Unrealized concepts of masked alkylidenes in (PNP)FeXY systems and alternative approaches to LnXmFe(IV)=CHR

Article abstract of DOI:10.1016/j.poly.2020.114460

Treatment of PNP ligands with iron(II) halides yielded pseudo tetrahedral {(Ph₂PCH₂)₂NR}FeX₂ (R″ = ^tBu, 1a-X; Me, 1b-X) and {(ⁱBu₂PCH₂)₂NR″}FeX₂ (R″ = ^tBu, 2a-X; Me, 2b-X) complexes, which could be mono- and dialkylated to afford various {(Ph₂PCH₂)₂NR″}FeCl(R) (R″ = ^tBu, 3a-R; Me, 3b-R), {(ⁱBu₂PCH₂)₂NR″}FeCl(R) (R″ = ^tBu, 4a-R; Me, 4b-R), {(Ph₂PCH₂)₂N^tBu}Fe(^neoPe)₂ (5a-^neoPe) and {(ⁱBu₂PCH₂)₂N^tBu}Fe(1-nor)₂ (6a-nor). All of the complexes were high spin (S = 2), and structural studies of {(Ph₂PCH₂)₂NMe}FeCl₂ (1b-Cl), {(ⁱBu₂PCH₂)₂N^tBu}FeCl₂ (2a-Cl), and {(ⁱBu₂PCH₂)₂N^tBu}FeCl(1-nor) (4a-nor), revealed chair conformations for the PNP ligands, which were calculated to have modest barriers to boat configurations via twist-boats as calculational minima. Attempts to convert these complexes to zwitterionic NCRR′Fe-containing species as “masked alkylidenes” failed to materialize. Alternative approaches involving structurally characterized diamagnetic {(ⁱBu₂PCH₂)₂N^tBu}(CO)₂FeCl(CO^neoPe) (8), the carbonylation product of 4a-^neoPe, and low-valent {(Ph₂PCH₂CH₂)₂N^tBu}Fe((H₂C=CHSiMe₂)₂O) (9) also failed. An analysis of orbital energies and overlap is given for alkylidenes. Covalence is identified as a crucial feature necessary for the alkylidene to metalacyclobutane transformation in metathesis.

Full text of DOI:10.1016/j.poly.2020.114460

Polyhedron 181 (2020) 114460
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Unrealized concepts of masked alkylidenes in (PNP)FeXY systems and
alternative approaches to LnXmFe(IV)=CHR
a
b
Gregory M. George a, Peter T. Wolczanski a, , Samantha N. MacMillan , Thomas R. Cundari
⇑
a Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853, USA
b Department of Chemistry, CASCaM, University of North Texas, Denton, TX 76201, USA
a r t i c l e i n f o
a b s t r a c t
Treatment of PNP ligands with iron(II) halides yielded pseudo tetrahedral {(Ph2PCH2)2NR}FeX2 (R00 = tBu,
1a-X; Me, 1b-X) and {(iBu2PCH2)2NR00}FeX2 (R00 = tBu, 2a-X; Me, 2b-X) complexes, which could be mono-
and dialkylated to afford various {(Ph2PCH2)2NR00}FeCl(R) (R00 = tBu, 3a-R; Me, 3b-R), {(iBu2PCH2)2NR00}FeCl
Article history:
Received 4 December 2019
Accepted 17 February 2020
Available online 21 February 2020
(R) (R00
=
tBu, 4a-R; Me, 4b-R), {(Ph2PCH2)2NtBu}Fe(neoPe)2 (5a-neoPe) and {(iBu2PCH2)2NtBu}Fe(1-nor)2
(6a-nor). All of the complexes were high spin (S = 2), and structural studies of {(Ph2PCH2)2NMe}FeCl2
(1b-Cl), {(iBu2PCH2)2NtBu}FeCl2 (2a-Cl), and {(iBu2PCH2)2NtBu}FeCl(1-nor) (4a-nor), revealed chair con-
formations for the PNP ligands, which were calculated to have modest barriers to boat configurations
via twist-boats as calculational minima. Attempts to convert these complexes to zwitterionic NCRR0Fe-
containing species as ‘‘masked alkylidenes” failed to materialize. Alternative approaches involving struc-
turally characterized diamagnetic {(iBu2PCH2)2NtBu}(CO)2FeCl(COneoPe) (8), the carbonylation product of
4a-neoPe, and low-valent {(Ph2PCH2CH2)2NtBu}Fe((H2C=CHSiMe2)2O) (9) also failed. An analysis of orbital
energies and overlap is given for alkylidenes. Covalence is identified as a crucial feature necessary for the
alkylidene to metalacyclobutane transformation in metathesis.
Dedicated to Prof. John E. Bercaw, for
teaching us how to be, and enjoy being,
scientists.
Keywords:
Iron
Phosphine
Zwitterion
Masking
Alkylidene
Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction
(IV) also failed to manifest reactivity other than some cyclopropa-
nation [11–14,31], and it is likely that such functionality is better
The application of transition metals to catalytic olefin metathe-
sis (OM) is practically limited to a select group of metals, most of
which are members of the second row [1–5]. While molybdenum
is relatively inexpensive and diverse in usage [1,2], ruthenium cat-
alysts are still the leading choice for most fine-chemicals synthesis
due to their lesser sensitivity, and relative ease of handling and
preparation [3,4]. Somewhat surprisingly, neither first row transi-
tion metal alternative – chromium in group 6 [6,7] or iron [8–22]
in group 8 – has been shown to exhibit similar 2 + 2, i.e. M=CRR0
+ olefin, chemistry, whereas vanadium alkylidenes [23–25] com-
plement metathesis reactivity observed initially from niobium
and tantalum [26], and titanium alkylidenes have shown modest
catalysis [27–29].
described as diarylcarbyl radicals antiferromagnetically coupled
to Fe(III) centers, i.e., LnXmFe"(III)(-C;Ar2) [14,32].
Work in these laboratories initially focused on generating
pseudo octahedral Fe(IV) alkylidenes using synthetic approaches
derived from Cp-based iron(IV) cation preparation. Three cationic
Fe(IV) chelates [20] and eight neutral Fe(IV) derivatives [21],
including five structurally characterized examples, were prepared,
yet none elicited reactivity that implicated metathesis. In addition,
calculations strongly indicated that Fe(IV) cationic alkylidenes can
be construed as Fe(II) stabilized carbocations, and the neutrals as
highly delocalized Fe(II) species [22]. Consequently, lower coordi-
nate derivatives akin to Grubbs’ type catalysts were sought, but
while Fe(IV) imido complexes, (IPr)Fe(=NAd)R2 (R = neoPe, 1-nor;
Ad = adamantyl) were synthesized [33], corresponding alkylidenes
could not be prepared, as shown in Scheme 1, even with a steric
mimic of AdN3, the aziridine Ad-cCN2 [34].
According to theoretical work by Hoffmann, dn (n ꢀ 4) com-
plexes are a requirement for metathesis activity [30]. Early work
on [CpL2Fe=CRR0)]+ and CpLXFe=CRR0 complexes [8–10,15–17],
including a structural characterization of [Cp(dppe)Fe=CHMe]+
[10], afforded examples of cyclopropanation, but no evidence of
2 + 2 chemistry. Subsequent exploration of diaryl carbenes of Fe
It is possible that Fe(IV)=CRR0 functionalities are not stable
enough in these coordination environments, or that the covalency
in the iron-carbon double bond needed for metathesis cannot be
obtained, as the alkylidene fragment is simply not oxidizing
enough. Since there is modest calculational support for the former,
approaches toward masking alkylidenes or carbenes, first
⇑
Corresponding author.
E-mail addresses: ptw2@cornell.edu (P.T. Wolczanski), t@unt.edu (T.R. Cundari).
https://doi.org/10.1016/j.poly.2020.114460
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

