Substituent effects in the hydrosilylation of coordinated dinitrogen in a ditantalum complex: Cleavage and functionalization of N2

Article abstract of DOI:10.1021/ja061508q

The dinitrogen complex ([NPN]Ta)₂(μ-η¹: η²-N₂)(μ-H)₂, 1, (where [NPN] = (PhNSiMe₂CH₂)₂PPh) undergoes hydrosilylation with primary and secondary alkyl- and arylsilanes, giving a new N-Si bond and a new terminal tantalum hydride derived from one Si-H unit. Various primary silanes can be employed to give isolable complexes of the general formula ([NPN]TaH)(μ-N-N-SiH_nR_3-n)(μ-H)₂(Ta[NPN]) (5, R=Bu, n = 2; 9, R=Ph, n = 2). Analogous complexes featuring secondary silanes are not isolable, because these products, and 5 and 9, are uniformly unstable toward reductive elimination of bridging hydrides as H₂, followed by cleavage of the N-N bond to give ([NPN]TaH)(μ-N)(μ-N-SiH _nR_3-n)(Ta[NPN]) (6, R=Bu, n = 2; 10, R=Ph, n = 2; 15, R=Ph, n = 1; 16, R=Ph and Me, n = 1). The bridging nitrido ligand in these complexes is itself a substrate for a second hydrosilylation when n = 2, and schemes leading to Ta(IV) complexes of the general formula ([NPN]Ta) ₂(μ-N-SiH₂R)(μ-N-SiH₂R') via elimination of H₂ are reported (4, R=R′ = Bu; 12, R=Bu, R′ = Ph; 13, R=Bu, R′ = CH₂CH₂SiH₃). At this point, the general reaction manifold for these compounds ramifies, with distinct outcomes occurring for different R groups-[NPN] ligand amide migration from Ta to RSi affords 11, ([NPN]Ta(μ-NSiH₂Ph)(μ-NSiH ₂Ph)Ta[NPN]), whereas stable complex 6 rearranges to give 7, ([NP(C₆H₄)N]Ta(BuSiH₂N-μ-SiHBu-μ-N) Ta[NPN]) in the presence of excess silane. Ethanediylbissilane reacts with 1 to give 14, ([NP(C₆H₄)N]Ta(N-SiH₂CH ₂CH₂Si(H)-μ-N)Ta[NPN]), isostructural to 7.

Full text of DOI:10.1021/ja061508q

Published on Web 06/30/2006
Substituent Effects in the Hydrosilylation of Coordinated
Dinitrogen in a Ditantalum Complex: Cleavage and
Functionalization of N2
Bruce A. MacKay, Rui F. Munha, and Michael D. Fryzuk*
Contribution from the Department of Chemistry, The UniVersity of British Columbia,
2036 Main Mall, VancouVer, British Columbia, Canada V6T 1Z1
Received March 3, 2006; E-mail: fryzuk@chem.ubc.ca
Abstract: The dinitrogen complex ([NPN]Ta)2(µ-η1:η2-N2)(µ-H)2, 1, (where [NPN] ) (PhNSiMe2CH2)2PPh)
undergoes hydrosilylation with primary and secondary alkyl- and arylsilanes, giving a new N-Si bond and
a new terminal tantalum hydride derived from one Si-H unit. Various primary silanes can be employed to
give isolable complexes of the general formula ([NPN]TaH)(µ-N-N-SiHnR3-n)(µ-H)2(Ta[NPN]) (5, RdBu,
n ) 2; 9, RdPh, n ) 2). Analogous complexes featuring secondary silanes are not isolable, because
these products, and 5 and 9, are uniformly unstable toward reductive elimination of bridging hydrides as
H2, followed by cleavage of the N-N bond to give ([NPN]TaH)(µ-N)(µ-N-SiHnR3-n)(Ta[NPN]) (6, RdBu,
n ) 2; 10, RdPh, n ) 2; 15, RdPh, n ) 1; 16, RdPh and Me, n ) 1). The bridging nitrido ligand in these
complexes is itself a substrate for a second hydrosilylation when n ) 2, and schemes leading to Ta(IV)
complexes of the general formula ([NPN]Ta)2(µ-N-SiH2R)(µ-N-SiH2R’) via elimination of H2 are reported
(4, RdR′ ) Bu; 12, RdBu, R′ ) Ph; 13, RdBu, R′ ) CH2CH2SiH3). At this point, the general reaction
manifold for these compounds ramifies, with distinct outcomes occurring for different R groupss[NPN]
ligand amide migration from Ta to RSi affords 11, ([NPN]Ta(µ-NSiH2Ph)(µ-NSiH2Ph)Ta[NPN]), whereas
stable complex 6 rearranges to give 7, ([NP(C6H4)N]Ta(BuSiH2N-µ-SiHBu-µ-N)Ta[NPN]) in the pres-
ence of excess silane. Ethanediylbissilane reacts with 1 to give 14, ([NP(C6H4)N]Ta(N-SiH2CH2CH2Si-
(H)-µ-N)Ta[NPN]), isostructural to 7.
of the dinitrogen unit,8,9 and there is one example of a system
that can be made to turnover to generate more than stoichio-
Introduction
Activation of molecular nitrogen by soluble metal complexes
has historically been concerned with the synthesis of new
dinitrogen-containing compounds.1-3 As a result of this exten-
sive early work, much is known about different bonding modes
for the N2 molecule and general synthetic methods for the
preparation of dinitrogen complexes.4 But what has lagged
behind is the development of reactivity patterns that result in
functionalization at bound N2 and/or N-N bond cleavage. Such
fundamental processes that involve transformation of the
dinitrogen unit are key to the direct utilization of molecular
nitrogen as a reagent in the formation of higher-value organo-
nitrogen materials.5
Despite the fact that there are hundreds of N2 complexes
known, there are only a handful of these derivatives that undergo
further reactivity at the N2 unit.6,7 Protonation of coordinated
dinitrogen has been used extensively in stoichiometric processes
involving group 6 complexes that result in further elaboration
metric quantities of NH3 with judicious choice of acid and
reducing agent.10,11 Reaction of coordinated N2 with various
electrophiles has been well studied and used to generate a variety
of new kinds of bonds N-X bonds.12-18 More recently, certain
zirconium19-22 and hafnium23 dinitrogen complexes have been
(8) Hidai, M.; Mizobe, Y. Can. J. Chem. 2005, 83, 358.
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9472
J. AM. CHEM. SOC. 2006, 128, 9472-9483
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Cleavage and Functionalization of N2
A R T I C L E S
found to react with H2 and silanes to generate new species with
N-H and N-Si bonds;20 these latter examples are important
as they involve functionalization of N2 via reaction with reagents
that have relatively nonpolar bonds. The reaction of terminal
alkynes can also result in functionalization of the dinitrogen
moiety to form either N-C bonds24 or N-H bonds,25 depending
on the ancillary ligand system utilized. Reductive cleavage of
the dinitrogen moiety has been examined with some group 5
and 6 metal complexes;26-30 in a few of these systems, inter-
mediates have been observed that have shed light on important
mechanistic issues.
Reaction of 1 with diisobutylaluminum hydride (DIBAL-H) at
low temperatures results in the formation of the analogous
organoaluminum adduct ([NPN]TaH)(µ-H)2(µ-η1:η2-NNAlBui2)-
(Ta[NPN]), 3; however, in this case, further rearrangement
involves [NPN] ligand amide migration from Ta to Al and
elimination of isobutene to generate a species with a nitride
bridging between Al and Ta.
In both hydroboration and hydroalumination, N-N bond
cleavage is observed after functionalization at nitrogen. In fact,
prior E-H addition to form N-B and N-Al bonds and the
terminal Ta-H bond in 2 and 3, respectively, is necessary to
promote loss of H2, which in turn provides the necessary two
electrons needed to cleave the N-N bond.
One of the more versatile dinitrogen complexes that shows a
remarkable breadth of reactivity patterns is the ditantalum
complex ([NPN]Ta)2(µ-η1:η2-N2)(µ-H)2, 1.31 The unique side-
We have previously communicated37 the preliminary results
of our study into the hydrosilylation reaction (E-H ) BuSiH3)
of dinitrogen complex 1. Although this example of E-H
addition shares some common features with both hydroboration
and hydroalumination, there are some intriguing differences for
the hydrosilylation process, which include substituent effects,
access to key intermediates, and isolation of different kinds of
products. In this work, we provide full details of the reaction
of primary, secondary, and tertiary silanes with 1.
on end-on bonding mode for the N2 moiety in this complex32
has been examined, and its Lewis basicity33 can be attributed
to the existence of a “lone pair” on the exposed terminal nitro-
gen atom in the molecular HOMO-1. In addition, there is
pronounced π character to the Ta-N bonding in the HOMO.34,35
Although this complex undergoes reaction with electrophiles
such as benzyl bromide to generate the expected N-C bond
of the benzyl-hydrazido unit in ([NPN]TaBr)(µ-H)2(µ-η1:η2-
NNCH2Ph)(Ta[NPN]),31 a more important reaction is the
addition of simple main-group hydrides of the general form-
ula E-H to the starting N2 complex 1. For example, when E-H
is 9-borabicyclo[3.3.1]nonane (9-BBN), the Ta-N π bond
is hydroborated34 to generate ([NPN]TaH)(µ-H)2(µ-η1:η2-
NNBC8H11)(Ta[NPN]), 2, in which the N2 unit is functionalized
with concomitant formation of a new tantalum hydride. This
reaction is general and proceeds identically for both primary
and secondary boranes.35 Remarkably, this simple adduct 2 is
unstable and undergoes a cascade of events that include
reductive elimination of the bridging hydride ligands as H2 and
eventual ancillary ligand degradation to ultimately generate
modest yields of imide-nitride derivatives.
