Subtle differences between Zr and Hf in early/late heterobimetallic complexes with cobalt

Article abstract of DOI:10.1021/ic200445x

The phosphinoamide-linked Co/Hf complexes ICo(Ph₂PN ⁱPr)₃HfCl (4), ICo(ⁱPr₂PNMes) ₃HfCl (5), and ICo(ⁱPr₂PNⁱPr) ₃HfCl (6) have been synthesized f

Full text of DOI:10.1021/ic200445x

ARTICLE
pubs.acs.org/IC
Subtle Differences Between Zr and Hf in Early/Late Heterobimetallic
Complexes with Cobalt
Vinay N. Setty, Wen Zhou, Bruce M. Foxman, and Christine M. Thomas*
Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02454, United States
S Supporting Information
b
ABSTRACT:
The phosphinoamide-linked Co/Hf complexes ICo(Ph2PNiPr)3HfCl (4), ICo(iPr2PNMes)3HfCl (5), and ICo(iPr2PNiPr)3HfCl
(6) have been synthesized from the corresponding tris(phosphinoamide)HfCl complexes (1ꢀ3) for comparison with the recently
reported tris(phosphinoamide) Co/Zr complexes. Very minor structural and electronic differences between the Zr and Hf
complexes were found when the N-iPr-substituted phosphinoamide ligands [Ph2PNiPr]ꢀ and [iPr2PNiPr]ꢀ were utilized. The
reduction products [(THF)4Na-{N2-Co(Ph2PNiPr)3HfCl}2]Na(THF)6 (7) and N2-Co(iPr2PNiPr)3Hf (9) are also remarkably
similar to the corresponding Zr/Co analogues. In the case of Hf/Co and Zr/Co complexes linked by the N-Mes ligand
[iPr2PNMes]ꢀ (Mes = 2,4,6-trimethylphenyl), however, more pronounced differences in structure, bonding, and reactivity are
observed. While differences associated with 5 are still modest, larger variations are observed when comparing the two-electron
reduction product [N2-Co(iPr2PNMes)3Hf-X][Na(THF)5] (8) with its Zr congener. In addition to structural and spectroscopic
differences, vastly different reactivity is observed, with 8 undergoing one-electron oxidation to form ClHf(MesNPiPr2)3CoN2 (11)
in the presence of MeI, while a two-electron oxidative addition process occurs in a similar reaction with the Zr derivative. The activity
of 5 toward Kumada coupling was investigated, finding significantly diminished activity in comparison to Co/Zr complexes.
’ INTRODUCTION
polar metalꢀmetal multiple bonds and unusual coordinatively
unsaturated geometries can be isolated.9 These reduced species are
thus, highly reactive toward a variety of small molecule substrates and
studies into their reactivity are ongoing.10,12 To better understand
the fundamental nature of the metalꢀmetal interactions in early/late
heterobimetallic complexes and to determine the factors that
strengthen and weaken these interactions, we have begun to explore
variations of both the early and late transition metal in phosphinoa-
mide-linked heterobimetallics.
From both a steric and electronic perspective, zirconium and its
heavier congener hafnium are quite similar, although hafnium
complexes are often known to be less reactive than their zirconium
analogues. For example, hafnium(IV) complexes have been shown,
in several cases, to be more difficult to reduce than structurally similar
zirconium(IV) compounds,13ꢀ15 and in general, Hf complexes are
significantly less active olefin polymerization catalysts.16ꢀ18 How-
ever, recent literature has suggested that subtle differences between
Zr and Hf may lead to substantial differences and enhanced reactivity
toward CꢀH and N2 activation, particularly in reduced species,
The combination of early and late transition metals in early/late
heterobimetallic complexes poses a unique strategy for tuning the
reactivity of transition metal complexes.1ꢀ3 Our group and others
have recently been exploring early/late heterobimetallic complexes
in which the early and late transition metal are linked by phosphi-
noamide ligands.4ꢀ11 It has been found that the bonds between the
metals in these complexes are polar in nature, with the electron-rich
late metal center donating electron density to the empty d orbital(s)
on the electron-deficient early metal center.8,9 In the case of
bis(phosphinoamide)-linked Zr/Pt species, we have found that the
electron-donating abilities of the ancillary ligands on Zr play a crucial
role in determining the strength of the PtfZr interaction.11
Nagashima and co-workers have documented a similar trend upon
varying the Pt-bound ligands in similar Zr/Pt complexes.4 We
hypothesized that metalꢀmetal interactions of this type should have
a dramatic effect on the redox activity of both transition metals;
indeed, we have found a ∼1 V shift to milder potentials in the
reduction of the Co/Zr heterobimetallic complex ICo(Ph2PNiPr)3
ZrCl (1Zr) as compared to its monometallic Co analogue ICo-
(Ph2PNHiPr)3.8 Moreover, we have found that upon two-electron
reduction of Co/Zr heterobimetallics, complexes featuring highly
Received: March 4, 2011
Published: April 20, 2011
r
2011 American Chemical Society
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ARTICLE
owing to relativistic effects promoting slightly better metalꢀligand σ
and π bonds in the case of Hf.19ꢀ21 While several early/late
heterobimetallic complexes featuring MfHf interactions have been
structurally characterized,22ꢀ25 an in-depth comparative study be-
tween M/Hf and M/Zr heterobimetallic complexes has not been
reported. Herein, we report the preparation of Co/Hf heterobime-
tallic complexes and a comparison of the metalꢀmetal interactions
in these species as compared to previously reported Co/Zr
complexes.
and iPrNHPiPr2 complexes 4 and 6 and 4Zr and 6Zr (Table 1).8
For example, complex 4 has a HfꢀCo distance of 2.7548(5) Å,
while 4Zr has a ZrꢀCo distance of 2.7315(5) Å, consistent with
the similar covalent radii of Zr and Hf. In contrast, complex 5 has
a HfꢀCo distance ∼0.06 Å longer than the metalꢀmetal
distance in 5Zr. Notably, the CoꢀHf distances in 4ꢀ6 are
substantially shorter than that in the lone example of a structu-
rally characterized Co/Hf heterobimetallic complex reported to
date (2.933(2) Å).23
The redox behavior of Hf/Co complexes 4ꢀ6 was examined
by cyclic voltammetry (CV) and compared with the analogous
Zr/Co complexes (Figure 2, Table 1). The cyclic voltammogram
of complex 4 is nearly identical to that of 4Zr, characterized by a
reversible two-electron reduction at ꢀ1.66 V (Figure 2). Both
the Zr and Hf heterobimetallics supported by [iPr2PNMes]ꢀ
ligands possess a reversible reduction followed by an quasi-reversible
reductive process. In the case of the [iPr2PNiPr]ꢀ derivatives,
both the Zr and Hf complexes show two sequential irreversible
reduction processes by cyclic voltammetry. Interestingly, while
the reductive features of complex 5 are shifted to slightly more
negative potentials than the corresponding reduction events
observed for 5Zr, the reductive processes in the CV of complex
6 are shifted to more positive potentials than those of 6Zr. The
shift of the redox events of complex 5 to more negative potentials
than those of the Zr analogue is consistent with the slightly longer
MꢀCo distance and, thus, weaker CofHf dative donation.