2
G.M. George et al. / Polyhedron 181 (2020) 114460
“R’HC:”
H
R‘
N
N
R
R
AdN3
- N2
N
N
Fe
R
R
Fe
N
Fe
N
R
R
N
R = neoPe
= 1-nor
N
e.g.
N
Scheme 1. Synthesis of (IPr)Fe(=NAd)R2 (R = neoPe, 1-nor) and failure to prepare an analogous alkylidene.
t
exemplified in chemistry from the Deng group [32], were sought.
In this chemistry, abbreviated in Scheme 2, di-tolyl- diazomethane
was utilized as the carbene source. The resulting cyclpropanations
FeX2 (R00 = Bu, X = Cl (86%), 1a-Cl; X = Br (85%), 1a-Br; R00 = Me,
X
= Cl (87%), 1b-Cl) in very good yields, affording pale-yellow
microcrystalline solids (Scheme 4). The related reaction of FeCl2
i
t
were rationalized via labeling studies on syn-
a
-deuterio styrene
and (R02PCH2)2NR00(R0 = Bu, R00 = Bu, Me) also resulted in {(iBu2-
cyclopropanation, Hammett substituent studies, and calculations
as having substantial radical character in a transient (PN2)Fe"(III)
(-C;Ar2). intermediate. Since previous diarylcarbene Fe(IV) com-
plexes were known to possess radical character [14], perhaps an
increase in the field strength at iron might allow ‘‘masked alkylide-
nes” to function in a metathesis capability. As a consequence, we
envisage utilizing known (R02PCH2)2NR‘‘ ligands as initial chelates,
as shown in Fig. 1.
Precedented P,N,P-type ligands containing diisobutyl and
diphenyl phosphine units were considered for appraisal [35–40],
and standard dihalides, {(R02PCH2)2NR00}FeX2, were targeted, noting
that subsequent alkylation would provide an additional means of
increasing the field strength of the iron core in order to increase
covalency. Three means to introduce the zwitterionic masked
alkylidene functionality were considered, as illustrated in
Scheme 3. The tetrahedral complexes could be subject to direct
alkylidene sources, such as conventional diazo compounds, R2CN2
[41,42], with displacement of N2 by the internal NR00 group serving
to mask the R2C unit and create the zwitterion. Alkylation of the
zwitterion should render a tetraalkylammonium cation, and het-
erolytic CH bond activation, perhaps triggered by an additional
base, should render the desired zwitterion. Finally, tetraalkylam-
monium formation with a dihaloalkane, such as CH2I2, would allow
a subsequent reductive coupling to introduce the zwitterion.
PCH2)2NR00}FeX2 (R00 tBu, X = Cl (89%), 2a-Cl; R00 = Me, X = Cl
=
(87%), 2b-Cl) as similarly colored solids. All pseudo tetrahedral
complexes were high spin (S = 2) according to Evans’ method mea-
surements [43]: 1a-Cl, 5.6 mB; 1a-Br, 5.5 mB; 1b-Cl, 5.2 mB; 2a-Cl, 5.0
mB; 2b-Cl, 5.6 mB).
2.1.2. Molecular structure of {(Ph2PCH2)2NMe}FeCl2 (1b-Cl)
The molecular view of {(Ph2PCH2)2NMe}FeCl2 (1b-Cl) in Fig. 2
reveals a pseudo tetrahedral structure with the PNP ligand in a
chair configuration with an equatorial NMe substituent and the
lone pair axial. In this geometry, any modest Me/Ph interaction is
minimized, and the Fe-Cl2 bond is ~ 90° to the NMe. The core angle
and distances are given in the caption of Fig. 2 and all distances are
consistent with a high spin iron center, with d(FeP)ave = 2.422(10)
Å and d(FeCl)ave = 2.228(14) Å. The bite angle of the PNP ligand is
90.024(15)°, causing some variation away from a regular tetrahe-
dron, with /Cl1-Fe-Cl2 = 125.73(2)°, and the Cl-Fe-P angles varying
from 99.977(17) – 118.454(19)°, with the angles to Cl1 wider than
those to Cl2, presumably due to the influence of the two adjacent
axial phenyl substituents.
2.1.3. Molecular structure of {(iBu2PCH2)2NtBu}FeCl2 2a-Cl)
Fig. 3 illustrates a molecular view of {(iBu2PCH2)2NtBu}FeCl2 2a-
Cl), and the diisobutylphosphine variant deviates little from 1b-Cl.
The bite angle of the PNP ligand is slightly wider at 92.405(13)°,
and its configuration is again a chair, with the tBu group in an
equatorial position, again roughly 90° with respect to the FeCl2
bond. The caption of Fig. 3 provides the remaining core distances
and angles, and any subtle variation can be rationalized on the
basis of slightly increased sterics among the isobutyl groups. As
Fig. 3 reveals, the chair is quite regular except for subtle variation
in the equatorial 2-propyl groups.
2. Results and discussion
2.1. {(R02PCH2)2NR00}FeXY
2.1.1. {(R02PCH2)2NR00}FeX2 syntheses
The addition of FeX2 (X = Cl, Br) and (R02PCH2)2NR00(R0 = Ph, R00 =
tBu, Me) in THF for 16 h in THF at 23 °C afforded {(Ph2PCH2)2NR00}
O
p-tolyl
p-tolyl
R
Ph
N N
Ph
P
Fe
O
P
R
Fe
O
p-tolyl
p-tolyl
N
N
N
16 examples
N
dfp
dfp
dfp
dfp
dfp = 2,6-difluorophenyl
Scheme 2. Cyclopropanation via Deng’s ‘‘masked” di-tolyl-carbene complex.

