Coordinatively and electronically unsaturated zwitterionic iron silyl complexes featuring the tripodal phosphine ligand [PhB(CH2PiPr2)3]-

Article abstract of DOI:10.1021/om030568r

A series of 14-electron, coordinatively unsaturated Fe(II) silyl complexes featuring the anionic, tripodal phosphine ligand [PhB(CH₂PⁱPr₂)₃]^- have been prepared and characterized; preliminary reactivity studies indicate that at least one such complex can undergo redox processes to generate isolable Fe(I) species with concomitant loss of the silyl ligand.

Full text of DOI:10.1021/om030568r

Organometallics 2003, 22, 4627-4629
4627
Coor d in a tively a n d Electr on ica lly Un sa tu r a ted
Zw itter ion ic Ir on Silyl Com p lexes F ea tu r in g th e
Tr ip od a l P h osp h in e Liga n d [P h B(CH2P iP r 2)3]-
Laura Turculet, J ay D. Feldman, and T. Don Tilley*
Department of Chemistry, University of California at Berkeley,
Berkeley, California 94720-1460
Received August 7, 2003
Summary: A series of 14-electron, coordinatively unsat-
urated Fe(II) silyl complexes featuring the anionic,
tripodal phosphine ligand [PhB(CH2PiPr2)3]- have been
prepared and characterized; preliminary reactivity stud-
ies indicate that at least one such complex can undergo
redox processes to generate isolable Fe(I) species with
concomitant loss of the silyl ligand.
cantly from these in that it coordinates via three soft
phosphine σ-donor groups. Accordingly, the PhBP3
ligand has enabled the isolation of transition-metal
complexes that have no counterpart in Cp- or Tp-ligated
systems, such as an Ir(III) silylene,5 and terminal Co
and Fe(III) imido complexes.4a,c We anticipate that
replacing the phenyl substituents on the phosphorus
donors with isopropyl groups will lead to significantly
different steric and electronic properties for this ligand
system.
Transition-metal complexes featuring metal-silicon
bonds are key components in a number of catalytic and
stoichiometric chemical transformations.1 The majority
of these silyl derivatives are coordinatively saturated
and are based on “traditional” coordination spheres
featuring cyclopentadienyl (Cp) or carbon monoxide
ancillary ligands. With the goal of developing new
reaction chemistry for metal-silicon-bonded compounds,
we have explored routes to electron-deficient silyl
complexes with unusual geometries and low coordina-
tion numbers.2
Yellow, paramagnetic [PhBP3′]FeBr (3) was obtained
in high yield by the direct reaction of [Li(THF)][[PhBP3′]
(2), prepared by addition of 3 equiv of LiCH2PiPr2 (1) to
dichlorophenylborane (eq 1), with FeBr2(THF)2 (Scheme
In this contribution, we report the synthesis and
preliminary reactivity studies of unusual 14-electron,
four-coordinate Fe(II) silyl complexes featuring the
anionic, tripodal phosphine ligand [PhB(CH2PiPr2)3]-
(abbreviated as PhBP3′).3,4 In previous reports we have
described the synthesis and reactivity of zwitterionic
iridium complexes featuring the diphenylphosphino
analogue of this ligand, [PhB(CH2PPh2)3]- (abbreviated
as PhBP3).5 Although isoelectronic with Cp and tris-
(pyrazolyl)borate (Tp)6 ligands, PhBP3 differs signifi-
1). The bromide complex 3 is readily isolated as a
crystalline solid from diethyl ether solution. The crys-
tallographically determined structure of 3 reveals a
monomeric, four-coordinate complex featuring a pseudo-
tetrahedral geometry at iron (Figure 1). The average
Fe-P bond distance is 2.420 Å, which is comparable to
the average Fe-P distances (2.425 Å) for [PhBP3]FeCl‚
C6H6 and [PhBP3′]FeCl.3,4 The effective magnetic sus-
ceptibility for 3 was measured in benzene-d6 solution
according to the Evans method and was used to deter-
mine an effective magnetic moment of 4.68 µB (300 K),
consistent with a high-spin Fe(II) center.7
(1) (a) Tilley, T. D. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 24, p
1415. (b) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S.,
Rappoport, Z., Eds.; Wiley: New York, 1991; Chapters 9 and 10, pp
245-309. (c) Pannell, K. H.; Sharma, H. Chem. Rev. 1995, 95, 1351.
