Addition of primary and secondary phosphines to dinuclear tantalum hydrides: Synthesis of phosphides and phosphinidenes via P-H bond activation

Article abstract of DOI:10.1021/om0490076

The dinuclear complex (^Rph[NPN]Ta)₂(μ-H) ₄ (where ^Rph[NPN] = [RP(CH₂SiMe ₂NPh)₂] and R = Ph, Cy) shows remarkable reactivity with molecular nitrogen, producing the side-on end-on bound dinitrogen complex ( ^Rph[NPN]Ta)₂(μ-η¹:η²- N₂)(μ-H)₂. This tantalum tetrahydride also promotes other small molecule activation. Addition of secondary phosphines HPR′₂ (R′ = Ph, Cy) to (^Rph[NPN]Ta) ₂(μ-H)₄ accesses trihydride phosphide systems ( ^Rph[NPN]Ta)₂(μ-H)₃(μ- PR″₂). Addition of primary phosphines H₂PR″ (R″ = Cy, Ad) gives bridging phosphinidene tantalum complexes ( ^Rph[NPN]Ta)₂(μ-H)₂(μ-PR″). Molecular structures as determined by X-ray crystallography for the complexes (^PhPh[NPN]Ta)₂(μ-H)₃(μ-PCy₂) and (^CyPh[NPN]-Ta)₂(μ-H)₂(μ-PAd) are presented. Isotopic labeling studies offer mechanistic insights into the reactivity of this tetrahydride system. Reaction of Cy₂PH with ( ^CyPh[NPN]Ta)₂(μ-D)₄, for instance, gives solely the monohydride dideuteride complex (^CyPh[NPN]Ta) ₂(μ-H)(M-D)₂(μPCy₂). This labeling study indicates that D₂ loss forms the reactive species, ( ^CyPh[NPN]Ta)₂-(μ-D)₂, which promotes small molecule activation. Addition of CyPH₂ to (^CyPh[NPN]Ta) ₂(μ-D)₄ gives a series of isotopomers, with an average composition of 0.67 H and 1.33 D, supporting a 1,2-H₂ elimination step to form the tantalum phosphinidenes. These results are tied into the general reactivity of (^Rph[NPN]Ta)₂(μ-H)₄.

Full text of DOI:10.1021/om0490076

Organometallics 2005, 24, 1419-1427
1419
Addition of Primary and Secondary Phosphines to
Dinuclear Tantalum Hydrides: Synthesis of Phosphides
and Phosphinidenes via P-H Bond Activation
Michael P. Shaver and Michael D. Fryzuk*
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, V6T 1Z1 Canada
Received December 14, 2004
The dinuclear complex (RPh[NPN]Ta)2(µ-H)4 (where RPh[NPN] ) [RP(CH2SiMe2NPh)2] and
R ) Ph, Cy) shows remarkable reactivity with molecular nitrogen, producing the side-on
end-on bound dinitrogen complex (RPh[NPN]Ta)2(µ-η1:η2-N2)(µ-H)2. This tantalum tetrahydride
also promotes other small molecule activation. Addition of secondary phosphines HPR′2 (R′
) Ph, Cy) to (RPh[NPN]Ta)2(µ-H)4 accesses trihydride phosphide systems (RPh[NPN]Ta)2(µ-
H)3(µ-PR′2). Addition of primary phosphines H2PR′′ (R′′ ) Cy, Ad) gives bridging phosphin-
idene tantalum complexes (RPh[NPN]Ta)2(µ-H)2(µ-PR′′). Molecular structures as determined
by X-ray crystallography for the complexes (PhPh[NPN]Ta)2(µ-H)3(µ-PCy2) and (CyPh[NPN]-
Ta)2(µ-H)2(µ-PAd) are presented. Isotopic labeling studies offer mechanistic insights into
the reactivity of this tetrahydride system. Reaction of Cy2PH with (CyPh[NPN]Ta)2(µ-D)4, for
instance, gives solely the monohydride dideuteride complex (CyPh[NPN]Ta)2(µ-H)(µ-D)2(µ-
PCy2). This labeling study indicates that D2 loss forms the reactive species, (CyPh[NPN]Ta)2-
(µ-D)2, which promotes small molecule activation. Addition of CyPH2 to (CyPh[NPN]Ta)2(µ-
D)4 gives a series of isotopomers, with an average composition of 0.67 H and 1.33 D,
supporting a 1,2-H2 elimination step to form the tantalum phosphinidenes. These results
are tied into the general reactivity of (RPh[NPN]Ta)2(µ-H)4.
Introduction
to generate (Cp*Fe)2(µ-H)2(PR2)2, a species with more
than one phosphido bridge.
Activation of small molecules by metal complexes that
contain more than one metal center is a mature area of
inorganic chemistry.1 Interest in this topic continues
because of the potential alacrity of such polymetallic
systems to engage in cooperative type interactions with
extremely stable, small molecules such as N2,2 CO2,3 and
alkanes.4 Of particular note are polynuclear metal
hydrides that can show enhanced reactivity due to the
presence of electron-deficient, bridging hydrides.
In the area of P-H bond activation, there are a
number of dinuclear systems that have been shown to
react with primary and secondary phosphines. A com-
mon outcome is the formation of phosphido-bridged
metal complexes by reaction with systems that contain
metal-metal multiple bonds.5,6 Formation of bridging
phosphido units by reaction of higher order metal
clusters with P-H containing species is also well
established.7-15 Similarly, dinuclear metal hydrides
such as (Cp*Fe)2(µ-H)4 undergo P-H bond addition16
The dinuclear tantalum tetrahydride complex
(
PhPh[NPN]Ta)2(µ-H)4, 1a (where PhPh[NPN] ) [PhP(CH2-
SiMe2NPh)2]), is a potent reducing agent able to for-
mally reduce dinitrogen by four electrons in the gen-
eration of (PhPh[NPN]Ta)2(µ-η1-η2-N2)(µ-H)2, a N2 complex
with side-on end-on bound dinitrogen.17,18 This reaction
is suspected to occur by initial loss of H2 to generate
the reactive, unobserved intermediate (PhPh[NPN]Ta)2-
(µ-H)2, 2a, a d2-d2 dimer that formally has a TadTa
double bond. To exploit the reducing power of the
starting tetrahydride complex, we have examined its
characteristic chemistry with a number of small mol-
ecules. In this paper, we present the reactions of
tetrahydride 1a and the related cyclohexyl derivative,
(8) Blum, T.; Braunstein, P.; Tiripicchio, A.; Tiripicchio, C. M.
Organometallics 1989, 8, 2504.
(9) Caffyn, A. J. M.; Mays, M. J.; Conole, G.; McPartlin, M.; Powell,
H. R. J. Organomet. Chem. 1992, 436, 83.
(10) Don, M. J.; Richmond, M. G.; Watson, W. H.; Nagl, A. J.
Organomet. Chem. 1989, 372, 417.
* Corresponding author. E-mail: fryzuk@chem.ubc.ca.
(1) Adams, R. D.; Cotton, F. A. Catalysis by Di- and Polynuclear
Metal Cluster Complexes; Wiley-VCH: New York, 1998.
(2) Gambarotta, S.; Scott, J. Angew. Chem., Int. Ed. 2004, 43, 5289-
5308.
(11) Ebsworth, E. A. V.; McIntosh, A. P.; Schroder, M. J. Organomet.
Chem. 1986, 312, C41.
(12) Horton, A. D.; Mays, M. J.; Raithby, P. R. J. Chem. Soc., Dalton
Trans. 1987, 1557.
(13) Jeffery, J. C.; Lawrence-Smith, J. G. J. Organomet. Chem. 1985,
280, C34.
(3) Doukov, T. I.; Iverson, T. M.; Seravalli, J.; Ragsdale, S. W.;
Drennan, C. L. Science 2002, 298, 567-572.
(4) Matsubara, K.; Inagaki, A.; Tanaka, M.; Suzuki, H. J. Am. Chem.