G.M. George et al. / Polyhedron 181 (2020) 114460
3
R”
R”
R”
N
N
R R
R
R
N
”R
N
R2P
Y
Fe
Y
R2P
R2’P
PR’2
Fe
Y
P
P
PPh2
PPh2
Y
Fe(IV)
Fe(II)
Fig. 1. Featured ligands toward potential masked alkylidenes.
Scheme 3. Three approaches to zwitterionic masked alkylidenes: 1) alkylidene precursors; 2) heterolytic CH-bond activation; and 3) reductive coupling.
Scheme 4. Di(phosphinomethyl)alkylamine iron dihalide syntheses.
2.1.4. {(R02PCH2)2NR00}FeXR syntheses
in Scheme 5. The bulkier alkyllithiums were chosen to lessen
the probability of aggregation difficulties, and treatment of
The dichlorides were chosen as substrates for alkylation with
RLi (R = 1-nor, neoPe, CH2SiMe3), and the reactions are illustrated
{(Ph2PCH2)2NR00}FeCl2 (R00
=
tBu, 1a-Cl; Me, 1b-Cl) generated

4
G.M. George et al. / Polyhedron 181 (2020) 114460
angle is slightly reduced to 88.684(12)°, the Cl-Fe-P angles average
100.8(37)°, the C23-Fe-P angles are 122.7(29)° (ave) and the C23-
Fe-Cl angle is only 115.86(4)°. The Fe-P and Fe-Cl distances are
essentially the same, and the unique d(Fe-C23) = 2.0509(13) Å, as
expected for high spin tetrahedral iron.
2.1.6. {(R02PCH2)2NR00}FeR2 syntheses
Dialkylations were also conducted with {(Ph2PCH2)2NR00}FeCl2
t
t
(R00 = Bu, 1a-Cl; Me, 1b-Cl) and {(iBu2PCH2)2NR00}FeCl2 (R00 = Bu,
2a-Cl; R00 = Me, 2b-Cl), but here the isolation of clean products,
as shown in Scheme 6, were limited to {(Ph2PCH2)2NtBu}Fe(neoPe)2
(5a-neoPe, 50%, m = 5.3 mB) and {(iBu2PCH2)2NtBu}Fe(1-nor)2 (6a-
nor, 58%), m = 5.5 mB). At this point, further exploration, purification
and isolation of alkyl and dialkyl derivatives was halted because
this approach to masked alkylidenes appeared fruitless (vide infra).
Fig. 2. Molecular view of {(Ph2PCH2)2NMe}FeCl2 (1b-Cl); ellipsoids at 50%. Core
interatomic distances (Å) and angles (°): FeCl1, 2.2180(5); FeCl2, 2.2380(5); FeP1,
2.4285(4); FeP2, 2.4144(4); Cl1-Fe-Cl2, 125.73(2); Cl1-Fe-P1, 118.454(19); Cl1-Fe-
P2, 113.632(19); Cl2-Fe-P1, 99.977(17); Cl2-Fe-P2, 102.332(18); P1-Fe-P2, 90.024
(15).
2.2. Attempts at masked alkylidenes
With several PNP-dihalides, alkyl-halides, and dialkyls in hand,
the approaches proffered in Scheme 3 were attempted. Without
elaboration, common diazo-species failed to deliver alkylidene
fragments, including every variant with Ph2CN2, and decomposi-
tion was common (Scheme 7). N-alkylation attempts were made
with a variety of RX and CH2I2, and surprisingly, no N-alkylation
was observed, even with the far less crowded N-Me ligands, and
even under elevated temperatures that eventually led to decompo-
sition. With certain highly reactive reagents, such as p-MeOC6H4-
CH2Br, phosphine alkylation was determined to occur upon
degradation.
{(Ph2PCH2)2NR00}FeCl(R) (R00
=
tBu, R = 1-nor, 3a-nor, 75%, neoPe,
3a-neoPe, 70%, CH2SiMe3, 3a-CH2SiMe3, 87%; R00 = Me, R = 1-nor,
3b-nor, 45%, CH2SiMe3, 3b-CH2SiMe3, 46%). Likewise, the addition
of RLi to {(iBu2PCH2)2NR00}FeCl2 (R00 = Bu, 2a-Cl; R00 = Me, 2b-Cl)
t
afforded {(iBu2PCH2)2NR00}FeCl(R) (R00
=
tBu, R = 1-nor, 4a-nor,
59%, neoPe, 4a-neoPe, 57%, CH2SiMe3, 4a-CH2SiMe3, 82%; R00 = Me,
R = 1-nor, 4b-nor, CH2SiMe3, 4b-CH2SiMe3, 63%). All alkylations
proceeded in modest yields, and while most produced yellow con-
taining microcrystalline samples, not all reactions were clean. For
example, a yield of pure 4b-nor could not be obtained, and in
virtually every alkylation, some Fe(1-nor)4 (typically ~5%) was pro-
duced [44], indicative of some disproportionation. Alkylation did
not appreciably increase the field strength of the pseudo tetrahe-
dral derivatives, as Evans’ method measurements [43] were consis-
tent with S = 2 iron centers: 3a-nor, m = 5.1 mB, 3a-neoPe, m = 5.6 mB,
2.3. Conformational studies
The lack of alkylation success prompted consideration of a
steric or conformational bias, and DFT calculations [45–47] were
brought to bear on the latter, with {(Ph2PCH2)2NtBu}FeCl2 (1b-Cl)
{(Ph2PCH2)2NMe}FeCl2 (1b-Cl) the subjects of the study in Fig. 5.
While the transition state ascribed to the chair to boat conforma-
tional change was only ~9.7 kcal/mol, the latter was not a stable
configuration, and twist-boat conformers were found to be stable
instead, at 3.7 kcal/mol relative to the chair. Since the interconver-
sions are relatively facile, there would appear to be no significant
impediment to alkylation. However, if the iron center acts as a
Lewis acid as expected, the orientation of the N-lone pair could
3a-CH2SiMe3,
m = 5.2 mB, 3b-nor, m = 5.3 mB, 3b-CH2SiMe3,
m = 5.6 mB, 4a-nor, m = 4.7 mB, 4a-neoPe, m = 4.8 mB, 4a-CH2SiMe3,
m = 5.4 mB, 4b-nor, m = 5.1 mB, 4b-CH2SiMe3, m = 5.1 mB.
2.1.5. Molecular structure of {(Ph2PCH2)2NMe}FeCl(1-nor) (4a-nor)
As in the cases of 1b-Cl and 2a-Cl, Fig. 4 indicates a pseudo
tetrahedral structure with near mirror symmetry. Its caption lists
the core metric values, and these are also similar, except that the
1-norbornyl ligand exerts modest steric influence. The PNP bite
Fig. 3. Molecular view of {(iBu2PCH2)2NtBu}FeCl2 (2a-Cl, a.) and another with phosphines eclipsed (b.); ellipsoids at 50%. Core interatomic distances (Å) and angles (°): FeCl1,
2.2271(5); FeCl2, 2.2553(5); FeP1, 2.4289(4); FeP2, 2.4459(4); Cl1-Fe-Cl2, 123.29(2); Cl1-Fe-P1, 114.414(17); Cl1-Fe-P2, 119.947(17); Cl2-Fe-P1, 106.091(17); Cl2-Fe-P2,
95.180(16); P1-Fe-P2, 92.405(13).

