Mononuclear tantalum(IV, d1) imido complexes supported by the monocyclopentadienyl, amidinate and guanidinate ligand sets as models to explore dinitrogen fixation by "end-on-bridged" dinuclear {[Ta(IV, d 1)]}2(μ-ν1:ν1-N 2) complexes

Article abstract of DOI:10.1021/om200961r

Two independent synthetic routes to the monocyclopentadienyl, amidinate (CpAm) and guanidinate (CpGu) mononuclear Ta(IV, d¹) terminal imido complexes Cp*Ta[N(ⁱPr)C(X)N(ⁱPr)][N(^tBu)] (Cp* = ν⁵-C₅Me₅) for X = Me (1) and NMe₂ (2), respectively, were developed. For 1, synthesis proceeded via the amido, chloride intermediate Cp*Ta[N(ⁱPr)C(Me)N( ⁱPr)](Cl)[NH(^tBu)] (4), which was kinetically deprotonated with LiN(ⁱPr)₂ to yield the enamido, amido species Cp*Ta[N(ⁱPr)C(CH₂)N(ⁱPr)[NH( ^tBu)] (5). In toluene solution, 5 underwent quantitative tautomerization to the desired CpAm terminal imido 1. For 2, the amido, chloride intermediate Cp*Ta[N(ⁱPr)C(NMe₂)N( ⁱPr)](Cl)-[NH(^tBu)] (8) was first synthesized and then reacted with TEMPO to provide the Ta(V) imido chloride Cp*Ta[N( ⁱPr)-C(NMe₂)N(ⁱPr)](Cl)[N(^tBu)] (9) through oxidative hydrogen-atom abstraction of the amido group. Chemical reduction of 9 with potassium graphite (KC₈) then served to provide the desired CpGu terminal imido 2. All new compounds were structurally characterized by single-crystal X-ray analysis. Although 1 and 2 proved to be unreactive toward hydrogenation or hydrosilyation involving the Ta=N bond, 2 was shown to engage in radical-based chemistry with MeI and PhS-SPh to yield the Ta(V) imido complexes Cp*Ta[N(ⁱPr)C(NMe₂)N( ⁱPr)](X)[N(^tBu)], where X = Me (10), I (11), and SPh (12). A similar radical-based reaction of {Cp*Ta[N(ⁱPr)C(Me)N( ⁱPr)]}₂(μ-ν¹:ν¹-N ₂) (I) with PhS-SPh yielded the product of formal 1,4-addition, {Cp*Ta[N(ⁱPr)C(Me)N(ⁱPr)](SPh)}₂(μ- ν¹:ν¹-N₂) (13).

Full text of DOI:10.1021/om200961r

Article
pubs.acs.org/Organometallics
Mononuclear Tantalum(IV, d1) Imido Complexes Supported by the
Monocyclopentadienyl, Amidinate and Guanidinate Ligand Sets As
Models to Explore Dinitrogen Fixation by “End-On-Bridged”
Dinuclear {[Ta(IV, d1)]}2(μ-η1:η1-N2) Complexes
Brendan L. Yonke, Andrew J. Keane, Peter Y. Zavalij, and Lawrence R. Sita*
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
S
* Supporting Information
ABSTRACT: Two independent synthetic routes to the
monocyclopentadienyl, amidinate (CpAm) and guanidinate
(CpGu) mononuclear Ta(IV, d1) terminal imido complexes
Cp*Ta[N(iPr)C(X)N(iPr)][N(tBu)] (Cp* = η5-C5Me5) for X
= Me (1) and NMe2 (2), respectively, were developed. For 1,
synthesis proceeded via the amido, chloride intermediate
Cp*Ta[N(iPr)C(Me)N(iPr)](Cl)[NH(tBu)] (4), which was
kinetically deprotonated with LiN(iPr)2 to yield the enamido,
amido species Cp*Ta[N(iPr)C(CH2)N(iPr)[NH(tBu)] (5).
In toluene solution, 5 underwent quantitative tautomerization
to the desired CpAm terminal imido 1. For 2, the amido,
chloride intermediate Cp*Ta[N(iPr)C(NMe2)N(iPr)](Cl)-
[NH(tBu)] (8) was first synthesized and then reacted with TEMPO to provide the Ta(V) imido chloride Cp*Ta[N(iPr)-
C(NMe2)N(iPr)](Cl)[N(tBu)] (9) through oxidative hydrogen-atom abstraction of the amido group. Chemical reduction of 9
with potassium graphite (KC8) then served to provide the desired CpGu terminal imido 2. All new compounds were structurally
characterized by single-crystal X-ray analysis. Although 1 and 2 proved to be unreactive toward hydrogenation or hydrosilyation
involving the TaN bond, 2 was shown to engage in radical-based chemistry with MeI and PhS-SPh to yield the Ta(V) imido
complexes Cp*Ta[N(iPr)C(NMe2)N(iPr)](X)[N(tBu)], where X = Me (10), I (11), and SPh (12). A similar radical-based
reaction of {Cp*Ta[N(iPr)C(Me)N(iPr)]}2(μ-η1:η1-N2) (I) with PhS-SPh yielded the product of formal 1,4-addition,
{Cp*Ta[N(iPr)C(Me)N(iPr)](SPh)}2(μ-η1:η1-N2) (13).
dinitrogen complexes of general formula {(η5-C5R5)M[N(R1)-
INTRODUCTION
Elucidation of the structural and electronic requirements for
metal-mediated activation of dinitrogen that serves to
■
C(X)N(R2)]}2(μ-N2), where X is an alkyl (R) or a
dialkylamino (NR′2) substituent, respectively, and M is a
metal from group 4, group 5, and group 6.6−8 Importantly, as
substantially lower energy barriers for subsequent NN
bond cleavage and efficient nitrogen-containing product
formation through N-atom functionalization reactionspro-
cesses that collectively constitute dinitrogen fixationremains
an important academic challenge with enormous societal
impact if realized.1−4 A successful strategy employed in these
efforts has been to utilize low-valent transition metal fragments
to formally reduce dinitrogen through bridging coordination
within dinuclear complexes of general formula [LnM]2(μ-N2).1
On the other hand, a survey of the magnitudes of crystallo-
graphically determined N−N distances in these dinuclear
complexes, which range from 1.20 to 1.65 Å, relative to that for
free N2 at 1.0971 Å,5 reveals that the extent of dinitrogen
activation that can be achieved in this manner is highly
dependent upon the nature of both the supporting metal
ligands and the mode of bridging dinitrogen coordination.
