Dinitrogen complexes supported by tris(phosphino)silyl ligands

Article abstract of DOI:10.1021/ic801855y

The tetradentate tris(phosphino)silyl ligand [SiP^Pr₃] ([SiP'^Pr₃] = [Si(o-C₆H₄P ⁱPr₂)_3]^- has been prepared, and its complexation with iron, cobalt

Full text of DOI:10.1021/ic801855y

Inorg. Chem. 2009, 48, 2507-2517
Dinitrogen Complexes Supported by Tris(phosphino)silyl Ligands
Matthew T. Whited,† Neal P. Mankad, Yunho Lee, Paul F. Oblad,† and Jonas C. Peters*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received September 29, 2008
The tetradentate tris(phosphino)silyl ligand [SiPiPr3] ([SiPiPr3] ) [Si(o-C6H4PiPr2)3]-) has been prepared, and its
complexation with iron, cobalt, nickel, and iridium precursors has been explored. Several coordination complexes
have been thoroughly characterized and are described. These include, for example, the divalent trigonal bipyramidal
metal chlorides [SiPiPr3]M-Cl (M ) Fe, Co, Ni), as well as the monovalent dinitrogen adducts [SiPiPr3]M-N2 (M )
Fe, Co, Ir), which are compared with related [SiPPh3]M-Cl and [SiPPh3]M-N2 species (M ) Fe, Co). Complexes
of this type represent the first examples of terminal dinitrogen adducts of monovalent iron, and the ligand architecture
allows examination of a unique class of dinitrogen adducts with a trans-disposed silyl donor. Oxidation of the
appropriate [SiPR ]M-N2 precursors affords the divalent iron triflate [SiPPh3]Fe(OTf) and trivalent cobalt triflate
3
{[SiPiPr3]Co(OTf)}{OTf} complexes, which are of interest for group transfer studies because of the presence of a
labile triflate ligand. Comparative electrochemical, structural, and spectroscopic data are provided for these
complexes.
I. Introduction
via proton/electron equivalents.2,6,7 In this context it is
interesting to consider terminal FeI-N2 complexes as
synthetic targets. Our group and Holland’s group have
reported the preparation of µ-N2 complexes of three- and
four-coordinate iron in which the iron centers may be
formally regarded as iron(I).5d,f More detailed spectroscopic
studies have suggested that the Holland system, supported
by ꢀ-diketiminato ligands, is perhaps better described as a
diiron(II) species with a reduced dinitrogen dianion.8 In
contrast, our phosphine-supported diiron system appears to
be better described by two S ) 3/2 iron centers that are very
weakly ferromagnetically coupled.6
A great deal of biochemical, theoretical, and synthetic
research has recently been focused on exploring the nature
of the inorganic cofactors of nitrogenase enzymes.1,2 Given
the presence of iron and molybdenum in FeMo-nitrogenase,
particular emphasis has been placed on elucidating the roles
these elements may play in N2 reduction.2,3 Yandulov and
Schrock recently reported a catalytic cycle for N2 reduction
involving a trivalent triamidoamine [NN3]Mo-N2 species
that is thought to shuttle between four different formal
oxidation states.4
We and others have established that, given a suitable set
of supporting ligands, single low-coordinate iron sites can
exhibit the necessary redox flexibility to facilitate multielec-
tron small molecule reductions,5,6 and we have begun to
target a synthetic scheme akin to the Chatt cycle in which a
monovalent iron center binds N2 and mediates its reduction
(5) (a) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc.
2003, 125, 322. (b) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc.
2005, 127, 1913. (c) Brown, S. D.; Mehn, M. P.; Peters, J. C. J. Am.
Chem. Soc. 2005, 127, 13146. (d) Betley, T. A.; Peters, J. C. J. Am.
Chem. Soc. 2003, 125, 10782. (e) Betley, T. A.; Peters, J. C. J. Am.
Chem. Soc. 2004, 126, 6252. (f) Smith, J. M.; Lachicotte, R. J.; Pittard,
K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland,
P. L. J. Am. Chem. Soc. 2001, 123, 9222. (g) Holland, P. L. Can.
J. Chem. 2005, 83, 296. (h) Smith, J. M.; Sadique, A. R.; Cundari,
T. R.; Rodgers, K. R.; Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschen-
riem, C. J.; Vela, J.; Holland, P. L. J. Am. Chem. Soc. 2006, 128,
756.
(6) Hendrich, M. P.; Gunderson, W.; Behan, R. K.; Green, M. T.; Mehn,
M. P.; Betley, T. A.; Lu, C. C.; Peters, J. C. Proc. Natl. Acad. Sci.
U.S.A. 2006, 103, 17107.
(7) MacBeth, C. E.; Harkins, S. B.; Peters, J. C. Can. J. Chem. 2005, 83,
332.
* To whom correspondence should be addressed. E-mail: jcpeters@
mit.edu.
†
California Institute of Technology, Pasadena, CA 91125.
(1) (a) Howard, J. B.; Rees, D. C. Chem. ReV 1996, 96, 2965; Burgess,
B. K., Lowe, D. J. Chem. ReV. 1996, 96, 2983. (b) Chatt, J.; Dilworth,
J. R.; Richards, R. L. Chem. ReV. 1978, 78, 589.
(2) Peters, J. C.; Mehn, M. P. In ActiVation of Small Molecules; Tolman,
W. B., Ed.; Wiley: New York, 2006; pp 81-120.
(3) MacKay, B. A.; Fryzuk, M. D. Chem. ReV. 2004, 104, 385.
(4) (a) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76. (b)
Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6252.
(c) Schrock, R. R. Acc. Chem. Res. 2005, 38, 955.
(8) Stoian, S. A.; Vela, J.; Smith, J. M.; Sadique, A. R.; Holland, P. L.;
Munck, E.; Bominaar, E. L. J. Am. Chem. Soc. 2006, 128, 10181.
10.1021/ic801855y CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 6, 2009 2507
Published on Web 02/11/2009

Whited et al.
Figure 1. Neutral and anionic tetradentate ligands related to the chemistry described herein.
Scheme 1
We have recently begun to consider whether monoanionic
tetradentate ligands might stabilize terminally bonded N2
adducts of monovalent iron. Accordingly, we have explored
two complementary approaches to introduce a single X-type
donor into a phosphine-rich, tetradentate framework (Figure
1).9,10 Recent results have established that tris(phosphino)silyl
ligands (abbreviated in general as [SiPR ]) are promising
3
scaffolds for the examination of N2 chemistry at iron.9b
Ligands of this type, the sole example of which was first
reported by Stobart,11 are also of general interest for catalytic
studies since the presence of a strongly trans-influencing silyl
donor should labilize bound substrates to facilitate turnover.
Herein we discuss the preparation and characterization of
the isopropyl-substituted [SiPiPr3] ligand derivative and probe
aspects of its fundamental coordination chemistry with di-
and trivalent iron, cobalt, nickel, and iridium. We present
complementary strategies for metalation of the silane ligand
and examine the monovalent, terminal dinitrogen complexes
[SiPiPr3]M-N2 (M ) Fe, Co, Ir). Comparisons are made
purification as an orange solid. Quenching this lithio species
with 1/3 equivalent of trichlorosilane in toluene, followed
by heating at 90 °C, afforded the target H[SiPiPr3] ligand (1,
where informative to related [SiPPh3] systems.9b
II. Results and Discussion
1
Scheme 1). Combustion analysis, H and 31P NMR spec-
Synthesis and Characterization of H[SiPiPr3]. (2-Bro-
mophenyl)phosphines serve as excellent precursors for a
variety of phosphine ligands since they are readily lithiated
with a single equivalent of n-butyllithium in diethyl ether at
low temperature.9a,12 Although the (2-bromophenyl)diphen-
troscopy, and an X-ray diffraction (XRD) study confirm the
assignment of the desired silane. The silyl proton is obscured
by aromatic proton resonances in the H NMR spectrum.