Results and Discussion
Hydrosilylation Using Butylsilane (BuSiH3). When dini-
trogen complex 1 is allowed to react with slightly more than 2
equiv of butylsilane (BuSiH3), the dinuclear disilylimide 4 can
be isolated as a crystalline red solid in good yields (eq 1).37
The solution NMR data indicate a highly symmetric structure
for 4, which was confirmed by the solid-state X-ray crystal
structure, as previously reported.
As compared to the aforementioned hydroboration and
hydroalumination reactions, wherein considerable ligand rear-
rangement accompanies functionalization and N-N bond cleav-
age, the hydrosilylation process is certainly more straightforward
and potentially more useful because both nitrogen atoms have
been silylated, despite their asymmetric activation in 1.
Extension of this E-H addition chemistry to hydroalumina-
tion of 136 follows a similar reaction course, at least initially.
Upon monitoring the reaction of 1 with butylsilane using 31
P
(23) Bernskoetter, W. H.; Olmos, A. V.; Lobkovsky, E.; Chirik, P. J. Organo-
metallics 2006, 25, 1021.
NMR spectroscopy, a number of intermediates were observed
to grow as a function of time. In fact, two of these intermediates
could be isolated by controlling stoichiometry, temperature, and
reaction times. Also evident by this experiment was the eventual
transformation of the intermediates to disilylimide 4, and under
certain conditions, the disilylimide itself undergoes a further
reaction. All of these processes are quite amenable to study using
31P NMR spectroscopy because each complex observed features
unique and characteristic chemical shifts and, in some cases,
JPP coupling constants. We have previously explored the
(24) Morello, L.; Love, J. B.; Patrick, B. O.; Fryzuk, M. D. J. Am. Chem. Soc.
2004, 126, 9480.
(25) Bernskoetter, W. H.; Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2005, 127, 7901.
(26) Kawaguchi, H.; Matsuo, T. Angew. Chem., Int. Ed. 2002, 41, 2792.
(27) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861.
(28) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.;
Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996,
118, 8623.
(29) Clentsmith, G. K. B.; Bates, V. M. E.; Hitchcock, P. B.; Cloke, F. G. N.
J. Am. Chem. Soc. 1999, 121, 10444.
(30) Mindiola, D. J.; Meyer, K.; Cherry, J.-P. F.; Baker, T. A.; Cummins, C. C.
Organometallics 2000, 19, 1622.
(31) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason, S. A.;
Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960.
(32) Studt, F.; MacKay, B.; Fryzuk, M. D.; Tuczek, F. J. Am. Chem. Soc. 2003,
126, 280.
(33) Studt, F.; Johnson, S. A.; MacKay, B.; Patrick, B. O.; Fryzuk, M. D.;
Tuczek, F. Chem.-Eur. J. 2005, 11, 385.
(34) Fryzuk, M. D.; MacKay, B. A.; Johnson, S. A.; Patrick, B. O. Angew.
Chem., Int. Ed. 2002, 41, 3709.
(35) MacKay, B.; Johnson, S. A.; Patrick, B. O.; Fryzuk, M. D. Can. J. Chem.
2005, 83, 315.
(36) MacKay, B. A.; Patrick, B. O.; Fryzuk, M. D. Organometallics 2005, 24,
3836.
(37) Fryzuk, M. D.; MacKay, B. A.; Patrick, B. O. J. Am. Chem. Soc. 2003,
125, 3234.
9
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A R T I C L E S
MacKay et al.
Figure 1. 31P NMR spectra (hourly; lowest plot is time 0) showing species
that arise in the reaction of 1 (brown) with an excess of butylsilane. The
rapid appearance of 5 (green), its conversion to 6 (violet), and the appearance
of 4 (red) and 7 (yellow) at the expense of 6 are evident. Conditions: 161
MHz (31P), 300 K, C6D6 solution.
Figure 2. ORTEP drawing (ellipsoids at 50%) of the solid-state molecular
structure of 6. Silyl methyls and phenyl ring carbons other than ipso omitted
for clarity. The hydride was modeled using XHYDEX.38 Selected bond
distances (Å), angles, and dihedral angles (deg): N1-N2 2.646(6);
Ta1-Ta2 2.9258(6); Ta1-N2 1.928(11); Ta2-N2 1.874(9); Ta1-N1
2.187(10); Ta2-N1 1.924(10); N1-Si1 1.741(10); Ta1-P1 2.709(3); Ta1-
N3 2.107(9); Ta1-N4 2.034(9); Ta2-P2 2.715(3); Ta2-N5 2.079(10);
Ta2-N6 2.052(9); Ta1-N1-Si1 127.7(5); Ta2-N1-Si1 139.2(5);
Ta1-N1-Ta1 90.5(4); Ta1-N2-Ta2 100.6(5); N1-Ta1-N2 80.0(4);
N1-Ta2-N2 88.6(4); N2-Ta2-P2 94.1(3); N2-Ta1-P1 170.7(3);
N3-Ta1-N4 100.3(4); N5-Ta2-N6 -118.0(4); N1-Ta1-Ta2-N2
174.0(6); P1-Ta1-Ta2-P2 -163.66(13).
mechanism of the hydroalumination of 1 by conducting a series
of consecutive timed 31P NMR experiments using an internal
standard for integration. Two intermediates, neither of which
is persistent enough at room temperature for isolation or study
using X-ray methods, were discovered using this technique.36
Figure 1 shows a stack plot of 31P NMR experiments
encompassing 13 h of the reaction between the starting
dinitrogen complex 1 and 4 equiv of butylsilane. Resonances
of 1 (brown) disappear rapidly at first, as resonances of the first
intermediate 5 (green) arise. As communicated, the first
intermediate 5 is then converted to a second intermediate, 6
(violet), and then conversion of this second intermediate 6 to
the disilylimide 4 (red) is evident. As will be discussed later,
when more than 2 equiv of butylsilane are used, a further
transformation of 4 to 7 (yellow) can be detected.
That this first intermediate 5 is thermally unstable and
rearranges to give the second intermediate 6 (eq 3) has already
been communicated. Loss of H2 from 5 provides the necessary
additional electrons required to cleave the intact N-N bond,
which leads to 6. The 15N NMR spectrum of 6 shows two
resonances but no homonuclear scalar coupling, consistent with
cleavage of the N-N bond. Since our original communication,37
conditions have been developed to allow isolation of X-ray
quality crystals of intermediate, 6, thus allowing complete
characterization of another mechanistically significant interme-
diate related to N-N bond cleavage in this system.
Optimal conditions for isolation of the first intermediate 5
were developed by the addition of approximately 1 equiv of
butylsilane to 1 and performing the reaction at -60 °C for 24
h; in this way, almost a quantitative yield of the monobutylsi-
lylhydride 5 could be obtained in analytically pure form by
precipitation from toluene using hexanes or pentane.
The solid-state molecular structure of 6 as determined by
X-ray crystallography is shown in Figure 2, and it confirms
our original assignment of this species as having separate
bridging nitride and bridging silylimide units. The N1‚‚‚N2
distance of 2.646(6) Å clearly indicates scission of the N-N
bond in 6. The absence of resonances consistent with bridging
hydrides in the 1H NMR spectrum of 6, and the observation of
Although a discussion of the solid-state X-ray structure of 5
will not be reiterated here, it should be noted that 5 is nearly
isostructural to the initial hydroboration addition product 2 and,
from its solution data, very similar in structure to the corre-
sponding hydroalumination adduct 3, characterized by low-
temperature NMR spectroscopy.36 An intact N-N bond in these
simple addition products is indicated by the 15N NMR spectra
of suitably labeled materials, in particular the presence of scalar
1
free H2 in H NMR spectra recorded concurrently during the
experiment illustrated in Figure 1, further supports our assertion
that reductive cleavage of the N-N bond of the first intermediate
5 is linked to elimination of the bridging hydrides. In the solid
state, the [NPN] ancillary ligands of 6 are arranged quasi trans
to each other, which was also observed for complex 5.
1
coupling between the nitrogen nuclei of JNN ) 16.6 Hz for
15N2-5.