Soc. 1999, 121, 7421-7422.
(14) King, J. D.; Mays, M. J.; Mo, C. Y.; Solan, G. A.; Conole, G.;
McPartlin, M. J. Organomet. Chem. 2002, 642, 227.
(15) Kwek, K.; Taylor, N. J.; Carty, A. J. Chem. Commun. 1986,
230.
(5) Garcia, M. E.; Riera, V.; Ruiz, M. A. Organometallics 2002, 21,
5515.
(16) Ohki, Y.; Suzuki, H. Angew. Chem., Int. Ed. 2000, 39, 3120.
(17) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc.
1998, 120, 11024.
(18) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.;
Mason, S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960.
(6) Angeles, A. Y.; Garcia, M. E.; Riera, V.; Ruiz, M. A. Organome-
tallics 2004, 23, 433.
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1986, 307, 219.
10.1021/om0490076 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 03/04/2005

1420 Organometallics, Vol. 24, No. 7, 2005
Shaver and Fryzuk
Scheme 1
Synthesis of Tantalum Phosphide Complexes.
Addition of dicyclohexylphosphine to an ethereal solu-
tion of (PhPh[NPN]Ta)2(µ-H)4, 1a, under Ar promotes an
immediate color change from purple to red along with
concomitant gas evolution. Removal of solvent and
collection of the pentane-soluble product gave a deep
red powder. 31P NMR spectroscopy of this product
showed three distinct phosphorus resonances at 10.47,
10.53, and 164.38 ppm. The peak at 164 ppm (PA)
appears as a doublet of doublets coupled to both PB (73.4
Hz) and PC (10.1 Hz). PB (10.53 ppm) appears as a
doublet and is assigned trans to PA due to the large
coupling constant; it overlaps with a doublet assigned
to PC (10.47 ppm) that exhibits a weaker cis coupling.
PA is in the characteristic range for bridging phosphide
metal complexes; for example, the heterobimetallic
complex Cp2TaH(µ-H)(µ-PPh2)Fe(CO)3 exhibits a 31P
chemical shift of 177.2 ppm.21 The product of 1a and
Cy2PH was initially assigned as (PhPh[NPN]Ta)2(µ-H)3-
(µ-PCy2), 3a.
(
CyPh[NPN]Ta)2(µ-H)4, 1b (where CyPh[NPN] ) [CyP(CH2-
SiMe2NPh)2]), with primary and secondary phosphines.
What results from this work are a number of new
phosphido-bridged dinuclear species as well as dinuclear
phosphinidene derivatives. Evidence is also presented
for the formation of the intermediate dihydride species
2, a key complex proposed in the activation of dinitrogen
by 1.
An analysis of the hydride region of the 1H NMR
spectrum supports this assignment. Two distinct hy-
dride environments at 10.5 and 8.7 ppm, integrating to
1H and 2H, respectively, indicate the presence of three
bridging hydrides. Further assignment and integration
1
of H and 31P NMR spectra confirmed the synthesis of
the phosphide complex, 4, in 84% yield. Other phosphido
tantalum complexes can be prepared similarly from the
addition of various secondary phosphines to the tanta-
lum tetrahydride complexes 1a and 1b. Addition of
HPCy2 to 1b, for example, affords (CyPh[NPN]Ta)2(µ-H)3-
(µ-PCy2), 3b. 31P NMR spectroscopy of this derivative
shows an upfield shift of the phosphide resonance from
164 to 149 ppm. The resonances due to the ligand
phosphines shift downfield, appearing at 16.3 and 18.7
ppm. Additions of HPPh2 to 1a and 1b afford (PhPh[NPN]-
Ta)2(µ-H)3(µ-PPh2), 4a, and (CyPh[NPN]Ta)2(µ-H)3(µ-
PPh2), 4b, respectively. A strong upfield shift of the
phosphide resonance also occurs for diphenylphosphine
derivatives; these resonances appear at 136.3 ppm for
4a and 115 ppm for 4b.
Results and Discussion
Synthesis of (CyPh[NPN]Ta)2(µ-H)4. Earlier studies
employing the diamidophosphine ligand set, [NPN],
have shown that variation of aryl or alkyl groups on the
amido and/or phosphine donors can drastically affect
complex stability and reactivity.19,20 In particular, sub-
stitution of the phenyl group at phosphorus by cyclo-
hexyl has already been shown to moderate the reactions
of complexes that incorporate this modified ligand
system as well as promote single-crystal formation in
certain cases. For these reasons, we include the syn-
thesis of (CyPh[NPN]Ta)2(µ-H)4 (1b), in which Cy )
C6H11. The sequence of reactions is identical to that
previously published preparation of 1a.18 Synthesis of
CyPh[NPN]TaMe3 is accomplished by the slow addition
of an ethereal solution of CyPh[NPN]Li2(OEt2) to a cold
(-78 °C) solution of TaMe3Cl2 in diethyl ether. The 31
P
NMR spectrum of CyPh[NPN]TaMe3 shows a singlet at
16.8 ppm, shifted upfield from 13.5 ppm in PhPh[NPN]-
TaMe3. Exposure of a solution of the trimethyl complex
to 4 atm of H2 promotes the generation of (CyPh[NPN]-
Ta)2(µ-H)4, 1b, evidenced by a color change from yellow
to purple. Upon workup, 1b is isolated as a bright purple
solid in 95% yield. Structure assignment was supported
1
by observation of a singlet at 9.9 ppm in the H{31P}
NMR spectrum corresponding to four bridging hydrides
along with two inequivalent silylmethyl resonances,
totalling 24 protons and the requisite cyclohexyl,
methylene, and phenyl resonances. A singlet in the 31P-
{1H} spectrum at 29.2 ppm due to the coordinated
phosphine is upfield from the 21.0 ppm peak observed
for (PhPh[NPN]Ta)2(µ-H)4. These reactions are shown in
Scheme 1.
(19) Fryzuk, M. D.; Shaver, M. P.; Patrick, B. O. Inorg. Chim. Acta
2003, 350, 293.
(20) Shaver, M. P.; Thomson, R. K.; Patrick, B. O.; Fryzuk, M. D.
Can. J. Chem. 2003, 81, 1431.

