Triple-bond reactivity of an AsP complex intermediate: Synthesis stemming from molecular arsenic, As4

Article abstract of DOI:10.1021/ja906550h

While P₄ is the stable molecular form of phosphorus, a recent study illustrated the possibility of P₂ generation for reactions in organic media under mild conditions. The heavier group 15 element arsenic can exist as As₄ molecules, but As₄ cannot be stored as a pure substance because it is both light-sensitive and reverts thermally to its stable, metallic gray form. Herein we report As4 activation giving rise to a μ-As₂ diniobium complex, serving in turn as precursor to a terminal arsenide anion complex of niobium. Functionalization of the latter provides the new AsPNMes* ligand, which when complexed with tungsten pentacarbonyl elicits extrusion of the (AsP)W(CO)₅ molecule as a reactive intermediate. Trapping reactions of the latter with organic dienes are found to furnish double Diels-Alder adducts in which the AsP unit is embedded in a polycyclic organic framework. Thermal generation of (AsP)W(CO)₅ in the presence of the neutral terminal phosphide complex P=Mo(N[ _iPr]Ar)₃ leads to the cyclo-AsP₂ complex (OC)₅W(cyclo-AsP₂)Mo(N[ⁱPr]Ar)₃. The (AsP)W(CO)₅ trapping products were crystallized and characterized by X-ray diffraction methods, and computational methods were applied for analysis of the As-As and As-P bonds in the complexes.

Full text of DOI:10.1021/ja906550h

Published on Web 10/20/2009
Triple-Bond Reactivity of an AsP Complex Intermediate:
Synthesis Stemming from Molecular Arsenic, As4
Heather A. Spinney, Nicholas A. Piro, and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139-4307
Received August 3, 2009; E-mail: ccummins@mit.edu
Abstract: While P4 is the stable molecular form of phosphorus, a recent study illustrated the possibility of
P2 generation for reactions in organic media under mild conditions. The heavier group 15 element arsenic
can exist as As4 molecules, but As4 cannot be stored as a pure substance because it is both light-sensitive
and reverts thermally to its stable, metallic gray form. Herein we report As4 activation giving rise to a µ-As2
diniobium complex, serving in turn as precursor to a terminal arsenide anion complex of niobium.
Functionalization of the latter provides the new AsPNMes* ligand, which when complexed with tungsten
pentacarbonyl elicits extrusion of the (AsP)W(CO)5 molecule as a reactive intermediate. Trapping reactions
of the latter with organic dienes are found to furnish double Diels-Alder adducts in which the AsP unit is
embedded in a polycyclic organic framework. Thermal generation of (AsP)W(CO)5 in the presence of the
neutral terminal phosphide complex PtMo(N[iPr]Ar)3 leads to the cyclo-AsP2 complex (OC)5W(cyclo-
AsP2)Mo(N[iPr]Ar)3. The (AsP)W(CO)5 trapping products were crystallized and characterized by X-ray
diffraction methods, and computational methods were applied for analysis of the As-As and As-P bonds
in the complexes.
) 3-6,8)7 and metal-arsenic clusters.8 Having prepared the first
complexes containing ligated cyclo-P3, Sacconi et al. reported
Introduction
Yellow arsenic (As4) is attractive as a soluble, molecular
source of this element for chemical synthesis in solution, but
its use is complicated because it readily polymerizes to the
metallic gray allotrope;1 As4 is photosensitive and thermally
unstable as a solid, but exhibits good thermal stability in solution
and in the gas phase. Bettendorff2 has been credited as the first
to report yellow arsenic,1 and an early discussion of arsenic
allotropes was provided by Linck.3 Stock and Siebert reported
a method of As4 generation using an electric arc,4 while
Erdmann and von Unruh diagrammed an As4 synthesis using
what is essentially a simple tube furnace.5 Scherer et al. have
utilized As4 solutions prepared in that manner6 in xylene or
decalin at reflux (140 or 170 °C, respectively) for reactions with
transition-metal carbonyls in syntheses of cyclo-Asn ligands (n
generation of As4 by the tube furnace method and its transfor-
mation to provide cyclo-As3 as a bridging ligand.9 West et al.
also described As4 activation by the disilene system
Mes2Si)SiMes2 in toluene solution.10 The foregoing is nearly
an exhaustive summary of the known solution-phase reaction
chemistry of the As4 molecule. Herein we add to this body of
knowledge that is derived from the tube furnace method of As4
generation.5
In prior work we showed that the niobaziridine-hydride
complex HNb(η2-tBuHCdNAr)(N[Np]Ar)2 (1, where Np )
t
CH2 Bu and Ar ) 3,5-C6H3Me2; Figure 1) activates P4 in organic
solvents at 20 °C, and that the product of this activation is a
µ-P2 diniobium complex.11 We went on to show that 2e
reduction of the µ-P2 diniobium complex elicited P-P bond
cleavage with formation of a terminal phosphide anion complex,
[PtNb(N[Np]Ar)3]-.12 The latter has proven to be quite
versatile for transfer of the phosphide phosphorus atom to
(1) Rodionov, A.; Kalendarev, R.; Eiduss, J.; Zhukovskii, Y. J. Mol. Struct.
1996, 380, 257–266.
(2) Bettendorff, A. Annalen Chem. Pharm. 1867, 144, 110–114.
(3) Linck, G. E. Chem. Ber. 1899, 32, 881–897.
(4) Stock, A.; Siebert, W. Chem. Ber. 1905, 38, 966–968.
(5) Erdmann, H.; von Unruh, M. Z. Anorg. Chem. 1902, 32, 437–452.
(6) Scherer, O. J.; Sitzmann, H.; Wolmersha¨user, G. J. Organomet. Chem.
1986, 309, 77–86.
(8) (a) Scherer, O. J.; Braun, J.; Wolmersha¨user, G. Chem. Ber. 1990,
123, 471–475. (b) Scherer, O. J.; Braun, J.; Walther, P.; Wolmer-
sha¨user, G. Chem. Ber. 1992, 125, 2661–2665. (c) Scherer, O. J.;
Keme´ny, G.; Wolmersha¨user, G. Chem. Ber. 1995, 128, 1145–1148.
(9) (a) Di Vaira, M.; Ghilardi, C. A.; Midollini, S.; Sacconi, L. J. Am.
Chem. Soc. 1978, 100, 2550–2551. (b) Di Vaira, M.; Midollini, S.;
Sacconi, L.; Zanobini, F. Angew. Chem., Int. Ed. Engl. 1978, 17, 676–
677.
(7) (a) Scherer, O. J.; Sitzmann, H.; Wolmersha¨user, G. Angew. Chem.,
Int. Ed. Engl. 1989, 28, 212–213. (b) Scherer, O. J.; Wiedemann, W.;
Wolmersha¨user, G. J. Organomet. Chem. 1989, 361, C11–C14. (c)
Scherer, O. J.; Vondung, J.; Wolmersha¨user, G. J. Organomet. Chem.