G.M. George et al. / Polyhedron 181 (2020) 114460
5
Scheme 5. Alkylations of {(R02PCH2)2NR00}FeX2 with RLi (R = 1-nor, neoPe, CH2SiMe3).
be critical to alkylation, and entropic factors leading to the proper
TS for attack at RX. . .Fe might be prohibitive since the complexes
are not that thermally stable.
2.4. Syntheses of {(Ph2PCH2CH2)2NtBu}FeXY and RX addition
With the possibility of conformational difficulties present, a
floppier PNP ligand was employed, and Scheme 8 reveals the syn-
theses of {(Ph2PCH2CH2)2NtBu}FeCl2 (7-Cl) [48] and {(Ph2PCH2-
CH2)2NtBu}FeCl(1-nor) (7-nor). Unfortunately, while the
dichloride was isolated cleanly (61%), the norbornyl-chloride was
prepared in low yield, as substantial quantities of (1-nor)4Fe [44]
were again produced as a byproduct. Nonetheless alkylation
attempts with the complexes were made without success, and
once again the highly reactive p-MeOC6H4CH2Br appeared to alky-
late at phosphorus.
2.5. Carbonylation of 4a-neoPe to {(iBu2PCH2)2NtBu}(CO)2FeCl
(COneoPe) (8)
Failure to achieve masking of alkylidenes via the routes exhib-
ited in Scheme 3 prompted consideration of elaboration of Fischer
carbene species, as shown in Scheme 9. Carbonylation of {(iBu2-
PCH2)2NtBu}FeCl(neoPe) (4a-neoPe) was explored as an entry into a
masked Fischer carbene. As as eq 1 reveals, carbonylation could
not be restricted to one CO, although acyl formation was noted.
Two terminal stretching frequencies at 2009 and 1954 cmꢁ1, and
Fig. 4. Molecular view of {(iBu2PCH2)2NtBu}FeCl(1-nor) (4a-nor); ellipsoids at 50%.
Core interatomic distances (Å) and angles (°): FeCl, 2.2840(4); FeC23, 2.0509(13);
FeP1, 2.4649(3); FeP2, 2.4439(4); Cl1-Fe-C23, 115.86(4); Cl1-Fe-P1, 103.399(13);
Cl1-Fe-P2, 98.207(13); C23-Fe-P1, 120.60(4); C23-Fe-P2, 124.74(4); P1-Fe-P2,
88.684(12).
Scheme 6. Dialkylations of {(R02PCH2)2NR00}FeX2 with RLi (R = 1-nor, neoPe, CH2SiMe3).

6
G.M. George et al. / Polyhedron 181 (2020) 114460
Scheme 7. Alkylidene transfer failed with Ph2CN2 and all variants. Aziridine Ad-cCHN2 [34] and PhCHN2 were also applied to selected species, albeit unsuccessfully. All N-
alkylation attempts also failed.
Fig. 5. Calculated conformational chair-to-boat calculations, shown ideally.
a
m
(CO) of 1599 cmꢁ1 corresponding to the acyl, were accompanied
plex, {(iBu2PCH2)2NtBu}(CO)2FeCl(COneoPe) (8). Overcarbonylation
of related alkyls was similarly observed despite attempts to control
stoichiometry.
by a 1H NMR spectrum of a diamagnetic complex, providing clear
indication of inclusion of three carbonyls to afford the 18e- com-
Scheme 8. Synthesis of {(Ph2PCH2CH2)2NtBu}FeCl2 (7-Cl) and {(Ph2PCH2CH2)2NtBu}FeCl(1-nor) (7-nor).

G.M. George et al. / Polyhedron 181 (2020) 114460
7
Fig. 6 illustrates a molecular view of acyl {(iBu2PCH2)2NtBu}
(CO)2FeCl(COneoPe) (8), with the acyl opposite one carbonyl and
the PNP ligand opposing the chloride and another CO. The stronger
trans-influence of CO vs. Cl is revealed by d(FeP2) = 2.2890(3) Å
being slightly greater than the d(FeP1) of 2.2273(3) Å. The config-
uration of the PNP ligand has completely changed from the Td
derivatives, as P1, P2, the methylene carbons, and iron are essen-
tially planar, resulting in an out-of-plane nitrogen. The core angles
of the pseudo octahedron are remarkably regular despite the
ligand variation, averaging 90.1(35)°, and ranging from 84.95(4)°
(P2-Fe-C3) to 96.688(12)°, the latter corresponding to the rela-
tively splayed P-Fe-P angle. The remaining core bonds are normal
for a low spin Fe(II) complex.
2.6. Synthesis and structure of {(Ph2PCH2CH2)2NtBu}Fe
((H2C = CHSiMe2)2O)
Successful efforts at preparing Fe(V) and Fe(IV) diimido com-
plexes by Deng [49] and Power [50] utilized Fe(I) and Fe(0) starting
materials, respectively, with adamantyl azide. Using {(Ph2PCH2-
CH2)2NtBu}FeCl2 (1a-Cl), reduction with KC8 in the presence of
bis-vinyldimethylsilyl-ether (dvtms) afforded {(Ph2PCH2CH2)2Nt-
Bu}-Fe((H2C=CHSiMe2)2O) (9) in 93% yield as a green powder. An
Evans’ method measurement was consistent with an intermediate
spin (S = 1) center, as meff = 3.1 mB. Calculations showed the S = 1
state to be well below the singlet (S = 0, +12.3 kcal/mol). Unfortu-
nately, exposure of 9 to various oxidizing alkylidene equivalents,
namely diazo derivatives, failed to yield new complexes, and most
were simply unreactive until decomposition. No evidence of 2 + 2
chemistry with the vinyl-silane functionalities could be discerned.
Crystals of {(Ph2PCH2CH2)2NtBu}Fe((H2C=CHSiMe2)2O) (9) were
grown from pentane, and a molecular view of the formally Fe(0)
complex is illustrated in Fig. 7. As expected, 9 is a highly distorted
tetrahedron with the olefin centroids taken as two sites. The phos-
phorus atoms are constrained at 91.958(14)° apart due to the che-
late, which is in a shallow boat configuration, and phosphine-olefin
centroid angles vary from 102.89° to 126.74°. The dvtms ligand is
asymmetric, with one olefin (C31-C32) essentially in a plane with
the iron and nitrogen, and C37-C8 effectively perpendicular to that
plane. This conformation renders the ether oxygen on the P1-side
of the pseudo-tetrahedron. The iron-phosphine distances are more
in line (2.2655(4), 2.2780(4) Å) with the previous high spin cases,
and the olefin CC distances are typical for first row complexes that
bind mostly via electrostatics [51].
Fig. 6. Molecular view of {(iBu2PCH2)2NtBu}(CO)2FeCl(COneoPe) (8); ellipsoids at
50%. Core interatomic distances (Å) and angles (°): FeCl, 2.3597(3); FeP1, 2.2273(3);
FeP2, 2.2890(3); FeC1, 1.8126(14); FeC2, 1.8359(14); FeC3, 2.0296(13); Cl-Fe-P1,
175.725(14); Cl-Fe-P2, 86.562(12); Cl-Fe-C1, 86.80(4); Cl-Fe-C2, 91.16(4); Cl-Fe-C3,
91.83(4); P1-Fe-P2, 96.688(12); P1-Fe-C1, 89.82(4); P1-Fe-C2, 91.67(4); P1-Fe-C3,
85.71(4); P2-Fe-C1, 172.89(4); P2-Fe-C2, 89.15(4); P2-Fe-C3, 84.95(4); C1-Fe-C2,
93.54(6); C1-Fe-C3, 92.72(5); C2-Fe-C3, 173.21(5).
was a surprising impediment to subsequent heterolytic CH bond
activation or reductive coupling. The orientation of the trisubsti-
tuted amine is inconsequential, as the inversion of the chelate ring
does not have a prohibitive barrier. It may be that conditions under
which alkylation can occur render the complexes susceptible to
decomposition, as the weak fields of the pseudo tetrahedral com-
plexes permit ready chelate arm dissociation. An obvious conse-
quence of this factor is that P-alkylation is sometimes observed.
If alkylation needs to proceed via an initial Lewis acid interaction
of RX with the iron center, the site of binding, and the approach
of the internal nucleophile could simply be unfavorable under con-
ditions where the chelate is stable.
Previous failures in generating olefin metathesis capability at Fe
(IV) complexes [20–22] that possess strong fields (vide supra)
prompted the switch to lower coordination number, as exemplified
by Scheme 1 and the attempts denoted herein, and supported by
calculations.[52] It must also be assumed that the Deng [32,49]
and Power [50] groups among others have been equally unsuccess-
ful in generating alkylidene species capable of olefin metathesis,
given their penchant and success at investigating 1st row transi-
tion metal ligand multiple bonds. What factor(s) is dominating
the lack of generating an Fe(IV)=CRR0 moiety capable of 2 + 2
chemistry?
3. Conclusions
3.1. Synthetic approach
The successful synthesis of a variety of PNP iron dihalides, alkyl-
halides and dialkyls was accomplished, yet none became the
desired platform for a masked alkylidene in zwitterionic form, as
planned via Scheme 3. Failure to alkylate at nitrogen, which has
to be done after complexation due to competing P-alkylation,
Scheme 9. Conceptual conversion of an acyl to masked carbene.

