could be identified by its 31P NMR spectrum,
showing a close similarity to that of the trans-rac
complex. As demonstrated by NMR spectroscopy, octa-
hedral trans coordination of [Fe(NCS)2(rac-prP4)] is
preferred over the cis configuration, and the cis com-
pound slowly converts into the trans isomer in solution.
This is in agreement with DFT calculations, which
predict that the trans isomer is 5 kcal lower in energy
than its cis counterpart.
1,1,4,8,11,11-H exa p h en yl-1,4,8,11-t et r a p h osp h a u n d e-
ca n e, P h 2P CH2CH2P (P h )CH2CH2CH2P (P h )CH2CH2P P h 2.
An 8.0 g portion of Ph2PCH2CH2Cl (0.032 mol) dissolved in
100 mL of thf was added dropwise over a period of 1 h to an
ice-cooled solution of Ph(Li)PCH2CH2CH2P(Li)Ph‚4THF (8.8
g, 0.016 mol) in 100 mL of thf. The solution was refluxed for
1 h and then cooled. A 30 mL portion of saturated NH4Cl(aq)
solution was added with vigorous stirring. The thf layer was
decanted, and the aqueous layer was washed with 20 mL of
thf. Both thf layers were combined and dried over MgSO4, and
the volume was reduced to 30 mL. After addition of 90 mL of
ethanol the solution was allowed to stand overnight at -40
°C. Next day the white precipitate which formed was filtered
and dried under vacuum. Anal. Calcd: C, 75.4; H, 6.5. Found:
C, 75.4; H, 6.5. Yield: 7.4 g (68%). Mp: 106-108 °C.
tr a n s- a n d cis-r Bis(isoth iocya n a to)(r a c-1,1,4,8,11,11-
h exaph en yl-1,4,8,11-tetr aph osph au n decan e)ir on (II), [Fe-
(NCS)2(r a c-p r P 4)]. A 48.6 mg portion (0.50 mmol) of potas-
sium thiocyanate dissolved in 10 mL of ethanol was added
dropwise to 31.7 mg (0.25 mmol) of FeCl2 and 171.2 mg (0.25
mmol) of rac-prP4, suspended in a mixture of 15 mL of ethanol
and 10 mL of dichloromethane. The color changed to red, and
the resulting precipitate was filtered. After some days red
crystals were obtained from the filtrate. These were filtered
and recrystallized from dichloromethane/ethanol. The cis
isomer 3 could be crystallized as light red needles in low yields
from solutions of the crude product, while the trans isomers 1
and 2 were obtained in better yields as dark red crystals from
concentrated solutions of the crude product. The precipitate
formed by the reaction yields a changing ratio for trans to cis
isomer, on the basis of 31P NMR of a solution of the primary
reaction product. Elemental analysis of the crude product is
as follows. Anal. Calcd: C, 63.1; H, 5.2; N, 3.3; S, 7.5. Found:
C, 62.9; H, 5.2; N, 3.4; S, 7.5. The reaction is nearly quantita-
tive.
In summary, a complete analysis of the stereochem-
istry of the prP4 ligand in mononuclear complexes has
been obtained and the spectroscopic properties of the
free ligand and its FeII(NCS)2 complexes have been
defined. Experiments are now underway to use this
ligand in dinitrogen complexes of Mo(0) and W(0)
relevant to nitrogen fixation.6
Exp er im en ta l Section
a . Syn th esis. All reactions were performed under an inert-
gas atmosphere using Schlenk techniques. The solvents were
dried and distilled under argon or dinitrogen gas. All other
reagents were used without further purification. The sample
preparation for vibrational, UV/vis, and NMR spectroscopy was
carried out in a glovebox. 15N-labeled thiocyanate (94.7%) was
bought from VEB Berlin Chemie, Berlin Adlershof, Germany.