Addition of Phosphines to Tantalum Hydrides
Organometallics, Vol. 24, No. 7, 2005 1421
Table 1. Selected Bond Distances (Å) and Angles (deg) for (PhPh[NPN]Ta)2(µ-H)3(µ-PCy2), 3a
length
angle
angle
Ta(1)-N(2)
Ta(1)-N(1)
Ta(1)-P(3)
Ta(1)-Ta(2)
Ta(1)-P(1)
Ta(2)-N(3)
Ta(2)-N(4)
Ta(2)-P(3)
Ta(2)-P(2)
P(3)-C(55)
P(3)-C(49)
P(1)-C(25)
P(2)-C(43)
2.160(4)
N(2)-Ta(1)-N(1)
N(2)-Ta(1)-P(3)
N(1)-Ta(1)-P(3)
N(2)-Ta(1)-Ta(2)
N(1)-Ta(1)-Ta(2)
P(3)-Ta(1)-Ta(2)
N(2)-Ta(1)-P(1)
N(1)-Ta(1)-P(1)
P(3)-Ta(1)-P(1)
Ta(2)-Ta(1)-P(1)
N(3)-Ta(2)-N(4)
N(3)-Ta(2)--P(3)
N(4)-Ta(2)-P(3)
115.00(14)
118.46(11)
86.91(10)
119.40(10)
119.40(10)
58.24(3)
83.65(11)
155.88(4)
74.10(10)
118.74(3)
115.72(15)
117.30(11)
123.58(11)
N(3)-Ta(2)-Ta(1)
N(4)-Ta(2)-Ta(1)
P(3)-Ta(2)-Ta(1)
N(3)-Ta(2)-P(2)
N(4)-Ta(2)-P(2)
P(3)-Ta(2)-P(2)
Ta(1)-Ta(2)-P(2)
C(55)-P(3)-C(49)
C(55)-P(3)-Ta(2)
C(49)-P(3)-Ta(2)
C(55)-P(3)-Ta(1)
C(49)-P(3)-Ta(1)
Ta(2)-P(3)-Ta(1)
115.41(10)
110.73(10)
60.57(3)
2.236(4)
2.5007(13)
2.5157(9)
2.6031(14)
2.114(4)
85.36(11)
79.60(11)
86.68(5)
2.185(4)
146.30(3)
107.3(2)
114.62(14)
120.06(16)
124.52(15)
122.26(16)
61.19(4)
2.4413(14)
2.5786(13)
1.925(5)
1.967(5)
1.790(5)
1.852(5)
Table 2. Crystallographic Data and Details of Refinement
PhPh[NPN]Ta)2(µ-H)3(µ-PCy2), 3a CyPh[NPN]Ta)2(µ-H)2(µ-PAd), 6b
(
(
empirical formula
C
60H84N4Si4P3Ta2
C58H91N4Si4P3Ta2
fw
1443.6
1411.5
cryst syst
space group
a-c (Å)
triclinic
P1h
monoclinic
P21/c
13.6335(27), 14.5644(29), 15.7177(31)
17.0490(7), 14.5931(5), 25.6519(9)
R,â,γ (deg)
91.361(30), 90.161(30), 94.016(30)
90.0, 97.550(1), 90.0
V, Å3
3112.39(20)
2
6326.81(9)
4
Z
Dcalc, g cm-3
µ(Mo KR), cm-1
T, K
1.54
1.48
3.708
173 ( 1
60.2
3.646
173 ( 1
55.8
2θ range (deg)
total no. of reflns
no. of unique reflns
no. of params
13 906
12 856
678
0.0368
0.0872
1.221
56 050
13 902
656
0.037
0.067
1.173
a
R1 a
Rw
goodness-of-fit
2
2
2 2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) ∑w(|Fo | - |Fc |)2/∑w|Fo | )1/2
.
Interestingly, the phosphide chemical shift has a
strong relationship to the nature of the [NPN] ligand
phosphine. Employing an alkylphosphine in the ligand
backbone results in an upfield shift of 15 and 21 ppm
for the additions of HPCy2 and HPPh2, respectively.
This upfield shift can be ascribed to the better σ-donor
properties of the cyclohexyl-substituted system, which
results in a more electron-rich tantalum center to
interact with the bridging phosphide.
Attempts were made to detect intermediates in the
reaction of the HPR2 with the tetrahydride. However,
the reaction appears to be faster than spectra could be
collected. Spectra collected directly after addition of
HPCy2 to an NMR tube containing a solution of 1a
(approximately 5 min reaction time) showed no peaks
for the starting tetrahydride complex, and no intermedi-
ates or decomposition products were detected. This
phosphido product is also quite stable, as evidenced by
the fact that only after thermolysis of a toluene solution
of 3a at 80 °C for 48 h does it slowly decompose, as
evidenced by the formation of the protonated ligand
precursor, PhPh[NPN]H2, as monitored by 31P NMR
spectroscopy.
Figure 1. ORTEP drawing (spheroids at 50% probability)
of (PhPh[NPN]Ta)2(µ-H)3(µ-PCy2), 3a. Silylmethyl, phenyl,
and cyclohexyl ring carbons other than ipso and methine
carbons have been omitted for clarity. Hydrides H(1), H(2),
and H(3) were located in the diffraction pattern and refined
isotropically.
Large deep red crystals of 3a were grown from a
saturated pentane solution and analyzed by X-ray
crystallography. An ORTEP22 depiction of the solid-state
molecular structure of 3a as determined by X-ray
crystallography is shown in Figure 1. Relevant bond
lengths and angles are listed in Table 1, and crystal-
lographic data are located in Table 2. The molecular
structure unambiguously shows the bridging phosphide
P(3) bound between Ta(1) and Ta(2) at distances of
2.4413(14) and 2.5007(13) Å, respectively, which are
considerably shorter than the tantalum phosphine bond
lengths of 2.6031(14) Å (Ta(1)-P(1)) and 2.5786(13) Å
(Ta(2)-P(2)) as expected for a bridging phosphide. By
comparison, the interatomic distances between [NPN]
phosphine donors and the tantalum are slightly longer
in 3a than those found in the dinitrogen complex
(21) Boni, G.; Blacque, O.; Sauvageot, P.; Poujaud, N.; Moise, C.;
Kubicki, M. M. Polyhedron 2002, 21, 371.
(22) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.

1422 Organometallics, Vol. 24, No. 7, 2005
Shaver and Fryzuk
the 1H NMR spectrum, the observation of a single peak
at 10.9 ppm integrating to two equivalent hydrides
supports the synthesis of (PhPh[NPN]Ta)2(µ-H)2(µ-PCy),
5a. Further NMR studies and elemental analysis con-
firmed this formulation.
Table 3. Shortest Reported Ta-Ta Interatomic
Distances and Selected Comparative Examples
complex
d(Ta-Ta), Å
25
(Cl2(PMe3)2Ta)2(µ-H)4
2.511(2)
2.5157(9)
2.5359(4)
2.545(1)
2.617(2)
2.621(1)
3.024(2)
a
(
PhPh[NPN]Ta)2(µ-H)3(µ-PCy)2
26
((cu)(PMe3)2(H)Ta)2(µ-H)4
Similarly, addition of H2PCy to 1b produced
27
(Cl2(PMe3)2Ta)2(µ-H)2
(
CyPh[NPN]Ta)2(µ-H)2(µ-PCy), 5b. 31P NMR resonances
28
([P2N2]Ta)2(µ-H)4
29
(Cl2(PMe3)2Ta)2(µ-H)2(µ-Cl)2
at 16.6, 22.1, and 436.4 ppm confirmed the genera-
tion of a tantalum phosphinidene complex. The down-
field resonance appears as a doublet of doublets, cou-
pling strongly to one ligand phosphine and weakly
to the other. Two equivalent hydrides are observed in
24
((η5-C5H4Me)(PPh2)Ta)2(µ-PPh2)2
a This work.
(
PhPh[NPN]Ta)2(µ-η1:η2-N2)(µ-H)2, the latter displaying
1
the H NMR spectrum at 10.4 ppm. They appear as a
Ta-P bond distances of 2.573(5) and 2.596(5) Å. Atoms
H(1), H(2), and H(3) were located in the diffraction
pattern and refined isotropically. To help ascertain the
presence of the bridging hydrides, a second crystal-
lographic refinement was performed, where atoms H(1),
H(2), and H(3) were deleted from the solution, and the
program X-HYDEX23 was used to predict the location
and number of bridging hydrides. The two studies were
in agreement with spectroscopic data; three hydrides
bridge the two tantalum centers.
1:1:1:2:1:1:1 septet, coupling to each of the three phos-
phorus nuclei, giving an overlapping doublet of doublets
of doublets. The high solubility of these phosphinidene
complexes has prevented characterization by X-ray
crystallography. The synthesis of complexes 5a and 5b
is shown in eq 2.
The Ta(1)-Ta(2) distance in 3a is 2.5157(9) Å, which
is drastically different from the only other crystallo-
graphically characterized example of a dinuclear tan-
talum phosphide; the complex ((η5-CpMe)(PPh2)Ta)2(µ-
PPh2)2 has a Ta-Ta bond length of 3.024(2) Å.24 In fact,
3a contains one of the shortest reported Ta-Ta single
bond distances. The only comparable bond length is
found in (Cl2(PMe3)2Ta)2(µ-H)4, with a Ta-Ta bond
distance of 2.511(2) Å. Interestingly, crystallographically
characterized systems containing the shortest Ta-Ta
interatomic distances (Table 3) all contain bridging
hydrides. Smaller bridging ligands help bind the two
metal centers close together (e.g., halogen-bridged com-
plexes generally have longer Ta-Ta bonds than com-
parable hydrido-bridged complexes). Also of interest, the
complexes with short metal-metal interactions contain
at least two phosphine donors supporting the early-
metal centers.