Recently, we have reported that the η5-cyclopentadienyl, η2-
amidinate (CpAm) and η2-guanidinate (CpGu) ligand sets are
valuable for providing access to several series of dinuclear
Chart 1 reveals, both “end-on-bridged” (μ-η1:η1-N2) (A) and
“side-on-bridged” (μ-η2:η2-N2) (B) coordination modes have
now been established for this family of [LnM]2(μ-N2)
complexes. Of these, the open-shell, [Ta(IV, d1), Ta(IV, d1)],
CpAm and CpGu end-on-bridged derivatives {Cp*Ta[N(iPr)-
C(X)N(iPr)]}2(μ-η1:η1-N2) (Cp* = η5-C5Me5), where X = Me
(I) and NMe2 (II), respectively, which are shown in Scheme 1,
are of particular interest due to an intrinsically low thermal
stability of these complexes in solution. More specifically, above
0 °C, both I and II quantitatively convert to the corresponding
diamagnetic [Ta(V, d0), Ta(V, d0)] bis(μ-nitrido) products
{Cp*Ta[N(iPr)C(X)N(iPr)](μ-N)}2 (III and IV, respectively),
in which N−N bonding is now completely absent.7,8 With
respect to potential N-atom functionalization reactions,
however, it was surprising to find that I does not participate
Received: October 11, 2011
Published: December 19, 2011
© 2011 American Chemical Society
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Chart 1
proceeds through selective Ta−N σ-bond metathesis to provide
the “ring-opened” product VII according to Scheme 1.7 As
presented in Scheme 2, this last reaction is of interest given that
a second σ-bond metathesis reaction of VII with an additional
equivalent of H3SiPh would now generate two equivalents of
the proposed mononuclear Ta(V) imido VIII in which the
origin of the silylated nitrogen atom is from the chemical
fixation of molecular N2. Finally, Scheme 2 presents the
additional goal of releasing a N-containing product through
chemical transformations of the TaN imido bond, such as
through exhaustive hydrogenolysis.
Scheme 1
In order to systematically, and extensively, investigate the
chemical reactivity profile of MN double bonds within
CpAm and CpGu early transition metal complexes that are
relevant to dinitrogen activation and N-atom functionalization,
we set out to synthesize and characterize a variety of group 4,
group 5, and group 6 mononuclear terminal imido complexes
of the general structure {(η5-C5R5)M[N(R1)C(X)N(R2)][N-
(R3)] (C) in Chart 1, in which the formal oxidation state of the
metal remains tetravalent across the series, i.e., M(IV).
Following a successful synthesis of the CpAm group 4 Zr(IV,
d0) terminal imido Cp*Zr[N(iPr)C(Me)N(iPr)][N(tBu)]
(IX),12 we next set sights on preparing analogous group 5
derivatives. In this regard, while the chemistry of first-row,
mononuclear V(IV) terminal imido complexes is well
established,13 examples of second-row, mononuclear Nb(IV)
terminal imido derivatives are extremely rare,14 and, quite
surprisingly, to the best of our knowledge, no reports have ever
appeared that detail the synthesis, isolation, and structural
characterization of a third-row, mononuclear Ta(IV, d1)
terminal imido species.15 This dearth of knowledge for second-
and third-row mononuclear group 5 metal imidos stands in
sharp contrast to the abundance of existing precedent for
dimeric M(IV) imido complexes in which two bridging imido
fragments (μ-NR) contribute to formation of a four-membered-
ring M2N2 core and a transannular M−M single bond.16 It is
also important to note that a significant wealth of experimental
data currently exists for the structures and chemical reactivities
of a variety of mononuclear Ta(V, d0) terminal imido
complexes.11,17
in 1,2-addition of either dihydrogen (H2) or phenylsilane
(H3SiPh) across the formal TaN double bonds,1−4 but
instead, this complex engages in dinuclear “1,4-addition” of
these reagents across the TaN−NTa framework to
generate the dinuclear μ-η1:η1-N2 dihydride and silyl, hydride
products V and VI, respectively, in which both metal centers
are formally oxidized to Ta(V, d0) (see Scheme 1).
Interestingly, while Fryzuk and co-workers1h have detailed a
variety of chemical transformations involving N-atom function-
alization within the novel “end-on, side-on-bridged” Ta(IV),
Ta(IV) dinuclear dinitrogen complex ([NPN]Ta)2(μ-H)2(μ-
η1:η2-N2) (where [NPN] = (PhNSiMe2CH2)2PPh) and with
some of these leading to complete N−N bond cleavage,
Schrock and co-workers9 did not report having observed either
the 1,2- or 1,4-addition of reagents across the formal TaN−
NTa framework of their seminal Ta(V), Ta(V) end-on-
bridged dinitrogen complex [Ta(CHCMe3)(PMe3)2Cl]2(μ-
η1:η1-N2).10
While sparse precedent does exist for the 1,2-addition of H2
and silanes across the TaN double bond of mononuclear
Ta(V) imido complexes,11 in the case of the Ta(V), Ta(V)
bis(μ-nitrido) complex III, reaction with H3SiPh alternatively
In this report, we now detail the successful synthesis and
structural characterization of the mononuclear CpAm and
CpGu Ta(IV) terminal imido derivatives Cp*Ta[N(iPr)C(X)-
Scheme 2
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Scheme 3
N(iPr)][N(tBu)] where X = Me (1) and NMe2 (2),
respectively. These results include the establishment of two
independent synthetic routes to 1 and 2 that was necessitated
by obvious, and more subtle, differences between the CpAm
and CpGu ligand sets. Importantly, the chemical reactivity
profiles displayed by 1 and 2 provide new insights that are
relevant to understanding the factors that dictate the observed
chemical properties of the dinuclear dinitrogen complexes I and
II. Finally, with 1 and 2 in hand, a valuable comparison could
be made between these group 5 derivatives of C and that of the
isostructural CpAm mononuclear group 4 Zr(IV, d0) terminal
imido, Cp*Zr[N(iPr)C(Me)N(iPr)][N(tBu)] (IX), that we
have previously prepared according to the synthetic route
shown in Scheme 3.12
In our previous studies, synthesis of the CpAm Zr(IV)
terminal imido IX proceeded through the bis(amido)
intermediate Cp*Zr[N(iPr)C(Me)N(iPr)][NH(tBu)]2 (X)12
(see Scheme 3); however, all attempts to prepare a Ta(IV)
analogue of X by reaction of 4 with a second equivalent of
LiNH(tBu), according to Scheme 4, produced only intractable
oils that failed to crystallize, thereby frustrating efforts to obtain
unequivocal structural verification of the desired paramagnetic
Ta(IV) product. Attempts to either alkylate or deprotonate 4
using a variety of organolithium reagents, LiR (R = Me, Et, and
nBu), likewise yielded a similar end result. On the other hand,
as Scheme 5 reveals, the reaction of 4 with slightly less than one
equivalent of lithium diisopropylamide, LiN(iPr)2, in tetrahy-
drofuran (THF) at −30 °C provided a good yield (66%) of the
deprotonated product 5 that could be isolated as a dark green
crystalline material, for which elemental analysis was consistent
with overall loss of HCl from 4.