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9474 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Cleavage and Functionalization of N2
A R T I C L E S
Scheme 1
Figure 3. ORTEP drawing (ellipsoids at 50%) of the solid-state molecular
structure of 7. Silyl methyls and phenyl ring carbons other than ipso omitted
for clarity. Selected bond distances (Å), angles, and dihedral angles (deg):
N1-N2 2.549(6); Ta1-Ta2 2.9511(5); Ta1-N1 2.103(8); Ta1-N2 2.073-
(7); Ta2-N2 1.972(8); N1-Si1 1.696(8); N1-Si2 1.703(9); N2-Si2 1.742-
(8); Ta1-N3 2.065(7); Ta1-N4 2.078(8); Ta1-P1 2.597(2); Ta2-N5
2.085(8); Ta2-N6 2.095(7); Ta2-P2 2.637(2); Ta2-C10 2.276(9);
Si1-N1-Si2 128.4(5); N1-Ta1-N2 75.2(3); N1-Si2-N2 95.4(4);
Ta1-N2-Ta2 93.7(3); Ta2-Ta1-P1 83.59(5) Ta1-Ta2-P2 129.09(5);
Ta1-Ta2-C10 92.3(2); N1-Ta1-Ta2-N2 4.1(4); N1-Ta1-Ta2-P2 9.2-
(3); P1-Ta1-Ta2-C 106.3(2); P1-Ta1-Ta2-P2 -171.06(9); Si1-N1-
Si2-N2 178.8(7).
Bimetallic complexes featuring µ-nitrido ligands are rare, but
the reported examples are dominated by group 5 com-
plexes.26,29,39,40 Mixed imido-nitrido derivatives featuring two
metals are unprecedented41 except for the ones that result from
E-H addition to 1. Examination of the Ta-N bond lengths in
the complexes described herein are unremarkable when com-
pared to other species with bridging nitrides in the literature.42,43
On the basis of the Cs symmetry apparent in the solution 1H
and 29Si-DEPT NMR spectra of 6, it appears that a fluxional
process involving a “rocking” motion of the [NPN] ligand bound
to Ta1, along with motion of hydride H1, occurs in solu-
tion. The atypical broadness (fwhm ) 28 Hz) of the 31P
NMR resonances of 6 as compared to the sharp lines (fwhm )
2 to 4 Hz) observed for the other silylated derivatives may also
be due to this processsthe broadness persists in the spectrum
of 15N2-6, suggesting that this line width does not result from
quadrupolar relaxation effects or coupling with 14N nuclei.
Generation of the disilylimide product 4 from the second
intermediate 6 requires the addition of a second equivalent of
butylsilane across the bridging nitride to generate the additional
µ-NSiH2Bu unit and a new Ta-H, as shown in 8 below (Scheme
1). Although 8 is not observed by the monitoring experiment
in Figure 1, loss of H2 via a dinuclear reductive elimination
would lead directly to 4.
A surprising finding from these sequential 31P NMR experi-
ments is the observation of additional peaks attributed to
complex 7 (yellow-colored singlet resonances at δ 71.15 and
24.23 in Figure 1). This species was not observed in previous
2:1 reactions between butylsilane and dinitrogen complex 1.
The solid-state molecular structure obtained by X-ray diffraction
(Figure 3) indicates that 7 involves a rearrangement of 4
accompanied by a final evolution of H2, rather than a species
resulting from further incorporation of free butylsilane into a
derivative of 1. The key differences between 7 and 4 are the
ortho-metalation of one phenyl substituent of an [NPN] lig-
and phosphine donor by the opposite tantalum center, the
eversion of one bridging silylimide to a new position, bridging
Ta1 and Si2, and the formation of a new bond between Si2
and N1. Both dinitrogen-derived N atoms are still silylated, but
they are no longer on opposite sides of the Ta‚‚‚Ta internuclear
axis.
1H/29Si HSQC NMR spectroscopy of 7 indicates one unique
silyl hydride bound to Si2, keeping the coordination number of
Si2 at four. In addition, two apparently equivalent silyl hydrides
remain on Si1, consistent with the observed Cs symmetry of
the molecule in solution.
Solutions of 1 in neat butylsilane form 7 as the major product
when observed by 31P NMR spectroscopy, implying that
complex 7 is the endpoint of this reaction manifold. Interest-
ingly, solutions of 4 are stable for weeks in the absence of excess
butylsilane, even at elevated temperatures. The fact that the
conversion of 4 to 7 requires additional BuSiH3, which is not
incorporated into the product, suggests a role for the silane;
As reported elsewhere, DFT analysis of the conversion of
the monobutylsilyl derivative 5 to the imide-nitride 6 has been
carried out and confirms that the intermediate shown in Scheme
1 is computationally accessible.44
(38) Orpen, A. G. Dalton Trans. 1980, 2509.
(39) Berno, P.; Gambarotta, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 822.
(40) Tayebani, M.; Feghali, K.; Gambarotta, S.; Bensimmon, C. Organometallics
1997, 16, 5084.
(41) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239.
(42) Banaszak Holl, M. M.; Wolczanski, P. T.; van Duyne, G. D. J. Am. Chem.
Soc. 1990, 112, 7989.
(43) Plenio, H.; Roesky, H. W.; noltmeyer, M.; Sheldrick, G. M. Angew. Chem.
1988, 100, 1377.
(44) Studt, F.; MacKay, B. A.; Fryzuk, M. D.; Tuczek, F. Dalton Trans. 2006,
1137.
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A R T I C L E S
MacKay et al.
Figure 4. ORTEP drawing (ellipsoids at 50%) of the solid-state molecular
structure of 9. Silyl methyls and phenyl ring carbons other than ipso omitted
for clarity. The hydrides were modeled using XHYDEX.38 Selected bond
distances (Å), angles, and dihedral angles (deg): N1-N2 1.354(10); Ta1-N
12.179(7); Ta1-Ta2 2.8393(14); Ta1-N2 2.214(6); Ta2-N2 1.860(8);
N1-Si1 1.736(7); Ta1-N3 2.137(7); Ta1-N4 2.080(7); Ta1-P1 2.590-
(2); Ta2-N5 2.049(7); Ta2-N6 2.041(8); Ta2-P2 2.635(3); Ta1-N1-
Si1 135.2(4); Ta1-N1-N2 73.5(4); Ta1-N2-Ta2 87.9(3); N1-Ta1-P1
80.96(18); N1-Ta1-N3 161.7(3); N1-Ta1-N4 84.9(3); N2-Ta2-P2
167.6(2); N2-Ta2-N5 103.6(3); N2-Ta2-N6 108.7(5); N1-Ta1-Ta2-
N2 -0.7(3); Ta2-Ta1-N1-Si1 106.9(5); P1-Ta1-Ta2-P2 -169.70-
(14).
Figure 5. ORTEP drawing (ellipsoids at 50%) of the solid-state molecular
structure of 11. Silyl methyls and phenyl ring carbons other than ipso omitted
for clarity. Selected bond distances (Å), angles, and dihedral angles (deg):
N1-N2 2.827(3); Ta1-Ta2 2.9837(3); Ta2-N2 1.903(5); Ta2-N1 1.911-
(5); Ta1-N2 2.253(5); Ta1-N1 2.158(4); N2-Si2 1.724(5); Si2-N5 1.758-
(5); N1-Si1 1.729(5); Si2-Ta1 2.527(3); Ta2-N3 2.050(5); Ta2-N4
2.035(5); Ta2-P2 2.7962(17); Ta1-N6 2.032(5); Ta1-P1 2.5034(17);
Ta1-N2-Ta2 91.4(2); Ta1-N1-Ta2 94.1(2); N1-Ta2-N2 95.17(19);
N1-Ta1-N2 79.30(18); N2-Si2-N5 118.9(2); Ta2-Ta1-N6 127.08-
(14); N3-Ta2-N4 114.61(19); N2-Ta2-P2 88.10(14); N6-Ta1-P1
82.47(15); Ta2-N1-Si1 146.1(3); N1-Ta1-N2-Ta2 177.9; P1-Ta1-
Ta2-P2 73.31(10); Ta2-N2-Si2-N5 -108.9(2); N2-Ta2-N1-Si1
155.2(5).
however, a mechanism for this process is not obvious. The net
reaction is shown in eq 4.
the thermal instability common to this family of compounds,
as it spontaneously converts to 10 in solution by the same
method previously invoked for conversion of 5 to 6 (see eq 3).
Although the solid-state molecular structure of 10 has not yet
been established, its structure is likely closely related to 6
because of the similarity of their NMR spectra. For example,
as for 15N2-6, 15N2-10 shows no scalar coupling between the
two resonances in its 15N NMR spectrum, and the chemical
shifts of the imido and nitrido atoms in these two homologues
are similar. Furthermore, resonances typical of bridging hydrido
ligands are absent in the 1H NMR spectrum of 10, and therefore,
it seems that the silylation chemistries up to this point do not
diverge when the silyl substituent changes.