1
However, the Si-H vibration (υSiH ) 2218 cm-1) is
prominent in the infrared spectrum and provides a convenient
handle with which to monitor consumption of the silane
ligand during metalation reactions (vide infra). The solid-
state molecular structure of 1 obtained by XRD analysis
(Scheme 1) reveals an arrangement in which the phosphine
arms adopt a propeller structure trisected by the Si-H unit
with the H-atom approximately equidistant from each P atom
(P-HSi(avg.) ) 3.14(2) Å), suggesting a convenient metal
binding site upon substitution of the silyl proton by a
transition metal ion. The P-HSi distances are sufficiently long
that no interaction in the solid state can be inferred.
ylphosphine precursor required for the synthesis of H[SiPPh
]
3
is most easily obtained by a Pd-catalyzed coupling
reaction,9a,13 we were unable to access the diisopropylphos-
phine derivative by such a route, and it was instead
i
synthesized by the addition of PrMgCl to the previously
reported (2-bromophenyl)dichlorophosphine.14 To prepare
the desired silane, 2-(diisopropylphosphino)phenyllithium
was generated at low temperature and isolated without further
(9) (a) Whited, M. T.; Rivard, E.; Peters, J. C. Chem. Commun. 2006,
1613. (b) Aspects of the current work have been previously com-
municated: Mankad, N. P.; Whited, M. T.; Peters, J. C. Angew. Chem.,
Int. Ed. 2007, 46, 5768.
(10) Arnold and coworkers have recently adopted a similar approach in
their synthesis of monoanionic, tetradentate ligands: (a) Chomitz,
W. A.; Arnold, J. Chem. Commun. 2007, 4797. (b) Chomitz, W. A.;
Mickenberg, S. F.; Arnold, J. Inorg. Chem. 2008, 47, 373.
(11) Joslin, F. L.; Stobart, S. R. J. Chem. Soc., Chem. Commun. 1989,
504.
Synthesis and Characterization of Di- and Trivalent
[SiPiPr3]M Complexes. Metalation of polydentate silyl
ligands has often been accomplished via chelate-assisted
Si-H bond oxidative addition at late metal precursors.15 This
method is a straightforward route for installing the [SiPiPr
]
3
ligand on Co, Ni, and Ir precursors, as shown in Scheme 2.
(12) Mankad, N. P.; Rivard, E.; Harkins, S. B.; Peters, J. C. J. Am. Chem.
Soc. 2005, 127, 16032.
(13) Brauer, D. J.; Hingst, M.; Kottsieper, K. W.; Liek, C.; Nickel, T.;
Tepper, M.; Stelzer, O.; Sheldrick, W. S. J. Organomet. Chem. 2002,
645, 14.
(14) Talay, R.; Rehder, D. Zeit. Naturforsch., B: Anorg. Chem. 1981, 36,
451.
(15) (a) Grundy, S. L.; Holmessmith, R. D.; Stobart, S. R.; Williams, M. A.
Inorg. Chem. 1991, 30, 3333. (b) Okazaki, M.; Ohshitanai, S.; Iwata,
M.; Tobita, H.; Ogino, H. Coord. Chem. ReV. 2002, 226, 167. (c)
Gossage, R. A.; McLennan, G. D.; Stobart, S. R. Inorg. Chem. 1996,
35, 1729.
2508 Inorganic Chemistry, Vol. 48, No. 6, 2009

Tris(phosphino)silyl Ligands
octahedral iridium(III) hydrido chloride complexes of chelat-
ing silyl ligands previously reported, in which the weakly
trans-influencing chloride ligand prefers the site opposite the
silyl donor.15c
Installation of the silyl ligand at iron precursors in high
yield proved more challenging. For instance, the reaction of
1 with Fe2Mes4 (Mes ) 2,4,6-trimethylphenyl), which proved
successful in the preparation of [SiPPh3]Fe(Mes),9b gave rise
to an ill-defined reaction mixture from which [SiPiPr3]Fe(Mes)
could not be readily isolated. The reaction of 1 with a single
equivalent of ferrous chloride in THF generated the bis(pho-
sphine) adduct complex (H[SiPiPr3])FeCl2 (5), whose structure
was established by X-ray crystallography (Scheme 3). For
comparison, the phenyl-substituted derivative H[SiPPh3] did
not react with FeCl2 under similar conditions. No dehydro-
halogenation was observed upon exposure of 5 to amine
bases such as Et3N, even at elevated temperatures. To induce
successful activation of the Si-H unit and thus install the
desired Fe-Si bond we explored the generation of a mixed
alkyl-chloride complex, akin to those examined by Kauff-
mann and co-workers,16 hoping that alkane elimination might
drive Fe-Si bond formation. Slow dropwise addition of
MeMgCl to a stirring solution of 5 at -78 °C led to an
immediate color change from pale yellow to bright orange,
characteristic of the trigonal bipyramidal [SiPiPr3]FeCl (6,
Scheme 3). The chloride complex 6 has been prepared in a
one-pot synthesis in respectable yield (43%) first by addition
of H[SiPiPr3] to FeCl2 at room temperature, followed by
cooling and addition of the Grignard reagent. This method
of installing the H[SiPiPr3] ligand appears to be of broad utility
and can also be used for the independent synthesis of
[SiPPh3]FeCl (8), as has been previously noted.9b
Figure 2. Displacement ellipsoid (35%) representation of [SiPiPr3]Ir(H)(Cl)
1
(4) with inset of IR stretch and H NMR signal for the Ir-H. Hydrogen
atoms and a molecule of benzene have been omitted for clarity. Selected
bond distances (Å) and angles (deg): Ir-Si 2.2749(8), Ir-P1 2.3597(8),
Ir-P2 2.3848(8), Ir-P3 2.3151(8), Ir-Cl 2.5433(7), P1-Ir-P2 107.43(3),
P1-Ir-P3 145.05(3), P2-Ir-P3 103.95(3), Si-Ir-Cl 178.46(3).
Scheme 2
For instance, monitoring a slurry of either CoCl2 or NiCl2
The solution magnetic moments and solid-state structural
data (Figure 3) for the [SiPiPr3]M-Cl complexes (Fe, Co,
Ni) indicate that the ligand confers a preference for trigonal
bipyramidal geometries at later first row metals, in contrast
to several previously reported, potentially tetradentate ligands
that show a tendency to dissociate a donor to accommodate
higher spin states with four-coordinate geometries.7,9a,17 The
spin states of the divalent iron (triplet, µeff ) 3.3 µB), cobalt
(doublet, µeff ) 1.8 µB), and nickel (singlet) chloride
complexes are in accord with the expected d-orbital splitting
pattern for trigonal bipyramidal complexes supported by
strong-field donor sets.18
in the presence of the neutral silane 1 and a sacrificial amine
base (e.g., Et3N or Pr2NEt) by IR spectroscopy established
i
the gradual decay of the Si-H vibration over a period of
16 h for CoCl2 at ambient temperature and 12 h for NiCl2 in
refluxing tetrahydrofuran (THF). By this method red
[SiPiPr3]CoCl (2) and pale red [SiPiPr3]NiCl (3) could be
isolated in analytically pure form and high yields (80% and
96%, respectively). The presence of a single resonance (δ
35.6 ppm) in the 31P NMR spectrum of diamagnetic complex
3 suggested a C3-symmetric solution structure, and XRD
analysis confirmed this assignment for both 2 and 3 in the
solid state (Figure 3).
Compared to our series of previously reported 4-coordinate
A potentially useful iridium synthon was prepared by the
addition of 1 to a stirring solution of orange [(cod)IrCl]2 (cod
) 1,4-cyclooctadiene), providing after 12 h the pale yellow
complex [SiPiPr3]Ir(H)(Cl) (4) in 95% yield. A single crystal
of 4 was analyzed by XRD methods. The hydride was not
located in the difference map, but its presence was indicated
by infrared spectroscopy (υIr-H ) 2145 cm-1). The distinctive
tris(phosphino)borate [PhBPR ]M-X complexes (M ) Mn,19
3
Fe,5a,20 Co,21 Ni22), the presence of the silyl donor draws
the coordinated metal ion into the plane of the three
(16) Kauffmann, T.; Laarmann, B.; Menges, D.; Neiteler, G. Chem. Ber.