Synthesis of Dinitrogen-Bound Co/Hf Heterobimetallic
Complexes 7ꢀ9 via Two-Electron Reduction. Complexes
4ꢀ6 were chemically reduced with excess Na/Hg amalgam to
compare their reduction products with the reduced Zr/Co
complexes 7Zrꢀ9Zr (Scheme 2). Reduction of complex 4 led
to a dinitrogen bound salt, [(THF)4Na-{N2-Co(Ph2PNiPr)3-
HfCl}2]Na(THF)6 (7). The solution IR of 7 revealed a ν(N2) at
2010 cmꢀ1, ∼5 cmꢀ1 lower than the ν(N2) of the Zr analogue
7Zr.10 This slight difference in CoꢀN2 backbonding is consistent
with the almost negligible differences between the Zr/Co and
Hf/Co complexes linked by [Ph2PNiPr]ꢀ ligands, as observed by
MꢀM distance and redox potentials. The solid state structure of
7 obtained via X-ray diffraction confirms that one THF-bound
Naþ countercation associates with the lone pairs on the distal
nitrogens of two symmetry-related molecules, while a second
THF-bound Naþ cation remains in an outersphere location
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Co/Hf Heterobimetallic
Complexes 4ꢀ6. The tris(phosphinoamide) hafnium chloride
metalloligands (Ph2PNiPr)3HfCl (1), (iPr2PNMes)3HfCl (2, Mes
= 2,4,6-trimethylphenyl), and (iPr2PNiPr)3HfCl (3) were synthe-
sized using the same procedure as reported previously for their Zr
congeners 1Zrꢀ3Zr:6,8 The R2PNHR0 phosphinoamines were de-
protonated with nBuLi, then treated with 1/3 equivalent of HfCl4.
Metalloligands 1ꢀ3were then treated with one equivalent of CoI2, to
generate the corresponding Hf/Co heterobimetallic complexes ICo-
(Ph2PNiPr)3HfCl (4), ICo(iPr2PNMes)3HfCl (5), and ICo-
(iPr2PNiPr)3HfCl (6), as shown in Scheme 1.26 Similar to the Zr/Co
derivatives 4Zrꢀ6Zr,8 complexes 4ꢀ6 are green in color, with
distinct dꢀd transitions in their UVꢀvis spectra. Single crystal
X-ray diffraction data were obtained for 4ꢀ6, and the resulting
solid state structures are shown in Figure 1. Upon comparison of
the interatomic distances and angles of 4ꢀ6 with the previously
reported Zr/Co derivatives 4Zrꢀ6Zr, it is apparent that there are
no notable differences between the structures of the iPrNHPPh2
Scheme 1
Figure 1. Displacement ellipsoid (50%) representation of 4, 5, and 6. All hydrogen atoms and substituents on all but one phosphinoamide ligand have
been omitted for clarity. Relevant interatomic distances (Å) and angles (deg). Complex 4: Hf1ꢀCo1, 2.7548(5); Hf1ꢀN1, 2.085(2); Hf1ꢀCl1,
2.3512(12); Co1ꢀP1:, 2.2886(6); Co1ꢀI1, 2.5541(6); N1ꢀHfꢀN1, 118.57(2); P1ꢀCo1ꢀP1:, 107.382(19). Complex 5: Hf1ꢀCo1, 2.6839(5);
Hf1ꢀN11, 2.1064(15); Hf1ꢀCl1, 2.3972(9); Co1ꢀP1, 2.3526(5); Co1ꢀI1, 2.5989(5); N11ꢀHf1ꢀN11, 119.155(14); P1ꢀCo1ꢀP1, 108.041(16).
Complex 6: Hf1ꢀCo1, 2.6370(7); Hf1ꢀN1, 2.090(3); Hf1ꢀN2, 2.082(4); Hf1ꢀCl1, 2.427(2); Co1ꢀP1, 2.3407(10); Co1ꢀP2, 2.3377(16);
Co1ꢀI1, 2.5570(8); N1ꢀHf1ꢀN1, 118.10(17); N1ꢀHf1ꢀN2, 120.23(9); P1ꢀCo1ꢀP1, 106.10(5); P1ꢀCo1ꢀP2, 109.26(4).
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Table 1. Comparison of Intermetallic Distances and Redox Potentials of Hf/Co Complexes 4ꢀ6 and Zr/Co Complexes 4Zrꢀ6Zr
and the ν(N2) of the Reduced Derivatives 7ꢀ9 and 7Zrꢀ9Zr
MꢀCo distance
E (V)
ν(N2) of reduced complex
phosphinoamide ligand
Zr9
Hf
Zr8
Hf
Zr8ꢀ10
Hf
[Ph2PNiPr]ꢀ
[iPr2PNMes]ꢀ
2.7315(5) Å
2.6280(5) Å
2.7548(5) Å
2.6839(5) Å
ꢀ1.65 Va
ꢀ1.64 Va
ꢀ1.87 Vb
ꢀ1.86 Vb
ꢀ2.07 Vb
ꢀ1.66 Va
ꢀ1.69 Va
ꢀ1.98 Vb
ꢀ1.79 Vb
ꢀ1.98 Vb
2015 cmꢀ1c
2023 cmꢀ1c
2010 cmꢀ1c
1992 cmꢀ1c
[iPr2PNiPr]ꢀ
2.6309(5) Å
2.6370(7) Å
2056 cmꢀ1d
2046 cmꢀ1d
a E1/2 values are reported for these reversible redox events. b E0 values are reported for these irreversible redox events. c Spectra were recorded in THF
using a KBr solution cell. d Spectra were recorded in C6H6 using a KBr solution cell.
(Figure 3). The HfꢀCo distance in 7 has contracted to
2.5470(3) Å from 2.7548(5) Å in 4, implying a substantial
increase in HfꢀCo bond order upon two-electron reduction.
While a solid state structure of the Zr/Co analogue 7Zr could not
be obtained for comparison,10 the similarity in spectroscopic
features suggests an analogous structure for the Zr and Hf
derivatives.
