Phosphine-induced ancillary ligand orthometalation at a tantalum-tantalum double bond

Article abstract of DOI:10.1021/om050045e

The dinuclear tantalum tetrahydride complexes (^RPh[NPN]Ta) ₂(μ-H)₄ ([NPN] = (PhNSiMe₂-CH ₂)₂PR; R = Ph, Cy) promote the asymmetric activation of molecular nitrogen to form (^RPh[NPN]Ta)₂(μ- η¹:η²-N₂)(μ-H)₂. The coordinatively unsaturated dinuclear dihydride, (^RPh[NPN]Ta) ₂(μ-H)₂, has been proposed as the reactive intermediate, with a tantalum-tantalum double bond storing four electrons with which to reduce N₂. Efforts to trap (^Cyph[NPN]Ta) ₂(μ-H)₂ with excess PMe₃ at low temperatures promoted the formation of a green product, proposed to be (^Cyph[NPN] Ta(PMe₃))₂(μ-H)₂. Attempts to isolate this product promoted the orthometalation of an ancillary ligand N-Ph ring, forming ^CyPh[NPN]Ta(μ-H)₂[μ-N(C₆H ₄)]Ta[PN](H)(PMe₃). Addition of 1 equiv of PMe₃ to (^PhPh[NPN]-Ta)₂(μ-H)₄ immediately produced the analogous orthometalation derivative, ^PhPh[NPN]Ta(μ- H)₂[μ-N(C₆H₄)]Ta[PN](H)(PMe₃).

Full text of DOI:10.1021/om050045e

2606
Organometallics 2005, 24, 2606-2611
Phosphine-Induced Ancillary Ligand Orthometalation at
a Tantalum-Tantalum Double Bond
Michael P. Shaver and Michael D. Fryzuk*
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, V6T 1Z1 Canada
Received January 21, 2005
The dinuclear tantalum tetrahydride complexes (RPh[NPN]Ta)2(µ-H)4 (RPh[NPN] ) (PhNSiMe2-
CH2)2PR; R ) Ph, Cy) promote the asymmetric activation of molecular nitrogen to form
(
(
RPh[NPN]Ta)2(µ-η1:η2-N2)(µ-H)2. The coordinatively unsaturated dinuclear dihydride,
RPh[NPN]Ta)2(µ-H)2, has been proposed as the reactive intermediate, with a tantalum-
tantalum double bond storing four electrons with which to reduce N2. Efforts to trap
(
CyPh[NPN]Ta)2(µ-H)2 with excess PMe3 at low temperatures promoted the formation of a
green product, proposed to be (CyPh[NPN]Ta(PMe3))2(µ-H)2. Attempts to isolate this
product promoted the orthometalation of an ancillary ligand N-Ph ring, forming
CyPh[NPN]Ta(µ-H)2[µ-N(C6H4)]Ta[PN](H)(PMe3). Addition of 1 equiv of PMe3 to (PhPh[NPN]-
Ta)2(µ-H)4 immediately produced the analogous orthometalation derivative, PhPh[NPN]-
Ta(µ-H)2[µ-N(C6H4)]Ta[PN](H)(PMe3).
Introduction
2 as possibly being involved.5 Given that the dinuclear
dihydride is coordinatively unsaturated, there is the
possibility that it could be trapped by addition of
suitable donors. The existence of the complex (Cl2(PMe3)2-
Ta)2(µ-H)2, a dinuclear dihydride Ta(III) complex with
a TadTa double bond, provides good support for this
idea.6 Other examples of reactive dinuclear Ta(III)
complexes that contain TadTa double bonds include
(η5-C5Me4R)2Ta2(µ-X)4 (R ) Me, Et; X ) Cl, Br)7-11 and
thioether and phosphine adducts of Ta(III).12-14 Revers-
ible addition of H2 has also been observed with ditung-
sten systems.15
The tantalum tetrahydride complexes (RPh[NPN]Ta)2-
(µ-H)4, (R ) Cy, 1a; R ) Ph, 1b) promote the coor-
dination and activation of molecular nitrogen to give
(
RPh[NPN]Ta)2(µ-η1:η2-N2)(µ-H)2.1,2 The asymmetrically
bound N2 fragment displays remarkable reactivity with
main-group hydrides. Addition of boranes or silanes
effects the formation of new N-B3 and N-Si4 bonds,
respectively, and ultimately results in N-N bond cleav-
age of the coordinated dinitrogen. It is suspected that
the reactive intermediate in the coordination and acti-
vation of N2 is (RPh[NPN]Ta)2(µ-H)2 (2), a dinuclear
dihydride complex with a TadTa double bond generated
via reductive elimination of H2, as shown in eq 1.