Recl. 1992, 125, 163.
(17) (a) Sacconi, L.; Divaira, M. Inorg. Chem. 1978, 17, 810. (b) Stoppioni,
P.; Mani, F.; Sacconi, L. Inorg. Chim. Acta 1974, 11, 227.
(18) Cotton, F. A. Chemical Applications of Group Theory, 3rd ed.; Wiley:
New York, 1990.
(19) Lu, C. C.; Peters, J. C. Inorg. Chem. 2006, 45, 8597.
(20) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074.
(21) (a) Shapiro, I. R.; Jenkins, D. M.; Thomas, J. C.; Day, M. W.; Peters,
J. C. Chem. Commun. 2001, 2152. (b) Jenkins, D. M.; Di Bilio, A. J.;
Allen, M. J.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124,
15336. (c) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2005, 127,
7148.
1
hydride signal observed by H NMR at δ -10.83 ppm (dt,
2
2JHP(trans) ) 118 Hz, JHP(cis) ) 24 Hz) reveals coupling to
two inequivalent 31P nuclei, and in combination with XRD
data this suggests a geometry in which the chloride ligand
is situated trans to the silyl donor and the hydride ligand
sits in the plane of the three phosphines, trans to one and
cis to the other two (Figure 2). This complex is similar to
Inorganic Chemistry, Vol. 48, No. 6, 2009 2509

Whited et al.
Figure 3. Displacement ellipsoid (35%) representations of (a) [SiPiPr3]FeCl (6), (b) [SiPiPr3]CoCl (2), and (c) [SiPiPr3]NiCl (3). Solvent molecules and
hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg), for 5: Fe-Si 2.305(1), Fe-P1 2.3627(7), Fe-P2 2.3438(7),
Fe-P3 2.3489(7), Fe-Cl 2.2820(9), P1-Fe-P2 115.67(2), P1-Fe-P3 116.70(2), P2-Fe-P3 118.09(2), Si-Fe-Cl 178.53(2). For 2: Co-Si 2.2262(9),
Co-P1 2.3684(8), Co-P2 2.3080(9), Co-P3 2.2734(8), Co-Cl 2.3091(8), P1-Co-P2 110.16(3), P1-Co-P3 131.26(3), P2-Co-P3 114.07(3), Si-Co-Cl
174.16(3). For 3: Ni-Si 2.2134(9), Ni-P1 2.2980(9), Ni-P2 2.2836(9), Ni-P3 2.2840(9), Ni-Cl 2.3143(9), P1-Ni-P2 117.09(3), P1-Ni-P3 118.66(3),
P2-Ni-P3 120.27(3), Si-Ni-Cl 178.59(3).
Scheme 3
phosphine donors and creates a far more sterically encum-
bered binding site. To illustrate this point, space-filling
models for the hexaisopropyl tris(phosphino)borate and
tris(phosphino)silyl iron(II) chloride complexes [PhBPiPr3]-
FeCl20 and [SiPiPr3]FeCl (6) are provided in Figure 4. The
figure reveals a substantially more protected chloride ligand
for the case of 6.23 A comparison of the solid-state molecular
structures of these complexes reveals slightly shorter Fe-P
bonds for complex 6 (Fe-Pavg ) 2.35 Å) relative to
[PhBPiPr3]FeCl (Fe-Pavg ) 2.43 Å), consistent with the lower
spin state of 6 (doublet versus quartet). Thus, it may be
possible to ascribe the slightly longer Fe-Cl bond in 6 (2.28
Å versus 2.22 Å for [PhBPiPr3]FeCl) at least in part to the
presence of a trans-disposed silyl donor. In any case, the
steric properties of the tris(phosphino)silyl ligands, combined
with the relatively soft nature of the donors, suggest that
monovalent, monomeric dinitrogen adducts might be gener-
instead appears to generate three distinct products, even at
scan rates as fast as 500 mV/s, whose respective oxidations
ally accessible.9b
Electrochemical Characterization of Divalent Iron
and Cobalt Complexes. Comparative cyclic voltammetry
data for the divalent iron complexes [SiPiPr3]FeCl (6) and
[SiPPh3]FeCl (8) are shown in Figure 5 and establish a
reversible FeIII/II redox couple for each complex. The
oxidation of divalent 6 occurs at E1/2 ) -670 mV, a 270
mV cathodic shift relative to the phenyl-substituted derivative
8 (E1/2 ) -400 mV). This difference was anticipated because
of the greater electron-releasing character of alkyl versus aryl
phosphines, and a similar magnitude shift has been observed
for the FeII/I couple upon substitution of isopropyl for phenyl
substituents in tris(phosphino)borate systems.20
can be seen at -2.40, -2.14, and -1.04 V on the return
scan (Figure 5a). To shed light on the identities of these
species, complex 6 was subjected to electrochemical inter-
rogation in the absence of N2 (Figure 5b), and under these
conditions both the FeIII/II and FeII/I couples were found to
be reversible and no other species were observed. In light
of our previously reported electrochemical results,9b we
assign the anodic wave observed -2.40 V to the reoxidation
of {[SiPiPr3]FeCl}- and the wave at -1.04 V to the oxidation
Complexes 6 and 8 display quite different behavior in their
cyclic voltammograms when scanned cathodically under N2.
Whereas phenyl-substituted complex 8 displays a reversible
FeII/I redox couple (E1/2 ) -2.10 V; Figure 5c), the reduction
of complex 6 at Emax ) -2.55 V is not fully reversible and
(22) MacBeth, C. E.; Thomas, J. C.; Betley, T. A.; Peters, J. C. Inorg.
Chem. 2004, 43, 4645.
Figure 4. Space-filling models of [PhBPiPr3]FeCl (left) and [SiPiPr3]FeCl
(6; right); [PhBPiPr3] ) [PhB(CH2PiPr2)3]-.
(23) We have previously provided a similar comparison of the [PhBP3]
and [SiPPh3] ligands (ref 9b).
2510 Inorganic Chemistry, Vol. 48, No. 6, 2009

Tris(phosphino)silyl Ligands
Figure 5. Cyclic voltammograms of (a) [SiPiPr3]FeCl (6) under N2, (b) [SiPiPr3]FeCl (6) under Ar, and (c) [SiPPh3]FeCl (8) under N2 (* denotes a trace
impurity in 8). Scans (a) and (b) were recorded at a scan rate of 500 mV/s, and scan (c) was recorded at 100 mV/s. All data were acquired in THF and
potentials are referenced to Fc/Fc+.
of monovalent [SiPiPr3]Fe(N2) (10) to its cation, {[SiPiPr3]-
Fe(N2)}+. These results also indicate that the species which
is oxidized at -2.14 V is generated only in the presence of
N2. Thus, since the position of the anodic wave is consistent
with a solvento [SiPiPr3]Fe(THF) or a chloride-dissociated
“[SiPiPr3]Fe” complex,24 the fact that the species is only
observed under N2 may indicate that chloride loss from
{[SiPiPr3]FeCl}- occurs by an associative, N2-assisted mech-
anism. Since this hypothesis would invoke an unstable 19-
electron intermediate complex, further studies will be
required to definitively assign this species.
electrochemical reduction of 9 in the absence of N2 is fully
reversible, and the oxidation wave ascribed to the [SiPPh3]-
Co(N2) species is no longer present. Interestingly, neither 2
nor 9 displays electrochemical evidence for the generation
of an alternative species such as that observed for the
[SiPiPr3]Fe system.