Reduction of the [iPr2PNiPr]ꢀcomplex 6 with excess Na/Hg
also followed a similar pattern to the Zr analogue, leading to a
diamagnetic N2-bound Co/Hf species N2-Co(iPr2PNiPr)3Hf
(9). While X-ray quality crystals of 9 were not obtained, the
absence of resonances corresponding to bound THF in the 1H
NMR spectrum implies an open coordination site at Hf, directly
analogous to the previously reported Zr derivative N2-Co-
(iPr2PNiPr)3Zr (9Zr).9 In addition, combustion analysis is con-
sistent with the proposed open coordination site formulation. In
contrast to 9Zr, the IR spectrum of 9 has a characteristic N2
Figure 2. Cyclic voltammetry of complexes 4ꢀ6 and their Zr/Co
analogues 4Zrꢀ6Zr (4 mM analyte concentration in 0.4 M [nBu4N][PF6]
in THF, scan rate: 100 mV/s).
stretch at 2046 cmꢀ1, indicative of slightly more backbonding
from Co to N2 and, thus, a weaker CofHf interaction.
In contrast to the two iso-propylamide-substituted derivatives,
substantial differences between Zr and Hf were observed upon
two-electron reduction of the mesitylanilide-substituted deriva-
tive 5. As for the Zr/Co analogue 5Zr,8 reduction of 5 with excess
Na/Hg in THF leads to formation of a monoanionic N2-bound
complex, [N2-Co(iPr2PNMes)3Hf-X][Na(THF)5] (8), in
which the Na countercation associates closely with the Zr-bound
halide (Figure 4). However, while the Zr/Co analogue 8Zr
undergoes rapid NaX loss to form N2-Co(iPr2PNMes)3Zr-
(THF) upon dissolution in benzene,9 the Hf/Co complex 8 is
insoluble in benzene and entirely inert toward a loss of NaX.
Another noteworthy difference is observed by comparing the
infrared N2 stretch of 8 with 8Zr: While exchange of Hf for Zr
only leads to minor changes in ν(N2) in complexes 7 and 9, the
ν(N2) of complex 8 is ∼30 cmꢀ1 lower in energy than the ν(N2)
of 8Zr (1992 cmꢀ1 vs 2023 cmꢀ1, Table 1). The increase in π-
backbonding from Co to N2 in 8 suggests that less Co-derived
electron density is involved in π-donation to Hf. This weaker
MfM interaction is also consistent with the loss in lability of the
NaX moiety as Co exerts less of a trans influence at Hf.
Structural characterization of 8 confirmed the hypotheses derived
by solution behavior. While the general formulation of 8 in the solid
state is very similar to that of 8Zr, there are a number of subtle but
noticeable differences in interatomic distances. For example, the
CoꢀHf distance in 8 is elongated by ∼0.04 Å from the CoꢀZr
distance in 8Zr. In addition, from the solid state structure of 8, it is
apparent that the dinitrogen moiety is bound more tightly to cobalt
in this species than in the Zr derivative 8Zr: The CoꢀN4 distance is
shorter (1.799(3) Å vs 1.8186(16) Å), and the N4ꢀN5 distance is
elongated (1.126(5) Å vs 1.120(2) Å) in 8 versus that in 8Zr.8 The
structures of both 8 and 8Zr contain significant disorder in the halide
bound to the group IV metal. In the 8Zr, the disorder was modeled
accurately with occupancies of 73% I and 27% Cl, while the refined
occupancies in the structure of 8 are 76% Cl and 24% I. Qualita-
tively, this suggests that while the NaX moiety in 8 cannot be
removed irreversibly to generate a neutral N2-Co(iPr2PMesN)3Hf-
(THF) species, NaX dissociation must be occurring reversibly to a
certain extent in solution to allow halide exchange to occur.
Additional differences in structure are apparent upon inspection
of the distances associated with the Hf/Zr-bound halide and the
associated Na(THF)5þ cation. The HfꢀCl and HfꢀI distances in 8
(2.5534(4) Å and 2.877(4) Å, respectively) are significantly shorter
than the ZrꢀCl and ZrꢀI distances in 8Zr (2.690(7) Å and
2.9722(7) Å, respectively).8 At the same time, the Naꢀhalide
distances in 8 are longer than those in 8Zr, as illustrated by the
NaꢀI distances of 3.211(4) Å and 3.2695(12) Å, respectively.
These structural parameters are consistent with the greater lability of
the NaX moiety in 8Zr as compared to 8 and are consistent with
assignment of weaker MꢀM bonding in the Hf derivative 8.
Representative Reactivity of 8 as Compared to Zr Analo-
gue 8Zr. To assess the similarities and differences between the
reactivity of reduced heterobimetallic complexes featuring Zr
and Hf, the reactivity of the reduced Co/Hf species 8 toward
oxidative addition was examined. Treatment of 8Zr with MeI
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Scheme 2
Figure 3. Displacement ellipsoid (50%) representation of 7. All hydrogen atoms, one Na(THF)6 countercation, and an outersphere THF solvent
molecule have been omitted for clarity. Relevant interatomic distances (Å): Hf1ꢀCo1, 2.5470(3); Hf1ꢀCl1, 2.4889(6); Hf1ꢀN1, 2.0971(19);
Hf1ꢀN2, 2.1245(19); Hf1ꢀN3, 2.1052(19); Co1ꢀP1, 2.1731(6); Co1ꢀP2, 2.1551(6); Co1ꢀP3, 2.1639(6); Co1ꢀN4, 1.788(2); N4ꢀN5, 1.113(3);
Na2ꢀN5, 2.473(2).
leads to quantitative formation of the two-electron oxidized
bridging methyl species (η2-MesNPiPr2)Zr(μ-CH3)(MesNPiPr2)2-
CoI (10, Scheme 3).10 In contrast, Hf derivative 8 reacts with
MeI via one-electron oxidation to yield XHf(MesNPiPr2)3CoN2
(11). Use of labeled 13CH3I reveals that the byproduct of this
reaction is ethane (see the Supporting Information). Complex 11
is a paramagnetic S = 1/2 complex, as confirmed by Evans’
method (μeff = 2.3 B. M.), with a characteristic ν(N2) indicative
of a weakly bound dinitrogen moiety (2067 cmꢀ1). Interestingly,
the N2 molecule is so weakly bound that it dissociates upon
exposure to a vacuum, as indicated by a dramatic color change
from pale orange to dark green in solution. Combustion analysis
data for 11 are consistent with the absence of dinitrogen in the
solid state.