Preparation of Tantalum Phosphinidene Com-
plexes. The generation of phosphides from secondary
phosphines can be extended to the synthesis of phos-
phinidenes by using primary phosphines and a similar
methodology. Addition of cyclohexylphosphine (H2PCy)
to either 1a or 1b produced a similar purple to red color
change and gas evolution. Initially, it was suspected
that a bridging phosphide would form with a µ-PH(Cy)
linkage. 31P NMR studies, however, on the addition of
H2PCy to 1 showed three peaks at 10.9, 15.0, and 448.9
ppm. While the three peaks showed coupling patterns
similar to those observed for the tantalum phosphides,
the resonance at approximately 450 ppm suggested the
presence of a phosphinidene. The 31P chemical shifts for
the trans-bis(phosphinidene) tantalum complexes
[Cp*TaCl(µ-PR)]2 are at 437.6, 482.0, 403.9, and 390.3
Synthesis of phosphinidene tantalum complexes with
phenylphosphine proved more difficult. Addition of H2-
PPh to 1a or 1b resulted in protonation of the amide
ligand donors of the starting complex. The only isolable
products were the protonated ligands RPh[NPN]H2.
Adamantyl groups have long been used to provide a
spherical steric constraint to transition metal systems
in order to control reactivity and promote crystal-
linity.31-36 For this reason, adamantylphosphine (H2-
PAd) was added to solutions of 1a and 1b. These
reactions gave the corresponding phosphinidene com-
(28) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics
2000, 19, 3931.
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1982, 21, 4179.
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2004, 23, 498.
t
ppm for R ) Cy, Bu, Ph, and Mes, respectively.24,30 In
(23) Orpen, A. G. J. Chem. Soc., Dalton Trans. 1980, 2509.
(24) Abdul-Hadi, G. A.; Fromm, K.; Blaurock, S.; Jelonek, S.; Hey-
Hawkins, E. Polyhedron 1997, 16, 721.
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2591.
(25) Scioly, A. J.; Luetkens, M. L.; Wilson, R. B.; Huffman, J. C.;
Sattelberger, A. P. Polyhedron 1987, 6, 741.
(33) Blaurock, S.; Hey-Hawkins, E. Zeit. Anorg. Allg. Chem. 2002,
628, 2515.
(26) Profilet, R. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1992,
11, 1559.
(34) Tellini, V. H. S.; Jover, A.; Galantini, L.; Meijida, F.; Tato, J.
V. Acta Crystallogr. 2004, B60, 204.
(27) Wilson, R. B.; Sattelberger, A. P.; Huffman, J. C. J. Am. Chem.
Soc. 1982, 104, 858.
(35) Grubb, T. L.; Mathias, L. J. J. Polym. Sci. A.: Polym. Chem.
2000, 35, 1743.

Addition of Phosphines to Tantalum Hydrides
Organometallics, Vol. 24, No. 7, 2005 1423
Table 4. Selected Bond Distances (Å) and Angles (deg) for (CyPh[NPN]Ta)2(µ-H)2(µ-PAd), 6b
length
angle
angle
Ta(1)-N(2)
Ta(1)-N(1)
Ta(1)-P(3)
Ta(1)-Ta(2)
Ta(1)-P(1)
Ta(2)-N(3)
Ta(2)-N(4)
Ta(2)-P(3)
Ta(2)-P(2)
P(3)-C(49)
P(1)-C(13)
P(2)-C(31)
2.084(3)
2.085(3)
2.3394(9)
2.7532(2)
2.6479(9)
2.094(3)
2.080(3)
2.4030(9)
2.6069(9)
1.883(3)
1.853(4)
1.849(4)
N(2)-Ta(1)-N(1)
N(2)-Ta(1)-P(3)
N(1)-Ta(1)-P(3)
N(2)-Ta(1)-Ta(2)
N(1)-Ta(1)-Ta(2)
P(3)-Ta(1)-Ta(2)
N(2)-Ta(1)-P(1)
N(1)-Ta(1)-P(1)
P(3)-Ta(1)-P(1)
Ta(2)-Ta(1)-P(1)
N(3)-Ta(2)-N(4)
N(3)-Ta(2)-P(3)
111.38(12)
103.18(9)
101.38(8)
123.93(8)
122.93(8)
55.60(2)
N(4)-Ta(2)-P(3)
N(3)-Ta(2)-Ta(1)
N(4)-Ta(2)-Ta(1)
P(3)-Ta(2)-Ta(1)
N(3)-Ta(2)-P(2)
N(4)-Ta(2)-P(2)
P(3)-Ta(2)-P(2)
Ta(1)-Ta(2)-P(2)
Ta(2)-P(3)-C(49)
Ta(1)-P(3)-C(49)
Ta(2)-P(3)-Ta(1)
∑P(3) angles
117.14(9)
117.82(8)
114.65(8)
53.44(2)
78.75(9)
77.21(9)
95.33(3)
148.77(2)
140.41(11)
148.21(11)
70.96(3)
359.58
81.67(9)
82.56(8)
171.91(3)
116.33(2)
113.10(12)
126.57(9)
plexes (PhPh[NPN]Ta)2(µ-H)2(µ-PAd), 6a, and (CyPh[NPN]-
Ta)2(µ-H)2(µ-PAd), 6b, in 90% and 95% yield, respec-
tively (eq 2). Characterization of 6a and 6b by NMR
showed patterns similar to the reaction of cyclohexyl-
phosphine with tantalum hydrides. For each phosphin-
idene, three resonances were observed in 31P{1H}
spectra: 7.35, 10.23, and 474.26 ppm for 6a and 14.37,
19.80, and 468.35 ppm for 6b.
sum of angles Ta(1)-P(3)-Ta(2), Ta(1)-P(3)-C(48), and
Ta(2)-P(3)-C(48) is 359.59°. The molecular structure
clearly supports the assignment of a phosphinidene
bridge.
The tantalum bonds to ligand phosphines P(1) and
P(2) have lengthened again to 2.6069(9) and 2.6479(9)
Å, respectively. The metal-metal bond has also length-
ened considerably, from 2.5157(9) Å in 3a to 2.7532(2)
Å in 6b. Presumably, eliminating one bridging hydride
from the system also removes some of the geometric
constraints that induced such a short Ta-Ta inter-
atomic distance in structure 3a. Hydride locations were
again confirmed by two methods: isotropic refinement
of diffraction pattern locations and using X-HYDEX.23
Structures 3a and 6b are rare examples of structurally
characterized dinuclear tantalum complexes bridged by
a single phosphide or phosphinidene.
Mechanistic Studies. Addition of primary and
secondary phosphines to the tantalum hydrides 1a and
1b clearly demonstrates that these complexes can
promote reactivity in addition to dinitrogen activation.
The initial premise behind the H-P additions focused
on exploiting the reactivity of the putative TadTa
double bond in (RR′[NPN]Ta)2(µ-H)2, formed as a reac-
tive, but undetected intermediate. To support this idea,
the mechanism of P-H addition in the synthesis of the
tantalum phosphide and phosphinidene complexes was
examined by analysis of product distributions using
isotopic labeling studies on the addition of HnPR3-n (n
) 1, 2) to (CyPh[NPN]Ta)2(µ-H)4. The deuterated isoto-
pomer (CyPh[NPN]Ta)2(µ-D)4, d4-1b, was prepared by
modifying the synthesis of 1b, employing D2 gas instead
of H2 gas. Spectroscopic characterization of deuteride
d4-1b showed little deviation from spectra for the
corresponding hydride complex, except the disappear-
ance of hydride resonances at 9.9 ppm. As will be seen,
reaction pathways can be supported by integration of
the hydride region in 1H NMR spectra following reaction
of phosphines with the tetradeuteride complex d4-1b.
Any observed hydrides must originate from the primary
or secondary phosphines, and the presence or absence
of these peaks can indicate whether D2, H2, or HD is
lost during the reaction. This assumes that the isotope
scrambling from the ancillary substituents does not
occur. Cursory experiments using the analogous phenyl-
substituted phosphine system d4-1a showed identical
patterns; one example is explicitly mentioned below.