In this work, we describe our attempts to generate
phosphine adducts of the dinuclear tantalum dihydride
2. What results is a phosphine-induced C-H activation
of the N-Ph unit of the ancillary ligand.
(1) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc.
1998, 120, 11024.
(2) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason,
S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960.
(3) Fryzuk, M. D.; MacKay, B. A.; Johnson, S. A.; Patrick, B. O.
Angew. Chem., Int. Ed. 2002, 41, 3709.
(4) Fryzuk, M. D.; MacKay, B. A.; Patrick, B. O. J. Am. Chem. Soc.
2003, 125, 3234.
(5) Shaver, M. P.; Fryzuk, M. D. Organometallics 2005, 24, 1419.
(6) Scioly, A. J.; Luetkens, M. L.; Wilson, R. B.; Huffman, J. C.;
Sattelberger, A. P. Polyhedron 1987, 6, 741.
(7) Ting, C.; Baenziger, N. C.; Messerle, L. J. Chem. Soc., Chem.
Commun. 1988, 1133.
(8) Ting, C.; Messerle, L. J. Am. Chem. Soc. 1987, 109, 6506.
(9) Ting, C.; Messerle, L. J. Am. Chem. Soc. 1989, 111, 3449.
(10) Huang, J.; Lee, T.; Swenson, D.; Messerle, L. Inorg. Chim. Acta
2003, 345, 209.
(11) Huang, J.; Luci, J. J.; Lee, T.; Swenson, D. C.; Jensen, J. H.;
Messerle, L. J. Am. Chem. Soc. 2003, 125, 1688.
(12) Messerle, L. Chem. Rev. 1988, 88, 1229.
(13) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. P. Inorg.
Chem. 1997, 36, 896-902.
Indirect evidence for the intermediacy of the dihydride
(
RPh[NPN]Ta)2(µ-H)2 was found in the reaction of 1a and
1b with primary and secondary phosphines. Isotopic
labeling studies of P-H bond activation pointed toward
(14) Cotton, F. A.; Falvello, L. R.; Najjar, R. C. Inorg. Chem. 1983,
22, 375-377.
(15) Green, M. L. H.; Mountford, P. J. Chem. Soc., Chem. Commun.
1989, 732-734.
* Corresponding author. E-mail: fryzuk@chem.ubc.ca.
10.1021/om050045e CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/15/2005

Phosphine-Induced Orthometalation at TadTa
Organometallics, Vol. 24, No. 11, 2005 2607
Results and Discussion
Addition of triphenylphosphine to 1a or 1b gave no
reaction. 31P NMR in situ experiments showed only
resonances due to free triphenylphosphine and unre-
acted tetrahydride; in addition, the characteristic deep
purple color for solutions of 1a and 1b remained
unchanged. Thermolysis of these mixtures did not
promote complex formation; instead, protonation of the
[NPN] ancillary ligand to form RR′[NPN]H2 was ob-
served. Similarly, the bulky tertiary phosphines PBut3
and PPri3 did not exhibit any reactivity with the
dinuclear tetrahydrides 1a and 1b.
Addition of an excess of sterically unencumbered
trimethylphosphine to (CyPh[NPN]Ta)2(µ-H)4, 1a, pro-
duced a different result. Cooling a mixture of 8 equiv of
PMe3 and 1a in Et2O to approximately -80 °C resulted
in the solution turning from deep purple to deep green.
Upon warming the solution, the color reverted back to
purple, indicative of the starting tetrahydride. As long
as the solution is sealed, the color could be cycled simply
by adjusting the temperature of the vessel.
Solution NMR studies of the green compound
(d8-toluene, -80 °C) showed broad resonances at -30
and +18 ppm in the 31P{1H} NMR spectrum, integrated
against an external standard to be 2P each, lending
support for the presence of the bis(trimethylphosphine)
adduct 3a (eq 2). Analysis of 1H{31P} spectra indicated
a new broad resonance at 8.5 ppm, integrating to 2H
against the SiMe resonances; this peak can be tenta-
tively assigned to the two bridging hydrides. Also
observed were broadened resonances for 1a and free
PMe3. Efforts to extract equilibrium constants and
thermodynamic data by following this transformation
by variable-temperature NMR spectroscopy were com-
plicated by a rearrangement process (vide infra).
Figure 1. ORTEP drawing (spheroids at 50% probability)
of 4a. Silylmethyl, N-phenyl, and P-cyclohexyl ring carbons
other than ipso and methine carbons have been omitted
for clarity. The bond between N4 and Ta1 is not explicitly
given.