However, in exploring this reaction manifold with sequential
31P NMR experiments, it was found that a phenylsilyl analogue
of the disilylimide 4 does not form. The imido nitrido complex
10 should be a substrate for a second hydrosilylation with
phenylsilane, giving a symmetric structure in which both dinitro-
gen atoms are equivalently phenylsilylated, as in the conversion
of 6 to 4 with butylsilane (Scheme 1). It is reasonable, therefore,
to suggest that such a species should have a 31P NMR chemical
shift near 0 ppm as was observed for 4. Instead, in the presence
of additional equivalents of phenylsilane, 10 converts to a new
complex 11, which has a 31P NMR spectrum consisting of two
sharp singlets (δ -10.46 and +15.88) of equal intensity. The
1H{31P} and 29Si-DEPT NMR spectra of this new species
indicate C1 solution symmetrysthe latter shows six different
silicon-29 resonances, four of which are typical of [NPN] ligand
silyl groups with chemical shifts from δ 6 to 30 and 2JPSi values
Hydrosilylation Using Phenylsilane (PhSiH3). Phenylsilane
reacts with the starting dinitrogen complex 1 in solution to give
the expected phenylsilyl derivative, 9 (equation 5). The solution
NMR spectral characteristics of 9 and its isotopologs are
completely analogous to the already discussed butylsilyl com-
plex 5; the solid-state molecular structure of 9 (Figure 4)
demonstrates that addition of phenylsilane to 1 results in a
complex that is isostructural not only with 4 but also with the
hydroboration adduct 2 previously reported.
The solid-state metric parameters of the phenylsilane adduct
9 match those of the butylsilyl analogue 5, within experimental
1
error (3σ). The JNN coupling in 15N2-9 is 17.4 Hz, which is
very near the value of 16.6 Hz reported for 15N2-5. 9 shares
9
9476 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Cleavage and Functionalization of N2
A R T I C L E S
Scheme 2
ranging from 0 to 20 Hz. Of the two remaining resonances,
only one is typical of a silylimide (δ -35.82). The other occurs
at δ 49.86 as a doublet (JPSi ) 41 Hz). The 15N NMR spectrum
of the appropriate enriched complex contains two resonances
suggestive of silylimides; these resonances display no scalar
coupling to each other. Fortunately, ochre crystals of 11 can be
obtained from benzene solutions, and the solid-state molecular
structure of 11 (Figure 5) reveals that an [NPN] ligand amide
group has migrated from tantalum to silicon.
There is no suggestion of a similar [NPN] ligand amide
migration in the chemistry of 1 with butylsilane. In fact, the
conversion of 6 to 9 suggests that when butyl groups are present
on the silyl addition reagent, the [NPN] ligand amides can
remain bound to the tantalum atoms through extreme rearrange-
ments of other portions of the molecule. The contrast between
this finding and the amide migration observed in 11 indicates
that silyl substituents play a role in organizing these complexes.
Two mechanistic possibilities arise. In the first scenario, 11
results directly from amide migration in an intramolecular re-
arrangement of a complex that is isostructural with 4. In the
second scenario, the second equivalent of phenylsilane interacts
with a ligand amide of 10 in an initial step and then with the
bridging nitride to give 11. This path implies poorer regio-
selectivity in the second hydrosilylation with phenylsilane
versus butylsilane. These two alternatives are expressed in
Scheme 2.
Silicon atom Si2 in Figure 5 is hypervalentsit is bound to a
phenyl substituent, two N atoms, and two distinct hydrides as
shown by the correlational1H/29Si-DEPT NMR spectrum of
11. The chemical shift of one of these hydrides is anomalously
downfield (δ 9.43) for a silyl hydride. This resonance displays
1
29Si satellites that indicate its JSiH coupling is 78.2 Hz, much
less than the typical values of 114 to 168 Hz observed in other
complexes in this study. These data and a small coupling to
31P indicate that this hydride is likely shared between Si2 and
Ta1, as shown in eq 8, although no artifacts indicating a hydride
in a suitable position were resolvable in the difference map in
the solid-state X-ray investigation. In retrospect, the 31P NMR
spectrum of 11 (both the chemical shift values and the absence
of scalar JPP coupling) is reminiscent of the final products of
hydroalumination, which also feature [NPN] ligand amide
migration (from Ta to Al). As for 6, a Ta-Ta bond, resulting
from elimination of two silane-derived hydrides as H2 after the
second hydrosilylation, is necessary to explain the observed
diamagnetic behavior in 11. Finally, the sequential 31P NMR
experiments described above show that 11 is the final complex
to occur in the reactions of 1 with excess phenylsilane.
In the butylsilane manifold, it has already been shown that
symmetrically functionalized complex 4 is available by direct
hydrosilylation of the dinitrogen-derived bridging nitrido ligand
of 6. This suggested that complexes isostructural to 4 but
incorporating two different substituents should be accessible.
To demonstrate this, the synthesis of complex 12 was undertaken
as shown in Scheme 3.
The 31P NMR spectrum of 12 comprises a doublet of doublets
(JPP ) 9.9 Hz), with both peaks very near the location of the
single resonance observed for 4. The 1H and 29Si NMR spectra
both imply Cs symmetry for this molecule. 12 is also available
from the complementary reaction between 10 and 1 equiv of
butylsilane, suggesting that the order of addition (butyl vs
phenyl) is unimportant (Scheme 3). This molecule is stable in
solution, which implies that no amide migration is occurring
(bottom of Scheme 2); this suggests that the second addition of
PhSiH3 to 10 leading to 11 probably proceeds at the ligand
amido donor rather than at the bridging nitride. Therefore, the
amide migration scenario in Scheme 2 is less likely in the
formation of 11, because it would appear that 12 does contain
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9477

A R T I C L E S
Scheme 3
MacKay et al.
Figure 6. ORTEP drawing (ellipsoids at 50%) of the solid-state molecular
structure of 13. Silyl methyls and phenyl ring carbons other than ipso omitted
for clarity. Selected bond distances (Å), angles, and dihedral angles (deg):
N1‚‚‚N2 2.979(4); Ta1-N1 2.022(7); Ta1-N2 2.031(7); Ta2-N1 2.013-
(7); Ta2-N2 2.053(7); N1-Si1 1.710(8); N2-Si2 1.700(7); Ta1-Ta2
2.6759(5); Ta1-P1 2.744(2); Ta1-N3 2.076(7); Ta1-N4 2.083(7);
Ta2-P2 2.688(2); Ta2-N5 2.092(7); Ta2-N6 2.078(7); Ta1-N1-Ta2
83.1(3); N1-Ta2-N2 94.3(3); Ta2-N2-Ta1 81.9(3); N2-Ta1-N1
74.7(3); Ta1-N1-Si1 142.4(4); Ta2-N2-Si2 136.2(4); Ta1-N2-Ta2-
N1 -155.3(4); P1-Ta1-Ta2-P2 146.30(10).
a bridging phenylsilyl imide and no migration occurs. This
shows that a very subtle level of control over [NPN] ligand
amide migration is at work, in that a small and perhaps remote
difference in substituents can suppress rearrangement of ancil-
lary ligands.
Hydrosilylation Using Ethanediylbissilane (H3SiCH2-
CH2SiH3). The ability to access dinuclear tantalum complexes
with mixed silylimide functionality, such as 12 via the sequence
in Scheme 3, suggested that two dinuclear complexes could be
linked by the used of a bifunctional disilane. The idea was to
examine the reaction of H3SiCH2CH2SiH3 with the isolable
imide-nitride 6 in an 1:2 stoichiometry to isolate a tetranuclear
complex of the formula {([NPN]Ta)2(µ-NSiH2Bu)}2(µ-NSiH2-
CH2)2. The reaction proceeds at room temperature in toluene
solution over a period of 4 days to generate a new complex
characterized by a singlet in the 31P NMR spectrum at δ -0.02
ppm. Isolation of crystalline material allowed determination of
the solid-state molecular structure by X-ray crystallography
(Figure 6). What is clearly evident is that the bifunctional silane
has only added at one end to generate the disilylimide 13, a
closely related analogue of the original disilylimide 4, which
was formed by the addition of two equiv of BuSiH3 to the
starting dinitrogen complex 1. This transformation is shown in
eq 7.
Figure 7. ORTEP drawing (ellipsoids at 50%) of the solid-state molecular
structure of 14. Silyl methyls and phenyl ring carbons other than ipso omitted
for clarity. Selected bond distances (Å), angles, and dihedral angles (deg):
N1-N2 2.609(6); Ta1-Ta2 2.9647(8); Ta1-N1 2.108(9); Ta1-N2 2.113-
(9); Ta2-N2 1.944(12); N1-Si1 1.672(10); N1-Si2 1.757(11); N2-Si2
1.720(12); Ta1-N3 2.062(9); Ta1-N4 2.103(9); Ta1-P1 2.597(3);
Ta2-N5 2.094(10); Ta2-N6 2.097(10); Ta2-P2 2.618(3); Ta2-C3 2.275-
(10); Si1-N1-Si2 108.0(5); N1-Ta1-N2 76.3(4); N1-Si2-N2 97.2(4);
Ta1-N2-Ta2 93.8(4); Ta2-Ta1-P1 83.40(7); Ta1-Ta2-P2 128.30(8);
Ta1-Ta2-C3 92.3(3); N1-Ta1-Ta2-N2 -15.5(6); N1-Ta1-Ta2-P2
16.8(3); P1-Ta1-Ta2-C10 6.3(2); P1-Ta1-Ta2-P2 -9.2(3); Si1-N1-
Si2-N2 166.6(6).