X-ray crystallography confirms the structure of 11 in the solid
state (Figure 5). Although complex 11 is presumed to be a 76/24
Cl/I mixture, on the basis of the disordered halide in the crystals of
the starting material 8, the solid state structure of 11 showed no sign
of disorder in the halide atom bound to Hf. We attribute this to
crystallization conditions, and on the basis of combustion analysis,
the bulk of the sample is likely a Cl/I mixture. A comparison of the
structures of the one-electron oxidized complex 11 with the fully
reduced complex 8 reveals substantial differences presented in
Table 2. Upon oxidation, the HfꢀCo distance elongates from
2.455(5) Å to 2.5624(4) Å. The HfꢀCl distance simultaneously
contracts in the absence of a strong CofHf interaction (2.553(4)ꢀ
2.4252(8) Å). The significantly weaker bonding between Co and
N2 implied by the high frequency ν(N2) stretch for 11 is also
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Scheme 3
Figure 4. Displacement ellipsoid (50%) representation of 8. All hydrogen atoms have been omitted for clarity.
Table 2. Comparison of Interatomic Distances in 8 and 11
8
11
HfꢀCo
HfꢀCl
2.455(5) Å
2.553(4) Å
1.799(3) Å
1.126(5) Å
2.138(3) Å
2.128(3) Å
2.124(3) Å
2.1948(11) Å
2.2010(10) Å
2.1977(11) Å
2.5624(4) Å
2.4252(8) Å
1.856(4) Å
1.063(7) Å
2.108(2) Å
2.138(3) Å
2.113(2) Å
2.2786(10) Å
2.2754(9) Å
2.797(9) Å
CoꢀN4
N4ꢀN5
HfꢀN1
HfꢀN2
HfꢀN3
CoꢀP1
CoꢀP2
CoꢀP3
addition of alkyl halides to [Co(CN)5]3ꢀ and Cr2(SO4)3 has been
well-documented.27,28 In a more recent example, Mashima and co-
workers observed the formation of IꢀPdIꢀMotMoꢀPdIꢀI along
with ethane upon the addition of excess MeI to Pd0ꢀMotMoꢀPd0
(where metals are connected by four 6-diphenylphosphino-
2-pyridonate ligands).29 On the basis of cyclic voltammetry data,
the likely origin of the difference between the reactivity of 8and 8Zr is
a combination of the weaker CofHf interaction and the larger
difference (0.29 V) between the two sequential reduction potentials
of 8 in comparison to 8Zr (0.23 V).
Figure 5. Displacement ellipsoid (50%) representation of 11. All
hydrogen atoms have been omitted for clarity.
evident in the solid state. The CoꢀN bond in 11 is ∼0.05 Å longer
than in 8 (1.856(4) Å vs 1.799(3) Å), while the NꢀN bond
distance in 11 is substantially shorter than that in 8 (1.063(7) Å vs
1.126(5) Å), implying much weaker π-backbonding from Co to the
N2 π* orbitals.
While MeI typically oxidatively adds to low valent transition
metals via a two-electron process, one-electron oxidation upon the
We recently reported that complexes 4Zrꢀ6Zr are active
precatalysts for the Kumada coupling of alkyl halides with alkyl
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Table 3. Comparison of Results of Kumada Coupling Reac-
tions Using Hf/Co Complex 5 and Zr/Co Complex 5Zr 12
bonding lead to vastly different reactivity, with 8 undergoing one-
electron oxidation in the presence of MeI while a two-electron
oxidative addition process occurs in a similar reaction with 8Zr.10
Consequently, the activity of 5 as a Kumada coupling catalyst is
considerably diminished compared to that of its Co analogue.12
While these Hf/Co complexes have not yet proven advantageous
over Zr/Co complexes, their more sluggish reactivity may prove
useful as a mechanistic probe in future studies.
cat: ð5 mol %Þ
n-OctMgBr þ R ꢀ X
n-Oct ꢀ R
ð30 mol%Þf
TMEDA
THF; rt
yielda
RꢀX
cat. 5
cat. 5Zr
1-bromopentane
2-bromobutane
1-chlorobutane
chlorocyclohexane
64.2%
10.3%
39.8%
11.7%
95.4%
42.7%
45.8%
55.9%
’ EXPERIMENTAL SECTION
General Considerations. All syntheses reported were carried out
using standard glovebox and Schlenk techniques in the absence of water
and dioxygen, unless otherwise noted. Benzene, pentane, diethyl ether,
tetrahydrofuran, dichloromethane, and toluene were degassed and dried
by sparging with N2 gas followed by passage through an activated
alumina column using a Glass Contour Seca Solvent System. All solvents
were stored over 3 Å molecular sieves. Deuterated benzene was
purchased from Cambridge Isotope Laboratories, Inc., degassed via
repeated freezeꢀpumpꢀthaw cycles, and dried over 3 Å molecular
sieves. THF-d8 was dried over CaH2, vacuum-transferred, and degassed
via repeated freezeꢀpumpꢀthaw cycles. Solvents were frequently tested
using a standard solution of sodium benzophenone ketyl in tetrahydro-
furan to confirm the absence of oxygen and moisture. Ph2PNHiPr,30,31
a Average of two values obtained via GC-MS analysis using tetradecane
as an internal integration standard.
Grignard reagents and show remarkable activity toward tradi-
tionally difficult alkyl chloride substrates.12 Notably, the mono-
metallic analogue ICo(Ph2PNHiPr)3 is inactive toward alkyl
chlorides, implying that the early metal component plays an
important role in catalysis. Given the pronounced differences in
MeI oxidative addition products between Hf/Co and Zr/Co
derivatives, variations in catalytic activity toward this cross-
coupling reaction were also anticipated. As shown in Table ,
representative Hf/Co complex 5 was screened as a catalyst for
the coupling of n-octylmagnesium bromide with both primary
and secondary alkyl chloride and bromide substrates. In all
cases, the yield of CꢀC coupled products was significantly lower
when the Hf/Co complex was utilized. This difference may be
attributed to either (1) the diminished MfM interaction in the
Hf/Co bimetallic as compared to the Zr/Co complex or (2) a
different substrate oxidative addition pathway operative in the
Hf/Co case. Further studies will be needed to distinguish the role
of the early metal in this catalytic process before definitive
conclusions can be drawn regarding the origin of the difference
in catalytic activity between Hf/Co and Zr/Co heterobimetallics.
iPr2PNHiPr,8 and Pr2PNHMes8 were synthesized using literature
i
procedures. All other chemicals were purchased from commercial
vendors and used without further purification. NMR spectra were
recorded at ambient temperature on a Varian Inova 400 MHz instru-
1
ment. H and 13C NMR chemical shifts were referenced to residual
solvent. 31P NMR chemical shifts were referenced to 85% H3PO4. IR
spectra were recorded on a Varian 640-IR spectrometer controlled by
Resolutions Pro software. UVꢀvis spectra were recorded on a Cary 50
UVꢀvis spectrophotometer using Cary WinUV software. Elemental
microanalyses were performed by Complete Analysis Laboratories, Inc.,
in Parsippany, New Jersey. Solution magnetic moments were measured
using the Evans’ method.32,33
Electrochemistry. Cyclic voltammetry measurements were carried
out in a glovebox under a dinitrogen atmosphere in a one-compartment
cell using a CH Instruments electrochemical analyzer. A glassy carbon
electrode and platinum wire were used as the working and auxiliary
electrodes, respectively. The reference electrode was Ag/AgNO3 in
THF. Solutions (THF) of electrolyte (0.40 M [nBu4N][PF6]) and
analyte (4 mM) were also prepared in the glovebox.