1
1
mixture gave a red solid analyzed by H, 31P, H/31P
HSQC, and 1H/1H COSY NMR experiments. Three
resonances are observed in the 31P{1H} NMR spec-
trum: a doublet at -20.2 ppm (PA, 1 P), a singlet at
10.5 ppm (PB, 1 P), and a doublet at 13.8 ppm (PC, 1 P).
In the 1H NMR spectrum, resonances assigned to eight
SiMe environments overlap with a large resonance (9
H) coupling strongly to PA, as observed in 1H/31P HSQC
experiments. From coupling constant and chemical shift
information, PA is likely a coordinated PMe3 group.
Cyclohexyl and methylene resonances couple to PB and
PC in the 1H/31P HSQC spectrum, indicating PB and PC
arise from the [NPN] ancillary ligand. These data are
inconsistent with the anticipated symmetrical structure
of the phosphine adduct 3a. Also not consistent with
3a was the observation of three separate hydride
resonances at 5.06, 8.15, and 11.24 ppm, each of which
is coupled to phosphorus-31: PA couples to HA at 5.06
ppm, PB couples to HB at 8.15 ppm, and PC couples to
HC at 11.24 ppm. If it is assumed that the three peaks
are all tantalum hydrides, the chemical shift of HA
indicates it is likely bound terminally. HB and HC can
be assigned with less certainty, but are most likely
bridging hydrides.
Fortunately, single crystals of this red product con-
taining 1.5 equiv of cocrystallized benzene were ob-
tained, which allowed analysis by X-ray diffraction
experiments. An ORTEP16 depiction of the solid-state
molecular structure of 4a is shown in Figure 1; this
transformation is also shown schematically in eq 3.
It can be assumed that the equilibrium in eq 2 is
extremely small at room temperature and that the
forward reaction is exothermic, as low temperatures and
a large excess of trimethylphosphine are required to
shift the equilibrium concentrations to the products.
Attempts to drive the equilibrium to the right to
increase the concentration of 3a by removal of H2
resulted in a new transformation.
Upon removal of H2 at ambient temperatures, the
solution turned from deep purple to red, not the
anticipated green color of 3a. Workup of the reaction

2608 Organometallics, Vol. 24, No. 11, 2005
Table 1. Selected Bond Distances (Å) and Angles (deg) for 4a
angles
Shaver and Fryzuk
lengths
angles
Ta(2)-N(3)
Ta(2)-N(4)
Ta(2)-P(2)
Ta(2)-P(3)
Ta(2)-Ta(1)
Ta(1)-N(2)
Ta(1)-N(1)
Ta(1)-C(48)
Ta(1)-N(4)
Ta(1)-P(1)
2.145(3)
2.235(3)
2.5796(9)
2.5919(9)
2.6778(3)
2.070(3)
2.169(3)
2.199(4)
2.410(3)
2.6318(9)
N(3)-Ta(2)-N(4)
126.15(12)
79.58(9)
Ta(1)-N(1)-C(19)
Ta(1)-N(1)-Si(1)
C(19)-N(1)-Si(1)
Ta(1)-N(2)-C(13)
Ta(1)-N(2)-Si(2)
C(13)-N(2)-Si(2)
Ta(2)-N(3)-C(31)
Ta(2)-N(3)-Si(3)
C(31)-N(3)-Si(3)
Ta(2)-N(4)-C(43)
Ta(2)-N(4)-Si(4)
C(43)-N(4)-Si(4)
115.6(3)
129.12(19)
115.1(3)
126.0(3)
122.18(19)
111.6(3)
123.7(3)
127.19(18)
109.1(3)
89.2(2)
N(3)-Ta(2)-P(2)
N(4)-Ta(2)-P(2)
N(3)-Ta(2)-P(3)
N(4)-Ta(2)-P(3)
P(2)-Ta(2)-P(3)
N(2)-Ta(1)-N(1)
N(2)-Ta(1)-N(4)
N(1)-Ta(1)-N(4)
N(2)-Ta(1)-P(1)
N(1)-Ta(1)-P(1)
N(4)-Ta(1)-P(1)
82.00(8)
93.67(9)
139.79(9)
102.49(4)
102.07(13)
163.43(12)
93.77(12)
85.35(10)
78.30(10)
93.22(8)
115.10(16)
122.0(3)
use of X-HYDEX.17 On the basis of the observed
diamagnetism and using oxidation state formalisms,
complex 4a is a Ta(IV)-Ta(IV) dimer with a Ta-Ta
single bond. The Ta1-Ta2 interatomic distance of
2.6778(3) Å is in the range reported for single Ta-Ta
bonds.6,13,18-21
Table 2. Crystallographic Data and Details of
Refinement
4a
4b
empirical formula
fw
cryst syst
space group
a-c (Å)
C
60H92N4Si4P3Ta2
C
54H72N4P3Si4Ta2
1436.55
triclinic
P1h
1345.43
monoclinic
P21/n
The formation of this cyclometalated product likely
results from dissociation of one PMe3 from the inter-
mediate dinuclear dihydride 3a, which opens up a site
for the ortho-hydrogen of an N-Ph to undergo C-H
activation across the TadTa double bond (Scheme 1).
