New mode of coordination for the dinitrogen ligand: Formation, bonding, and reactivity of a tantalum complex with a bridging N2 unit that is both side-on and end-on

Article abstract of DOI:10.1021/ja0041371

The reaction of a mixture of 1 equiv of PhPH₂ and 2 equiv of PhNHSiMe₂CH₂Cl with 4 equiv of BuⁿLi followed by the addition of THF generates the lithiated ligand precursor [NPN]Li₂·(THF)₂ (w

Full text of DOI:10.1021/ja0041371

3960
J. Am. Chem. Soc. 2001, 123, 3960-3973
New Mode of Coordination for the Dinitrogen Ligand: Formation,
Bonding, and Reactivity of a Tantalum Complex with a Bridging N2
Unit That Is Both Side-On and End-On
Michael D. Fryzuk,*,† Samuel A. Johnson,† Brian O. Patrick,†, Alberto Albinati,‡
Sax A. Mason,§ and Thomas F. Koetzle|
Contribution from the Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall,
VancouVer, B.C., Canada V6T 1Z1, Istituto di Chimica Farmaceutica, UniVersita` di Milano,
42 Viale Abruzzi, I-20131 Milano, Italy, Institut Max Von Laue-Paul LangeVin,
38042 Grenoble Cedex 9, France, and Department of Chemistry, BrookhaVen
National Laboratory, P.O. Box 5000, Upton, New York, 11973-5000
ReceiVed December 1, 2000
Abstract: The reaction of a mixture of 1 equiv of PhPH2 and 2 equiv of PhNHSiMe2CH2Cl with 4 equiv of
BunLi followed by the addition of THF generates the lithiated ligand precursor [NPN]Li2‚(THF)2 (where [NPN]
) PhP(CH2SiMe2NPh)2). The reaction of [NPN]Li2‚(THF)2 with TaMe3Cl2 produces [NPN]TaMe3, which
reacts under H2 to yield the diamagnetic dinuclear Ta(IV) tetrahydride ([NPN]Ta)2(µ-H)4. This hydride reacts
with N2 with the loss of H2 to produce ([NPN]Ta(µ-H))2(µ-η1:η2-N2), which was characterized both in solution
and in the solid state, and contains strongly activated N2 bound in the unprecedented side-on end-on dinuclear
bonding mode. A density functional theory calculation on the model complex [(H3P)(H2N)2Ta(µ-H)]2-
(µ-η1:η2-N2) provides insight into the molecular orbital interactions involved in the side-on end-on bonding
mode of dinitrogen. The reaction of ([NPN]Ta(µ-H))2(µ-η1:η2-N2) with propene generates the end-on bound
dinitrogen complex ([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-N2), and the reaction of [NPN]Li2‚(THF)2 with NbCl3-
(DME) generates the end-on bound dinitrogen complex ([NPN]NbCl)2(µ-η1:η1-N2). These two end-on bound
dinitrogen complexes provide evidence that the bridging hydride ligands are responsible for the unusual bonding
mode of dinitrogen in ([NPN]Ta(µ-H))2(µ-η1:η2-N2). The dinitrogen moiety in the side-on end-on mode is
amenable to functionalization; the reaction of ([NPN]Ta(µ-H))2(µ-η1:η2-N2) with PhCH2Br results in C-N
bond formation to yield [NPN]Ta(µ-η1:η2-N2CH2Ph)(µ-H)2TaBr[NPN]. Nitrogen-15 NMR spectral data are
provided for all the tantalum-dinitrogen complexes and derivatives described.
Introduction
these are early transition metal complexes that require strongly
reducing conditions for their synthesis. Although a few catalytic
processes have been reported none are commercially interesting
largely because of the presence of stoichiometric quantities of
reducing agent.17 Clearly, any commercially viable process that
functionalizes dinitrogen under mild conditions will require
activation via coordination to a metal complex and this step
must be facile.
Serendipity has figured prominently in the synthesis of
dinitrogen complexes, from the first reported dinitrogen com-
plex, [Ru(NH3)5N2]2+ 1,2
to the recent reports of the cleavage
,
of dinitrogen.3-7 Out of the hundreds of dinitrogen derivatives
that have been prepared either rationally or via serendipity, only
a handful have displayed reactivities that result in functional-
ization of the coordinated dinitrogen,8-16 and the majority of
We recently reported the discovery of a facile process for
the preparation of early transition metal dinitrogen complexes.
The reaction of dihydrogen with the tantalum(V) derivative
[NPN]TaMe3 (where [NPN] ) PhP(CH2SiMe2NPh)2) generates
the dinuclear tetrahydride ([NPN]Ta)2(µ-H)4 that reacts spon-
taneously with N2 to generate ([NPN]Ta)2(µ-H)2(µ-η1:η2-N2),
a complex with the dinitrogen unit end-on bound to one tantalum
† University of British Columbia.
Professional Officer: UBC X-ray Structural Laboratory.
‡ Universita` di Milano.
§ Institut Max von Laue-Paul Langevin.
| Brookhaven National Laboratory.
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10.1021/ja0041371 CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/05/2001

New Mode of Coordination for the Dinitrogen Ligand
J. Am. Chem. Soc., Vol. 123, No. 17, 2001 3961
Scheme 1
Scheme 2
center and side-on bound to the other tantalum center.18 This
process is remarkable for several reasons: (i) the reduction of
the dinitrogen moiety occurs via reductive elimination of H2,
so that no strong reducing agent is required in the formation of
this dinitrogen complex from a tantalum(V) precursor, and (ii)
unlike many other dinitrogen complexes formed by the reductive
elimination of H2 from one or more transition metal centers,19-23
the dinitrogen moiety in this complex is activated toward further
functionalization. In this paper we discuss the full details of
the synthesis of ([NPN]Ta)2(µ-H)2(µ-η1:η2-N2), some of the
reactivity patterns of the Ta2(µ-H)2(µ-η1:η2-N2) unit, and a
description of the bonding in this unique example of an end-on
side-on bound dinitrogen complex.
Results and Discussion
precursor is shown in Scheme 2.27 The silylated aniline ClCH2-
SiMe2NHPh is prepared by the reaction of LiNHPh with ClCH2-
SiMe2Cl. The reaction of 4 equiv of BunLi with a mixture of 2
equiv of ClCH2SiMe2NHPh and 1 equiv of PhPH2 in diethyl
ether was expected to provide [NPN]Li2, after separation of the
LiCl by filtration; however, this species proved difficult to
characterize. An easily isolated lithiated ligand precursor is
prepared via the addition of approximately 4 equiv of THF to
a slurry of the presumed [NPN]Li2 crude product in hexanes.
The solid immediately dissolves, and later precipitates in 85%
yield as a colorless crystalline solid identified by 1H, 31P{1H},
and 7Li{1H} NMR spectroscopies, elemental analysis, and X-ray
crystallography (see Supporting Information) as [NPN]Li2‚
(THF)2 (1).
Our recent published work on tantalum chemistry has focused
on transformations of [P2N2]TaMe3,24,25 where [P2N2] ) PhP(CH2-
SiMe2NSiMe2CH2)2PPh,26 a macrocyclic ligand with two amido
and two phosphine donors. Many of the tantalum complexes
prepared with this [P2N2] ligand are unreactive. For example,
the dinuclear Ta(IV) tetrahydride ([P2N2]Ta)2(µ-H)4 is coordi-
natively saturated and does not bind and activate small
molecules; however, the electrons stored in the tantalum-
tantalum bond of this complex allow it to behave as a reducing
agent, as shown in Scheme 1.25
To produce more reactive systems we investigated a new
ligand system in which one of the phosphine donors was
removed to generate a tridentate donor set. It was anticipated
that enhanced reactivity would result with [NPN] systems as
compared to [P2N2] analogues simply because the former must
be more coordinatively unsaturated.
Synthesis and Characterization of [NPN]TaMe3 (2). The
synthesis of [NPN]TaMe3 was accomplished by the reaction of
1 with TaMe3Cl2, as shown in eq 1. Complex 2 is a pale yellow
light-sensitive solid that was isolated in 80% yield.
Synthesis and Characterization of [PhP(CH2SiMe2NPh)2]-
Li2‚(THF)2 (1). A convenient route to the lithiated [NPN] ligand
(18) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc. 1998,
120, 11024-11025.
(19) Sacco, A.; Rossi, M. J. Chem. Soc., Chem. Commun. 1967, 316.
(20) (a) de Wolf, J. M.; Blaauw, R.; Meetsma, A.; Teuben, J. H.; Gyepes,
R.; Varga, V.; Mach, K.; Veldma, N.; Spek, A. L. Organometallics 1996,
15, 5, 4977. (b) Chirik, P. J.; Henling, L. M.; Bercaw, J. E. Organometallics
2001, 20, 534-544.
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15, 1839.
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J. A.; Peruzzini, M. J. Am. Chem. Soc. 1997, 119, 8459.
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18, 4059-4067.
An ORTEP depiction of the solid-state molecular structure
of 2 as determined by X-ray crystallography is shown in Figure
1 and crystal data are given in Table 1. The [NPN] ligand in 2
(25) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 2000,
19, 3931-3941.
(26) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. J. Chem. Soc., Chem.
Commun. 1996, 2783.
(27) Schrock, R. R.; Seidel, S. W.; Schrodi, Y.; Davis, W. M. Organo-
metallics 1999, 18, 428-437.

3962 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Fryzuk et al.
The P(1) atom is approximately trans disposed relative to C(25)
as indicated by the P(1)-Ta(1)-C(25) angle of 160.74(14)°,
and so the structure is probably best described as a distorted
octahedron rather than a distorted trigonal prism. The Ta-P
distance of 2.7713(13) Å in 2 is longer than that observed in
the seven-coordinate complex [P2N2]TaMe3, where the two
Ta-P distances were 2.6180(8) and 2.6088(9) Å.24 In contrast,
both the Ta-N and Ta-Me distances are slightly shorter than
those found in [P2N2]TaMe3.
1
The room-temperature H NMR spectrum of 2 provides no
evidence for the lack of symmetry observed in the solid-state
molecular structure, which indicates that there is some fluxional
process that occurs in 2. A single resonance is observed for the
three tantalum-bound methyl groups, and there are two silyl
methyl environments and two ligand methylene resonances.
There is only one set of ortho, meta, and para protons for the
nitrogen phenyl groups, and one set of ortho, meta, and para
protons for the phosphorus phenyl group. At 185 K, two
resonances are observed for the tantalum-bound methyl groups
Figure 1. ORTEP depiction of the solid-state molecular structure of
[NPN]TaMe3 (2) as determined by X-ray crystallography. Selected bond
lengths (Å) and bond angles (deg): Ta(1)-P(1), 2.7713(13); Ta(1)-
N(1), 2.078(4); Ta(1)-N(2), 2.025(4); Ta(1)-C(25), 2.224(5); Ta(1)-
C(26), 2.228(5); Ta(1)-C(27), 2.204(5); N(1)-Ta(1)-N(2), 113.0(2);
P(1)-Ta(1)-N(1), 81.72(11); P(1)-Ta(1)-N(2), 70.3(1); P(1)-
Ta(1)-C(25), 160.74(14); P(1)-Ta(1)-C(26), 119.28(15); P(1)-
Ta(1)-C(27), 78.94(15); N(1)-Ta(1)-C(25), 82.6(2); N(1)-Ta(1)-
C(26), 152.4(2); N(1)-Ta(1)-C(27), 89.6(2); N(2)-Ta(1)-C(25),
106.0(2); N(2)-Ta(1)-C(26), 92.0(2); N(2)-Ta(1)-C(27), 137.7(2);
C(25)-Ta(1)-C(26), 79.3(2); C(25)-Ta(1)-C(27), 112.2(2); C(26)-
Ta(1)-C(27), 78.2(2).
1
in the H NMR spectrum, in a 2:1 ratio, consistent with either
an octahedral or trigonal prismatic geometry. Variable-temper-
ature 1H NMR also reveals that the resonances due to the ortho
and meta protons of the NPh groups broaden and decoalesce at
low temperature, which is indicative of hindered rotation of these
phenyl groups. The overlapping nature of these resonances has
prevented the accurate determination of kinetic parameters.
Synthesis and Characterization of ([NPN]Ta)2(µ-H)4 (3).
The hydrogenation of 2 was performed by adding 4 atm of H2
gas to a diethyl ether solution of 2. This reaction is complete
within 12 h, which is much faster than the several days required
for the complete hydrogenation of [P2N2]TaMe3.25 The solution
darkens from pale yellow to dark purple, suggestive of a
reduction from Ta(V) to Ta(IV). A single resonance in the room-
temperature 31P{1H} NMR spectrum of this species is consistent
with a symmetrical structure; this singlet is unchanged even at
180 K. The corresponding 1H NMR spectrum is likewise
unchanged by variations in temperature, and the observation of
only two silyl methyl resonances is also consistent with a highly
symmetrical species. The room-temperature 1H NMR contains
the expected ligand resonances, as well as resonance at δ 10.62,
which can be assigned to four bridging hydride ligands. This
resonance is a broad singlet, with no coupling to 31P resolved.
On the basis of these data, a dinuclear structure of the formula
([NPN]Ta)2(µ-H)4 (3) is proposed (eq 2).
Table 1. X-ray Diffraction Crystal Data and Structure Refinement
for [NPN]Li2(C4H8O)2 (1), [NPN]TaMe3 (2), and
([NPN]Ta(µ-H))2(µ-η1:η2-N2) (4)
compound
formula
fw
color, habit
cryst size, mm
cryst syst
space group
a, Å
2a
4a
C33H46N2P2Si2Ta
738.83
C48H64N6P2Si4Ta2
1261.26
yellow, block
0.15 × 0.35 × 0.50
orthorhombic
Pna21 (No. 33)
22.1812(3)
8.6093(3)
brown, block
0.50 × 0.20 × 0.20
monoclinic
P21/n (No. 14)
13.2532(4)
19.3894(3)
21.4134(2)
b, Å
c, Å
18.0832
R, deg
â, deg
95.9773(6)
γ, deg
V, Å3
3453.25(11)
4
5472.7(2)
4
Z
T, °C
-93
20
1.53
2504.00
Mo
41.73
0.6218-1.0000
55.8
48443
12845
0.065
F
calc, g/cm3
1.42
F(000)
1496.00
Mo
33.17
0.4463-1.000
60.0
29324
5245
0.039
radiation
µ, cm-1
transmission factors
2θmax, deg
total no. of reflns
no. of unique reflcns
Rmerge
no. with I g nθ(I)
no. of variables
R
Rw
GOF
5994 (n ) 3)
12473 (n ) 0)
To date, the solid-state molecular structure of 3 has not been
determined by X-ray crystallography. A structure with four
bridging hydrides seems probable by comparison to the related
species ([P2N2]Ta)2(µ-H)4 produced by the hydrogenation of
[P2N2]TaMe3,25 as well as from similar structures for other
dinuclear Ta(IV) hydrides;28,29 however, a structure with bridg-
ing and terminal hydrides in rapid exchange cannot be entirely
351
559
0.031a
0.024a
1.71
0.04
0.69, -0.72
0.065a
0.090a
1.01
0.01
2.71, -2.68
max ∆/σ
residual density, e/Å3
a Rigaku/ADSC CCD diffractometer, R ) ∑||Fo | - |Fc2||/∑|Fo |;
2
2
Rw ) (∑w(|Fo | - |Fc2|)2/∑w|Fo | )1/2
.
2
2 2
(28) Scioly, A. J.; Luetkens, M. L. J.; Wilson, R. B.; Huffman, J. C.;
Sattelberger, A. P. Polyhedron 1987, 6, 741-757.
(29) Profilet, R. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1992,
11, 1559-1561.
is facially bound and the methyl groups are all cis disposed.
The overall geometry at the tantalum center is very distorted.