Single-crystal X-ray analysis of 5 served to confirm that
deprotonation of 4 occurred on the methyl group of the
amidinate fragment to provide the enamido, amido structure
Cp*Ta[N(iPr)C(CH2)N(iPr)][NH(tBu)], which is depicted in
Scheme 5. The solid-state molecular structure and selected
geometric parameters of 5 are provided in Figure 1b and Table
1, respectively. We have previously reported similar deproto-
nations of the acetamidinate ligand within different classes of
group 4 and group 5 metal CpAm derivatives,9,17,19 and in fact,
this includes the synthesis and structural characterization of the
group 4 Zr(IV) analogue of 5, Cp*Zr[N(iPr)C(CH2)N(iPr)]-
[NH(tBu)] (XI) (see Scheme 3).12 In comparing the structures
of 5 and XI, the former compound exhibits a small contraction
of the metal−ligand bonding distances relative to the latter due
to differences in covalent radii as expected [cf. for 5, Ta(1)−
N(1) is 1.9804(13) Å and for XI the corresponding Zr−N
bond length is 2.0533(16) Å].20 Both compounds also possess
a more reduced M−N−C bond angle associated with the
NH(tBu) fragment than that displayed by 4 [cf. Ta(1)−N(1)−
C(19), 136.72(10)° for 5 vs 147.82(19)° for 4]. Finally, the
Ct(1)−Ta(1)−N(1)−H(1) torsion angle for 5 is 0.2°, and this
serves to direct the N−H bond directly toward the Cp*
fragment (see Figure 1b).
RESULTS AND DISCUSSION
■
A. Synthesis and Characterization of Cp*Ta[N(iPr)C-
(Me)N(iPr)][N(tBu)] (1). The synthetic route to 1 is presented
in Scheme 4. Thus, to begin, reaction of the CpAm Ta(IV)
Scheme 4
dichloride Cp*Ta[N(iPr)C(Me)N(iPr)]Cl2 (3)7,18 with one
equivalent of lithium tert-butylamide, LiNH(tBu), in diethyl
ether (Et2O) at −30 °C provided a good yield (64%) of the
paramagnetic Ta(IV) amido chloride Cp*Ta[N(iPr)C(Me)N-
(iPr)][NH(tBu)]Cl (4), as a dark red, crystalline material for
which elemental analysis was fully consistent with the indicated
empirical formula. Single crystals of 4 were further subjected to
X-ray crystallography, and Figure 1a and Table 1 provide the
solid-state molecular structure and selected geometric param-
eters for this compound. Of special interest with respect to
structural comparisons that will be made in this report is the
Ta(1)−N(1) bond distance of 1.960(2) Å and the Ta(1)−
N(1)−C(19) bond angle of 147.82(19)° for 4. It is also
interesting to note that the torsion angle for Ct(1)−Ta(1)−
N(1)−H(1) is 88.1°, which orients the N−H bond of the
NH(tBu) group “outward” and pointed away from both the
Cp* and Cl ligands (see Figure 1a).
As depicted in Scheme 5, it can be quickly appreciated that
compound 5 is the tautomer of the desired CpAm Ta(IV)
terminal imido 1. In our previous studies with the
corresponding tautomeric group 4 Zr(IV) analogues, IX and
XI, no evidence was obtained for a dynamic solution
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Figure 1. Molecular structures (30% thermal ellipsoids) of (a) 4, (b) 5, and (c) 1. Hydrogen atoms have been removed for the sake of clarity, except
for selected ones for 4 and 5, which are represented by spheres of arbitrary size.
Table 1. Selected Geometric Parameters for Compounds 1, 2, 4, 5, and 8−12
parameter
1
2
4
5
8
9
10
11
12
Bond Lengths (Å)
Ta(1)−N(1)
Ta(1)−N(2)
Ta(1)−N(3)
C(11)−C(12)
C(11)−N(2)
C(11)−N(3)
1.790(3)
2.180(2)
2.180(2)
1.499(5)
1.326(3)
1.326(3)
1.785(2)
2.188(2)
2.190(2)
1.960(2)
2.1644(18)
2.2050(17)
1.506(3)
1.341(3)
1.329(3)
1.9804(13)
2.0432(12)
2.0471(13)
1.353(2)
1.978(2)
1.775(2)
2.183(2)
2.220(2)
1.793(2)
2.205(2)
2.219(2)
1.779(2)
2.226(2)
2.178(2)
1.7937(13)
2.2061(13)
2.2132(13)
2.2127(18)
2.1607(19)
1.406(2)
1.330(3)
1.332(3)
1.344(4)
1.326(3)
1.328(3)
1.338(3)
1.329(4)
1.348(4)
1.335(2)
1.343(2)
1.4009(19)
Bond Angles (deg)
136.72(10)
Ta(1)−N(1)−C(19)
Ta(1)−N(1)−C(20)
177.3(3)
147.82(19)
176.2(2)
143.90(17)
165.0(2)
168.4(2)
169.9(2)
171.34(12)
Scheme 5
equilibrium existing between the two forms in solution.12 In
contrast, in the present work, it was determined that, in
benzene solution at 25 °C, 5 quantitatively converted after ∼18
h to 1, with the latter being isolated as a bright red, crystalline
material (see Scheme 5). These contrasting results for the
group 4 and group 5 isostructural tautomeric systems strongly
suggest that a lower barrier for tautomerization of the enamido,
amido kinetically deprotonated product 5 to the more
thermodynamically favored amidinate, imido species 1 exists
reaction conditions, it is also possible that the isomerization of
5 to 1 is catalyzed by the base that is present.