Attempts to determine the origin of the terminal hydride
with isotopic labeling studies were inconclusive. Em-
ploying (CyPh[NPN]Ta)2(µ-D)4 in an analogous experi-
ment showed isotopic mixing across all three hydridic
11.2799(2),
13.4464(4),
22.9421(7)
81.286(4),
77.613(4),
73.064(4)
3236.53(48)
2
1.37
3.559
173 ( 1
55.76
25 544
18 249
673
11.1558(8),
23.4870(16),
22.5891(17)
90.0,
84.011(4),
90.0
5886.4(7)
4
1.517
3.915
173 ( 1
60.04
19 536
11 663
615
0.0554
0.0963
1.038
R, â, γ (deg)
V, Å3
Z
Dcalc, g cm-3
µ(Mo KR), cm-1
T, K
1
resonances; integration of H{31P} NMR peaks for HA,
2θ range (deg)
total reflns
unique reflns
parameters
HB, and HC summed to ∼1H, supporting orthometala-
tion followed by isotopic mixing to form three isoto-
pomers. We have not seen any evidence of dissociation
into mononuclear fragments for any of this chemistry;
for example, mixing 1a and 1b did not produce any cross
products indicative of fragmentation.
a
R1
0.0330
0.0705
1.134
a
Rw
goodness-of-fit
2
2
2 2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) ∑w(|Fo | - |Fc |)2/∑w|Fo | )1/2
.
While the formation of 4a needed to be encouraged
by the removal of H2 in vacuo, the same transforma-
tion of the phenylphosphine-substituted tetrahydride
The structure shows two CyPh[NPN] ligands and a
single PMe3 group consistent with the 31P{1H} NMR
data. However, the lack of symmetry arises from an
N-Ph ring that has undergone C-H activation to
generate a Ta[µ-N(C6H4)]Ta moiety. Relevant bond
lengths and angles are listed in Table 1, and crystal-
lographic data are located in Table 2. The Ta1-C48
distance of 2.199(4) Å is within standard Ta-C bond
lengths. The amido donor attached to this ring appears
to be bridging the two tantalum centers. Amido
donors N1, N2, and N3 are planar; the sum of angles
around each are 359.82°, 359.78°, and 359.99°, respec-
tively. The unique amido nitrogen, N4, is pyramidal,
with the sum of angles Ta2-N4-Si4, C43-N4-Si4, and
C43-N4-Ta2 being 326.3°. Therefore, the amido lone
pair on N4 is not π-donating to Ta2, but rather is
directed at Ta1. The Ta1-N4 distance of 2.410(3) Å is
quite long, much longer than the Ta2-N4 bond length
of 2.235(3) Å; nevertheless, the orientation and distance
still support a bridging amide.
(
PhPh[NPN]Ta)2(µ-H)4, 1b, with PMe3 proved more facile.
Addition of just 1 equiv of PMe3 to an ethereal solution
of 1b promoted an instantaneous color change from
purple to red. Analysis of the product showed it was
similar to the orthometalated complex 4a. 31P{1H} NMR
spectra showed three resonances at -18.25 (d), 6.18 (s),
and 10.87 (d) ppm, respectively. As in 4a, three hydride
resonances were observed (11.46, 8.41, and 5.29 ppm).
Integration of the peaks between -0.5 and 0.8 ppm
indicated eight silylmethyl resonances and one coordi-
nated PMe3. 2D NMR experiments clearly indicated
orthometalation of a ligand phenyl ring, analogous to
that observed in 4a; however, it was uncertain whether
the N-Ph or P-Ph ring had undergone C-H activation.