1989, 376, C35–C38. (d) Scherer, O. J.; Blath, C.; Wolmersha¨user,
G. J. Organomet. Chem. 1990, 387, C21–C24. (e) Scherer, O. J.;
Wiedemann, W.; Wolmersha¨user, G. Chem. Ber. 1990, 123, 3–6. (f)
Scherer, O. J.; Winter, R.; Heckmann, G.; Wolmersha¨user, G. Angew.
Chem., Int. Ed. Engl. 1991, 30, 850–852. (g) Scherer, O. J.; Blath,
C.; Heckmann, G.; Wolmersha¨user, G. J. Organomet. Chem. 1991,
409, C15–C18. (h) Scherer, O. J. Acc. Chem. Res. 1999, 32, 751–
762.
(10) (a) Tan, R. P.; Comerlato, N. M.; Powell, D. R.; West, R. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1217–1218. (b) Fanta, A. D.; Tan,
R. P.; Comerlato, N. M.; Driess, M.; Powell, D. R.; West, R. Inorg.
Chim. Acta 1992, 198-200, 733–739.
(11) Figueroa, J. S.; Cummins, C. C. J. Am. Chem. Soc. 2003, 125, 4020–
4021.
(12) Figueroa, J. S.; Cummins, C. C. Angew. Chem., Int. Ed. 2004, 43,
984–988.
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A R T I C L E S
Spinney et al.
Figure 1. Three-step synthesis of the η2-AsPNMes* complex 4 in a sequence encompassing As4 activation, µ-As2 ligand reductive cleavage, and terminal
arsenide functionalization with the [PNMes*]+ electrophile.
various electrophiles, resulting in interesting bound ligands or
free main-group species.13 Of particular note is the function-
alization of [PtNb(N[Np]Ar)3]- with Niecke’s chloroimino-
and plays an important role in the system.19 In contrast, the
homonuclear diatomic molecules P2 and As2 have negligible
concentration at these temperatures. An AsP solid solution phase
has also been described for the arsenic-phosphorus system under
the high-pressure conditions needed to suppress sublimation of
the elements near 600 °C.20 Vibrational signatures for the AsP
molecule were first observed using a microwave discharge
through a flowing mixture of AsCl3 and PCl3,21 and the molecule
has been generated more recently by laser ablation of arsenic
14
t
phosphane, ClPdNMes* (Mes* ) 2,4,6-C6H2 Bu3), leading
to the synthesis of a diphosphaazide complex of niobium, (η2-
PPNMes*)Nb(N[Np]Ar)3, which upon thermolysis rearranges
to the niobium(V) imido, (Mes*N)Nb(N[Np]Ar)3, with loss of
the P2 molecule.15 As represented in a series of papers, we have
gathered convincing evidence for the intermediacy of
the free P2 molecule as a reactive transient engaging in Diels-
Alder reactions with organic dienes and undergoing addition
to terminal phosphide MtP triple bonds to yield cyclo-P3
complexes.15,16 Similar triple-bond reactivity has likewise been
exhibited by the tungsten pentacarbonyl complex of P2, having
the formula (P2)W(CO)5, a species also proposed to be accessible
as a reactive transient intermediate.15-17 Accordingly, a primary
aim of the present work was to investigate the As4 activation
chemistry of 1 in search of parallels with the prior line of inquiry
based on P4 activation.
in the presence of PH3.22 AsP has a measured dissociation
23
energy of 429.7 ( 12.6 kJ mol-1
,
and an equilibrium bond
distance of 1.9995440(2) Å.22 Bimetallic complexes containing
bridging AsP ligands have been synthesized by treating the salts
[Li][M2Cp2(CO)4(µ-PH2)] (M ) Mo, W) with an equivalent of
AsCl3;24 however, no investigations of AsP triple-bond reactivity
under preferred conditions for solution-phase organic reactions
(25 °C, 1 atm) have been described.
Results and Discussion
A second aim of the present work was to discover if our
established scheme for P2 generation/transfer could be extended
to provide access to the diatomic molecule AsP, either in free
form or else stabilized as its tungsten pentacarbonyl complex.
In the gas-phase phosphorus-arsenic system at 600-850 °C, it
has been shown that the major species present are the tetrahedral
molecules AsnP4-n (n ) 0 - 4; a selective, low-temperature route
to AsP3 was reported recently),18 but that the diatomic molecule
AsP is also present as a significant percentage of the mixture
Reaction of Niobaziridine Hydride with As4. To a cold
(-30 °C) solution of As4 in toluene was added niobaziridine-
hydride complex 1 Via a solid addition tube, and then the
reaction mixture was allowed to warm to room temperature in
the dark (Figure 1). After stirring for 16 h, it was observed that
the reaction mixture had attained a dark green color, indicative
of µ-As2 complex (µ2:η2,η2-As2)[Nb(N[Np]Ar)3]2, 2. Diarsenide
complex 2 was then isolated as a bright green powder in 52%
yield. A single-crystal X-ray diffraction study of complex 2 has
not yet resulted in a high quality structure solution, but one
(13) (a) Figueroa, J. S.; Cummins, C. C. J. Am. Chem. Soc. 2004, 126,
13916–13917. (b) Figueroa, J. S.; Cummins, C. C. Angew. Chem.,
Int. Ed. 2005, 44, 4592–4596. (c) Figueroa, J. S.; Cummins, C. C.
Dalton Trans. 2006, 2161–2168.
(19) Ugai, Y. A.; Semenova, G. V.; Berendt, E.; Goncharov, E. G. Russ.
J. Phys. Chem. 1979, 53, 576–577.
(20) Karakaya, I.; Thompson, W. T. J. Phase Equilib. 1991, 12, 343–6.
(21) Yee, K. K.; Jones, W. E. Chem. Commun. 1969, 586a.
(22) Leung, F.; Cooke, S. A.; Gerry, M. C. L. J. Mol. Spectrosc. 2006,
238, 36–41.
(14) Niecke, E.; Nieger, M.; Reichert, F. Angew. Chem., Int. Ed. Engl.
1988, 27, 1715–1716.
(15) Piro, N. A.; Figueroa, J. S.; McKellar, J. T.; Cummins, C. C. Science
2006, 313, 1276–1279.
(23) Gingerich, K. A.; Cocke, D. L.; Kordis, J. J. Phys. Chem. 1974, 78,
603–606.
(16) Piro, N. A.; Cummins, C. C. J. Am. Chem. Soc. 2008, 130, 9524–
9535.
(24) (a) Davies, J. E.; Kerr, L. C.; Mays, M. J.; Raithby, P. R.; Tompkin,
P. K.; Woods, A. D. Angew. Chem., Int. Ed. 1998, 37, 1428–1429.
(b) Davies, J. E.; Mays, M. J.; Raithby, P. R.; Shields, G. P.; Tompkin,
P. K.; Woods, A. D. J. Chem. Soc., Dalton Trans. 2000, 1925–1930.
(17) Piro, N. A.; Cummins, C. C. Inorg. Chem. 2007, 46, 7387–7393.
(18) Cossairt, B. M.; Diawara, M.-C.; Cummins, C. C. Science 2009, 323,
602.
