Characterization of the terminal iron(IV) imides {[PhBPtBu 2(pz′)]FeIV≡NAd}+

Article abstract of DOI:10.1021/ja0604358

New hybrid bis(phosphine)(pyrazole)borate tripodal ligands ([PhBP^tBu₂(pz′)]^-) are reported that support pseudotetrahedral iron in the oxidation states +1, +2, +3, and +4. The higher oxidation states are stabilized by a ter

Full text of DOI:10.1021/ja0604358

Published on Web 03/28/2006
Characterization of the Terminal Iron(IV) Imides {[PhBPtBu2(pz′)]FeIVtNAd}+
Christine M. Thomas, Neal P. Mankad, and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received January 27, 2006; E-mail: jpeters@caltech.edu
Although implicated as intermediates in group transfer reactions,1
Scheme 1
mid-to-late first row transition metals (e.g., Mn, Fe, Co, Ni)
featuring terminal imido/nitrene functionalities are rare.2 To date,
the only examples of structurally characterized mononuclear iron
imides are those supported by tris(phosphino)borate ligands.3 These
species have been reported in the Fe(III) and Fe(II) oxidation states.
In the Fe(III) state, they have been accessed via oxidative nitrene
transfer from organic azides using low valent FeI precursors. The
[PhBPR ]FeIII(NR) complexes that have been isolated all show
3
electrochemically reversible FeIII/II couples, and chemical reduction
typically provides their corresponding d6 {[PhBPR ]FeII(NR)}-
3
analogues in high yield. Well-defined FeIVdNR species have proven
generally more elusive,4 though thoroughly characterized examples
of FeIVdO species are now well-known.5 To the best of our
knowledge, the single report of a complex that can be formulated
as an Fe(IV) imide concerns Lee’s tetranuclear cluster Fe4(µ3-Nt-
Bu)4(NtBu)Cl3, isolated in only 1-2% yield.6 Mo¨ssbauer data for
this species were consistent with a cluster featuring three Fe(III)
centers and one Fe(IV) center. Herein we describe a new hybrid
pyrazolyl/phosphinoborate ligand that can support pseudotetrahedral
iron in the +1, +2, +3, and +4 oxidation states. The +3 and +4
oxidation states are stabilized by the terminal FetNR imide linkage.
Access to the required bis(phosphino)pyrazolylborate7 ligand is
achieved by initial preparation of the bis(phosphino)borane precur-
sor PhB(CH2PtBu2)2 (1) via metathesis between PhBCl2 and 2 equiv
of LiCH2PtBu2 (Scheme 1). Reaction of [pz]Li with 1 (pz ) pyraz-
olyl), followed immediately by salt metathesis with TlPF6, leads
to the clean formation of solid white [PhBPtBu2(pz)]Tl (2a) in 66%
isolated yield.8 The use of the bulky LiCH2PtBu2 carbanion is criti-
cally important in the preparation of this type of hybrid borate ligand
because (i) effective di- rather than trisubstitution at boron can be
achieved, which could not be realized using less hindered carban-
ions, such as LiCH2PiPr2 and LiCH2PPh2; (ii) the borane product,
PhB(CH2PtBu2)2 (1), does not appear to dimerize to an appreciable
degree in solution. This fact allows the efficient introduction of a
third donor arm.
Metathesis of 2a with FeCl2 leads to the clean formation of the
high spin (S ) 2) complex [PhBPtBu2(pz)]FeCl (3a), isolated in
69% yield as a yellow crystalline solid. Solution 1H NMR data for
3a are suggestive of a monomeric structure in solution, in accord
with the calculated solution Evans’ method magnetic moment at
295 K (5.2 µB).9 However, the 100 K X-ray crystal structure of 3a
establishes a dimeric structure with bridging chlorides and a
crystallographic center of symmetry in the solid state.