8
G.M. George et al. / Polyhedron 181 (2020) 114460
romagnetic coupling (e.g. Fe"(III)(-C;Ar2)) has been used as an
alternative to a bond, in part based on chemical reactivity, as cyclo-
propanations are observed. In these weak overlap situations, the
GS is not necessarily covalent, as the overlap integral is so small
that substantial ionic character contributes. If /M and /L deviate
in the slightest, the ionic character can become dominant, likely
leading to cyclopropanation.
On the left in Fig. 8 is the standard strong overlap case where
olefin metathesis occurs, and the only electrons in the plane of
the metalacyclobutane originate from the M@C and C@C p-bonds.
Note that the total energies of the Wb and CCpb orbitals’ electrons
are shown roughly the same as WbS and WbA ensuring that each com-
ponent of the catalytic cycle is similar. Strong covalent interactions
result in excellent M/C and C/C overlap in metallacycle formation,
and that the symmetric (S) and antisymmetric (A) orbitals reveal
the two new bonds (see Fig. 9).
Assume the weak overlap ‘‘covalent” situation actually results
in an interaction that has substantial ionic character. Fig. 8 shows
that the combination of poor orbital overlap, in part due to energy
mismatches between metal and carbon double bond orbitals, and
also due to the ionic character of the orbitals, presents significant
problems. In the worst case, if the interaction is weak enough,
the requisite WbA orbital may not even be populated, as
W* may
S
be filled instead, resulting in no net bonding. The asymmetry in
the ground state of the alkylidene clearly impacts overlap, and cyclo-
propanation is a likely consequence.
The problem is deceptively simple, as the coordination sphere
must be strongly donating enough to engender low spin deriva-
tives covalent enough to undergo metalacyclobutane formation
[52], yet low coordination environments are affiliated with weak
field, high spin systems, except under extraordinary circum-
Fig. 7. Molecular view of {(Ph2PCH2CH2)2NtBu}Fe((H2C=CHSiMe2)2O) (9); ellipsoids
at 50%. Core interatomic distances (Å) and angles (°): FeP1, 2.2655(4); FeP2, 2.2780
(4); FeC31, 2.1213(15); FeC32, 2.0628(15); FeC37, 2.0881(15); FeC38, 2.0540(16);
C31-C32, 1.417(2); C37-C38, 1.413(2); P1-Fe-P2, 91.958(14); P1-Fe-C31, 88.08(5);
P1-Fe-C32, 117.71(5); P1-Fe-C37, 99.01(5); P1-Fe-C38, 126.20(6); P2-Fe-C31, 88.73
(4); P2-Fe-C32, 111.76(5); P2-Fe-C37, 126.59(5); P2-Fe-C38, 93.20(5); C31-Fe-C32,
39.56(6); C31-Fe-C37, 143.30(6); C31-Fe-C38, 145.51(7); C32-Fe-C37, 108.53(7);
C32-Fe-C38, 109.61(7), C37-Fe-C38, 39.89(7).
stances. Energies of the metal d
p orbital and carbon 2pp orbital
must be closely matched to elicit as much covalence as possible
in the alkylidene. Second, the alkylidene must be electronically
3.2. Are Fe(IV) olefin metathesis catalysts plausible?
oxidizing enough to share the
p-electrons with the iron, a factor
that is difficult for all first-row transition metals, especially the
later ones.[62] To what extent geometric considerations, such as
the shorter MꢁC bonds in the first row, contribute to metalacy-
clobutane formation and its rearrangement are unknown. It may
be necessary to turn toward anionic iron(IV) derivatives to expand
the 3d orbitals such that appropriate p-overlap between iron and
carbon can be achieved, albeit with the introduction of a new prob-
lem, that of increased electron–electron repulsions.
Covalency is typically defined as ‘‘atoms sharing a pair of elec-
trons”, a statement that if not nebulous, is certainly broadly
defined. In support, this simple statement regarding a common
type of chemical bond is at least understandable in contrast to
some recent definitions.[53] Perhaps a readily understood con-
struct that can represent covalency is the overlap integral, which
at least has intrinsic components of energy and overlap.
The best rationale for lack of metathesis activity infers a lack of
covalency in the iron-carbon interaction, more specifically the
p-
bond, but exactly why covalency may be critical to metathesis is
less transparent. To gain perspective, consider the interaction of
4. Experimental
a metal d
p orbital and its interaction with the corresponding car-
4.1. General considerations
bon -orbital. At the crux of this interaction is a 3d orbital that is
p
small with respect to the 2p orbital of carbon, leading to two lim-
iting cases of weak overlap, and one of potentially maximum cova-
lence, as illustrated in Fig. 7.
All manipulations were performed using either glovebox, Sch-
lenk, or high vacuum line techniques, unless stated otherwise. All
glassware was oven dried at 180 °CꢂTHF and ether were distilled
under nitrogen from purple sodium benzophenone ketyl and vac-
uum transferred from the same prior to use. Hydrocarbon solvents
were treated in the same manner with the addition of 1–2 mL/L
tetraglyme. Benzene d6 was dried over sodium, vacuum transferred
and stored over sodiumꢂTHF-d8 was dried over sodium, and vac-
uum transferred from sodium benzophenone ketyl prior to use.
Acetonitrile d3 was dried over refluxing CaH2, vacuum distilled
and stored over CaH2, and chloroform d1 (Cambridge Isotope Labo-
ratories) was used as received. PNP ligands were prepared via liter-
ature preparations [35–40].
In the normal configuration, the 2p orbital is well below the 3d,
resulting in carbanion character in the M = CRR0 bond. Early metal
2nd row olefin metathesis catalysts possess this polar covalent
interaction, as evidenced by reactions with electrophiles and Wit-
tig-like reactivity [1,2]. At the other extreme, in an inverted field
[54–57], cationic character of the alkylidene is intrinsic to the orbi-
tal ordering [22], leading to potential carbocation transfers, likely
observed as cyclopropanations. It is the covalent representation
that invites scrutiny, as it may provide the key to generating an
active metathesis catalyst. The
p-interaction may be interpreted
within the framework of the 2-orbital, 2-electron, 4-state paradigm
first utilized in assessing dihydrogen [58], and later applied to
NMR spectra were obtained using Varian 300 MHz (Mercury),
400 MHz (Inova), 500 MHz (Inova) and 600 MHz (Inova) spectrom-
eters. 1H and 13C NMR shifts are referenced to benzene d6 (1H,
d7.16 ppm; 13C, d128.39 ppm), toluene d8 (1H, d2.09 ppm; 13C,
dimolybdenum d- [59,60] and
p-bonds [61]. For weak overlap
situations, such as Chirik’s and Deng’s [14,32], the model of antifer-