New Mode of Coordination for the Dinitrogen Ligand
J. Am. Chem. Soc., Vol. 123, No. 17, 2001 3963
1
ruled out from the data obtained from the H NMR spectrum.
The IR spectra of 3 and the deuterated analogue formed from
the reaction of 2 with D2 (3-d4) contain no absorptions in the
region anticipated for a terminal tantalum hydride or tantalum
deuteride, respectively.30 Unfortunately, the region below 1400
cm-1, where bridging hydride or deuteride absorptions would
be expected, is obscured by ligand absorptions.
It was possible to provide additional evidence that the
structure of 3 contains 4 bridging hydrides by examining the
degree of isotopic perturbation of resonance caused by the
replacement of hydride ligands with deuteride ligands. The
reaction of 2 with a mixture of H2 and D2 gas created a series
of isotopomers: ([NPN]Ta)2(µ-H)4, ([NPN]Ta)2(µ-H)3(µ-D),
([NPN]Ta)2(µ-H)2(µ-D)2, ([NPN]Ta)2(µ-H)(µ-D)3, and ([NPN]-
Ta)2(µ-D)4. This mixture of isotopomers can also be generated
1
by the addition of D2 gas to 3. The H NMR spectrum of this
Figure 2. ORTEP depiction of the solid-state molecular structure of
([NPN]Ta(µ-H))2N2 (4) as determined by neutron crystallography. The
silyl methyl groups are omitted for clarity and only the ipso carbons
of the PPh and NPh groups and tantalum bound hydrogens are shown.
Selected bond lengths (Å), bond angles (deg), and dihedral angles
(deg): N(5)-N(6), 1.319(4); Ta(1)-N(5), 2.139(4); Ta(1)-N(6),
1.966(4); Ta(2)-N(5), 1.887(4); Ta(1)‚‚‚Ta(2), 2.830(4); Ta(1)-P(1),
2.573(5); Ta(2)-P(2), 2.596(5); Ta(1)-N(1), 2.079(4); Ta(1)-N(2),
2.031(4); Ta(2)-N(3), 2.069(4); Ta(2)-N(4), 2.049(4); Ta(1)-
HM(1), 2.020(8); Ta(1)-HM(2), 2.016(8); Ta(2)-HM(1), 1.952(7);
Ta(2)-HM(2), 1.886(8); Ta(2)-N(5)-N(6), 150.2(2); Ta(1)-N(5)-
Ta(2), 89.1(2); Ta(1)-N(6)-N(5), 78.6(2); P(1)-Ta(1)-N(1),
78.8(2); P(1)-Ta(1)-N(2), 77.5(2); P(1)-Ta(1)-N(6), 74.6(2); P(2)-
Ta(2)-N(5), 160.3(2); Ta(2)-Ta(1)-P(1), 152.8(2); Ta(2)-Ta(1)-
N(1), 122.9(2); Ta(2)-Ta(1)-N(2), 106.9(2); Ta(1)-Ta(2)-P(2),
114.7(2); Ta(1)-Ta(2)-N(3), 128.1(2); Ta(1)-Ta(2)-N(4), 123.4(2);
N(1)-Ta(1)-N(2), 108.2(2); N(1)-Ta(1)-N(6), 133.0(2); N(2)-
Ta(1)-N(6), P(2)-Ta(2)-N(3), 86.2(2); P(2)-Ta(2)-N(4), 76.3(2);
N(3)-Ta(2)-N(4), 107.2(2); N(3)-Ta(2)-N(5), 112.6(2); N(4)-Ta-
(2)-N(5), 102.6(2); HM(1)-Ta(1)-HM(2), 60.0(3); HM(1)-Ta(2)-
HM(2), 63.4(3); Ta(1)-HM(1)-Ta(2), 90.9(3); Ta(1)-HM(2)-Ta(2),
92.9(2); HM(1)-Ta(1)-Ta(2), 43.6(2); HM(2)-Ta(1)-Ta(2),
41.7(2); HM(1)-Ta(2)-Ta(1), 45.5(2); HM(2)-Ta(2)-Ta(1),
45.3(2); N(1)-Ta(1)-HM(1), 81.4(2); N(1)-Ta(1)-HM(2), 104.7(2);
P(1)-Ta(1)-HM(1), 142.4(3); P(1)-Ta(1)-HM(2), 156.7(3); N(2)-
Ta(1)-HM(1), 139.5(3); N(2)-Ta(1)-HM(2), 79.6(2); N(3)-Ta(2)-
HM(1), 156.8(3); N(3)-Ta(2)-HM(2), 96.5(3); P(2)-Ta(2)-HM(1),
80.2(2); P(2)-Ta(2)-HM(2), 83.2(3); N(4)-Ta(2)-HM(1), 87.8(2);
N(4)-Ta(2)-HM(2), 147.3(3); Ta(1)-Ta(2)-N(5)-N(6), 25.8(4);
P(1)-Ta(1)-Ta(2)-P(2), -162.6(3).
mixture reveals that the isotopic shifts of the hydride resonances
are only very slightly different for each of these isotopomers;
the difference in the chemical shift for each is between 0.02
and 0.03 ppm. These data are consistent with the hydrides all
occupying bridging positions, rather than there being two
different chemical environments for the hydride ligands (bridg-
ing and terminal), which should produce a much larger isotopic
perturbation of resonance.31,32
Synthesis and Characterization of ([NPN]Ta(µ-H))2-
(µ-η1:η2-N2) (4). Attempts to crystallize complex 3 under a
nitrogen atmosphere resulted in a visible color change. Hydride
3 is dark purple; however, after storage of solutions of 3 under
N2 gas for 1 h at 15 °C, the solution is noticeably lighter colored,
and turns brown within 24 h. The brown dinitrogen compound
([NPN]Ta(µ-H))2(µ-η1:η2-N2) can be isolated in 75% yield from
this reaction. The yield can be improved to 90% when this
reaction is performed under a 9:1 mixture of N2 and H2 gas, as
shown in eq 3. The role of H2 gas in this reaction is under further
investigation.
2 and crystal data are given in Table 2. The corresponding X-ray
diffraction data are presented in Table 1. The most intriguing
aspect of this structure is the bonding mode of the dinitrogen
moiety; it is end-on bound to one tantalum and side-on bound
to the other tantalum. In contrast, the majority of dinuclear
dinitrogen complexes bind dinitrogen in the end-on mode,
particularly derivatives of the group 5 metals.33-44 This
µ-η1:η2 bonding mode appears unprecedented in bimetallic
(33) Fryzuk, M. D.; Haddad, T. S.; Mylvaganam, M.; McConville, D.
H.; Rettig, S. J. J. Am. Chem. Soc. 1993, 115, 2782.
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7812.
(35) Churchill, M. R.; Wasserman, H. J. Inorg. Chem. 1981, 20, 2899-
2904.
An ORTEP depiction of the solid-state molecular structure
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(37) Rocklage, S. M.; Turner, H. W.; Fellmann, J. D.; Schrock, R. R.
Organometallics 1982, 1, 703-707.
of 4 as determined by neutron crystallography is shown in Figure
(30) Hlatky, G. G.; Crabtree, R. H. Coord. Chem. ReV. 1985, 65, 1-48.
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354.
(38) Rocklage, S. M.; Schrock, R. R. J. Am. Chem. Soc. 1982, 104, 3077.
(39) Schrock, R. R.; Wesolek, M.; Liu, A. H.; Wallace, K. C.; Dewan,
J. C. Inorg. Chem. 1988, 27, 2050.
(40) Dilworth, J. R.; Henderson, R. A.; Hills, A.; Hughes, D. L.;
Macdonald, C.; Stephens, A. N.; Walton, D. R. M. J. Chem. Soc., Dalton
1990, 1077.

