Rare Examples of Fe(IV) Alkyl-Imide Migratory Insertions: Impact of Fe - C Covalency in (Me2IPr)Fe(=NAd)R2 (R = neoPe, 1-nor)

Article abstract of DOI:10.1021/jacs.7b06960

The iron(IV) imide complexes, (Me₂IPr)-R₂Fe=NAd (R = ^neoPe (3a), 1-nor (3b)) undergo migratory insertion to iron(II) amides (Me₂IPr)RFe{NR(Ad)} (R = ^neoPe (4a), 1-nor (4b)) without evidence of imidyl

Full text of DOI:10.1021/jacs.7b06960

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Communication
Rare Examples of Fe(IV) Alkyl-Imide Migratory Insertions:
2
2
neo
Impact of Fe-C Covalency in (MeIPr)Fe(=NAd)R (R = Pe, 1-nor)
Brian P. Jacobs, Peter T Wolczanski, Quan Jiang, Thomas R. Cundari, and Samantha N MacMillan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06960 • Publication Date (Web): 10 Aug 2017
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Rare Examples of Fe(IV) Alkyl-Imide Migratory Insertions: Impact
of Fe-C Covalency in (Me2IPr)Fe(=NAd)R2 (R = neoPe, 1-nor)
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Brian P. Jacobs,a Peter T. Wolczanski*,a Quan Jiang,b Thomas R. Cundari*,b and
Samantha N. MacMillana
aDepartment of Chemistry & Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853
bDepartment of Chemistry, CASCaM, University of North Texas, Denton, Texas, 76201
Supporting Information Placeholder
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(
neoPe)2 (1, S = 2, Scheme 1) to AdN3 (Ad = 1-adaman-
ABSTRACT: The iron(IV) imide complexes, (Me2IPr)-
R2Fe=NAd (R = neoPe (3a), 1-nor (3b)) undergo migra-
tory insertion to iron(II) amides (Me2IPr)RFe{NR(Ad)}
(R = neoPe (4a), 1-nor (4b)) without evidence of imidyl
or free nitrene character. By increasing the field strength
about iron, odd-electron reactivity was circumvented via
increased covalency.
tyl) afforded AdN=CHtBu, and no R-R coupling, sug-
gestive of a nitrene insertion process.
Scheme 1. Syntheses of LnFeR2.
2 RLi
N
1/2 [LnFeCl2]2
LnFe
Fe
Recent investigations of iron imido complexes1 fea-
ture species exhibiting imidyl (nitrogen radical, RN-.)
reactivity in the context of carbon-hydrogen bond acti-
vation,2-7 to remarkably stable high oxidation state spe-
cies.8-12 The imidyl, "nitrene-insertion" reactivity, such
as that described by Betley,2 can be construed as a re-
bound process in which hydrogen atom transfer (HAT)
is followed by coupling of carbon- and nitrogen-based
radicals. Aziridinations2-7,13 often accompany or appear
related to HAT initiated reactions, and can be considered
as complementary, stepwise radical additions to olefins.
Classical imido (RN2-) complexes are often unreac-
tive, but in combination with electrophilic (d0) early
transition metal sites, can activate aliphatic CH bonds,
including that of CH4, in a clear concerted fashion.14,15
Can iron complexes also exhibit such behavior? The
covalency of related Fe(IV) imides must be extremely
high in order to attenuate imidyl character, and strong
field ligands such as alkyls and N-heterocyclic carbenes
are attractive ancillary ligands.16 Herein are described
formally Fe(IV) imido complexes that exhibit migratory
insertion behavior best construed as concerted. The reac-
tions are rare examples of LmMn(=X)R → LmM(n-2)-X-R
(X = O,17 NR')18-20 insertions that incur a formal oxida-
tion state change, and portend the possibility of reactions
such as olefin metathesis with first-row transition metal
systems.
