Hydrosilylation of a dinuclear tantalum dinitrogen complex: Cleavage of N2 and functionalization of both nitrogen atoms

Article abstract of DOI:10.1021/ja034303f

Hydrosilylation of the ditantalum dinitrogen complex ([NPN]Ta)₂(μ-H)₂(μ-η¹:η²-N₂) proceeds via an addition reaction to produce ([NPN]TaH)(μ-H)₂(μ-η¹:η²-N-NSiH₂Bu)(Ta[NPN]), which contains a new N-Si bond and a terminal tantalum hydride; this species has been characterized by NMR spectroscopy and X-ray diffraction. This complex undergoes reductive elimination of H₂ followed by N-N bond cleavage to generate a new intermediate with the formula ([NPN]TaH)(μ-N)(μ-NSiH₂Bu)(Ta[NPN]); confirmation of N-N bond cleavage is evident from the ¹⁵N-labeled isotopomer that displays an absence of ¹⁵N-¹⁵N scalar coupling in the ¹⁵N NMR spectrum. In the presence of additional silane, a second hydrosilylation and reductive elimination results to give ([NPN]Ta)₂(μ-NSiH₂Bu)₂, a species in which each dinitrogen-derived N atom has been converted to a bridging silylimide ligand. This latter complex displays C_2h symmetry both in solution and in the solid state. Copyright

Full text of DOI:10.1021/ja034303f

Published on Web 02/21/2003
Hydrosilylation of a Dinuclear Tantalum Dinitrogen Complex: Cleavage of N2
and Functionalization of Both Nitrogen Atoms
Michael D. Fryzuk,* Bruce A. MacKay, and Brian O. Patrick
Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer, BC, Canada V6T 1Z1
Received January 23, 2003 ; E-mail: fryzuk@chem.ubc.ca
Research into the activation of molecular nitrogen by metal
complexes has reached a high level of sophistication.1 While early
work concentrated on the discovery of new examples of N2
coordinated to metals in a variety of bonding modes,2 it is clear
that further advancement in this area will require systems that show
enhanced reactivity patterns. In particular, discovery of metal
dinitrogen complexes that undergo transformations resulting in the
cleavage and/or functionalization3 of the coordinated N2 unit is of
interest especially if the conditions are mild and potentially
applicable to catalytic deployment.4
We recently reported the hydroboration of the tantalum dinitrogen
complex ([NPN]Ta)2(µ-H)2(µ-η1:η2-N2), 1 (where [NPN] )
(PhNSiMe2CH2)2PPh), in which addition of the B-H bond of
9-BBN across a metal-dinitrogen π bond gave a new N-B bond
and a terminal tantalum hydride.5 A key finding was that this
Figure 1. ORTEP drawing of 2 (ellipsoids at 50% probability, silyl methyl
and phenyl carbons other than ipso omitted for clarity). Selected bond
lengths (Å) and angles (deg): Ta1-N1 1.997(9), Ta1-N2 2.007(9), Ta2-
N1 2.038(10), Ta2-N2 1.990(9), N1‚‚‚N2 2.935(4), N1-Si1 1.729(10),
reaction triggered a cascade of events including N-N bond
cleavage. Unfortunately, an ancillary [NPN] ligand was irreversibly
rearranged via silyl migration to a metal nitrido ligand, limiting
the usefulness of this process. However, the success of this reaction
provided the incentive to examine other simple hydride reagents
that might add E-H bonds to 1 without the unattractive features
of the hydroboration chemistry. We have previously reported
hydrosilylation of dinitrogen complexes of zirconium,6 and thus
exploring addition reactions with primary silanes seemed attractive.
Herein we report preliminary results that extend this reactivity
pattern for the N2 ligand and establish new possibilities for cleavage
and complete functionalization of coordinated dinitrogen without
the involvement of the ancillary ligands.