G.M. George et al. / Polyhedron 181 (2020) 114460
9
Fig. 8. Normal, covalent, and inverted field combinations of a 1st row transition metal dp orbital and a carbon 2pp orbital.
Fig. 9. The generation of a metalacyclobutane via alkylidenes (strong and weak) and an ethylene.

10
G.M. George et al. / Polyhedron 181 (2020) 114460
d20.40 ppm), acetonitrile d3 (1H, d1.94 ppm; 13C, d118.26 ppm),
tetrahydrofuran d8 (1H, d3.58 ppm; 13C, d67.57 ppm),
dichloromethane d2 (1H, d5.32 ppm; 13C, d53.84 ppm), deuterium
oxide (1H, d4.79 ppm; 13C, CH3CN spike, d1.79 ppm). Solution mag-
netic measurements were conducted via Evans’ method in the
same solvent as the 1H NMR was conducted.[43] Elemental analy-
ses were performed by Robertson Microlit Laboratories, Madison,
New Jersey.
4.2.5. {(iBu2PCH2)2NMe}FeCl2 (2b-Cl)
To a 100 mL round bottom flask charged with anhydrous FeCl2
(0.693 g, 5.47 mmol) and (iBu2PCH2)2NMe (1.920 g, 5.53 mmol)
was added 50 mL of freshly distilled THF at ꢁ78 °C. The reaction
mixture was allowed to warm slowly to 23 °C and stirred for
16 h, resulting in a pale yellow solution. The volatiles were evapo-
rated, and the off-white residue was triturated with pentane
(2 ꢃ 10 mL) and then with Et2O (3 ꢃ 10 mL). Taking the residue
up in Et2O, cooling the solution to ꢁ78 °C followed by filtering
the solution yields a white powder (1.778 g, 69%). 1H NMR (THF-
4.2. Procedures
d8) d -0.89 (23H), 48.98 (10H), 122.54 (10H). leff (Evans) = 5.6 lB
.
4.2.1. {(Ph2PCH2)2NtBu}FeCl2 (1a-Cl)
4.2.6. {(R02PCH2)2NR00}FeR(Y) general procedure
To a 100 mL round bottom flask charged with anhydrous FeCl2
(0.964 g, 7.61 mmol) and (Ph2PCH2)2NtBu (3.929 g, 8.37 mmol) was
added 50 mL of freshly distilled THF at ꢁ78 °C. The reaction mix-
ture was allowed to warm slowly to 23 °C and stirred for 16 h,
resulting in a yellow solution. The volatiles were evaporated, and
the yellow residue was triturated with pentane (2 ꢃ 10 mL). The
residue was suspended in pentane, filtered, and washed
(2 ꢃ 10 mL) to isolate the yellow powder (4.159 g, 92%). 1H NMR
(C6D6) d -3.68 (7H), 2.67 (14H), 8.11 (2H), 14.92 (10H). leff
To a 50 mL round bottom flask charged with {(R02PCH2)2NR00}
FeCl2 (0.287 g, 2.26 mmol) and one or two equivalents of RLi was
added 25 mL of freshly distilled Et2O at ꢁ78 °C. The reaction mix-
ture was allowed to warm slowly to 23 °C and stirred for 16 h. The
solution was filtered, and the volatiles evaporated. Taking the resi-
due up in minimal pentane, cooling the solution to ꢁ78 °C followed
by filtering yields a microcrystalline solid.
4.2.6.1. {(Ph2PCH2)2NtBu}FeCl(1-nor) (3a-nor). 1H NMR (C6D6)
d -9.76 (2H), -5.61 (3H), -2.31 (3H), 3.12 (17H), 11.41 (6H), 14.84
(1H), 15.56 (6H), 19.10 (2H), 34.27 (3H), 64.75 (1H). leff
(Evans) = 5.6
lB. Anal. for C30H33P2NFeCl2 (calc.) C 60.43, H 5.58,
N 2.35; (found) C 59.73, H 5.51, N 2.21.
(Evans) = 5.1
N 2.14; (found) C 66.51, H 6.82, N 1.80.
lB. Anal. for C37H44P2NFeCl (calc.) C 67.74, H 6.76,
4.2.2. {(Ph2PCH2)2NtBu}FeBr2 (1a-Br)
To a 50 mL round bottom flask charged with anhydrous FeBr2
(130 mg, 0.603 mmol) and (Ph2PCH2)2NtBu (300 mg, 0.639 mmol)
was added 20 mL of freshly distilled THF at ꢁ78 °C. The reaction
mixture was allowed to warm slowly to 23 °C and stirred for
16 h, resulting in a yellow solution. The volatiles were evaporated,
and the residue was triturated with pentane (2 ꢃ 10 mL). The resi-
due was suspended in pentane, filtered, and washed (2 ꢃ 10 mL) to
isolate the pale yellow powder (366 mg, 85%). 1H NMR (C6D6)