Although both 1 and 5 possess the same empirical formula,
1
noticeable differences in the paramagnetically shifted H NMR
resonances for these two compounds could be used to follow
the tautomerization process. Unequivocal confirmation of the
monomeric terminal imido structure of 1, however, was
provided by a single-crystal X-ray analysis, and Figure 1c and
Table 1 present the solid-state molecular structure and selected
geometric parameters for this compound, respectively. The
most notable feature of 1 is that, relative to 5, the Ta(1)−N(1)
bond distance is shorter at 1.790(3) Å, while the Ta(1)−
N(1)−C(19) bond angle is more linear at 177.3(3)° (see Table
1). Finally, in comparing 1 with the Zr(IV) analogue IX, the
two have nearly superimposable structures, but with the former
compound once again possessing slightly contracted metal−
ligand bond distances [cf. for IX: Zr−N(tBu), 1.839(2) Å].12
B. Synthesis and Characterization of Cp*Ta[N(iPr)C-
(NMe2)N(iPr)][N(tBu)] (2). Although the enamido, amido to
amidinate, imido tautomerization route to the CpAm Ta(IV)
terminal imido 1 was successful, it could not be adopted for
providing access to the corresponding CpGu derivative 2 due
for the more electron-rich Ta(IV, d1) center, vis-a-
̀
vis the Zr(IV,
d0) metal center of IX, by virtue of the enamido group in 5 also
being more nucleophilic and, hence, more susceptible to
tautomerization through bimolecular proton transfer than IX.21
A further important point to make is that, to date, the desired
CpAm Ta(IV) terminal imido 1 is accessible only through the
two-step kinetic deprotonation/tautomerization pathway of
Scheme 5. In practice, a “one-pot” procedure was employed to
provide a moderate yield (33%) of 1 that entailed first treating
4 with a slight excess (1.25 equiv) of LiN(iPr)2 in THF at −30
°C, followed by stirring overnight at room temperature, and
then isolation of 1 through standard workup. Under these
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to the lack of a similar enolizable proton on the Gu ligand.
Accordingly, it was not known a priori whether the absence of
deprotonation pathways would either help or hinder the
development of a different synthetic route to 2. As Scheme 6
specifically, while benzene solutions of the CpAm derivative 5
were indefinitely stable at 25 °C, at a similar solution
concentration, the CpGu analogue 8 underwent complete
decomposition over the course of 18 h at room temperature.
The resulting crude material, however, consisted of only two
major degradation products, and one of these could be isolated
in analytically pure form as a diamagnetic, white crystalline
compound. Subsequent single-crystal X-ray and elemental
analyses served to establish its identity as being the
mononuclear CpGu Ta(V) imido chloride Cp*Ta[N(iPr)C-
(NMe2)N(iPr)][N(tBu)]Cl (9), which is depicted in Scheme 5.
Figure 2b and Table 1 present the solid-state molecular
structure and selected geometric parameters, respectively, for 9,
and here it can be noted that the Ta(1)−N(1) bond distance of
1.775(2) Å and the more acute Ta(1)−N(1)−C(20) bond
angle of 165.0(2)° that are associated with the TaN(tBu)
imido moiety are in keeping with the corresponding parameters
obtained for other known mononuclear Ta(V) imido
species.17d Finally, as with the CpAm derivative 5, attempts
to either alkylate or deprotonate 8 by employing a variety of
organolithium reagents, RLi, were unsuccessful and only
yielded complex mixtures.
Scheme 6
Formal one-electron-based hydrogen atom abstraction
(HAA) from a transition metal amido species, followed by
formal one-electron oxidation of the metal, and, alternatively,
metal oxidation followed by proton (H+) loss have been
established as viable pathways for production of early and late
transition metal imido complexes.22−24 Following the lead
provided by others,22,24 we sought to develop the 8 → 9
degradative transformation into a synthetically useful proce-
dure. Thus, addition of one equivalent of (2,2,6,6-tetramethyl-
piperidin-1-yl)oxyl (TEMPO) to a pentane solution of 8 at
room temperature provided an excellent yield (92%) of 9 that
could be isolated in simple fashion according to Scheme 6. The
desired CpGu Ta(IV) terminal imido 2 was then obtained in
likewise excellent yield (85%) through chemical reduction of 9
with potassium graphite (KC8) in THF at −30 °C (see Scheme
6). Elemental and single-crystal X-ray analyses served to
confirm the identity of 2, and Figure 2c and Table 1 provide the
solid-state molecular structure and selected geometric param-
eters for this compound, respectively. Interestingly, 2 displays a
Ta(1)−N(1) bond length of 1.785(2) Å, which is nearly
identical to that of 1, thereby suggesting a minimal, if any, effect
on the nature of TaN multiple bonding on going from the
reveals, reaction of the CpGu Ta(IV) dichloride Cp*Ta[N-
(iPr)C(NMe2)N(iPr)]Cl2 (7), obtained from sodium amalgam
(NaHg) reduction of the corresponding CpGu Ta(V)
trichloride Cp*Ta[N(iPr)C(NMe2)N(iPr)]Cl3 (6), with one
equivalent of LiNH(tBu) in Et2O provided an excellent yield
(92%) of the corresponding CpGu Ta(IV) amido chloride
Cp*Ta[N(iPr)C(NMe2)N(iPr)][NH(tBu)]Cl (8), which was
isolated as a dark purple, crystalline material. Given the
paramagnetic nature of 8, single-crystal X-ray analysis was once
again used to confirm the composition and molecular structure
of this compound according to the data presented in Figure 2a
and Table 1. In short, only small differences in the Ta(1)−
N(1), Ta(1)−N(1)−C(20), and Ct(1)−Ta(1)−N(1)−H(1)
bond distance, bond angle, and torsion angle parameters were
observed for 8, vis-a-vis the corresponding values already
̀
presented for 5 (cf. Table 1).
Although any discernible structural differences between the
CpAm and CpGu Ta(IV) amido chloride derivatives 5 and 8
are very slight, the two compounds displayed pronounced
differences in solution with respect to thermal stability. More
Figure 2. Molecular structures (30% thermal ellipsoids) of (a) 8, (b) 9, and (c) 2. Hydrogen atoms have been removed for the sake of clarity, except
for the amido hydrogen atom of 8, which is represented by a sphere of arbitrary size.
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CpAm to the CpGu ligand set. As a final note, an exploratory
study confirmed that the CpAm derivative 5 reacts in similar
fashion with TEMPO to provide a corresponding diamagnetic
CpAm Ta(V) imido chloride product; however, the complete
analysis of this latter compound was not pursued further.
C. Chemical Reactivity of Cp*Ta[N(iPr)C(X)N(iPr)][N-
(tBu)]. Having successfully synthesized the CpAm and CpGu
Ta(IV) terminal imido complexes 1 and 2, it was of significant
interest to probe the chemistry associated with the TaN
paramagnetic resonance (EPR) spectrum presented in Figure 3
reveals that the unpaired electron is strongly coupled [giso
=
̀
double bond of these open-shell compounds, vis-a-vis that was
previously established for the [Ta(IV), Ta(IV)](μ-η1:η1-N2)
complexes, I and II, and for the isostructural CpAm Zr(IV)
terminal imido derivative IX. For these studies, the CpGu
derivative 2 was generally preferred over the use of 1 given the
greater ease in obtaining larger, analytically pure quantities of
the former compound through the synthetic route of Scheme 5.