N2-Si2 1.739(10), Ta1-Ta2 2.6677(6), Ta1-P1 2.735(3), Ta1-N3 2.070-
(9), Ta1-N4 2.045(9), Ta2-P2 2.687(3), Ta2-N5 2.077(9), Ta2-N6
2.069(8), Ta1-N1-Ta2 82.8(4), N1-Ta2-N2 93.5(4), Ta2-N2-Ta1 83.7-
(3), N2-Ta1-N1 94.3(4), Ta1-N1-Si1 143.2(6), Ta2-N2-Si2 134.8-
(5), Ta1-N2-Ta2-N1 155.8(5).
trogen complex 15N2-1 clearly indicate that the coordinated dini-
trogen is the source of the N-atoms of the bridging silylimide units
of 2.
Other silanes such as PhSiH3 and Ph2SiH2 also add across the
Ta2N2 moiety with similar outcomes as will be reported in due
course.
The reaction between 2 equiv of butylsilane (BuSiH3) and the
ditantalum dinitrogen complex 1 proceeds cleanly over 36 h to
produce 2 in 63% yield (eq 1). The NMR spectral data of 2,
particularly the presence of a single [NPN] silyl resonance and a
single butylsilyl resonance in the 29Si{1H} NMR spectrum, are
consistent with C2h symmetry in solution.7 No 1H NMR resonances
typical of hydrides, either bridging or terminal, are observed.
Insight into the intervening processes in this reaction was
obtained by periodically observing the 31P{1H} NMR spectra of
the reaction of butylsilane and 1. The first step is an addition
reaction analogous to the hydroboration sequence5 that gives a new
N-Si bond and new terminal hydride; the characteristic 31P
resonances of 1 convert cleanly to those of complex 3 within 8 h
at room temperature. This species has been isolated in excellent
yield as crystals from a cooled solution of pentane; its solid-state
molecular structure is shown in Figure 2.9 Complex 3 maps closely
onto the N-boryl congener, but the butylsilyl group is clearly not
coplanar with the Ta2N2 core. The Ta-N-Si angle is 133.6(5)°
versus 163.1(10)° in the N-boryl analogue, and this may indicate a
lack of N-Si π overlap and thus a change in hybridization at N1
between the two otherwise very similar complexes. The hydride
ligands shown in Figure 2 were not located in the diffraction
experiment but were successfully modeled using XHYDEX10 and
The solid-state molecular structure of 2, determined from a
twinned crystal, is shown in Figure 1.8 It is evident that the
dinitrogen fragment has been cleaved and that both dinitrogen-
derived N atoms have been functionalized in the same manner
despite their asymmetric activation in parent complex 1. These
bridging units can be considered as bridging silylimides bracketing
a Ta(IV)-Ta(IV) metal-metal bond, thus accounting for the
observed diamagnetism. Labeling studies using the enriched dini-
1
are strongly implied by the H NMR spectrum of 3.
Complex 3 is thermally unstable and undergoes spontaneous
elimination of H2, cleanly converting to 4 over 8 h at room
temperature; this process occurs both in solution and in the solid
1
state. This new species exhibits Cs symmetry in solution in its H
NMR spectrum; a resonance typical of a terminal tantalum hydride
is observed at δ 17.4 ppm, but no resonances suggestive of bridging
9
3234
J. AM. CHEM. SOC. 2003, 125, 3234-3235
10.1021/ja034303f CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Acknowledgment. The authors thank NSERC of Canada for
funding (Discovery Grant to M.D.F., PGS-A/B to B.A.M.), Mr.
Christopher D. Carmichael for assistance with the crystallography,
Mr. Marshall Lapawa for mass spectrometry, and Mr. Peter Borda
and Mr. Minaz Lakha for elemental analyses.
