A tetrahedrally coordinated L3Fe-Nx platform that accommodates terminal nitride (FeIV≡N) and dinitrogen (Fe I-N2-FeI) ligands

Article abstract of DOI:10.1021/ja048713v

A tetrahedrally coordinated L₃Fe-N_x platform that accommodates both terminal nitride (L₃Fe^IV≡N) and dinitrogen (L₃Fe^I-N₂-Fe^IL₃) functionalities is described. The diamagnetic L₃Fe^IV≡N species featured has been characterized in solution under ambient conditions by multinuclear NMR (¹H, 31P, and 15N) and infrared spectroscopy. The electronic structure of the title complex has also been explored using DFT. The terminal nitride complex oxidatively couples to generate the previously reported L₃Fe^I-N₂-Fe^IL₃ species. This reaction constitutes a six-electron transformation mediated by two iron centers. Reductive protonation of the nitride complex releases NH₃ as a significant reaction product. Copyright

Full text of DOI:10.1021/ja048713v

Published on Web 04/30/2004
A Tetrahedrally Coordinated L3Fe-Nx Platform that Accommodates Terminal
Nitride (FeIVtN) and Dinitrogen (FeI-N2-FeI) Ligands
Theodore A. Betley 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 March 5, 2004; E-mail: jpeters@caltech.edu
Scheme 1
High-oxidation state iron complexes featuring metal-to-ligand
multiple bonds (e.g., FedE/FetE) are proposed as key intermedi-
ates in numerous biocatalytic transformations.1 Because such
intermediates are typically too reactive to be directly observed,2a
much effort has focused on developing low-molecular weight model
complexes that enable the more systematic study of their physical
characteristics and reactivity patterns.1,2b The vast majority of work
in this field has been devoted to reactive ferryls (FeIVdO) as these
species are postulated as critical intermediates in a variety of
enzymes that reduce dioxygen.3 The reduction of dinitrogen
constitutes an equally fascinating biocatalytic transformation.4 A
highly redox-active molybdenum center (MoIII to MoVI) has been
suggested as the site of N2 binding and reduction in the well-studied
FeMo cofactor.5 Schrock’s demonstration of catalytic ammonia
production using a well-defined tris(amido)amine molybdenum
complex elegantly establishes that such a scenario is chemically
feasible.6 One of the various alternative possibilities to consider is
that a single low-valent iron site5a,7 initiates the required redox
transformations to convert N2 to NH3 by successive H+/e- transfer
steps (e.g., FeI-N2 + 3 H+ + 3 e- f FeIVtN + NH3; FeIVtN +
3 H+ + 3 e- f FeI-NH3).
previously used by Cummins as an N-atom transfer agent,11b
provided clean access to the desired reaction manifold (Scheme
1). Addition of Li(dbabh) to yellow [PhBPiPr3]FeCl in THF (or
THF-d8) at ca. -100 °C formed a slurry which, upon warming to
-35 °C, generated a red species that is formulated as the high-
spin amide complex [PhBPiPr3]Fe(dbabh). For comparison, a
structurally related and thermally stable red iron amide complex,
[PhBPiPr3]Fe(NPh2), has been isolated and structurally characterized
(see Supporting Information for details). The reaction between
Li(dbabh) and [PhBPiPr3]FeCl to generate [PhBPiPr3]Fe(dbabh) can
be monitored in situ by NMR spectroscopy in THF-d8 and proceeds
cleanly. A broad optical band associated with [PhBPiPr3]Fe(dbabh)
is observed at 475 nm (ꢀ ) 2300 M-1 cm-1), well-separated from
yellow [PhBPiPr3]FeCl (420 nm) and Li(dbabh) (370 nm).
