Fe(iv) alkylidenes via protonation of Fe(ii) vinyl chelates and a comparative M?ssbauer spectroscopic study

Article abstract of DOI:10.1039/c5sc01268f

Treatment of cis-Me₂Fe(PMe₃)₄ with di-1,2-(E-2-(pyridin-2-yl)vinyl)benzene ((bdvp)H₂), a tetradentate ligand precursor, afforded (bdvp)Fe(PMe₃)₂ (1-PMe₃) and 2 equiv. CH₄

Full text of DOI:10.1039/c5sc01268f

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Fe(IV) alkylidenes via protonation of Fe(II) vinyl
chelates and a comparative Mossbauer
spectroscopic study†
¨
Cite this: DOI: 10.1039/c5sc01268f
a
a
a
a
*
Brian M. Lindley, Ala'aeddeen Swidan, Emil B. Lobkovsky, Peter T. Wolczanski,
Mario Adelhardt,b Jorg Sutterb and Karsten Meyerb
¨
Treatment of cis-Me2Fe(PMe3)4 with di-1,2-(E-2-(pyridin-2-yl)vinyl)benzene ((bdvp)H2), a tetradentate
ligand precursor, afforded (bdvp)Fe(PMe3)2 (1-PMe3) and 2 equiv. CH4, via C–H bond activation. Similar
treatments with tridentate ligand precursors PhCH]NCH2(E-CH]CHPh) ((pipp)H2) and PhCH]N(2-
CCMe-Ph) ((pipa)H) under dinitrogen provided trans-(pipp)Fe(PMe3)2N2 (2) and trans-(pipvd)
Fe(PMe3)2N2 (3), respectively; the latter via one C–H bond activation, and a subsequent insertion of the
alkyne into the remaining Fe–Me bond. All three Fe(II) vinyl species were protonated with H[BArF4] to
form the corresponding Fe(IV) alkylidene cations, [(bavp)Fe(PMe3)2][BArF4] (4-PMe3), [(piap)Fe(PMe3)3]-
Received 8th April 2015
Accepted 20th May 2015
[BArF4] (5), and [(pipad)Fe(PMe3)3][BArF4] (6). Mossbauer spectroscopic measurements on the formally
¨
DOI: 10.1039/c5sc01268f
Fe(II) and Fe(IV) derivatives revealed isomer shifts within 0.1 mm sꢀ1, reflecting the similarity in their bond
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distances.
carbene transfers, and some transformations that hint at
radical reactions.
Introduction
In an effort to expand the scope of Fe(IV) alkylidenes, and to
develop new synthetic paths, Fe(II) vinyl chelates have been
explored as potential precursors to cationic Fe(IV) alkylidenes
via protonation.14–18 Entry into ferrous vinyl derivatives was
Homogeneous alkylidene compounds that catalyze olen
metathesis1–5 typically contain 2nd row transition metals that
have modest limitations regarding functionality tolerance (e.g.,
Mo),1,2 and relative abundance (e.g., Ru).3,4 Applications for
commodity chemicals production have been hampered by these
factors, and inexpensive rst row transition metal alternatives
hold great promise for solving some of the problems. Thus far,
the synthesis of 1st row transition metal alkylidene complexes
has presented a signicant challenge to the organometallic
community, especially in the case of iron.
Electronic structure analysis by Hoffmann et al.6 suggests that
metathesis catalysts need to be dn (n # 4), hence Fe(IV) alkylidenes
are the target of interest, especially in analogy to their 2nd row
congeners. Several Fe(IV) alkylidenes have been synthesized, with
two routes utilized in the cases of those structurally characterized
(Fig. 1): (1) conversion of [CpLL0Fe]C(OR)R0]+ via hydride or alkyl
anion reagents,7–9 and (2) the addition of diazoalkanes, typically
Ph2CN2, to coordinatively unsaturated complexes or labile
precursors.10–13 The subsequent chemistry has been limited to
aDepartment of Chemistry & Chemical Biology, Baker Laboratory, Cornell University,
Ithaca, NY, 14850, USA. E-mail: ptw2@cornell.edu; Fax: +1 607 255 4137; Tel: +1 607
255 7220
bDepartment of Chemistry & Pharmacy, Friedrich Alexander University Erlangen-
¨
Nurnberg (FAU), Egerlandstr. 1, D-91058 Erlangen, Germany
† Electronic supplementary information (ESI) available. CCDC 1057828–1057831.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5sc01268f
Fig. 1 Some iron alkylidenes and methods of synthesis.
