Neutral Fe(iv) alkylidenes, including some that bind dinitrogen

Article abstract of DOI:10.1039/c5cc08825a

Neutral, formally Fe(iv) alkylidene species are sought as plausible olefin metathesis catalysts, and the synthesis of several is described herein. The complexes are prepared via nucleophilic attack (Nu = MeLi, PhCH₂K, 2-picolyllithium, Me₂PCH₂Li, MePhPCH₂Li, Ph₂PCH₂Li) at the imine of cationic [mer-{κ-C,N,C-(C₆H₄-yl)-2-CH=N(2-C₆H₄-C(ⁱPr)=)}Fe(PMe₃)₃][B(3,5-CF₃-C₆H₃)₄]. In contrast, MeMgCl and mesityllithium displaced and deprotonated bound PMe₃, respectively. Structural details are provided for mer-{κ-C,N,C-(C₆H₄-yl)-2-CH(Bn)N(2-C₆H₄-C(ⁱPr)=)}Fe{trans-(PMe₃)₂}N₂ and {κ-C,N,C,P-(C₆H₄-yl)-2-CH(CH₂PMe₂)N(2-C₆H₄-C(ⁱPr)=)}Fe(PMe₃)₂.

Full text of DOI:10.1039/c5cc08825a

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Neutral Fe(IV) alkylidenes, including some that bind dinitrogen†
Brian M. Lindley, Brian P. Jacobs, Samantha N. MacMillan, and Peter T. Wolczanski*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Neutral, formally Fe(IV) alkylidene species are sought as plausible
olefin metathesis catalysts, and the synthesis of several is
described herein. The complexes are prepared via nucleophilic
Nu: attack at imine Nu:
Nu
[BArF
]
4
N
Fe
P'
N
P'
attack (Nu
MePhPCH2Li, Ph2PCH2Li) at the imine of cationic [mer-{κ-C,N,C-
(C6H4-yl-2-CH=N(2-C6H4-C(iPr)=}Fe(PMe3)3][B(3,5-CF3-C6H3)4].
In
=
MeLi, PhCH2K, 2-picolyllithium, Me2PCH2Li,
P'
Nu:
Fe
P'
P'
P'
2
H
H
H[BArF
]
4
contrast, MeMgCl and mesityllithium displaced and deprotonated
bound PMe3, respectively. Structural details are provided for mer-
{κ-C,N,C-(C6H4-yl-2-CH(Bn)N(2-C6H4-C(iPr)=}Fe{trans-(PMe3)2}N2
and {κ-C,N,C,P-(C6H4-yl-2-CH(CH2PMe2)N(2-C6H4-C(iPr)=}Fe(PMe3)2.
P' = PMe3
Nu:
- NuH
- P'
N
Fe
P'
Nu:
P'
P'
Nu
N
P'
P'
H
N
Fe
Nu: attack
at alkylidene
A Holy Grail of base metal catalysis is the generation of iron
P'
1
Fe
complexes capable of catalysing olefin metathesis,1-3
a
P'
deprotonation of iPr
P'
H
reaction currently moderated best by Ru2,4 and Mo3,5 species.
Hoffmann’s critique of olefin metathesis6 convincingly suggests
that dn (n≤4) is a necessary constraint for active catalysts,
hence Fe(IV) species are crucial targets. Recent work from
these laboratories7 has featured Fe(IV) tetra- and tri-dentate
cationic chelates that contain alkylidene fragments created via
the protonation8-12 of Fe(II) vinyl precursors. While the
compounds were not metathesis-active, they are the only non-
diphenylcarbene,13-16 alkylidene complexes to be structurally
characterized besides [Cp*(dppe)Fe=CH(Me)]PF6.17
Nu: substitution of P'
Nu
Scheme 1. Plausible reactions of a Nu: with the cationic Fe(IV)
alkylidene, [mer-{κ-C,C-(C6H4-yl-2-CH=N(2-C6H4-C(iPr)=}Fe(PMe3)3] [B(3,5-
CF3-C6H3)4] (2).