3964 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Fryzuk et al.
Table 2. Crystallographic and Experimental Data for the Neutron
Diffraction Study of ([NPN]Ta(µ-H))2(µ-η1:η2-N2) (4)
The bridging hydride ligands are unsymmetrically bound to
the two tantalum centers, with the Ta(1)-HM(1) and Ta(1)-
HM(2) bond distances of 2.020(8) and 2.016(8) Å only slightly
longer than the Ta(2)-HM(1) and Ta(2)-HM(2) distances of
1.952(7) and 1.886(8) Å. The Ta(1)-HM(1)-Ta(2) and
Ta(1)-HM(2)-Ta(2) bond angles are 90.9(3)° and 92.9(2)°,
respectively.
compound
formula
fw
crystal size, mm
crystal system
space group
data colln, T (K)
a, Å
4
C48H64N6P2Si4Ta2
1266.00
1.8 × 1.2 × 0.9
monoclinic
P21/n
The room-temperature 1H NMR spectrum of dinitrogen
complex 4 is indicative of a species of lower symmetry than
hydride 3; however, more symmetry is observed than in the
solid-state molecular structure, which exhibits C1 symmetry. In
the room-temperature 1H NMR spectrum four silyl methyl
resonances are observed, consistent with a species of Cs
symmetry. A single hydride signal is observed at δ 10.85 that
is coupled to two different 31P environments and accounts for
two hydrides by integration. The chemical shift of the hydride
resonance (similar to that observed in 3) and the coupling of
this hydride resonance to both phosphine donors together
provide evidence that the two hydride ligands bridge the metal
centers. At 190 K, the 1H NMR spectrum is indicative of a less
symmetrical species. There are eight silyl methyl resonances,
and two bridging hydride resonances, consistent with a species
of C1 symmetry, as was observed in the solid-state molecular
structure. Presumably this fluxional process primarily involves
the rocking of the [NPN] ligands about the Ta(1)-Ta(2) axis.
This is best illustrated by noting the difference between the
N(1)-Ta(1)-Ta(2)-P(2) and N(2)-Ta(1)-Ta(2)-P(2) dihe-
dral angles, which are 23.9(3)° and -101.82(19)°, respectively;
the N(1)-Ta(1) bond nearly resides in the plane defined by
P(2)-Ta(2)-Ta(1), whereas the N(2)-Ta(2) bond is almost
perpendicular to this plane. The fluxional process that results
20(2)
13.0847(3)
19.2086(5)
21.1087(5)
94.853(2)
5286.4(2)
4
b, Å
c, Å
â, deg
V, Å3
Z
F
calc, g cm-3
1.591
wavelength λ, Å
1.5334 (1) (Cu(200)
monochromated)
2.469
109.9
10427
5918
5194
1135
absorption coeff. µ, cm-1
2θmax, deg
no. of reflcs (no)
no. of indep. reflcns
no. of obsd reflcns [I > 2.0 σ(I)]
no. of parameters refined (nv)
max ∆p/σ(p)
<0.1
0.053
Rav
R(obs)
0.059
0.070
Rw2(obs)
GOF
1.104
a Rav ) ∑|Fo2 - Fo ,av|/∑i|Fo |. R ) ∑(|Fo - (1/k)Fc|)/∑|Fo|. Rw2
)
2
2
[∑w(Fo - (1/k)Fc2)2/∑w|Fo | ], where w ) [σ2(Fo ) + (0.0394P)2 +
2
2 2
2
113.2818P]-1 and P ){[Fo + 2.0(Fc2)]/3.0}GOF ) [∑w(Fo
-
2
2
(1/k)Fc2)2/(no - nv)]1/2
.
dinitrogen complexes, although similar bonding modes have
been observed for the isoelectronic CO and NO+ and CN-
ligands.45-48 Related but distinctly different binding modes for
N2 have been found in heterobimetallic complexes of Ni with
Li,49 and Co with K,50 as well as an unusual titanocene
derivative.51
The N(5)-N(6) distance of 1.319(4) Å is consistent with a
formal assignment of the bridging dinitrogen moiety as
(N2).4-14,34,37,38,52 The Ta(1)-Ta(2) distance of 2.830(4) Å is
short enough for a metal-metal bonding interaction; however,
because the two Ta centers are Ta(V) in this formalism, no
electrons are available for a metal-metal bonding interaction.
The shortest of the tantalum-dinitrogen interactions is the “end-
on” Ta(2)-N(5) bond, at 1.887(4) Å. This distance is consistent
with the Ta(2)-N(5) bond having considerable double-bond
character. The side-on bonding interactions are both longer: the
Ta(1)-N(6) bond length is 1.966(4) Å and the Ta(1)-N(5) bond
is longer yet at 2.139(4) Å. To compare, the average of the
Ta(1)-N(1), Ta(1)-N(2), Ta(2)-N(3), and Ta(2)-N(4) dis-
tances is 2.057 Å.
1
in Cs symmetry in the room-temperature H NMR spectrum
must interchange these environments.
The 31P{1H} NMR spectrum of 4 shows two chemical
environments (δ 7.8 and 11.0) and it is unusual for two reasons.
First, the resonances are significantly broader than those in
hydride 3, and this broadness is not affected by temperature.
This broadness of these peaks is presumably due to the
quadrupolar 14N nuclei of the dinitrogen moiety; quadrupolar
nuclei are known to cause rapid nuclear spin relaxation, and
therefore line broadening.53 Most unusual about the 31P{1H}
NMR spectrum of this product is that although the peak at δ
11.0 is a doublet due to coupling to the second 31P environment,
the other peak is apparently a poorly resolved 1:2:2:1 quartet.
The presence of an apparent 1:2:2:1 quartet in the 31P{1H} NMR
spectrum is consistent with the coupling of this 31P resonance
to a single 14N environment (I ) 1, 99.6% abundance). The
coupling constant is coincidentally nearly identical with the
coupling constant between the two phosphorus atoms, and
combination of a 1:1:1 triplet from coupling to 14N with a 1:1
doublet due to coupling to 31P results in the unusual appearance
of the multiplet. The coupling of 14N to 31P is not commonly
observed.54
(41) Edema, J. J. H.; Meetsma, A.; Gambarotta, S. J. Am. Chem. Soc.
1989, 111, 6878-80.
(42) Buijink, J.-K. F.; Meetsma, A.; Teuben, J. H. Organometallics 1993,
12, 2004.
(43) Berno, P.; Gambarotta, S. Organometallics 1995, 14, 2159.
(44) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.; Re,
N.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1997, 119, 10104.
(45) Marsella, J. A.; Huffman, J. C.; Caulton, K. G.; Longato, B.; Norton,
J. R. J. Am. Chem. Soc. 1982, 104, 6360.
(46) Aspinall, H. C.; Deeming, A. J.; Donovan-Mtunzi, S. J. Chem. Soc.,
Dalton Trans. 1983, 2669.
Although 15N NMR spectroscopy has been applied to only a
minority of dinitrogen complexes,33,54,55 it should provide a
means of identifying the binding mode of dinitrogen in the
solution structure of 4. The 15N NMR spectrum of the isotopi-
cally enriched complex ([NPN]TaH)2(µ-η1:η2-15N2) (4-15N2),
(47) Deeming, A. J.; Donovan-Mtunzi, S. Organometallics 1985, 4, 693.
(48) Legzdins, P.; Rettig, S. J.; Veltheer, J. E.; Batchelor, J. J.; Einstein,
F. W. B. Organometallics 1993, 12, 3575.
(49) Jonas, K. Angew. Chem., Int. Ed. Engl. 1973, 12, 997.
(50) Klein, H. F.; Hammer, R.; Weiniger, J.; Friedrich, P.; Huttner, G.
Z. Naturforsch. 1978, 33b, 1267.
(52) Turner, H. W.; Fellmann, J. D.; Rocklage, S. M.; Schrock, R. R.;
Churchill, M. R.; Wasserman, H. J. J. Am. Chem. Soc. 1980, 102, 7809.
(53) Ebsworth, E. A. V.; Rankin, D. W. H.; Cradock, S. Structural
Methods in Inorganic Chemistry; CRC press: Boston, 1991.
(54) Mason, J. Chem. ReV. 1981, 81, 205-227.
(55) von Philipsborn, W.; Mu¨ller, R. Angew. Chem., Int. Ed. Engl. 1986,
25, 383-486.
(51) Pez, G. P.; Apgar, P.; Crissey, R. K. J. Am. Chem. Soc. 1982, 104,
482.

New Mode of Coordination for the Dinitrogen Ligand
J. Am. Chem. Soc., Vol. 123, No. 17, 2001 3965
Table 3. Selected Bond Lengths, Bond Angles, and Dihedral
Angles for the ab Initio DFT Geometry Optimization of the Model
Complex [(H3P)(H2N)2Ta(µ-H)]2(µ-η1:η2-N2) (4A)
atom
atom
distance (Å)
atom
atom
distance (Å)
N(5)
N(6)
N(5)
N(6)
N(5)
Ta(2)
1.371
2.193
1.988
1.873
2.891
Ta(1)
Ta(2)
Ta(1)
Ta(1)
Ta(2)
Ta(2)
P(1)
P(2)
N(1)
N(2)
N(3)
N(4)
2.820
2.748
2.020
2.005
2.008
2.023
Ta(1)
Ta(1)
Ta(2)
Ta(1)
Figure 3. Depiction of an isosurface of the HOMO for model complex
4A (left) and an illustration clarifying the metal-dinitrogen overlaps
in the HOMO (right).
atom atom atom angle (deg) atom atom atom angle (deg)
Ta(2) N(5) N(6)
Ta(1) N(5) Ta(2)
Ta(1) N(6) N(5)
P(1) Ta(1) N(1)
P(1) Ta(1) N(2)
P(1) Ta(1) N(6)
P(2) Ta(2) N(5)
Ta(2) Ta(1) P(1)
Ta(2) Ta(1) N(1)
Ta(2) Ta(1) N(2)
152.91
90.29
79.19
78.78
79.00
Ta(1) Ta(2) P(2)
Ta(1) Ta(2) N(3)
Ta(1) Ta(2) N(4)
N(1) Ta(1) N(2)
N(1) Ta(1) N(6)
N(2) Ta(1) N(6)
P(2) Ta(2) N(3)
P(2) Ta(2) N(4)
N(3) Ta(2) N(4)
N(3) Ta(2) N(5)
N(4) Ta(2) N(5)
106.91
126.58
123.31
109.31
123.18
110.20
92.14
prepared by the addition of 15N2 to hydride 3, contains two
chemical shifts for the dinitrogen moiety, separated by 184 ppm.
The terminal nitrogen resonance (where terminal refers to N(6)
in Figure 3) is a multiplet at δ 163.6 (relative to nitromethane
at 0 ppm) with a one-bond coupling to the adjacent 15N of the
dinitrogen moiety of 21.5 Hz, a strong coupling to one ligand
31P nucleus (JNP ) 21.2 Hz), and a weak coupling to the other
ligand 31P nucleus (JNP ) 3.5 Hz). The differences in the
magnitudes of the couplings to 31P are presumably the result of
the former being a two-bond coupling while the latter coupling
is through three bonds. The end-on nitrogen resonance (where
end-on refers to N(5) in Figure 3) occurs at δ -20.4 and is a
multiplet, with the previously mentioned coupling to the adjacent
15N of 21.5 Hz, a strong coupling to one ligand 31P nucleus
(JNP ) 24.6 Hz), and a weak coupling to the other ligand 31P
nucleus (JNP ) 6.6 Hz). The difference in magnitude of 15N-
31P couplings in this case cannot be ascribed to the distance
between the nuclei, as these are both two-bond couplings. The
larger coupling is due to the 31P nucleus that is trans disposed
to the 15N nucleus (P(2) in Figure 2). This 15N-31P coupling
constant of 24.6 Hz corresponds to a 14N-31P coupling constant
of 17.5 Hz, due the difference in magnetogyric ratio between
these two nuclei, and is consistent with the overlapping multiplet
observed in the 31P{1H} NMR spectrum of unlabeled 4.
The loss of H2 upon binding of dinitrogen is common for
the late transition metals;11,14 however, these reactions do not
result in the N2 moiety being strongly activated. With the early
transition metals H2 loss to generate a dinitrogen complex has
been observed with a titanium complex and more recently with
a zirconium derivative; however, in both of these examples the
N2 moiety is only weakly bound, and not amenable to further
reactivity.20 No example of H2 loss to give such a highly
activated dinitrogen species has been reported. Typical proce-
dures for the generation of highly activated dinitrogen complexes
of the early transition metals involve the use of harsh alkali
metal reducing agents, or utilizing transition metal precursors
in low oxidation states.11,12,14 The reaction of 3 with N2 to
produce 4 generates a strongly activated dinitrogen complex
without the requirement of strong reducing agents. Of the four
electrons involved in the reduction of dinitrogen to form 4, two
are obtained from the elimination of H2 from 3, and two are
stored in the tantalum-tantalum bond in 3, which resulted from
the reduction of the Ta(V) precursor [NPN]TaMe3 upon
hydrogenation. The reduction potentials of the electrons in the
tantalum-tantalum bond of the closely related dinuclear tet-
rahydride ([P2N2]Ta)2(µ-H)4 are known, and are only mildly
reducing (-1.18 and -0.33 V relative to the SCE).25
70.65
156.23
148.84
120.14
112.77
82.61
108.08
103.50
108.66
atom
atom
atom
atom
dihedral angle (deg)
Ta(1)
P(1)
Ta(2)
Ta(1)
N(5)
Ta(2)
N(6)
P(2)
8.29
176.18
into the bonding of the N2 moiety in the new side-on end-on
mode. To simplify the calculations the model complex
[(H3P)(H2N)2Ta(µ-H)]2(µ-η1:η2-N2) (4A) was studied with
restrictions placed on the Ta-Ta-N-H dihedral angles. These
restrictions were intended to mimic the rigid geometry imposed
by the [NPN] ligand, which prevents the amido donors from
freely rotating.
The geometry of model complex 4A was optimized (with
the exception of the previously stated constraints) with C1
symmetry by using the Gaussian 98 program59 and the hybrid
functional B3LYP method.60 The basis functions and effective
core potentials (ECP) used were those in the LANL2DZ basis
set61 but with additional d-polarization functions added to P
atoms with the exponent of the d-functions set at 0.37. This
level of theory has been used previously in the study of metal-
metal bonded species and dinuclear tantalum hydride com-
plexes.25,62
Selected optimized bond lengths, bond angles, and dihedral
angles for the model complex 4A are shown in Table 3. The
numbering scheme used is the same as that for the structure of
complex 4 shown in Figure 3. The general features of the
optimized geometry of this model complex are in good
agreement with the structure of complex 4, with the exception
(56) Yamabe, T.; Hori, K.; Minato, T.; Fukui, K. Inorg. Chem. 1980,
19, 2154-2159.
(57) Goldberg, K. I.; Hoffman, D. M.; Hoffmann, R. Inorg. Chem. 1982,
21, 3863.
(58) Pelika´n, P.; Boca, R. Coord. Chem. ReV. 1984, 55, 55.
(59) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K.
N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Petersson, V. A.; Ayala, P. Y.;
Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.;
Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Repogle, E. S.; Pople, J.
A. Gaussian 98, Revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998.
(60) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
Density Functional Theory Analysis of the Bonding in the
Side-On End-On Mode. The bonding of dinitrogen in the side-
on dinuclear mode and end-on dinuclear mode has been
described in detail in the literature.33,56-58 A density functional
theory (DFT) calculation was performed to gain similar insight
(61) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(62) Cotton, F. A.; Feng, X. J. Am. Chem. Soc. 1997, 119, 7514-7520.