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Triple-Bond Reactivity of an AsP Complex Intermediate
A R T I C L E S
Table 1. Topological Properties of Selected As-As and As-P Bonds
c
Fb
e
g
qAs
g
moleculea
distanceb
nd
32F
εf
qP
As2
E-HAs)AsH
H2AsAsH2
As4
AsP
E-HAs)PH
H2AsPH2
2.108
2.255
2.482
2.437
2.006
2.155
2.366
2.324
2.364
2.138
2.328
2.244
2.297
0.134
0.110
0.081
0.079
0.153
0.125
0.094
0.091
0.084
0.120
0.090
0.103
0.106
3.00
2.00
1.00
1.03
3.00
2.00
1.00
1.04
1.14
1.72
1.03
1.28
1.35
-0.042
-0.056
-0.043
-0.003
-0.139
-0.110
-0.071
-0.015
+0.009
-0.048
-0.013
-0.041
-0.088
0.00
0.33
0.00
0.07
0.00
0.33
0.03
0.11
0.36
0.06
0.14
0.02
0.04
0.00
+0.31
+0.51
+0.00
+0.10
+0.35
+0.55
+0.17
-0.53
-0.25
-0.00
-0.00
+0.52
-0.10
+0.37
+0.86
-0.06
AsP3
(µ-As2)[Nb(N[Me]Ph)3]2
(η2-AsPNMes)Nb(N[Me]Ph)3
(AsP[PW(CO)5])Mo(N[Me]Ph)3
+0.67
-0.21
-0.37
+0.36
h
(As[PW(CO)5])(C6H8)2
a All molecules geometry-optimized without symmetry constraints at the spin-restricted all-electron OLYP ZORA level of theory with QZ4P basis for
c
As, P, Nb, TZ2P basis for C, N, W, and O and DZP basis for H atoms. b Distances given in Å. Electron density in e/a30 at the (3, -1) As-As or
As-P bond critical point. d Bond order n according to the definition n ) exp(A × (Fb - B)). For As-As bonds, A ) 19.7363 and B ) 0.0774921 as
determined by least-squares fitting using As2, E-HAsdAsH, and H2AsAsH2 as reference compounds for triple, double, and single bonds, respectively.
For As-P bonds, A ) 17.23 and B ) 0.088469 using AsP, E-HAsdPH, and H2AsPH2 as reference systems. e The value of the Laplacian of the electron
density at the (3, -1) As-As or As-P bond critical point in e/a5o. f Ellipticity at the (3, -1) As-As or As-P bond critical point. g Atomic charge
obtained by integration of the electron density over the respective As or P atomic basin. h First line of data refers to the As-P bond involving the P
atom that is not ligated by W(CO)5.
study did confirm the connectivity proposed for the molecule
as drawn (Figure 1).
interarsenic single bond. These values compare well with those
calculated similarly for the spherical-aromatic32 As4 molecule:
rAs-As ) 2.4372 Å (exp. ) 2.44 Å).33
The most important structural parameter for complex 2 is
the As-As interatomic distance, so the model system (µ2:η2,η2-
As2)[Nb(N[Me]Ph)3]2 was constructed and subjected to geom-
etry optimization using density functional theory (DFT) methods
to obtain rAs-As (calcd) ) 2.365 Å. In the activation of As4 by
niobaziridine-hydride complex 1, a rare example is generated
of a molecule containing an M2As2 butterfly core. The only
structurally characterized prior examples of such a molecule
are (µ-As2)[Mn(CO)2Cp*]225 and (µ-As2)[Pd(PPh3)2]2,26 which
have interarsenic distances of 2.225(1) and 2.274(1) Å, respec-
tively. A related structural type is represented by the complex
Terminal Arsenide Anion Synthesis. Two-electron reduction
of diarsenide 2 was achieved by treatment with an excess of
1% Na/Hg in tetrahydrofuran (THF) solvent. This led to the
isolation in 68% yield of the [Na(THF)]+ salt of the arsenide
complex anion [AstNb(N[Np]Ar)3]-, 3, which was found to
crystallize as a close-contact ion-paired dimer by X-ray dif-
fraction analysis (rAs-Nb ) 2.3106(3) Å).34 Also isolated and
structurally characterized was the [Na(12-crown-4)2]+ salt of
anion 3, and in this case there are no close contacts involving
the one-coordinate As atom of the terminal arsenide anion
(rAs-Nb ) 2.3078(5) Å, see Figure 2).
Terminal arsenide complexes—characterized by one-coordi-
nate As that is triply bonded to a metal atom—are rare, but
known examples include the triamidoamine-supported com-
plexes AstMo[(Me3SiNCH2CH2)3N] (rAs-Mo ) 2.252(3) Å)35
and AstW[(Me3SiNCH2CH2)3N] (rAs-W ) 2.2903(11) Å).36
Unlike the niobium arsenide 3, the latter two complexes were
not prepared Via As4 activation chemistry, but by treatment of
the corresponding metal chloride compounds with lithium
organoarsenide salts. A series of transition-metal Zintl phases
containing one-coordinate As atoms (e.g., [Cs7][As(InAs)3-
NbtAs], rAs-Nb ) 2.390(2) Å) have also been reported,37 and
these represent the only instances other than our new complex
3 in which a metal terminal arsenide is also a component of an
anion. For our purposes, this feature is expected to facilitate
functionalization of the terminal arsenide unit.
27
i
[AsMoCp′(CO)2]2 (Cp′ ) PrC5H4), in which there exists a
Mo-Mo bond and the central As2Mo2 unit is an approximate
tetrahedron with rAs-As ) 2.302(1) Å. The single-bond covalent
radius for As is 1.21 Å,28 while the double-bond covalent radius
is 1.14 Å.29 As such, As-As bonds are expected to be ca. 2.42
Å long, while AsdAs bonds are expected to be ca. 2.28 Å long,
and based on the crystallographic data for the diarsane (Mes)2-
As-As(Mes)2 (2.472(3) Å)30 and the diarsene Mes*AsdAsMes*
(2.2634(3) Å),31 these are indeed acceptable values. By these
criteria, our complex 2 (calculated rAs-As ) 2.365 Å) might be
assigned some As-As double bond character. To assess this in
more detail, we turned to a topological analysis of the calculated
electron density; calculated properties for several molecules from
the present work as well as for some reference molecules are
collected in Table 1. Consideration of the electron density Fb at
the As-As bond critical point for our model system representing
2 gives a bond order n ) 1.14 (Table 1). This is consistent
with the representation of 2 as drawn in Figure 1 with an
(32) Hirsch, A.; Chen, Z.; Jiao, H. Angew. Chem., Int. Ed. 2001, 40, 2834–
2838.
(25) Herrmann, W. A.; Koumbouris, B.; Zahn, T.; Ziegler, M. L. Angew.
Chem., Int. Ed. Engl. 1984, 23, 812–814.
(33) Maxwell, L. R.; Hendricks, S. B.; Mosley, V. M. J. Chem. Phys. 1935,
3, 699–709.
(26) Fenske, D.; Fleischer, H.; Persau, C. Angew. Chem., Int. Ed. Engl.
1989, 28, 1665–1667.
(34) Please see the Supporting Information.