As with all of the iron and cobalt complexes described,
cyclic voltammetry of [SiPiPr3]NiCl (3) revealed a reversible
NiIII/II redox couple (E1/2 ) -245 mV), indicating that
chemical oxidation of this complex should be relatively
facile. However, as expected for an 18-electron complex, 3
was not well-behaved upon electrochemical reduction.
Chemical Reduction to Generate Monovalent [SiPiPr3]-
M(N2) (M ) Fe, Co, Ir). At the outset of our work, it was
not clear whether a strongly trans-influencing silyl ligand
would support a coordinated dinitrogen ligand in a position
trans to the silyl linkage. Indeed, there have been few
examples of M-N2 complexes that also feature a supporting
silyl ligand.25 None of these contains an N2 ligand and silyl
donor in a trans orientation.
Figure 6 shows related voltammetry data for the
i
[SiPR ]CoCl derivatives (R ) Pr (2) or Ph (9)), where 9
3
can be prepared in a fashion analogous to 2. As with the
iron systems, both complexes exhibit reversible CoIII/II redox
couples with the half-wave potential for 2 (E1/2 ) -710 mV)
being about 260 mV negative of that for 9 (E1/2 ) -450
mV). Additionally, while 9 exhibits a quasi-reversible CoII/I
event, the electrochemical reduction of 2 is completely
irreversible. Both are connected with reoxidation events
slightly negative of the III/II couple, consistent with the
generation of a new Co(I) species by halide expulsion. By
(24) A similarly dramatic shift in redox potential upon binding of N2 at an
available Mo(III) coordination site has been described by Peters et al.
Peters, J. C.; Cherry, J. P. F.; Thomas, J. C.; Baraldo, L.; Mindiola,
D. J.; Davis, W. M.; Cummins, C. C. J. Am. Chem. Soc. 1999, 121,
10053.
analogy with 6, we assign the new waves to [SiPR ]Co(N2)
3
complexes in each case. This assignment is supported by
the observation that, as with complex 6 (vide supra),
Inorganic Chemistry, Vol. 48, No. 6, 2009 2511

Whited et al.
Figure 6. Cyclic voltammograms of (a) [SiPiPr3]CoCl (2) and (b) [SiPPh3]CoCl (9) recorded at a scan rate of 100 mV/s (* denotes a trace impurity in 2).
Both scans were recorded in THF under an N2 atmosphere. Potentials are referenced to Fc/Fc+.
Figure 7. Displacement ellipsoid (35%) representations of (a) [SiPiPr3]Fe(N2) (10),26 (b) [SiPiPr3]Co(N2) (12), and (c) [SiPiPr3]Ir(N2) (14). Hydrogen atoms
and solvent molecules have been omitted for clarity. Selected bond distances (Å) and angles (deg), for 10: Fe-Si 2.2918(6), Fe-P1 2.3174(5), Fe-P2
2.2954(5), Fe-P3 2.3078(5), Fe-N1 1.817(4), N1-N2 1.065(5), P1-Fe-P2 115.14(1), P1-Fe-P3 119.18(2), P2-Fe-P3 117.82(2), Si-Fe-N1 178.83(6).
For 12: Co-Si 2.2327(6), Co-P1 2.2376(6), Co-P2 2.2277(6), Co-P3 2.2342(6), Co-N1 1.813(2), N1-N2 1.123(3), P1-Co-P2 117.48(2), P1-Co-P3
118.01(2), P2-Co-P3 119.51(2), Si-Co-N1 179.13(6). For 14: Ir-Si 2.304(2), Ir-P1 2.336(2), Ir-P2 2.328(2), Ir-P3 2.315(2), Ir-N1 2.033(4), N1-N2
1.094(4), P1-Ir-P2 116.00(4), P1-Ir-P3 121.61(6), P2-Ir-P3 117.31(5), Si-Ir-N1 177.05(9).
Exposure of [SiPiPr3]FeCl (6) to an equivalent of sodium
naphthalide results in an immediate darkening of the bright
orange solution and the appearance of a prominent infrared
stretch (υNN ) 2008 cm-1) consistent with a terminal
dinitrogen ligand. NMR and XRD analyses reveal the desired
S ) 1/2 terminal N2 complex, [SiPiPr3]Fe(N2) (10) (Figure
7).26 The hexaphenyl complex 8 can be reduced in the same
fashion, though the reaction is effected by a weaker reductant
(Na/Hg amalgam), consistent with the cyclic voltammetry
data. The resulting N2 complex, [SiPPh3]Fe(N2) (11), reveals
an intense infrared stretch at higher energy (υNN ) 2041
cm-1), indicating that the N2 ligand is coordinated to a
substantially less electron-rich metal center in this complex.
Complexes 10 and 11 are the first examples of d7 iron
complexes containing terminally bonded dinitrogen ligands.
Thesynthesisandstructureof11wasrecentlycommunicated.9b
The cobalt complexes [SiPiPr3]CoCl (2) and [SiPPh3]CoCl
(9) are reduced by Na/Hg amalgam in a similar fashion to
afford the diamagnetic d8 dinitrogen complexes [SiPiPr3]-
Co(N2) (12) and [SiPPh3]Co(N2) (13). Again, the N2 stretching
frequencies provide a quantitative measure of the relative
electron-releasing properties of each ligand, as a 32 cm-1
difference is observed between 12 (υNN ) 2063 cm-1) and
13 (υNN ) 2095 cm-1).
(25) (a) Dioumaev, V. K.; Plossl, K.; Carroll, P. J.; Berry, D. H. J. Am.
Chem. Soc. 1999, 121, 8391. (b) Yoo, H.; Carroll, P. J.; Berry, D. H.
J. Am. Chem. Soc. 2006, 128, 6038. (c) Trovitch, R. J.; Lobkovsky,
E.; Chirik, P. J. Inorg. Chem. 2006, 45, 7252. (d) Gusev, D. G.;
Fontaine, F. G.; Lough, A. J.; Zargarian, D. Angew. Chem., Int. Ed.
2003, 42, 216.
Since divalent iridium halide complexes of [SiPiPr3] were
not readily available, a somewhat different synthetic strategy
was required to access the IrI-N2 complex. In a reaction
reminiscent of that utilized by Stobart and co-workers to
(26) Compound 10 co-crystallized with a small amount of [SiPiPr3]FeCl
(6) (refined to 4% occupancy). In Figure 8, the chloride ligand
(which was refined isotropically because of low occupancy) has
been omitted.
2512 Inorganic Chemistry, Vol. 48, No. 6, 2009

Tris(phosphino)silyl Ligands
Scheme 4
Displacement of the N2 ligand by CO addition was
examined for the case of [SiPiPr3]Co(N2) (12). Thus, exposure
of 12 to an atmosphere of carbon monoxide resulted in the
rapid and quantitative consumption of the N2 complex with
the appearance of a single new species, [SiPiPr3]Co(CO) (15),
characterized by 31P NMR (δ 78.2 ppm), IR spectroscopy
(υCO ) 1896 cm-1), and XRD analysis (Figure 8). Addition-
ally, oxidation of 12 with silver triflate (either 1 or 2 equiv)
effected N2 loss as evident by infrared spectroscopy and the
concomitant formation of either the Co(II) or Co(III) triflato
complexes. The cobalt(III) complex {[SiPiPr3]Co(OTf)}{OTf}
(16) was thoroughly characterized, and XRD analysis (Figure
8) revealed a trigonal bipyramidal complex with one
coordinated (κ1) and one uncoordinated triflate anion. The
reluctance of the second triflate ligand to bind, which would
afford an octahedral, 18-electron complex, can likely be
ascribed to the steric bulk enshrouding the cobalt ion. The
utility of this complex as a Co(III) synthon is currently under
exploration.
prepare the tris(phosphinoalkyl)silyl rhodium monocarbonyl
((Ph2PCH2CH2)3Si)Rh(CO),11 [SiPiPr3]Ir(H)(Cl) (4) was found
to react with 1 equiv of methyl Grignard under a dinitrogen
atmosphere to afford the diamagnetic N2 adduct [SiPiPr3]-
Ir(N2) (14) (Scheme 4). The presence of 14 was indicated
by a single resonance in the 31P NMR spectrum (δ 38.7 ppm)
and an intense N2 infrared stretch (υNN ) 2122 cm-1). Its
identity was confirmed by XRD analysis, revealing a
structurally unusual example of a trigonal bipyramidal
dinitrogen adduct of iridium(I) (Figure 7).27
Table 1 summarizes some pertinent parameters for the N2
adducts 10-14 reported here and related literature compounds.