The availability of this bifunctional silylating agent also
suggested that its chemistry with the starting dinitrogen complex
1 should be explored. Although the isolation of 13 suggests
that linking multiple dinuclear tantalum complexes in an
intermolecular fashion is unlikely, the possibility of an intramo-
lecular disilylation was not initially precluded. The reaction
between H3SiCH2CH2SiH3 and 1 in toluene proceeds initially
as for butylsilane, forming complexes that are congeners of 5
The 29Si{1H} NMR spectrum of isolated product 13 indicates
the presence of three discrete alkylsilyl groups, which is con-
sistent with the dinuclear formulation rather than the anticipated
linked-tetranuclear target mentioned above. It is likely that the
potentially reactive SiH3 group that remains in 13 is not steric-
ally available to other complexes with reactive TadN bonds
because of the presence of the [NPN]-ligand phenyl groups.
9
9478 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Cleavage and Functionalization of N2
A R T I C L E S
Scheme 4
and 6 by their 31P NMR spectra. On the basis of the spectral
data for 4, 12, and 13, resonances near 0 ppm would be expected
for a second hydrosilylation, whether this second reaction were
intramolecular or intermolecular. However, no such resonances
were detected. Instead, resonances reminiscent of the cyclom-
etalated complex 7 are observed; upon workup, the related
complex 14 could be isolated as crystals. An ORTEP drawing
of its solid-state molecular structure is shown in Figure 7. This
complex exhibits the cyclometalation and imide displacement
silanes. Thus, the analogous first intermediates observed for
primary silanes could not be isolated because they also possess
the instability toward reductive elimination of H2 coupled to
N-N bond cleavage that is considered to be a hallmark of this
type of complex. The rate at which this reaction occurs is not
noticeably affected by the change from primary to secondary
silanes, and therefore, conversion of the hydrosilylated complex
to analogues of 6 and 10 is rapid as compared to hydrosilylation
when secondary silanes are employed.
1
observed in 7. The H NMR spectra of the two species are
similar, showing unique aryl resonances for the cyclometalated
phenyl ring and apparent Cs molecular symmetry. As shown in
Scheme 4, one possible explanation for the formation of this
product is that the second silylation occurs intramolecularly but
the strain of the ethanediyl bridge facilitates the eversion of
the second imide nitrogen and promotes orthometalation of the
phosphorus-phenyl. This is, of course, speculation.
Hydrosilylation Using Secondary Silanes. From the discus-
sion above, it is clear that even within the primary silane
manifold, a substituent change from butyl to phenyl at silicon
affects rearrangements later in the hydrosilylation process. To
probe this further, this reaction was extended to secondary
silanes, in particulary diphenylsilane (Ph2SiH2) and methylphen-
ylsilane (MePhSiH2). Monitoring the reactions of each of these
reagents with the starting dinitrogen complex 1 via 31P NMR
spectroscopy shows that only a transient amount of the hydro-
silylated product homologous to 5 and 9 is ever observed; the
decrease in the intensities of the resonances of 1 is matched by
the appearance of resonances associated with complexes 15 (R1
) R2 ) Ph) and 16 (R1 ) Me; R2 ) Ph) as shown in eq 8.
These reactions with either diphenylisilane or methylphenylsi-
lane are very sluggish as compared to the reactions with butyl-
and phenylsilane that generate 5 or 9, respectively, which is
likely due to steric effects and the decreased availability of
reactive Si-H bonds in these reagents versus the primary
The imide-nitride complexes 15 (diphenylsilyl) and 16
(methylphenylsilyl) have been isolated, and the solid-state
molecular structure of 16 is shown in Figure 8. In contrast to
the primary silane homologues 6 and 10, 16 features cis-disposed
phosphines. Although this has not been observed elsewhere in
the hydrosilylation chemistry, we have reported complexes
resulting from hydroalumination36 of 1 that also feature phos-
phines that are not as strictly trans oriented with respect to the
Ta-Ta internuclear axis, as most derivatives of 1 appear to be.
The diphenylsilyl derivative 15 resembles the bridging nitrido
1
complexes 6 and 10 by comparison of their H and 31P NMR
spectra. The solid-state molecular structure of this complex has
not yet been established. Although 15 and 16 should feature a
reactive bridging nitride moiety, they do not react with additional
equivalents of either primary or secondary silane. It is possible
that the additional bulk of the secondary silane is compressing
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9479

A R T I C L E S
MacKay et al.
The reaction of the starting dinitrogen complex with tertiary
silanes such as Me3SiH does not proceed under any circum-
stances. Given the slow progress of the reaction with secondary
silanes, it is likely that access to the coordinated N2 unit is
impeded by the bulkier tertiary silane.
Conclusions
The results of this study show that hydrosilylation of the
dinitrogen complex 1 results in the formation of N-Si bonds
and concomitant N-N bond cleavage. This process is compa-
rable to the hydroboration and hydroalumination chemistries
previously reported,35,36 at least in the early stages. In particular,
the first step in all of these processes is common and involves
E-H addition across the Ta-N π-bond at the terminal nitrogen
of the side-on end-on N2 ligand, which initiates a series of steps
that involves H2 elimination and N-N bond cleavage. The
hydrosilylation process discussed in this work has provided
important details on these initial steps along with an understand-
ing of what happens subsequently. The isolation and solid-state
structural characterization of the key imide-nitride species 6
provides considerable insight into further functionalization of
the bridging nitride of 6 by reaction with additional silane. For
example, addition of butylsilane and subsequent elimination of
H2 result in the formation of the symmetrical disilylimide 4;
moreover, unsymmetrical disilylimide species such as 12 and
13 are also accessible due to isolation of intermediates such as
6 (and 10).
Figure 8. ORTEP drawing (ellipsoids at 50%) of the solid-state molecular
structure of 16. Silyl methyls and phenyl ring carbons other than ipso (will
be) omitted for clarity. Selected bond distances (Å), angles, and dihedral
angles (deg): Ta1-Ta2 2.914(3); Ta1-N3 2.040(5); Ta1-N1 1.961(6);
Ta1-N4 2.055(5); Ta1-N2 1.829(5); Ta1-P1 2.866(3); Ta2-N1 2.129-
(5); Ta2-N5 2.087(5); Ta2-N2 1.960(6); Ta2-N6 2.093(6); N1-Si1 1.726-
(6); Ta2-P2 2.729(3); N1-Ta2-N5 95.2(2); Ta1-N1-Ta2 90.8(2);
N1-Ta2-N6 165.4(2); N1-Ta2-N2 -80.2(2); N1-Ta2-P2 99.95(16);
Ta2-N2-Ta1 100.5(2); H101-Ta2-N5 142.5(3); N2-Ta1-N1 -88.1-
(2); N2-Ta1-N3 105.9(2); Ta1-N1-Si1 -140.1(3); N2-Ta1-N4 97.0-
(2); Ta2-N1-Si1 127.0(3); N2-Ta1-P1 169.31(16); N1-Ta1-N3 114.3-
(2); N1-Ta1-N4 129.8(2); N3-Ta1-N4 112.1(2).
One of the remarkable conclusions that arises from this work
is how changes in the silane structure can affect the outcome
of the reactions. Within the primary silane manifold, a change
from butylsilane to phenylsilane changes the course of the
addition of the second equivalent of silane, as evidenced by
the isolation of the unsymmetrical product 11 in the reaction
of PhSiH3. How a remote substituent change can influence the
mechanism is unknown, but one could speculate that transmis-
sion through the ancillary NPN ligands by some conformation
change in the ligand binding could be responsible. Even more
dramatic is the change in outcome when additional substituents
are employed on the starting silane. For secondary silanes such
as Ph2SiH2 and MePhSiH2, the reaction slows down consider-
ably and stops at the addition of one equivalent of silane as
evidenced by the formation of the imide nitride derivatives 15
and 16. The importance of steric effects in the silane structure
is also apparent even with the primary silanes in that mesityl-
silane is unreactive while tert-butylsilane undergoes more
complicated transformations.
The initial observation that addition of two equiv of butyl-
silane to the dinitrogen complex 1 resulted in the formation of
the symmetrical disilylimide species 4 suggested that perhaps
additional silane might be able to further functionalize the
nitrogen atoms and perhaps even release silylated amine.
However, our attempts to accomplish this were thwarted by a
intramolecular transformation rearrangement that involves cy-
clometalation of the P-Ph group across that Ta2(µ-NSi)2 core,
elimination of H2, and rearrangement of the imide functionality.
This process also was observed to occur for the reaction of
ethanediylbissilane with 1. The fact that this transformation
requires additional silane to occur without the silane being
incorporated has yet to be rationalized.
the [NPN] ligand phenyl groups in a manner that prevents
reactions at the bridging nitride position. In the particular case
of 16, it may be that cis-disposed phosphines negate further
reactivity. In any case, the fundamental questions of how the
silyl substituent affects the cascade of reactions experienced by
derivatives of 1 remains unanswered. Resonances suggestive
of complexes related to 4, 7, or 11 are not observed, even over
weeks in the presence of butylsilane or phenylsilane. Although
the lack of cyclometalation is significant, these complexes do
not offer a cleaner route to homologues of 4 because they are
unreactive.