(27) DiMaio, A.-J.; Baldacchini, C. J.; Rheingold, A. L. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1990, 46, 492–494.
(28) Pyykko¨, P.; Atsumi, M. Chem.sEur. J. 2009, 15, 186–197.
(29) Pyykko¨, P.; Atsumi, M. Chem.sEur. J., accepted.
(30) Chen, H.; Olmstead, M. M.; Pestana, D. C.; Power, P. P. Inorg. Chem.
1991, 30, 1783–1787.
(35) Scheer, M.; Mu¨ller, J.; Ha¨ser, M. Angew. Chem., Int. Ed. Engl. 1996,
35, 2492–2496.
(36) Mo¨sch-Zanetti, N. C.; Schrock, R. R.; Davis, W. M.; Wanninger, K.;
Seidel, S. W.; O’Donoghue, M. B. J. Am. Chem. Soc. 1997, 119,
11037–11048.
(37) (a) Gascoin, F.; Sevov, S. C. Angew. Chem., Int. Ed. 2002, 41, 1232–
1234. (b) Gascoin, F.; Sevov, S. C. Inorg. Chem. 2003, 42, 904–907.
(c) Gascoin, F.; Sevov, S. C. Inorg. Chem. 2003, 42, 8567–8571.
(31) Bouslikhane, M.; Gornitzka, H.; Escudie, J.; Ranaivonjatovo, H. J.
Organomet. Chem. 2001, 619, 275–279.
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A R T I C L E S
Spinney et al.
Figure 2. Molecular structure of anion 3 as found in crystals of the salt
[Na(12-crown-4)2][AstNb(N[Np]Ar)3] shown with 50% probability thermal
ellipsoids. Selected interatomic distances (Å) and angles (deg): Nb1-As1,
2.3078(5); Nb1-N1, 2.107(2); Nb1-N2, 2.058(2); Nb1-N3, 2.051(2);
As1-Nb1-N1, 108.10(7); As1-Nb1-N2, 103.48(7); As1-Nb1-N3,
100.89(7).
To model the structure and bonding in anion 3, the structure
of anion [AstNb(N[Me]Ph)3]- was geometry optimized with
no counterion giving rise to a calculated rAs-Nb ) 2.295 Å. This
agrees well with the values observed in both the [Na(12-crown-
4)2]+ salt of anion 3 (rAs-Nb ) 2.3078(5) Å) and the close-contact
ion-paired dimer (rAs-Nb ) 2.3106(3) Å) and suggests that
cation-anion interactions do little to perturb the strong Nb-As
bond. Addition of the triple-bond covalent radii for Nb (1.16
Å) and As (1.06 Å) gives a predicted value for a NbtAs bond
of 2.24 Å,38 while similar addition of the double-bond covalent
radii gives a predicted value for a NbdAs bond of 2.39
Å.29 Thus we can conclude that the Nb-As bond in [AstNb-
(N[Np]Ar)3]-, 3, contains significant triple bond character.
Figure 3. Molecular structure of complex 4 shown with 50% probability
thermal ellipsoids. Selected interatomic distances (Å) and angles (deg):
As1-P1, 2.1296(6); P1-N4, 1.564(2); Nb1-As1, 2.6704(3); Nb1-P1,
2.4799(6);As1-P1-N4,140.56(8);As1-P1-Nb1,70.335(19);Nb1-As1-P1,
60.989(17).
Preparation of the AsdPdNMes* Complex. Functionalization
of the terminal As atom in 3 was achieved by treatment of its
[Na(THF)]+ salt with the phosphorus electrophile14 ClPdNMes*
(Figure 1). This provided the desired complex, formulated as
(η2-AsPNMes*)Nb(N[Np]Ar)3, 4. After separation from NaCl,
complex 4 was isolated in 51% yield as an orange-red solid. A
spectroscopic signature of complex 4 is a 31P NMR singlet at δ
) 348 ppm (benzene-d6, 20 °C). For comparison, the corre-
sponding central phosphorus atom in diphosphaazide complex
(η2-PPNMes*)Nb(N[Np]Ar)3 resonates at δ ) 335 ppm.15 X-ray
diffraction analysis (Figure 3) revealed arsaphosphaazide com-
plex 4 to be nearly isostructural with the diphosphaazide
complex reported previously, with the AsPNMes* ligand
coordinated in an η2 fashion to niobium Via its terminal AsP
Figure 4. Plot of the Laplacian of the electron density in units of e/a5o for
the model system of 4, (η2-AsPNMes)Nb(N[Me]Ph)3; the plot is in a plane
defined by the Nb, As, and P nuclei. Two small arrows indicate areas of
valence-shell charge concentration in the P-Nb and P-As bonding regions.
moiety. The As-P and P-N bond distances in 4 (rAs-P
)
2.1296(6) Å, rP-N ) 1.564(2) Å) are essentially identical
in length to those reported in the phosphaarsene (Me3Si)2-
HCAsdPMes* (rAs-P ) 2.124(2) Å)39 and the iminophosphane
(Me3Si)3CPdNMes* (rP-N ) 1.566(3) Å),40 and as such the
AsPNMes* ligand is best represented as a heterocumulene
containing As-P and P-N double bonds. A geometry optimi-
zation performed on the model compound (η2-AsPNMes*)Nb-
(N[Me]Ph)3 gave a calculated rAs-P ) 2.138 Å and a topological
analysis performed on the same molecule gave an As-P bond
order n ) 1.72 (Table 1). A plot of the Laplacian of the electron
density for the model system for 4 is shown in Figure 4. The
plot is in a plane defined by the Nb, As, and P nuclei and
accumulation of valence-shell charge between P and As, as well
as P and Nb, is clearly visible. The arsaphosphaazide ligand,
AsPNMes*, is a new addition to a small group of heavy azide
analogues,41,42 which also includes the diphosphaazide ligand,
PPNMes*.
(38) Pyykko¨, P.; Riedel, S.; Patzschke, M. Chem.sEur. J. 2005, 11, 3511–
3520.
(39) Cowley, A. H.; Lasch, J. G.; Norman, N. C.; Pakulski, M.; Whittlesey,
B. R. J. Chem. Soc., Chem. Commun. 1983, 881–882.
(40) Schinkels, B.; Ruban, A.; Nieger, M.; Niecke, E. Chem. Commun.
1997, 293–294.
(41) Brask, J. K.; Fickes, M. G.; Sangtrirutnugul, P.; Dura`-Vila`, V.; Odom,
A. L.; Cummins, C. C. Chem. Commun. 2001, 1676–1677.
(42) Agarwal, P.; Piro, N.; Meyer, K.; Mu¨ller, P.; Cummins, C. Angew.
Chem., Int. Ed. 2007, 46, 3111–3114.
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Triple-Bond Reactivity of an AsP Complex Intermediate
A R T I C L E S
Figure 5. Thermolysis of the AsdPdNMes* complex 4 results in fragmentation to the niobium imido 5 and the putative AstP molecule. The latter
fragment is absorbed by molybdenum terminal phosphide 6 to yield the new cyclo-AsP2 complex 7.
