Dinitrogen complexes of sulfur-ligated iron

Article abstract of DOI:10.1021/ja2020907

We report a unique class of dinitrogen complexes of iron featuring sulfur donors in the ancillary ligand. The ligands utilized are related to the recently studied tris(phosphino)silyl ligands (2-R₂PC₆H ₄)₃Si (R

Full text of DOI:10.1021/ja2020907

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Dinitrogen Complexes of Sulfur-Ligated Iron
Ayumi Takaoka, Neal P. Mankad, and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S Supporting Information
b
ABSTRACT: We report a unique class of dinitrogen
complexes of iron featuring sulfur donors in the ancillary
ligand. The ligands utilized are related to the recently
studied tris(phosphino)silyl ligands (2-R2PC6H4)3Si (R =
Ph, iPr) but have one or two phosphine arms replaced with
thioether donors. Depending on the number of phosphine
arms replaced, both mononuclear and dinuclear iron com-
plexes with dinitrogen are accessible. These complexes
contribute to a desirable class of model complexes that
possess both dinitrogen and sulfur ligands in the immediate
iron coordination sphere.
Figure 1. (left) Hypothetical binding mode of N2 at the FeMoco and
(right) a hypothetical model complex. Whether any of the S atoms
shown in red for FeMoco (left) are protonated during catalysis is
unknown.
Scheme 1
he intimate mechanism of nitrogen fixation by the nitrogen-
T
ase enzymes remains a fascinating puzzle. Recent theoretical
and experimental studies on the FeMo nitrogenase cofactor
(FeMoco) have suggested N2 binding at iron (Figure 1),1
resulting in a growing interest in model complexes of iron with
nitrogenous ligands (NxHy).2 To date, such nitrogenase model
chemistry has been dominated by complexes with phosphorus
and nitrogen donors because of their propensity to afford N2
complexes. This situation is striking given the sulfur-rich environ-
ment of the iron centers in the cofactor of FeMo nitrogenase.3
Noteworthy in this context is the work by Sellmann4 and more
recently by Qu,5 who have reported a number of iron complexes
ligated by multiple sulfur donors and nitrogenous ligands.6
Dinitrogen as a ligand remains a notable exception in these
systems. In general, synthetic transition metal complexes with
sulfur atom donors rarely afford N2 adduct complexes; examples
are known but remain comparatively uncommon.7 With the
exception of a single tetrahydrothiophene adduct of an FeꢀN2
complex,8 the SꢀFeꢀN2 linkage is unknown, regardless of the
number of SꢀFe interactions.
ligand, (2-R2PC6H4)3Si ([SiPR ]; R = Ph, iPr), that has proven
3
exceptionally successful in stabilizing terminal trigonal-bipyramidal
{[SiPR ]FeꢀN2}n complexes (n = ꢀ1, 0, þ1).12 Herein we present
3
a new class of iron complexes featuring the SꢀFeꢀN2 linkage
supported by such hybrid ligands.
Precursors of the desired ligands were conveniently synthe-
sized by lithiation of the aryl bromides 2-iPr2P(C6H4Br) (1) and
2-AdS(C6H4Br) (2) with n-BuLi followed by quenching with 0.5
equiv of HSiCl3, which yielded the chlorosilanes (2-iPr2PC6H4)2-
Si(H)(Cl) (3) and (AdSC6H4)2Si(H)(Cl) (4) in quantitative
yield (Scheme 1). Addition of another equivalent of the lithiation
product of 2 and 1 to 3 and 4, respectively, afforded the hybrid
ligands (2-iPr2PC6H4)2(2-AdSC6H4)SiH ([SiPiPr2SAd]H, 5)
and (2-iPr2PC6H4)(2-AdSC6H4)2SiH ([SiPiPrSAd2]H, 6) in
high yield. The tris(thioether)silane ligand (2-AdSC6H4)3SiH,
([SiSAd3]H, 7) was also synthesized by addition of 0.33 equiv of
HSiCl3 to the lithiation product of 2.13
Relative to phosphines and amines, π-donating sulfides and
thiolates are weak-field ligands9 that typically yield high-spin
complexes with long FeꢀL bonds.10 Such a scenario is undesir-
able with respect to the favorable π back-bonding needed for a
metal center to coordinate N2. Accordingly, terminal N2 adducts
of transition metals do not populate high-spin states. In this
regard, an electron-releasing and sulfur-containing ancillary
ligand that yields low-spin metal centers may prove useful. Such
scaffolds may help stabilize sulfur-ligated N2 adducts of iron,
especially in cases where N2 is terminally bonded. Thioethers are
particularly appealing since they are σ-donating and (weakly) π-
accepting11 and thus would favor states with lower spin relative to
thiolates and sulfides. To test this idea, we targeted hybrid
thioether/phosphine relatives of a tetradentate tris(phosphino)silyl
Received: March 7, 2011
Published: May 16, 2011
r
2011 American Chemical Society
8440
dx.doi.org/10.1021/ja2020907 J. Am. Chem. Soc. 2011, 133, 8440–8443
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Scheme 2
Scheme 3
Scheme 4
which exhibits a more rigorous TBP geometry.12d This difference
likely reflects the smaller steric influence of a thioether relative to
a phosphine donor, resulting in expansion of the PꢀFeꢀP angle
in 10 relative to {[SiPiPr3]Fe(N2)}[BArF ].