Hydrosilylation Using Bulky Primary Silanes and Tertiary
Silanes. A number of other simple silanes were examined for
their ability to functionalize the starting dinitrogen complex 1.
Although mesitylsilane (2,4,6-Me3C6H2SiH3) reacts with 1, there
is no evidence for an addition product comparable to prototypical
hydrosilylated complexes 5 and 9. In fact, the 31P NMR
spectrum of the reaction mixture after 24 h contains a plethora
of resonances, none of which could be tentatively identified as
belonging to the family of complexes already described. This
reaction was abandoned after several fruitless attempts at
separating these compounds.
By contrast, tert-butylsilane (Me3CSiH3) appears to react with
1 via hydrosilylation as suggested by resonances comparable
to those of 5 and 9 in the 31P NMR spectra of 2:1 reactions.
However, this material converts rapidly into other species. The
lone isolated and fully characterized product features substantial
degradation of ancillary ligands as well as N-N bond cleavage.
It is presented in the Supporting Information rather than in the
main text for brevity, as it offers little to this discussion other
than to suggest once more that rearrangements of derivatives
of 1 are governed by silyl substituents.
The transformations reported in this work, namely N-N bond
cleavage and N-Si bond formation, hold promise for potential
9
9480 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Cleavage and Functionalization of N2
A R T I C L E S
of a solution of phenylsilane in approximately 0.25 mL C6D6 was added
as a bolus through the septum using a 20-gauge hypodermic needle.
The reagents were mixed by brief inversion of the tube before the
sample was returned to the probe and automated acquisition was begun.
Individual resonances were integrated with respect to the internal
standard. Similar experiments using D2-1 and 15N2-1 were also
conducted.
discovery of homogeneous catalysts that can convert molecular
nitrogen into higher value organonitrogen complexes. But there
are still major hurdles to surmount. One avenue that we are
exploring is iterative ancillary ligand design, which might
mitigate some of the unproductive rearrangements reported
herein.
Synthesis of ([NPN]TaH)(µ-H)2(µ-η1:η2-NNSiH2Ph)(Ta[NPN]), 9.
To a stirred solution of 441 mg (0.350 mmol) 1 in 25 mL toluene was
added 37.9 mg (0.35 mmol, 1 equiv) of phenylsilane in roughly 2 mL
toluene under N2. The flask was stored for 24 h in a freezer, after which
the dark-brown solution had turned red-orange. Solvent was removed
under vacuum, leaving a pink residue that was triturated under hexanes
and left overnight. The resulting red crystalline material was recovered
on a glass frit, yielding 488 mg (0.345 mmol, 99% yield) 9.
Cocrystallized hexanes (0.5 equiv) could not be removed from the solid
under full vacuum, and its presence was confirmed by NMR and X-ray
crystallography. 1H NMR (C7D8, -60 °C, 400 MHz): δ -0.12, -0.07,
-0.02, 0.01, 0.02, 0.05, 0.09, 0.52 (s, 3H each, total 24 SiCH3),
0.47, 0.81, 1.25, 1.29, 1.37, 1.58, 1.74, and 1.89 (AMX, 1H each,
Experimental Section
General Considerations. Unless otherwise stated, all manipulations
were performed under an atmosphere of dry oxygen-free dinitrogen
by means of standard Schlenk or glovebox techniques (Vacuum
Atmospheres HE-553-2 glovebox equipped with a MO-40-2H
purification system and a -60 °C freezer). Anhydrous hexanes and
toluene were purchased from Aldrich, sparged with dinitrogen, and
passed through columns containing activated alumina and Ridox catalyst
before use. Anhydrous diethyl ether was stored over sieves and distilled
from sodium benzophenone ketyl under argon. Pentane was stored over
sieves and distilled from sodium benzophenone ketyl solublized by
tetraglyme under dry dinitrogen prior to storage over a potassium mirror.
Tetrahydrofuran was heated at reflux over CaH2 prior to distillation
from sodium benzophenone ketyl under argon. Nitrogen gas was dried
and deoxygenated by passage through a column containing activated
molecular sieves and MnO. The syntheses and characterization of
compounds 4, 5, and 6 have been reported in our preliminary
communication34 along with the X-ray structures of 4 and 6 and details
of their acquisition and solution; nevertheless, the Supporting Informa-
tion for this manuscript has the preparations and spectral data
reproduced for completeness.
2
SiCH2P), 4.62 and 5.38 (d, JHH 10.9 Hz, 1H each, NSiH2Ph), 6.76,
(o-C6H5-Si), 6.82, 7.11 (C6H5-Si), 7.38 (d, JHH ) 7.3, o-C6H5-P)
8.40 (d, JHH ) 5.0, o-C6H5-P) 6.47, 6.76, 6.82, 6.95, 7.13, 7.18, 7.20,
7.28, 7.46 (overlapping m, PPh-H and NPh-H), 11.24 (ddd, 2H, JHH
2
) 10.1 and 5.24 Hz, JHP ) 22.1 Hz, TaHTa) 14.30 (dd, JHP ) 18.1
Hz, JHH ) 5.24 Hz, 1H, Ta-Ht).13C NMR (C7D8, -60 °C, 100.61
MHz): -0.33, 0.52, 0.80, 0.97, 2.19, 2.86, 3.85, 4.14 (s, SiCH3), 15.06,
15.39, 16.24, 17.18, 22.32, 22.05, 19.58, 20.69 (d, SiCH2P), 130.02
(o-C6H5-Si) 132.03 and 133.60 (o-C6H5-P) 116.44, 117.86, 121.87,
122.46, 123.01, 126.31, 129.22, 130.02, 132.65, 1334.16, 136.31,
138.44, 139.89, 143.98, 145.77, 146.21, 147.04, 147.69, 152.37, 155.69,
156.81, 157.31 (P(C6H5) and N(C6H5). 31P{1H} NMR (C7D8, -60 °C,
161.97 MHz): δ 23.27 (d, JPP ) 17.8 Hz), 8.86 (d, JPP ) 17.8 Hz).
29Si NMR (C7D8, -60 °C, 79.5 MHz) δ -9.26 (s, NSiH2Ph) 6.73 (d,
Synthesis of [NP(C6H4)N]Ta(BuSiH2N-µ-SiHBu-µ-N)Ta[NPN], 7.
To a stirred solution of 235 mg (0.186 mmol) 1 in 15 mL toluene was
added 65.5 mg (0.744 mmol, 4 equiv) of butylsilane in roughly 2 mL
toluene under N2. The dark-brown solution turned red initially and then
dark brown over the course of 4 days. Solvent was removed under
vacuum, leaving a brown residue that was triturated under hexanes.
The resulting precipitate was recovered on a glass frit and rinsed with
minimal hexanes to afford 112 mg (0.078 mmol, 42% yield) of 7. In
the assignment of the 1H NMR spectrum, the protons on the metalated
phenyl ring are labeled R, â, δ, and γ, with the proton nearest the
metalated carbon labeled R. 1H{31P} NMR (C7D8, 30 °C, 400
MHz): δ -0.41, 0.11, 0.14, 0.29 (s, 6H each, SiCH3), 0.68, 0.91,
1.31, 1.46 (d, 2H each, SiCH2P), 3.37 (s, 2H, SiAH2), 4.62 (s, 1H,
SiBH), 0.49 (m, overlapping, 2H, SiACH2CH2-), 1.82 (br m, 2H,
SiACH2CH2CH2-), 1.04 (br, 2H, SiACH2CH2CH2CH3), 1.30, (d, 3H,
SiACH2CH2CH2CH3), 0.13 (m, overlapping, 2H, SiBCH2CH2-), 1.43
(br m, 2H, SiBCH2CH2CH2-), 1.78 (br, 2H, SiBCH2CH2CH2CH3), 1.36,
(d, 3H, SiBCH2CH2CH2CH3), 7.61 (dd, 1H, HR), 7.21 (d, 1H, Hâ), 7.21
(d, 1H, Hγ), 6.38 (d, 1H, Hδ), 7.11, 7.72, 7.91 (P-C6H5) 6.89, 6.95,
7.05, 7.13, 7.20, 7.22 (d, t, 23H total, some overlap with solvent,
N-C6H5). 31P{1H} NMR (C6D6, 30 °C, 161.97 MHz): δ 24.23 (s),
71.15 (s). Anal. Calcd for C56H82N6P2Si6Ta2: C 46.98; H 5.77; N 5.87.
Found: C 47.12; H 5.91; N 5.89.
