Reaction of a sterically encumbered iron(I) aryl/arene with organoazides: Formation of an iron(v) bis(imide)

Article abstract of DOI:10.1039/b810941a

Reaction of 3,5-Prⁱ₂Ar*Fe(η⁶- C₆H₆)(3,5-Prⁱ₂Ar* = C ₆H₁-2,6-(C₆H₂-2,4,6-Pr ⁱ₃)₂-3,5-Prⁱ₂) with N₃C₆H₃-2,6-Mes₂ (Mes = C ₆H₂-2,4,6-Me₃) afforded the dimeric iron(ii) amido/aryl complex {CH₂C₆H₂-2(C ₆H₃-2-N(H)FeAr*-3,5-Prⁱ ₂)-3,5-Me₂}₂ (1) which arises via methyl hydrogen abstraction by nitrogen and dimerization of the radical via C-C bond formation; in contrast, reaction of 3,5-Prⁱ₂Ar *Fe(η⁶-C₆H₆) with N₃(1-Ad) (1-Ad = 1-adamantanyl) gave the iron(v) bis(imido) complex 3,5-Pr ⁱ₂Ar*Fe{N(1-Ad)}₂ (2). The Royal Society of Chemistry.

Full text of DOI:10.1039/b810941a

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Reaction of a sterically encumbered iron(I) aryl/arene with organoazides:
formation of an iron(V) bis(imide)w
Chengbao Ni,a James C. Fettinger,a Gary J. Long,b Marcin Bryndaa and
Philip P. Power*a
Received (in Berkeley, CA, USA) 27th June 2008, Accepted 29th September 2008
First published as an Advance Article on the web 21st October 2008
DOI: 10.1039/b810941a
Reaction of 3,5-Pri2Ar*Fe(g6-C6H6)(3,5-Pri2Ar* = C6H1-2,6-
(C6H2-2,4,6-Pri3)2-3,5-Pri2) with N3C6H3-2,6-Mes2 (Mes =
C6H2-2,4,6-Me3) afforded the dimeric iron(II) amido/aryl
complex {CH2C6H2-2(C6H3-2-N(H)FeAr*-3,5-Pri2)-3,5-Me2}2
(1) which arises via methyl hydrogen abstraction by nitrogen
and dimerization of the radical via C–C bond formation; in
contrast, reaction of 3,5-Pri2Ar*Fe(g6-C6H6) with N3(1-Ad)
(1-Ad = 1-adamantanyl) gave the iron(V) bis(imido) complex
3,5-Pri2Ar*Fe{N(1-Ad)}2 (2).
The products 1 and 2 were initially characterized by X-ray
crystallography and elemental analysis. For the reaction with
N3C6H3-2,6-Mes2 the intention was to synthesize the stable
two-coordinate Fe(III) imido complex 3,5-Pri2Ar*-FeQ
NC6H3-2,6-Mes2. However 1, which is dimerized through an
intermolecular C–C bond involving ortho methyl groups from
the flanking mesityl rings (Fig. 1) was the only product
isolated (46% yield) from the reaction mixture. Such behavior
is precedented by that of the unstable species [HC{C(Me)
N(C6H3-2,6-Pri2)}2FeN(1-Ad)]15 which undergoes intra-
molecular H abstraction from an ortho Pri group to afford
an iron amide. The formation of 1 also has similarities to the
behavior of the unstable Co(III) terminal imido complexes
{HB(pyrazolyl-3-R-5-Me)3}CoN(SiMe3) (R = Pri or But)
which rearrange by H atom abstraction from a Pri or But
group to form, in the case of Pri, a product dimerized via C–C
bonding.16 Similarly, the first step in the formation of 1 may be
H abstraction from an ortho-Me group of a flanking mesityl
ring by the nitrogen center. The radical thus generated may
dimerize intermolecularly rather than interact with the metal
which may be sterically less favorable. The iron(II) centers in 1
have a rare two-coordinate, bent geometry (N(1)–Fe(1)–C(1) =
138.91(8) 1). There is a relatively long interaction (Fe–C(49) =
2.866(2) A) with the ipso-carbon of one of the flanking mesityl
rings. Bending of the metal geometry in two coordinate transi-
tion metal species is often observed because of the tendency of
the metals, which have low numbers of valence electrons (10 or
14 in the case of 1), to interact with electron rich moieties.17 The
Fe–N and Fe–C bond lengths are similar to those reported for
