New mode of coordination for the dinitrogen ligand: A dinuclear tantalum complex with a bridging N2 unit that is both side-on and end-on [25]

Full text of DOI:10.1021/ja982377z

11024
J. Am. Chem. Soc. 1998, 120, 11024-11025
Scheme 1
New Mode of Coordination for the Dinitrogen
Ligand: A Dinuclear Tantalum Complex with a
Bridging N2 Unit That Is Both Side-On and End-On
Michael D. Fryzuk,* Samuel A. Johnson, and Steven J. Rettig†
Department of Chemistry
UniVersity of British Columbia, 2036 Main Mall
VancouVer, B.C., Canada, V6T 1Z1
ReceiVed July 7, 1998
Interest in the coordination chemistry of the abundant yet inert
nitrogen molecule is undergoing an apparent renaissance. Among
the most notable recent achievements have been the splitting of
the N2 unit upon coordination1-3 and its reactivity with H2 and
silanes to generate N-H and N-Si bonds, respectively.4 While
most dinuclear dinitrogen complexes have the N2 bound in an
end-on, bridging fashion,5-7 the side-on mode of bonding is
becoming more prevalent.8-15 In this work, we detail a new
coordination mode for the dinitrogen ligand when bound to two
metals. In addition, the mode of formation of this early metal
N2 complex is unusual in that it entails the displacement of H2
by N2.
Scheme 2
We have recently described the logical progression of ligand
design from the PNP system (A in Scheme 1) to the P2N2
macrocycle (B).16 It occurred to us that another combination
within this group of donor types would be the NPN system (C)
easily seen as the leftmost (or rightmost) quadrant of the
macrocycle. One of the advantages of this system is the possible
flexible coordination modes as well as the fact that there is the
opportunity for coordinative unsaturation, at least as compared
to the P2N2 system.
The synthesis of the NPN ligand is shown in Scheme 2; the
reaction of 4 equiv of BuLi with a mixture of 2 equiv of
PhNHSiMe2CH2Cl and 1 equiv of PhPH2 in diethyl ether followed
by the addition of THF results in 85% yield of the ligand precursor
Li2(THF)2[NPN] (1) ([NPN] ) PhP(CH2SiMe2NPh)2). Coordina-
tion to tantalum(V) was achieved by the addition of 1 to TaMe3-
Cl2 to produce light-sensitive [NPN]TaMe3 (2) as a pale yellow
solid in 80% yield. The solid-state molecular structure of 2
demonstrates that the [NPN] ligand binds to Ta in a facial
manner.17
A diethyl ether solution of 2 reacts cleanly with dihydrogen
(1-4 atm) with elimination of methane to produce the diamagnetic
purple dinuclear Ta(IV) hydride {[NPN]TaH2}2 (3). The 31P and
1H NMR spectra of 3 are consistent with a symmetrical complex
with identical chemical environments at the two Ta centers. The
1
hydride resonance at 10.6 ppm in the H NMR spectrum shows
† Professional Officer: UBC Structural Chemistry Laboratory.
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no change upon cooling to -80 °C, which suggests that the
hydrides are all bridging rather than a fluxional combination of
bridging and terminal hydrides.18-20 This conclusion is supported
by the IR spectra of both 3 and 3-D4 for which no terminal Ta-H
or Ta-D stretches could be identified;21 assignment of the Ta(µ-
H)Ta modes was not possible due to overlapping peaks.
Upon exposure of a solution of 3 to N2 gas, the color of 3
changes from purple to brown, with the elimination of H2 (1 equiv
by stoichiometry). After less than 1 h under an atmosphere of
N2, 3 was converted to the dinitrogen compound {[NPN]TaH}2N2
(4) (Scheme 3). From the spectroscopic data, it is quite evident
that the N2 is bound unsymmetrically. The 31P NMR spectrum
shows two 31P environments, a broad doublet and an apparent
broad quartet with peaks in a 1:2:2:1 ratio, with an average
coupling constant of 17.6 Hz, the result of near coincident
(17) Experimental details are available in Supporting Information.