2
2
2JPSi ) 9.6 Hz), 7.16 (d, JPSi ) 9.6 Hz) 10.68 (d, JPSi ) 11.3 Hz),
11.30 (d, JPSi ) 9.6 Hz). UV-Vis: λmax ) 500 nm, ꢀ ) 619 M-1
2
cm-1. Anal. Calcd for C54H72N6P2Si5Ta2‚(C6H14)0.5 C 48.47; H 5.64; N
5.95. Found: C 48.63; H 5.20; N 5.48.
Synthesis of 15N2-9. A solution of 15N2-1 was treated in a manner
similar to the preparation of 9. 15N NMR (C7D8, -60 °C, 40 MHz) δ
1
2
1
-139.7 (d, JNN ) 17.4 Hz) -50.5 (dd, JPN ) 26.1 Hz, JNN ) 17.4
Hz). Additional coupling of 26.1 Hz was observed in the δ 23.27 ppm
resonance in the 31P{1H} spectrum, as was an additional coupling of
4.9 Hz in the δ -9.26 ppm resonance of the 29Si{1H} spectrum.
Synthesis of ([NPN]TaH)(µ-N)(µ-NSiH2Ph)(Ta[NPN]), 10. A
red-orange 25 mL toluene solution of 9 (488 mg, 0.345 mmol) was
left at 15 °C under N2 for 36 h, after which the solvent was removed
under vacuum, leaving a yellow-brown residue, which was triturated
under hexanes. Solid powdery 10 was recovered on a glass frit and
rinsed with hot hexanes to remove a brown impurity (473 mg, 0.335
mmol, 97% yield). 1H NMR (C6D6, 30 °C, 400 MHz): δ -0.18, -0.04,
0.03 and 0.41 (s, 6H each, 24H total, SiCH3), 0.98, 1.16, 1.34, and
1.48 (d, 2H each, SiCH2P), 4.76 (b, 2H, NSiH2Ph), 6.48, 6.92, 7.08,
Synthesis of 15N2-7. A solution of 15N2-1 was treated in a manner
similar to the preparation of 7. 31P{1H} NMR (C6D6, 30 °C, 161.97
MHz): δ 24.23 (s), 71.15 (d, 2JPN ) 9.9 Hz). 15N NMR (C6D6, 30 °C,
7.47, 6.94, 7.09, 7.15, 7.38, and 7.61 (overlapping m, PPh-H and
2
NPh-H) 7.92 and 7.41 (m, 2H each, PPh-o-H), 18.21 (d, JHP
)
47.3 Hz, 1H, Ta-Ht). 31P{1H} NMR (C6D6, 30 °C, 161.97 MHz): δ
-5.4 (b), -12.8 (b). 29Si NMR (C6D6, 30 °C, 79.5 MHz) δ -38.62 (s,
NSiH2Ph) 11.51 (d, 2JPSi ) 12.1 Hz), 10.81 (d, 2JPSi ) 12.4 Hz). Anal.
Calcd for C54H70N6P2Si5Ta2: C 47.43; H 5.16; N 6.15. Found: C 47.80;
H 5.51; N 5.94.
2
40 MHz) δ -17.5 (d, JPN ) 9.9 Hz), 233.4 (s).
31P NMR Spectroscopic Investigation of the Reaction of 1 with
Butylsilane or Phenylsilane. A 9” Wilmad NMR tube was charged
with ∼40 mg 1 in roughly 0.8 mL C6D6 and a sealed glass capillary
tube containing neat P(OMe)3 as an internal reference. The tube was
sealed with a 5 mm rubber septum and wrapped with ParaFilm
laboratory film and inserted into the probe of a Bruker AVA-400 NMR
spectrometer. The spectrometer was programmed to observe consecutive
sets of 1H{31P} and 31P{1H} spectra every 15 min for 25 h. After initial
spectrometer calibration was performed, the sample was ejected and
14 mg neat butylsilane (roughly 4.5 equivs) or a comparable amount
Synthesis of 15N2-10. A solution of 15N2-9 in C6D6 was allowed
to decompose overnight at room temperature. 15N NMR (C6D6, 30 °C,
40 MHz) δ 312.2 (b) -36.9 (d, 2JPN ) 17.2 Hz,). Additional coupling
of 17.2 Hz was observed in the δ -12.8 ppm resonance in the 31P{1H}
spectrum, as was an additional coupling of 4.1 Hz in the δ -38.6 ppm
resonance of the 29Si{1H} spectrum.
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9481

A R T I C L E S
MacKay et al.
SiH3), 0.34 (d, 2H, N-SiH2CH2CH2SiH3) 1.63 ppm (AMX, 8H,
Synthesis of ([NPN]Ta)(µ-NSiH2Ph)(µ-NSiHPh)(Ta[NPN]), 11.
To a stirred solution of 312 mg (0.247 mmol) 1 in 15 mL toluene was
added 55.0 mg (0.510 mmol, 2.1 equiv) of phenylsilane in roughly 2
mL toluene under N2. The dark-brown solution turned dark red over
the course of 48 h, and the solvent was removed under vacuum, leaving
a brown residue, which was triturated under hexanes. An ochre
precipitate was recovered on a glass frit, giving 205 mg (0.139 mmol,
2
2
SiCH2P), 3.63 (t, JHH ) 3.8 Hz, 3H, H3CSiEtSiH2-N), 4.81 (d, JHH
) 3.56 Hz, H3CSiEtSiH2-N), 4.89 (s, 2H, SiH2Bu), 7.62 (d, 4H,
o-HPhP), 6.90, 6.92, 7.01, 7.06, 7.10, 7.12, 7.19, 7.22 (overlapping m,
PPh-H and NPh-H). 13C NMR (C6D6, 30 °C, 100.61 MHz): δ -2.42,
1.52, 4.85, 5.12 (SiCH3), 25.11, 26.20 (CH2-P) 25.11, 26.20, 13.96,
3.87 (BuSiH2), 3.87 (H3SiCH2), 12.89 (H3SiCH2CH2SiH2-N), 131.46,
131.57 (o-Ph-P), 122.44, 122.54, 123.61, 125.21, 128.10, 128.50,
128.66, 128.84 (PPh and NPh). 31P{1H} NMR (C6D6, 30 °C, 161.97
MHz): δ -0.021 ppm (m). 29Si-DEPT NMR (C6D6, 30 °C, 79.5 MHz)
1
56% yield) of analytically pure 11. H{31P} NMR (C6D6, 30 °C, 400
MHz): δ -0.39, -0.24, -0.23, -0.19, -0.13, -0.01, 0.22, 0.59 (s,
3H each, SiCH3), 1.13, 1.23, 1.28, 1.43, 1.61, 1.94, 2.29, and 2.79
2
δ -2.14 (H3SiEtSiH2-N), -0.01 (BuSiH2-N) 8.08 (d, JPSi ) 15.7
2
ppm (AMX, 1H each, SiCH2P), 4.60 and 5.68 (d, 1H each, JHH
)
2
Hz, [NPN] ligand) 8.79 (d, JPSi ) 17.4 Hz, [NPN] ligand). H3Si-Et
13.3 Hz, PhSiH2-N), 5.59 (s, N-PhSi(H)-N), 7.54, 7.62 (d, 2H each,
o-C6H5-P) 9.43 (s, 2H o-C6H5-SiH2-N), 6.52, 6.69, 6.79, 6.88,
silyl resonance not located. Anal. Calcd for C54H82N6P2Si7Ta2: C 45.18;
H 5.76; N 5.85. Found: C 45.23; H 5.91; N 5.96.
6.93, 7.04, 7.21, 7.36, 8.18 (overlapping m, PPh-H and NPh-H). 13
C
NMR (C6D6, 30 °C, 100.61 MHz): δ -1.56, -0.59, -0.38, 0.87, 3.30,
4.03, 4.12, 4.81 (SiCH3), 15.22, 18.14, 24.22, 35.17 (CH2-P), 125.91
(N-o-C6H5Si(H)-N) 129.07 (o-C6H5-SiH2-N), 123.47, 124.44,
126.39, 127.85, 129.58, 132.48, 133.69, 137.58 (P-C6H5 and
Synthesis of [NP(C6H4)N]Ta(NSiH2CH2CH2Si(H)-µ-N)Ta[NPN],
14. To a stirred 10 mL toluene solution of 184 mg (146 mmol) 1 was
added neat ethanediylbissilane (13 mg, 146 mmol, 1 equiv). The
solution was allowed to sit overnight, and solvent was removed under
vacuum to give a yellow-brown residue that yielded crystalline 14 (138
mg, 105 mmol, 72% yield) after trituration under hexanes and 24 h at
-60 °C in a glovebox freezer.1H{31P} NMR (C7D8, 30 °C, 400 MHz):
δ -0.80, -0.08, 0.13, 0.47 (s, 6H each, 24H total, SiCH3), 0.68, 0.92,
1.34, 1.76 (d, 2H each, -CH2P), 3.52 (s, 2H, SiH2), 4.11 (s, 1H, SiH)
7.74 (d, 2H, o-C6H5-P), 7.72 (dd, 1H, HR), 7.03 (d, 1H, Hâ), 6.47 (d,
1H, Hγ), 6.79 (d, 1H, Hδ) 6.82, 6.84, 6.91, 7.02, 7.14, 7.31, 7.44 (d, t,
overlapping with solvent resonances, C6H5-N and C6H5-Ph). 31P{1H}
NMR (C7D8, 30 °C, 400 MHz): δ 21.7 (s), 72.8 (s). Elemental analysis
was not obtained.