3966 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Fryzuk et al.
Scheme 3
As might be expected, the bonding interactions in the side-
on end-on mode contain features of both the side-on and end-
on bonding modes of dinitrogen. Unexpectedly absent is a
δ-symmetry interaction with the side-on bound dinitrogen
moiety, which was anticipated because of the presence of this
interaction in side-on bound dinitrogen complexes.33 Instead, a
π-interaction occurs from the terminal nitrogen to the leftmost
metal center. These orbital interactions account for the range
of Ta-N bond lengths observed for complex 4. The bonding
of the dinitrogen moiety in species 4 can be adequately described
by the resonance structures A and B shown in Scheme 3.
Factors Which Favor the Side-On End-On Bonding Mode
for Dinitrogen. The absence of a δ-bonding interaction between
the side-on bound dinitrogen and the tantalum center in the
calculated molecular orbitals of the model complex 4A raises
the question as to why the side-on end-on bonding mode of
dinitrogen is preferred over the end-on dinuclear bonding mode
in 4. One possible explanation is that the bridging hydride
ligands force the two tantalum centers so close together that
the end-on dinuclear bonding mode becomes impossible.
Presumably, the energetic advantage of maintaining bridging
hydride ligands outweighs the energy difference between the
end-on dinuclear bonding mode and the side-on end-on binding
mode. Two complexes are described below that provide
evidence that bridging ligands are a requirement for the side-
on end-on bonding mode observed in complex 4.
Figure 4. Depiction of an isosurface of the HOMO-1 for model
complex 4A (left) and an illustration clarifying the metal-dinitrogen
overlaps in the HOMO-1 (right).
Figure 5. Simplified depictions of the orbital overlaps in the two
σ-bonding interactions between the dinitrogen moiety and the tantalum
centers in 4A.
of the longer Ta-P bonds in model 4A, and a slightly larger
Ta(1)-Ta(2) distance. The bond lengths and angles of the
dinitrogen moiety are well reproduced, although Ta(1), Ta(2),
N(5), N(6), P(1), and P(2) are closer to occupying the same
plane in the model complex, which almost gives this model
complex a mirror plane of symmetry.
The HOMO of model complex 4A is shown in Figure 4, and
can be described as the π-overlap between two metal-based
d-orbitals and a π-antibonding orbital of the dinitrogen moiety.
The π-overlap of the rightmost tantalum with the end-on
nitrogen (corresponding to Ta(2)-N(5) in Figure 2) is visibly
larger than the π-overlap of the leftmost tantalum center with
the terminal nitrogen (corresponding to Ta(1)-N(6) in Figure
2). Both the larger contribution of the d-orbital on the rightmost
tantalum as well as its orientation relative to the dinitrogen
moiety contribute to the greater interaction of this tantalum
center with this dinitrogen π-antibonding orbital compared to
the leftmost tantalum center.
Reaction of 4 with Propene. The reaction of 4 with propene
occurs over a period of two weeks to produce a crystalline red
product. A similar reaction with ethylene was observed;
however, several unidentified side-products were also generated,
and the initial product could not be isolated. The product of
the reaction of 4 with propene is ([NPN]Ta(CH2CH2CH3))2-
(µ-η1:η1-N2) (5), which results from the insertion of propene
into tantalum hydride bonds. This reaction is illustrated in
eq 4.
The HOMO-1 is shown in Figure 4. Similar to the HOMO,
the orbital contribution from the dinitrogen moiety is π-anti-
bonding with respect to the N-N bond; however, this π-anti-
bonding contribution is perpendicular to the one present in the
HOMO. The dinitrogen-based π-antibonding orbital interacts
with a metal-based d-orbital on the leftmost tantalum. Although
the rightmost metal center makes the largest π-bonding con-
tribution to the dinitrogen moiety in the HOMO, in the HOMO-1
there is comparatively little contribution from the rightmost
metal center; this orbital also contains small contributions from
the amido-donor lone-pairs, which are negligible in the HOMO.
The approximate metal-dinitrogen σ-bonding interactions are
depicted in Figure 5. The calculated molecular orbitals of model
complex 4A that contain σ-bonding interactions between the
metal center and the dinitrogen moiety are complicated by
contributions from other orbitals and are much lower in energy
than the HOMO and HOMO-1. A depiction of the metal-based
d-orbital and dinitrogen-based orbitals in the side-on σ-bonding
interaction is shown on the left side of Figure 5, and involves
a contribution from the dinitrogen moiety that is π-bonding in
nature. The right side of Figure 5 depicts the end-on σ-bonding
interaction, which occurs with the dinitrogen-based molecular
orbital that is approximately nonbonding in nature.
The solid-state molecular structure of complex 5 as deter-
mined by X-ray crystallography is shown in Figure 6, and crystal
data are presented in Table 4. As in solution, the solid-state
structure of 5 contains two chemically unique tantalum centers,
despite the fact that both tantalum centers are coordinated to a

New Mode of Coordination for the Dinitrogen Ligand
J. Am. Chem. Soc., Vol. 123, No. 17, 2001 3967
Table 4. X-ray Diffraction Crystal Data and Structure Refinement for ([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-N2) (5), ([NPN]NbCl)2(µ-η1:η1-N2)
(6), [NPN]Ta(µ-η1:η2-N2CH2Ph)(µ-H)2TaBr[NPN] (7)
compound
formula
fw
color, habit
cryst size, mm
cryst syst
space group
a, Å
5a
6a
7a
C54H76N6P2Si4Ta2
1345.42
red, chip
0.50 × 0.30 × 0.20
monoclinic
P21/a (No. 14)
20.2474(6)
12.6106(3)
24.1729(6)
C54H68Cl2N6Nb2P2Si4
1232.18
brown, block
0.10 × 0.25 × 0.35
triclinic
P1h (No. 2)
11.1226(7)
11.1940(11)
12.6930(14)
72.592(4)
81.6611(11)
81.7194(9)
1483.5(2)
1
C73H89N6P2BrSi4Ta2
1666.54
red, prism
0.60 × 0.40 × 0.20
monoclinic
P21/n (No. 14)
14.6365(5)
27.3586(7)
18.6731(4)
b, Å
c, Å
R, deg
â, deg
105.085(2)
95.771(2)
γ, deg
V, Å3
5959.4(3)
4
7439.5(3)
4
Z
T, °C
-100
-93
-100
F
calc, g/cm3
1.50
1.38
1.49
F(000)
2696.00
Mo
38.37
0.5942-1.0000
55.8
41275
12334
0.066
636.00
Mo
6.51
0.8455-1.000
61.0
13822
6821
3344.00
Mo
36.25
0.5919-1.000
55.8
53841
16114
0.037
radiation
µ, cm-1
transmission factors
2θmax, deg
total no. of reflns
no. of unique reflcns
Rmerge
0.041
no. with I g nθ(I)
no. of variables
R
Rw
GOF
7481 (n ) 3)
4483 (n ) 3)
12522 (n ) 3)
613
316
801
0.060a
0.072a
0.87
0.062a
0.044a
0.068a
1.11
0.071a
1.21
max ∆/σ
0.00
1.83, -1.51
0.0004
1.01, -1.06
0.05
1.85, -0.93
residual density, e/Å3
a Rigaku/ADSC CCD diffractometer, R ) ∑||Fo | - |Fc2||/∑|Fo |; Rw ) (∑w(|Fo | - |Fc2|)2/∑w|Fo | )1/2
.
2
2
2
2 2
bipyramidal, where the P(1) and N(5) atoms occupy the axial
positions, and N(2), N(1), and C(25) occupy the equatorial
positions. The stereochemistry at Ta(2) is best described as
square-based pyramidal, where the square base is defined by
P(2), N(3), C(52), and N(4), and is capped by N(6). The N(5)-
Ta(1)-P(1) angle is 175.1(3)° and, in contrast, the N(6)-
Ta(2)-P(2) angle is 109.3(1) Å. The Ta(1)-P(1) distance of
2.734(1) Å is significantly longer than the Ta(2)-P(2) distance
of 2.640(1) Å, probably due to the strong trans influence of the
dinitrogen moiety for the former.
Complex 5 provides an opportunity to compare bond distances
in the end-on dinuclear bonding mode of dinitrogen with the
side-on end-on mode for electronically similar complexes. The
N(5)-N(6) bond length of 1.289(6) Å is not significantly shorter
than the N-N bond length in 4 of 1.319(6) Å, which indicates
the side-on end-on mode activates dinitrogen to nearly the same
degree as the end-on dinuclear mode. On the other hand, the
Ta(1)-N(5) and Ta(2)-N(6) distances in 5 are 1.818(4) and
1.815(4) Å, respectively, which are significantly shorter than
the end-on Ta(2)-N(5) distance of 1.888(5) Å in 4, and much
shorter than the side-on Ta(1)-N(5) and Ta(1)-N(6) inter-
actions of 2.141(4) and 1.975(5) Å, respectively.
Figure 6. ORTEP depiction of the solid-state molecular structure of
([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-N2) (5) as determined by X-ray
crystallography. The silyl methyl groups are omitted for clarity, and
only the ipso carbons of the PPh and NPh groups are shown. Selected
bond lengths (Å), bond angles (deg), and dihedral angles (deg): N(5)-
N(6), 1.289(6); Ta(1)-N(5), 1.818(4); Ta(2)-N(6), 1.815(4); Ta(1)-
P(1), 2.734(1); Ta(2)-P(2), 2.640(1); Ta(1)-N(1), 2.057(4); Ta(1)-
N(2), 2.061(4); Ta(2)-N(3), 2.092(4); Ta(2)-N(4), 2.061(4); Ta(1)-
C(25), 2.194(5); Ta(2)-C(52), 2.193(5); Ta(1)-N(5)-N(6), 174.6(3);
N(5)-Ta(1)-P(1), 173.5(1); N(5)-Ta(1)-N(1), 98.2(2);); N(5)-
Ta(1)-N(2), 101.5(2); N(5)-Ta(1)-C(25), 98.4(2); P(1)-Ta(1)-N(1),
78.2(1); P(1)-Ta(1)-N(2), 77.3(1); P(1)-Ta(1)-C(25), 88.0(1); N(1)-
Ta(1)-N(2), 130.9(2); N(1)-Ta(1)-C(25), 111.5(2); N(2)-Ta(1)-
C(25), 109.5(2); Ta(1)-C(25)-C(26), 119.8(4); Ta(2)-N(6)-N(5),
171.8(4); N(6)-Ta(2)-P(2), 109.3(1); N(6)-Ta(2)-N(3), 108.3(2);
N(6)-Ta(2)-N(4), 107.3(2); N(6)-Ta(2)-C(52), 108.4(2); P(2)-
Ta(2)-N(3), 77.4(1); P(2)-Ta(2)-N(4), 74.6(1); P(2)-Ta(2)-C(52),
142.3(1); N(3)-Ta(2)-N(4), 140.2(2); N(3)-Ta(2)-C(52), 91.7(2);
N(4)-Ta(2)-C(52), 93.5(2); Ta(2)-C(52)-C(53), 108.4(4); C(52)-
Ta(2)-Ta(1)-C(25), -119.5(4).
The 31P{1H} NMR spectrum of 5 contains two resonances,
at δ 14.2 and δ 19.7, that, in contrast to complex 4, are not
coupled. To confirm the bonding mode of dinitrogen, the 15N2
labeled species ([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-15N2) was
prepared from the reaction of 4-15N2 with propene. In the 31P-
{1H} NMR spectrum of 5-15N2, the resonance at δ 19.7 is a
singlet, whereas the resonance at δ 14.2 is coupled to two
different 15N2 environments, with coupling constants of 30.5
and 6.6 Hz. The 15N NMR spectrum of 5-15N2 contains two
doublets of doublets at δ 16.2 and 28.0. The resonance at δ
[NPN] ligand, a propyl ligand, and an end-on bound dinitrogen
unit. The stereochemistry at Ta(1) is best described as trigonal
2
16.2 is coupled to a single 31P environment, with a JPN value