4
Figure 2. Solid-state structures of (left) 10 and (right) 13 (50%
For the bis(thioether) complex 9, protonation in Et2O led
probability; H atoms and solvent for 10 and 13 and BArF ꢀ for 10 have
instead to the solvent adduct {[SiPiPrSAd2]Fe(Et2O)}BArF
4
4
been removed). Selected bond lengths (Å) and angles (deg) for 10:
FeꢀN1, 1.954(3); FeꢀSi, 2.3106(9); FeꢀP1, 2.353(1); FeꢀP2,
2.3542(9); FeꢀS, 2.2941(9); SiꢀFeꢀN1, 173.66(9); SꢀFeꢀP1,
119.42(4); SꢀFeꢀP2, 104.80, P1ꢀFeꢀP2, 129.76(4). For 13: FeꢀN1,
1.828(2); FeꢀSi, 2.2157(8); FeꢀP, 2.185(7); FeꢀS1, 2.3002(7);
FeꢀS2, 2.2887(7); N1ꢀN2, 1.116(3); SiꢀFeꢀN1, 177.79(7).
(11). The lack of N2 binding is likely dictated by the slightly
reduced electron density at the iron center chelated by 6 relative
to that by 5 due to the smaller number of phosphine donors.
To increase the electron-richness of the iron center, the addition
of a hydride donor was explored.
Accordingly, addition of NaEt3BH to 10 and 11 resulted in
clean conversion to the corresponding neutral and diamagnetic
hydrideꢀN2 complexes [SiPiPr2SAd]Fe(H)(N2) (12) and
Metalation with iron was found to be facile for 5 and 6.
Addition of 2 equiv of MeMgCl to a solution of FeCl2 and 5 or 6
at ꢀ78 °C yielded the paramagnetic (S = 1) iron(II) methyl
complex [SiPiPr2SAd]FeMe (8) or [SiPiPrSAd2]FeMe (9), respec-
tively (Scheme 2). The tris(thioether)silane 7 was not metalated
under similar conditions, perhaps underscoring the need of a
phosphine donor to aid the chelate-assisted SiꢀH bond
activation.12b
[SiPiPrSAd2]Fe(H)(N2) (13), which exhibited νN values of
2
2055 and 2060 cmꢀ1, respectively. While isomers in which the
hydride ligand is trans to either a thioether or a phosphine are
conceivable, only one hydride signal (triplet for 12 and doublet
1
for 13) was observable in their respective H NMR spectra at
ca. ꢀ19 ppm. The solid-state structure of 13 featuring two
thioether donors (Figure 2 right) exhibits a hydride ligand trans
to one of the thioether ligands, as would be expected on the basis
of the greater trans influence of a phosphine.16 Density functional
theory calculations indicated an energy difference of 17.5 kcal/mol
in favor of the observed isomer [see the Supporting Informa-
tion (SI) for details]. The structure of 12 is presumed to be
similar in light of the equivalence of the P atoms in the 31P{1H}
spectrum.
Complexes 8 and 9 served as convenient entry points into the
FeꢀN2 chemistry of interest. Protonation of the methyl ligand in
8 and 9 with HBArF [BArF = tetrakis(3,5-trifluoromethyl-
4
4
phenyl)borate] in Et2O resulted in loss of methane. For com-
plex 8, loss of methane was followed by binding of N2 to
yield the cationic, paramagnetic (S = 1) dinitrogen complex
{[SiPiPr2SAd]Fe(N2)}BArF4 (10), as evident from the N2 stretch
in its IR spectrum at νN = 2156 cmꢀ1. Consistent with the high
2
Despite the stability of [SiPiPr3]FeI(N2),12a,b,d the correspond-
ing iron(I) complexes using ligands 5 and 6 did not prove to be
accessible. For 10, strong reductants such as KC8 and Na/Hg
resulted in a mixture of products. One of these products was
determined by X-ray diffraction to be a dimeric complex formed
by cleavage of the SꢀC(alkyl) bond of the thioether arm to yield
a thiolate ligand that bridges two iron centers (Scheme 3).
IR frequency, the N2 ligand was appreciably labile, and a rapid
color change from green to orange occurred under reduced
pressure.