To begin, both 1 and 2 were somewhat surprisingly found to
be completely inert toward H2 in toluene solution, even at
temperatures of up to 100 °C for extended periods of time (ca.
72 h). Compounds 1 and 2 also proved to be inert in toluene
solution toward hydrosilylation employing either H3SiPh or n-
hexylsilane, H3Si(n-C6H13), nor did any observable reaction
occur under CO pressure. A similar lack of reactivity of the
CpAm Zr(IV) terminal imido IX toward hydrogenation and
hydrosilylation was previously observed.12 Mountford and co-
workers25 have previously established a rich cycloaddition
chemistry associated with the TiNR moiety of the first-row
group 4 congener, Cp*Ti[N(iPr)C(NMe2)N(iPr)][N(tBu)]
(XII); however, studies relating to the hydrogenation or
hydrosilylation of these compounds have not yet been reported
by this group. Finally, it is interesting to note that, in sharp
contrast to the apparent general lack of reactivity of group 4
and group 5 CpAm and CpGu derivatives of C (see Chart 1)
toward hydrogenation, Chirik and co-workers26 found the
opposite to be true for the bis(η5-cyclopentadienyl) titanium
imido complex {[η5-C5H3-1,3-(SiMe3)2]}2Ti[N(SiMe3)]
(XIII), which undergoes rapid 1,2-addition of H2 across the
formal TiN bond.
Figure 3. X-band ESR spectrum (toluene, 25 °C) of compound 1; giso
= 1.975, Aiso(181Ta) = 217 G.
1.975, Aiso(181Ta) = 217 G] to the Ta metal center, whereas
with I, SQUID magnetic susceptibility data recorded between 4
and 300 K are consistent with a singlet ground state that
coexists with a low-lying triplet state that is thermally populated
at room temperature.7 Given these similar characteristics, it was
of interest to determine whether the mononuclear imido and
dinuclear dinitrogen systems might engage in similar one-
electron chemistry.
To begin, compound 2 was found to react cleanly with one
equivalent of methyl iodide in pentane solution to provide a 1:1
mixture of the CpGu Ta(IV) imido, methyl and imido, iodo
complexes Cp*Ta[N(iPr)C(NMe2)N(iPr)](X)[N(tBu)],
where X = Me (10) and I (11), respectively, according to
Scheme 7. This result is supportive of a one-electron halogen
abstraction mechanism28 that has been previously reported for
17-electron tantalum29 and chromium30 complexes supported
by a closely related η5-C5H5, η2-β-diketiminate [N(Ar)C-
(Me)]2CH (Ar = 2,6-diisopropylphenyl) ligand set. Efforts to
obtain analytically pure samples of 10 and 11 were only
partially successful in that multiple fractional recrystallizations
provided cocrystals enriched in either 10 (88%) or 11 (84%),
as confirmed by both elemental and single-crystal X-ray
analyses. Figure 4a and b present the solid-state molecular
structures of 10 and 11 as derived from cocrystals in which each
is present as the major component (i.e., the structure of 10 in
Figure 4a was obtained from a 90:10 cocrystal of 10:11, while
that in Figure 4b was obtained from a 10:90 cocrystal ratio).
Table 1 provides the values for selected geometric parameters
for these two compounds.
When a pentane solution of I was treated with MeI in similar
fashion to 2, an intractable complex mixture of compounds was
obtained that appeared to include products arising from
indiscriminate radical abstraction chemistry, e.g., those with
(I)[Ta](μ-N2)[Ta](Me), (I)[Ta](μ-N2)[Ta](I), and (Me)-
[Ta](μ-N2)[Ta](Me) frameworks.31 Unfortunately, all at-
tempts to separate this product mixture into analytically pure
samples that could then be subjected to analytical and
crystallographic structural analyses were unsuccessful. This
impasse, however, motivated efforts to investigate other classes
of reagents for radical chemistry that might yield more
straightforward results. Thus, as Scheme 8 shows, when a
solution of 2 in pentane was treated with one equivalent of
diphenyldisulfide, PhS-SPh, at room temperature, near
It is obvious from the present results that the Ta(IV) CpAm
and CpGu imido derivatives 1 and 2, respectively, are not viable
models for investigating a mechanism for hydrogenation of I
that involves initial 1,2-addition across the formal TaN
bonds. Indeed, Lei and Musaev and co-workers27 just very
recently published the results of a computational investigation
that suggest that the mechanistic pathway for the I → V
transformation is far more complicated than either 1,2-addition
or simple hydrogen atom abstraction by a metal-centered
radical and, more importantly, that this transformation is
dependent upon the dinuclear nature of the substrate. More
specifically, these authors conclude that the first step involves
structural rearrangement of the initial end-on-bridged [Ta]2(μ-
η1:η1-N2) framework of I to an “end-on, side-on” [Ta]2(μ-η1:η2-
N2) bonding motif that then undergoes 1,1-oxidative addition
of H2 at one of the metal centers in a manner that involves a
concomitant change in the formal oxidation states of both the
N−N bridging group and the other Ta center. Subsequent H-
atom migration from the newly formed Ta(V) dihydride center
to the other Ta center then proceeds through formation of a
bridging hydride group within a dinuclear μ-H intermediate.27
One feature that is common to both the mononuclear Ta(IV,
d1) imido complexes 1 and 2 and the dinuclear dinitrogen
complexes I and II is that at ambient temperature these are all
open-shell paramagnetic species. In the case of 1, the electron
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Organometallics
Article
Scheme 7
Figure 4. Molecular structures (30% thermal ellipsoids) of (a) 10, (b) 11, and (c) 12. Hydrogen atoms have been removed for the sake of clarity.
Scheme 8
quantitative conversion into the Ta(V) imido, sulfido complex
12 occurred after only a few minutes. Single-crystal X-ray
analysis of 12 confirmed the solid-state molecular structure of
this species that is presented in Figure 4c, and Table 1 presents
selected bond lengths and bond angles.
Gratifyingly, addition of one equivalent of PhS-SPh to a
pentane solution of the dinuclear dinitrogen complex I also
resulted in a rapid reaction that led to complete conversion to
two new compounds that were easily separated on the basis of
Figure 5. Molecular structure (30% thermal ellipsoids) of meso-13.