1-Ch lor o-2-(d ip h en ylp h osp h in o)et h a n e,
(C6H 5)2P -
(CH2)2Cl. A 5.75 g portion (0.25 mol) of sodium was added to
250 mL of condensed ammonia at -78 °C. The solution turned
immediately blue and was rapidly stirred for 1 h, until all
sodium was dissolved. Then 32.75 g of triphenylphosphine
(0.125 mol) was added over a 15 min period, and the resulting
red solution was again stirred for 1 h. After the mixture was
treated with 6.25 g (0.118 mol) of NH4Cl (dried overnight at
100 °C) to destroy the C6H5Na formed as a cleavage product,
it was stirred for another 1 h. The orange solution of sodium
diphenylphosphide was added to a solution of 25 mL of ClCH2-
CH2Cl in 25 mL of toluene at -78 °C. Then the solution was
allowed to stand overnight to remove all NH3 before 50 mL of
toluene was added. The resulting mixture was washed twice
b. Sp ectr oscop y a n d Sin gle-Cr ysta l X-r a y Str u ctu r e
Deter m in a tion s. NMR Sp ectr oscop y. NMR spectra were
recorded on a Bruker Avance 400 pulse Fourier transform
1
spectrometer operating at a H frequency of 400.13 MHz using
a 5 mm inverse triple-resonance probe head. References as
external standards: H3PO4 85% pure, δ(31P) 0 ppm; LiCl-
saturated D2O, δ(7Li) 0 ppm; CH3NO2 neat, δ(15N) 0 ppm.
Samples were measured at 300 K. Spectral simulations were
performed using the WinDNMR Calculation V7.1.6 software
package by Hans J . Reich on a personal computer.
X-r a y Str u ctu r e An a lysis. Intensity data were collected
using a STOE imaging plate diffraction system with Mo KR
radiation. The structures were solved by direct methods using
(6) Habeck, C.; Hoberg, C.; Tuczek, F. To be submitted for publica-
tion.

3258 Organometallics, Vol. 23, No. 13, 2004
Habeck et al.
SHELXS-97,7 and refinement was performed against F2 using
SHELXL-97.7 All non-H atoms were refined anisotropically.
The hydrogen atoms were located with idealized geometry and
refined with isotropic displacement parameters using a riding
model. The crystals of compound 1 contain additional dichlo-
romethane molecules as solvent which are extremely disor-
dered. Because no reliable split model was found, the data were
corrected for disordered solvent molecules using the squeeze
option in Platon. The content of CH2Cl2 was estimated to about
0.5 per each formula unit.
Cryogenics helium cryostat, and all spectra were recorded at
10 K with resolution was set to 2 cm-1
.
Ra m a n Sp ectr oscop y. Raman spectra were obtained on
a Bruker IFS 66/CS near-IR FT-Raman spectrometer at 270
K. The setup involves a 350 mW Nd:YAG laser with an
excitation wavelength of 1064 nm. The samples were pressed
into the groove of a holder which could be sealed with a glass
plate to ensure inert gas conditions.
Ack n ow led gm en t. We want to thank the DFG
SPP1118 (Tu58-13) for funding of this research.
UV-Vis Sp ectr oscop y. The UV/vis spectra were recorded
on a Analytik J ena Specord S100 spectrometer using a quartz
cuvette under an inert gas atmosphere. The resolution was
set to 1 nm.
Su p p or tin g In for m a tion Ava ila ble: IR and FT Raman
spectra of the compounds 1 and 3 as well as a UV-vis
spectrum of 1, procedures, the resulting force constants, and
tables of calculated and experimental frequencies for a normal
coordinate analysis of 1, structure determination details,
atomic coordinates, isotropic and anisotropic displacement
parameters, and bond lengths and angles for all structures,
and additional 31P NMR spectra of the cis/ trans conversion.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IR Sp ectr oscop y. Mid-IR spectra were obtained from RbI
pellets using a Mattson Genesis Typ I spectrometer. Far-IR
spectra were obtained from RbI pellets using a Bruker IFS 66
FTIR spectrometer. Both instruments were equipped with a
(7) Sheldrick, G. M. SHELXS-97 and SHELXL-97: Program for the
Solution and Refinement of Crystal Structures; University of Go¨ttin-
gen, Go¨ttingen, Germany, 1997.
OM030579K