X-ray quality crystals containing 0.5 equiv of cocrys-
tallized solvent were grown from the slow evaporation
of a benzene solution of the crude product. Analysis of
these crystals by X-ray crystallography confirmed the
formation of the analogous C-H activated product to
The terminal hydride was located in the diffraction
pattern and refined isotropically. H101 is located on the
same Ta center as the PMe3 group, between N4, P3, and
P2 donors. Unfortunately, no evidence for the bridging
hydrides could be found in the diffraction pattern,
although calculated positions were suggested with the
4a, namely, PhPh[NPN]Ta(µ-H)2[µ-N(C6H4)]Ta[PN](H)-
(PMe3), 4b. An ORTEP16 depiction of the solid-state
molecular structure of 4b is shown in Figure 2 (see also
eq 3).
(16) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(17) Orpen, A. G. J. Chem. Soc., Dalton Trans. 1980, 2509.

Phosphine-Induced Orthometalation at TadTa
Organometallics, Vol. 24, No. 11, 2005 2609
Scheme 1
Table 3. Selected Bond Distances (Å) and Angles (deg) for 4b
lengths
angles
angles
Ta(2)-N(3)
Ta(2)-N(4)
Ta(2)-P(2)
Ta(2)-P(3)
Ta(2)-Ta(1)
Ta(1)-N(2)
Ta(1)-N(1)
Ta(1)-C(45)
Ta(1)-N(4)
Ta(1)-P(1)
2.140(8)
2.223(7)
2.570(3)
2.593(2)
2.6689(5)
2.079(8)
2.145(8)
2.167(11)
2.353(8)
2.618(2)
N(3)-Ta(2)-N(4)
N(3)-Ta(2)-P(2)
N(4)-Ta(2)-P(2)
N(3)-Ta(2)-P(3)
N(4)-Ta(2)-P(3)
P(2)-Ta(2)-P(3)
N(2)-Ta(1)-N(1)
N(2)-Ta(1)-N(1)
N(2)-Ta(1)-N(4)
N(1)-Ta(1)-N(4)
N(2)-Ta(1)-P(1)
N(1)-Ta(1)-P(1)
N(4)-Ta(1)-P(1)
126.4(3)
80.1(2)
Ta(1)-N(1)-C(22)
117.9(6)
128.3(4)
113.6(7)
123.0(5)
124.8(4)
112.1(6)
122.6(9)
127.5(5)
109.9(6)
88.4(5)
Ta(1)-N(1)-Si(1)
C(22)-N(1)-Si(1)
Ta(1)-N(2)-C(16)
Ta(1)-N(2)-Si(2)
C(16)-N(2)-Si(2)
Ta(2)-N(3)-C(34)
Ta(2)-N(3)-Si(3)
C(34)-N(3)-Si(3)
Ta(2)-N(4)-C(40)
Ta(2)-N(4)-Si(4)
C(40)-N(4)-Si(4)
Ta(2)-N(4)-Ta(1)
81.8(2)
92.0(2)
140.8(2)
99.94(8)
103.7(3)
103.7(3)
156.2(3)
97.3(3)
84.0(2)
117.1(4)
120.0(7)
71.3(2)
79.5(2)
88.98(17)
Relevant bond lengths and angles are listed in Table
3, and crystallographic data are located in Table 2.
The structures of 4a and 4b are quite similar. The
Ta(1)-C(45) distance in 4b is 2.167(11) Å and the
Ta(1)-Ta(2) distance is 2.6689(5) Å, both differing only
slightly from previously observed bond lengths. The
Ta(1)-N(3) and Ta(2)-N(3) distances of 2.353(8) and
2.223(7) Å, respectively, indicate a more symmetrical
bridging by the amido ligand. This is again supported
by the planarity of N(1), N(2), and N(3) in comparison
to N(4); the sum of angles around N(4) is 326.2°.
Unfortunately, none of the hydrides were located within
the diffraction pattern due to the low quality of the
crystal. Three hydrides were located in the diffraction
pattern by X-HYDEX:17 one terminally bound to Ta(2)
and two located bridging the tantalum centers.
Changing the phosphine donor from cyclohexyl to the
inferior σ-donor phenyl drastically increases the procliv-
ity for the orthometalation reaction to occur. Presum-
ably, the cyclohexyl phosphine donor better stabilizes
the reactive intermediates, and ligand decomposition is
less favorable.
Attempts to characterize decomposition products in
the absence of phosphine traps have been unsuccessful.