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implemented via precedented C–H bond activations by Karsch's
cis-Me2Fe(PMe3)4 (ref. 19) complex.20–23 While viable olen
metathesis catalysts containing Fe have not yet been realized,
the generality of this approach suggests that incremental
advances may yet prove successful.
Results and discussion
Di-1,2-(E-2-(pyridin-2-yl)vinyl)benzene: tetradentate ligand
precursor
As Scheme 1 illustrates, incorporation of two vinyl groups into a
tetradentate chelate precursor was predicated on successful
implementation of a Horner–Wadsworth–Emmons reaction to
achieve the requisite E-stereochemistry. The modied 2-pyr-
idinyl-methyl reagent was prepared according to a literature
procedure24 from Na[OP(OEt)2] and 2-pyridyl-methylchloride in
70% yield. Its addition to 1,2-benzene-dialdehyde afforded di-
1,2-(E-2-(pyridin-2-yl)vinyl)benzene ((bdvp)H2, E/Z > 19 : 1) in
37% yield upon crystallization from ether/hexane.
Scheme 2 Preparation of the tridentate precursors, PhCH]NCH2(E-
CH]CHPh) ((pipp)H2) and PhCH]N(2-CCMe-Ph) ((pipa)H).
possessing o-Me groups on the pyridines (Scheme 1) failed to
metalate, and decomposition of cis-Me2Fe(PMe3)4 was instead
observed.
Tridentate ligand precursors: PhCHNCH2(E-CH]CHPh) and
PhCH]N(2-CCMe-Ph)
A
similar exposure of cis-Me2Fe(PMe3)4 (ref. 19) to
PhCH]NCH2(E-CH]CHPh) ((pipp)H2) in benzene at 23 ꢁC
aer 20 h afforded a purple solid upon subsequent concentra-
tion (Scheme 3). Dissolution in THF under a dinitrogen atmo-
sphere provided brown trans-(pipp)Fe(PMe3)2N2 (2) in 63% yield
aer removal of solvent. It is likely that the tris-PMe3 derivative
is formed initially, but N2 replaces PMe3 in a probable disso-
ciative process. Previous examples have shown that steric
factors – in this case the phenyl substituent – labilize the
phosphine opposite the imine.23 The dinitrogen ligand is
Condensation routes afforded the two additional tridentate
ligands used in this study, as shown in Scheme 2. Cinnamyl
amine25 and benzaldehyde were used to synthesize
PhCH]NCH2(E-CH]CHPh) ((pipp)H2),26 while 2-propynyl-
aniline, prepared from Pd-catalyzed cross-coupling27 of propyne
and 2-iodo-aniline, and benzaldehyde were used to generate
PhCH]N(2-CCMe-Ph) ((pipa)H). A number of other potential
imine and pyridine-containing tridentate ligand precursors
were similarly made, but the subsequent C–H bond activations
proved to be too slow or ineffective, allowing for competitive cis-
Me2Fe(PMe3)4 degradation.
Metalation via C–H bond activation and insertion
Treatment of cis-Me2Fe(PMe3)4 (ref. 19) with the tetradentate
precursor 1,2-(E-2-(pyridin-2-yl)vinyl)benzene ((bdvp)H2) was
ꢁ
undertaken at ꢀ20 C in toluene. Aer 10 h, the solution was
warmed to 23 ꢁC and concentrated to afford (bdvp)Fe(PMe3)2 (1-
PMe3) in 84% yield as purple microcrystals (Scheme 3). The
reaction is quite sensitive to steric bulk, as use of a precursor
Scheme 3 Methods employed in synthesizing vinyl precursors derived
Scheme 1 Preparation of the divinyl ligand precursor, 1,2-(E-2-(pyr- from CH-bond activation/metalation of cis-Me2Fe(PMe3)4 and
idin-2-yl)vinyl)benzene ((bdvp)H2).
acetylene insertion.
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readily discerned via its IR spectrum, which reveals a n(NN) at pyridine are 81.01(13)ꢁ (ave), and the phenyl-divinyl bite angle is
2048 cmꢀ1
.
85.61(14)ꢁ, hence the chelate angles sum to 360.1ꢁ.