3968 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Fryzuk et al.
result of the binding mode of dinitrogen in 4. More importantly,
this compound demonstrates that the bridging hydrides in
complex 4 are necessary for maintaining the side-on end-on
dinitrogen bonding mode.
Synthesis of ([NPN]NbCl)2(µ-η1:η1-N2) (6). In parallel
studies, we are examining [NPN] complexes of niobium. The
product of the reaction of [NPN]Li2‚(THF)2 with NbCl3(DME)
is the diamagnetic dinitrogen complex ([NPN]Nb)2(µ-η1:η1-N2)
(6), as shown in eq 5.
The 1H, 13C{1H}, and 31P{1H} NMR spectra of 6 are
consistent with a species of high symmetry. A single resonance
is observed in the 31P{1H}NMR spectrum, and only two ligand
silyl methyl environments are observed in the 1H NMR
spectrum, which is unlike both species 4 and species 5. The C,
H, and N elemental analyses are consistent with a dinitrogen
complex.
Figure 7. ORTEP depiction of the solid-state molecular structure of
([NPN]NbCl)2(µ-η1:η1-N2) (6) as determined by X-ray crystallography.
Selected bond lengths (Å) and bond angles (deg): N(3)-N(3)*,
1.237(4); N(1)-N(3), 1.843(2); Nb(1)-P(1), 2.7189(8); Nb(1)-N(1),
2.023(2); Nb(1)-N(2), 2.053(2); Nb(1)-Cl(1), 2.3922(9); Nb(1)-
N(3)-N(3)*, 176.4(2); P(1)-Nb(1)-N(3), 171.57(8); Cl(1)-Nb(1)-
N(3), 99.03(8); N(1)-Nb(1)-N(3), 95.70(9); N(2)-Nb(1)-N(3),
99.44(9); P(1)-Nb(1)-N(1), 76.16(7); P(1)-Nb(1)-N(2) 81.83(7);
Cl(1)-Nb(1)-P(1), 87.07(3); Cl(1)-Nb(1)-N(1), 119.31(7); Cl(1)-
Nb(1)-N(2) 123.18(7); N(1)-Nb(1)-N(2), 111.59(10).
An ORTEP depiction of the solid-state molecular structure
of 6 as determined by X-ray crystallography is shown in Figure
7, and crystal data are presented in Table 4. In the solid-state
structure, the two halves of the molecule are related by an
inversion center that lies in the middle of the N-N bond. Unlike
complex 5, which is unsymmetrical, the geometry at both Nb
centers in 6 is best described as trigonal bipyramidal, with the
P(1) phosphine donor and the N(3) atom of the dinitrogen
moiety in axial positions and the N(1), N(2), and Cl(1) atoms
in equatorial positions. The N(3)-N(3)* bond distance of
1.237(4) Å is slightly shorter than that observed for the
dinitrogen moiety in 5 (1.289(6) Å). Although chloride ligands
can behave as bridging ligands, they are terminal in 6; bridging
chlorides would require the bonding mode of dinitrogen to
change in 6. The difference in dinitrogen bonding mode between
4 and 6 indicates that not any ligand capable of bridging the
metal centers will favor the side-on end-on mode; the side-on
end-on bonding mode is presumably less thermodynamically
favorable than the end-on dinuclear bonding mode of dinitrogen.
Clearly the possible thermodynamic advantage of bridging
versus terminal chloride ligands is not sufficient to overcome
the thermodynamic disadvantage of the side-on end-on binding
mode versus the end-on dinuclear binding mode of dinitrogen,
and therefore the end-on dinuclear mode is observed in complex
6. This example confirms the necessity of a more thermo-
dynamically favored bridging ligand to induce the side-on end-
on mode. The hydride ligands perform this role in complex 4.
Reaction of 4 with Benzylbromide. Does the side-on end-
on bonding mode of dinitrogen in complex 4 open up new
possibilities for dinitrogen-based reactivity? A simplified picture
of the bonding of dinitrogen in this mode has the terminal
nitrogen bearing a nonbonding pair of electrons (resonance
structure B in Scheme 3) and this nitrogen should be expected
to react with electrophiles. The availability of nonbonding
electrons on the dinitrogen moiety in the side-on end-on mode
is a feature shared with the mononuclear end-on and dinuclear
1
of 30.5 Hz, as well as to 15N with a JNN value 11.3 Hz; the
resonance at δ 28.0 also exhibits the identical one-bond coupling
to 15N (1JNN ) 11.3 Hz), as well as a two-bond coupling to 31
P
(2JPP ) 6.6 Hz). The JNN value of 11.3 Hz is much smaller
than the analogous 21.5 Hz coupling observed for 4. As noted
previously, the N(5)-Ta(1)-P(1) angle is 175.1(3)°, and it is
presumably this phosphorus, approximately trans disposed to
the dinitrogen moiety, that couples to both the N(5) and N(6)
1
1
nuclei in the 15N and 31P{1H} spectra of 5-15N2. The H and
13C{1H} NMR spectra are consistent with two ligand and two
propyl environments and were assigned with the assistance of
1
1
a H-COSY spectrum and a H,13C-HSQC spectrum.
The solution structure of 5 contains only a mirror plane of
symmetry, which is consistent with the different geometries
observed at the two tantalum centers in the solid-state structure
of 5. This lower symmetry can be explained if the orientation
of the [NPN] ligands with respect to the bridging hydrides in 4
is maintained when the propene inserts into the Ta-H bonds
to produce 5. In Figure 6, P(1) is cis disposed to the propyl
group, and trans disposed relative to the dinitrogen moiety, so
this corresponds to P(2) of complex 4 in Figure 2, which is cis
disposed to the bridging hydride ligands and trans disposed to
the dinitrogen moiety. Similarly, in Figure 7 P(2) is trans
disposed to the propyl group, and cis disposed relative to the
dinitrogen moiety, and this corresponds to P(1) in Figure 2. The
stereoselectivity of this reaction generates an unsymmetrical
dinuclear end-on dinitrogen compound that might not be
accessible by other routes; the lack of symmetry in 5 is a direct

New Mode of Coordination for the Dinitrogen Ligand
J. Am. Chem. Soc., Vol. 123, No. 17, 2001 3969
NMR spectrum is indicative of a species with no symmetry,
because eight ligand silyl methyl resonances are observed. The
two methylene protons of the benzyl fragment appear at δ 5.03
and 5.45 and are coupled to each other with a 13.4 Hz coupling
constant. This species can tentatively be assigned as [NPN]Ta-
(µ-η1:η2-N2CH2Ph)(µ-H)2TaBr[NPN] (7), where a carbon-
nitrogen bond has been formed between the terminal nitrogen
and the benzyl methylene carbon, and the bromide is transferred
to the metal center. This reaction product is shown in eq 6.
Figure 8. Comparison of the end-on, side-on, and side-on end-on
binuclear bonding modes of dinitrogen. Unlike the most prevalent end-
on bonding mode, both the side-on and side-on end-on bound dinitrogen
moiety bear electron pairs that are nonbonding with respect to the metal
centers.
side-on bonding modes of dinitrogen, in contrast to the dinuclear
end-on bonding mode. The presence of dinitrogen-based non-
bonding electrons should render reactions that form bonds to
the dinitrogen moiety more thermodynamically favorable for
dinuclear side-on end-on complexes than in dinuclear end-on
complexes, where no dinitrogen-based nonbonding electrons are
available, and therefore metal-dinitrogen bonding interactions
must be broken to form new bonds to the activated dinitrogen
moiety. Indeed, many more transformations have been reported
for mononuclear end-on bound dinitrogen complexes11,12,14,63
than dinuclear end-on bound dinitrogen complexes38 and recent
reports confirm the high reactivity of a dinuclear side-on bound
N2 complex.64 Illustrations of the bonding in the common
dinuclear end-on and dinuclear side-on bonding modes are
shown in Figure 8, and are compared with the side-on end-on
bonding mode with respect to the availability of electron pairs
based on the dinitrogen moiety that cannot form bonding
interactions to the metal centers.
The synthesis of the 15N2 labeled species [NPN]Ta(µ-η1:η2-
15N2CH2Ph)(µ-H)2TaBr[NPN] (7-15N2) was performed to con-
firm this assignment. In the 1H NMR spectrum, one of the two
1
diastereotopic benzyl methylene H resonances is coupled to
both 15N nuclei, with coupling constants of 5.0 and 3.2 Hz. The
13C{1H} NMR spectrum of 7-15N2 provides further evidence that
carbon-nitrogen bond formation has occurred. The methylene
carbon of the benzyl group appears as a doublet of doublets at
δ 69.6, with coupling to both 15N nuclei coincidentally identical
at 12 Hz. The 31P{1H} spectrum of 7-15N2 consists of two
coupled resonances at δ -0.2 and 12.1; this latter resonance is
The reaction of mononuclear dinitrogen compounds with
alkylhalides is known, and frequently occurs via a photochemical
mechanism.11,65,66 The benzylation of a dinitrogen complex with
benzylbromide appears to be less common; this reaction has
only been reported on one occasion,67 although attempts to
perform this reaction with other dinitrogen complexes have been
noted.11,68-70 The reaction of 4 with benzylbromide in toluene
occurs over about 8 h at room temperature or within minutes
by heating the solution to 70 °C. The extent of the reaction can
be monitored visually; initially, the solution is dark brown,
which gradually lightens to generate a pale red mixture. The
31P{1H} and 1H NMR spectra are indicative of a single reaction
product. The 31P{1H} NMR spectrum contains two resonances
2
also coupled to 15N with a JNP value of 27.1 Hz, typical of a
31P nucleus trans disposed to the end-on dinitrogen as in the
side-on end-on dinitrogen complexes previously described. The
15N{1H} spectrum of 7-15N2 contains a resonance for the end-
on nitrogen that is a doublet of doublets at δ -45.3, with a
27.1 Hz coupling to the trans disposed 31P nucleus, and a 17.5
Hz coupling to the adjacent 15N nucleus. The second resonance
in the 15N{1H} NMR spectrum is a doublet at δ -112.4, with
a 17.5 Hz coupling to the adjacent 15N nucleus. The JNN coupling
constant is much larger for 7-15N2 than was observed in 4-15N2.
The chemical shift of the end-on resonance is very close to that
observed in 4-15N2; however, the chemical shift of the previously
terminal nitrogen is very different than that in 4-15N2, and may
be characteristic of C-N bond formation.
1
at δ -0.2 and 12.1 that are coupled (JPP ) 22.9 Hz). The H
(63) Hidai, M. Coord. Chem. ReV. 1999, 185-186, 99-108.
(64) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science
1997, 275, 1445.
(65) Chatt, J.; Diamantis, A. A.; Heath, G. A.; Hooper, N. E.; Leigh, G.
J. J. Chem. Soc., Dalton Trans. 1977, 688.
(66) Bossard, G. E.; Busby, D. C.; Chang, M.; George, T. A.; Iske, S.
D. A. J. J. Am. Chem. Soc. 1980, 102, 1001.
(67) Yoshida, T.; Adachi, T.; Ueda, T.; Kaminaka, M.; Sasaki, N.;
Higuchi, T.; Aoshima, T.; Mega, I.; Mizobe, Y.; Hidai, M. Angew. Chem.,
Int. Ed. Engl. 1989, 101, 1053-5.
(68) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. ReV. 1978, 78,
589.
(69) Chatt, J.; Head, R. A.; Leigh, G. J.; Pickett, C. J. J. Chem. Soc.,
Chem. Commun. 1977, 299.
(70) The reaction of benzylbromide with Mo and W dinitrogen complexes
bearing phosphine ligands results in the formation of bibenzyl, due to the
radical pathway by which these complexes react with alkyl halides.
An ORTEP depiction of the solid-state molecular structure
of 7 as determined by X-ray crystallography is shown in Figure
9, and crystal data are presented in Table 4. The X-ray structure
of complex 7 demonstrates that the dinitrogen moiety maintains
the side-on end-on bonding mode with the two tantalum centers,
and also that the terminal nitrogen has formed a bond to carbon.
The bromide ligand is attached to the tantalum that is side-on
bound to the dinitrogen moiety. The bonding of the dinitrogen
moiety to Ta(2) appears largely unchanged relative to the parent
complex 4; the Ta(2)-N(5) bond length of 1.865(3) Å is very
similar to that in complex 4 (1.888(5) Å). On the other hand,
the Ta(1)-N(5) distance of 2.204(3) Å and the Ta(1)-N(6)
distance of 2.102(3) Å are both longer than the analogous