to a stoichiometric equivalent of (1-Ad)NdPMe3. The EPR spec-
trum of 5a displays a rhombic signal that was simulated (see SI) to
provide the g values g1 ) 2.96, g2 ) 1.95, and g3 ) 1.88 (Figure 1A),
a spectrum similar to those of related [PhBPR ]FeIIItNR imides.3
3
The cyclic voltammetry of 5a reveals very different features from
those observed for [PhBPR ]FeIII(NR) imides (Figure 1B). For ex-
3
ample, the cyclic voltammogram of previously reported [PhBPiPr3]-
FetNAd3b features a fully reversible reductive wave at -1.79 V
and an irreversible oxidative wave at ca. -0.45 V. This latter pro-
cess presumably reflects a one-electron oxidation to an unstable
Fe(IV) species. By contrast, complex 5a exhibits a completely irre-
Versible reductive wave at -2.20 V, indicating that the Fe(II) imide
anion is, in this case, unstable. The oxidative irreversible wave at
-1.26 V appears only after scanning through the -2.20 V wave,
indicating that it represents a byproduct of the one-electron reduction
of 5a. More interesting, however, is the presence of a quasi-rever-
sible feature at -0.72 V for 5a (at 100 mV/s; 22 °C). This wave
becomes fully reversible at ambient temperature when the scan rate
isincreasedto500mV/s.ItrepresentsanFeIV/III redoxcoupleandsug-
gests that “{[PhBPtBu2(pz)]FeIVtNAd}+” might be modestly stable.
In accord with these electrochemical data, 5a can be chemically
oxidized with [Fc][B(ArF)4] (ArF ) 3,5-(CF3)2-C6H3) at low tem-
perature (-50 °C) in THF solution to generate a green, cationic
species formulated as {[PhBPtBu2(pz)]FeIVtNAd}{B(ArF)4} (6a).
A single set of paramagnetic resonances is observed for 6a in its
1H NMR spectrum at -50 °C (see SI), distinct from the resonances
observed for 5a. Using an optical dip-probe assembly, the appear-
ance of absorption bands at 580 and 677 nm are readily observed
at low temperature upon addition of the ferrocenium oxidant (see
SI). We also established that the addition of 1 equiv of CoCp2 to
6a generated in situ in THF-d8 solution regenerated 5a cleanly.
The half-life of 6a is approximately 50 min at -40 °C in THF-d8,
and it must therefore be manipulated at temperatures below -40
Following an overall methodology that proved effective for the
synthesis of Fe(III) imides previously,3 one-electron reduction of
3a with sodium/mercury amalgam in the presence of excess PMe3
generates the high spin d7 precursor [PhBPtBu2(pz)]Fe(PMe3) (4a)
as a pale green crystalline solid (65% isolated yield; µeff ) 4.2 µB).
The reaction between 4a and 2 equiv of 1-adamantyl azide (1-
AdN3) generates the expected Fe(III) imide [PhBPtBu2(pz)]FetNAd
(5a) as a red-brown low spin species (2.0 µB in C6D6), in addition
°C. Solution magnetic data collected at low temperature (µeff
)
3.1(2) µB in THF-d8, 222 K; avg of 4 runs) indicate two unpaired
9
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J. AM. CHEM. SOC. 2006, 128, 4956-4957
10.1021/ja0604358 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. (A) EPR spectrum of 5a with g1 ) 2.96, g2 ) 1.95, and g3 )
1.88 (in 2-methyltetrahydrofuran glass at 20 K, 9.474 GHz). (B) Cyclic
voltammetry of 5a (0.40 M [nBuN4][ClO4] in THF, scan rate ) 100 mV/s
(full), 500 mV/s (inset)) and [PhBPiPr3]FetNAd (0.40 M [nBuN4][PF6] in
THF).
electrons (S ) 1), consistent with the ground state electronic
configuration (dz2)2(dxy)1(dx2-y2)1(dxz)0(dyz)0.
We undertook a crystallographic investigation of 5a and 6a to
confirm their connectivities and to examine their Fe-N bond
distances and Fe-Nimide-C bond angles for comparison with
[PhBPR ]FetNR imides. For the case of 5a, the crystals that we
3
obtained were twinned, regardless of the method employed for
crystallization. Its structure could nonetheless be refined isotropi-
cally to confirm its pseudotetrahedral geometry (see SI). Evident
from its isotropic structure is an expectedly short Fe-Nimide bond
distance (1.63 Å) and a nearly linear C-N3-Fe angle (169°).3
Owing to the thermal instability of 6a, its XRD analysis proved
to be a more challenging experiment. Single green crystals could
be obtained by storing a THF/petroleum ether solution at -78 °C
for several days, and XRD analysis confirmed its proposed
assignment (see SI). To obtain a better quality data set, we set out
to prepare an analogue of 6a of greater kinetic stability. We thus
pursued a [PhBPtBu2(pz′)]- derivative substituted by methyl groups
at the 3 and 5 positions. The required precursor ([PhBPtBu2(pzMe2)]-
Tl, 2b) and an analogous series of iron complexes (3b-6b, Scheme
1) were readily prepared by the same procedures described above.