Selected bond lengths (Å) and bond angles (deg): Ta(1)−N(1)
1.831(2), Ta(1)−S(1) 2.5192(8), Ta(2)−N(2) 1.837(2), Ta(2)−S(2)
2.5235(8), N(1)−N(2) 1.297(3); Ta(1)−N(1)−N(2) 175.9(2),
Ta(2)−N(2)−N(1) 169.2(2).
their striking difference in solubility in pentane. Elemental
1
analyses and H NMR spectra of the pure products strongly
suggested that they were most likely diastereomers of the
ditantalum dinitrogen disulfide 13, which is depicted in Scheme
9, and indeed, recrystallization of the more soluble product
provided single crystals for X-ray analysis that confirmed it to
be meso-13 on the basis of the solid-state molecular structure
and selected geometric parameters that are presented in Figure
Scheme 9
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Cp*Ta[N(iPr)C(Me)N(iPr)][NH(tBu)]Cl (4). A solution of LiNH-
(tBu) (74 mg, 0.911 mmol) in 10 mL of Et2O was cooled to −30 °C
and added dropwise to a solution of 3 (471 g, 0.891 mmol) in 50 mL
of Et2O, precooled to −30 °C, over a period of 5 min. The reaction
mixture was allowed to warm to room temperature and stirred for 3 h
to produce a red-purple-colored solution. Volatiles were removed in
vacuo to yield a red-purple solid. The crude product was then taken up
in pentane and filtered through a short pad of Celite on a glass frit.
The collected dark red filtrate was concentrated and cooled to −30 °C
to provide dark red crystals of 4 (324 mg, 64% yield). Anal. Calcd for
C22H42N3ClTa: C, 46.75; H, 7.50; N, 7.44. Found: C, 46.94; H, 7.39;
N, 7.22.
5. Unfortunately, efforts to obtain single crystals of the other
isomeric product for a similar analysis have so far been
unsuccessful.
The above results confirm that each of the metal centers in I
can be viewed as having a formal Ta(IV, d1) electronic
configuration and that the reaction of I with PhS-SPh likely
proceeds via a radical abstraction pathway that leads to formal
(nonconcerted) 1,4-addition across the TaN−NTa frame-
work. In the disulfide product, the metal centers in meso-13 are
now formally Ta(V, d0) and the N−N bond distance of the
bridging group is 1.297(3) Å, which is similar to the values
obtained for other CpAm [Ta(V, d0), Ta(V, d0)] dinitrogen
derivatives, such as V, N−N 1.307(6) Å; VI, N−N 1.284(4) Å;
and {Cp*Ta(Cl)[N(iPr)C(Me)N(iPr)]}2(μ-η1:η1-N2), N−N
1.288(10) Å.7
Cp*Ta[N(iPr)C(CH2)N(iPr)][NH(tBu)] (5). A −30 °C solution of
LiN(iPr)2 (43 mg, 0.402 mmol) in 5 mL of THF was added dropwise
to a −30 °C solution of 4 (226 mg, 0.403 mmol) in 15 mL of THF.
The reaction mixture was warmed to room temperature and stirred at
this temperature for 1 h to produce a dark-green-colored solution.
Volatiles were removed in vacuo to yield a dark green solid. The crude
product was dissolved in minimal pentane and filtered through a short
pad of Celite on a glass frit. The collected filtrate was concentrated and
cooled to −30 °C to give dark green crystals of 5 (140 mg, 66%, yield).
Anal. Calcd for C22H41N3Ta: C, 49.99; H, 7.82; N, 7.95. Found: C,
50.21; H, 7.67; N, 7.93.
CONCLUSION
■
The present report demonstrates the ability to utilize the
CpAm and CpGu ligand sets to access, apparently for the first
time, mononuclear Ta(IV, d1) terminal imido complexes.
Although these complexes were found to be inaccessible using
traditional routes of α-abstraction or direct base deprotonation
of an amido group, novel routes involving an enamide, amido
to amidinate, imido tautomerization in the case of the CpAm
derivative 1, and an amido group oxidative hydrogen atom
abstraction followed by chemical reduction in the case of the
CpGu derivative 2, have now been established. Unfortunately,
the lack of reactivity of 1 and 2 toward hydrogenation or
hydrosilyation of the TaN bond limits the utility of these
compounds as models for similar reactions of group 5 dinuclear
dinitrogen complexes. On the other hand, the radical-based
chemistry of CpAm and CpGu Ta(IV, d1) imido complexes
appears rich and currently unexplored. Importantly, making
direct parallels of this chemistry to that of formal [Ta(IV, d1),
Ta(IV, d1)](μ-η1:η1-N2) complexes looks promising, and this
avenue of investigation may ultimately provide transformations
that can aid the objective of developing new classes of catalysts
for dinitrogen fixation. Efforts along these lines are currently in
progress.
Cp*Ta[N(iPr)C(Me)N(iPr)][N(tBu)] (1). A solution of LiN(iPr)2
(53 mg, 0.496 mmol) in 10 mL of Et2O, precooled to −30 °C, was
added dropwise to a solution of 4 (224 mg, 0.396 mmol) in 20 mL of
Et2O, precooled to −30 °C, over a period of 5 min. The reaction
mixture was allowed to warm to room temperature and stirred for 2 h
to produce a green-brown-colored solution. Volatiles were removed in
vacuo to give a brown oil. The oil was dissolved in pentane and filtered
through Celite. The collected dark brown filtrate was dried in vacuo to
give a brown solid, which was then dissolved in toluene and left at
room temperature for 16 h to produce a dark-red-colored solution, and
then the solvent was removed in vacuo to give a bright red oil. The oil
was dissolved in a minimal amount of pentane and cooled to −30 °C
for 1 week to give bright red crystals of 1 (76 mg, 33% yield). Anal.
Calcd for C22H41N3Ta: C, 49.97; H, 7.82; N, 7.95. Found: C, 49.76; H,
7.51; N, 7.86.
Cp*Ta[N(iPr)C(NMe2)N(iPr)]Cl3 (6). A solution of N′,N′-diisopro-
pylcarbodiimide (746 mg, 4.60 mmol) in 10 mL of THF, precooled to
−30 °C, was added dropwise to a solution of LiNMe2 (235 mg, 4.60
mmol) in 10 mL of THF, precooled to −30 °C. The reaction mixture
was allowed to warm to room temperature and stirred for 1 h. The
resulting pale yellow solution was then cooled to −30 °C and added
dropwise to a solution of Cp*TaCl4 (2.01 g, 4.38 mmol) in 80 mL of
THF, precooled to −30 °C, over a period of 5 min. The reaction
mixture was allowed to warm to room temperature and stirred for 16 h
to produce a red-colored solution, after which time, volatiles were
removed in vacuo to give a red solid. The red solid was dissolved in
toluene and filtered through a short pad of Celite on a glass frit. The
filtrate was concentrated, pentane was layered on top, and then the
mixture was cooled to −30 °C to provide red crystals of 6 (2.18 g, 84%
yield). Anal. Calcd for C19H35Cl3N3Ta: C, 38.50; H, 5.95; N, 7.09.