Exposing 1a and 1b to vacuum at low concentrations
did result in a purple to brown color change, but
analysis of this crude material indicated a myriad of
products due to decomposition. This observation, how-
ever, helps to explain previous observations2 in the
synthesis of the side-on end-on dinitrogen complex
(
PhPh[NPN]Ta)2(µ-η1:η2-N2)(µ-H)2. It was noted that the
yields and purity of this complex dramatically im-
proved when reaction with dinitrogen is conducted
under a 90/10 N2/H2 mixture instead of a pure N2 atmos-
phere. The activation of N2 by 1b is a slow reaction and
Figure 2. ORTEP drawing (spheroids at 50% probability)
of 4b. Silylmethyl and N- and P-phenyl ring carbons other
than ipso carbons have been omitted for clarity.

2610 Organometallics, Vol. 24, No. 11, 2005
Shaver and Fryzuk
addition of a toluene solution of sodium benzophenone ketyl
prior to use to ensure absence of oxygen and water. Alterna-
tively, anhydrous diethyl ether was stored over sieves and
distilled from sodium benzophenone ketyl under argon. Tet-
rahydrofuran was refluxed over CaH2 prior to distillation from
sodium benzophenone ketyl under argon, and pentane was
stored over sieves and distilled from sodium benzophenone
ketyl solubilized by tetraglyme under argon prior to storage
over a potassium mirror. Nitrogen gas was dried and deoxy-
genated by passage through a column containing activated
molecular sieves and MnO. Deuterated benzene was dried by
heating at reflux with sodium/potassium alloy 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}, and 31P{1H} NMR spectra were
recorded on a Bruker AMX-500 instrument with a 5 mm BBI
probe operating at 500.1 MHz for H. H NMR spectra were
referenced to residual protons in C6D5H (δ 7.15 ppm) with
respect to tetramethylsilane at δ 0.0 ppm. 31P NMR spectra
were referenced to either external or internal P(OMe)3 (δ 141.0
ppm with respect to 85% H3PO4 at δ 0.0 ppm). Elemental
analyses were performed by Mr. M. Lakha of the University
of British Columbia, Department of Chemistry. Complexes
is hindered further by the low solubility of molecular
nitrogen in diethyl ether or toluene. The presence of H2
in the system kinetically stabilizes the dihydride species
2b, swinging the equilibrium toward the relatively
stable tetrahydride complex 1b (eq 1). With the con-
centration of reactive TadTa bonds low, the poor
solubility of N2 in the reaction solvent is marginalized,
and the decomposition reaction is minimized.
Concluding Remarks
The results of this study provide further experi-
mental evidence for the intermediacy of the reactive
dinuclear dihydride species (RPh[NPN]Ta)2(µ-H)2 in the
activation of small molecules by the highly reducing
tetrahydride (RPh[NPN]Ta)2(µ-H)4 (R ) Cy, 1a; R )
Ph, 1b). The addition of PMe3 to 1a at low tempera-
tures results in a color change from deep purple to
green, and spectroscopic evidence is consistent with the
formation of (CyPh[NPN]Ta(PMe3))2(µ-H)2 (3a). However,
3a is unstable except at low temperatures, and upon
warming under vacuum, one of the N-Ph substitu-
ents is orthometalated to generate CyPh[NPN]-
1
1
(
PhPh[NPN]Ta)2(µ-H)42 and (CyPh[NPN]Ta)2(µ-H)45 were prepared
by literature procedures. PMe3 was purchased from Aldrich
and distilled under N2 prior to use.
Ta(µ-H)2[µ-N(C6H4)]Ta[PN](H)(PMe3), 4a. This same
Reaction of (CyPh[NPN]Ta)2(µ-H)4 with Excess PMe3. A
solution of PMe3 (29.0 mg, 0.381 mmol) in d8-toluene (1.5 mL)
was vacuum transferred into a sealable NMR tube containing
transformation is obtained upon addition of PMe3 to
(
PhPh[NPN]Ta)2(µ-H)4, 1a; however, in this case the
(
CyPh[NPN]Ta)2(µ-H)4 (50.0 mg, 0.0381 mmol). The NMR tube
formation of PhPh[NPN]Ta(µ-H)2[µ-N(C6H4)]Ta[PN](H)-
was sealed under vacuum and allowed to warm to room
temperature briefly, and then cooled back to -78 °C, promoting
a color change from deep purple to dark green. The crude
mixture was analyzed by low-temperature 31P{1H} and
1H{31P} NMR spectroscopy. 1H NMR (C7D8, -80 °C, 500 MHz,
selected resonance): 8.5 (br, 2H, TaHTa). 31P{1H} NMR (C7D8,
-80 °C, 202.5 MHz): δ -62 (s, in excess), -30 (br, 2P), 18 (br,
2P), 29 (br, in excess).