two-coordinate iron amides18 or aryls.19 The magnetic proper-
ties of 1 were measured from 6 to 320 K in a 0.001 T applied
The chemistry of late transition metal imido (NR2ꢀ) deriva-
tives and their amido (NR2ꢀ) counterparts is considerably less
developed than that of their earlier metal analogues.1,2 This is
thought to be due to the lack of stabilizing p-interactions by
the nitrogen lone pair(s) with metal d-orbitals that are either
energetically disparate or already occupied.3–5 Until recently
this lack of development was particularly marked for the later
first row elements where stable complexes having terminal
imido ligands were unknown. In 2000, Lee and co-workers
showed that the cubane cluster Fe4(m3-NBut)4(NBut)Cl3,
which carries a terminal imido group at one of the four irons,
could be isolated from FeCl3/LiNHBut reaction mixtures.6
The terminal NBut group featured
a short Fe(IV)–N
bond length of 1.635(4) A and an almost linear
(Fe–N–C = 178.6(3)1) imido ligand geometry. Hillhouse and
Mindiola reported the stable, mononuclear nickel(II) complex
(But2PCH2CH2PBut2)NiNC6H3-2,6-Pri2 featuring
a short
(1.702(2) A) Ni–N bond and a wide (162.8(8)1) Ni–N–C angle
indicating a nitrogen lone-pair p-interaction with a nickel
d-orbital.7 These two reports heralded several important stable
terminal imido metal complexes, particularly those of Fe(III)
and Co(III), which are stabilized by multidentate ligands.8–12
We now report that the reaction (Scheme 1) of the
recently synthesized Fe(I) species 3,5-Pri2Ar*Fe(Z6-C6H6)
(Ar*-3,5-Pri2 = C6H-2,6(C6H2-2,4,6-Pri3)2-3,5-Pri2)13 with
the organo azides N3C6H3-2,6-Mes2 (Mes = C6H2-2,4,6-Me3)14
or N3(1-Ad) afforded, the Fe(II) amido derivative 1 and Fe(V)
bis(imido) complex 2 with N2 elimination.z
a Department of Chemistry, University of California, One Shields
Avenue, Davis, CA 95616, USA. E-mail: pppower@ucdavis.edu
b Department of Chemistry, Missouri University of Science and
Technology, Rolla, MO 65409, USA. E-mail: glong@umr.edu
w Electronic supplementary information (ESI) available: Magnetic
data for 1 and 2. CCDC 693020 and 693021. For ESI and crystal-
lographic data in CIF or other electronic format see DOI:
10.1039/b810941a
Scheme 1 Synthesis of compounds 1 and 2.
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The coordination planes at the imido nitrogens are almost
coincident (maximum deviation 7.21) with the central
C(1)Fe(1)N(1)N(2) plane. However, the plane of the central
aryl ring of the terphenyl ligand subtends an angle of ca. 551
with respect to the metal plane. The Fe–C distance is margin-
ally shorter than that in 1, whereas the iron nitrogen bond
lengths, Fe(1)–N(1) = 1.642(2) and Fe(1)–N(2) = 1.619(2) A
are considerably shorter than the single Fe–N bond (1.909(2) A)
in 1. The shorter Fe–N bond length is associated with the
wider Fe–N–C bending angle. The Fe–N distances may be
compared to those in the above mentioned Fe4(m3-NBut)4
(NBut)Cl3 (Fe(IV)–N = 1.635(4) A),6 [{PhB(CH2PBut2)2(3,5-
Me2-pyrazolyl)}FeN(1-Ad)][B{C6H3-3,5-CF3}2]4] (Fe(IV)–N =
1.634(4) A)9b {PhB(CH2PPh2)3}FeNC6H4-4-Me (Fe–N =
1.6578(2) A),9b the Fe(III) complexes {PhB(CH2PPri2)3}FeN
(1-Ad) (Fe–N = 1.638(2) A),9c {PhB(CH2PPh2)3}FeN(1-Ad)
(Fe–N = 1.641(2) A),9d and the Fe(II) species [NBun ]
4
Fig. 1 Thermal ellipsoid (30%) plot of 1. The C1 ring substituents
and H atoms (except N–H) are not shown. Selected bond lengths (A)