Ln = Me2IPr, tmeda
Et2O
N
-78 -> 23°C
-2 LiCl
R = neoPe, Ln = tmeda (1, 53%, colorless, 5.3 µB)
Me2IPr (2a, 54%, yellow, 5.0 µB)
R = 1-nor, Ln = Me2IPr (2b, 63%, tan, 5.6 µB),
In order to generate a strong field, three-coordinate
environment likely to induce imide formation, bulky
Me2IPr26-29 was utilized as an ancillary ligand. 1-
Norbornyl (nor) was another featured alkyl, as β-H-
elimination is obviated by the formation of bridgehead
olefins, and there is a history of 1-nor providing excep-
tionally strong fields,30 as in Fe(1-nor)4.23,31,32
118.78(7)°
2.0620(18)
2.0489(18)
120.32(7)°
2.1167(16)
119.59(7)°
Fig. 1. Molecular view of (Me2IPr)Fe(1-nor)2 (2b); se-
lected bond distances (black, Å) and angles (red) are given.
(Me2IPr)Fe(neoPe)2 (2a) and (Me2IPr)Fe (nor)2 (2b)
were prepared from metathesis of [(Me2IPr)FeCl2]2 with
neoPeLi and (1-nor)Li, respectively (Scheme 1) and pos-
sess µeff values of 5.0 (2a) and 5.6 µB (2b),33 indicative
of S = 2 centers. A molecular view of 2b is given in Fig.
1, along with core interatomic distances and angles.
Three-coordinate 2b is nearly trigonal despite the steric
bulk of the tertiary 1-norbornyl substituents. A similar
A variety of pseudo-tetrahedral LnFe(neoPe)2 complex-
es were targeted to avoid β-H-elimination paths, and
synthesized via metathetical procedures21-27 from corre-
sponding halides. Upon treatment with various oxidants,
including azides,28 2,2,5,5-tetramethylhexane was de-
tected with few exceptions. Exposure of (tmeda)Fe-
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geometry is observed for (Me2IPr)Fe(neoPe)2 (2a, X-ray)
dark green (Me23I3Pr)Fe(=NAd)(1-nor)2 (3b, 81%), and
but severe twinning problems hampered refinement.
Exposure of (Me2IPr)Fe(neoPe)2 (2a) to AdN3 afforded
a persistent dark green solution at -78°C that slowly be-
its µeff of 2.9 µB revealed an intermediate spin (S = 1)
system. Imide 3b was thermally converted to the yellow
insertion product, (Me2IPr)Fe{N(Ad)(1-nor)}(1-nor)
(4b, 57%, S = 2, µeff = 5.5 µB), and protic quenching of
4b provided AdNH(1-nor). Spectroscopic characteristics
relating 3b to 3a suggested the latter to be the thermally
unstable imide, (Me2IPr)Fe(=NAd)(neoPe)2 (3a).
1
came yellow at 23°C. H NMR spectroscopy, protic
quenching studies that yielded AdNH(neoPe), and X-ray
crystallography revealed the migratory insertion product,
(Me2IPr)Fe{N(Ad)neoPe}(neoPe) (4a, 65%), as shown in
Scheme 2. Its µeff of 5.1 µB33 was consistent with an S =
2 ferrous center.
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Scheme 2. (Me2IPr)Fe(=NAdR2 to (Me2IPr)Fe-{N(Ad)-
R}R (R = neoPe, 3a→4a; 1-nor, 3b→4b).
1.6723(18)
120.62(8)°
2.074(2)
107.90(9)°
(3a)
N
N
N
kinsert
N
Fe
N
Fe
yellow
N
µeff = 5.1 µB
(4a)
65%
dark green
N
N
R
R
R = neoPe (2a)
= 1-nor (2b)
- 78°- 23°
toluene, - N2
Fe
+
Fig. 3. Molecular view of (Me2IPr)Fe(=NAd)(1-nor)2
(3b). Selected bond distances (black, Å) and angles (red, °):
Fe-C12, 2.005(2); Fe-C19, 2.024(2); N3-C26, 1.437(3);
N3-Fe-C12, 105.09(9); C12-Fe-C19, 103.77(9); C1-Fe-
C12, 96.98(8); C1-Fe-C19, 119.23(9); Fe-N3-C26,
168.04(15).
AdN3
N
kinsert
N
Fe
N
N
Fe
N
N
(4b)
dark green
µeff = 2.9 µB
yellow
57%
Fig. 3 shows the distorted tetrahedral structure of
(Me2IPr)Fe(=NAd)(1-nor)2 (3b), highlighting its short
(1.6723(18) Å), nearly linear (168.04(15°)) iron imide
linkage. The distorted core appears to reflect steric inter-
actions, as the adamantyl unit is tipped away from the
iPr group on the Me2IPr, although the 1-nor ligands span
a C12-Fe-C19 angle of only 103.77(9)°. Favorable dis-
persion forces, specifically in Fe(IV) complexes, appear
to play a role in stabilization of hindered systems.23,34
Calculations (B3PW91-GD3/6-31+G(d)) of (Me2IPr)-
Fe(=NAd)(1-nor)2 (3b) confirm an S = 1 ground state.