(PMe3), 4b, occurs rapidly and with no detectable PMe3
intermediates. Such an effect of the substituent at
phosphorus in the NPN ancillary ligand backbone is
likely due to the ability of the more electron rich
CyPh[NPN] system to stabilize the dinuclear dihydride
2a. Orthometalation of ancillary N-Ph amide ligands
is rare to our knowledge; typically, cyclometalation of
ortho-substituted ligands is more common.22-24
Synthesis of CyPh[NPN]Ta(µ-H)2[µ-N(C6H4)]Ta[PN](H)-
(PMe3), 4a. A solution of PMe3 (290.0 mg, 3.81 mmol) in Et2O
(50 mL) was vacuum transferred into a vessel containing
CyPh[NPN]Ta)2(µ-H)4 (500.0 mg, 0.381 mmol). The contents of
Experimental Section
(
the flask were frozen, and the vessel was evacuated and sealed.
Allowing the contents to warm to room temperature, followed
by stirring for 1 h, promoted a color change from deep purple
to red. Removal of volatiles after 2 h gave a dark red powder,
which was dissolved in benzene. Slow evaporation of the
benzene solution gave crystals (85%) of 4a suitable for X-ray
General Considerations. Unless otherwise stated, all
manipulations were performed under an inert atmosphere of
dry, oxygen-free dinitrogen or argon by means of standard
Schlenk or glovebox techniques. Where choice of atmosphere
affects reaction outcomes, the distinction between dinitrogen
and argon will be made. Anhydrous hexanes and toluene were
purchased from Aldrich, sparged with dinitrogen, and passed
through activated alumina and Ridox catalyst columns under
a positive pressure of nitrogen prior to use.25 Anhydrous
pentane, benzene, tetrahydrofuran, and diethyl ether were
purchased from Aldrich, sparged with dinitrogen, and passed
through an Innovative Technologies Pure-Solv 400 solvent
purification system. All organic solvents were tested with
1
diffraction. H{31P} NMR (C6D6, 25 °C, 500 MHz): δ -0.34,
0.17, 0.16, 0.03, 0.18, 0.27, 0.53, 0.72 (s, 24H total, SiCH3),
0.36 (s, 9H, P(CH3)3), 0.08-1.75 (m, 30H, PCH2, PC6H11), 5.06
(d, 1H, TaH), 6.55-7.63 (m, 19H, NPh-H), 8.15 (s, 1H, TaH),
11.24 (d, 1H, TaH). 31P{1H} NMR (C6D6, 25 °C, 202.5 MHz):
2
2
δ -20.2 (d, JPP ) 188 Hz, PMe3), 10.5 (s, NPN), 13.8 (d, JPP
) 18.8 Hz, NPN). 1H,31P HSQC: 31P{1H} δ -20.2 (0.36, 5.06),
10.5 (0.18, 0.27, 1.09-1.75, 8.15), 13.8 (-0.17, 0.03, 1.09-1.75,
11.24). H,1H COSY (selected correlations): δ 7.63 (â, 7.48),
1
(18) Profilet, R. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1992,
11, 1559.
7.48 (R, 7.63; δ, 6.76), 6.76 (â, 7.48; γ, 6.55), 6.55 (δ, 6.76). Anal.
Calcd for C51H85N4P3Si4Ta2: C, 46.36; H, 6.48; N, 4.24.
Found: C, 46.80; H, 6.81; N, 4.02.
(19) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics
2000, 19, 3931.
(20) Chadeayne, A. R.; Wolczanski, P. T.; Lobkovsky, E. B. Inorg.
Chem. 2004, 43, 3421-3431.
Synthesis of PhPh[NPN]Ta(µ-H)2[µ-N(C6H4)]Ta[PN](H)-
(21) Miller, R. L.; Toreki, R.; LaPointe, R. E.; Wolczanski, P. T.; van
Duyne, G. D.; Roe, D. C. J. Am. Chem. Soc. 1993, 115, 5.
(22) Chesnut, R. W.; Jacob, G. G.; Yu, J. S.; Fanwick, P. E.; Rothwell,
I. P. Organometallics 1991, 10, 321-8.
(PMe3), 4b. A solution of PMe3 (29.3 mg, 0.385 mmol) in Et2O
(50 mL) was vacuum transferred into a vessel containing
(
PhPh[NPN]Ta)2(µ-H)4 (500.0 mg, 0.385 mmol). Allowing the
(23) Abbenhuis, H. C. L.; Van Belzen, R.; Grove, D. M.; Klomp, A.