and angles (1): Fe1–C1 2.0447(18), Fe–N1 1.9086(17), N1–C43
1.380(2), C50–C55 1.510(3), C55–C55A 1.537(4); C1–Fe1–N1
138.91(8), Fe1–N1–C43 132.26(14), Fe1–N1–H1 120.0(13),
C43–N1–H1 107.4(14), C50–C55–C55A 112.2(2)
[{PhB(CH2PPh2)3}FeN(1-Ad)] (Fe(II)–N = 1.651(3) A).9e It
is noteworthy that only 0.02 A separate the longest Fe(II)–N =
1.651(3) A and shortest Fe(IV)–N (1.634(4) A) distances. The
shortest Fe–N bond length in 2 is 1.618(2) A and this distance
approaches the ca. 1.61 A observed in the Fe(V) nitride
[trans-(cyclam-OAc)FeN]+,22 although it is ca. 0.05 A longer
than the 1.57 A measured by EXAFS in the Fe(VI) nitride
field. A plot of the inverse molar susceptibility vs. temperature
revealed an almost linear relationship consistent with the pre-
sence of two paramagnetic iron(II) centers with very weak
coupling (J = ꢀ1.11 cmꢀ1). The meff = 5.29 mB/Fe is consistent
with a high-spin d6 electron configuration.
species [(Me3-cyclam-OAc)FeN]2+ 23,24
,
and considerably
longer than the ca. 1.51–1.55 A in Fe(IV) nitrides.25,26 Magnetic
studies of 2 afford a meff which decreases linearly from 2.74 mB at
8 K to 2.43 mB at 320 K. A plot of 1/wM vs. temperature is not
strictly linear but the data indicate a low-spin d3 configuration.
The S = 1/2 ground state is consistent with that of
[trans-(cyclam-OAcFeN]+ which is also low spin.23
The compound 2 (Fig. 2) is notable not only because it is a
stable Fe(V) imide, but it is also a rare example of a well
characterized Fe(V) molecule.y20,21 The iron has a planar three-
coordinate geometry with interligand angles that have a
maximum deviation of ca. 1.41 from the idealized 1201 value.
The electronic structure of 2 was probed using DFT meth-
ods (the detailed description of the computational procedures,
optimized geometrical parameters and molecular orbitals are
presented in the ESIw). Geometry optimizations of the model
species Ar#Fe(NBut)2 (Ar# = C6H3-2,6-Ph2) yielded bond
lengths (Fe–C = 1.974 A, Fe–N = 1.614 A) and bond angles
in reasonable agreement with the experimental values, except
for the N(1)FeN(2) angle, which is wider than it is in the model
species (by 12.2 to 18.51, depending on the level of theory
used). Such a discrepancy is not surprising, since no con-
straints were applied in the gas-phase optimized structures
allowing a free rotation around the Cipso–Fe bond, and
indicate that the narrower experimental N(1)FeN(2) angle
(121.86(10)1) is most probably a consequence of the very large
size of the 3,5-Pri2Ar* substituent. The close-to-planar struc-
ture of the CipsoFeN(1)N(2) core is also well reproduced with
the unpaired electron almost completely localized in the iron
dxz orbital of approximate B1 symmetry and only very slight
spin polarization on the nitrogen atoms.
The isolation of 1 and 2 emphasize the importance of steric
effects in determining the type of product obtained. In 1, the
coordination of only one NC6H3-2,6-Mes2 imide to the iron is
allowed because of the large size of the nitrogen substituent.
However, use of the less bulky imide group N(1-Ad) allows
association of a second imido ligand to afford 2 which has 17
valence electrons (including the N lone pairs). The stability of
2 is probably a consequence of the sterically crowding, mono-
dentate nature of the stabilizing 3,5-Pri2Ar* ligand which
Fig. 2 Thermal ellipsoid (30%) plot of 2; H atoms are not shown.