The imide cation 6b indeed exhibits far greater thermal stability
than 6a. Its cyclic voltammetry is very similar to that of 6a, and
the FeIV/III redox couple is reversible even at slower scan rates (e.g.,
100 mV/s). 6b can even be isolated in pure form at ambient
temperature (86.7% yield) and manipulated without appreciable
degradation for short periods. X-ray data sets were obtained for
5b (disordered structure; see SI) and 6b. The anisotropically refined
X-ray crystal structure of {[PhBPtBu2(pzMe2)]FeIVtNAd}{B(ArF)4}
6b is shown in Figure 2. The structure reveals the anticipated
pseudotetrahedral iron cation and its tetra(aryl)borate counteranion.
The Fe-N3 bond distance (1.634(4) Å) and the Fe-Nimide-C bond
angle (176.2(3)°) are similar to the parameters obtained for
crystallographically characterized [PhBP3]Fe imides in the +3 and
+2 oxidation states.3 Assuming that the two unpaired electrons of
6b reside in relatively nonbonding d-orbitals, as predicted from
simple MO considerations,2b,3c the Fe-N bond should retain its
triple bond character (i.e., FeIVtNR+) and the Fe-N distance is
therefore not expected to change to a large extent upon oxidation.
In summary, whereas iron imides had been previously obtained
Figure 2. Anisotropically refined thermal ellipsoid representation of
{[PhBPtBu2(pzMe )]FeIVtNAd}{B(ArF)4} (6b); two molecules of THF and
2
all hydrogen atoms have been omitted for clarity; Fe-N3 1.634(4) Å; Fe-
N3-C1 176.2(3)°.
symmetry of the [PhBPtBu2(pz′)] ligand, and the compatibility of
this lower symmetry with a d4 triplet electronic configuration.
Acknowledgment. We acknowledge Larry Henling for crystal-
lographic assistance, and Dr. Mark Mehn for assistance with EPR
spectroscopy. We thank the NIH for financial support (GM 070757),
and N.P.M. is grateful for an NSF graduate fellowship.
Supporting Information Available: Detailed experimental pro-
cedures for 1, 2a,b-6a,b, all characterization data, and crystallographic
details for 3a, 5a, 5b, 6a, and 6b. This material is available free of
charge via the Internet at http://pubs.acs.org.
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in the +3 and +2 oxidation states using [PhBPR ]Fe systems, we
3
now find that imides in the +3 and +4 oxidation states are acces-
sible using [PhBPtBu2(pz′)]Fe systems. It is remarkable that termi-
nally bonded L3FetNR species have now been characterized in
three distinct oxidation states using phosphine-borate ligands given
the paucity of such species more generally. The cause of the in-
creased stability of the FeIVtNR linkage in the [PhBPtBu2(pz′)]Fe
system described herein is an interesting issue and might in part
be attributed to (i) a cathodic shift in the FeIV/III potential by compar-
(7) A related bis(pyrazole)(phosphine)borate ligand was reported recently:
Casado, M. A.; Hack, V.; Camerano, J. A.; Ciriano, M. A.; Tejel, C.;
Oro, L. A. Inorg. Chem. 2005, 44, 9122.
(8) In this paper, we adopt the ligand abbreviations [PhBPtBu2(pz)] and
[PhBPtBu2(pzMe2)] to refer to a parent pyrazole and a 3,5-dimethyl-
substituted derivative, respectively. [PhBPtBu2(pz′)] is a general notation
to represent any pyrazole derivative.
(9) (a) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (b) Evans, D. F. J. Chem.
Soc. 1959, 2003.
ison to previous [PhBPR ]Fe imide systems, which in turn might
3
lend added stability to the ligand borate unit and/or (ii) to the lower
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