J. A.; Van Mier, G. P. M.; Spek, A. L.; Van Koten, G. Organometallics
1993, 12, 210-19.
contents to warm to room temperature promoted an immediate
color change from deep purple to red. Removal of volatiles after
30 min gave a dark red powder, which was dissolved in
toluene. Removal of solvent gave 4b in >85% yield. Slow
evaporation of a benzene solution of 4b gave crystals suitable
(24) Kawaguchi, H.; Matsuo, T. J. Am. Chem. Soc. 2003, 125,
14254-14255.
(25) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.

Phosphine-Induced Orthometalation at TadTa
Organometallics, Vol. 24, No. 11, 2005 2611
for X-ray diffraction. 1H{31P} NMR (C6D6, 25 °C, 500 MHz): δ
-0.30, -0.12, -0.10, 0.09, 0.23, 0.33, 0.42, 0.58 (s, 24H total,
SiCH3), 0.40 (s, 9H, P(CH3)3), 1.11-2.26 (m, 8H, PCH2), 5.29
(d, 1H, TaH), 6.51-7.67 (m, 19H, NPh-H, PPh-H), 8.41 (s, 1H,
TaH), 11.46 (d, 1H, TaH). 31P{1H} NMR (C6D6, 25 °C, 202.5
MHz): δ -18.25 (d, 2JPP ) 19.1 Hz, PMe3), 6.18 (s, NPN), 10.87
anisotropically using SHELXL-97.31 When hydrogen atom
locations were important to determine compound composition
or connectivity, hydrogen atoms were located and refined
isotropically. Otherwise, hydrogen atoms were included in
fixed positions. Their positional parameters were calculated
with fixed C-H bond distances of 0.99 Å for sp2 C, 0.98 Å for
sp3 C, and 0.95 Å for aromatic sp C, with Uiso set to 1.2 times
Ueq of the attached sp or sp2 C and 1.5 times the Ueq values of
the attached sp3 C atoms. Methyl hydrogen torsion angles were
determined from electron density. Structure solution and
refinements were conducted using the WinGX software pack-
age, version 1.64.05.32
2
(d, JPP ) 19.1 Hz, NPN). 1H,31P HSQC: 31P{1H} δ -18.25
(0.40, 5.29), 6.18 (0.23, 0.33, 1.11-1.58, 8.41), 10.87 (-0.12,
0.09, 1.11-1.58, 11.46). 1H,1H COSY (selected correlations):
δ 7.67 (â, 7.35), 7.35 (R, 7.67; δ, 6.88), 6.88 (â, 7.35; γ, 6.51),
6.51 (δ, 6.88). Anal. Calcd for C51H73N4P3Si4Ta2: C, 46.78; H,
5.62; N, 4.28. Found: C, 47.05; H, 5.99; N, 4.14.
X-ray Crystal Structure Experimental Information.
Suitable crystals were selected and coated in Paratone-N oil
or an acceptable substitute under an inert atmosphere.
Crystals were then mounted on a glass fiber external to the
glovebox environment. All measurements were made on a
Rigaku/ADSC CCD area detector with graphite-monochro-
mated Mo KR radiation. Diffraction data for compounds 4a
and 4b were collected at a temperature of -100 ( 1 °C. The
data were processed using the d*TREK program26 in the
CrystalClear software package (v. 1.3.5) and corrected for
Lorentz and polarization effects and absorption. Neutral atom
scattering factors for all non-hydrogen atoms were taken from
Acknowledgment. We thank NSERC of Canada for
funding (Discovery Grant to M.D.F., PGS B Scholarship
to M.P.S.). M.P.S. was also supported by a Killam
Predoctoral Fellowship and a Li Tse Fong Fellowship.
Thanks are also extended to Mr. Christopher D. Car-
michael for collection of the X-ray data.
Supporting Information Available: X-ray crystallo-
graphic data for 4a and 4b in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM050045E
Cromer and Waber.27 Anomalous dispersion effects were
(29) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1993, 26,
343.
(30) Burla, M. C.; Camalli, M.; Carrozzini, G. L.; Cascarano, G. L.;
Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2003, 36,
1103.
(31) Sheldrick, G. M. Shelx97, Programs for Crystal Structure
Analysis; University of Gottingen: Germany, 1997.
(32) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
28
included in Fcalc
.
The structures were solved by direct
methods using SIR9229 or SIR200230 and expanded using
Fourier techniques. All non-hydrogen atoms were refined
(26) Pflugrath, J. W. Acta Crystallogr. 1999, D55, 1718-1725.
(27) Cromer, D. T.; Waber, J. T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, 1974; Vol. IV.
(28) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781.