Thermolysis of the AsdPdNMes* Complex with Conversion
to a Niobium Imido Complex. Thermolysis of complex 4 was
carried out under various conditions. Simple thermolysis in
benzene-d6 solution (60 °C, 3 h) gave near quantitative
conversion to the known imido complex (Mes*N)Nb(N[Np]-
Ar)3, 5, a molecule differing in its chemical formula from that
of precursor 4 by only the AsP diatomic unit. An observed
kinetic parameter for the first-order disappearance of 4 (benzene-
d6, 60 °C) is the rate constant k ) 2.9(1) × 10-4 s-1. Under
these conditions, the 31P NMR signal for starting 4 was observed
to decay away, while no new 31P NMR resonances were
observed to grow in.
Thermolysis of 4 (65 °C, 3 h) in neat 1,3-cyclohexadiene or
neat 2,3-dimethyl-1,3-butadiene did not lead to efficient AsP
trapping. The 31P NMR spectra of the reaction mixtures showed
disappearance of starting material and formation of a multitude
of phosphorus-containing products. Attempts to isolate indi-
vidual compounds from the mixtures were unsuccessful. Reso-
nances at δ ) -79 ppm (1,3-cyclohexadiene) and δ ) -52
ppm (2,3-dimethyl-1,3-butadiene) were tentatively assigned to
the double Diels-Alder adducts of AsP with the respective dienes
based on the corresponding shifts for the P2 adducts, P2(C6H8)2
(δ ) -80 ppm) and P2(C6H10)2 (δ ) -52 ppm).15 Integration
of these 31P NMR resonances versus an internal standard (PPh3)
revealed that the proposed AsP trapping products, AsP(C6H8)2
and AsP(C6H10)2, were formed in 16% and 19% yields,
respectively.43
The AsdPdNMes* complex 4 and the molybdenum phosphide
6 do not react upon mixing at room temperature; however,
following completion of thermolysis (70 °C, 3 h) two new
singlets appeared in the 31P NMR spectrum at δ ) -137 and δ
) -185 ppm. The former is assigned to the new complex
(cyclo-AsP2)Mo(N[iPr]Ar)3, 7, while the latter is assigned to
the previously reported complex (cyclo-P3)Mo(N[iPr]Ar)3, which
has been prepared both Via thermolysis of the diphosphaazide
complex (η2-PPNMes*)Nb(N[Np]Ar)3 in the presence of 616
and Via P4 activation by Mo(H)(η2-Me2C)NAr)(N[iPr]Ar)2.45
Monitoring formation of 7 by 31P NMR (PPh3 as internal
standard) gave an overall yield of 10% for the cyclo-AsP2
compound, compared to 20% for the cyclo-P3 compound. The
mixture of (cyclo-E3)Mo(N[iPr]Ar)3 complexes was separated
from the (Mes*N)Nb(N[Np]Ar)3 coproduct, 5, by dissolving the
crude solids from the reaction mixture in Et2O and cooling
the solution to -35 °C, whereupon white, fibrous crystals of
the (cyclo-E3)Mo(N[iPr]Ar)3 complexes precipitated. The (cyclo-
AsP2)Mo(N[iPr]Ar)3, 7, and (cyclo-P3)Mo(N[iPr]Ar)3 compounds
1
are indistinguishable by H and 13C NMR spectroscopy; only
one set of resonances for the anilide ligands was observed when
the mixture of the two compounds was dissolved in THF-d8.
Further attempts to separate 7 and (cyclo-P3)Mo(N[iPr]Ar)3 by
fractional crystallization were unsuccessful.
The only reported example of a cyclo-AsP2 complex in the
literature, (cyclo-AsP2)CrCp(CO)2, was also formed concomi-
tantly with its cyclo-P3 analogue.46 The pair was synthesized
by treating the tetrahedral complex [CrCp(CO)2]2(µ,η2-P2) with
one equivalent of AsCl3, and the cyclo-AsP2 complex (δ ) -242
ppm) appeared 43 ppm downfield from the cyclo-P3 complex
(δ ) -285 ppm) in the 31P NMR spectrum. Crystals of (cyclo-
AsP2)CrCp(CO)2 were obtained in 10% yield Via fractional
crystallization of the reaction mixture. The formation of (cyclo-
AsP2)Mo(N[iPr]Ar)3, 7, can be explained by a suprafacial [2 +
We have discovered previously that MtP bonds are more
effective traps for the P2 molecule than are organic dienes, and
react with P2 in a one-to-one fashion to form cyclo-P3 metal
complexes.16 As such, 4 was also thermolyzed in benzene in
the presence of PtMo(N[iPr]Ar)3, 6,44 with the goal of trapping
AsP to form a cyclo-AsP2 molybdenum complex (Figure 5).
(43) As commented by a reviewer, the trapping of the AsP fragment with
the dienes is somewhat tenuous, since it is based solely on the NMR
shift obtained in a mixture of compounds.
(44) Cherry, J.-P.; Stephens, F. H.; Johnson, M. J. A.; Diaconescu, P. L.;
Cummins, C. C. Inorg. Chem. 2001, 40, 6860–6862.
(45) Stephens, F. H.; Johnson, M. J. A.; Cummins, C. C.; Kryatova, O. P.;
Kryatov, S. V.; Rybak-Akimova, E. V.; McDonough, J. E.; Hoff, C. D.
J. Am. Chem. Soc. 2005, 127, 15191–15200.
(46) Umbarkar, S.; Sekar, P.; Scheer, M. J. Chem. Soc., Dalton Trans.
2000, 1135–1137.
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A R T I C L E S
Spinney et al.
Figure 6. Thermal fragmentation of complex 4-W(CO)5 in the presence of various trapping agents. Yields shown for the ostensible products of (AsP)W(CO)5
trapping are isolated yields of pure material after separation from coproduct imido 5.
2] cycloaddition reaction between AstP and PtMo(N[iPr]Ar)3,
followed by rapid intramolecular isomerization to the molyb-
denum cyclo-AsP2 complex.16 The mechanism for As/P ex-
change to form (cyclo-P3)Mo(N[iPr]Ar)3 has not been elucidated.
Synthesis of the (AsP)W(CO)5 Synthon. In an effort to
stabilize AsP in solution after its extrusion from the Nb(N-
[Np]Ar)3 platform, we investigated the possibility of its genera-
tion as a tungsten pentacarbonyl complex. We have observed
previously that coordination of the W(CO)5 fragment to the
terminal P atom of (η2-PPNMes*)Nb(N[Np]Ar)3 results in room
temperature fragmentation of the complex to the (P2)W(CO)5
molecule and the niobium imido 5.15 Binding of W(CO)5
sufficiently stabilizes the P2 molecule to extend its lifetime in
solution, but does not shut down the triple bond reactivity of
the diphosphorus moiety. Higher yields of trapping products
are generally observed for (P2)W(CO)5 than for the unsupported
P2 molecule.15-17
Treatment of [Na(THF)][AstNb(N[Np]Ar)3], [Na(THF)][3],
with a solution of W(CO)5(THF) resulted in conversion to the
W(CO)5-capped arsenide salt [Na(THF)][(OC)5WAstNb-
(N[Np]Ar)3], [Na(THF)][3-W(CO)5], which can be isolated as
a red solid in 55% yield. Addition of one equivalent of
ClPdNMes* to a solution of [Na(THF)][3-W(CO)5] in thawing
Et2O resulted in formation of (OC)5W(η2-AsPNMes*)Nb-
(N[Np]Ar)3, 4-W(CO)5, via an NaCl elimination reaction.