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10.1021/ja982377z CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/08/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 42, 1998 11025
Scheme 3
Figure 1. Molecular structure of {[(PhNSiMe2CH2)2PPh]2TaH}2N2 (4)
with thermal ellipsoids at the 50% probability level. Silyl methyls have
been omitted and only the ipso carbon atoms of the phenyl rings attached
to phosphorus and nitrogen are shown. Selected distances (Å) and angles
(deg): N(5)-N(6), 1.319(6); Ta(1)-N(5), 2.141(4); Ta(1)-N(6), 1.975(5);
Ta(2)-N(5), 1.888(5); Ta(1)-N(1), 2.066(5); Ta(1)-N(2), 2.040(5);
Ta(2)-N(3), 2.072(5); Ta(2)-N(4), 2.086(5); Ta(1)-Ta(2), 2.8311(3);
Ta(2)-N(5)-N(6), 151.9(4); Ta(1)-N(5)-N(6), 64.6(3); Ta(1)-N(6)-
N(5), 78.3(3); N(5)-Ta(1)-N(6); Ta(1)-N(5)-Ta(2), 89.0(2).
coupling both to 31P and quadrupolar 14N. The 15N NMR
spectrum of 15N-labeled 4 also shows two resonances at -20.4
and 163.6 ppm with 1JNN ) 21.5 Hz. The two bridging hydrides
are chemically equivalent in the room temperature 1H NMR
spectrum and appear as a doublet of doublets at 10.85 ppm.
The X-ray crystal structure of 4, shown in Figure 1,22 confirmed
the unsymmetrical binding of N2, but we were unable to locate
the hydrides. The Ta2N2 core is approximately planar, and the
Ta(2)-N(5)-N(6) angle is 151.9(4)°. The η1 bound Ta(2)-N(5)
distance of 1.888(5) Å is indicative of some double bond
character, whereas the η2 Ta(1)-N(6) and Ta(1)-N(5) distances
of 1.975(5) Å and 2.141(4) Å are slightly longer. The N(5)-
N(6) bond distance of 1.319(6) Å is typical for end-on bound N2
N2 would appear to be unprecedented, there are examples of this
kind of activation for the isoelectronic moieties CO, NO+, and
CN- in dinuclear systems.33-36 Related but distinctly different
binding modes for N2 have been found in heterobimetallic
complexes of Ni with Li37 and Co with K,38 as well as an unusual
titanocene derivative.39
Also important is the manner in which this dinitrogen com-
pound was formed. Although late transition metal hydrides are
known to eliminate hydrogen and bind dinitrogen,40,41 this usually
occurs with minimal activation of the N-N bond. To our
knowledge, no isolable early transition metal hydrides have
previously been reported to bind N2 by displacing coordinated
hydride ligands. Such a transformation might be an important
method of binding N2 to a transition metal complex in a catalytic
system. The second step in the proposed catalytic cycle would
involve addition of reagents such as hydrogen or silanes to form
N-H or N-Si bonds4 and regenerate the original hydride. We
are pursuing such goals.
units7 and can be considered formally as the result of reduction
4-
of coordinated N2 to N2
.
The majority of dinuclear complexes bind dinitrogen in a
bridging end-on manner, particularly derivatives of the group 5
metals.9,23-32 The use of the less bulky tridentate NPN ligand
has allowed for the characterization of a new (µ-η2:η1) binding
mode for dinitrogen, the only example of a Ta dinitrogen complex
not containing (µ-η1:η1) bound N2. While this bonding mode for
Acknowledgment. Funding for this research was provided by NSERC
of Canada in the form of a Research grant to M.D.F. and a postgraduate
scholarship to S.A.J.
(22) Crystals of (µ-N2)[(PhNSiMe2CH2)2PPh)TaH]2‚2C6H6 (C60H74N6P2-
Si4Ta2) are orthorhombic, space group P212121, a ) 12.9290(7) Å, b )
15.11740(10) Å, c ) 32.5768(4) Å, Z ) 4, T ) -93 °C, Rigaku AFC7/
ADSC Quantum 1 CCD diffractometer. The structure was solved by the
Patterson method and was refined by full-matrix least-squares procedures to
Rw (F2) ) 0.065 for all 15996 reflections (including all available Friedel pairs)
(R (F) ) 0.037 for 12264 reflections with I g 3σ(I)). The absolute
configuration was determined by Flack parameter refinement.
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Supporting Information Available: Full experimental details for the
preparation of all new compounds; details of the single-crystal X-ray
analyses of 2 and 4 (30 pages, print/PDF). See any current masthead
page for ordering information and Web access instructions.
JA982377Z
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