28,29
A
third, different metalation was conducted with
Considerable distortion in the chelate is evident, as the
˚
PhCH]N(2-CCMe-Ph) ((pipa)H) and cis-Me2Fe(PMe3)4. The iron–carbon bonds are quite short at 1.883(8) A (ave), while
precedented imine-directed Ar-H activation occurred, followed the C6–C7–C8 and C13–C14–C15 angles of 127.4(4)ꢁ (ave)
by insertion of the pendant acetylene into the Fe–Me bond. The deviate signicantly from 120ꢁ. Fig. 3 illustrates the chelate
resulting complex, trans-(pipvd)Fe(PMe3)2N2 (3), contains a distances and angles in comparison to those of the related
dimethyl-vinyl group as the precursor to a potential alkylidene. alkylidene complex (vide infra). All the angles about the Fe–C
Green-brown 3 was prepared in 79% yield aer metalation for bonds are distorted in response to the proximity of the vinyl
20 h at 23 ꢁC, and as in the previous case, it is likely an initially carbons to the iron. Note that the pyridines are not perfectly
formed tris-PMe3 complex lost a phosphine in the presence aligned as donors, as the Fe–N–C angles open to an average of
of N2 to afford the dinitrogen complex,23 whose n(NN) is at 130.6(5)ꢁ.
2046 cmꢀ1
.
All three precursors feature downeld 13C NMR chemical
shis for the vinyl carbons bound to iron. A triplet (JPC ¼ 23 Hz)
corresponding to the Fe–C(Ar)] unit in (bdvp)Fe(PMe3)2 (1-
PMe3) was located at d 258.3, an unusual shi that may be
intrinsic to the metrics of its tetradentate chelation, i.e.,
reecting a very short d(Fe–C). The Fe–vinyl carbon of the tri-
dentate chelate in trans-(pipp)Fe(PMe3)2N2 (2) also manifests a
signicant downeld shi at d 207.9 (t, JPC ¼ 18 Hz), while the
Fe–C(Ar)] carbon of trans-(pipvd)Fe(PMe3)2N2 (3), the most
sterically hindered vinyl, resonates at d 167.9 (t, JPC ¼ 17 Hz).
Vinyl protonations lead to Fe(IV) alkylidenes
The vinyl precursors synthesized via the C–H bond activation
and insertion processes were protonated14–18 to yield cationic
Fe(IV) alkylidenes, as illustrated in Scheme 4. The tetradentate
chelate complex (bdvp)Fe(PMe3)2 (1-PMe3) was treated with
H[BArF
] in THF to afford orange [(bavp)Fe(PMe3)2][BAr 4] (4-
F
30
4
PMe3) in 80% yield. The lability of 1-PMe3 was tested with excess
PMe2Ph (10 equiv.), and repeated thermolyses in reuxing
toluene, including periodic removal of PMe3, were required to
generate (bdvp)Fe(PMe2Ph)2 (1-PMe2Ph). The dimethylphenyl-
phosphine derivative 1-PMe2Ph was not isolated, and charac-
terization using NMR spectroscopy proved elusive due to
broadened and overlapping resonances. As a consequence, it
was generated in situ and treated with H[BArF4] in THF to yield
analytically pure [(bavp)Fe(PMe2Ph)2][BArF4] (4-PMe2Ph, 94%).
A related protonation of the tridentate chelate species trans-
(pipp)Fe(PMe3)2N2 (2) in THF initially gave a complex mixture
that exhibited 31P{1H} NMR spectral resonances consistent with
starting material, a tri-phosphine complex, and degradation
products. The addition of PMe3 to the reaction resulted in one
major product, [(piap)Fe(PMe3)3][BArF4] (5) that was isolated as
yellow-orange microcrystals in 72% yield. It is likely that an
initial dinitrogen-containing Fe(IV) product, [(piap)Fe(PMe3)2N2]
Structure of (bdvp)Fe(PMe3)2 (1-PMe3)
Shown in Fig. 2 is the molecular structure of (bdvp)Fe(PMe3)2 (1-
PMe3), as determined by single crystal X-ray crystallography.
The tetradentate ligand essentially resides in a plane of the
pseudo-octahedral structure (:C/N–Fe–P ¼ 90.0(11)ꢁ (ave);
:P1–Fe–P2 ¼ 179.24(3)ꢁ), accompanied by trans-PMe3 groups
˚
at d(Fe–P) ¼ 2.2283(8) A (ave). The d(Fe–N) of 2.060(4) (ave) are
normal, but there is a splay in the N1–Fe–N2 angle (112.43(11)ꢁ)
indicative of a strain in the chelate. The bite angles of the vinyl-
Fig.