Complex 4-W(CO)5 is thermally unstable and readily fragments
to (Mes*N)Nb(N[Np]Ar)3, 5, and the (AsP)W(CO)5 unit, even
at temperatures below 0 °C. At 10 °C in toluene-d8 solution,
4-W(CO)5 is observed to decay following clean first-order
kinetics with a rate constant k ) 5.5(3) × 10-4 s-1. The free
(AsP)W(CO)5 molecule was not directly observed by 31P NMR
spectroscopy.
of one equivalent of 6 to a thawing Et2O solution of 4-W(CO)5
yieldedamixtureof(Mes*N)Nb(N[Np]Ar)3,5,and(OC)5W(cyclo-
AsP2)Mo(N[iPr]Ar)3, 7-W(CO)5 (Figure 6). Although 7-W(CO)5
was observed to form in 70% yield by 1H NMR spectroscopy,
the required separation from 5 resulted in a much lower isolated
yield for the red, crystalline solid of 31%. At 20 °C in benzene-
d6, 7-W(CO)5 appears as a broad resonance at δ ) -169 ppm
in the 31P NMR spectrum, presumably resulting from circu-
mambulation of the W(CO)5 fragment about the cyclo-AsP2 ring.
For comparison, the related cyclo-P3 complex, (OC)5W(cyclo-
P3)Mo(N[iPr]Ar)3, formed by the trapping of (P2)W(CO)5 by 6,
resonates at δ ) -212 ppm (20 °C, benzene-d6).16 A small
amount of (cyclo-AsP2)Mo(N[iPr]Ar)3 missing the W(CO)5 cap
(δ ) -137 ppm) is always observable in the baseline of 31P
NMR spectra of 7-W(CO)5, despite the established purity of
the sample by elemental analysis.
A solution of 7-W(CO)5 in toluene-d8 was analyzed by variable-
temperature 31P NMR spectroscopy. At 50 °C, the resonance at δ
) -169 ppm sharpened significantly, revealing other small peaks
in the baseline at δ ) -185 and δ ) -212 ppm, which have
been assigned as (cyclo-P3)Mo(N[iPr]Ar)3 and (OC)5W(cyclo-
P3)Mo(N[iPr]Ar)3, respectively. This demonstrated that As/P
exchange may also be occurring in the (AsP)W(CO)5 trapping
reaction with 6, but to a much lesser extent than with the
unsupported AsP molecule. Locking out of the different phosphorus
environments in 7-W(CO)5 did not occur until the temperature of
the sample was lowered to -80 °C. At this temperature, a complex
pattern of sharp doublets (JPP ) ca. 300 Hz) emerged.34 At low
temperature, a propeller-like C3 conformation of the anilide ligands
is locked out as well as the migration of the W(CO)5 about the
AsP2 ring. This makes 7-W(CO)5 a chiral molecule with two
inequivalent phosphorus nuclei. Locking out of the W(CO)5
fragment on each of As, P1, and P2 gives rise to three distinct
isomers of 7-W(CO)5 with six expected 31P NMR resonances. The
additional resonances observed are most likely a consequence of
Trapping of the (AsP)W(CO)5 Molecule. Having already
obtained a measure of success in trapping unsupported AsP with
PtMo(N[iPr]Ar)3, 6, we turned our attention to trapping
(AsP)W(CO)5 with the same molybdenum complex. Addition
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16238 J. AM. CHEM. SOC. VOL. 131, NO. 44, 2009

Triple-Bond Reactivity of an AsP Complex Intermediate
A R T I C L E S
Figure 8. Molecular structure of complex 8-W(CO)5 shown with 50%
probability thermal ellipsoids. Selected interatomic distances (Å) and angles
(deg): As1-P1, 2.3010(6); P1-W1, 2.5405(5); As1-P1-W1, 116.22(2).
Figure 7. Molecular structure of complex 7-W(CO)5 shown with 50%
probability thermal ellipsoids. The atoms As1 and P1 are site disordered in
the solid state structure with the major contributor (57%) shown. Selected
interatomic distances (Å) and angles (deg): As1-P1, 2.284(9); As1-P2,
2.227(3); P1-P2, 2.223(12); P2-W1, 2.5463(7); Mo1-As1, 2.611(2);
Mo1-P1,2.636(12);Mo1-P2,2.4759(7);P2-P1-As1,59.2(3);P1-P2-As1,
61.8(3); P1-P2-W1, 130.6(3), P2-As1-P1, 59.0(3).
respectively. Monitoring formation of 8-W(CO)5 and 9-W(CO)5
in solution by 31P NMR spectroscopy (PPh3 as internal standard)
gave much higher yields of 46% and 52%, indicating that
(AsP)W(CO)5 is trapped much more efficiently by organic
dienes than AsP. The formulations of 8-W(CO)5 and 9-W(CO)5
were confirmed by high-resolution electron impact mass spec-
trometry; the molecular ions of both species were located and
matched the predicted isotope patterns for (OC)5-
W(AsP)(C6H8)2 and (OC)5W(AsP)(C6H10)2. Of note, the
(AsP)W(CO)5 radical cation was also observed in the mass
spectrum of both 8-W(CO)5 and 9-W(CO)5.
the small amounts of (cyclo-AsP2)Mo(N[iPr]Ar)3, (cyclo-P3)Mo-
(N[iPr]Ar)3, and (OC)5W(cyclo-P3)Mo(N[iPr]Ar)3 also present in
solution.
The solid state structure of 7-W(CO)5 exhibits the expected
geometry, with the cyclo-AsP2 ring η3-coordinated to the
molybdenum trisanilide platform (Figure 7). Of note, the
W(CO)5 fragment is η1-coordinated to phosphorus and not to
the arsenic atom to which it was bound in [3-W(CO)5]-. This
is not surprising considering the relative nucleophilicities of
phosphorus and arsenic, and the established ability of the
W(CO)5 fragment to circumambulate the cyclo-AsP2 ring in
solution. Atoms P1 and As1 are site disordered in the crystal
structure of 7-W(CO)5, with the major contributor (57%) being
shown in Figure 7. As/P site disorder is also observed in the
solid state structure of the cyclo-AsP2 complex (cyclo-
AsP2)CrCp(CO)2; however, in this case the arsenic atom is
disordered over all three positions in the ring.46 The bonds about
the cyclo-AsP2 ring in the solid state structure of 7-W(CO)5 are
The 31P NMR spectra of compounds 8-W(CO)5 (δ ) -23
ppm, JWP ) 219 Hz) and 9-W(CO)5 (δ ) -12 ppm, JWP ) 225
Hz) exhibit singlets flanked by 183W satellites, indicating that
migration of the W(CO)5 fragment from arsenic to phosphorus
has occurred during the course of the reaction. Indeed, the solid-
state structures of the double Diels-Alder adducts 8-W(CO)5
(Figure 8) and 9-W(CO)5 (Figure 9) confirm that W(CO)5 is
bound to phosphorus in the final products. The (P2)W(CO)5 and
(As2)W(CO)5 molecules have been the subject of a theoretical
study concluding that, in contrast to N2, the P2 and As2 ligands
preferentially bind side-on to the W center.47 If the same is
assumed to be true for (AsP)W(CO)5, it is easy to envision
migration of the W(CO)5 fragment along the As-P bond to the
more nucleophilic phosphorus atom during the trapping reactions.