3970 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Fryzuk et al.
of reaction is remarkable because it results in the formation of
a reactive strongly activated dinitrogen complex of an early
transition element without the use of strong reducing agents.71
To access the side-on end-on bonding mode requires the
availability of donors that can act as efficient bridging ligands;
in this system, bridging hydrides are necessary to enforce this
unusual ligation. Although chloride ligands are known to bridge,
in the closely related niobium system they remain terminal and
the ubiquitous end-on bridging N2 bonding mode is observed
in the diniobium derivative ([NPN]NbCl)2(µ-η1:η1-N2). Further
substantiation of the crucial role that bridging ligands play in
the observation of this side-on end-on mode is the fact that the
reaction of ([NPN]Ta(µ-H))2(µ-η1:η2-N2) with propene, which
occurs by migratory insertion into the bridging hydride bonds
to produce terminal propyl groups, is accompanied by an
isomerization to generate the end-on dinitrogen derivative
([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-N2).
Figure 9. ORTEP depiction of the solid-state molecular structure of
[NPN]Ta(µ-η1:η2-N2CH2Ph)(µ-H)2TaBr[NPN] (7) as determined by
X-ray crystallography. The silyl methyl groups are omitted for clarity
and only the ipso carbons of the PPh and NPh groups are shown.
Selected bond lengths (Å), bond angles (deg), and dihedral angles
(deg): N(5)-N(6), 1.353(4); Ta(1)-N(5), 2.204(3); Ta(1)-N(6),
2.102(3); Ta(2)-N(5), 1.865(3); Ta(1)-Br, 2.7185(3); N(6)-C(49),
1.466(4); Ta(1)-Ta(2), 2.8449(2); Ta(1)-P(1), 2.6914(8); Ta(1)-P(2),
2.6358(9); Ta(1)-N(1), 2.061(3); Ta(1)-N(2), 2.079(2); Ta(2)-N(3),
2.048(2); Ta(2)-N(4), 2.042(3); Ta(1)-H(88), 1.92(3); Ta(1)-H(89),
1.84(4); Ta(2)-H(88), 1.78(3); Ta(2)-H(89), 1.81(4); Ta(2)-N(5)-
N(6), 155.6(2); N(5)-N(6)-C(49), 115.2(3); Ta(1)-N(6)-C(49),
134.3(2); Ta(1)-N(6)-N(5), 75.9(2); P(1)-Ta(1)-N(1), 81.75(8);
P(1)-Ta(1)-N(2), 73.02(7); P(1)-Ta(1)-Br, 87.64(2); P(1)-Ta(1)-
N(6), 78.44(7); P(1)-Ta(1)-N(5), 114.95(7); N(1)-Ta(1)-N(2), 95.3-
(1); N(1)-Ta(1)-Br, 85.56(7); N(1)-Ta(1)-N(5), 161.1(1); N(1)-
Ta(1)-N(6), 158.2(1); N(2)-Ta(1)-Br, 160.29(7); P(1)-Ta(1)-H(88),
150.0(9); P(1)-Ta(1)-H(89), 158(1); P(2)-Ta(2)-N(5), 166.80(8);
P(2)-Ta(2)-N(3), 84.21(8); P(2)-Ta(2)-N(4), 77.36(9); N(3)-
Ta(2)-N(4), 108.8(1); N(3)-Ta(2)-N(5), 104.3(1); N(4)-Ta(2)-N(5),
108.6(1); N(3)-Ta(2)-H(89), 154(1); N(4)-Ta(2)-H(89), 92(1);
N(3)-Ta(2)-H(88), 102(1); Ta(2)-N(5)-N(6)-C(49), 142.2(4).
Although reactivity studies of the side-on end-on dinitrogen
complex 4 are ongoing, we note that the reaction with benzyl
bromide results in the formation of a C-N bond, presumably
as a result of the nucleophilic nature of the dinitrogen fragment.
The bridging benzylhydrazido fragment remains side-on end-
on bound and shows only minor changes in structure from the
parent dinitrogen unit. This suggests that the side-on end-on
mode of bonding is somewhat robust to those reactions directed
at the N2 moiety, despite precedent for a variety of other binding
modes for diazenido ligands (RNN-) in related dinuclear
systems.72
Finally, it is useful to speculate on the broader implications
of the formation of dinitrogen complex 4. From the point of
view of ultimately generating a catalytic system for the
functionalization of dinitrogen, we suggest that this work
provides a first step toward this goal because the coordination
and activation of the substrate N2 have been demonstrated to
occur in a facile manner. It is also notable that there is a
connection to the biological fixation of dinitrogen. It is known
that the nitrogenase enzyme produces at least 1 equiv of H2
gas in the conversion of N2 to NH3. Although there is some
debate as to whether this arises from a necessary step in this
reaction or from a side-reaction of this highly reducing enzyme
with H+,11,73,74 there is a similarity to the formation of ([NPN]-
Ta(µ-H))2(µ-η1:η2-N2) because H2 is also eliminated for the latter
reaction. Does the iron-sulfide-molybdenum core of the
nitrogenase enzyme lose H2 from a hydride intermediate before
N2 binds and is finally reduced? Is so, then our work provides
a precedent for this process. Furthermore, it has also been
suggested that the electrons that reduce dinitrogen in the iron-
sulfide core of nitrogenase are initially stored in metal-metal
bonds,75,76 much like the tantalum-tantalum bonding electrons
in hydride 3 that are transferred to dinitrogen to form complex
4. These ideas must await further details on the biological
system.
distances in complex 4 (2.141(4) and 1.975(5) Å, respectively).
The most dramatic change is the Ta(1)-N(6) distance; this
tantalum-nitrogen bond length is consistent with a single bond,
compared to the partial double bond character observed in
complex 4. Other features of the solid-state molecular structure
of 7 are consistent with the assignment of the Ta(1)-N(6) bond
as a single bond. In particular, the bond angles around N(6) are
consistent with the presence of a stereochemically active lone
pair. The sum of the three bond angles at N(6) is 325.4(7)°,
which is much smaller than the 360° anticipated for a planar
nitrogen, and quite close to that predicted for tetrahedral bond
angles of 109.5°, where the sum of the bond angles would be
328.5°.
The results of this preliminary study indicate that the polarized
dinitrogen moiety in complex 4 is susceptible to electrophilic
attack at the terminal end of the dinitrogen moiety. Further
studies are underway to extend the reactivity of complex 4.
(71) It should be noted that the precursor TaMe3Cl2 is prepared from
TaCl5 with ZnMe2. This is a metathesis reaction, which results in the
formation of ZnCl2. A reducing agent is not necessary to convert Ta(V) to
Ta(III) because the 4 electrons required to reduce dinitrogen in complex 4
are obtained by two consecutive reductiVe eliminations of H2. Reductive
elimination reactions allow for metal centers to be reduced in the absence
of an added reducing agent.
(72) Schollhammer, P.; Guenin, E.; Petillon, F. Y.; Talarmin, J.; Muir,
K. W.; Yufit, D. S. Organometallics 1998, 17, 1922.
(73) Leigh, G. J. Sci. Prog. (Oxford) 1989.
(74) Sellmann, D.; Fursattel, A. Angew. Chem., Int. Ed. Engl. 1999, 38,
2023-2026.
(75) Han, J.; Beck, K.; Ockwig, N.; Coucouvanis, D. J. Am. Chem. Soc.
1999, 121, 10448.
Conclusions
In this work we document the facile synthesis of a new
dinuclear dinitrogen complex with the unprecedented side-on
end-on mode of bonding for the N2 unit. The ditantalum
derivative ([NPN]Ta(µ-H))2(µ-η1:η2-N2) results from the direct
reaction of N2 with the dinuclear tetrahydride complex ([NPN]-
Ta)2(µ-H)4, which in turn was prepared by the reaction of H2
with the Ta(V) starting derivative [NPN]TaMe3. This sequence
(76) Tyson, M. A.; Coucouvanis, D. Inorg. Chem. 1997, 37, 33808.

New Mode of Coordination for the Dinitrogen Ligand
J. Am. Chem. Soc., Vol. 123, No. 17, 2001 3971
[NPN]Li2‚(C4H8O)2 (7.02 g, 11.8 mmol) dissolved in 800 mL of Et2O.
When the solution was warmed a white precipitate appeared. The
solution was evaporated to dryness. The remaining solid was extracted
into 80 mL of toluene and filtered through Celite. The solvent was
removed and the resulting solid was rinsed with 10 mL of hexanes,
which removed a dark brown impurity. The remaining pale yellow solid
was dried under vacuum to yield [NPN]TaMe3 (6.33 g, 80%). X-ray
quality crystals were obtained by the slow evaporation of a benzene/
hexamethyldisiloxane solution and contained 1 equiv of cocrystallized
benzene; the crystalline solid rapidly desolvated in the absence of
benzene. 1H NMR (C6D6, 30 °C, 500 MHz): δ 0.05, 0.25 (s, 12H total,
SiCH3), 1.19 (d, 3JPH ) 3.4 Hz, 9H, Ta-CH3), 1.23, 1.18 (ABX, 2JHH
) 14.3 Hz, 4H total, SiCH2P), 6.91 (m, 2H, NPh p-H), 7.10 (m,4H,
NPh o-H), 7.12 (m, 5H, PPh p-H and NPh m-H), 7.18 (m, 2H, PPh
m-H), 7.58 (m, 2H, PPh o-H). 31P{1H} NMR (C6D6, 30 °C, 500
MHz): δ (ppm) 13.5 (s). 13C{1H} NMR (C6D6, 30 °C, 500 MHz): δ
1.3 and 3.5 (d, JPC ) 4.3 and 3.3 Hz, Si(CH3)2), 13.9 (d, JPC ) 6.7
Hz, SiCH2P), 69.5 (d, 2JPC ) 9.5 Hz, TaCH3), 124.3, 128.3, and 128.9
(s, NPh o-C, m-C, and p-C), 128.8 (d, 3JPC ) 8.1 Hz, PPh m-C), 130.0
(d, 4JPC ) 1.9 Hz, PPh p-C), 131.6 (d, 2JPC ) 10.5 Hz, PPh o-C), 136.6
(d, 1JPC ) 17.6 Hz, PPh i-C), 149.1 (d, 4JPC ) 10.5 Hz, NPh i-C). Anal.
Calcd for C27H40N2PSi2Ta: C, 49.08; H, 6.10; N, 4.24. Found: C, 49.26;
H, 6.13; N, 4.22
Experimental Section
General Procedures. 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
-40 °C freezer). Hexanes were predried by refluxing over CaH2 and
then distilled under argon from sodium benzophenone ketyl with
tetraglyme added to solubilize the ketyl. Anhydrous diethyl ether was
stored over sieves and distilled from sodium benzophenone ketyl under
argon. Toluene was predried by refluxing over CaH2 and then distilled
from sodium under argon. Nitrogen was dried and deoxygenated by
passage through a column containing activated molecular sieves and
MnO. Deuterated benzene and toluene were dried by refluxing with
molten potassium metal and molten sodium metal, respectively, in a
sealed vessel under partial pressure, then trap-to-trap distilled and
freeze-pump-thaw-degassed three times. Unless otherwise stated,
1H, 31P, 1H{31P}, 13C{1H}, 13C, 15N, 15N{1H}, and variable-temperature
NMR spectra were recorded on a Bruker AMX-500 instrument
operating at 500.1 MHz for 1H spectra. 1H NMR spectra were referenced
to internal C6D5H (7.15 ppm) and C7D7H (2.09 ppm), 31P{1H} NMR
spectra to external P(OMe)3 (141.0 ppm with respect to 85% H3PO4 at
0.0 ppm), 13C NMR spectra to 13CC5D6 (128.4 ppm), and 15N and 15N-
{1H} spectra to external 15NH4Cl in D2O (-352.9 ppm with respect to
nitromethane at 0.0 ppm). UV-visible spectra were obtained using a
Hewlett-Packard 8453 UV-visible spectrophotometer and quartz
cuvettes with Teflon valves. Elemental analyses were performed by
Mr. P. Borda of this department.
3
1
([NPN]Ta)2(µ-H)4 (3). A solution of [NPN]TaMe3 (0.75 g, 1.13
mmol) dissolved in 30 mL of Et2O was transferred into a 200 mL thick
wall glass vessel equipped with a Teflon valve and thoroughly degassed
via 3 freeze-pump-thaw cycles. The vessel was cooled in liquid
nitrogen and H2 gas was added. The vessel was then sealed, warmed
to room temperature, and stirred for 10 h, over which time the solution
turned dark purple. The solvent was removed in vacuo, and the
remaining solid dried, to afford ([NPN]2Ta)2(µ-H)4 as a nitrogen-
Starting Materials and Reagents. The reagents PhNH2, ClSiMe2-
CH2Cl, and PhCH2Br were purchased from Aldrich and purified by
distillation. Solutions of BunLi (1.6 M in hexanes) were obtained from
Acros Organics and used as received. Hydrogen gas, 10% H2/N2 gas
mixture, and propene gas were purchased from Praxair. The compounds
TaMe3Cl2,77 PhPH2,78 and NbCl3(DME)79 were prepared by literature
methods.
sensitive purple solid in quantitative yield. 1H NMR (C6D6, 30 °C, 500
2
MHz): δ 0.15, 0.12 (s, 24H total, SiCH3), 0.99, 1.05 (ABX, JHH
)
13.6, 8H, SiCH2P), 6.90 (m,4H, NPh p-H), 7.21 (m, 6H, PPh m-H and
p-H), 7.26 (m, 8H, NPh m-H), 7.33 (m,8H,NPh o-H), 7.38 (m, 4H,
PPh o-H), 10.62 (s, 4H, TaH). 31P{1H} NMR (C6D6, 20 °C): δ 21.0
(s). Anal. Calcd for C48H66N4P2Si4Ta2: C, 46.67; H, 5.39; N, 4.54.
Found: C, 46.23, H, 5.47; N, 4.61.
([NPN]Ta)2(µ-D)4 (3-d4). A sample of ([NPN]2Ta)2(µ-D)4 was
prepared in a manner identical to that for ([NPN]2Ta)2(µ-H)4 with D2
gas.
[NPN]Li2‚(C4H8O)2 (1). To an ice-cooled stirred solution of aniline
(14.9 g, 0.16 mol) in 50 mL of Et2O and 30 mL of THF was slowly
added BunLi (100 mL, 1.6 M in hexanes). The resulting colorless
solution was stirred for 1 h at room temperature, then cannula transferred
dropwise into a stirred solution of ClSiMe2CH2Cl in 50 mL of Et2O at
-78 °C. The solution was warmed to room temperature then evaporated
to dryness. The residue was dissolved in 100 mL of hexanes and filtered
through Celite. The solvent was removed and the remaining oil was
distilled under vacuum at 72-74 °C, to afford PhNHSiMe2CH2Cl in
Reaction of ([NPN]Ta)2(µ-H)4 with D2. A NMR tube containing
([NPN]Ta)2(µ-H)4 in C6D6 was frozen in liquid N2 and sealed under
an atmosphere of D2 gas. The solvent was thawed and then the sealed
1
90% yield. H NMR (C6D6, 20 °C, 200 MHz): δ 0.10 (s, 6H, Si-
1
NMR tube was transferred into the NMR probe. H NMR, hydride
CH3), 2.57 (s, 2H, CH2), 3.03 (b, 1H, NH), 6.46 (m, 2H, o-H), 6.75
(m, 1H, p-H), 7.08 (m, 2H, m-H).
region (C6D6, 300 K, 500 MHz): δ 10.61 (s, Ta(µ-D)3(µ-H)), 10.59
(s, Ta(µ-D)2(µ-H)2), 10.56 (s, Ta(µ-D)1(µ-H)3), 10.53 (s, Ta(µ-H)4).
([NPN]Ta(µ-H))2(µ-η1:η2-N2) (4). A solution of ([NPN]Ta)2(µ-H)4
(0.70 g, 0.56 mmol) in 10 mL of Et2O was exposed to a mixture of
90% N2 gas with 10% H2 gas for 24 h, with rapid stirring, resulting in
a color change from purple to brown. The solvent was removed and
the remaining solid was rinsed with minimal hexanes and dried, to
afford ([NPN]2TaH)2N2 in 90% yield. 1H NMR (C6D6, 30 °C, 500
MHz): δ -0.21, -0.05, 0.02, 0.1 (s, 24H total, SiCH3), 0.65 (AMX,
2JHH ) 13.5, 2JHP ) 10.6, 2H, SiCH2P), 1.48 (AMX, 2JHH ) 13.5, 2JHP
) 12.2, 2H, SiCH2P), 1.59, 1.59 (m, 4H total, SiCH2P), 6.74 (m, 2H,
PhH), 6.83 (b, 2H, PPh o-H), 6.89 (m, 2H, PhH), 6.91 (d, 4H, NPh
o-H), 7.10 to 7.27 (m, 18H, PhH), 8.24 (m, 2H, PPh o-H), 10.85 (dd,
2JHP ) 20.3, 14.3, 2H, TaH). 31P{1H} NMR (C6D6, 20 °C): δ 11.0 (d,
3JPP ) 21.2 Hz, 1P), 7.8 (m, 1P). Anal. Calcd for C48H64N6P2Si4Ta2:
C, 45.71; H, 5.11; N, 6.66. Found: C, 46.05; H, 5.26; N, 6.69.
([NPN]Ta(µ-H))215N2 (4-15N2). A solution of ([NPN]Ta)2(µ-H)4 (0.30
g, 0.56 mmol) in 5 mL of Et2O was exposed to 15N2 gas, in a manner
similar to that used with ([NPN]Ta(µ-H))2N2, and afforded ([NPN]-
TaH)215N2 in 80% yield. No change was observed in the 1H NMR
To an ice-cooled stirred mixture of PhNHSiMe2CH2Cl (15.6 g, 0.078
mol) and PhPH2 (4.3 g, 0.5 equiv) in 150 mL of Et2O was added
dropwise 100 mL of BunLi (1.6 M in hexanes, 2 equiv). The solution
was stirred for 2 h and then thoroughly dried in vacuo. The remaining
solids were extracted into 70 mL of toluene and filtered through Celite.
The solvent was removed and 20 mL of hexanes was added, to
precipitate a white solid from the yellow solution. The addition of an
excess of THF (∼4 equiv) initially dissolved the solid, to produce a
clear solution. The solution was cooled and over the course of an hour
[NPN]Li2‚(C4H8O)2 precipitated as a colorless crystalline solid in 85%
1
yield. H NMR (C6D6, 25 °C, 500 MHz): δ 0.43 and 0.46 (s, 12H
total, SiCH3), 1.09 (m, 8H, THF-OCH2CH2), 1.24 (m, 4H, PCH2Si),
3.31 (m, 8H, THF-OCH2CH2), 6.71 (m, 2H, NPh p-H), 6.92 (m, 4H,
NPh o-H), 7.13 (m, 1H, PPh p-H), 7.23 (m, 6H, NPh m-H and PPh
m-H ), 7.65 (m,2H, PPh o-H). 31P{1H} NMR (C6D6, 25 °C, 500
MHz): δ -37.7 (quartet, JPLi ) 37.7 Hz). Li NMR (C6D6, 25 °C,
500 MHz): δ -1.2 (s, 1Li), -0.9 (d, JLiP ) 37.7 Hz, 1Li). Anal.
Calcd for C32H47Li2N2O2PSi2: C, 64.84; H, 7.99; N, 4.73. Found: C,
65.05; H, 8.02; N, 4.66.
[NPN]TaMe3 (2). A solution of TaMe3Cl2 (3.51 g, 11.8 mmol) in
50 mL of Et2O was added dropwise to a -78 °C stirred solution of
1
7
1
3
spectrum. 31P{1H} NMR (C6D6, 30 °C): δ 11.0 (ABXY, JPP ) 21.2,
2
JPN ) 6.6, JPN ) 21.2), 7.8 (ABXY, JPP ) 21.2, JPN ) 3.5, JPN
)
24.6). 15N NMR (C6D6, 30 °C): δ (ppm) -20.4 (ABXY, JNP ) 6.6,
JNP ) 24.6, JNN ) 21.5), 163.6 (ABXY, JNP ) 3.5, JNP ) 21.2, JNN
(77) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389.
(78) Taylor, R. C.; Kolodny, R.; Walters, D. B. Synth. Inorg. Met.-Org.
Chem. 1973, 3, 175-179.
)
21.5).
(79) Roskamp, E. J.; Pederson, S. F. J. Am. Chem. Soc. 1987, 109, 6551.
([NPN]Ta(µ-D))2N2 (4-d2). A sample of ([NPN]Ta(µ-D))2(µ-N2) was