2 Molecular view of (bdvp)Fe(PMe3)2 (1-PMe3). Interatomic
distances (A˚) and angles (ꢁ): Fe–N1, 2.057(3); Fe–N2, 2.062(3); Fe–C7,
1.877(3); Fe–C14, 1.888(3); Fe–P1, 2.2285(8); Fe–P2, 2.2280(8); N1–
Fe–C7, 81.10(13); N1–Fe–C14, 166.62(13); N1–Fe–N2, 112.43(11); N1–
Fe–P1, 89.45(8); N1–Fe–P2, 91.17(8); C7–Fe–C14, 85.61(14); C7–Fe– Fig. 3 Comparative ligand metric parameters (distances (A˚), dashed
N2, 166.35(13); C7–Fe–P1, 90.06(10); C7–Fe–P2, 89.61(10); C14–Fe– black lines; angles (ꢁ, italics), curved black lines) for Fe(II) (bdvp)
N2, 80.91(13); C14–Fe–P1, 89.03(9); C14–Fe–P2, 90.26(9); N2–Fe– Fe(PMe3)2 (1-PMe3, black) vs. Fe(IV) [(bavp)Fe(PMe3)2][BArF4] (4-PMe3,
P1, 91.84(8); N2–Fe–P2, 88.32(8); P1–Fe–P2, 179.24(3).
red).
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Fig. 4 Molecular view of the cation pertaining to [(bavp)Fe(PMe3)2]
[BArF4] (4-PMe3); the PMe3 methyl groups have been removed for
clarity. Interatomic distances (A˚) and angles (ꢁ): Fe–N1, 2.083(3); Fe–
N2, 2.129(4); Fe–C7, 1.809(4); Fe–C14, 1.858(4); Fe–P1, 2.2671(11); Fe–
P2, 2.2725(11); N1–Fe–C7, 81.47(17); N1–Fe–C14, 168.31(19); N1–Fe–
N2, 110.98(14); N1–Fe–P1, 88.53(9); N1–Fe–P2, 91.32(9); C7–Fe–C14,
86.9(2); N2–Fe–C7, 167.48(17); C7–Fe–P1, 91.87(13); C7–Fe–P2,
87.81(13); N2–Fe–C14, 80.71(18); C14–Fe–P1, 91.49(13); C14–Fe–P2,
88.60(13); N2–Fe–P1, 89.89(10); N2–Fe–P2, 90.44(10); P1–Fe–P2,
179.66(5).
Scheme
alkylidenes.
4 ] afforded cationic Fe(IV)
Protonation with H[BArF
4
[BArF4], readily loses N2, and through redistribution generates 5
along with decomposition products.
corresponding angles in 1-PMe3, and the remaining chelate
distances and angles change in concert.
Protonation of (pipvd)Fe(PMe3)2N2 (3) was conducted with
H[BArF4] in diethyl ether, and a mixture whose NMR spectra is
related to that of the initial protonation of 2 was discerned.
Again, the addition of PMe3 to the reaction mixture permitted
the isolation of [(pipvd)Fe(PMe3)3][BArF4] (6) in 74% yield as
orange microcrystals.
Structure of [(piap)Fe(PMe3)3][BArF4] (5)
Fig. 5 displays a molecular view of the cation corresponding to
[(piap)Fe(PMe3)3][BArF4] (5), and reveals its pseudo-octahedral
geometry with the piap ligand occupying a mer-conguration
about iron. The critical alkylidene distance, d(Fe–C10), is
Denitive spectral characterization of the isolated Fe(IV)
alkylidene complexes was predicated on observation of diag-
13
˚
nostic downeld C NMR resonances31,32 attributed to the
1.867(7) A, which is signicantly shorter than d(Fe–C1) ¼
˚
M]CRR0 functionality (Scheme 4). The spectral signatures were
difficult to detect, requiring indirect methods, but the alkyli-
dene chemical shis for 4-PMe3, 4-PMe2Ph, 5, and 6 were
eventually observed at d 348.4 (JPC ¼ 31 Hz), d 350.6 (JPC ¼ 31
Hz), d 352.6 (JPC ¼ 21 Hz), and d 348.4 (br), respectively.