An intriguing intermediate to consider under the latter scenario
is the iron nitride (FetN). At present, the only spectroscopic
evidence for terminally bound nitrides of iron comes from frozen
matrix experiments.8 Nakamato and co-workers reported the
resonance Raman detection of an octaethylporphyrinato nitride
(OEP)FeV(N) formed via photochemically induced N2 expulsion
from a coordinated azide ligand.8a Wieghardt and co-workers used
a similar photochemical strategy to generate a high-valent iron
species assigned as an FeV(N) nitride based upon low-temperature
EPR and Mo¨ssbauer data.8b The conditions under which these
FeV(N) species were produced, in addition to their high degree of
thermal instability, vitiated their more thorough spectroscopic and
chemical interrogation. Herein we report the room-temperature
observation of what is, to our knowledge, the first terminal iron-
(IV) nitride, [PhBPiPr3]FeIVtN ([PhBPiPr3] ) [PhB(CH2PiPr2)3]-).9
The pseudotetrahedral L3Fe-Nx platform described shuttles between
the formal oxidation states FeIV and FeI as the terminal Nx ligand
is transformed from the nitride functionality (FeIVtN) to dinitrogen
(FeI-N2-FeI). In addition, the FeIVtN subunit can serve as a
source of NH3 in the presence of proton and electron equivalents.
To explore the viability of a pseudotetrahedral iron nitride
L3FeIVtN, we sought an anionic X-type ligand that would un-
dergo clean oxidative N-atom transfer once coordinated to the
“[PhBPiPr3]Fe” template. Choice of the “[PhBPiPr3]Fe” subunit as
a suitable N-atom acceptor stemmed from prior work by our lab
concerning the preparation of pseudotetrahedral, low-spin S )
1/2Fe(III) imides of the type [PhB(CH2PPh2)3]FetNR and [PhBPiPr3]-
FetNR.7c,10 The lithium amide reagent Li(dbabh) (dbabh ) 2,3:
5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene),11 which has been
[PhBPiPr3]Fe(dbabh) is thermally unstable and exhibits clean first-
order decay when warmed to room temperature in solution (t1/2
≈
11 min at 22 °C)12 to produce a stoichiometric equivalent of
anthracene (NMR integration) and a new diamagnetic iron species
assigned as the tan nitride complex [PhBPiPr3]FetN on the basis
of its NMR data. The 31P NMR spectrum of the reaction solution
(in THF-d8) shows a single sharp resonance at 84 ppm (Figure 1a
inset) that is close in chemical shift to the structurally related
complex [PhBPiPr3]CotN-p-tolyl (85 ppm).7c The 1H NMR
resonances for the chelated [PhBPiPr3] ligand are also indicative of
chemically equivalent phosphine donors (Figure 2), even at low
temperature (-80 °C). While these NMR data are consistent with
a three-fold symmetric, pseudotetrahedral structure type, a 15N-
labeling experiment was critical to more firmly establish the
FetN functionality. Terminally bound nitrides give rise to signature
resonances by 15N NMR spectroscopy.13 The diamagnetic nature
of the d4 [PhBPiPr3]FetN complex therefore renders it particularly
well-suited to direct NMR detection of the terminal nitride ligand
9
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C O M M U N I C A T I O N S
transformed from a π-basic nitride to π-acidic N2. While the
converse of this pathway (i.e., FeI-NtN-FeI f 2 FeIVtN) is
therefore kinetically competent by microscopic reversibility, it may
not be thermally accessible in the present system. Heating a THF
solution of {[PhBPiPr3]Fe}2(µ-N2) at 60 °C brought about its gradual
degradation; however, no [PhBPiPr3]FetN was detected, and the
various reaction products produced were ill-defined. Evidence for
a viable FeI-to-FeIV redox couple was demonstrated by the addition
of a high-valent MntN source,16 (trans-[1,2-cyclohexanediamino-
N,N′-bis(4-diethylaminosalicylidene])MnVtN, to the N2-bridged
complex {[PhBPiPr3]Fe}2(µ-N2). Mixing a stoichiometric equivalent
of these two complexes in THF solution generated [PhBPiPr3]FeIVt
N in minutes (∼40% based on Fe). The reaction did not proceed
to completion, and the reaction mixture likely contained equilibrat-
ing species.17
Figure 1. (a) 15N NMR and 31P NMR (inset) for [PhBPiPr3]Fet15N. (b)
IR (pentane/KBr): (blue line) [PhBPiPr3]Fet14N, (red line) [PhBPiPr3]-
Fet15N.