’ CONCLUSIONS
In summary, in early/late heterobimetallic complexes combin-
ing group IV metals and Co (4ꢀ6, 4Zrꢀ6Zr), there are only
subtle structural and electronic differences between Zr and Hf
when the N-iPr-substituted phosphinoamide ligands [Ph2PNiPr]ꢀ
and [iPr2PNiPr]ꢀ are utilized. CoꢀM distances vary by <0.02 Å,
and reduction potentials vary by <0.1 V between Co/Zr and Co/
Hf complexes. The reduction products 7/7Zr and 9/9Zr are also
remarkably similar. In the case of complexes linked by the N-Mes
ligand [iPr2PNMes]ꢀ, however, more pronounced differences in
structure, bonding, and reactivity are observed. For example, the
CoꢀM distance lengthens by ∼0.05 Å upon moving from Zr to
Hf, and the potential difference between the first and second
reduction event increases. While these differences are modest,
larger differences are observed in comparing the two electron
reduction products 8 and 8Zr. The vastly different ν(N2)
stretches of 8 and 8Zr implicate stronger backbonding to N2 in
the Hf derivative, accompanied by a weaker CofHf interaction
as evident in an elongated CoꢀHf distance. While NaX can
readily be removed from 8Zr to generate the neutral species
N2ꢀCo(iPr2PNMes)3Zr(THF), the NaX unit in 8 is more
tightly bound to Hf. This can be deduced by both the reluctance
of NaX to dissociate and also by the shorter HfꢀX contacts in the
solid state structure of 8. These differences in structure and
X-Ray Crystallography Procedures. All operations were per-
formed on a Bruker-Nonius Kappa Apex2 diffractometer, using graphite-
monochromated Mo KR radiation. All diffractometer manipulations,
including data collection, integration, scaling, and absorption correc-
tions were carried out using the Bruker Apex2 software.34 Preliminary
cell constants were obtained from three sets of 12 frames. Structures
were solved using SIR9235 or SuperFlip,36 and refinements (full-matrix-
least-squares) were carried out using the Oxford University Crystals for
Windows program.37 All non-hydrogen atoms were refined using
anisotropic displacement parameters. Experimental details are provided
in Tables 4 and 5, and data collection, solution, and refinement details
are supplied in the Supporting Information.
(Ph2PNiPr)3HfCl (1). A solution of iPrNHPPh2 (6.10 g, 25.0 mmol)
n
in Et2O (250 mL) was cooled to ꢀ78 °C. To this was added BuLi
(17.2 mL, 1.6 M in hexanes, 27.5 mmol) dropwise over 10 min. The
resulting yellow/orange solution was warmed to room temperature and
stirred for 2 h. The solution was then cooled again to ꢀ78 °C, and HfCl4
(2.68 g, 8.30 mmol) was added portionwise as a solid. The reaction mixture
was warmed to room temperature and stirred for 12 h. Volatiles were
removed from the solution in vacuo, and the resulting solids were extracted
with CH2Cl2 (20 mL) and filtered through a pad of Celite to remove LiCl.
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Table 4. Crystallographic Details for Complexes 4, 5, and 6
4
5
6
chemical formula
fw
C
45H51ClCoHfIN3P3
C45H75ClCoHfIN3P3
C27H63ClCoHfIN3P3
1126.62
120
1150.81
120
922.52
120
T (K)
λ (Å)
0.71073
20.8180(3)
20.8180(3)
20.8180(3)
90
0.71073
12.0541(3)
12.0541(3)
19.7252(7)
90
0.71073
15.1818(4)
18.1649(5)
13.5297(4)
90
a (Å)
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
90
90
90
90
120
90
9022.3(2)
Pa3
2482.11(12
P-3
3731.17.(18)
Pnma
space group
Z
8
2
4
Dcalcd (g/cm3)
1.659
1.540
1.642
μ (mmꢀ1
R1, wR2a (I > 2σ(I))
a R1 = ∑ Fo| ꢀ |Fo /∑ |Fo|, wR2 = {∑[w(Fo ꢀ Fc2)2]/∑[w(Fo ) ]}1/2
)
3.552
3.229
4.273
0.0232, 0.0534
2
0.0200, 0.0431
0.0321, 0.0624
2 2
.
Table 5. Crystallographic Details for Complexes 7, 8, and 11
7 THF
8
11 1.5Et2O
3
3
chemical formula
fw
C
69H99ClCoHfN5NaO6P3
C65H115Cl0.76CoHfI0.24N5NaO5P3
C51H90ClCoHfI0.02N4.97O1.5P3
1483.36
120
1457.79
120
1164.62
120
T (K)
λ (Å)
0.71073
12.7902(8)
13.8837(8)
22.7745(14)
99.384(3)
102.512(3)
100.827(3)
3789.3(4)
P-1
0.71073
14.6354(4)
24.6754(7)
19.2156(6)
90
0.71073
27.9112(11)
15.4306(6)
26.3171(10)
90
a (Å)
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
90.2243(16
90
90
90
6939.4(3)
P121/n1
4
11334.4(8)
Pbcn
space group
Z
2
8
Dcalcd (g/cm3)
1.300
1.395
1.365
μ (mmꢀ1
R1, wR2a (I > 2σ(I))
a R1 = ∑ Fo| ꢀ |Fo /∑ |Fo|, wR2 = {∑[w(Fo ꢀ Fc2)2]/∑[w(Fo ) ]}1/2
)
1.741
1.997
2.303
0.0287, 0.0724
2
0.0398, 0.0645
0.0318, 0.0611
2 2
.