However, the main discussion focuses on the cyclohexyl-
substituted phosphine system.
Figure 2. ORTEP drawing (spheroids at 50% probability)
of (CyPh[NPN]Ta)2(µ-H)2(µ-PAd), 6b. Silylmethyl, phenyl,
and cyclohexyl ring carbons other than ipso and methine
carbons have been omitted for clarity. Hydrides H(1) and
H(2) were located in the diffraction pattern and refined
isotropically.
As was observed in phosphide systems, the choice of
phosphine in the [NPN] ancillary ligand produces a
marked effect in phosphinidene chemical shift. Systems
stabilized by the CyPh[NPN] ligand appear upfield by 12
ppm for the addition of H2PCy to the tantalum hydrides.
The effect is somewhat muted for adamantyl phos-
phine: an upfield shift of 6 ppm is observed.
Dark red crystals of 6b were grown from pentane
solutions and were studied by X-ray crystallography. An
ORTEP22 depiction of the solid-state molecular structure
of 6b as determined by X-ray crystallography is shown
in Figure 2. Relevant bond lengths and angles are listed
in Table 3, and crystallographic data are located in
Table 2. The bridging phosphinidene P(3) has Ta(1)-
P(3) and Ta(2)-P(3) bond lengths of 2.3394(9) and
2.4030(9) Å, respectively, shortened significantly from
the phosphide derivative. P(3) is also quite planar; the
(36) Gessmann, R.; Petratos, K.; Hanodrakas, S. J.; Tsitsa, P.;
Antoniadou-Vyza, E. Acta Crystallogr. 1992, C48, 347.

1424 Organometallics, Vol. 24, No. 7, 2005
Shaver and Fryzuk
Scheme 2
Scheme 3
Two mechanisms are proposed for the addition of
dicyclohexylphosphine to 1a or 1b that differ only in
the step where H2 or D2 is eliminated. In mechanism
A, P-H addition across the Ta-Ta single bond in d4-
1b would give a phosphide-monohydride-tetradeuteride
intermediate, as shown in Scheme 2. Loss of either D2
or HD from this species would generate the final
product. Statistically, there are 10 different eliminations
possible, forming two isotopomers; six of these would
eliminate D2 and leave H in the resultant product. In
other words, there is a 60% possibility that D2 will be
lost instead of HD. This analysis assumes that the re-
addition of D2 or HD does not occur.
Mechanism B proposes that D2 loss precedes P-H
addition. Loss of D2 followed by P-H addition generates
the final product. Thus, only D2 would be lost, forming
a dideuteride intermediate with a Ta-Ta double bond.
P-H addition would give a single isotopomer: the
hydride from the phosphine addition would appear in
the final product 100% of the time.
Specifically not considered is the prior association of
the primary or secondary phosphine with the tetradeu-
teride d4-1b or with dideuteride d2-3 as a neutral donor.
While such adducts are likely, our isotopic labeling
experiments do not provide any evidence of their
participation. However, there is some precedent for such
a species; sodium-mercury amalgam reduction of the
complex (Cl2(PMe3)2Ta)2(µ-H)2(µ-Cl)2 gives (Cl2(PMe3)2-
Ta)2(µ-H)2, a dihydride dinuclear Ta(III) complex with
a metal-metal double bond. This species can be hydro-
genated to generate (Cl2(PMe3)2Ta)2(µ-H)4, a complex
isoelectronic to 1.25
nism B. Loss of D2 generates a TadTa double bond,
which is the reactive intermediate in this hydrophos-
phination reaction.
To employ complex d4-1b to make mechanistic con-
clusions based on observed product distributions, the
presence of a kinetic isotope effect in the hydrophos-
phination reactions was examined. To accomplish this,
the following experiment shown in Scheme 3 was
devised: addition of 1 equiv of Cy2PH to (CyPh[NPN]Ta)2-
(µ-H)4, 1b, and (CyPh[NPN]Ta)2(µ-D)4, d4-1b, would give
respectively
CyPh[NPN]Ta)2(µ-D)2(µ-H)(µ-PCy2), d2-3b. This competi-
(
CyPh[NPN]Ta)2(µ-H)3(µ-PCy2), 3b, and
(
tion experiment will determine if the tetrahydride and
its deuterated isotopomer react at different rates and
thus exhibit a measurable kinetic isotope effect. Equimo-
lar amounts of 1b and d4-1b were added to an NMR
tube containing ferrocene as an internal standard, a
solution of HPCy2 in C6D6 was subsequently added via
syringe through a rubber septum, and the reaction was
monitored by NMR spectroscopy. The reaction to form
3b and d2-3b is much faster than any isotopic exchange
reactions between 1b and d4-1b.
Integration of the hydride resonances at δ 8.67 and
10.47 against the ferrocene internal standard totals
2.07H. With simple algebra, one can now solve for the
ratio of 3b and d2-3b produced during the reaction. If x
is the amount of 3b that has formed and y is the amount
of d2-3b formed, then the sum of the hydride integra-
tions must be 3x + y. As only 1 equiv of phosphine was
added into the mixture, x + y ) 1. Solving for x and y
reveals the products formed in 53.5% and 46.5% yield,
respectively. If complexes 1b and d4-1b reacted at the
same rate, a 50/50 ratio of products should be observed.
While a minor kinetic isotope effect is observed, the
slight difference does not account for the quantitative
transfer of hydride to form phosphides in the previous
experiment. With confidence, one can now propose that
the formation of tantalum phosphides proceeds through
a dideuteride complex, as shown in mechanism B.
Indirectly, the observation of a small isotope effect in
this competition experiment suggests that the rate-
determining step occurs prior to P-H addition.
Experimentally, addition of Cy2PH to a C6D6 solution
of d4-1b in an NMR tube fitted with a rubber septum
allows for monitoring of this reaction by NMR spectros-
copy. The observed spectra are similar to 3b, except for
the hydride region. Careful integration of the two
hydride resonances supports the formation of a mono-
hydride-dideuteride complex. Relative to a silylmethyl
proton peak (3H), the peak at δ 8.67 integrates to 0.65H
and the peak at δ 10.47 integrates to 0.33H, totaling
0.98H. Similar results were observed for the addition
of Cy2PH to d4-1a (δ 8.56, 0.63H; δ 10.35, 0.33H). In
both cases, the hydride from P-H addition is not lost
as HD; the transfer is quantitative. The formation of
Generation of phosphinidene tantalum complexes
presents another mechanistic question. For the addition
of a primary phosphine to d4-1b, the first step can be
(
CyPh[NPN]Ta)2(H)(D)2(PCy2), d2-3b, supports mecha-

Addition of Phosphines to Tantalum Hydrides
Organometallics, Vol. 24, No. 7, 2005 1425
Scheme 4
assumed to proceed analogously to mechanism B, again
presuming readdition of H2, HD, or D2 does not occur.
From this monohydride-dideuteride tantalum complex,
there are again two possible pathways to tantalum
phosphinidenes. The addition of the remaining P-H
bond across the latent Ta-Ta single bond would form
an intermediate complex with two hydrides, two deu-
terides, and a bridging phosphinidene. Elimination of
H2, HD, or D2 from this complex would generate the
observed tantalum phosphinidene. This potential mech-
anism is shown in Scheme 4 as mechanism C. In this
case, the potential elimination of H2 (16.7%), HD
(66.7%), and D2 (16.7%) would generate three isoto-
pomers. On average, one hydride would remain in the
final product. If mechanism C is in action, a monohy-
dride-monodeuteride isotopic composition, (RR′[NPN]-
Ta)2(µ-H)(µ-D)(µ-PCy), would be observed.
composition of (CyPh[NPN]Ta)2(µ-H)0.67(µ-D)1.33(µ-PCy),
dx-6b. Similarily, addition of CyPH2 to d4-1a supports
these results (δ 10.47, 0.69H). If the product contains
only two-thirds of a hydride on average, mechanism D
may be operating, presuming a negligible kinetic isotope
effect. The concerted 1,2-H2 elimination from Ta-H and
P-H bonds generates the phosphinidene complex.