(d) Eisen, M. S. In The Chemistry of Organic Silicon Compounds;
Apeloig, Y., Rappoport, Z., Eds.; Wiley: New York, 1998; Vol. 2,
Chapter 35, p 2037. (e) Corey, J . Y.; Braddock-Wilking, J . Chem. Rev.
1999, 99, 175.
Surprisingly, the bromide derivative 3 reacted with
KSi(SiMe3)3 in benzene solvent to give an orange,
diamagnetic product (4). An X-ray crystallographic
study of 4 reveals η5 coordination of a silylated benzene
moiety to the iron center (Figure 2) to form a 6-exo-
cyclohexadienyl complex. Complex 4 likely forms via
nucleophilic addition of (Me3Si)3Si- to the cationic 18-
electron species {[PhBP3′]Fe(η6-C6H6)}+, which may
exist in equilibrium with 3 in benzene solution. Signifi-
cant precedent exists for addition of nucleophiles to
analogous CpFe+(η6-arene) cations to form the corre-
sponding 6-exo-cyclohexadienyl complexes.8 However,
(2) (a) Roddick, D. M.; Tilley, T. D.; Rheingold, A. L.; Geib, S. J . J .
Am. Chem. Soc. 1987, 109, 945. (b) Heyn, R. H.; Tilley, T. D. Inorg.
Chim. Acta 2002, 341, 91.
(3) The syntheses of [PhBP3′]Tl and of [PhBP3′]FeCl have just
appeared: Betley, T. A.; Peters, J . C. Inorg. Chem. 2003, 42, 5074.
(4) Four-coordinate Fe and Co complexes featuring the [PhB(CH2-
PPh2)3]- ligand (abbreviated as PhBP3) have been reported by Peters
and co-workers: (a) Brown, S. D.; Betley, T. A.; Peters, J . C. J . Am.
Chem. Soc. 2003, 125, 322. (b) J enkins, D. M.; Di Bilio, A. M.; Allen,
M. J .; Betley, T. A. Peters, J . C. J . Am. Chem. Soc. 2002, 124, 15336.
(c) J enkins, D. M.; Betley, T. A.; Peters, J . C. J . Am. Chem. Soc. 2002,
124, 11238. (d) Shapiro, I. R.; J enkins, D. M.; Thomas, J . C.; Day, M.
W.; Peters, J . C. Chem. Commun. 2001, 2152.
(5) (a) Feldman, J . D.; Peters, J . C.; Tilley, T. D. Organometallics
2002, 21, 4065. (b) Feldman, J . D.; Peters, J . C.; Tilley, T. D.
Organometallics 2002, 21, 4050. (c) Peters, J . C.; Feldman, J . D.; Tilley,
T. D. J . Am. Chem. Soc. 1999, 121, 9871.
1
there was no indication (by H NMR) for the formation
of the diamagnetic {[PhBP3′]Fe(η6-C6H6)}+ cation upon
(7) (a) Evans, D. F. J . Chem. Soc. 1959, 2003. (b) Sur, S. K. J . Magn.
Reson. 1989, 82, 169.
(6) Trofimenko, S. Chem. Rev. 1993, 93, 943.
(8) Astruc, D. Tetrahedron 1983, 39, 4027 and references therein.
10.1021/om030568r CCC: $25.00 © 2003 American Chemical Society
Publication on Web 10/14/2003

4628 Organometallics, Vol. 22, No. 23, 2003
Communications
Sch em e 1
dissolving 3 in benzene-d6. Additionally, no reaction was
observed when 1 equiv of NaBPh4 was added to a
benzene/THF solution of 3.