2.106(6) A, but on par with the iron–vinyl carbon distances in 1-
˚
˚
PMe3 (1.883(8) A (ave)) and 4-PMe3 (1.858(4) A). The tridentate
chelate is strained, as the C1–Fe–C10 angle is 161.6(3)ꢁ, and the
Fe–C10–C9 and Fe–C10–C11 angles of 115.7(5)ꢁ and 136.4(5)ꢁ,
respectively, indicate that the alkylidene is not perfectly
oriented. Note that the C10–Fe–P2 angles are 100.20(4)ꢁ; as a
consequence, the d-orbital that comprises the iron portion of
the Fe]C p-bond has some Fe–P s* character that aids in
producing better overlap with the carbon p-orbital.
Structure of [(bavp)Fe(PMe3)2][BArF4] (4-PMe3)
A molecular view of the cation pertaining to [(bavp)Fe(PMe3)2]
[BArF4] (4-PMe3) is illustrated in Fig. 4, showing its distorted
octahedral structure, with the tetradentate chelate occupying a
single plane. The P–Fe–C/N angles average 90.0(15)ꢁ, and there
Structure of [(pipad)Fe(PMe3)3][BArF4] (6)
is a splay in the bavp ligand indicated by the N1–Fe–N2 angle of A molecular view of the cation of [(pipad)Fe(PMe3)3][BArF4] (6) is
110.98(14)ꢁ, and the trans-N–Fe–C angles of 168.31(19) and provided in Fig. 6, which indicates the mer-octahedral structure
167.48(17)ꢁ.
of the iron alkylidene. The tridentate pipad chelate is essentially
˚
˚
The critical d(Fe]C7) is 1.809(4) A, which is ꢂ0.05 A shorter planar, and the isopropyl-aryl alkylidene possesses a d(Fe]C) of
˚
˚
than the adjacent iron–vinyl carbon distance of 1.858(4) A. Both 1.899(3) A, which is longer than the iron–vinyl carbon distances
are shorter than the iron–carbon bond lengths in 1-PMe3, in in 1-PMe3 and 4-PMe3. Again, the chelate exhibits strain about
contrast to the d(Fe–N), which are longer at 2.083(3) and the core, as the C1–Fe–C17 angle is 163.36(15)ꢁ, and its isopropyl
˚
2.129(4) A. As these changes and the comparison between 1- group exerts a steric inuence on the unique PMe3, as the
PMe3 and 4-PMe3 in Fig. 3 reveal, the chelate has pinched in to a C1–Fe–P1 angle is 104.46(12)ꢁ. The Fe–C1–C5 and Fe–C1–C2
slightly greater extent in the cation. The angles C6–C7–C8 and angles are 112.8(2)ꢁ and 131.0(3)ꢁ, respectively, showing that the
C13–C14–C15 are 2.3 and 1.4ꢁ less, respectively, than the alkylidene is at an imperfect orientation with respect to the iron.
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a carbene radical antiferromagnetically (AF) coupled to a metal
dp-electron of appropriate symmetry. Modern calculations have
not been employed on Floriani's calix[4]arane diphenycarbene
complexes,11 but they are high spin, and therefore are likely to
conform to an AF coupling model.
Of the remaining diamagnetic complexes, some relative
distances appear to be a clear consequence of the trans-inu-
ence. When no ligand is opposite the diphenylcarbene, the
distance is short, as in the cases of (tmtaa)Fe]CPh2 (B)10 and
(TPFPP)Fe]CPh2 (A).12 As the methylimidazole adduct of the
latter (i.e., (TPFPP)Fe(]CPh2)(MeIm) (A))12 indicates, the
˚
distance is increased by 0.55 A. It is not surprising that the
˚
Fig. 5 Molecular view of the cation pertaining to [(piap)Fe(PMe3)3]
[BArF4] (5); the methyl groups of the trans-PMe3 ligands have been
removed for clarity. Interatomic distances (A˚) and angles (ꢁ): Fe–C1,
2.106(6); Fe–N1, 1.949(6); Fe–C10, 1.867(7); Fe–P1, 2.281(2); Fe–P2,
2.2733(16); C7–N1, 1.307(9); C1–Fe–N1, 78.4(3); C1–Fe–C10, 161.6(3);
C1–Fe–P1, 103.7(2); N1–Fe–C10, 83.2(3); N1–Fe–P1, 177.94(17); C10–
Fe–P1, 94.8(2); C1–Fe–P2, 79.47(4); N1–Fe–P2, 88.54(5); C10–Fe–P2,
100.20(4); P1–Fe–P2, 91.83(5); P2–Fe–P2, 158.89(8); Fe–C1–C2,
133.8(5); Fe–C1–C6, 110.4(5); Fe–C10–C9, 115.7(5); Fe–C10–C11,
136.4(5).
complexes herein have d(Fe]C) that range from 1.809–1.899 A,
given the presence of a strong trans-inuence ligand (an aryl).