Scheme 1 illustrates possible outcomes of a reaction of an
anionic nucleophile with the cationic alkylidene [mer-{κ-C,N,C-
(C6H4-yl-2-CH=N(2-C6H4-C(iPr)=}Fe(PMe3)3][B(3,5-CF3-C6H3)4] (2): 1)
deprotonation by Nu:- can regenerate the Fe(II) vinyl precursor (1);
2) the Nu:- can substitute for PMe3; 3) Nu:- attack at the alkylidene
affords an Fe(II) species; and 4) Nu:- attack at the imine provides an
Fe(IV) amide complex. Complex 27 was chosen as the starting
alkylidene because its bulky isopropyl substituent should hamper
deprotonation and attack at the Fe=C(Ar)iPr. Since substitution is
likely to be a dissociative process, imine to amide conversion is the
favoured outcome.
In order to eventually obtain catalytically active Fe(IV)
alkylidenes, it was suggested7 that some or all of three
additional criteria must be met: 1) coordinative unsaturation is
significant, based on the known 16e- Ru precursors and 14e-
Mo catalysts; 2) complexes must be neutral or anionic, such
that the 3d orbitals of Fe are not contracted; and 3) hydrogen
substituted alkylidenes, e.g., Fe=CHR, are more likely to react.
In this communication, neutral alkylidenes have been
generated, and some reveal instabilities likely associated with
coordinative unsaturation.
Scheme 2 illustrates the synthesis of several neutral Fe(IV)
alkylidenes derived from [mer-{κ-C,N,C-(C6H4-yl-2-CH=N(2-C6H4-
C(iPr)=}Fe(PMe3)3] [B(3,5-CF3-C6H3)4] (2).7 Treatment of 2 with MeLi,
2-picolyllithium,18 and potassium benzyl afforded the Fe(IV)
dinitrogen alkylidenes mer-{κ-C,N,C-(C6H4-yl-2-CH(R)N(2-C6H4-
C(iPr)=}Fe{trans-(PMe3)2}N2 (R = Me, 3 (red); 2-pic, 4 (red-orange)
Bn, 5 (red-brown)). The substitution of a bulky PMe3 opposite the
amide is expected, based on dinitrogen substitutions in Fe(II) imine-
i
phosphine complexes.7,19 The bulky Pr group labilizes the PMe3
opposite the N-donor, enabling binding by the thin dinitrogen
ligand. Curiously, in the case of MeMgCl, direct substitution of the
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CH3
CH3Li, - Li[BArF
Et2O
]
3
4
The new amide-alkylidenes manifest alkylidene 13C shifts
significantly lower20,21 than previous cationic derivatives (e.g.,
2
N
N
-78 °C -> 23 °C
12 h
P'
P'
2
,
δ
δ
348.4): 3,
δ
315.55, JPC = 14 Hz;
313.77, JPC = 14 Hz; 4,
δ 313.95, JPC = 13 Hz;
Fe
Fe
[BArF
]
5,
6,
δ
313.17, td, JPC = 12, 23 Hz; 7, δ
P'
P'
4
72%
N
H
H
N
N
P'
(2-pic)Li, - Li[BArF
Et2O
12 h
]
4
321.15, JPC = 31 Hz. The asymmetry of each dinitrogen-amide
complex is revealed by inequivalent 31P signals at ~13 and ~15
ppm, with JPP ~ 124-127 Hz.
Et2O
-78 °C ->
23 °C
MeMgCl12
h
- ClMg[BArF
]
4
-78 °C ->
23 °C
N
6 h
P'
Fig. 1 illustrates the
amide and dinitrogen. While the Fe=C bond is readily
understood as a classic dxy±C(p ) interaction, the -bonding to
N2 is more complicated. The dinitrogen ligands exhibit
considerable backbonding, with (NN) = 2058, 2056, and 2058
cm-1 for the respective methyl, 2-picolyl, and benzyl
derivatives. The amide dyz-N(p ) interaction renders the
predominantly dyz-orbital closer in energy to the N2 *-orbital.