of paramagnetic Fe silyl complexes are species of the
type {[(Me3Si)3Si]2FeX}- (X ) Cl, OTf), [(Me3Si)3SiFe-
Cl2(pyr)]-, and [(Me3Si)3Si]2FeL (L ) DME, Et2O,
THF).2,9 Thus, the successful isolation of the coordina-
tively unsaturated silyl complex 5 prompted us to
examine syntheses of additional four-coordinate iron
silyl complexes. The reaction of 3 with either Ph3SiLi-
(THF)3 or Mes2SiHLi(THF)2 in pentane led to the
formation of the corresponding silyl (6) and hydrosilyl
(7) complexes (Scheme 1), indicating that silyllithium
reagents can also be utilized for the preparation of
[PhBP3′]Fe-silyl complexes. Compounds 6 and 7 pos-
sess magnetic moments of 5.32 and 4.81 µB, respectively
(Evans method, 300 K).
When the reaction of 3 with KSi(SiMe3)3 was carried
out in pentane solvent, paramagnetic [PhBP3′]FeSi-
(SiMe3)3 (5) was isolated as purple needles in 58% yield
1
(Scheme 1). Complex 5 has been characterized by H
NMR and IR spectroscopy, by its magnetic moment
(4.85 µB, Evans method, 300 K), and by elemental
analysis. The NMR spectrum of 5 in benzene-d6 contains
a broad resonance at 1.45 ppm that can be attributed
to the methyl protons on the Si(SiMe3)3 ligand. The IR
spectrum of 5 reveals absorptions characteristic of the
Si(SiMe3)3 group at 831, 671, and 622 cm-1. No reaction
occurred over the course of 16 h upon exposure of 5 to
benzene at room temperature, suggesting that 5 is not
an intermediate in the formation of the cyclohexadienyl
complex 4.
As previously reported, coordinatively unsaturated
iron silyl complexes form adducts with π-acids such as
CO and xylyl isocyanide.2 Exposure of a red diethyl
ether solution of 7 to an atmosphere of CO resulted in
Paramagnetic silyl complexes are quite rare, and to
our knowledge the only previously reported examples
F igu r e 2. Crystallographically determined structure of 4,
depicted with 50% thermal ellipsoids. All hydrogen atoms
have been omitted for clarity, with the exception of the
hydrogen atoms on the η5-cyclohexadienyl ring. Selected
bond distances (Å) and angles (deg): Fe(1)-P(1), 2.314(1);
Fe(1)-P(2), 2.323(1); Fe(1)-P(3), 2.335(1); C(1)-C(2), 1.509-
(6); C(1)-C(6), 1.521(7); C(2)-C(3), 1.400(6); C(3)-C(4),
1.404(7); C(4)-C(5), 1.406(7); C(5)-C(6), 1.374(7); Si(1)-
C(1), 1.933(5); P(1)-Fe(1)-P(2), 88.15(5); P(1)-Fe(1)-P(3),
88.92(5); P(2)-Fe(1)-P(3), 93.53(5).
F igu r e 1. Crystallographically determined structure of 3,
depicted with 50% thermal ellipsoids. All hydrogen atoms
have been omitted for clarity. Selected bond distances (Å)
and angles (deg): Fe(1)-Br(1), 2.375(1); Fe(1)-P(1),
2.419(2); Fe(1)-P(2), 2.411(2); Fe(1)-P(3), 2.430(2); Br(1)-
Fe(1)-P(1), 120.11(6); Br(1)-Fe(1)-P(2), 123.36(7); Br(1)-
Fe(1)-P(3), 123.52(7); P(1)-Fe(1)-P(2), 94.08(7); P(1)-
Fe(1)-P(3), 93.84(7); P(2)-Fe(1)-P(3), 94.22(7).

Communications
Organometallics, Vol. 22, No. 23, 2003 4629
an immediate color change to green; however, no
crystalline product could be isolated from this reaction
mixture. The room-temperature addition of xylyl iso-
cyanide to a red benzene solution of 7 resulted in a
similar immediate color change to green, followed by a
rapid (less than 1 min) color change to red-brown.