There is no straightforward correlation of d(Fe]C) to its
respective 13C NMR spectroscopic shi.
¨
Mossbauer analysis of Fe(II) and Fe(IV) chelates
¨
Shown in Fig. 7 are Mossbauer spectra of the related Fe(II) and
Fe(IV) complexes trans-(pipvd)Fe(PMe3)2N2 (3) and [(pipad)
Fe(PMe3)3][BArF4] (6), respectively. In Table 2, all Mossbauer
¨
parameters for the corresponding Fe(II) and Fe(IV) compounds
are listed. The data in Table 2 reveal isomer shis for the
diamagnetic species all within Dd of 0.1 mm sꢀ1, and provide a
textbook example of why they should not be simply correlated
with formal oxidation state, but are strong indicators of cova-
lency.34,35 “Iron–ligand bond lengths play a decisive role for the
isomer shi of a compound”,35 and the data in Table 2 and
Fig. 2–4 bear this out. Minimal bond distance changes occur in
the protonation of (bdvp)Fe(PMe3)2 (1-PMe3) to afford [(bavp)
Fe(PMe3)2][BArF
] (4-PMe3), and similarly small d(Fe-L/X)
4
changes are likely in the related protonations, leading to small
isomer shi differences.
One counter argument regarding interpretation of isomer
shis pertains to the somewhat arbitrary formalism of treating
a Schrock alkylidene as a (2ꢀ) ligand, whereas a Fischer car-
bene, in which conjugated lone pairs can donate to the carbon
Fig. 6 Molecular view of the cation pertaining to [(pipad)Fe(PMe3)3]
[BArF4] (6); the methyl groups of the trans-PMe3 ligands have been
removed for clarity. Interatomic distances (A˚) and angles (ꢁ): Fe–C1,
1.899(3); Fe–N1, 1.933(3); Fe–C17, 2.059(3); Fe–P1, 2.317(2); Fe–P2,
2.226(3); Fe–P3, 2.367(3); N1–C11, 1.307(5); C1–C2, 1.525(5); N1–Fe–
C17, 80.04(14); C1–Fe–C17, 163.36(15); C17–Fe–P1, 92.03(12); C17–
Fe–P2, 88.28(13); C17–Fe–P3, 87.16(12); N1–Fe–C1, 83.34(14); N1–
Fe–P1, 167.04(14); N1–Fe–P2, 96.52(16); N1–Fe–P3, 79.34(14); C1–
Fe–P1, 104.46(12); C1–Fe–P2, 92.87(12); C1–Fe–P3, 90.53(13); P1–
Fe–P2, 93.45(14); P1–Fe–P3, 90.11(13); P2–Fe–P3, 174.32(13); Fe–
C17–C16, 136.0(3); Fe–C17–C12, 110.8(3); Fe–C1–C5, 112.8(2); Fe–
C1–C2, 131.0(3).
Table 1 Comparison of iron-alkylidene d(Fe]C) and 13C NMR shift (d)
Compounda
d(Fe]C) (A) d (13C]Fe)
˚
(tmtaa)Fe]CPh2 (B)b
1.794(3)
1.767(3)
1.787(8)
1.809(4)
1.827(5)
1.867(7)
1.899(3)
1.9205(19)
1.9234(18)
1.9357(18)
1.943(8)
313.2
359.0
336.6
350.6
385.4
352.6
348.4
Para
Para
Para
Para
Para
(TPFPP)Fe]CPh2 (A)c
[Cp*(dppe)Fe]CH(Me)]PF6 (E)d
[(bavp)Fe(PMe3)2][BArF4] (4-PMe3)
(TPFPP)Fe(]CPh2)(MeIm) (A)c
[(piap)Fe(PMe3)3][BArF4] (5)
[(pipad)Fe(PMe3)3][BArF4] (6)
(EtPDI)Fe]CPh2 (C)e
Structural comparisons
(
MeEtPDI)Fe]CPh2 (C)e
In Table 1, a comparison of known Fe(IV) alkylidenes is given
[p-tBu-calix[4](O)2(OMe)2]Fe]CPh2 (D)f
13
with reference to d(Fe]C) and C NMR chemical shi (d).29,30
[p-tBu-calix[4](O)2(OSiMe3)2]Fe]CPh2 (D)f 1.958(5)
Paramagnetic derivatives are on the long side of the bond
distance values, and the electronic structure analysis by Chirik
et al.13 of the PDI derivatives suggests that these species are best
considered carbene radicals.33 The p-interaction is construed as
1.973(5)
Para
a
See Fig. 1 for ligand structural types corresponding to A–D. b Ref. 10.