Compounds 8-W(CO)5 and 9-W(CO)5 are isostructural to the
related (P2)W(CO)5 double Diels-Alder adducts of 1,3-cyclo-
hexadiene and 2,3-dimethyl-1,3-butadiene,15 with the exception
of the slightly longer As-P bond distances (8-W(CO)5, rAs-P
) 2.3010(6) Å; 9-W(CO)5, rAs-P ) 2.3195(11) Å). The double
1,3-cyclohexadiene adduct, 8-W(CO)5, exhibits a tetracyclic
structure with a cofacial pair of CdC π bonds, while the double
2,3-dimethyl-1,3-butadiene adduct, 9-W(CO)5, exhibits a more
open bicyclic framework. The structure of 8-W(CO)5 was
geometry optimized using DFT calculations (Table 1), giving
a calculated rAs-P ) 2.297 Å, which agrees well with the As-P
all essentially equal in length (rAs-P1 ) 2.284(9) Å, rAs-P2
)
2.227(3) Å, rP1-P2 ) 2.223(12) Å), which is likely a consequence
of the site disorder and not a true reflection of the bonding
situation in the complex. A geometry optimization performed
on the model complex (OC)5W(cyclo-AsP2)Mo(N[Me]Ph)3
predicted two different As-P bond lengths in this structure
(rAs-P ) 2.328 Å, rAs-P ) 2.244 Å), with the shorter distance
involving the phosphorus atom bound to W(CO)5 (Table 1).
Trapping of the (AsP)W(CO)5 molecule can also be achieved
by addition of 10 equivalents of either 1,3-cyclohexadiene
(C6H8) or 2,3-dimethyl-1,3-butadiene (C6H10) to a thawing Et2O
solution of 4-W(CO)5 (Figure 6) and allowing the reaction
mixture to warm slowly to room temperature. Following
separation from (Mes*N)Nb(N[Np]Ar)3 (5), the W(CO)5-
complexed double Diels-Alder adducts of AsP, (OC)5W(AsP)-
(C6H8)2 (8-W(CO)5) and (OC)5W(AsP)(C6H10)2 (9-W(CO)5)
were isolated as white crystalline solids in 14% and 17% yields,
(47) Esterhuysen, C.; Frenking, G. Chem.sEur. J. 2003, 9, 3518–3529.
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A R T I C L E S
Spinney et al.
Deuterated solvents for NMR spectroscopy were purchased from
Cambridge Isotope Laboratories. Benzene-d6 and toluene-d8 were
degassed and stored over molecular sieves for at least 2 days prior
to use. THF-d8 was distilled from sodium/benzophenone and stored
over molecular sieves at -35 °C. Celite 435 (EM Science), 4 Å
molecular sieves (Aldrich), and alumina (EM Science) were dried
by heating at 200 °C under dynamic vacuum for at least 24 h prior
to use. 1,3-Cyclohexadiene and 2,3-dimethyl-1,3-butadiene were
purchased from Aldrich, distilled from NaBH4, and stored over
molecular sieves at -35 °C prior to use. Tungsten hexacarbonyl
was purchased from Strem and used without further purification.
The compounds HNb(η2-tBuHCdNAr)(N[Np]Ar)2 (1),11 Mes*-
NPCl,14 and PtMo(N[iPr]Ar)3 (6)44 were prepared according to
literature procedures. All glassware was oven-dried at temperatures
greater than 170 °C prior to use. NMR spectra were obtained on
Bruker Avance 400, Varian Mercury 300, or Varian Inova 500
spectrometers. 1H NMR spectra were referenced to residual solvent
resonances (C6D5H, 7.16 ppm, C4D7HO, 3.58 ppm, C7D7H, 2.09
ppm); 13C NMR spectra were referenced to C6D6 (128.39 ppm)
and C4D8O (67.57 ppm); and 31P NMR spectra were referenced
externally to 85% H3PO4 (0 ppm). Elemental analyses were
performed by Midwest Microlab, LLC, Indianapolis, Indiana.
As4 Generation and Trapping. Yellow arsenic (As4) is gener-
ated by subliming gray arsenic in the apparatus shown in Supporting
Information Figure S1. In a typical run, approximately 5 g of gray
arsenic is loaded into the middle of the inner horizontal steel tube
and heated to a temperature of 515-550 °C for a period of three
hours. A stream of argon gas (flow rate: 1 L/min) carries the As4
vapors into the T-joint, where they meet another stream of room
temperature argon (flow rate: 1 L/min). The argon/As4 mixture is
bubbled through a flask containing 500 mL of chilled toluene (-30
°C, bath: 50/50 water/ethylene glycol + liquid N2), where the As4
vapors condense in solution. The argon stream leaves the flask and
passes through a series of three bleach-filled bubblers to scrub out
any escaping arsenic vapors. The entire procedure is performed in
the dark to prevent conversion of As4 to gray arsenic. The saturated
solution of As4 in toluene is then used in the subsequent synthesis
of 2, without the isolation of solid As4. It is apparent that not all
the As4 remains in solution, as gram quantities of gray arsenic are
always observed in the flask after sublimation.
Figure 9. Molecular structure of complex 9-W(CO)5 shown with 50%
probability thermal ellipsoids. Selected interatomic distances (Å) and angles
(deg): As1-P1, 2.3195(11); P1-W1, 2.5191(11); As1-P1-W1, 112.64(4).
bond lengths observed in the solid state structures of 8-W(CO)5
and 9-W(CO)5 and the predicted As-P bond length from single
bond covalent radii (2.32 Å).28
Conclusions
In conclusion, we have used the niobium-mediated activation
chemistry of As4 to access the unsaturated intermediate (AsP)-
W(CO)5 in solution. Despite being bound by tungsten pentac-
arbonyl, the AsP moiety retains its triple bond reactivity, as
witnessed by its ability to react with two equivalents of organic
diene in Diels-Alder reactions. The (AsP)W(CO)5 molecule also
reacts in a one-to-one fashion with PtMo(N[iPr]Ar)3 (6) to
generate the new cyclo-AsP2 molybdenum complex, (OC)5W-
(cyclo-AsP2)Mo(N[iPr]Ar)3 (7-W(CO)5), which has been struc-
turally characterized. In addition to accessing triple-bond
reactivity of the proposed (AsP)W(CO)5 intermediate, this work
has yielded a simple mononuclear niobium terminal arsenide
anion, [AstNb(N[Np]Ar)3]- (3), which contains a Nb-As triple
bond. The negative charge in 3 facilitates functionalization of
the terminal arsenide unit, allowing for the synthesis of the new
AsdPdNMes* ligand atop the niobium trisanilide framework.