These experimental results rule out another mecha-
nism not explicitly detailed in Scheme 4. After phos-
phide formation, if release of HD or D2 occurs prior to
P-H addition, then one would predict a mixture of
(
CyPh[NPN]Ta)2(µ-H)(µ-D)(µ-PCy), d1-6b, and (CyPh[NPN]-
Ta)2(µ-H)2(µ-PCy), 6b, with hydride resonances inte-
grating to 1.5H, clearly at odds with the observed
formation of products with 0.67 ( 0.02H.
Concluding Remarks
Alternatively, 1,2-H2 elimination could happen prior
to P-H addition, as shown by mechanism D in Scheme
4. The P-H bond and a tantalum hydride or deuteride
could eliminate in a concerted fashion to give the
observed product. In this case, only loss of H2 or HD is
possible and would lead to the formation of two isoto-
pomers with an average isotopic composition of 0.67 H
and 1.33 D. Integration of the hydride region would
show 0.67 H if mechanism D was operating. This
mechanism has some support in the literature: a 1,2-
H2 elimination has been proposed for the decomposition
of (silox)3TaH(PHPh) to (silox)3TadPPh.37 In this case,
only 1,2-H2 elimination could occur, as the Ta(V)
complex cannot be further oxidized. This would preclude
the P-H oxidative addition step proposed in mechanism
C.
Reactions of primary and secondary phosphines with
[NPN]-supported dinuclear tantalum tetrahydride com-
plexes readily access bridging phosphido and phosphin-
idene compounds via P-H bond activation. The forma-
tion of the phosphide-trihydride derivatives ([NPN]Ta)2(µ-
H)3(µ-PR2) is consistent with initial loss of H2 from the
tetrahydride ([NPN]Ta)2(µ-H)4 via reductive elimina-
tion to generate the unobserved TadTa intermediate,
([NPN]Ta)2(µ-H)2; this is followed by oxidative addition
of a P-H bond across the TadTa double bond. In the
case of primary phosphines, phosphinidene complexes
([NPN]Ta)2(µ-H)2(µ-PR) are generated by a 1,2-H2 elimi-
nation following formation of the phosphide-trihydride
intermediate. By probing these reactions using isotopic
labeling studies, information on the intimate details of
these processes has been obtained.
Experimentally, addition of H2PCy to d4-1b is analo-
gous to the previous isotopic labeling studies. In this
case, the spectrum shows only one peak in the hydride
region at δ 10.87 that integrates to 0.65H relative to
24 silylmethyl protons. The product formed in this
reaction is a mixture of two isotopomers with an average
Experimental Section
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
(37) Bonanno, J. B.; Wolczanski, P. T.; Lobkovsky, E. B. J. Am.
Chem. Soc. 1994, 116, 11159.

1426 Organometallics, Vol. 24, No. 7, 2005
Shaver and Fryzuk
and argon use 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.38 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
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 solublized 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. Selected
complexes in this paper do not have elemental analysis data
due to problems with sample handling and the high air- and
nitrogen-sensitivity of these complexes. Complexes (PhPh[NPN]-
Ta)2(µ-H)4,18 TaCl2Me3,39 CyPh[NPN]Li2(OEt2),19 H2PPh,40 and
H2PAd41,42 were prepared by literature procedures. H2PCy was
purchased from Strem and distilled under N2 prior to use. All
other reagents were purchased from Aldrich and used as
supplied.
Preparation of CyPh[NPN]TaMe3. A solution of TaMe3Cl2
(5.00 g, 16.84 mmol) in 50 mL of Et2O was added dropwise to
a solution of CyPh[NPN]Li2(OEt2) (8.90 g, 16.84 mmol) in 800
mL of Et2O at -78 °C. The solution was then warmed to room
temperature and a white precipitate appeared. The solution
was evaporated to dryness, and the remaining solids were
extracted into 50 mL of toluene and filtered through Celite.
The solvent was removed, and the solids were washed with
pentane to afford a yellow solid, CyPh[NPN]TaMe3, in 85% yield
(9.54 g, 14.31 mmol). 1H NMR (C6D6, 25 °C, 500 MHz): δ -0.22
(s, 6H, SiCH3), 0.01 (s, 6H, SiCH3), 0.61 (s, 9H, TaCH3), 0.63-
1.75 (m, 15H, C6H11, PCH2Si), 6.65-6.80 (m, 6H, NPh), 6.95-
7.05 (m, 4H, NPh). 31P{1H} NMR (C6D6, 25 °C, 202.5 MHz): δ
16.78 (s). Anal. Calcd for C27H46N2PSi2Ta: C, 48.64; H, 6.95;
N, 4.20. Found: C, 48.45; H, 6.67; N, 4.60.
Preparation of (CyPh[NPN]Ta)2(µ-H)4, 1b. A solution of
CyPh[NPN]TaMe3 (2.50 g, 3.75 mmol) in 50 mL of Et2O was
transferred into a 200 mL thick-walled glass vessel equipped
with a Kontes valve and thoroughly degassed via three freeze-
pump-thaw cycles. The vessel was cooled in liquid nitrogen
and H2 gas added. The vessel was sealed, warmed to room
temperature, and stirred for 24 h, over which time the solution
turned to deep purple. The solvent was removed in vacuo, and
the product extracted into degassed pentane and filtered
through Celite. Removal of pentane affords (CyPh[NPN]Ta)2(µ-
H)4 in 97% yield (2.27 g, 1.82 mmol). The product was highly
nitrogen-sensitive in solution, but is stable under nitrogen in
solid form at -35 °C. 1H NMR (C6D6, 25 °C, 500 MHz): δ 0.18
(s, 12H, SiCH3), 0.24 (s, 12H, SiCH3), 0.35-1.90 (m, 30H,
C6H11, PCH2Si), 6.96 (s, 4H, NPh), 7.18 (s, 8H, NPh), 7.36 (s,
8H, NPh), 9.90 (s, 4H, TaHTa). 31P{1H} NMR (C6D6, 25 °C,
202.5 MHz): δ 29.20 (s). Anal. Calcd for C27H46N2PSi2Ta: C,
46.22; H, 6.30; N, 4.49. Found: C, 45.89; H, 6.20; N, 4.34.
Preparation of (PhPh[NPN]Ta)2(µ-H)3(µ-PCy2), 3a. The
following procedure is representative of the synthesis of
compounds 3 and 4. A solution of (PhPh[NPN]Ta)2(µ-H)4 (500
mg, 0.405 mmol) in 20 mL of Et2O under Ar was prepared in
a vessel equipped with a Kontes valve. Dicyclohexylphosphine
(80.3 mg, 0.405 mmol) was added to the flask via microsyringe,
the vessel was sealed, and its contents were stirred for 30 min.
A color change from deep purple to red was observed, along
with the evolution of a gas. Removal of solvent in vacuo left a
red, oily solid. This solid was triturated with pentane and dried
under vacuum to afford (PhPh[NPN]Ta)2(µ-H)3(µ-PCy2) as a red
powder in 95% yield (551 mg). Crystals of 3a suitable for X-ray
diffraction were grown from slow evaporation of a concentrated
solution of the phosphide in layered benzene/HMDS. 1H NMR
(C6D6, 25 °C, 500 MHz): δ -0.21 (s, 3H, SiCH3), -0.02 (s, 3H,
SiCH3), 0.07 (s, 3H, SiCH3), 0.17 (s, 3H, SiCH3), 0.23 (s, 3H,
SiCH3), 0.31 (s, 3H, SiCH3), 0.88-2.65 (m, 30H, C6H11, PCH2-
Si), 6.65-7.62 (m, 30H, NPh, PPh), 8.67 (m, 2H, TaHTa), 10.47
(m, 1H, TaHTa). 31P{1H} NMR (C6D6, 25 °C, 202.5 MHz): δ
10.47 (d, 2JPP ) 12 Hz) 10.53 (d, 2JPP ) 83 Hz), 164.38 (d of d,
2JPP ) 12 Hz, 83 Hz). Anal. Calcd for C60H87N4P3Si4Ta2: C,
50.34; H, 6.13; N, 3.91. Found: C, 50.09; H, 6.20; N, 4.34.