c
Ref. 12. d Ref. 9. e Ref. 13. f Ref. 11.
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the complex is notably altered due to the change from a
symmetric divinyl coordination to that of the alkylidene and
vinyl arrangement.
Conclusions
Protonation of Fe(II) chelate complexes in which iron–vinyl
bonds are present led to the formation of four cationic Fe(IV)
alkylidene complexes, three of which are structurally charac-
terized. Prior to this study, [Cp*(dppe)Fe]CH(Me)]PF6 was the
only non-aryl Fe(IV) alkylidene that had undergone X-ray
diffraction structural analysis, although numerous related
[CpLL0Fe]CHR]+ complexes have been synthesized.7–9,14–16
This study confers condence in iron–vinyl protonation as a
viable, general route to Fe(IV) alkylidenes in non-Cp systems.
The compounds herein (i.e., 4-PMe3, 5 and 6) were not active
towards metathesis (e.g. cis-2-pentene and RCCR; R ¼ Me, Ph) or
cycloprotonation, primarily because PMe3 is not sufficiently
labile, as expected. In order to implement this route toward
viable olen metathesis catalysts, future syntheses must
address three factors: (1) complexes must be coordinatively
unsaturated, with 14eꢀ species the obvious targets based on
ruthenium analogues; (2) complexes must be neutral or
anionic, where the d-orbitals are less contracted; and (3)
Fe]CHR moieties must be targeted.
Fig. 7 Mossbauer spectra of Fe(II) trans-(pipvd)Fe(PMe3)2N2 (3, d ¼ 0.07(1)
¨
mm sꢀ1; DEQ ¼ 1.97(1) mm sꢀ1), and the corresponding Fe(IV) cation
[(pipad)Fe(PMe3)3][BArF4] (6, d ¼ 0.07(1) mm sꢀ1; DEQ ¼ 2.20(1) mm sꢀ1).
(i.e., M]CX(R) 4 M(ꢀ)–C]X(+) (R)), is neutral. While one can
argue there is some conjugation for 3 and 6, the other cases are
less readily interpreted in this fashion, especially given the
orientation of the phenyl group of 5 as roughly orthogonal to
the Fe]C interaction. There can be little question that two pairs
of electrons – one sigma and one pi – exist between iron and
carbon in these compounds, and that the parameters of the
Experimental section
Experimental details, full spectral characterizations, and a
¨
Mossbauer spectra correlate with a strong degree of covalency.
¨
description of the Mossbauer spectroscopic analysis are given
Previously characterized alkylidene species are limited to those
reported by Chirik et al.,13 whose S ¼ 1 systems are sufficiently
different to be essentially incomparable.
in the ESI.† For general descriptions, consult the schemes.
Some crystallographic information is given below.
Interpretation of the quadrupole splitting (DEQ), a measure
of the electric eld gradient at the nucleus,35 is less transparent.
The changes in ligand coordination, principally PMe3 for N2 in
the conversion of 2 and 3 to 5 and 6, respectively, are apparently
signicant enough to offset changes to the Ar-Fe(–Vy/]C) axes.