This three-orbital interaction is responsible for increased
π-bonds affiliated with the alkylidene,
Et2O
Fe
N
KBn, - K[BArF
]
4
P'
P'
62%
H
N
N
Fe
-78 °C -> 23 °C
5-12 h
π
π
P'
4
H
7
CH3
ν
P' = PMe3
N
N2
- PMe3
N
P'
π
P'
Fe
Fe
P'
π
P'
P'
H
77%
N
N
H
6
5
overlap for backbonding to dinitrogen, and some net N(p
π)-
>Fe(dyz) bonding.
Scheme 2. Syntheses of neutral Fe(IV) alkylidene complexes via attack
at the chelate imine and substitution of PMe3.
unique PMe3 occurred, affording thermally unstable mer-{κ-C,N,C-
(C6H4-yl-2-CH=N(2-C6H4-C(iPr)=}Fe{trans-(PMe3)2}CH3 (7) in ~80%
purity. A triplet (JPH = 11 Hz) at δ -0.65 characterized the Fe-CH3 in
1
the H NMR, and only one 31P signal was observed, consistent with
Cs symmetry.
In order to show that the initial product of imine attack is
the mer-(PMe3)3 species, dinitrogen was scrupulously kept out
of the reaction of
2
with KBn, and magenta mer-{κ-C,C-(C6H4-yl-
) was generated in
2-CH(R)N(2-C6H4-C(iPr)=}Fe{trans-(PMe3)3
(
6
~80-90% purity. A key feature is an ABC pattern in the 31P NMR
spectrum that reflects the low symmetry of the mer-(PMe3)3
derivative.
z
R
N
x
P'
Fe
NN2(π*)
P'
H
N
N
Fig. 2. Molecular view of mer-{κ-C,N,C-(C6H4-yl-2-CH(Bn)N(2-C6H4-
C(iPr)=}Fe{trans-(PMe3)2}N2 (5). Pertinent interatomic distances (Å) and
angles (°): Fe-N1, 1.9299(13); Fe-N2, 1.7925(15); Fe-C4, 1.9535(16); Fe-C24,
2.0206(16); Fe-P1, 2.2679(5); Fe-P2, 2.2523(5); N1-C11, 1.471(2); N1-C10,
1.326(2); C9-C10, 1.434(2); C8-C9, 1.362(3); C7-C8, 1.405(3); C6-C7, 1.364(3);
C5-C6, 1.434(2); C5-C10, 1.441(2); C4-C5, 1.420(2); C19-C24, 1.413(2); C11-
C19, 1.515(2); N1-Fe-N2, 173.91(6); P1-Fe-P2, 175.45(2); C4-Fe-C24,
163.25(7); L/X-Fe-L/X, 90.0(58) (ave).
FeC(π*)
N
FeNN2(π*)
N
N
+ FeNam(π*)
H
Fig. 2 illustrates a molecular view of pseudo-octahedral
mer-{κ-C,N,C-(C6H4-yl-2-CH(Bn)N(2-C6H4-C(iPr)=}Fe{trans-(PMe3)2}N2
(5), and pertinent metrics are listed in its caption. Of crucial
importance are the d(N1-C10) = 1.326(2) Å and d(C4-C5) = 1.420(2)
Å, which are 0.01-0.02 Å and 0.05 Å shorter than expected,22 and
arene distances that hint at a significant contribution from an Fe(II)
representation (Fig. 3). The valence bond structures of the Fe(IV)
and Fe(II) resonance forms in Fig. 3. depict whether the respective
FeC(π*) (i.e., d4) or FeC(πb) (i.e., d6) in Fig. 1 carry the brunt of the d-
orbital contribution.
π-bonding
C(π)
manifolds
N
N
Fe
FeC(πb)
H
H
N
Nam(π)
Fe
FeN(πb)
N
N
N
N
Fig. 1. The iron-alkylidene and amide-iron-dinitrogen
manifolds of -R-N2.