Having prepared the desired dinitrogen complexes of iron,
cobalt, and iridium, we briefly canvassed the lability of the
N2 unit. Early evidence that the trans silyl donor would serve
to labilize the bound dinitrogen ligand came from the results
of combustion analysis data, which consistently indicated
lower levels of nitrogen than expected for the empirical
formulas of 10-14 (between 25% and 50% of the values
predicted for the monomeric N2 complexes). This lability is
particularly evident for [SiPiPr3]Ir(N2) (14), where N2 loss is
quickly apparent upon application of vacuum by a color
change of the solution from light yellow to dark green. This
change in color is reversed upon reintroduction of nitrogen,
both in solution and in the solid state. While it is possible
that application of vacuum results in the formation of the
µ-N2 dimer species {[SiPiPr3]Ir}2(µ-N2), similar to complexes
known for rhodium and iridium pincer systems,28 the
sterically crowded environment of the binding site in the
resulting species would likely prevent dimerization. Instead
we suspect that the favored species is a four-coordinate,
trigonal pyramidal iridium(I) complex. No direct character-
ization data is yet in hand, however.
Related chemistry is also prevalent for the iron system.
For instance, we have communicated that [SiPPh3]Fe(N2)
reacts with CO rapidly to afford the terminal carbonyl
[SiPPh3]Fe(CO) (νCO ) 1881 cm-1).9b [SiPPh3]Fe(N2) can also
be oxidized by AgOTf to provide the divalent, structurally
characterized triflato species [SiPPh3]Fe(OTf) (17) with
release of N2 (Figure 8).
III. Conclusions
We have described synthetic methods and characterization
data for a family of complexes supported by tetradentate
tris(phosphino)silyl ligands. In particular, we have introduced
the newly prepared hexaisopropyl [SiPiPr3] ligand and
characterized several coordination complexes derived from
this scaffold. The collective spectroscopic and electrochemi-
cal data available indicate that, as expected, [SiPiPr3] is
considerably more electron-rich than the related hexaphenyl
[SiPPh3] derivative, whose synthesis was recently com-
municated.9b Importantly, complementary strategies for the
installation of the M-Si linkage on later transition metals
have been developed, and these strategies provide access to
[SiPR ] complexes of iron, cobalt, nickel, and iridium.
3
Reduction of the di- and trivalent [SiPR ]M halide complexes
3
also provides access to unusual trigonal bipyramidal dini-
trogen adducts of monovalent iron, cobalt, and iridium, and
in all three cases the N2 ligand appears to be labilized by a
strongly trans-influencing silyl donor. The propensity of the
(27) Collman, J. P.; Kubota, M.; Vastine, F. D.; Sun, J. Y.; Kang, J. W.
J. Am. Chem. Soc. 1968, 90, 5430.
[SiPR ] ligand to give rise to a single and labile reaction site
3
(28) (a) Cohen, R.; Rybtchinski, B.; Gandelman, M.; Rozenberg, H.; Martin,
J. M. L.; Milstein, D. J. Am. Chem. Soc. 2003, 125, 6532. (b) van der
Boom, M. E.; Liou, S. Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein,
D. J. Am. Chem. Soc. 1998, 120, 6531. (c) Ghosh, R.; Kanzelberger,
M.; Emge, T. J.; Hall, G. S.; Goldman, A. S. Organometallics 2006,
25, 5668.
(29) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Sur, S. K. J. Magn.
Reson. 1989, 82, 169.
(30) George, T. A.; Rose, D. J.; Chang, Y. D.; Chen, Q.; Zubieta, J. Inorg.
Chem. 1995, 34, 1295.
is one we hope to exploit in the context of future reactivity
studies. Specifically, it will be of interest to examine group
transfer reactions to both the mid- and low-valent synthons
described.
Experimental Section
All manipulations were carried out using standard Schlenk or
glovebox techniques under a dinitrogen atmostphere. Unless
otherwise noted, solvents were deoxygenated and dried by thorough
sparging with N2 gas followed by passage through an activated
alumina column. Non-halogenated solvents were typically tested
(31) Bianchini, C.; Mealli, C.; Meli, A.; Peruzzini, M.; Zanobini, F. J. Am.
Chem. Soc. 1988, 110, 8725.
(32) Lee, D. W.; Kaska, W. C.; Jensen, C. M. Organometallics 1998,
17, 1.
(33) Gutierrez-Puebla, E.; Monge, A.; Nicasio, M. C.; Perez, P. J.; Poveda,
M. L.; Carmona, E. Chem.sEur. J. 1998, 4, 2225.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2513

Whited et al.
Figure 8. Displacement ellipsoid (35%) representations of [SiPiPr3]Co(CO) (15), {[SiPiPr3]Co(OTf)}{OTf} (16), and [SiPPh3]Fe(OTf) (17). All hydrogen
atoms and co-solvent molecules omitted for clarity. See combined cif file (Supporting Information) for details.
Table 1. Representative Dinitrogen Stretching Frequencies for
The crystals were mounted on a glass fiber with Paratone N oil.
Structures were determined using direct methods with standard
Fourier techniques using the Bruker AXS software package. In some
cases, Patterson maps were used in place of the direct methods
procedure. Table 2 contains the XRD experimental details and CIF
files are provided in the Supporting Information.
Complexes of Fe, Co, and Ir
entry
complex
υNN (cm-1
)
reference
1
2
3
4
5
6
7
8
[SiPPh3]Fe(N2) (11)
[SiPiPr3]Fe(N2) (10)
[SiPPh3]Co(N2) (13)
[SiPiPr3]Co(N2) (12)
[SiPiPr3]Ir(N2) (14)
[NPPh3]Fe(N2)
2041
2008
2095
2063
2122
1967
2090
2090
2128
2190
2078
2190
this work
this work
this work
this work
Tris(o-(diisopropylphosphino)phenyl)silane (H[SiPiPr3], 1). (2-
Bromophenyl)diisopropylphosphine (4.1429 g, 15.164 mmol) was
dissolved in 100 mL of diethyl ether and chilled to -78 °C, and
n-butyllithium (1.60 M in hexanes, 9.50 mL, 15.2 mmol) was added
dropwise, causing a darkening of the mixture and gradual precipita-
tion of solids. After 15 min, the slurry was brought to room
temperature and stirred for 2 h, after which volatiles were removed
in vacuo to yield a pale red powder. The powder was redissolved
in toluene (80 mL), chilled to -35 °C, and trichlorosilane (511
µL, 5.06 mmol) was added in one portion, resulting in the
immediate precipitation of white solids and lightening of the
solution. The mixture was stirred at room temperature for 30 min,
then at 90 °C for 15 h, and filtered through Celite to give a clear,
light orange solution. Solvents were removed in vacuo to give an
orange oil, and the addition of petroleum ether (20 mL) caused the
precipitation of white solids. These were isolated on a frit and
washed with petroleum ether (2 × 5 mL) to yield 1 as a white
powder. Subsequent crops could be isolated by crystallization from
concentrated petroleum ether solutions at -35 °C (0.5439 g, 18%).