The volatiles were removed from the resulting filtrate in vacuo, and the
resulting solids were washed with pentane (3 ꢁ 50 mL) to yield analytically
pure 1as a white solid (5.437 g, 69.6%). 1H NMR (400 MHz, C6D6):δ7.46
(m, 12H, o-Ph), 6.96 (m, 12H, m-Ph), 6.94 (m, 6H, p-Ph), 4.22 (m, 3H,
CH(CH3)2), 1.52 (d, J = 6 Hz, 18H, CH(CH3)2). 31P{1H} NMR (161.8
MHz, C6D6): δ ꢀ4.2 (s). 13C NMR (100.5 MHz, C6D6): δ 137.4 (d,
1JPꢀC = 11.5 Hz, ipso-Ph), 134.0 (d, m, o-Ph), 129.6 (s, p-Ph), 128.3 (s, m-
Ph), 53.7 (s, NCH(CH3)2), 27.6 (s, NCH(CH3)2).
extracted with CH2Cl2 (20 mL) and filtered through a pad of Celite to
remove LiCl. The volatiles were removed from the resulting filtrate in vacuo,
and the resulting solids were washed with pentane (3 ꢁ 50 mL) to yield
analytically pure 2 as a white solid (8.276 g, 53.6%). 1H NMR (400 MHz,
C6D6): δ 6.87 (s, 6H, Mes), 2.55 (s, 18H, Mes-Me), 2.41 (m, 6H,
PCH(CH3)2), 2.22 (s, 9H, Mes-Me), 1.30 (m, 18H, PCH(CH3)2), 1.17
(m, 18H, PCH(CH3)2). 31P{1H} NMR (161.8 MHz, C6D6): δ 16.6 (s).
13C NMR (100.5 MHz, C6D6): δ 148.7 (ipso-Mes), 134.2 (o-Mes), 132.3
(iPr2PNMes)3HfCl (2). A solution of MesNHPiPr2 (12.198 g, 48.5
mmol) in Et2O (200 mL) was cooled to ꢀ78 °C. To this was added nBuLi
(33.3 mL, 1.6 M in hexanes, 53.3 mmol) dropwise over 10 min. The
resulting yellow/orange solution was warmed to room temperature and
stirred for 2 h. The solution was then cooled again to ꢀ78 °C, and HfCl4
(5.158 g, 16.0 mmol) was added portionwise as a solid. The reaction
mixture was warmed to room temperature and stirred for 12 h. Volatiles
were removed from the solution in vacuo, and the resulting solids were
(p-Mes), 129.6 (m-Mes), 38.8 (PCH(CH3)2), 22.1 (PCH(CH3)2
þ
MesMe overlapping), 21.3 (PCH(CH3)2 þ MesMe overlapping). Anal.
Calcd for C45H75ClHfN3P3: C, 56.01; H, 7.83; N, 4.35. Found: C, 56.06; H,
7.96; N, 4.51.
i
i
i
( Pr2PN Pr)3HfCl (3). A solution of PrNHPiPr2 (4.496 g, 25.7
mmol) in Et2O (150 mL) was cooled to ꢀ78 °C. To this was added
nBuLi (14.8 mL, 1.6 M in hexanes, 26.9 mmol) dropwise over 10 min.
The resulting yellow/orange solution was warmed to room temperature
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and stirred for 2 h. The solution was then cooled again to ꢀ78 °C, and
HfCl4 (2.739 g, 8.55 mmol) was added portionwise as a solid. The
reaction mixture was warmed to room temperature and stirred for 12 h.
Volatiles were removed from the solution in vacuo, and the resulting
solids were extracted with CH2Cl2 (20 mL) and filtered through a pad of
Celite to remove LiCl. The volume of the filtrate was reduced to 5 mL in
vacuo. The resulting supersaturated solution was layered with pentane
and cooled to ꢀ35 °C to yield analytically pure colorless crystals of 3
(3.126 g, 49.6%). 1H NMR (400 MHz, C6D6): δ 4.09 (m, 3H, NCH-
(CH3)2), 2.16 (m, 6H, PCH(CH3)2), 1.59 (d, J = 6.4 Hz, 18H,
NCH(CH3)2), 1.11ꢀ1.25 (m, 36H, PCH(CH3)2). 31P{1H} NMR
(161.8 MHz, C6D6): δ 7.5 (s). 13C NMR (100.5 MHz, C6D6): δ 53.1
(NCH(CH3)2), 29.1 (NCH(IH3)2), 27.5 (PCH(CH3)2), 22.2
was filtered away from the amalgam, and the solvent was removed from
the filtrate in vacuo. Solids were extracted back into THF and filtered
through Celite. Layering the resulting concentrated red solution with
pentane and cooling to ꢀ35 °C resulted in red crystals of 7 (0.480 g,
60.4%). 1H NMR (400 MHz, THF-d8): δ 7.42 (m, 24H, o-Ph), 6.84 (m,
12H, p-Ph), 6.72 (m, 24H, m-Ph), p-Ph), 4.22 (m, 6H, CH(CH3)2), 3.54
(m, 8H, THF), 1.68 (m, 8H, THF), 1.78 (d, J = 6.4 Hz, 36H,
CH(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 44 ppm (br s). 13
C
NMR(100.5 MHz, THF): δ 143.6(m, ipso-Ph), 133.0(s, o-Ph), 126.3 (s,
m-Ph), 125.8 (s, m-Ph), 49.8 (s, NCH(CH3)2), 27.0 (s, NCH(CH3)2. IR
(KBr solution cell, THF): 2010 cmꢀ1. UVꢀvis (λ, nm (ε, cmꢀ1 Mꢀ1)):
445 (14 600), 672 (190). Satisfactory combustion analysis data could not
be obtained on repeated attempts. This is likely a result of the extreme
sensitivity ofthiscomplextoairandmoisture andthelability ofthe bound
N2 unit.
1
(PCH(CH3)2, JPC = 11 Hz), 19.7 (PCH(CH3)2). Anal. Calcd for
C27H63ClHfN3P3: C, 44.02; H, 8.62; N, 5.70. Found: C, 43.95; H, 8.58;
N, 5.44.