4.2.6.2. {(Ph2PCH2)2NtBu}FeCl(neoPe) (3a-neoPe). 1H NMR (C6D6)
d -4.37 (3H), -4.24 (3H), -3.65 (1H) 2.69 (3H), 3.37 (12H), 10.81
(6H), 14.92 (2H), 15.85 (5H), 39.90 (9H). leff (Evans) = 5.6 lB
.
4.2.6.3. {(Ph2PCH2)2NtBu}FeCl(CH2SiMe3) (3a-CH2SiMe3). 1H NMR
(C6D6) d -4.23 (6H), 2.62 (12H), 12.55 (17H), 15.51 (9H). leff
(Evans) = 5.2 lB
.
d -3.54 (6H), 2.89 (14H), 15.10 (13H). leff (Evans) = 5.5 lB
.
4.2.6.4. {(Ph2PCH2)2NMe}FeCl(1-nor) (3b-nor). 1H NMR (C6D6)
d -4.73 (1H), -2.91 (1H), 2.77 (16H), 3.44 (16H), 9.66 (2H), 15.39
4.2.3. {(Ph2PCH2)2NMe}FeCl2 (1b-Cl)
(2H). leff (Evans) = 5.3 lB
.
To a 100 mL round bottom flask charged with anhydrous FeCl2
(0.287 g, 2.26 mmol) and (Ph2PCH2)2NtBu (1.061 g, 2.48 mmol) was
added 50 mL of freshly distilled THF at ꢁ78 °C. The reaction mix-
ture was allowed to warm slowly to 23 °C and stirred for 16 h,
resulting in a yellow solution. The volatiles were evaporated, and
the residue was triturated with pentane (2 ꢃ 10 mL). The residue
was suspended in pentane, filtered, and washed (2 ꢃ 10 mL) to iso-
late the yellow powder (1.179 g, 87%). Crystals suitable for X-ray
diffraction were obtained via slow evaporation of a concentrated
THF solution at 23 °C. 1H NMR (THF-d8) d -2.59 (6H), -1.86 (5H),
4.2.6.5. {(Ph2PCH2)2NMe}FeCl(CH2SiMe3) (3b-CH2SiMe3). 1H NMR
(C6D6) d -5.24 (3H), -3.56 (3H), 2.66 (3H), 3.36 (3H), 4.60 (3H),
7.46 (6H), 10.19 (4H), 16.11 (13H). leff (Evans) = 5.6 lB
.
4.2.6.6. {(iBu2PCH2)2NtBu}FeCl(1-nor) (4a-nor). 1H NMR (C6D6)
d -12.67 (4H), -7.25 (9H), -5.53 (10H), -3.88 (10H), 4.68 (17H),
19.89 (3H), 35.67 (4H), 62.06 (1H), 99.74 (2H). leff (Evans) = 4.7
lB. Crystals suitable for X-ray diffraction were grown from slow
evaporation of a concentrated pentane solution at ꢁ35 °C.
5.20 (4H), 14.30 (12H). leff (Evans) = 5.2
lB. Anal. for C27H27P2-
NFeCl2 (calc.) C 58.51, H 4.91, N 2.53; (found) C 57.30, H 4.79, N
2.27.
4.2.6.7. {(iBu2PCH2)2NtBu}FeCl(neoPe) (4a-neoPe). 1H NMR (C6D6)
d -5.86 (11H), -4.22 (9H), -2.68 (9H), 1.37 (15H), 16.66 (2H),
37.81 (10H). leff (Evans) = 4.8 lB
.
4.2.4. {(iBu2PCH2)2NtBu}FeCl2 (2a-Cl)
To a 100 mL round bottom flask charged with anhydrous FeCl2
(1.498 g, 11.82 mmol) and (iBu2PCH2)2NtBu (5.00 g, 12.83 mmol)
was added 50 mL of freshly distilled Et2O at ꢁ78 °C. The reaction
mixture was allowed to warm slowly to 23 °C and stirred for
16 h, resulting in a pale yellow solution with a white suspension.
The volatiles were evaporated, and the off-white residue was trit-
urated with pentane (2 ꢃ 10 mL). The residue was suspended in
pentane, filtered, and washed (2 ꢃ 10 mL) to isolate the off-white
powder (5.806 g, 89%). crystals suitable for X-ray diffraction were
grown from slow evaporation of a concentrated pentane solution
at ꢁ35 °C. 1H NMR (C6D6) d -1.57 (18H), 1.45 (13H), 2.84 (18H) leff
4.2.6.8. {(iBu2PCH2)2NtBu}FeCl(CH2SiMe3) (4a-CH2SiMe3). 1H NMR
(C6D6) d -5.56 (8H), -3.86 (9H), -2.63 (8H), 3.03 (8H), 3.84 (16H)
14.99 (11H). leff (Evans) = 5.4 lB
.
4.2.6.9. {(iBu2PCH2)2NMe}FeCl(1-nor) (4b-nor). 1H NMR (C6D6)
d -6.64 (10H), -5.21 (8H), -0.66 (17H), 3.91 (3H), 18.76 (3H),
34.20 (4H), 46.74 (3H), 61.57 (2H), 67.29 (2H), 81.74 (2H). leff
(Evans) = 5.1 lB
4.2.6.10. {(iBu2PCH2)2NMe}FeCl(CH2SiMe3) (4b-CH2SiMe3). 1H NMR
(C6D6) d -9.21 (1H), -5.34 (7H), -3.31 (6H), -0.74 (7H), 2.82 (8H),
3.57 (8H), 15.13 (11H), 21.84 (1H), 70.69 (1H), 79.29 (2H),
(Evans) = 5.0
lB. Anal. for C22H49P2NFeCl2 (calc.) C 51.18, H 9.57, N
2.71; (found) C 51.45, H 9.82, N 2.71.
105.74 (2H). leff (Evans) = 5.1 lB
.