There is an impressive admixture of 1-nor and Me2IPr
character to the Fe(dπ)-N(pπ)frontier orbitals of 3b (6 α-
and 4 β-orbitals span 0.10 eV), such that correlating
spins and defining a ligand field (dn) are subjective. The
significant amounts of iron and carbon character are
consistent with strong covalent interactions that belie the
formal Fe(IV) oxidation state.35
(3b)
81%
µeff = 5.5 µB
Fig. 2 illustrates a molecular view of (Me2IPr)-
Fe{N(Ad)neoPe}(neoPe) (4a), which exhibits a slightly
distorted trigonal structure with the Me2IPr ligand again
vertical to the trigonal plane. Core bond distances are
normal for a low coordinate high spin ferrous center, and
the d(FeC) for the neopentyl ligand is ~0.07 Å shorter
than the corresponding NHC interaction.
2.1409(19)
119.48(7)°
1.9361(15)
112.77(8)°
127.41(8)°
2.068(2)
Fig. 2. View of (Me2IPr)Fe{N(Ad)neoPe}(neoPe) (4a); se-
lected bond distances (black, Å) and angles (red) are given.
Failure to isolate dark green 3a prompted a switch to
(Me2IPr)Fe(1-nor)2 (2b).23,31,32 Treatment of 2b with
AdN3 under identical conditions permitted isolation of
a.
b.
Fig. 4. CAS(14,14) natural orbitals of 3a (contour value =
0.03): a. FeC(σ) (neoPe) with FeN(π); b. FeN(π*).
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4a when an equiv of 2b was present, indicative of an
intramolecular migratory insertion.
MCSCF calculations were conducted at the DFT-
optimized geometry of 3a. Within the CAS(14,14) ac-
tive space, MCSCF natural orbitals show significant
delocalization, particularly four nearly doubly occupied
(natural orbital population ~ 1.9 e-) orbitals with mixed
FeC(σ), (neoPe), and FeN(π) character (Fig. 4a.). Next
highest in energy are two singly occupied orbitals that
are primarily Fe(3d) in character, followed by two natu-
ral orbitals (occupation ~ 0.1 e-) that are FeN(π*) (Fig.
4b.). The total Mulliken population of Fe among the 14
active space natural orbitals is 6.9 e-,35 implying signifi-
cant covalency and charge delocalization among all
iron-ligand bonds.
25.6
26.2
1.8
∆G
∆H
∆S
20.1
20.2
0.5
16.7
17.4
2.3
20.7
20.9
1.8
Triplet
Quintet
R = 1-nor
3b->4b
2.0
4.1
7.0
∆G
∆H
∆S
2.9
4.2
4.4
R = neoPe
3a->4a
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0.0
0.0
N
N
N
kinsert
R
Fe
N
Fe
N
R
R
R
-21.7
-22.6
-3.1
N
-20.6
-21.6
-1.6
Table 1. Migratory insertion rates of (Me2IPr)Fe(=N-
Ad)R2 (R = neoPe, 3a; 1-nor, 3b) to (Me2IPr)-
Fe{N(R)Ad}R2 (R = neoPe, 4a; 1-nor, 4b).a
-29.9
-28.4
4.8
∆G, ∆H, and ∆S
relative to triplet 3a
and 3b (G = 0,
H = 0, S = 0)
-32.3
-32.0
0.9
rxn
T(±1.0 °C)
-10.0
1.0
k (s-1)
∆G‡
Fig. 5. Triplet (green) and quintet (blue) surfaces for the
3b→4b and 3a→4a migratory insertions; ∆G and ∆H are in
kcal/mol and ∆S is in eu; transition states are in italics.