Figure 2. ORTEP drawing of 3 (ellipsoids at 50% probability, silyl methyl
and phenyl carbons other than ipso omitted for clarity). Hydrides were placed
using XHYDEX. Selected bond lengths (Å) and angles (deg): N1-N2
1.363(13), Ta1-N1 2.181(10), Ta1-N2 2.179(10), Ta2-N2 1.835(10),
N1-Si1 1.732(11), Ta1‚‚‚Ta2 2.8430(9), Ta1-P1 2.583(3), Ta1-N3 2.028-
(10), Ta1-N4 1.236(11), Ta2-P2 2.632(3), Ta2-N5 2.044(8), Ta2-N6
2.075(7), Ta1-N1-Si1 133.6(5), Ta1-N1-N2 71.7(6), Ta1-N2-Ta2
89.7(4), N1-Ta1-P1 79.3(3), N1-Ta1-N3 159.5(4), N1-Ta1-N4 87.0-
(4), N2-Ta2-P2 166.6(3), N2-Ta2-N5 108.4(4), N2-Ta2-N6 105.8-
(4), N1-Ta1-Ta2-N2 1.0(5), Ta2-Ta1-N1-Si1 105.7(8), P1-Ta1-
Ta2-P2 -170.6(2).
Supporting Information Available: X-ray and complete experi-
mental details for 2, 3, 4, and isotopomers (PDF). Structural information
for 2 and 4 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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that loss of the bridging hydride ligands via reductive elimination
as H2 provides the final two electrons required for overall reductive
cleavage of the N-N bond. Evidence for this cleavage event comes
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prepared using enriched dinitrogen complex 15N2-1. Scalar coupling
of 1JNN ) 16.6 Hz between the two resonances in 15N2-3 presumably
results from the intact N-N bond. However, the spectrum of 15N2-4
shows no N-N scalar coupling and therefore indicates that N-N
bond scission has occurred.11
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Disilylimide 2 therefore appears to derive from intermediate 4
via a second addition of BuSiH3 across the Ta-NdTa unit,12
a
(7) 29Si NMR spectral data for 2: (C6D6, 30 °C, 79.5 MHz) δ 8.70, (d, 2JPSi
2
2
) 7.0 Hz, NSiMe2), -1.74 (dd, JPSi ) 4.1 Hz, dd, JPSi ) 3.9 Hz,
BuSiµN).
dinuclear reductive elimination of the hydride ligands as H2
completes the process (Scheme 1).13
(8) Crystallographic data for 2: C56H84N6P2Si6Ta2, triclinic, space group P-1,
a ) 11.843(1) Å, b ) 11.853(1) Å, c ) 23.382(2) Å, R ) 86.68(1)°, â
) 76.24(1)°, γ ) 79.36(1)°, Z ) 4, two-component twin, related by 44°
rotation normal to (-0.1, -1.0, -1.2), R1, 0.097 (I > 2.00σ(I)), wR2
0.211.
These results suggest three general conclusions regarding the
addition of E-H bonds across the dinitrogen unit of 1. First, the
initial reaction involves a common intermediate formed by the
addition of 1 equiv of E-H across the π-bond of the ditantalum
dinitrogen moiety. Second, this addition triggers loss of the bridging
hydride ligands as H2, which provides the two electrons necessary
to completely cleave the N-N bond. Third, the reducing power of
this system is “built-in” via the reductive elimination of H2, which
can occur repeatedly. As 1 is itself derived from a tantalum(IV)
hydride dimer that forms via reductive elimination,14 this finding
may bear strongly on the development of catalytic processes for
the incorporation of dinitrogen-derived N atoms into other sub-
strates. Such studies are in progress as are investigations into
reactions of other organometallic E-H reagents with 1.
(9) Crystallographic data for 3: C52H76N6P2Si5Ta2, orthorhombic, space group
Pna21, a ) 27.600(6) Å, b ) 17.262(4) Å, c ) 14.701(3) Å, Z ) 4, R1,
0.0566 (I > 2.00σ(I)), wR2 0.1353.
(10) Orpen, A. G. J. Chem. Soc., Dalton Trans. 1980, 2509.
(11) 15N NMR spectral data for 15N2-3: (C7D8, -60 °C, 40 MHz) δ -163.5
1
2
1
(d, JNN ) 16.6 Hz), -52.5 (dd, JPN ) 26.4 Hz, JNN ) 16.6 Hz). 15N
NMR spectral data for 15N2-4: (C6D6, 30 °C, 40 MHz) δ 284.4 (b), -44.8
2
(d, JPN ) 18.7 Hz).
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12841.
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A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960-3973.
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