Preparation of (CyPh[NPN]Ta)2(µ-H)3(µ-PCy2), 3b. This
reaction was conducted analogously to the synthesis of 3a.
1
1
(
CyPh[NPN]Ta)2(µ-H)4 (500 mg, 0.401 mmol) and dicyclohexyl-
phosphine (79.49 mg, 0.401 mmol) were employed to afford
CyPh[NPN]Ta)2(µ-H)3(µ-PCy2) in 84% yield (486 mg). 1H NMR
(
(C6D6, 25 °C, 500 MHz): δ -0.02 (s, 3H, SiCH3), 0.05 (s, 6H,
SiCH3), 0.15 (s, 6H, SiCH3), 0.26 (s, 3H, SiCH3), 0.70-2.38 (m,
52H, 4 C6H11, 4 PCH2Si), 6.90-7.50 (m, 20H, NPh), 8.56 (m,
2H, TaHTa), 10.35 (m, 1H, TaHTa). 31P{1H} NMR (C6D6, 25
°C, 202.5 MHz): δ 16.28 (d, JPP ) 4 Hz), 18.66 (d, JPP ) 86
Hz), 149.02 (d of d, JPP ) 4 Hz, 86 Hz). Anal. Calcd for
2
2
2
C60H99N4P3Si4Ta2: C, 49.92; H, 6.91; N, 3.88. Found: C, 49.81;
H, 6.67; N, 4.24.
Preparation of (PhPh[NPN]Ta)2(µ-H)3(µ-PPh2), 4a. This
reaction was conducted analogously to the synthesis of 3a.
(
PhPh[NPN]Ta)2(µ-H)4 (500 mg, 0.405 mmol) and diphenylphos-
phine (75.41 mg, 0.405 mmol) were employed to afford
(
PhPh[NPN]Ta)2(µ-H)3(µ-PPh2) in 94% yield (542 mg). 1H NMR
(C6D6, 25 °C, 500 MHz): δ -0.12 (s, 3H, SiCH3), -0.08 (s, 3H,
SiCH3), 0.01 (s, 9H, SiCH3), 0.28 (s, 9H, SiCH3), 0.34 (s, 3H,
SiCH3), 0.37 (s, 3H, SiCH3), 1.13-1.45 (m, 8H, PCH2Si), 6.48,
6.60, 6.85, 6.96, 7.10, 7.38, 7.68, 7.75, 7.80 (m, 40H, NPh, PPh,
PPh2) 9.31 (m, 2H, TaHTa), 10.46 (br, 1H, TaHTa). 31P{1H}
2
NMR (C6D6, 25 °C, 202.5 MHz): δ 13.10 (d, JPP ) 87 Hz),
2
2
14.05 (d, JPP ) 12 Hz), 136.27 (d of d, JPP ) 12 Hz, 87 Hz).
Preparation of (CyPh[NPN]Ta)2(µ-H)3(µ-PPh2), 4b. This
reaction was conducted analogously to the synthesis of 3a.
(
CyPh[NPN]Ta)2(µ-H)4 (500. mg, 0.401 mmol) and diphenylphos-
phine (74.6 mg, 0.401 mmol) were employed to afford
(
CyPh[NPN]Ta)2(µ-H)3(µ-PPh2) in 96% yield (554 mg). 1H NMR
(C6D6, 25 °C, 500 MHz): δ 0.08 (s, 9H, SiCH3), 0.15 (s, 3H,
SiCH3), 0.21 (s, 6H, SiCH3), 0.34 (s, 3H, SiCH3), 0.70-1.80 (m,
30H, C6H11, PCH2Si), 6.80, 7.00, 7.05, 7.18, 7.35, 7.52 (m, 30H,
NPh, PPh), 9.15 (m, 2H, TaHTa), 10.20 (s, 1H, TaHTa). 31P-
{1H} NMR (C6D6, 25 °C, 202.5 MHz): δ 19.38 (d, 2JPP ) 2 Hz),
2
2
21.41 (d, JPP ) 89 Hz), 115.39 (d of d, JPP ) 2 Hz, 89 Hz).
Anal. Calcd for C60H87N4P3Si4Ta2: C, 50.34; H, 6.13; N, 3.91.
Found: C, 50.00; H, 5.95; N, 4.24.
(38) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(39) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389.
(40) Baudler, M.; Zarkdas, A. Chem. Ber. 1965, 104, 1034.
(41) Ohashi, A.; Matsukawa, S.; Imamoto, T. Heterocycles 2000, 52,
905.
Preparation of (PhPh[NPN]Ta)2(µ-H)2(µ-PCy), 5a. The
following procedure is representative of the synthesis of
compounds 5 and 6. Cyclohexylphosphine (47.01 mg, 0.405
(42) Setter, H.; Last, W. D. Chem. Ber. 1969, 102, 3364.

Addition of Phosphines to Tantalum Hydrides
Organometallics, Vol. 24, No. 7, 2005 1427
mmol) was added under Ar via microsyringe to a solution of
0.705 mmol). The product is highly nitrogen-sensitive in
solution, but is stable under nitrogen in solid form at -35 °C.
1H NMR (C6D6, 25 °C, 500 MHz): δ 0.18 (s, 12H, SiCH3), 0.24
(s, 12H, SiCH3), 0.37-1.89 (m, 30H, C6H11, PCH2Si), 6.95 (s,
4H, NPh), 7.17 (s, 8H, NPh), 7.35 (s, 8H, NPh). 31P{1H} NMR
(C6D6, 25 °C, 202.5 MHz): δ 29.30 (s).
(
PhPh[NPN]Ta)2(µ-H)4 (500.0 mg, 0.405 mmol) in 20 mL of Et2O
prepared in a vessel equipped with a Kontes valve. An
immediate color change from purple to red was observed along
with concomitant gas evolution. The solution was stirred for
another 30 min to ensure complete reaction, and the solvent
was removed. The resulting red solid was dissolved in pentane
and then dried under vacuum to afford (PhPh[NPN]Ta)2(µ-H)2-
Preparation of (CyPh[NPN]Ta)2(µ-D)2(µ-H)(µ-PCy2), d2-
3b. A solution of (PhPh[NPN]Ta)2(µ-D)4 (100 mg, 0.081 mmol)
in degassed C6D6 (1 mL) was prepared in a Wilmad 5 mm NMR
tube fitted with a rubber septum. To this solution, dicyclo-
hexylphosphine (16.0 mg, 0.081 mmol) in degassed C6D6 (1 mL)
was added via microsyringe through the septum. Mixing of
the solutions promoted a color change from purple to red, and
the reaction was then monitored by 1H and 31P NMR spec-
1
(µ-PCy) (89%, 487 mg). H NMR (C6D6, 25 °C, 500 MHz): δ
-0.08 (s, 6H, SiCH3), 0.15 (s, 6H, SiCH3), 0.20 (s, 6H, SiCH3),
0.22 (s, 6H, SiCH3), 0.96-1.70 (m, 19H, C6H11, PCH2Si), 6.82,
6.89, 7.05, 7.19, 7.25, 7.32, 7.51, 7.67 (m, 30H, NPh, PPh),
10.87 (m, 2H, TaHTa). 31P{1H} NMR (C6D6, 25 °C, 202.5
2
2
MHz): δ 10.74 (d, JPP ) 152 Hz), 15.01 (d, JPP ) 17 Hz),
2
1
449.0 (d of d, JPP ) 17 Hz, 152 Hz).
troscopy. H NMR (C6D6, 25 °C, 500 MHz): δ -0.21, -0.02,
Preparation of (CyPh[NPN]Ta)2(µ-H)2(µ-PCy), 5b. This
reaction was conducted analogously to the synthesis of 5a.