π-bonding
3
2 | J. Name., 2012, 00, 1-3
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“t”d, JPC = 18, 11 Hz; 8-MePh (major), δ 306.48, “t”d, JPC = 20, 12 Hz;
8-Ph2, δ 306.48, “t”d, JPC = 20, 12 Hz.
R
R
N
N
P'
P'
Fe
Fe
P'
P'
H
H
N
N
N
Fe(II)
Fe(IV)
N
Fig. 3. Fe(IV) and Fe(II) resonance structures for 3-5 and related complexes.
The d(Fe=C) is long (1.9535(16) Å) relative to compound 2
(1.899(2) Å), consistent with the change from cation to neutral. The
Fe-Nam distance of 1.9299(13) Å is normal, as is the iron-nitrogen
(N2) distance of 1.7925(15) Å. The remaining metrics reflect the
pseudo-octahedral arrangement, although the C4-Fe-C24 angle is
163.25(7)°, which enables a trace of σ*-character to dxz, a factor
that also contributes to effective dinitrogen backbonding.7
The use of 2-picolyl in Scheme 2 stemmed from the possibility
that it could act as a chelate arm in displacing one of the trans-
PMe3 groups. Even though py is a weaker donor than PMe3, the
latter can be removed in vacuo, thereby generating a potentially
labile py ligand cis to the alkylidene.
When the same approach was taken with nucleophiles
LiCH2PRR’ (R = R’ = Me,23 Ph;24 R = Me, R’ = Ph)22 and 2,
displacement of a PMe3 was affected, resulting in {κ-C,N,C,P-(C6H4-
yl-2-CH(CH2PRR’)N(2-C6H4-C(iPr)=}Fe(PMe3)2 (R = R’ = Me, 8-Me2
(magenta); R = Me, R’ = Ph, 8-MePh (red-purple); R = R’ = Ph, 8-Ph2
(oily red), as illustrated in Scheme 3. In a surprising reaction,
treatment of 2 with mesityllithium25 did not result in attack at the
imine, but deprotonation of PMe3 occurred, providing an alternate
route to 8-Me2. While the tetradentate chelate 8-PMe2 showed
appreciable thermal stability, and the diastereomeric mixture 8-
PMePh was also reasonably stable, 8-Ph2 was thermally sensitive,
and any attempts at purification simply increased the amount of
degradation products.
Fig. 4. Molecular view of one of two independent enantiomers of {κ-
C,N,C,P-(C6H4-yl-2-CH(CH2PMe2)N(2-C6H4-C(iPr)=}Fe(PMe3)2 (8-Me2)
with PMe3 methyl groups removed for clarity. Pertinent interatomic
distances (Å) and angles (°): Fe2-C30, 1.947(3); Fe2-C43, 2.083(3); Fe2-N2,
1.916(2); Fe2-P4, 2.2142(9); Fe2-P5, 2.2008(9); Fe2-P6, 2.2254(9); N2-C36,
1.329(4); C35-C36, 1.409(4); C34-C35, 1.344(5); C33-C34, 1.390(5); C32-C33,
1.364(4); C31-C32, 1.421(4); C31-C36, 1.420(4); C30-C31, 1.421(4); N2-C37,
1.472(4); C37-C38, 1.493(5); C38-C43, 1.383(5); C37-C44, 1.583(6); P4-C44,
1.820(4); N2-Fe2-P5, 174.84(8); C30-Fe2-C43, 161.30(12); P4-Fe2-P6,
159.69(4).
Two independent enantiomers occupy the asymmetric unit of
{κ-C,N,C,P-(C6H4-yl-2-CH(CH2PMe2)N(2-C6H4-C(iPr)=}Fe(PMe3)2 (8-
Me2), and the view of one in Fig. 4 shows the unusual tetradentate
chelate that ligates to all but cis-sites on an octahedron. The
average metrics for 8-Me2 essentially parallel those of 5, except
that P4, P6, C30 and C43 are all canted away (97.8(29)° ave) from
P5. The alkylidene is again long (1.943(6) Å ave), and the short N-Car
(1.329(4) Å ave) and long Calk-Car of 1.426(6) Å (ave) also suggest
that the Fe(II) resonance form in Fig. 3 plays a significant role.