3972 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Fryzuk et al.
prepared in a manner identical to that used for ([NPN]Ta(µ-H))2(µ-
N2) with ([NPN]Ta)2(µ-D)4.
(overlapping m, 6H total, PPh p-H and NPh p-H), 8.15 (m, 4H, PPh
o-H). 31P{1H} NMR (C4D8O, 301.6 K): δ (ppm) 16.0 (s). 13C{1H}
NMR (C4D8O, 301.6 K): δ 2.2 (d, Si(CH3)2), 2.4 (s, Si(CH3)2), 15.0
(m, SiCH2P), 122.8, 127.8, 129.1, 129.2, 130.6, and 133.2 (NPh and
PPh o-C, m-C, and p-C). Anal. Calcd for C24H31ClN3NbPSi2‚
(C6H6)0.5: C, 52.64; H, 5.56; N, 6.82. Found: C, 52.32; H, 5.64; N,
6.66.
([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-N2) (5). A solution of ([NPN]-
Ta)2(µ-H)2(µ-η1:η2-N2) (1.10 g, 0.872 mmol) in 50 mL of toluene was
added to a glass vessel equipped with a Teflon valve. The vessel was
evacuated by 2 freeze-pump-thaw cycles and 1 atm of propene gas
was added at room temperature. The vessel was sealed and the solution
was stirred. Over the course of two weeks the color changed from dark
brown to red. The solvent and excess propene gas were then removed
by vacuum and the remaining solid was dissolved in a mixture of
hexamethyldisiloxane (4 mL) and benzene (10 mL). Slow evaporation
of this solution generated crystalline ([NPN]Ta(CH2CH2CH3))2(µ-η1:
[NPN]Ta(µ-η1:η2-N2CH2Ph)(µ-H)2TaBr[NPN] (7). To a dark
brown solution of ([NPN]Ta(µ-H))2N2 (1.00 g, 0.793 mmol) in 20 mL
of toluene was added a 3-fold excess of benzylbromide. Over the course
of 8 h the solution turned dark red. The reaction could also be performed
in 5 min by heating the solution to 70 °C. The solvent was removed
under vacuum, and the resulting red solid was rinsed with hexanes
and cooled to -40 °C. The red solid was collected on a glass filter
and dried under high vacuum (1.01 g, 89%) and identified as [NPN]-
Ta(µ-η1:η2-N2CH2Ph)(µ-H)2TaBr[NPN]. Single crystals suitable for
X-ray analysis were obtained by the slow evaporation of a benzene
and hexamethyldisiloxane solution. 1H NMR (500 MHz, C6D6, 25
°C): δ -0.37, -0.27, -0.05, -0.02, 0.01, 0.14, 0.39, 0.74 (s, 24H
total, SiCH3), 0.42, 0.81, 1.31, 1.37, 1.40, 1.51, 1.73, 1.90 (ABX, 8H
1
η1-N2) (1.04 g, 89%). H NMR (C6D6, 30 °C, 500 MHz): δ -0.24,
0.04, 0.28, 0.38 (s, 24H total, SiCH3), 0.00 (br m, 2H, TaBCH2CH2-
CH3), 1.15 and 1.23 (overlapping m, 4H total, SiCH2P), 1.16 (over-
lapping m, 3H, TaBCH2CH2CH3), 1.23 and 1.34 (overlapping m, 4H
total, SiCH2P), 1.30 (overlapping m, 3H, TaACH2CH2CH3), 1.78
(overlapping m, 2H, TaBCH2CH2CH3), 1.82 (overlapping m, 2H,
TaACH2CH2CH3), 2.48 (m, 2H, TaACH2CH2CH3), 6.64 (t, 2H, NPh
p-H), 6.72 (m, 4H, NPh m-H), 6.89 (d, 4H, NPh o-H), 6.95 (t, 2H,
NPh p-H), 7.05 (d, 4H, NPh o-H), 7.13 (m, 1H, PPh p-H), 7.20 (m,
1H PPh, p-H), 7.22 (m, 2H, PPh m-H), 7.29 (m, 4H, NPh m-H), 7.44
(m, 2H, PPh m-H), 7.72 (AMX, 2H, PPh o-H), 7.91 (AMX, 2H, PPh
o-H). 31P{1H} NMR (C6D6, 30 °C): δ 14.2 (s, TaA-P), 19.7 (s, TaB-
P). 13C{1H} NMR (C6D6, 30 °C): δ 0.2, 1.8, and 3.9 (d, SiCH3), 5.7
(s, SiCH3), 14.7 (d, SiCH2P), 17.3 (s, SiCH2P), 21.6 (s, TaBCH2CH2-
CH3), 22.2 (s, TaBCH2CH2CH3), 22.7 (s, TaACH2CH2CH3), 26.0 (s, TaA-
2
total, SiCH2P), 5.03 and 5.45 (AB, JHH ) 13.4 Hz, 2H total, N2CH2-
Ph), 5.56 (m, 1H, phenyl proton), 6.48 (m, 1H, phenyl proton), 6.61
(m, 1H, phenyl proton), 6.74 (m, 1H, phenyl proton), 6.79 (d, 2H,
phenyl protons), 6.84 (m, 2H, phenyl protons), 6.93 (dd, 2H, PPh o-H),
6.97 (d, 2H, phenyl protons), 7.02 (m, 1H, phenyl proton), 7.07-7.2
(overlapping peaks, phenyl protons), 7.25 (m, 1H, phenyl proton), 7.36
(m, 1H, phenyl proton), 7.46 (m, 1H, phenyl proton), 7.59 (m, 1H,
phenyl proton), 7.70 (m, 2H, phenyl protons), 8.15 (m, 1H, phenyl
proton), 8.46 (dd, 2H, PPh o-H), 11.90 (ABXY, 2JHH ) 16.3 Hz, 2JHP
2
2
CH2CH2CH3), 59.1 (d, JPC ) 5.0 Hz, TaACH2CH2CH3), 76.2 (d, JPC
) 25.2 Hz, TaBCH2CH2CH3), 121.8 (s, NPh p-H), 121.8 (s, NPh p-H),
126.2 (s, NPh o-H), 127.3 (s, NPh o-H), 128.4 (s, NPh m-H), 128.5 (s,
NPh m-H), 129.0 (overlapping d, PPh m-H), 129.0 (overlapping d, PPh
m-H), 130.1 (s, PPh p-H), 130.7 (s, PPh p-H), 131.7 (d, 2JCP ) 12 Hz,
2
2
) 13.0 Hz, JHP ) 24.9 Hz, 1H, TaHTa), 13.00 (ABXY, JHH ) 16.3
Hz, 2JHP ) 11.7 Hz, 2JHP ) 16.6 Hz, 1H, TaHTa). 31P{1H} NMR (C6D6,
25 °C): δ -0.2 (d, JPP ) 23.6 Hz), 12.1 (d, JPP ) 23.6 Hz). 13C NMR
(C6D6, 25 °C): δ 0.5 (d, JCP ) 7.1 Hz, SiCH3), 1.0 (s, SiCH3), 1.1 (d,
JCP ) 1.1 Hz, SiCH3), 1.5 (d, JCP ) 1.1 Hz, SiCH3), 1.9 (d, JCP ) 10.2
Hz, SiCH3), 3.0 (d, JCP ) 2.7 Hz, SiCH3), 4.5 (s, SiCH3), 4.8 (d, JCP
) 3.3 Hz, SiCH3), 14.9, 15.2, 17.3, and 19.4 (s, SiCH2P), 69.6 (s, N2-
CH2Ph), 122.0 (s, phenyl), 122.3 (s, phenyl), 122.5 (s, phenyl), 124.2
(s, phenyl), 125.6 (s, phenyl), 126.7 (s, phenyl), 127.6 (s, phenyl), 128.3
(s, phenyl), 128.4 (s, phenyl), 128.7 (s, phenyl), 128.8 (s, phenyl), 128.9
(br, phenyl), 129.0 (br, phenyl), 129.3 (br, phenyl), 129.4 (br, phenyl),
129.5 (s, phenyl), 130.1 (d, JCP ) 2.2 Hz, phenyl), 130.2 (s, phenyl),
130.5 (d, JCP ) 2.2 Hz, phenyl), 133.7 to 133.8 (overlapping peaks,
phenyl), 134.6 (d, JCP ) 33.2 Hz, PPh), 137.2 (d, JCP ) 25.1 Hz, PPh),
144.9 (s, phenyl), 153.2 (d, JCP ) 4.9 Hz, phenyl), 154.8 (d, JPC ) 4.4
Hz, phenyl), 156.4 (d, JPC ) 2.7 Hz, phenyl), 162.3 (d, JCP ) 3.8 Hz,
phenyl). Anal. Calcd for C55H71N6P2Si4Ta2: C, 46.12; H, 5.00; N, 5.87.
Found: C, 46.36; H, 5.24; N, 5.81.
2
PPh o-H), 134.1 (d, JCP ) 15 Hz, PPh o-H). HSQC NMR (30
°C,C6D6): δ (1H,13C) (-0.24; 0.2), (0.04; 1.8), (0.28; 3.9), (0.38; 5.7),
(1.34 and 1.23; 14.7), (1.15 and 1.23; 17.3), (1.78; 21.6), (1.16; 22.2),
(1.30; 22.7), (2.48; 26.0), (1.82; 59.1), (0.00; 76.2), (6.64 and 6.95;
121.8), (6.89; 126.2), (7.05; 127.3), (6.72 and 7.29; 128.4 and 128.5),
(7.22 and 7.44; 129.0 and 129.0), (7.13; 130.1), (7.20; 130.7), (7.72;
131.7), (7.91; 134.1). COSY NMR (30 °C, C6D6): δ (1H;1H) (0.00;
1.78), (1.15; 1.23), (1.16; 1.78), (1.23; 1.34 and 1.15), (1.30; 2.48),
(1.34; 1.23), (1.78; 0.00 and 1.16), (1.82; 2.48), (2.48; 1.30 and 1.82),
(6.64; 6.72), (6.72; 6.64 and 6.89), (6.89; 6.72), (6.95; 7.29), (7.05;
7.29), (7.13; 7.22), (7.20; 7.44), (7.22; 7.13 and 7.72), (7.29; 6.95 and
7.05), (7.44; 7.20 and 7.91), (7.72; 7.22), (7.91; 7.44). Anal. Calcd for
C52H72N6P2Si4Ta2: C, 47.41; H, 5.51; N, 6.38. Found: C, 47.78; H,
5.74; N, 6.41.
([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-15N2) (5-15N2). A sample of
([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-15N2) (23-15N2) was prepared in a
manner identical to that for ([NPN]Ta(CH2CH2CH3))2(µ-η1:η1-N2),
except with ([NPN]Ta)2(µ-H)2(µ-η1:η2-15N2). 31P{1H} NMR (C6D6, 30
[NPN]Ta(µ-η1:η2-15N2CH2Ph)(µ-H)2TaBr[NPN] (7-15N2). The 15
N
labeled analogue was prepared in a manner identical to that used for
the unlabeled material except for using the 15N labeled precursor ([NPN]-
TaH)215N2. 1H{31P} NMR, only peaks affected by coupling to 15N are
shown, Lorentz-Gauss enhanced80 (LB ) -5.1, GB ) 16.4) (C6D6, 25
2
3
°C): δ 14.2 (AXY, JNP ) 30.5 Hz, JNP ) 6.6 Hz), 19.7 (s). 15N
1
2
NMR: δ 16.2 (AMX, JNN ) 11.3 Hz, JNP ) 30.5 Hz), 28.0 (AMX,
1JNN ) 11.3 Hz, JNP ) 6.6 Hz).
2
2
°C): δ 5.03 (ABXY, JHH ) 13.4 Hz, JHN ) 5.0 Hz, JHN ) 3.2 Hz).
{[NPN]NbCl}2(µ-η1:η1-N2) (6). A room-temperature solution of
[NPN]Li2‚(C4H8O)2 (2.048 g, 3.455 mmol) in 30 mL of Et2O was added
to a slurry of NbCl3(DME) (1.00 g, 3.46 mmol) in 20 mL of Et2O
under a N2 atmosphere. The solution turned brown within a few minutes
after being stirred at room temperature. The solution was evaporated
to dryness, the remaining solid was extracted into 150 mL of toluene,
and the solution was filtered through Celite. The solvent was removed
and the remaining material was crystallized from a benzene/hexam-
ethyldisiloxane mixture. The compound {[NPN]NbCl}2(µ-η1:η1-N2)‚
(C6H6) was isolated as a brown crystalline solid (2.98 g, 70%) that is
only moderately soluble in aromatic solvents, but has good solubility
in THF. The cocrystallized C6H6 found present in the crystal structure
2
31P{1H} NMR (C6H6, 25 °C): δ -0.2 (d, JPP ) 22.9 Hz), 12.1 (dd,
2
2JPP ) 22.9 Hz, JPN ) 27.1 Hz). 15N{1H} NMR (C6D6, 25 °C): δ
1
2
1
-35.6 (dd, JNN ) 18.1 Hz, JNP ) 27.1 Hz), -112.4 (d, JNN ) 18.1
Hz). 13C NMR, selected peak only (C6D6, 25 °C): δ 69.6 (dd, JCN
12 Hz, JCN ) 12 Hz, N2CH2Ph).
)
Calculations on [(H3P)(H2N)2Ta(µ-H)]2(µ-η1:η2-N2) (4A). The ab
initio DFT calculations on the model compound [(H3P)(H2N)2Ta(µ-
H)]2(µ-η1:η2-N2) were performed with the hybrid functional B3LYP
method60 with use of the Gaussian 98 package.59 The basis functions
and effective core potentials (ECP) used were those in the LANL2DZ
basis set developed by Hay and Wadt61 and provided with the Gaussian
98 program, but with an additional d polarization function added to P
atoms. The exponent of the d function was 0.37. This level of theory
has been shown to be adequate for modeling complexes containing
metal-metal bonds of various orders.62 Model complex 4A was
1
and identified in the H NMR spectrum could not be removed under
1
high vacuum at room temperature. H NMR (C4D8O, 301.6 K, 500
2
MHz): δ -0.09, 0.06 (s, 24H total, SiCH3), 1.42 (ABX, JHH ) 13.7
Hz, 2JPH ) 8.3 Hz, 4H, SiCH2P), 1.59 (ABX, 2JHH ) 13.7 Hz, 2JHH
)
11.6 Hz, 4H, SiCH2P), 6.74 (m, 8H, NPh o-H), 6.89 (m,4H, PPh m-H),
7.16 (m, 8H, NPh m-H), 7.32 (s, 6H, cocrystallized C6H6), 7.43
(80) Braun, S.; Kalinowski, H.-O.; Berger, S. 150 and More Basic NMR
Experiments; Wiley-VCH: Toronto, 1998.