For 1-PMe3 and 4-PMe3, there is a consequential change from
DEQ ¼ 1.96(1) mm sꢀ1 to 2.67(1) mm sꢀ1, as the rhombicity of
Crystal data for 1-PMe3
ꢀ
C26H32N2P2Fe, M ¼ 490.33, triclinic, P1, a ¼ 10.2138(8), b ¼
ꢁ
ꢁ
˚
10.6014(8), c ¼ 12.4208(10) A, a ¼ 88.674(4) b ¼ 67.062(3) , g ¼
3
ꢁ
˚
˚
89.687(4) , V ¼ 1238.24(17) A , T ¼ 203(2) K, l ¼ 0.71073 A, Z ¼
2, Rint ¼ 0.0311, 22 420 reections, 6098 independent, R1(all
data) ¼ 0.0663, wR2 ¼ 0.1766, GOF ¼ 1.077.†
Table 2 Comparison of Fe(II) and Fe(IV) alkylidene Mossbauer parameters
¨
Compound
d (mm sꢀ1
)
DEQ (mm sꢀ1
)
GFWHM (mm sꢀ1
)
(bdvp)Fe(PMe3)2 (1-PMe3)
0.09(1)
0.08(1)
0.07(1)
0.01(1)
0.06(1)
0.07(1)
1.96(1)
2.14(1)
1.97(1)
2.67(1)
2.02(1)
2.20(1)
0.45(1)
0.31(1)
0.33(1)
0.28(1)
0.29(1)
0.28(1)
trans-(pipp)Fe(PMe3)2N2 (2)a
trans-(pipvd)Fe(PMe3)2N2 (3)
[(bavp)Fe(PMe3)2][BArF4] (4-PMe3)b
[(piap)Fe(PMe3)3][BArF4] (5)c
[(pipad)Fe(PMe3)3][BArF4] (6)
a
b
Sample contained 20% of a high spin Fe(II) species: d ¼ 1.23(1) mm sꢀ1, DEQ ¼ 2.40(1) mm sꢀ1, GFWHM ¼ 0.73(1) mm sꢀ1
. Sample contained 18%
of a high spin Fe(II) species: d ¼ 1.28(1) mm sꢀ1, DEQ ¼ 2.70(1) mm sꢀ1, GFWHM ¼ 0.54(1) mm sꢀ1
.
Sample contained 35% of a high spin Fe(II)
c
species: d ¼ 1.25(1) mm sꢀ1, DEQ ¼ 2.42(1) mm sꢀ1, GFWHM ¼ 0.51(1) mm sꢀ1
.
Chem. Sci.
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13 S. K. Russell, J. M. Hoyt, S. C. Bart, C. Milsmann,
S. C. E. Stieber, S. P. Semproni, S. DeBeer and P. J. Chirik,
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Crystal data for 4-PMe3
C58H45N2F24BP2Fe, M ¼ 1354.56, monoclinic, P21/c, a ¼
ꢁ
˚
19.6517(7), b ¼ 12.5655(4), c ¼ 25.3645(7) A, b ¼ 109.7450(10) ,
3
˚
˚
V ¼ 5895.1(3) A , T ¼ 203(2) K, l ¼ 0.71073 A, Z ¼ 4, Rint
¼
0.0365, 49 817 reections, 12 059 independent, R1(all data) ¼
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Crystal data for 5(THF)
C
61H60NOF24BP3Fe, M ¼ 1438.67, monoclinic, C2/m, a ¼
ꢁ
˚
19.963(5), b ¼ 17.492(6), c ¼ 19.586(6) A, b ¼ 93.869(14) , V ¼
3
˚
˚
6824(4) A , T ¼ 203(2) K, l ¼ 0.71073 A, Z ¼ 4, Rint ¼ 0.0579,
21 278 reections, 5076 independent, R1(all data) ¼ 0.0899, wR2
¼ 0.1923, GOF ¼ 1.155.†
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Crystal data for 6
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20 (a) G. Xu, H. Sun and X. Li, Organometallics, 2009, 28, 6090–
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and X. Li, Dalton Trans., 2013, 3417–3428.
C
58H55NF24BP3Fe,
M
¼
1381.60, monoclinic, P21/c,
a
¼
ꢁ
˚
18.4232(6), b ¼ 13.0618(4), c ¼ 25.9802(8) A, b ¼ 99.5300(10) , V
3
˚
˚
¼ 6165.6(3) A , T ¼ 233(2) K, l ¼ 0.71073 A, Z ¼ 4, Rint ¼ 0.0393,
35 706 reections, 9178 independent, R1(all data) ¼ 0.0780, wR2
¼ 0.1273, GOF ¼ 1.050.†
¨
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and U. Florke, Angew. Chem., Int. Ed., 2005, 44, 975–977; (d)
Support from the National Science Foundation (CHE-1402149),
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