The incipient coordinative unsaturation evidenced by the
thermal instability of 8-Ph2 was tested by exposure to alkynes and
alkenes. No evidence of olefin metathesis or carbene transfer26 was
obtained, and similar tests of the remaining 8-R,R’ failed to elicit
evidence of alkylidene reactivity. Dinitrogen is typically a labile
ligand in related systems,7,27,28 and 3-5 was subjected to olefins,
but again no substantive reactivity was observed up to degradation
temperatures.
LiCH2PMePh
2
- Li[BArF
]
4
[BArF
]
4
N
Fe
P'
Et2O/THF
N
Fe
P'
P'
P'
P'
-78 °C -> 23 °C
12 h
P
P'
Me
Ph
crude
1.4:1
rextal
>4.5:1
52%
H
H
H
LiCH2PPh2
- Li[BArF
Ph
Et2O
]
4
Et2O, 6h
-78 °C ->
23 °C
N
Fe
P'
-78 °C -> 23 °C
LiCH2PMe2
- Li[BArF
Previously, it was suggested that the switch from cationic Fe(IV)
alkylidenes to neutral might be crucial to the observation of
metathesis reactivity. While the compounds herein have some Fe(II)
character (Figs. 1 and 3), the requisite orbital for acceptance of an
olefin and generation of a metallacyclobutane is present. It appears
MesLi
12 h
- Li[BArF
]
4
P
]
4
Me
N
Fe
P'
8-MePh
P'
N
75%
P'
P
i
Ph
8-Ph2
H
Fe
P'
certain that the substituents on the alkylidene, in particular the Pr
Ph
P
Me
group on the neutral species, are deleterious to productive
reactivity. As a consequence, the next steps in this synthetic series
will be to prepare related Fe(IV)=CHR species that are
coordinatively unsaturated or labile.
The National Science Foundation (CHE-1402149) is gratefully
acknowledged for financial support of this work.
H
8-Me2
~80% crude; ~80% purity
Me
Scheme 3. Syntheses of tetradentate chelate complexes {κ-C,N,C,P-(C6H4-yl-
2-CH(CH2PRR’)N(2-C6H4-C(iPr)=}Fe(PMe3)2 (5-R,R’).
The chelates were readily identified by an ABC pattern in the 31
P
NMR spectrum, with shifts at δ ~12-14, 19-22 and 40-42 for 8-Me2
and 8-MePh, while related signals for 8-Ph2 resonated at δ 13.35,
17.42, and 56.62. The alkylidene 13C NMR resonances for 8-R,R’ are
again lower than the related cationic chelates: 8-Me2, δ 305.79,
Notes and references
Crystal data for 2-Bn-N2: C30H41N3P2Fe, M = 561.45, monoclinic, P21/n, a =
10.6131(6) Å, b = 14.2296(9) Å, c = 19.7148(12) Å, β = 97.830(3)°, V =
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2949.6(3) Å3, T = 223(2) K, λ = 0.71073 Å, Z = 4, 35866 reflections, 8606
independent, Rint = 0.0320, R1(all data) = 0.0571, wR2 = 0.1183, GOF = 1.032,
CCDC- 1430257.
Crystal data for 5-Me2: C26H42NP3Fe, M = 517.37, monoclinic, P21/c, a =
17.283(3) Å, b = 19.422(3) Å, c = 15.933(3) Å, β = 92.103(8)°, V = 5344.5(16)
Å3, T = 223(2) K, λ = 0.71073 Å, Z = 8, 49309 reflections, 10926 independent,
Rint = 0.0520, R1(all data) = 0.0698, wR2 = 0.1331, GOF = 1.040, CCDC-
1430256.
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