The solid-state structure of 10 (Figure 2 left) reveals a
distorted trigonal-bipyramidal (TBP) geometry (τ = 0.7314)
with a terminal N2 ligand.15 The distorted structure is in contrast
to that of the corresponding {[SiPiPr3]Fe(N2)}[BArF ] complex,
4
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3
independence of the solid-state data suggests an S = /2 state
that is largely separated from the other spins states. The N2-
bridged diiron complex {[SiPPhSAd2]Fe}2(N2)}[BArF ] (15)
4
([SiPPhSAd2]H = (2-Ph2PC6H4)(2-AdSC6H4)2SiH) featuring
phenyl groups on the phosphine donors was synthesized analogously
to 14 and exhibited similar spectroscopic features (see the SI).
To conclude, a class of dinitrogen complexes of iron chelated
by hybrid silyl ligands that include sulfur and phosphine donors
has been characterized. Noteworthy are the ironꢀN2 adducts 13,
14, and 15, which possess multiple sulfur donors per Fe; these
complexes are unique in this regard. Additionally, 14 and 15
represent unusual examples of formally Fe(I)/Fe(II) mixed-
valent dinitrogen complexes. The work herein illustrates that
mononuclear and dinuclear dinitrogen complexes of sulfur-
ligated iron are accessible in various spin states (S = 3/2, S = 1,
and S = 0) using sulfur-containing scaffolds that induce relatively
electron-rich metal centers. The structural relevance of thioether
donors as models of the local environment of the iron centers in
the FeMo cofactor in part depends on whether inorganic sulfide
is protonated during catalytic turnover,20 which in turn could
result in HS f Fe dative interactions (Figure 1). Regardless, the
use of thioethers in the present synthetic context provides steric
protection while conserving a low-valent iron center.
Figure 3. Variable-temperature SQUID data (fit in black) and (inset)
EPR data (20 K in 2-Me-THF) for 14.
Related dimeric iron cores with bridging thiolates that are
chelated by tripodal tris(thiolate) ligands have been reported
elsewhere.17 In contrast, addition of CoCp2 to 10 unexpectedly
led to transmetalation of one Cp ligand with concomitant
displacement of a thioether arm (see the SI). These observations
underscore some of the problems associated with stabilizing
dinitrogen complexes using thiolates/thioethers, as thiolates
tend to bridge metal centers and occupy sites that may otherwise
be available for N2 binding while thioethers can be labile in
comparison with phosphines or undergo reductive SꢀC
cleavage.
’ ASSOCIATED CONTENT
S
Supporting Information.
Synthetic and spectroscopic
b
details and data, computational details, solid-state structures, and
crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
Reduction of the solvent adduct 11 using CoCp2 or Cr-
(C6H6)2 proved to be more interesting and led to a mixed-valent
Fe(II)/Fe(I) complex with a bridging dinitrogen ligand,
’ AUTHOR INFORMATION
[{[SiPiPrSAd2]Fe}2(N2)]BArF (14) (Scheme 4).18 The com-
4
bustion analysis data and stoichiometry of the reaction were
consistent with our formulation of 14, the latter requiring 0.5
equiv each of reductant and N2 (Toepler pump analysis) per Fe
center. While crystals suitable for X-ray diffraction invariably led
to disorder/twinning problems arising from crystallization in a
cubic space group, insights into the precise coordination environ-
ment of 14 were gained through its spectroscopic properties.
Complex 14 exhibited a weak IR stretch at 1881 cmꢀ1 that
shifted to 1819 cmꢀ1 when the reaction was performed under
Corresponding Author
jpeters@caltech.edu
’ ACKNOWLEDGMENT
This work was supported by the NIH (GM070757). Charlene
Tsay and Larry Henling are acknowledged for crystallographic
assistance. Prof. George Rossman is acknowledged for use of his
NIR spectrometer. Dr. Dave Harris is acknowledged for useful
discussions of the magnetic data.
15N2 (calcd harmonic oscillator model: 1818 cmꢀ1). This νN
2
stretch was much lower in energy and significantly weaker in
intensity than that for the terminal Fe(I) dinitrogen complex
[SiPiPr3]Fe(N2) (2008 cmꢀ1); the frequency was closer to the
value for the corresponding Fe(0) complex [(SiPiPr3)Fe0(N2)]ꢀ
(1891 cmꢀ1), highlighting the influence of a second metal
center.12d Since a molecule with an inversion center cannot yield
an IR-active N2 stretch, complex 14 must be asymmetric on the
IR time scale, presumably via an asymmetric orientation of the
phosphine/thioether arms (Scheme 4). Additionally, an inter-
valence charge-transfer band, characteristic of a mixed-valent
species, was observed at 1360 nm in the NIR spectrum of 14 in
Et2O, and its assignment was supported by the observation of a
solvent-dependent λmax. The 20 K EPR spectrum of 14 exhibited
features at g = 4.23, 3.98, and 2.02 due to the Kramer’s doublet
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