1H NMR (C6D6): δ 7.46-7.34 (m, 3H, Ar-H). 7.30-7.20 (m, 3H,
Ar-H), 7.20-7.10 (td, J ) 1.2 and 7.2 Hz, 3H, Ar-H), 7.02-6.92
(tt, J ) 1.5 and 7.5 Hz, 3H, Ar-H), 2.05-1.85 (doublet of septets,
this work
30
28
7
31
27
32
33
{[NPPh3]Fe(H)(N2)}+
{[NPiPr3]Fe(H)(N2)}+
{[PP3]Co(N2)}+
9
10
11
12
IrCl(N2)(C8H12O4)(PPh3)2
[PCP]Ir(N2)
[Tp*]Ir(Ph)2(N2)
a [NPR3] ) N(CH2CH2PR2)3. b [PP3] ) P(CH2CH2PPh2)3. c [PCP] )
C6H3-2,6-(CH2PtBu2)2. d [Tp*] ) tris(3,5-dimethylpyrazol)yl borate.
with a standard purple solution of sodium benzophenone ketyl in
tetrahydrofuran to confirm effective oxygen and moisture removal.
(2-bromophenyl)diisopropylphosphine12 and (2-bromophenyl)di-
phenylphosphine9a were prepared according to literature procedures.
All other reagents were purchased from commercial vendors and
used without further purification unless otherwise noted. Deuterated
solvents were purchased from Cambridge Isotopes Laboratories,
Inc. and were degassed and stored over activated 3 Å molecular
sieves prior to use.
Physical Methods. Elemental analyses were performed by Desert
Analytics, Tucson, AZ. A Varian Mercury-300 spectrometer was
1
used to record H, 31P{1H}, and 13C{1H} spectra at room temper-
ature unless otherwise noted. 1H and 13C{1H} chemical shifts were
referenced to residual solvent. 31P{1H} NMR are reported relative
to an external standard of 85% H3PO4 (0 ppm). Abbreviations for
reported signal multiplicities are as follows: s, singlet; d, doublet;
t, triplet; q, quartet; m, multiplet; br, broad. IR spectra were recorded
on a Bio-Rad Excalibur FTS 3000 spectrometer controlled by Win-
IR Pro software using a KBr solution cell. Solution magnetic
moments were measured at 298 K following the Evans method.29
UV-vis measurements were recorded on a Varian Cary 50 Bio
Spectrophotometer controlled by Cary WinUV Software. All
measurements were recorded using a quartz cell fitted with a Teflon
cap. XRD studies were carried out in the Beckman Institute
Crystallographic Facility on a Bruker Smart 1000 CCD diffracto-
meter. Electrochemical analysis was performed on a CHI 600B
Potentiostat/Galvanostat using a glassy carbon working electrode,
a platinum wire auxiliary electrode, and an Ag/AgNO3 non-aqueous
reference electrode filled with THF and Bu4NPF6.
3
2JHP ) 6.9 Hz, JHH ) 3.0 Hz, 6H, -CH(CH3)2), 1.20-1.02 (m,
18H, -CH(CH3)2), 1.00-0.88 (m, 18H, -CH(CH3)2). 31P NMR
(C6D6): δ 1.7 (s). IR (THF, KBr, cm-1) υ(Si-H): 2218. Anal. Calcd
for C36H55P3Si: C, 71.02; H, 9.11. Found: C, 71.09; H, 9.38.
[SiPiPr3]CoCl (2). H[SiPiPr3] (102.6 mg, 0.1685 mmol) and
triethylamine (100 µL, 1.35 mmol) were dissolved in THF (5 mL)
and added by pipet to a stirring solution of cobaltous chloride (21.9
mg, 0.169 mmol) in THF (10 mL), causing an immediate color
change from light blue to dark green. Over a period of 30 min, the
solution gradually adopted a maroon hue and was allowed to stir
12 h. Volatiles were removed in vacuo, and the residues extracted
into diethyl ether (15 mL), filtered, and dried to an analytically
pure red powder (95.2 mg, 80%). Crystals suitable for XRD were
grown by slow evaporation of benzene from a concentrated solution.
1H NMR (C6D6): δ 8.7, 8.0, 7.8, 7.4, 7.3, 6.7, 2.8, 2.6, 2.3, 1.6,
X-ray Crystallography Procedures. X-ray quality crystals were
grown as indicated in the experimental procedures for each complex.
2514 Inorganic Chemistry, Vol. 48, No. 6, 2009

Tris(phosphino)silyl Ligands
1.4, 1.2, 1.0, 0.6. UV-vis (THF) λmax, nm (ε, M-1 cm-1): 508
(1100), 420 (1700). µeff (C6D6): 1.8 µB. Anal. Calcd for
C36H54ClCoP3Si: C, 61.58; H, 7.75. Found: C, 61.29; H, 7.61.
[SiPiPr3]NiCl (3). To a stirring slurry of nickel(II) chloride in
THF (5 mL) was added a solution of H[SiPiPr3] (34.5 mg, 0.0567
mmol) in THF (3 mL). Triethylamine (ca. 200 µL) was added and
the solution was heated at reflux for 12 h with stirring, causing a
color change to bright red. Volatiles were removed in Vacuo and
the residues extracted into benzene (5 mL), filtered through Celite
and lyophilized to yield 3 as a pale red powder (38.0 mg, 96%).
Crystals suitable for XRD were grown by slow evaporation of
benzene from a concentrated solution. 1H NMR (C6D6): δ 7.78-7.70
(m, 3H, Ar-H), 7.30-7.24 (m, 6H, Ar-H), 7.15-7.00 (m, 3H,
Ar-H), 2.70-2.55 (m, 6H, -CH(CH3)2), 1.35-1.20 (m, 18H,
-CH(CH3)2), 1.00-0.80 (m, 18H, -CH(CH3)2). 31P NMR (C6D6):
δ 35.6 (s). UV-vis (THF) λmax, nm (ε, M-1 cm-1): 482 (1500),
349 (2000). Anal. Calcd for C36H54ClNiP3Si: C, 61.60; H, 7.75.
Found: C, 61.55; H, 7.73.
[SiPiPr3]Ir(H)(Cl) (4). To a stirring solution of [(cod)IrCl]2
(60.1 mg, 0.0895 mmol) was added a solution of H[SiPiPr3] (108.8
mg, 0.1787 mmol) in THF (3 mL), causing an immediate color
change from orange to yellow. After 12 h, volatiles were
removed in vacuo, and the residues dissolved in benzene (10
mL), filtered through Celite, and lyophilized to yield 4 as a pale
yellow powder (142.4 mg, 95%). Analytically pure sample was
obtained as colorless crystals by vapor diffusion of petroleum
ether into a concentrated benzene solution, and analysis is
1
reported for 4 · (C6H6). H NMR (C6D6): δ 8.10 (d, J ) 7.2 Hz,
2H, Ar-H), 7.92 (d, J ) 6.9 Hz, 1H, Ar-H), 7.38-7.28 (m,
2H, Ar-H), 7.26-7.20 (m, 1H, Ar-H), 7.14-6.94 (m, 6H,
Ar-H), 3.00-2.85 (m, 2H, -CH(CH3)2), 2.80-2.65 (m, 2H,
-CH(CH3)2), 2.55-2.40 (m, 2H, -CH(CH3)2), 1.90-1.75 (m,
6H, -CH(CH3)2), 1.45-1.25 (m, 12H, -CH(CH3)2), 1.05-0.90
(m, 12H, -CH(CH3)2), 0.60-0.45 (m, 6H, -CH(CH3)2), -10.83
2
2
(dt, JHP(trans) ) 118 Hz, JHP(cis) ) 24 Hz, 1H, Ir-H). 31P NMR
2
(THF): δ 28.9 (br s, 1P, P-Ir-Htrans), 25.5 (d, JHP ) 100 Hz,
2P, P-Ir-Hcis). UV-vis (THF) λmax, nm (ε, M-1 cm-1): 495
(18), 428 (48), 367 (120). IR (THF, KBr, cm-1) υ(Ir-H): 2145.