While the FeIVtN coupling reaction to produce FeI-NtN-
FeI under argon further substantiates our assignment of the terminal
nitride functionality, it has also frustrated our efforts to isolate
[PhBPiPr3]FetN in crystalline form. To obtain satisfactory vibra-
tional characterization of the FetN functional unit it was necessary
to reconstitute the nitride complex in a straight-chain hydrocarbon.
To do this while circumventing its concentration-dependent deg-
radation to {[PhBPiPr3]Fe}2(µ-N2), a ∼0.02 M solution of [PhBPiPr3]-
Fe(dbabh) was generated at -35 °C in THF and then dried by
removal of the reaction volatiles without warming the solution. The
remaining residue was reconstituted in cold pentane and subse-
quently warmed to 22 °C to generate a solution containing [PhBPiPr3]-
FetN and the anthracene byproduct (see Supporting Informa-
tion). Examination of the respective IR spectra of pentane solu-
tions of [PhBPiPr3]FetN and [PhBPiPr3]Fet15N prepared in this
fashion reveals an ν(FetN) vibration at 1034 cm-1 and an
ν(Fet15N) vibration at 1007 cm-1 (Figure 1b). The isotopically
labeled derivative thus shifts to lower frequency by 27 cm-1, the
amount predicted from the reduced mass calculation for a simple
harmonic oscillator model (28 cm-1). The energy of the FetN
vibration is reflective of the FetN triple bond (vide infra) and is
to be compared with related data for other bona fide MtN triple
bonds. For example, the MntN vibration in (TPP)MntN is 1036
cm-1 (15N: 1008) and the MotN vibration in Schrock’s
[IPTN3N]MotN is 1013 cm-1 (15N: 986).13b,18,19 Nakamoto
reported a much lower vibration (876 cm-1) for an (TPP)FeV(N)
species detected in a frozen glass.8a The lower frequency for (TPP)-
FeV(N) likely results from attenuation of the Fe-N bond order
(FedN) due to occupation of the Fe-N π* orbitals by two
electrons.
Figure 2. 1H NMR of [PhBPiPr3]Fet15N. S ) residual solvent.
Figure 3. Molecular representation of the solid-state structure of {[Ph-
BPiPr3]Fe}2(µ2-N2). Fe1-N1 ) 1.811(5) Å; Fe2-N2 ) 1.818(5) Å; N1-
N2 ) 1.138(6) Å. See Supporting Information for details.
of interest. A 15N-labeled sample of Li(dbabh) was prepared from
commercially available 15N-labeled (ca. 98% 15N) potassium
phthalimide (see Supporting Information). Addition of 15N-Li-
(dbabh) to [PhBPiPr3]FeCl at low temperature in THF and subse-
quent warming of the sample to 22 °C for 25 min produced the
expected [PhBPiPr3]Fet15N nitride product (31P NMR). The sample
was then cooled to -5 °C, and a high-quality 15N NMR spectrum
was obtained over a period of 8 h. The 15N NMR spectrum is shown
in Figure 1a and exhibits a single sharp resonance at 952 ppm
(referenced to nitromethane at 380 ppm), cementing our assignment
of the complex as cylindrically symmetric with a terminally bound
FetN linkage.14
Akin to several high-valent osmium and ruthenium nitrides,15
[PhBPiPr3]FetN exhibits a propensity to undergo bimolecular
condensation via nitride coupling. This process generates the
previously reported N2-bridged complex {[PhBPiPr3]Fe}2(µ-N2).7c
The coupling reaction takes place under an argon atmosphere or
upon concentration under vacuum (Scheme 1). The solid-state
molecular structure of {[PhBPiPr3]Fe}2(µ-N2) has been obtained by
XRD analysis, and its X-ray structural representation is shown in
Figure 3. The FetN coupling reaction is striking in that it
constitutes what is, to our knowledge, the only example of a
6-electron redox process mediated by two iron centers. Each iron
center formally shuttles from FeIV to FeI as the Nx ligand is
Both five- and even six-coordinate iron complexes can be
prepared using the [PhBPiPr3] ligand scaffold,9 and we therefore
suggest that there is an electronic stabilization of the FetN linkage
under three-fold symmetry that arises from the possibility to achieve
one σ- and two π-bonding interactions. That this should be the
case can be easily predicted from simple symmetry considerations
and is also supported by DFT calculations. A DFT-minimized
geometry for [PhBPiPr3]FetN has been determined using the atomic
1
coordinates reported previously for the S )
/ imide complex
2
[PhBPiPr3]FetNAd as an initial HF guess.7c The calculation
provides the geometric and electronic structure information provided
in Figure 4. The Fe-N bond distance is calculated at 1.49 Å,
appreciably shorter than the distance for its crystallographically
characterized relative [PhBPiPr3]FetNAd (1.64 Å).7c The P-Fe-P
angles are appreciably expanded (99-101°) in the calculated nitride
structure by comparison to their more typical angles in a host of
other [PhBPiPr3]Fe complexes (90-95°),7c,9 lending it a conforma-
tion that is somewhat more tetrahedral in nature. This fact, in
9
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C O M M U N I C A T I O N S
T.A.B. is grateful for a graduate research fellowship from the
Department of Defense. Professor Seth N. Brown provided several
2
stimulating discussions on the topic of dz .