3a → 4ab
3a → 4ab
3a → 4ab
3a → 4ab
3a → 4ab
3b → 4bc
3b → 4bc
3b → 4bc
3b → 4bc
3b → 4bc
4.85(8)x10-5
2.84(29)x10-4
1.17(8)x10-3
4.78(30)x10-3
1.37(8)x10-2
4.85(8)x10-5
2.84(29)x10-4
1.17(8)x10-3
4.78(30)x10-3
1.37(8)x10-2
20.5(8)
20.4(8)
20.3(8)
20.3(8)
20.3(8)
24.0(6)
24.0(6)
24.0(6)
24.0(6)
24.0(6)
10.0
21.0
30.0
23.0
40.0
50.0
60.0
Calculations were conducted to gain insight into the
reaction coordinate for insertion. Following the success
of Power et al.,34 B3PW91-GD3/G-31+G(d) simulations
were applied to the migratory insertions. Fig. 5 shows
that the computations reproduce the lower barrier for
3a→4a (R = neoPe) over 3b→4b (R = 1-nor):
∆∆G‡(298K) = 3.6 kcal/mol, ∆∆G‡(calc) = 4.0 kcal/mol.
The requisite spin-flip from the triplet imide surface to
the quintet amide surface occurs prior to the transition
state for insertion in both instances.
The favorable ∆∆G° of -2.4 kcal/mol for 4a formation
relative to 4b is mostly enthalpic in origin, and the ∆∆H‡
of -3.5 kcal/mol favoring the neoPe-based conversion
closely corresponds to the accompanying ∆∆H° of -3.6
kcal/mol. Subtle entropic factors also favor the less rigid
neoPe system as the change from 4- to 3-coordinate frees
up conformational space.
71.0
aToluene-d8. [3a] = 0.057 M; ∆H‡ = 22.3(8), ∆S‡ = 6.5(1)
eu. c[3b] = 0.047 M; ∆H‡ = 24.4(6), ∆S‡ = 1.3(1) eu.
b
(Me2IPr)Fe(=NAd)(neoPe)2 (3a) could not be isolated,
but it persisted in solution for a time sufficient to evalu-
ate the insertion kinetics. (Me2IPr)Fe(=NAd)(1-nor)2
(3b) could be similarly analyzed, and the results are
listed in Table 1. The migratory insertion of 3a to
(Me2IPr)Fe{N(Ad)neoPe}(neoPe) (4a, ∆G‡(25°) = 20.4(8)
kcal/mol) is ~3.6 kcal/mol more favorable than the con-
version of 3b to (Me2IPr)Fe{N(Ad)(1-nor)})(1-nor) (4b,
∆G‡(25°) = 24.0(6) kcal/mol). Enthalpy (∆∆H‡ ~ 2.1)
and entropy (∆∆S‡ ~ 5.2 eu) differences impart roughly
the same changes in ∆∆G‡.
Only additions of AdN3 to 2e and 3e enabled spectro-
scopic observation of the Fe(IV) imide, but exposure of
R'N3 to various dialkyls (Me2IPr)FeR2 (R = neoPe, 2a; 1-
nor, 2b; mesityl (Mes)) enabled rough relative amide
formation rates to be obtained: R = Mes ~ neoPe > 1-nor;
R' = Ph > SiMe3 > Ad. In the presence of cyclohexene,
1,4-cyclohexadiene, or Et3SiH, the 3b to 4b conversion
was unaffected, and products from nitrene trapping were
not observed. Slow alkyl exchanges are rampant, as 2a
and 2b equilibrate with (Me2IPr)Fe(neoPe)(1-nor) (2ab),
2b and 4a with (Me2IPr)Fe{NAd(neoPe)}(1-nor) and
2ab, and 4b and 2a with (Me2IPr)Fe{NAd(1-
nor)}(neoPe) and 2ab in near statistical mixtures. Despite
these complications, transiently generated 3a provided
Table 2. Calculateda BDEs and BDFEs for ground states
(GSs) of (Me2IPr)YFe-R ([Fe] = (Me2IPr)Fe).b
iron alkyl cmpd
BDE
51.1
57.0
36.5c
32.3d
43.5
39.5
53.8
55.5
BDFE
35.5
[Fe](neoPe)2 (2a)
[Fe](1-nor)2 (2b)
42.2
(AdN=)[Fe](neoPe)2 (3a)
18.1 c
15.2 d
26.0
(AdN=)[Fe](1-nor)2 (3b)
24.0
36.2
{N(Ad)neoPe}[Fe](neoPe) (4a)
{N(Ad)(1-nor)}[Fe](1-nor) (4b)
41.2
a
b
c
B3PW91-GD3/G-31+G(d). [Fe] = (Me2IPr)Fe. Triplet
GS. dQuintet excited state (ES).