Cyclohexylphosphine (46.6 mg, 0.401 mmol) and (CyPh[NPN]-
Ta)2(µ-H)4 (500 mg, 0.401 mmol) were employed to afford
0.07, 0.17, 0.23, 0.31 (s, 24H, SiCH3’s), 0.88-2.65 (m, 30H,
C6H11, PCH2Si), 6.65-7.62 (m, 30H, NPh, PPh), 8.67 (m,
0.65H, TaHTa), 10.47 (m, 0.33H, TaHTa). 31P{1H} NMR (C6D6,
25 °C, 202.5 MHz): δ 10.47 (d, 2JPP ) 12 Hz), 10.53 (d, 2JPP
)
2
(
CyPh[NPN]Ta)2(µ-H)2(µ-PCy) (93%, 508 mg). 1H NMR (C6D6,
83 Hz), 164.38 (d of d, JPP ) 12 Hz, 83 Hz). Similar results
25 °C, 500 MHz): δ -0.18 (s, 6H, SiCH3), 0.22 (s, 12H, SiCH3),
0.23 (s, 6H, SiCH3), 0.65-1.95 (m, 30H, C6H11, PCH2Si), 6.85
(m, 2H, NPh), 7.14 (m, 4H, NPh), 7.30 (m, 4H, NPh), 10.47
(m, 2H, TaHTa). 31P{1H} NMR (C6D6, 25 °C, 202.5 MHz): δ
were observed for the addition of dicyclohexylphosphine to
(
PhPh[NPN]Ta)2(µ-D)4. 1H NMR: δ 8.56 (m, 0.63H, TaHTa),
10.35 (m, 0.33H, TaHTa).
Details of Competition Experiment between 1b and
d4-1b. A solution of dicyclohexylphosphine (8.0 mg, 0.0405)
in C6D6 (2 mL) was added via syringe through a rubber septum
into an NMR tube containing an intimate mixture of
2
2
16.58 (d, JPP ) 154 Hz), 22.09 (d, JPP ) 18 Hz), 436.38 (d of
2
d, JPP ) 18 Hz, 154 Hz). Anal. Calcd for C54H87N4P3Si4Ta2:
C, 47.71; H, 6.45; N; 4.12. Found: C, 47.44; H, 6.29; N, 4.27.
Preparation of (PhPh[NPN]Ta)2(µ-H)2(µ-PAd), 6a. This
reaction was conducted analogously to the synthesis of 5a.
Adamantylphosphine (68.1 mg, 0.405 mmol) and (PhPh[NPN]-
Ta)2(µ-H)4 (500 mg, 0.405 mmol) were employed to afford
(
CyPh[NPN]Ta)2(µ-H)4 (50.5 mg, 0.0405 mmol), (CyPh[NPN]Ta)2-
(µ-D)4, (50.0 mg, 0.0405 mmol), and ferrocene (7.5 mg, 0.0405
1
mmol). The crude contents of the flask were analyzed by H-
{31P} NMR spectroscopy. 1H NMR: δ 8.67 (m, 1.38H, TaHTa),
10.47 (m, 0.69H, TaHTa).
(
PhPh[NPN]Ta)2(µ-H)2(µ-PAd) (88%, 499.9 mg). 1H NMR (C6D6,
25 °C, 500 MHz): δ -0.11 (s, 6H, SiCH3), 0.22 (s, 12H, SiCH3),
0.28 (s, 6H, SiCH3) 0.95 (s, 6H, PC(CH2)3), 1.29-1.57 (m, 8H,
PCH2), 1.68 (s, 6H, CHCH2CH), 1.75 (s, 3H, CH2CH(CH2)2),
6.86 (m, 8H, NPh), 7.09-7.40 (m, 14H, NPh, PPh), 7.77 (m,
4H, PPh), 10.93 (m, 2H, TaHTa). 31P{1H} NMR (C6D6, 25 °C,
Preparation of (CyPh[NPN]Ta)2(µ-H)0.67(µ-D)1.33(µ-PCy),
dx-6b. A solution of (CyPh[NPN]Ta)2(µ-D)4 (100 mg, 0.081 mmol)
in degassed C6D6 (1 mL) was prepared in a Wilmad 5 mm NMR
tube fitted with a rubber septum. To this solution, cyclohexyl-
phosphine (9.4 mg, 0.081 mmol) in degassed C6D6 (1 mL) was
added via microsyringe through the septum. Mixing of the
solutions promoted a color change from purple to red, and the
reaction was then monitored by 1H and 31P NMR spectroscopy.
1H NMR (C6D6, 25 °C, 500 MHz): δ -0.08, 0.15, 0.20, 0.22 (s,
24H, SiCH3’s), 0.96-1.70 (m, 19H, C6H11, PCH2Si), 6.82-7.67
(m, 30H, NPh, PPh), 10.87 (m, 0.65H, TaHTa). 31P{1H} NMR
2
2
202.5 MHz): δ 7.35 (d, JPP ) 162 Hz), 10.23 (d, JPP ) 12
2
Hz), 474.26 (d of d, JPP ) 12 Hz, 162 Hz). Anal. Calcd for
C58H79N4P3Si4Ta2: C, 49.78; H, 5.69; N, 4.00. Found: C, 49.78;
H, 5.49; N, 4.14.
Preparation of (CyPh[NPN]Ta)2(µ-H)2(µ-PAd), 6b. This
reaction was conducted analogously to the synthesis of 5a.
Adamantylphosphine (67.5 mg, 0.401 mmol) and (CyPh[NPN]-
Ta)2(µ-H)4 (500 mg, 0.401 mmol) were employed to afford
2
(C6D6, 25 °C, 202.5 MHz): δ 10.74 (d, JPP ) 152 Hz), 15.01
(d, 2JPP ) 17 Hz), 449.0 (d of d, 2JPP ) 17 Hz, 152 Hz). Similar
results were observed for the addition of cyclohexylphosphine
to (PhPh[NPN]Ta)2(µ-D)4. 1H NMR: δ 10.47 (m, 0.69H, TaHTa).
(
CyPh[NPN]Ta)2(µ-H)2(µ-PAd) (97%, 549 mg). Crystals of 6b
suitable for X-ray diffraction were grown from slow evapora-
tion of a concentrated solution of 6b in pentane. 1H NMR
(C6D6, 25 °C, 500 MHz): δ -0.11 (s, 6H, SiCH3), 0.25 (s, 6H,
SiCH3), 0.30 (s, 6H, SiCH3), 0.31 (s, 6H, SiCH3) 1.08-1.88 (m,
30H, C6H11, PCH2Si), 1.39 (s, 6H, PC(CH2)3), 1.78 (s, 6H,
CHCH2CH), 2.06 (s, 3H, CH2CH(CH2)2), 6.87-6.91 (m, 4H,
NPh), 7.15-7.23 (m, 8H, NPh), 7.29-7.36 (m, 8H, NPh), 10.61
(m, 2H, TaHTa). 31P{1H} NMR (C6D6, 25 °C, 202.5 MHz): δ
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 X-ray data.
2
2
14.37 (d, JPP ) 160 Hz), 19.80 (s), 468.35 (d, JPP ) 160 Hz).
Preparation of (CyPh[NPN]Ta)2(µ-D)4, d4-1b. (CyPh[NPN]-
Ta)2(µ-D)4 is prepared analogously to 1b, except D2 gas is
added to the diethyl ether solution of CyPh[NPN]TaMe3 (1.00
g, 1.50 mmol), affording a bright purple solution after 24 h.
The solvent was removed in vacuo, and the product extracted
into degassed pentane and filtered through Celite. Removal
of pentane affords (CyPh[NPN]Ta)2(µ-D)4 in 94% yield (0.88 g,
Supporting Information Available: Information on X-
ray data collection and processing for 3a and 6b. X-ray
crystallographic data for 3a and 6b in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM0490076