N-C6H5). 31P{1H} NMR (C6D6, 30 °C, 161.97 MHz): δ -10.46, 15.88
2
ppm (s). 29Si NMR (C6D6, 30 °C, 79.5 MHz) δ -35.82, (dd, JPSi
)
2
12.2, 16.6 Hz, N-PhSi(H)-N) 6.07 (s, [NPN] ligand) 12.93 (d, JPSi
) 14.8 Hz, [NPN] ligand), 17.07 (d, JPSi ) 14.8 Hz, [NPN] ligand),
18.30 (d, JPSi ) 20.1 Hz, [NPN] ligand), 49.86 (d, JPSi ) 41.0 Hz,
PhSiH2-N). Anal. Calcd for C60H74N6P2Si6Ta2: C 48.90; H 5.20; N
5.70. Found: C 48.63; H 5.20; N 5.48.
2
2
2
Synthesis of 15N2-11. A solution of 15N2-1 was treated in a manner
similar to the preparation of 11. 31P{1H} NMR (C6D6, 30 °C, 161.97
MHz): δ -10.46 (d, JPN ) 17.86 Hz), 15.88 ppm (s). 15N NMR (C6D6,
30 °C, 40 MHz) δ -61.21 (d, JPN ) 17.86 Hz), -35.84 (s). 29Si NMR
(C6D6, 30 °C, 79.5 MHz) δ -35.82, (ddd, 2JPSi ) 12.2, 16.6 Hz, 1JNSi
Synthesis of ([NPN]TaH)(µ-N)(µ-NSiHPh2)(Ta[NPN]), 15. To a
15 mL toluene solution of 268 mg (212 mmol) 1 was added 39.3 mg
(213 mmol, ∼1 equiv) Ph2SiH2 in roughly 2 mL toluene. The mixture
was agitated briefly and allowed to stand under N2 at ambient
temperature. The 31P NMR spectrum of a portion of the solution
withdrawn after 2 weeks indicated conversion to a new product, and
solvent was removed under vacuum. Brown residues were triturated
under hexanes and 191 mg (131 mmol, 62% yield) of yellow-white
powdery 15 were isolated on a frit after rinsing with minimal hexanes.
1H{31P} NMR (C7D8, 30 °C, 400 MHz): δ -0.28, -0.03, 0.16, 0.31
(s, 6H each, 24H total, SiCH3), 0.84, 0.92, 1.36, 1.49 (d, 2H each,
-CH2P), 3.49 (s, 1H, SiH) 7.83, 7.90 (d, 2H each, o-C6H5-P), 6.85,
6.92, 7.14, 7.33, 7.38, 7.41 (d, t, overlapping with solvent resonances,
C6H5-N and C6H5-Ph), 6.43, 7.11, 7.34 (d, t, 10H total, C6H5Si).
31P{1H} NMR (C7D8, 30 °C, 400 MHz): δ - 16.54 (vb, fwhm 36.2
Hz). -9.47 (vb, fwhm 36.8 Hz). Anal. Calcd for C60H84N6P2Si5Ta2: C
49.58; H 5.82; N 5.78. Found: C 49.33; H 5.91; N 5.86.
2
) 10.5 Hz, N-PhSi(H)-N), 49.86 (dd, JPSi ) 41.0 Hz, PhSiH2-N),
[NPN] ligand silyl resonances unaffected.
Synthesis of ([NPN]Ta)2(µ-NSiH2Bu)(µ-NSiH2Ph), 12. To a stirred
20 mL toluene solution of 126 mg (0.094 mmol, ∼1 equiv) 6 was added
9 mg (0.10 mmol) phenylsilane. The stirred solution turned deep red
over 2 days. The solvent was evaporated, and the dark solids were
triturated under pentane, giving 97.8 mg 12 (0.067 mmol, 72%
yield). Alternately, 10 was combined with butylsilane in C6D6 in an
NMR tube, and the 31P NMR resonances of 12 were observed after 2
1
days. No yield is reported in the absence of an internal standard. H-
{31P} NMR (C6D6, 30 °C, 400 MHz): δ -0.14, -0.12, 0.02, 0.10, (s,
6H each, SiCH3), -0.31 (br, 2H, SiCH2CH2-), 0.62 (br m, 2H,
-CH2CH2CH2-), 1.24 (br, 2H, -CH2CH2CH3), 0.856 (d, 3H, CH2CH3),
1.02, 1.16, 1.31, and 1.64 ppm (d, 2H each, SiCH2P), 4.78 (s, 2H,
PhSiH2-N), 4.41 (s, 2H BuSiH2-N), 6.37 (d, 2H, o-C6H5-Si), 7.34
and 7.49 (d, 2H each, o-C6H5-Si) 4.91 (s, 2H, SiH2) 6.941 (d, 4H,
p-HPhN), 7.11 (dd, 8H, m-HPhN), 6.52 (d, 8H, m-HPhN), 7.12 (d,
2H, p-HPhP), 7.20 (dd, 4H, m-HPhP) and 7.65 (d, 2H, o-HPhP),
6.87, 6.93, 6.95, 6.97, 6.99, 7.05, 7.08, 7.23, 7.27 (overlapping m,
PPh-H and NPh-H). 31P{1H} NMR (C6D6, 30 °C, 161.97 MHz): δ
-1.17 ppm (d, JPP ) 9.9 Hz), -0.70 (d, JPP ) 9.9 Hz). 29Si NMR
([NPN]TaH)(µ-N)(µ-NSiH(Me)Ph)(Ta[NPN]), 16. To a dark brown
solution of 1 (0.20 g, 0.16 mmol) in 5 mL of toluene was added 0.019
g of methylphenylsilane under N2. Over the course of 4 days, the
solution turned red. The solvent was removed under vacuum, and the
resulting red solid was dissolved in hexanes. Yellowish-red crystals
were obtained (0.080 g, 37%) over a course of 48 h at room temperature
and were collected in a glass frit and rinsed with pentane. A pentane
solution of 16 was cooled to -30 °C, and single crystals suitable for
2
(C6D6, 30 °C, 79.5 MHz) δ -11.3, (d, JPSi ) 8.0 Hz, PhSiH2-N),
-1.8 (d, 2JPSi ) 8.8 Hz, BuSiH2-N), 10.93 (d, 2JPSi ) 14.8 Hz, [NPN]
2
ligand) 12.9 (d, JPSi ) 12.4 Hz [NPN] ligand) Anal. Calcd for
1
C58H80N6P2Si6Ta2: C 47.92; H 5.55; N 5.78. Found: C 47.60; H 5.15;
N 6.07.
X-ray analysis were obtained. H NMR (C7D8, -70 °C, 500 MHz):
δ(ppm) -0.15, -0.09, -0.02, 0.08, 0.13, 0.28, 0.33, 0.40, 0.44
(overlapping s, 24H total, SiCH3), 0.99, 1.06, 1.17, 1.29, 1.45, 1.54,
1.64, 1.74 (overlapping, 11H total, SiCH2P and NSiCH3), 4.58, 4.88,
5.24, 5.32 (s, 1H total, NSiH), 6.0 to 8.1 (overlapping, SiPh-H, PPh-H
and NPh-H), 16.79, 16.86, 16.95, 17.44, 17.51 17.65 (overlapping m,
1H total, Ta-Ht). 31P{1H} NMR (C7D8, -70 °C, 500 MHz) δ(ppm)
1.64, -4.70, -11.03, 11.70, -15.78, -16.92, -21.35. 31P COSY
correlation between δ(ppm) -1.64 and -16.92, -11.03 and -21.35,
-15.39 and -15.78. Anal. Calcd for C55H71N6P2Si5Ta2: C, 47.82; H,
5.25; N, 6.08. Found: C, 47.92; H, 5.58; N, 6.19
Synthesis of ([NPN]Ta)2(µ-NSiH2Bu)(µ-NSiH2CH2CH2SiH3), 13.
To a stirred 15 mL toluene solution of 383 mg (0.284 mmol, 1 equiv)
6 was added 25.7 mg (0.284 mmol) ethanediylbissilane. The stirred
solution turned deep red over 4 days. The solvent was evaporated,
and the dark solids were triturated under pentane, giving 289 mg
13 (0.202 mmol, 71% yield). 1H{31P} NMR (C6D6, 30 °C, 400
MHz): δ 0.00, 0.11, 0.38, 0.40, (s, 6H each, SiCH3), -0.35 (br, 2H,
SiCH2CH2-), -0.12 (br m, 2H, -CH2CH2CH2-), 1.58 (br, 2H,
-CH2CH2CH3), 1.02 (d, 3H, CH2CH3), -0.05 (d, 2H, N-SiH2CH2CH2-
9
9482 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Cleavage and Functionalization of N2
A R T I C L E S
Acknowledgment. We are grateful to NSERC of Canada for
funding (Discovery Grant for M.D.F., PGS-A and PGS-B to
B.A.M.) and to Dr. Christopher D. Carmichael for his assistance
with the diffraction experiments.
for the X-ray crystal structure analyses; CIF files for com-
plexes 6, 7, 9, 11, 13, 14, and 16; experimental details for
t
the reaction of BuSiH3 with the dinitrogen complex 1. This
material is available free of charge via the Internet at
http://acs.pubs.org.
Supporting Information Available: Full experimental details
for the synthesis of complexes 4, 5, and 6; experimental details
JA061508Q
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9483