Why does the neopentyl system manifest a greater in-
sertion rate and more favorable thermodynamics? In-
sight was obtained via BDE and BDFE calculations of
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4538-4539. (c) Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50,
the various D(FeR). The favorable thermodynamics for
3a→4a vs. 3b→4b tracks with iron-alkyl bond energy
differences as the iron(IV) imide converts to the
iron(II)amide (Table 2). The iron-alkyl bonds exhibit
greater changes (∆∆H = -17.3 vs. -12.0 kcal/mol; ∆∆G =
-18.1 vs. -15.2 kcal/mol) in the neoPe system over the 1-
norbornyl complexes, despite the latter having larger
magnitudes due to the greater s-character of the con-
strained bridgehead position.
In summary, an increase in covalency enables formal-
ly Fe(IV) dialkyl-imides to manifest a rare migratory
insertion reaction intrinsic to strong field systems. The
chemistry provides hope that 2e- reactivity, such as ole-
fin metathesis, may be possible in formally high oxida-
tion state first-row transition metal systems.
2063-2067. (d) Thomas, C. M.; Mankad, N. P.; Peters, J. C. J. Am. Chem.
Soc. 2006, 128, 4956-4957.
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Zhang, Q; Wang, L.; Deng, L. Angew. Chem. Int. Ed. 2014, 53, 8432-8436.
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(13) Heins, S. P.; Morris, W. D.; Wolczanski, P. T.; Lobkovsky, E. B. An-
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ASSOCIATED CONTENT
(16) (a) Lindley, B. M.; Swidan, A.; Lobkovsky, E. B.; Wolczanski, P. T.;
Adelhardt, M.; Sutter, J.; Meyer, K. Chem. Sci. 2015, 6, 4730-4736. (b) Lind-
ley, B. M.; Jacobs, B. P.; MacMillan, S. N.; Wolczanski, P. T. Chem. Com-
mun. 2016, 52, 3891-3894.
Supporting Information. Experimental details on proce-
dures and reactions; X-ray crystallographic information
pertaining to 1, 2a, 2b, 3b, and 4a.
(17) (a) Brown, S. N.; Mayer, J. M. J. Am. Chem. Soc. 1996, 118, 12119-
12133. (b) Brown, S. N.; Mayer, J. M. J. Am. Chem. Soc. 1994, 116, 2219-
2220.
AUTHOR INFORMATION
(18) Hu, X.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322-16323.
(19) (a) Matsunaga, P. T.; Hess, C. R.; Hillhouse, G. L. J. Am. Chem. Soc.
1994, 116, 3665-3666. (b) Koo, K.; Hillhouse, G. L. Organometallics 1996,
15, 2669-2271.
Corresponding Authors
e-mail: ptw2@cornell.edu; t@unt.edu
Notes
(20) Cenini, S.; Gallo, E.; Caselli, A.; Ragaini, F.; Fantauzzi, S.; Piangio-
lino, C. Coord. Chem. Rev. 2006, 250, 1234-1253.
The synthesis and structure of (tmeda)Fe(neoPe)2 (1) have
also been done by E. B. Hulley et al. (Univ. Wyoming).
(21) Fürstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.; Leh-
mann, C. W. J. Am. Chem. Soc. 2008, 130, 8773-8787.
(22) (a) Al-Afyouni, M. H.; Fillman, K. L.; Brennessel, W. W.; Neidig, M. L.
J. Am. Chem. Soc. 2014, 136, 15457-15460. (b) Muñoz III, S. B.; Daifuku, S.
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ACKNOWLEDGMENT
We thank the NSF (PTW: CHE-1402149; CHE-1664580.
TRC: CHE-1531468 (CASCaM)), and the DOE (TRC: DE-
FG02-03ER15387) for financial support.
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Table of Contents Graphic
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8
intramolecular
∆∆G‡ = -2.4 kcal/mol
neoPe > 1-nor
N
N
N
N
N
kinsert
AdN3
- N2
R
R
N
R
II
Fe
II
Fe
IV
R
9
N
Fe
R
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
R
N
R = neoPe, 1-nor
ACS Paragon Plus Environment