Found: C, 38.81; H, 5.68; N, 7.22. 1H NMR (400 MHz, benzene-d6):
1.17 (6H, d, CH(CH3), J = 6.7 Hz), 1.88 (6H, d, CH(CH3)2, J = 6.9
Hz), 2.22 (15H, s, C5(CH3)5), 2.32 (6H, s, N(CH3)2), 4.26 (1H, sep,
CH(CH3)2, J = 6.7 Hz), 4.57 (1H, sep, CH(CH3)2, J = 6.9 Hz).
Cp*Ta[N(iPr)C(NMe2)N(iPr)]Cl2 (7). Sodium amalgam, 0.5% (w/
w) Na/Hg (16.7 g, 3.64 mmol), was added dropwise to a solution of 6
(1.99 g, 3.36 mmol) in 100 mL of Et2O, precooled to −30 °C. The
solution was allowed to warm to room temperature and stirred for 16
h to give a dark-orange-colored solution, at which time the volatiles
were removed in vacuo to give a brown-orange solid. The solid was
dissolved in toluene and filtered through a short pad of Celite on a
glass frit. The dark red filtrate was concentrated and cooled to −30 °C
to give dark orange crystals of 7 (1.59 g, 85% yield). Anal. Calcd for
C19H35N3Cl2Ta: C, 40.94; H, 6.33; N, 7.54. Found: C, 41.11; H, 6.20;
N, 7.43.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations with air- and
moisture-sensitive compounds were carried out under an inert
atmosphere of N2 or Ar using standard Schlenk-line or glovebox
techniques. All solvents were dried (Na for toluene and Na/
benzophenone for pentane, Et2O, and THF) and distilled under N2
prior to use. Benzene-d6 was dried over Na/K alloy and isolated by
vacuum transfer. Celite was oven-dried (150 °C for several days), and
all amines were dried over CaH2 and distilled by short-path distillation
prior to use. Cooling for reactions was achieved using the internal
freezer of a glovebox maintained at −30 °C. TEMPO, N,N-
diisopropylcarbodiimide, and LiNMe2 were purchased from Sigma-
Aldrich, and Cp*TaCl4 was purchased from Strem Chemicals. All
chemicals were used as received. Cp*Ta[N(iPr)C(Me)N(iPr)]Cl2 (3)
was prepared according to previously reported procedures.17 All H
1
NMR spectra were recorded at 400 MHz. Solution EPR spectra were
recorded at Georgetown University on a JEOL JES-FA200 continuous
wave spectrometer equipped with an X-band Gunn oscillator bridge
and a cylindrical mode cavity employing a modulation frequency of
100 kHz. All spectra were obtained from freshly prepared toluene
solutions (5 mM) in a nitrogen-filled glovebox. Aiso(181Ta) values in
gauss were determined by the separation of the middle two lines of the
eight-line pattern. Elemental analyses (C, H, and N) were performed
by Midwest Microlab, LLC.
Cp*Ta[N(iPr)C(NMe2)N(iPr)][NH(tBu)]Cl (8). A solution of
LiNH(tBu) (123 mg, 1.55 mmol) in 20 mL of Et2O, precooled to
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−30 °C, was added dropwise to a solution of 7 (823 mg, 1.48 mmol)
in 35 mL of Et2O, precooled to −30 °C, over a period of 5 min. The
reaction mixture was allowed to warm to room temperature and stirred
for 4 h to give a purple-colored solution, at which time the volatiles
were removed in vacuo to yield a purple solid. The solid was dissolved
in pentane and filtered through a short pad of Celite on a glass frit.
The dark purple filtrate was concentrated and cooled to −30 °C to
give dark purple crystals of 8 (805 mg, 92% yield). Anal. Calcd for
C23H45N4ClTa: C, 46.48; H, 7.64; N, 9.43. Found: C, 46.41; H, 7.39;
N, 9.32.
0.91 (3H, d, CH(CH3)2, J = 7.0 Hz), 0.95 (3H, d, CH(CH3)2, J = 6.7
Hz), 1.27 (9H, s, C(CH3)3), 1.30 (3H, d, CH(CH3)2, J = 6.7 Hz), 1.33
(3H, d, CH(CH3)2, J = 7.0 Hz), 2.07 (15H, s, C5(CH3)5), 2.33 (6H, s,
N(CH3)2), 4.04 (1H, sep, CH(CH3)2, J = 7.0 Hz), 4.12 (1H, sep,
CH(CH3)2, J = 6.7 Hz), 6.96 (1H, m, S(C6H5)), 7.20 (2H, m,
S(C6H5)), 8.03 (2H, m, S(C6H5)).
{Cp*Ta[N(iPr)C(Me)N(iPr)](SPh)}2(μ-η1:η1-N2) (13). Diphenyl di-
sulfide (24 mg, 0.111 mmol) was added to a solution of
{Cp*Ta[N(iPr)C(Me)N(iPr)]}2(μ-η1:η1-N2) (I) (104 mg, 0.111
mmol) in 5 mL of pentane that was precooled to −30 °C. After
warming to room temperature and stirring for 5 min, the volatiles were
removed in vacuo to provide a mixture of a dark orange oil and a light
orange powder (isomer). The mixture was taken up in 5 mL of Et2O
and cooled to −30 °C, whereupon the solids were isolated by filtration
through a glass-frit. The dark orange filtrate was concentrated in vacuo
and then cooled to −30 °C to provide dark orange crystals of meso-13
(33 mg, 25% yield). The light orange solids were collected and washed
with first pentane and then Et2O. The solids were then dissolved in a
minimal amount of toluene and allowed to precipitate from solution at
−30 °C to provide an analytically pure orange powder that is an
isomer of 13 (44 mg, 44% yield). For meso-13: Anal. Calcd for
C48H74N6S2Ta2: C, 49.65; H, 6.42; N, 7.24. Found: C, 49.15; H, 6.47;
N, 7.08. 1H NMR (400 MHz, benzene-d6): 0.67 (6H, d, CH(CH3)2, J
= 7.2 Hz), 1.00 (6H, d, CH(CH3)2, J = 7.2 Hz), 1.34 (6H, d,
CH(CH3)2, J = 7.0 Hz), 1.43 (6H, d, CH(CH3)2, J = 7.0 Hz), 1.69
(6H, s, N(iPr)C(CH3)N(iPr)), 2.13 (30H, s, C5(CH3)5), 3.86 (2H,
sep, CH(CH3)2, J = 7.0 Hz), 4.48 (2H, sep, CH(CH3)2, J = 7.2 Hz),
6.95 (2H, m, S(C6H5)), 7.21 (4H, m, S(C6H5)), 8.07 (4H, m,
S(C6H5)). Isomer of 13: Anal. Calcd for C48H74N6S2Ta2: C, 49.65; H,
6.42; N, 7.24. Found: C, 50.14; H, 6.65; N, 7.45. 1H NMR (400 MHz,
benzene-d6): 0.67 (6H, d, CH(CH3)2, J = 6.6 Hz), 0.94 (6H, d,
CH(CH3)2, J = 6.8 Hz), 1.37 (6H, d, CH(CH3)2, J = 6.6 Hz), 1.64
(6H, br, CH(CH3)2), 1.69 (6H, s, N(iPr)C(CH3)N(iPr)), 2.05 (30H,
s, C5(CH3)5), 4.03 (2H, br, CH(CH3)2), 4.41 (2H, br, CH(CH3)2),
7.00 (2H, m, S(C6H5)), 7.14 (4H, m, S(C6H5)), 8.00 (4H, d, S(C6H5),
J = 7.6).
Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)]Cl (9). TEMPO (192 mg,
1.23 mmol) was added to a solution of 10 (688 mg, 1.16 mmol) in 30
mL of pentane at room temperature and stirred. The solution turned a
transparent orange color, and the volatiles were removed in vacuo to
give an orange-white solid, which was then dissolved in a minimal
amount of toluene. A small amount of pentane was added, and the
mixture cooled to −30 °C to provide white crystals of 9 (632 mg, 92%
yield). Anal. Calcd for C23H44N4ClTa: C, 46.58; H, 7.48; N, 9.45.
Found: C, 46.77; H, 7.28; N, 9.67. 1H NMR (400 MHz, benzene-d6):
1.21 (3H, d, CH(CH3)2, J = 6.6 Hz), 1.28 (3H, d, CH(CH3)2, J = 6.6
Hz), 1.33 (3H, d, CH(CH3)2, J = 6.6 Hz), 1.41 (3H, d, CH(CH3)2, J =
6.7 Hz), 1.44 (9H, s, C(CH3)3), 2.08 (15H, s, C5(CH3)5), 2.35 (6H, s,
N(CH3)2), 3.68 (1H, sep, CH(CH3)2, J = 6.6 Hz), 3.80 (1H, sep,
CH(CH3)2, J = 6.6 Hz).
Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)] (2). Potassium graphite
(KC8) (90 mg, 0.666 mmol) was transferred over a period of 5 min
to a solution of 9 (247 mg, 0. 416 mmol) in 15 mL of THF, precooled
to −30 °C, by slurrying with 15 mL of THF that had been precooled
to −30 °C. The stirred suspension was allowed to warm to room
temperature and stirred for an additional 3 h to provide a red-colored
solution containing a black suspension. The volatiles were removed in
vacuo, and the crude material was taken up in pentane and filtered
through a short pad of Celite on a glass frit. The red filtrate was
concentrated and cooled to −30 °C overnight, and any unreacted 9
that crystallized out was removed by decanting. The red mother liquor
was then concentrated and cooled to −30 °C for three days to provide
red crystals of 2 (197 mg, 85% yield). Anal. Calcd for C23H44N4Ta: C,
49.55; H, 7.95; N, 10.05. Found: C, 49.62; H, 7.71; N, 10.02.
Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)](Me) (10) and Cp*Ta[N-
(iPr)C(NMe2)N(iPr)][N(tBu)](I) (11). Methyl iodide (6.0 μL, 0.096
mmol) was added to a solution of 2 (54 mg, 0.096 mmol) in 3 mL of
pentane at room temperature to provide a clear solution. The volatiles
were removed in vacuo to give a colorless solid, and then this crude
product was fractionally crystallized in pentane at −30 °C to provide
opaque light yellow cocrystals that were enriched with 11 in a 10:90
10:11 ratio (26 mg, 44% yield). The mother liquor was then allowed
to slowly evaporate at −30 °C, whereupon white cocrystals enriched
with 10 in a 90:10 10:11 ratio were obtained (15 mg, 30% yield). For
enriched-10 containing 12 mol % of 11: Anal. Calcd for
C23.9H46.6N4I0.1Ta: C, 49.01; H, 8.03; N, 9.57. Found: C, 49.14; H,
7.96; N, 9.50. 1H NMR (400 MHz, benzene-d6): 0.54 (3H, s, TaCH3),
1.12 (6H, d, CH(CH3)2, J = 6.8 Hz), 1.25 (3H, d, CH(CH3)2, J = 6.6
Hz), 1.41 (3H, d, CH(CH3)2, J = 6.6 Hz), 1.42 (9H, s, C(CH3)3), 2.00
(15H, s, C5(CH3)5), 2.35 (6H, s, N(CH3)2), 3.59 (1H, sep,
CH(CH3)2, J = 6.8 Hz), 3.78 (1H, sep, CH(CH3)2, J = 6.6 Hz).
For enriched-11 containing 12 mol % of 10: Anal. Calcd for
C23.1H44.4N4I0.9Ta: C, 41.56; H, 6.69; N, 8.38. Found: C, 41.61; H,
6.50; N, 8.40. 1H NMR (400 MHz, benzene-d6): 1.22 (3H, d,
CH(CH3)2, J = 6.7 Hz), 1.24 (3H, d, CH(CH3)2, J = 6.8 Hz), 1.27
(3H, d, CH(CH3)2, J = 6.7 Hz), 1.34 (3H, d, CH(CH3)2, J = 6.8 Hz),
1.52 (9H, s, C(CH3)3), 2.12 (15H, s, C5(CH3)5), 2.29 (6H, s,
N(CH3)2), 3.70 (1H, sep, CH(CH3)2, J = 6.8 Hz), 3.86 (1H, sep,
CH(CH3)2, J = 6.7 Hz).
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental details and X-ray crystallographic
reports, including tables of bond lengths, angles, and
anisotropic displacement parameters, for complexes 1, 2, 4, 5,
8−12, and meso-13. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: lsita@umd.edu.
■
ACKNOWLEDGMENTS
■
Funding for this work was provided by the DOE (BES-SIS-GR
Grant DE-SC0002217), for which we are grateful. We thank
Kamille D. Williams and Prof. Timothy H. Warren for
recording EPR spectra of compounds 1 and 2 at Georgetown
University; they are grateful to the NSF for an award (CHE-
0840453) to purchase the EPR spectrometer.
REFERENCES
■
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mg, 0.182 mmol) in 5 mL of pentane at room temperature. After
stirring for 2 min, the colorless solution was concentrated in vacuo and
then cooled to −30 °C to provide white crystals of 12 (115 mg, 94%
yield). Anal. Calcd for C29H49N4STa: C, 52.24; H, 7.41; N, 8.40.
Found: C, 52.32; H, 7.68; N, 8.54. 1H NMR (400 MHz, benzene-d6):
353
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