New Mode of Coordination for the Dinitrogen Ligand
J. Am. Chem. Soc., Vol. 123, No. 17, 2001 3973
optimized with C1 symmetry. Orbital depictions were created with use
of the MOLDEN program.81
Mad) to pre-set monitor counts, in equatorial or normal-beam geometry,
using ω scans. The time per step in ω increased with the scattering
angle to 10 s per step at high scattering angle. Most of the unique
reflections up to 2θ = 100° were recorded and some further reflections
were measured up to 2θ = 110°. Three standard reflections were
monitored regularly, and showed no significant variation.
The unit cell dimension were precisely determined from the centroids
in 3D of 3176 strong reflections (in the range 3.1 e θ e 54.95°) by
using the program Rafd19, and the collected intensities were integrated
in 3D by using the program Retreat developed at ILL.90 In view of the
small size of the crystal and of the small value of the µ coefficient, no
absorption correction was necessary. Further crystallographic data and
experimental details are summarized in Table ZZ and Table SN1 in
the Supporting Information.
The starting structural model for the refinement was based on the
atomic coordinates for the heavy atoms taken from the X-ray structural
determination. The structure was refined by full matrix least squares,
minimizing the function [∑w(Fo2 - (1/k)Fc2)2] and using all independent
data. During the refinement the Fourier difference maps clearly revealed
all the H atoms positions of the ligand and showed clearly two negative
peaks between the two Ta atoms, corresponding to the bridging hydride
ligands. The final structure model included coordinates and anisotropic
displacement parameters for all atoms. No extinction correction was
deemed necessary. Upon convergence the final Fourier difference map
showed no significant features. The coherent scattering amplitudes used
were those tabulated by Sears.91 All calculations were carried out by
using the PC version of the SHELX-97 programs.92
X-ray Crystallographic Analyses. Crystallographic data appear in
Table 1 for 2 and 4 and in Table 4 for 5, 6, and 7. All measurements
were made on a Rigaku/ADSC CCD area detector with graphite
monochromated Mo KR radiation. The final unit-cell parameters were
obtained by least-squares methods on the setting angles for 19510
reflections (2), 6013 reflections (4), 18889 reflections (5), 9572
reflections (6), and 31949 reflections (7). The data were processed82
and corrected for Lorentz and polarization effects. Structures 4, 5, and
7 were solved by direct methods83 and expanded by using Fourier84
techniques. Structures 2 and 6 were solved by heavy atom Patterson
methods85 and also expanded by using Fourier84 techniques. All non-
hydrogen atoms were refined with anisotropic thermal parameters. The
tantalum bound hydrogen atoms in 4 could not be located and were
omitted from the model. The tantalum bound hydrogen atoms in 7 were
located in a difference map and refined with isotropic thermal
parameters. All remaining hydrogen atoms were fixed in calculated
positions with C-H ) 0.98 Å. Neutral atom scattering factors and
anomalous dispersion corrections were taken from the International
Tables for X-ray Crystallography.86,87 Atomic coordinates, anisotropic
thermal parameters, complete bond lengths and bond angles, torsion
angles, intermolecular contacts, and least-squares planes are included
as Supporting Information.
Neutron Diffraction Study of ([NPN]Ta(µ-H))2(µ-η1:η2-N2) (4).
A dark brown, transparent, platelike crystal of 4, of volume 1.8 mm3,
obtained by slow evaporation of a toluene solution, was glued (using
the two component glue Kwikfill) on a vanadium pin under a nitrogen
atmosphere and then sealed in a SiO2 thin-walled capillary. The sample
was transferred to a DISPLEX cryorefrigerator88 on the thermal-beam
instrument D19, at the ILL reactor, equipped with a 4° × 64° position
sensitive detector.89 A Cu(200) monochromator in reflection (nominal
takeoff angle 75°) was used to select a neutron beam of wavelength
1.5334(1) Å.
Acknowledgment. We gratefully acknowledge both the
NSERC of Canada and the Petroleum Research Fund, admin-
istered by the American Chemical Society, for funding in the
form of research grants. Financial support was also provided
by the NSERC of Canada in the form of a postgraduate
scholarship for S. A. Johnson, as well as the University of British
Columbia in the form of a University Graduate Fellowship.
The sample was slowly cooled (∼1 °C/min) to 20 K, while
monitoring the strong 2 1 -2 reflection. No change in mosaic or
splitting of the peak was observed. The room-temperature space group
P21/n was confirmed at 20 K.
Supporting Information Available: Full experimental
details, tables of parameters, fractional coordinates and thermal
parameters, bond lengths, bond angles, torsion angles, and
ORTEP diagrams for the X-ray structures of 1, 2, 4, 5, 6, and
7 as well as for the neutron structure of 4; density functional
theory calculation results for model complex 4A (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Reflections were measured (using the ILL programs Hklgen and
(81) Schaftenaar, G. Molden, 3.5 ed.; CAOS/CAMM Center, University
of Nijmegen, The Netherlands, 1991.
(82) teXsan: Crystal Structure Analysis Package; Molecular Structure
Corp.: The Woodlands, TX, 1996.
(83) Altomare, A.; Burla, M. C.; Cammali, G.; Cascarano, M.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, A.
SIR97, an integrated package of computer programs for the solution and
refinement of crystal structures using single-crystal data, 1999.
(84) Beurkens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; Smits, J. M. M. DIRDIF94; The DIRDIF-94 program
system, Technical Report of the Crystallography Laboratory, University of
Nijmegen, The Netherlands, 1994.
(85) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. PATTY;
The DIRDIF program system, Technical Report of the Crystallography
Laboratory, University of Nijmegen, The Netherlands, 1992.
(86) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, U.K. (present distributor Kluwer Academic: Boston, MA),
1974; Vol. IV.
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(88) Archer, J.; Lehmann, M. S. J. Appl. Crystallogr. 1986, 19, 456-
458.
(89) Thomas, M.; Stansfield, R. F. D.; Berneron, M.; Filhol, A.;
Greenwood, G.; Jacobe, J.; Feltin, D.; Mason, S. A. Position-SensitiVe
Detection of Thermal Neutrons; Academic Press: London, 1983.
(90) Wilkinson, C.; Khamis, H. W.; Stansfield, R. F. D.; McIntyre, G.
J. J. Appl. Crystallogr. 1988, 21, 471-478.
(91) Sears, V. F. Neutron News 1992, 3, 26-27.
(92) Sheldrick, G. M. SHELX-97, Structure Solution and Refinement
Package; Georg-August Universita¨t Go¨ttingen, 1997.
(87) International Tables for Crystallography; Kluwer Academic: Bos-
ton, MA, 1992; Vol. C.