Anal. Calcd for C42H61ClP3IrSi: C, 55.15; H, 6.72. Found: C,
55.11; H, 6.66.
[SiPiPr3]FeCl (6). To a stirring slurry of ferrous chloride (30.6
mg, 0.241 mmol) in THF (10 mL) was added a solution of H[SiPiPr
]
3
(147.2 mg, 0.2418 mmol) in THF (5 mL), causing a color change
to yellow as complex 5 was generated (n.b. Crystalline samples of
5 could be isolated at this point by removal of solvent and
crystallization from benzene). The resulting solution was chilled
to -78 °C, and MeMgCl (3.0 M in THF, 81 µL, 0.24 mmol) was
diluted in THF (1 mL) and added dropwise, causing an immediate
darkening of the solution. The reaction was stirred at -78 °C for
1 h, then warmed to room temperature and stirred 3 h to give a
dark orange solution. The solution was filtered through Celite and
concentrated to an orange film in vacuo. The residues were extracted
into benzene, filtered, lyophilized, and washed with petroleum ether
(3 × 3 mL) to yield 6 as an orange powder (72.5 mg, 43%). Crystals
suitable for XRD were obtained by slow evaporation of benzene
from a concentrated solution. 1H NMR (C6D6): δ 7.3, 5.4, 2.6, -2.7
ppm. UV-vis (THF) λmax, nm (ε, M-1 cm-1): 471 (990), 379
(2100). µeff (C6D6): 3.3 µB. Anal. Calcd for C36H54ClFeP3Si: C,
61.85; H, 7.79. Found: C, 62.34; H, 7.96.
Tris(o-(diphenylphosphino)phenyl)silane (H[SiPPh3], 7). (2-
Bromophenyl)diphenylphosphine (10.24 g, 30.0 mmol) was dis-
solved in diethyl ether (150 mL) and cooled to -78 °C. n-Butyl-
lithium (1.60 M in hexanes,18.8 mL, 30.0 mmol) was added slowly,
Inorganic Chemistry, Vol. 48, No. 6, 2009 2515

Whited et al.
giving a light orange solution with a tan-colored precipitate. This
mixture was allowed to warm gradually to room temperature and
then stirred for 1 h, after which the volatiles were removed in vacuo.
Toluene (150 mL) was added, and the cloudy orange solution was
cooled back to -78 °C. Trichlorosilane (1.01 mL, 10.0 mmol) was
added in one portion, and the resulting mixture was warmed to
room temperature gradually. After stirring for 30 min, the reaction
was heated in a sealed reaction bomb to 110 °C for 15 h. The
resulting yellow solution and white precipitate were cooled to room
temperature and filtered through Celite, and the filtrate was
concentrated to white solids. Petroleum ether (100 mL) was added,
and the resulting mixture was stirred vigorously for 20 min, at which
point tan solids were collected on a sintered glass frit and washed
with additional petroleum ether (2 × 30 mL) to afford 7 as a fine
an orange solution of 6 (46.8 mg, 0.0669 mmol) in THF (5 mL),
causing the color of the solution to change to dark orange over
a period of several minutes. The reaction was allowed to proceed
overnight, filtered, and volatiles removed in vacuo to give an
orange-red film. The residues were extracted into benzene (5
mL), filtered, and dried. The residues were triturated with
petroleum ether (1 × 5 mL) to give a red powder that was
washed with petroleum ether (2 × 3 mL) to yield spectroscopi-
cally pure 10 (10.5 mg, 23%). Crystals suitable for XRD were
obtained by slow evaporation of benzene from a concentrated
1
solution. H NMR (C6D6): δ 10, 7.1, 7.0, 3.8, 2.1, 1.1 ppm. µeff
(C6D6): 2.2 µB. IR (THF, KBr, cm-1) υ(N2): 2008. UV-vis
(THF) λmax, nm (ε, M-1 cm-1): 468 (1800), 380 (3500).
[SiPPh3]Fe(N2) (11). Sodium (8.3 mg, 0.36 mmol) and mercury
(0.714 g) were combined in THF (1 mL). Solid 8 (0.322 g, 0.357
mmol) was added, and the total volume was brought up to 15
mL. After vigorous stirring for 6 h at room temperature, a brown
supernatant was decanted off the Na/Hg amalgam and concen-
trated in vacuo to brown solids. Benzene (10 mL) was added,
and the resulting cloudy solution was filtered through Celite.
The resulting red-orange filtrate was lyophilized, providing
spectroscopically pure 11 as a fluffy red-orange solid (0.278 g,
87%). Crystals suitable for XRD were obtained by slow diffusion
of petroleum ether vapors into a THF solution. 1H NMR (C6D6):
δ 10.48, 7.98, 7.42, 6.17, 4.46, -1.85 (br), -1.86. µeff (C6D6):
1.8 µB. UV-vis (toluene) λmax, nm (ε, M-1 cm-1): 347 (9400).
IR (KBr, cm-1) υ(N2): 2041.
1
tan powder (6.15 g, 76%). H NMR (C6D6): δ 7.63 (dm, J ) 1.5
and 6.3 Hz, 3H), 7.34 (ddm, J ) 1.0, 3.9, and 7.8 Hz, 3H),
7.25-7.20 (m, 12H), 7.05 (td, J ) 1.5 and 7.3 Hz, 6H), 7.02-6.95
(m, 19H). 13C NMR (C6D6): δ 145.5 (d, J ) 11.4 Hz), 144.3 (t, J
) 4.0 Hz), 144.0 (t, J ) 4.0 Hz), 138.8 (d, J ) 14.6 Hz), 138.5 (d,
J ) 12.8 Hz), 134.7, 134.5 (d, J ) 19.2 Hz), 130.4, 128.8, 128.6
(d, J ) 17.3 Hz). 31P NMR (C6D6): δ -10.4 (s). IR (KBr, cm-1
)
υ(Si-H): 2170. Anal. Calcd for C54H43P3Si: C, 79.78; H, 5.33.
Found: C, 79.39; H, 5.61.
[SiPPh3]FeCl (8). H[SiPPh3] (2.19 g, 2.69 mmol) and FeCl2 (0.341
g, 2.69 mmol) were combined in THF (50 mL) and cooled to -78
°C. n-Butyllithium (1.60 M in hexanes, 1.68 mL, 2.69 mmol) was
added slowly, resulting in an immediate color change to dark red.
The mixture was allowed to warm to room temperature, and after
stirring for 2 h was concentrated to oily red solids. Benzene (50
mL) was added, and the resulting solution was filtered through
Celite and concentrated. Diethyl ether (40 mL) was added, and the
mixture was stirred vigorously, yielding an orange precipitate and
a red supernatant. The orange solids were collected on a sintered
glass frit and washed with additional diethyl ether portions, yielding
pure 8 as a light orange powder (1.28 g, 53%). Crystals suitable
for XRD were obtained by slow diffusion of petroleum ether vapors
[SiPiPr3]Co(N2) (12). A red solution of 2 (154.3 mg, 0.2197
mmol) in THF (10 mL) was added onto a 0.27 weight % Na/Hg
amalgam (0.0057 g, 0.25 mmol sodium dissolved in 2.0875 g of
mercury) with stirring. Over a period of 2 h, the color of the solution
changed from red to golden. After 12 h, the solution was filtered
to remove insoluble residues and volatiles were removed in vacuo
to give a golden film. The residues were extracted into benzene,
filtered, and lyophilized to an orange powder, which was subse-
quently washed with petroleum ether (3 × 10 mL) to yield 12 as
a bright orange powder (90.4 mg, 60%). Crystals suitable for XRD
were obtained by slow evaporation of benzene from a concentrated
1
into a dichloromethane solution. H NMR (C6D6): δ 12.32, 7.61,
6.99, 4.67, 3.29, -2.09, -5.03. µeff (C6D6): 2.9 µB. UV-vis
(toluene) λmax, nm (ε, M-1 cm-1): 479 (5700), 426 (4700). Anal.