Selected bond lengths (A) and angles (1): Fe1–C1 2.013(2), Fe1–N1
1.642(2), Fe1–N2 1.619(2), N1–C43 1.448(3), N2–C53 1.455(3);
C1–Fe1–N1 118.61(9), C1–Fe1–N2 119.46(9), N1–Fe1–N2
121.86(10), Fe1–N1–C43 156.6(2), Fe1–N2–C53 160.7(2).
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permits coordination of two N(1-Ad) groups without exceed-
ing 18 valence electrons. Investigations of the chemistry of 2
and other high valent late transition metal imides are in hand.
We thank the National Science Foundation (CHE-0641020)
for financial support.
8 X. Hu and K. Meyer, J. Am. Chem. Soc., 2004, 126, 16322.
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Notes and references
z All manipulations were carried out under strictly anhydrous and
anaerobic conditions. (Z6-C6H6)FeAr*-3,5-Pri2 and N3C6H3-2,6-Mes2
were prepared by published procedures.13,14
1: To a mixture of N3C6H3-2,6-Mes2 (Mes = mesityl, 2,4,6-Me3C6H2)
(0.153 g, 0.43 mmol) and (Z6-C6H6)FeAr*-3,5-Pri2 (0.300 g, 0.43 mmol),
ca. 30 mL of hexanes was added. The mixture was stirred at ca. 25 1C.
overnight by which time the solution had become red. The solution
11 S. C. Bart, E. Lobkovsky, E. Bill and P. J. Chirik, J. Am. Chem.
Soc., 2006, 128, 5302.
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13 C. Ni, B. D. Ellis, J. C. Fettinger, G. J. Long and P. P. Power,
Chem. Commun., 2008, 1014.
was filtered and concentrated to ca.
3 mL, which afforded
X-ray quality red crystals of 1 after storage for 1 day at 7 1C. Yield
0.188 g (46.1%). The compound melts at 128 1C. UV-Vis (hexane, nm
14 J. Gavenonis and T. D. Tilley, Organometallics, 2002, 21, 5549.
15 N. A. Eckert, S. Vaddadi, S. Stoian, R. J. Lachiotte, T. R. Cundari
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J. Am. Chem. Soc., 2003, 125, 4440.
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1988, 27, 1782; (c) H. Chen, R. A. Bartlett, H. V. R. Dias,
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[e/Mꢀ1 cmꢀ1]): 450 (1500)
2: A solution of N3(1-Ad) (0.152 g, 0.86 mmol) in ca. 15 mL of hexanes
was added dropwise to an orange solution of (Z6-C6H6)FeAr*-3,5-Pri2
(0.300 g, 0.43 mmol) in ca. 10 mL hexanes at room temperature. The
mixture was stirred at room temperature for about 18 h by which time
the solution had turned deep brown. The solution was filtered and
concentrated to ca. 4 mL, which afforded X-ray quality dark brown
crystals of 2 after storage for 3 days at 7 1C. Yield 0.107 g (27.0%). The
compound melts at 155 1C. Calc. for C62H91FeN2: C, 80.92; H, 9.97;
N, 3.04. Found: C, 81.3; H, 9.63; N, 3.11. UV-Vis (hexane, nm
[e/Mꢀ1 cmꢀ1]): 416 (11 700)
Crystal data for 1 and 2 at 90 K with Mo-Ka (l = 0.71073 A)
radiation. 1: monoclinic, space group I2/a; a = 25.135(2), b =
28.011(3), c = 25.235(2) A, b = 118.871(11)1, Z = 4, R1 = 0.0612
ꢀ
for 12 369 (I 4 2s(I)) data. 2: triclinic, space group, P1, a =
19 (a) W. Seidel, H. Muller and H. Gorls, Angew. Chem., Int. Ed.
¨
¨
13.9437(14), b = 14.9787(15), c = 15.5738(16) A, a = 82.3806(15),
b = 66.0829(14), g = 65.8148(14)1, Z = 2, R1 = 0.0387 for 8050
(I 4 2s(I)) data.
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y The complex may be viewed as an [3,5-iPr2Ar*Fe]4+ fragment
20 F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochmann,
Advanced Inorganic Chemistry, Wiley, New York, 1999, 6th edn,
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bound to two [N(1-Ad)]2ꢀ ions or
a neutral Fe (I) moiety
3,5-iPr2Ar*Fe bound to two neutral N(1-Ad) nitrenes.
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