Supporting Information Available: Synthetic protocols and
characterization data; crystallographic data including a CIF file;
spectroscopic data; details for DFT study. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Figure 4. Theoretically predicted geometry and electronic structure (DFT,
JAGUAR 5.0, B3LYP/LACVP**) for S ) 0 [PhBPiPr3]FetN. Lobal
representations correspond to the frontier orbitals (energies in eV). Structural
parameters: Fe-P ) 2.28, 2.28, 2.29 Å; N-P-Fe ) 117, 117, 119°;
P-Fe-P ) 99, 101, 101°; Fe-N ) 1.490 Å.
addition to the distinct N-atom hybridization in the nitride complex
by comparison to related imide structures,21 dramatically destabilizes
2
the iron-centered a1 orbital of dz parentage. This situation gives
rise to a large HOMO-LUMO gap for [PhBPiPr3]FetN and a
favorable ground-state (xy)2(x2 - y2)2(z2)0(xz)0(yz)0 electronic con-
figuration consistent with a d4, S ) 0 FeIVtN subunit. The
diamagnetic d4 complex (mesityl)3IrVtO presumably owes its
stability to similar electronic arguments.22
While we have yet to thoroughly survey the reactivity of
[PhBPiPr3]FetN, we note the following preliminary observations.
The addition of PPh3 or PEt3 to THF solutions of [PhBPiPr3]FetN
appears to cleanly (1H NMR) generate the corresponding S ) 2
Fe(II) phosphiniminatos [PhBPiPr3]Fe-NdPR3 (R ) PPh3 or PEt3).
The presence of the phosphiniminato functionality is in each case
confirmed by the presence of intense IR vibrations (KBr/C6H6, ν-
(Ph3PdN) ) 1223 cm-1, ν(Et3PdN) ) 1214 cm-1). Also, the
corresponding R3PdNH2+ protonolysis products are liberated and
detected by subjecting the reaction solutions to positive mode ES-
(9) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074.
(10) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125,
322.
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G.; Dufresne, R. F.; Thoman, C. J. J. Org. Chem. 1988, 53, 2565. (b)
Mindiola, D. J.; Cummins, C. C. Angew. Chem., Int. Ed. 1998, 37, 945.
(12) The first-order decay of [PhBPiPr3]Fe(dbabh) was established by monitoring
the decay of its optical absorption at 475 nm (k ) 0.067 M-1 s-1, 22 °C,
1.2 mM solution; See Supporting Information for details).
(13) (a) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861. (b) Yandulov,
+
+
MS (electrospray: Ph3PNH2 ) m/z 278, Et3PNH2 ) m/z 120).
Perhaps most interesting to note is that the nitride ligand serves as
a source of NH3 upon the addition of proton and electron
equivalents. For example, the addition of solid [LutH][BPh4] and
CoCp2 (3 equiv of each) to a room-temperature C6D6 solution of
[PhBPiPr3]FetN produced after 2 h an appreciable quantity of NH3,
D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6252.