N2Co(iPr2PNMes)3HfX-Na(THF)5 (8). A 0.5% Na/Hg amalgam
was prepared from 26 mg of Na (1.1 mmol) and 5.1 g of Hg. To this
vigorously stirred amalgam in 5 mL of THF was added a solution of 5
(0.511 g, 0.444 mmol) in THF (2 mL). The solution immediately began
to change color from green to red/orange. After 2 h, the resulting red/
orange solution was filtered away from the amalgam, and the solvent was
removed from the filtrate in vacuo. Solids were extracted back into THF
and filtered through Celite. Layering the resulting concentrated red
solution with pentane and cooling to ꢀ35 °C resulted in orange crystals
of 8 (0.487 g, 74.5%). Crystals suitable for X-ray diffraction were grown
by vapor diffusion of pentane into a concentrated THF solution of
N2Co(iPr2PNMes)3HfX-Na(THF)5 at ꢀ35 °C. 1H NMR (400 MHz,
THF-d8): δ 6.55 (s, 6H, Mes), 3.61 (Na-THF), 2.76 (m, 6H, PCH-
(CH3)2), 2.38 (s, 18H, Mes-Me), 2.10 (s, 9H, Mes-Me), 1.50 (m, 18H,
PCH(CH3)2), 1.37 (m, 18H, PCH(CH3)2). 31P{1H} NMR (162 MHz,
C6D6): δ 50.1 (br s). 13C NMR (100.5 MHz, THF): δ 135.0 (s, o-Mes),
134.9 (s, ipso-Mes), 128.7 (s, p-Mes), 127.8 (s, m-Mes), 44.5 (m,
PCH(CH3)2), 25.0 (PCH(CH3)2 overlapping with THF peak), 23.8
(s, MesMe) 22.8 (s, PCH(CH3)2), 20.1 (s, MesMe). IR (KBr solution
cell, THF): 1992 cmꢀ1. UVꢀvis (λ, nm (ε, cmꢀ1 Mꢀ1)): 441 (1312).
Satisfactory combustion analysis data could not be obtained on repeated
attempts. This is likely a result of the extreme sensitivity of this complex
to air and moisture and the lability of the bound N2 unit.
ICo(Ph2PNiPr)3HfCl (4). Solid 1 (5.437 g, 5.78 mmol) and solid
CoI2 (1.82 g, 5.78 mmol) were combined in THF (200 mL) and stirred
for 12 h at room temperature. The resulting dark green reaction solution
was filtered through Celite, and solvent was removed from the volatiles
in vacuo. The remaining green solids were extracted with toluene
(10 mL) and filtered through Celite. The filtrate was cooled to
ꢀ35 °C overnight to yield 4 as dark green crystalline solids (5.771 g,
88.7%). Crystals for X-ray diffraction were grown by cooling a toluene/
pentane solution to ꢀ35 °C. 1H NMR (400 MHz, C6D6): δ 14.5 (br s,
Ph), 5.1 (br, (CH(CH3)2), 1.2 (br s, (CH(CH3)2), ꢀ5.5 (br, Ph), ꢀ7.3
(br s, Ph). UVꢀvis (λ, nm (ε, cmꢀ1 Mꢀ1)): 578 (120), 690 (180), 708
(170), 735 (190), 750 (220), 776 (300), 867 (350). Evans’ method
(C6D6): 3.0 μB. Anal. Calcd for C45H51ClCoHfIN3P3: C, 47.97; H 4.56,
N, 3.74. Found: C, 47.94; H 4.67, N, 3.71.
ICo(iPr2PNMes)3HfCl (5). Solid 2 (0.0846 g, 0.0877 mmol) and
solid CoI2 (0.0274 g, 0.0877 mmol) were combined in CH2Cl2 (4 mL)
and stirred for 48 h at room temperature. The resulting dark green
reaction solution was filtered through Celite and solvent was removed
from the volatiles in vacuo. The remaining green solids were washed
with copious pentane and dried in vacuo to yield analytically pure 5 as a
green powder (0.0864 g, 85.7%). Cyrstals suitable for X-ray diffraction
1
were grown from CH2Cl2 and Et2O at room temperature. H NMR
N2Co(iPr2PNiPr)3Hf (9). A 0.5% Na/Hg amalgam was prepared
from 10 mg of Na (0.45 mmol) and 2.0 g of Hg. To this vigorously
stirred amalgam in 5 mL of THF was added a solution of 6 (0.149 g,
0.179 mmol) in THF (2 mL). The solution immediately began to
change color from green to red. After 2 h, the resulting red solution was
filtered away from the amalgam, and the solvent was removed from the
filtrate in vacuo. Solvent was extracted into C6H6 and filtered through
Celite. Solvent was removed in vacuo to yield analytically pure product
as a red powder (0.0805 g, 58.3%). Crystals suitable for X-ray diffraction
were grown via vapor diffusion of pentane into a concentrated solution
of 9 in THF at ꢀ35 °C. 1H NMR (400 MHz, C6D6): δ 3.67 (m, 3H,
NCH(CH3)2), 2.76 (m, 6H, PCH(CH3)2), 1.42 (m, 36H, PCH-
(CH3)2), 1.11 (d, J = 4 Hz, 18H, NCH(CH3)2), 1.11ꢀ1.25 (m, 36H,
(400 MHz, C6D6): δ 13.6 (br s, PCH(CH3)2), 7.0 (Mes-Me), 2.8 (br s,
PCH(CH3)2), 2.1 (Mes-Me), ꢀ2.0 (br, Mes-Ar) . UVꢀvis (λ, nm
(ε, cmꢀ1 Mꢀ1)): 342 (6900), 382 (4800), 648 (150), 783 (140), 802
(150), 890 (320). Evans’ method (C6D6): 3.3 μB. Anal. Calcd for
C45H75ClCoHfIN3P3: C, 46.97; H, 6.57; N, 3.65. Found: C, 46.87; H,
6.73; N, 3.59
ICo(iPr2PNiPr)3HfCl (6). Solid 3 (1.1373 g, 1.544 mmol) and solid
CoI2 (0.483 g, 1.544 mmol) were combined in CH2Cl2 (20 mL) and
stirred for 48 h at room temperature. The resulting dark green reaction
solution was filtered through Celite, and solvent was removed from the
volatiles in vacuo. The remaining green solids were extracted with
toluene (10 mL) and filtered through Celite. The filtrate was cooled to
ꢀ35 °C overnight to yield 6 as dark green crystalline solids (0.427 g,
30.0%). Crystals suitable for X-ray diffraction were grown from toluene/
PCH(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 38.7 ppm (br s). 13
C
1
NMR (100.5 MHz, C6D6): δ 48.4 (NCH(CH3)2), 32.0 (PCH(CH3)2),
31.1 (PCH(CH3)2), 21.4 (NCH(CH3)2), 20.0 (PCH(CH3)2). IR (KBr
solution cell, C6H6): 2046 cmꢀ1. UVꢀvis (λ, nm (ε, cmꢀ1 Mꢀ1)): 296
(10000), 336 (6000), 442 (1000), 520 (650), 755 (80). Anal. Calcd for
C27H63CoHfN5P3: C, 41.14; H, 8.06; N, 8.89. Found: C, 41.25; H, 8.19;
N, 8.83.
pentane at ꢀ35 °C. H NMR (400 MHz, C6D6): δ 72 (br s, PCH-
(CH3)2), 8.6 (br s, PCH(CH3)2), 5.1 (br s, NCH(CH3)2), 1.7 (br s,
NCH(CH3)2), ꢀ2.2 (br s, PCH(CH3)2) . UVꢀvis (λ, nm (ε, cmꢀ1
M
ꢀ1)): 337 (3100), 439 (670), 691 (210), 728 (250), 743 (280), 769
(350), 887 (170). Evans’ method (C6D6): 3.0 μB. Anal. Calcd for
C27H63ClCoHfIN3P3: C, 35.15; H, 6.88; N, 4.55. Found: C, 35.43; H,
7.01; N, 4.70.