Surprisingly, the isolated product of this reaction is not
an adduct of 7 but, rather, a five-coordinate Fe(I)
complex, [PhBP3′]Fe(CNXyl)2 (8), which has lost the
-SiHMes2 ligand (eq 2). Monitoring the reaction at room
F igu r e 3. Crystallographically determined structure of 8,
depicted with 50% thermal ellipsoids. All hydrogen atoms
have been omitted for clarity. Selected bond distances (Å)
and angles (deg): Fe(1)-P(1), 2.307(1); Fe(1)-P(2),
2.294(1); Fe(1)-P(3), 2.322(1); Fe(1)-C(1), 1.811(4); Fe(1)-
C(2), 1.795(4); P(1)-Fe(1)-P(2), 89.97(4); P(1)-Fe(1)-P(3),
90.34(4); P(2)-Fe(1)-P(3), 92.99(4); C(1)-Fe(1)-C(2),
82.9(2); Fe(1)-C(1)-N(1), 171.4(4); Fe(1)-C(2)-N(2),
174.5(4).
temperature by 1H NMR spectroscopy (benzene-d6)
revealed the presence of a diamagnetic product contain-
ing an SiH resonance at 5.37 ppm (1J SiH ) 98 Hz).
However, thus far we have been unable to conclusively
determine the fate of the -SiHMes2 ligand in this
reaction.
enzyme reactivity, there has been increasing interest
in the chemistry of this unusual oxidation state of iron.11
In summary, we have prepared a new class of four-
coordinate, 14-electron Fe(II) silyl complexes featuring
the tripodal phosphine ligand [PhB(CH2PiPr2)3]-
([PhBP3′]-). These unique complexes are highly reactive,
as indicated by the facile reduction of [PhBP3′]FeSiHMes2
in the presence of xylyl isocyanide to form an isolable
five-coordinate Fe(I) complex. We are presently examin-
ing the chemistry and redox reactivity of these and
related low-coordinate transition-metal silyl complexes
in search of new chemical transformations.
The crystallographically determined structure of 8
(Figure 3) reveals a distorted-square-pyramidal geom-
etry at iron. The magnetic moment determined for 8 is
1.70 µB, which is consistent with a low-spin Fe(I) center.
Although several electronically and coordinatively un-
saturated monovalent iron complexes have been previ-
ously reported, few have been structurally characterized
by X-ray diffraction methods.4a,9a,10 Similar reactivity
has been observed by Parkin and co-workers for a tris-
(pyrazolyl)borate iron methyl complex that reacted with
CO at 120 °C to generate a four-coordinate FeI-CO
complex with loss of the Me ligand.9a Additionally, four-
coordinate [PhBP3]FePPh3 and five-coordinate [PhBP3]-
Fe(CO)2 have been recently reported.4a Given the sus-
pected role of low-coordinate Fe(I) species in metallo-
Ack n ow led gm en t is made to the National Science
Foundation for their generous support of this work. We
thank Dr. Frederick J . Hollander and Dr. Allen G.
Oliver for assistance with the X-ray structure determi-
nations.
Su p p or tin g In for m a tion Ava ila ble: Text giving detailed
experimental procedures and characterization data and tables
of crystal, data collection, and refinement parameters, atomic
coordinates, bond distances, bond angles, and anisotropic
displacement parameters for complexes 3, 4, and 8. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(9) Analogous four-coordinate Fe(II) alkyl complexes supported by
Tp-derived ligands have recently been reported: (a) Kisko, J . L.;
Hascall, T.; Parkin, G. J . Am. Chem. Soc. 1998, 120, 10561. (b) Akita,
M.; Shirasawa, N.; Hikichi, S.; Moro-oka, Y. Chem. Commun. 1998,
973. (c) Shirasawa, N.; Akita, M.; Hikichi, S.; Moro-oka, Y. Chem.
Commun. 1999, 417. (d) Shirasawa, N.; Nguyet, T. T.; Hikichi, S.;
Moro-oka, Y.; Akita, M. Organometallics 2001, 20, 3582.
OM030568R
(11) (a) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J . Am. Chem.
Soc. 1999, 121, 9736. (b) Lyon, E. J .; Georgakaki, I. P.; Reibenspies,
J . H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 1999, 38, 3178.
(10) Lappert, M. F.; MacQuitty, J . J .; Pye, P. L. J . Chem. Soc., Dalton
Trans. 1981, 1583 and references therein.