Calcd for C54H42ClFeP3Si: C, 71.81; H, 4.69. Found: C, 71.82; H,
4.41.
1
solution. H NMR (C6D6): δ 7.89 (d, J ) 6.9 Hz, 3H, Ar-H),
7.21-6.95 (m, 9H, Ar-H), 2.51 (br, 6H, -CH(CH3)2), 1.11 (br,
18H, -CH(CH3)2), 0.65 (br, 18H, -CH(CH3)2). 31P NMR (C6D6):
[SiPPh3]CoCl (9). H[SiPPh3] (0.483 g, 0.595 mmol) and CoCl2
(0.0830 g, 0.639 mmol) were combined in THF (30 mL) with
iPr2NEt (110 µL, 0.666 mmol). After several minutes, the blue
solution adopted a red hue and was stirred for 16 h. Solvent was
removed under reduced pressure, and the remaining solid was
dissolved in benzene (30 mL). This solution was filtered through a
glass microfilter and lyophilized to yield spectroscopically pure 9
as a fluffy red powder (0.4686 g, 87%). Crystals suitable XRD
were obtained by vapor diffusion of petroleum ether into a saturated
solution of 9 in methylene chloride. 1H NMR (C6D6): δ 10.3, 8.1,
7.7, 5.6, 3.0, -1.2. UV-vis (C6H6) λmax, nm (ε, M-1 cm-1): 402
(5700), 506 (3800).
δ 65.7 (s). IR (THF, KBr, cm-1) υ(N2): 2063. UV-vis (THF) λmax
nm (ε, M-1 cm-1): 381 (2800).
,
[SiPPh3]Co(N2) (13). A red solution of 9 (0.035 g, 0.039 mmol)
in THF (10 mL) was added onto a 0.5 weight % Na/Hg amalgam
(0.0015 g, 0.066 mmol sodium dissolved in 0.3025 g of mercury)
with stirring, causing a gradual color change to brown. After 12 h,
the mixture was decanted from the amalgam and filtered through
Celite to give an orange solution. Solvent was removed in vacuo,
and the residues were extracted into benzene, filtered, and lyoph-
1
ilized to afford 13 as an orange powder (0.014 g, 45%). H NMR
(C6D6): δ 8.1, 7.4, 7.2, 7.0, 6.9, 6.8. 31P NMR (C6D6): δ 63.5.
UV-vis (C6H6) λmax, nm (ε, M-1 cm-1): 378 (2000). IR (KBr, cm-1
υ(N2): 2095.
)
As we have noted previously,9b complexes 10-14 proved
unstable to extended exposure to vacuum because of the lability
of the N2 ligand. Although the reported complexes were
spectroscopically pure, suitable elemental analyses were not
obtained.
[SiPiPr3]Fe(N2) (10). A dark green solution of sodium naph-
thalide was prepared by stirring a colorless solution of naph-
thalene (8.6 mg, 0.067 mmol) in THF (3 mL) over excess sodium
metal (8.0 mg, 0.35 mmol) for 3 h. The resulting naphthalide
solution was filtered away from sodium and added dropwise to
[SiPiPr3]Ir(N2) (14). A colorless solution of 4 (101.2 mg, 0.1210
mmol) in THF (5 mL) was chilled to -78 °C. MeMgCl (3.0 M in
THF, 44 µL, 0.132 mmol) was diluted in 1 mL of THF and added
dropwise to the stirring solution. The reaction was allowed to
proceed at -78 °C for 30 min with no color change, then warmed
to room temperature, and stirred for 4 h, causing a change in color
to bright yellow. The mixture was concentrated in vacuo to give a
green film which was subsequently extracted into benzene/
petroleum ether (2:1, 20 mL). The solution was filtered through
2516 Inorganic Chemistry, Vol. 48, No. 6, 2009

Tris(phosphino)silyl Ligands
Celite and dried to afford 14 as an electric yellow powder (87.5
mg, 87%). H NMR (C6D6): δ 8.09 (d, J ) 6.9 Hz, 3H, Ar-H),
7.30-7.20 (m, 3H, Ar-H), 7.20-7.10 (m, 3H, Ar-H), 7.10-7.00
(m, 3H, Ar-H), 2.52 (br, 6H, -CH(CH3)2), 1.06 (br, 18H,
-CH(CH3)2), 0.69 (br, 18H, -CH(CH3)2). 31P NMR (C6D6): δ 38.7
(s). IR (THF, KBr, cm-1) υ(N2): 2122. UV-vis (THF) λmax, nm
(ε, M-1 cm-1): 310 (7600).
810 (220), 611 (330), 517 (390), 373 (2300). µeff (THF-d8): 3.3 µB.
Anal. Calcd for C38H54CoF6O6P3S2Si: C, 47.30; H, 5.64. Found:
C, 47.17; H, 5.73.
1
[SiPPh3]Fe(OTf) (17). In separate scintillation vials, [SiPPh3]FeN2
(29.5 mg, 0.0329 mmol) and AgOTf (8.5 mg, 33 mmol) were
dissolved in THF (1 mL each) and cooled to -35 °C. The AgOTf
solution was then added slowly to the [SiPPh3]FeN2 solution with
stirring. After stirring for 5 h at room temperature, volatiles were
removed. Et2O (10 mL) was added, and black solids were filtered
away from the resulting suspension to yield an orange filtrate.
Volatiles were removed to yield 17 as an orange powder (22.2 mg,
66%). 1H NMR (C6D6, δ): 12.4, 5.4, 5.1, -4.3. µeff (C6D6): 3.0 µB.
UV-vis (toluene, nm(M-1cm-1): 432(1440), 489(1840). IR (KBr,
cm-1): 3050, 2958, 1586, 1482, 1435, 1324, 1232 (OTf), 1210
(OTf), 1179 (OTf), 1108, 1027. Anal. Calcd for C55H42F3FeO3P3SSi:
C, 64.97; H, 4.16. Found:.C 64.08; H, 3.76.
[SiPiPr3]Co(CO) (15). In a sealed NMR tube, an orange solution
of 12 in THF (1 mL) was frozen, and the headspace evacuated and
backfilled with carbon monoxide. Over a period of 12 h, the hue
of the solution lightened from orange to yellow. Consumption of
12 and appearance of 15 was confirmed by NMR and IR, and
yellow X-ray quality crystals were grown by storing a concentrated
solution of 15 in petroleum ether at -35 °C. 31P NMR (THF): δ
78.2 (s). IR (THF, KBr, cm-1) υ(CO): 1896.
[SiPiPr3]Co(OTf)2 (16). To a solution of 12 (45.1 mg, 0.0649
mmol) in THF (5 mL) was added a colorless solution of silver
triflate (33.4 mg, 0.130 mmol) in THF (5 mL). The color of the
mixture immediately changed from orange to green as it became
cloudy. After 5 h, the solution was filtered through Celite and
concentrated in vacuo to a green film. The product was triturated
with petroleum ether (1 × 5 mL) and washed with petroleum ether
(3 × 5 mL) and benzene (1 × 5 mL) to give 16 as a fine green
powder (81.0 mg, 84%). X-ray quality crystals were grown by vapor
diffusion of petroleum ether into a concentrated solution of 16 in
Acknowledgment. We acknowledge the NIH (GM-
070757), the Moore Foundation (fellowship to M.T.W.), and
the National Science Foundation (fellowship to N.P.M.).
Larry Henling provided crystallographic assistance.
Supporting Information Available: CIF files for complexes
1-6, 10, 12, 14, 15, 16, and 17. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
THF. H NMR (CD3CN): δ 11.9, 7.8, 7.4, 7.2, 7.0, 3.8, 3.4, 3.0,
2.7, 1.9, 1.2, 0.9, 0.6, -3.5. UV-vis (THF) λmax, nm (ε, M-1 cm-1):
IC801855Y
Inorganic Chemistry, Vol. 48, No. 6, 2009 2517