N2Co(iPr2NMes)3HfX (11). Complex 8 (0.1309 g, 0.0913 mmol)
was dissolved in THF (5 mL). To this was added dropwise a dilute
THF solution of MeI (4.5 μL, 0.073 mmol) in 2 mL of THF. A
substoichiometric amount of MeI resulted in the most satisfactory yields
owing to the difficulty in determining the exact MW of samples of 8 due
to the mixture of Clꢀ/Iꢀ. After 30 min of stirring at room temperature,
[(THF)4Na{N2Co(Ph2PNiPr)3HfCl}2]Na(THF)6] (7). A 0.5% Na/Hg
amalgam was prepared from 32 mg of Na (1.4 mmol) and 6.5 g of Hg.
To this vigorously stirred amalgam in 5 mL of THF was added a solution
of 4 (0.635 g, 0.564 mmol) in THF (2 mL). The solution immediately
began to change color from green to red. After 2 h, the resulting red solution
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ARTICLE
volatiles were removed from the mixture in vacuo. The remaining orange
solids were extracted with benzene (3 mL) and filtered through Celite to
remove NaX salts. Volatiles were removed in vacuo to yield spectro-
scopically pure product as a green/brown powder (0.0587 g, 70.3%
based on MeI). Note: In solution, N2 is bound, leading to an orange
product. Upon exposure to a vacuum, a color change to green was
observed, suggesting dissociation of N2 (consistent with elemental
analysis results). Crystals suitable for X-ray diffraction were grown via
slow evaporation of a concentrated Et2O solution at ꢀ35 °C. 1H NMR
(400 MHz, C6D6): δ 7.75 (s, m-Mes), 5.82 (s, iPr-Me), 5.14 (br s, iPr-
(11) Cooper, B. G.; Fafard, C. M.; Foxman, B. M.; Thomas, C. M.
Organometallics 2010, 29, 5179–5186.
(12) Zhou, W.; Napoline, J. W.; Thomas, C. M. Eur. J. Inorg. Chem.
2011, DOI: 10.1002.ejic.201100109.
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1980, 1284–1285.
(14) Silveira, F.; Simplício, L. M. T.; Rocha, Z. N. d.; Santos, J. H. Z.
d. Appl. Catal., A 2008, 344, 98–106.
(15) Lappert, M. F.; Raston, C. L.; Skelton, B. W.; White, A. H.
J. Chem. Soc., Dalton Trans. 1984, 893–902.
(16) Weiss, K.; Neugebauer, U.; Blau, S.; Lang, H. J. Organomet.
Chem. 1996, 520, 171–179.
i
CH), 2.92 (s, Pr-Me), 2.24 (s, p-Mes), ꢀ1.70 (s, o-Mes). IR (KBr
solution cell, C6H6): 2067 cmꢀ1. Evans’ method (C6D6): 2.3 μB. Anal.
Calcd for C45H75CoHfI0.24Cl0.76N3P3: C, 50.22; H, 6.98; N, 3.91.
Found: C, 48.79; H, 6.88; N, 3.61. Note: Repeated combustion analysis
showed evidence for a loss of dinitrogen prior to analysis. Satisfactory
analysis was obtained if a 76:24 Cl/I mixture (resulting from composi-
tion of the I/Cl mixtures in the starting material, 8) is considered.
Typical Procedure for Kumada Coupling Reactions. In a
nitrogen-filled glovebox, alkyl halide (0.47 mmol) and TMEDA (16.5 mg,
0.14 mmol, 30 mol %) were weighed into a 20 mL vial. Dry THF (7 mL)
and complex 5 (0.024 mmol, 5 mol %) were added, and the mixture was
stirred for 10 min. To this mixture, n-OctMgBr (2 M in Et2O, 0.28 mL,
0.56 mmol) was added by syringe pump at a rate of 2 mL per hour. Upon
completion of this addition, the reaction was removed from the glove-
box, and saturated aqueous NH4Cl was added to quench the reaction.
The organic layer was passed through a silica plug, and the resulting
mixture was analyzed using GC-MS. Yields were determined via
comparison to an internal standard (tetradecane).
(17) Jany, G.; Gustafsson, M.; Repo, T.; Aitola, E.; Dobado, J. A.;
Klinga, M.; Leskel€a, M. J. Organomet. Chem. 1998, 553, 173–178.
(18) Ransom, P.; Ashley, A. E.; Brown, N. D.; Thompson, A. L.;
O’Hare, D. Organometallics 2011, 30, 800ꢀ814.
(19) Marschner, C. Angew. Chem., Int. Ed. 2007, 46, 6770–6771.
(20) Krueger, C.; Mueller, G.; Erker, G.; Dorf, U.; Engel, K.
Organometallics 1985, 4, 215–223.
(21) Jacobsen, H.; Berke, H.; Brackemeyer, T.; Eisenbl€atter, T.;
Erker, G.; Fr€ohlich, R.; Meyer, O.; Bergander, K. Helv. Chim. Acta 1998,
81, 1692–1709.
(22) Oishi, M.; Kato, T.; Nakagawa, M.; Suzuki, H. Organometallics
2008, 27, 6046–6049.
(23) Lang, H.; Herres, M.; Imhof, W. J. Organomet. Chem. 1994,
465, 283–287.
(24) Gelmini, L.; Stephan, D. W. Organometallics 1988, 7, 849–855.
(25) Baker, R. T.; Fultz, W. C.; Marder, T. B.; Williams, I. D.
Organometallics 1990, 9, 2357–2367.
(26) Much like the reactions to form 4Zrꢀ6Zr, the fate of the second
Iꢀ ion and the origin of the reduction from CoII to CoI has not been
unambiguously determined. However, since similar products are not
formed from CoCl2, we suggest that the 2Iꢀ f I2 redox couple plays a
role in the reduction of CoI. The formation of I2 could not be monitored,
however, since I2 reacts readily with the tris(phosphinoamide)MCl
starting materials.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files and additional crys-
b
tallographic details for complexes 4ꢀ8 and 11. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
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’ ACKNOWLEDGMENT
This material is based upon work supported by the Depart-
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