2
(14) JPN coupling is not observed in either the 15N NMR spectrum or the 31
P
NMR spectrum for [PhBPiPr3]Fet15N. A number of phosphine-supported
imides and nitrides that feature other metals (e.g., Mo, W) have been
prepared for which 15N and 31P NMR data is available. JPN coupling is
2
not observed in these systems. See: (a) Rocklage, S. M.; Schrock, R. R.
J. Am. Chem. Soc. 1982, 104, 3077. (b) Donovan-Mtunzi, S.; Richards,
R. L.; Mason, J. J. Chem. Soc., Dalton Trans. 1984, 1329.
(15) (a) Buhr, J. D.; Taube, H. Inorg. Chem. 1979, 18, 2208. (b) Man, W.-L.;
Tang, T.-M.; Wong, T.-W.; Lau, T.-C.; Peng, S.-M.; Wong, W.-T. J. Am.
Chem. Soc. 2004, 126, 478. (c) Seymore, S. B.; Brown, S. N. Inorg. Chem.
2002, 41, 462.
(16) (a) (trans-[1,2-Cyclohexanediamino-N,N′-bis(4-diethylaminosalicylidene])-
MnVtN was synthesized from trans-[1,2-cyclohexanediamino-N,N′-bis-
(4-diethylaminosalicylidene] according to: Du Bois, J.; Hong, J.; Carreira,
E. M.; Day, M. W. J. Am. Chem. Soc. 1996, 118, 915. (b) Bendix, J. J.
Am. Chem. Soc. 2003, 125, 13348. (c) Chang, C. J.; Low, D. W.; Gray,
H. B. Inorg. Chem. 1997, 36, 270.
(17) For instance, we suspect the reaction solution also contains a bridged nitride
species (i.e., FedNdMn). Studies are underway to resolve the complete
product profile.
(18) [TPP] ) tetraphenylporphyrinato; [HIPTN3N] ) tris(2-(2,4,6,2′′,4′′,6′′-
hexaisopropyl-1,1′:3′,1′′-terphenyl-5′-amino)ethyl)amine.
(19) Hill, C. L.; Hollander, F. J. J. Am. Chem. Soc. 1982, 104, 7318.
(20) See Supporting Information for details. Jaguar, version 5.0, Schrod-
inger: Portland, OR, 2002.
1
easily identified by H NMR in d6-DMSO as its NH4Cl salt after
vacuum transfer of the reaction volatiles into an ethereal solution
of HCl (ca. 1 M).13b Two independent runs using these nonopti-
mized conditions afforded 41 and 45% of NH3 based upon NMR
integration versus an internal standard. In a potentially related
reaction, the release of p-toluidine by hydrogenation of the iron-
(III) imide [PhB(CH2PPh2)3]FetN-p-tolyl was observed.23
To conclude, the “[PhBPiPr3]Fe-Nx” chemistry discussed herein
and that which we have reported previously7c collectively illustrate
that the redox chemistry available to a pseudotetrahedral iron site
can be remarkably rich. Examples of “[PhBPiPr3]Fe-Nx” complexes
have now been characterized that feature five formal iron oxidation
-
states based upon the magnetic data available:24 [PhBPiPr3]Fe0N2
(S ) 1), [PhBPiPr3]FeI-N2-FeI[PhBPiPr3] (S ) 3/2 per iron center),
[PhBPiPr3]FeII-N2Me (S ) 2), [PhBPiPr3]FeIIItNAd (S ) 1/2), and
[PhBPiPr3]FeIVtN (S ) 0). For each of these complexes, the
dominant structural modification pertains to the nature of the fourth
Nx ligand: π-acidic N2 favors the lower oxidation states, whereas
π-basic nitride (or imide) favors the higher oxidation states.
(21) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124,
11238.
(22) Hay-Motherwell, R. S.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse,
M. B. Polyhedron 1993, 12, 2009.
(23) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 4538.
(24) While their ground spin-state assignments (EPR, SQUID) are consistent
with these formal oxidation states, studies are underway to further probe
this family of [PhBPiPr3]Fe-Nx species by Mo¨ssbauer spectroscopy.
Acknowledgment. This work was supported by the NSF (CHE-
01232216), the Sloan Foundation, and the Dreyfus foundation.
JA048713V
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