G.M. George et al. / Polyhedron 181 (2020) 114460
11
4.2.6.11. {(Ph2PCH2)2NtBu}Fe(neoPe)2 (5a-neoPe). 1H NMR (C6D6)
-4.25 (10H), 2.52 (17H), 15.08 (13H), 28.18 (15H). leff
(Evans) = 5.3 B. Anal. for C40H55P2NFe (calc.) C 71.96, H 8.30, N
4.3.3. {(iBu2PCH2)2NtBu}FeCl(1-nor) (4a-nor)
d
A yellow plate measuring 0.26 ꢃ 0.11 ꢃ 0.07 mm3 was obtained
from pentane. Crystal data for C29H60ClNP2Fe, M = 576.02, mono-
clinic, P21/n, a = 9.91359(4), b = 24.79084(8), c = 13.98973(6) Å,
b = 106.1234(4)°, V = 3302.96(2) Å3, T = 100(2)K, k = 1.54184 Å,
Z = 4, qcalc = 1.158 Mg/m3, m = 5.428 mmꢁ1, 147,953 reflections,
l
2.10; (found) C 70.57, H 8.26, N 1.92.
4.2.6.12. {(iBu2PCH2)2NtBu}Fe(1-nor)2 (6a-nor). 1H NMR (C6D6)
d -4.32 (11 H), -1.59 (2H), 1.58 (21H), 16.67 (13H), 28.42 (6H),
7066 independent (Rint
= 0.0409), R1 (all data) = 0.0281,
51.99 (2H). leff (Evans) = 5.5 lB
.
wR2 = 0.0744, R1 (I > 2 I) = 0.0276, wR2 = 0.0740, GOF = 1.053.
r
4.2.7. {(iBu2PCH2)2NtBu}(CO)2FeCl(COneoPe) (8)
4.3.4. {(iBu2PCH2)2NtBu}(CO)2FeCl(COneoPe) (8)
To a 25 mL round bottom charged with 4a (50 mg, 0.097 mmol)
was added 10 mL freshly distilled C6H6. The flask was allowed to
warm to room temperature and under 1 atm of CO. An immediate
color change was observed, the solution was stirred for 16 h. The
volatiles were removed, and the residue was washed with pentane
(2 ꢃ 10 mL), and removed to give an orange powder. The volatiles
were evaporated to give an orange powder. Orange crystals suit-
able for X-ray diffraction were grown from slow evaporation of a
An orange block measuring 0.29 ꢃ 0.25 ꢃ 0.11 mm3 was
obtained from diethyl ether. Crystal data for C30H60ClNO3P2Fe,
M = 636.03, monoclinic, P21/n, a = 11.92710(10), b = 18.0981(2),
c = 16.6852(2) Å, b = 93.1790(10)°, V = 3594.30(7) Å3, T = 100(2)
K, k = 0.71073 Å, Z = 4, qcalc = 1.175 Mg/m3, m = 0.611 mmꢁ1
84,638 reflections, 8892 independent (Rint = 0.0356), R1 (all
data) = 0.0345, wR2 = 0.0724, R1 (I > 2rI) = 0.0285, wR2 = 0.0692,
GOF = 1.032.
,
concentrated Et2O solution at ꢁ35 °C. IR nujol mull
m(CO)
2009 cmꢁ1, 1954 cmꢁ1, (acyl) 1599 cmꢁ1
.
1H NMR (C6D6) d 0.80
4.3.5. Ph2PCH2CH2)2NtBu}Fe((H2C = CHSiMe2)2O) (9)
(s, 9H NtBu-CH3), {0.90, 0.90, 0.92, 0.95, 0.98, 1.00, 1.13, 1.14
An orange block measuring 0.29 ꢃ 0.25 ꢃ 0.11 mm3 was
(24H iBu-CH3)}, 1.25 (s, 9H neoPen-CH3), {1.94, 1.99, 2.01, 2.06
i
obtained from pentane. Crystal data for
C38H51NOP2Si2Fe,
(m, 4H Bu-CH)}, {1.62, 1.84, 1.85, 1.88, 2.02, 2.24, 2.47, 2.58 (8H
iBu-CH2)}, {2.94, 3.01, 3.09, 3.12 (4H, PCH2N)}, {3.64, 4.13 (d,
J = 18.01 Hz 2H neoPen-CH2)}. 13C NMR (C6D6) {24.47, 25.24,
25.25, 25.39, 25.64, 26.65, 25.87, 26.41 (iBu-CH3)}, 26.29
M = 711.76, triclinic, P1bar, a = 10.61500(10), b = 12.73230(10),
c = 15.6085(2) Å, = 72.4270(10)°, b = 84.5660(10)°, = 68.3740
(10)° V = 1869.21(4) Å3, T = 100(2)K, k = 0.71073 Å, Z = 2,
calc = 1.265 Mg/m3, m = 0.583 mmꢁ1, 77,875 reflections, 8244 inde-
pendent (Rint = 0.0359), R1 (all data) = 0.0347, wR2 = 0.0799, R1
(I > 2 I) = 0.0317, wR2 = 0.0776, GOF = 1.035.
a
c
(NtBu-CH3), 29.78
(
neoPen-CH3), {30.00, 30.66, 31.54, 32.50
q
(iBu-CH(CH3)2)}, 32.45
(
neoPen-C(CH3)3), {33.48, 34.67, 36.66,
37.37 (iBu-CH2)}, {47.15, 47.77 (PCH2N)}, 56.57 (NtBu-C(CH3)3),
71.94 (neoPen-CH2), 185.00 (acyl) {212.57, 212.92 (CO)}.
r
4.4. Computational methods
4.2.8. {(Ph2PCH2CH2)2NtBu}Fe((H2C = CHSiMe2)2O) (9)
To a 50 mL two necked flask charged with {(Ph2PCH2)2NtBu}
FeCl2 (315 mg, 0.528 mmol) and 1,3-Divinyltetramethyldisiloxane
(100 mg, 0.536 mmol) and equipped with a solid addition finger
charged with KC8 (152 mg, 1.12 mmol) was added 20 mL freshly
distilled THF at ꢁ78 °C. The KC8 was added at ꢁ78 °C and the reac-
tion mixture was allowed to warm slowly to 23 °C and stirred for
16 h. The volatiles were evaporated, the residue was taken up in
pentane and filtered through a silica plug to give a green solution.
The volatiles were evaporated to give a green powder (0.351 g,
93%). 1H NMR (C6D6) d -7.85 (1H), -1.87 (5H), -0.99 (1H), 0.66
(8H), 0.95 (8H), 1.38 (13H), 4.06 (1H), 9.98 (9H), 11.78 (3H),
Computations described herein employed the Gaussian 16 code
(revision A.03) [45]. The B3PW91 [46] functional with the GD3
empirical dispersion correction [47] was utilized in conjunction
with the 6-31+G(d) basis set. Initial computations indicated a quin-
tet ground state, so that unrestricted DFT methods were employed
with no evidence of spin contamination as evidenced by inspection
of the <S2> expectation value. All minima and transition states
were optimized with neither symmetry nor geometric constraint,
and were verified for the correct number of imaginary frequencies
via computation of the energy Hessian. All reported energetics
assumed 1 atm and 298.15 K.
13.25 (1H), 31.70 (1H). leff (Evans) = 3.1
lB. Crystal suitable for
x-ray diffraction were grown from slow evaporation of a concen-
trated pentane solution at ꢁ35 °C.
CRediT authorship contribution statement
4.3. X-ray crystal structure determinations
Gregory M. George: Experimental. Peter T. Wolczanski: Con-
ceptualization. Samantha N. MacMillan: X-ray crystallography.
Thomas R. Cundari: Calculations.
4.3.1. {(Ph2PCH2)2NMe}FeCl2 (1b-Cl)
A yellow plate measuring 0.46 ꢃ 0.15 ꢃ 0.09 mm3 was obtained
from THF. Crystal data for C27H27Cl2NP2Fe, M = 554.18, monoclinic,
P21/c, a = 14.72110(10), b = 11.06180(10), c = 16.53250(10) Å,
b = 96.0520(10)°, V = 2677.18(3) Å3, T = 173(2)K, k = 1.54184 Å,
Z = 4, qcalc = 1.375 Mg/m3, m = 7.599 mmꢁ1, 108,554 reflections,
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
5702 independent (Rint
= 0.0562), R1 (all data) = 0.0305,
wR2 = 0.0816, R1 (I > 2 I) = 0.0295, wR2 = 0.0808, GOF = 1.096.
r
4.3.2. {(iBu2PCH2)2NtBu}FeCl2 (2a-Cl)
Acknowledgements
A yellow plate measuring 0.24 ꢃ 0.08 ꢃ 0.07 mm3 was obtained
from pentane. Crystal data for C22H49Cl2NP2Fe, M = 516.31, mono-
clinic, P21/n, a = 11.36310(10), b = 15.32800(10), c = 16.79070(10)
Å, b = 94.2580(10)°, V = 2916.43(3) Å3, T = 222(2)K, k = 1.54184 Å,
Z = 4, qcalc = 1.176 Mg/m3, m = 6.916 mmꢁ1, 123,225 reflections,
We thank Dr. Spencer P. Heins for initiating this work. Support
from the National Science Foundation (CHE-1664580 (PTW), CHE-
1464943 (TRC)) and Cornell University is gratefully acknowledged.
NSF support for UNT CASCaM HPC cluster via CHE-1531468, and
the CCB NMR facility (NSF-MRI CHE-1531632) is appreciated, as
is technical aid from Ivan Keresztes.
5538 independent (Rint
= 0.0581), R1 (all data) = 0.0270,
wR2 = 0.0714, R1 (I > 2 I) = 0.0257, wR2 = 0.0706, GOF = 1.059.
r

12
G.M. George et al. / Polyhedron 181 (2020) 114460
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