Thermolysis of the arsaphosphaazide complex (η2-AsPNMes*)Nb-
(N[Np]Ar)3 (4) results in near quantitative conversion to the
niobium imido (Mes*N)Nb(N[Np]Ar)3 (5), with concomitant
loss of the AsP diatomic unit. Trapping reactions involving the
AstP molecule gave very low yields or complex mixtures of
products, suggesting that this molecule is less tractable in terms
of trapping or transfer than was the case for PtP when accessed
using similar methodology. The synthetic explorations reported
herein represent a dual effort to discover triple-bond reactivity
for low-coordinate arsenic while defining methods for arsenic
incorporation that stem from the element itself.
Synthesis of (µ2:η2,η2-As2)[Nb(N[Np]Ar)3]2, 2. In the dark,
HNb(η2-tBuHC)NAr)(N[Np]Ar)2 (1, 1.81 g, 2.73 mmol), was
added under an argon stream to a cold solution of As4 in toluene
(-30 °C; generated as described above) by means of a solid addition
funnel. The reaction mixture was kept in the dark, allowed to warm
to room temperature, and stirred for 16 h, during which time the
color of the solution changed from orange to dark green. The
product of the reaction is not light-sensitive, and no further
precautions to exclude light were taken during the subsequent
workup. The toluene was removed under vacuum, and the resulting
green-black residue was taken up in n-hexane (400 mL) and filtered
through Celite to remove solid arsenic and other insoluble materials.
The dark green solution was then evaporated to dryness and the
solids obtained thereby were slurried in n-pentane (60 mL). The
pentane slurry was placed in the freezer for 16 h at -35 °C to
induce further precipitation. A bright-green powder was isolated
by filtration and washed with cold n-pentane. The filtrate was
concentrated, placed back in the freezer and a second crop of the
green powder was isolated. Total yield: 1.04 g, 0.704 mmol, 52%.
Elem. Anal. Calcd for C78H120N6As2Nb2: C, 63.41; H, 8.19; N, 5.69.
1
Found: C, 62.61; H, 8.07; N, 5.72. H NMR (benzene-d6, 20 °C,
500 MHz): δ ) 6.95 (bs, 12H, o-Ar), 6.58 (s, 6H, p-Ar), 4.26 (bs,
12H, N-CH2), 2.23 (s, 36H, Ar-CH3), 0.99 (s, 54H, C(CH3)3) ppm.
13C{1H} NMR (benzene-d6, 20 °C, 125.8 MHz): δ ) 154.2 (bs,
aryl ipso), 138.2 (s, m-Ar), 126.6 (s, p-Ar), 124.2 (bs, o-Ar), 73.5
(bs, N-CH2), 37.3 (bs, C(CH3)3), 30.7 (s, C(CH3)3), 22.1 (s, Ar-
CH3) ppm. FTIR (KBr windows, thin film): 2945(s), 1585(s),
1473(s), 1394(m), 1363(m), 1286(m), 1154(m), 1064(m), 999(s),
Experimental Section
General Procedures. All manipulations were performed in a
Vacuum Atmospheres model MO-40 M glovebox under an
atmosphere of purified N2. Solvents were obtained anhydrous and
oxygen-free from a Contour Glass Solvent Purification System.
842(s), 683(s) cm-1
.
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Triple-Bond Reactivity of an AsP Complex Intermediate
A R T I C L E S
Synthesis of [Na(THF)][AstNb(N[Np]Ar)3], [Na(THF)][3].
Freshly prepared 1% sodium amalgam (Na: 0.25 g, 11 mmol, 15
equiv/Nb) was added to a dark-green THF (30 mL) solution of
(µ2:η2,η2-As2)[Nb(N[Np]Ar)3]2 (2, 1.09 g, 0.740 mmol). The mixture
was stirred vigorously for 2 h, during which time the color gradually
changed from green to bright red. The supernatant solution was
decanted from the amalgam and evaporated to dryness, leaving
behind a red residue. The red residue was extracted with 50 mL of
n-pentane and filtered through Celite. The filtrate was concentrated
to a volume of 3 mL and placed in the freezer for 16 h at -35 °C,
during which time a bright orange microcrystalline precipitate
deposited. The orange precipitate was isolated by filtration and
washed with 1 mL of cold n-pentane. The filtrate was concentrated,
placed back in the freezer and a second crop of orange powder
was isolated. Total yield: 0.836 g, 1.00 mmol, 68%. Crystals suitable
for X-ray diffraction were grown over a period of 16 h from a
concentrated pentane solution kept at -35 °C. Elem. Anal. Calcd
for C43H68N3ONaAsNb: C, 61.94; H, 8.22; N, 5.04. Found: C,
solution was filtered, concentrated to a volume of 4 mL, and placed
back in the freezer. More W(CO)6 was removed by filtration and
the red supernatant solution was pumped down to dryness to yield
an oily red solid. The red solid was slurried in 2 mL of n-pentane
and placed in the freezer for 16 h at -35 °C. A fine red powder
precipitated and was isolated by filtration and washed with a further
2 mL of cold n-pentane. Total yield: 0.373 g, 0.344 mmol, 55%.
The red powder was further purified by heating for 2 h at 70 °C in
a sublimation apparatus under dynamic vacuum. A few additional
mg of W(CO)6 contaminant was collected on the coldfinger. Elem.
Anal. Calcd for C44H60N3O5NaAsNbW: C, 48.68; H, 5.57; N, 3.87.
1
Found: C, 48.88; H, 5.66; N, 3.25. H NMR (benzene-d6, 20 °C,
500 MHz): δ ) 6.98 (s, 6H, o-Ar), 6.46 (s, 3H, p-Ar), 4.09 (s, 6H,
N-CH2), 2.18 (s, 18H, Ar-CH3), 1.10 (s, 27H, C(CH3)3) ppm.
13C{1H} NMR (benzene-d6, 20 °C, 125.8 MHz): δ ) 208.3 (s, trans
CO), 202.3 (s, cis CO, JCW ) 125 Hz), 157.7 (s, aryl ipso), 139.2
(s, m-Ar), 124.3 (s, p-Ar), 122.0 (s, o-Ar), 73.8 (bs, N-CH2), 36.4
(s, C(CH3)3), 30.0 (s, C(CH3)3), 21.9 (s, Ar-CH3) ppm. FTIR (KBr
windows, nujol mull): 2349(m), 2052(s), 1968(s), 1928(vs), 1809(vs),
1
61.98; H, 8.33; N, 4.96. H NMR (benzene-d6, 20 °C, 400 MHz):
δ ) 6.99 (s, 6H, o-Ar), 6.48 (s, 3H, p-Ar), 4.37 (s, 6H, N-CH2),
3.67 (m, 4H, THF), 2.24 (s, 18H, Ar-CH3), 1.56 (m, 4H, THF),
1.13 (s, 27H, C(CH3)3) ppm. 13C{1H} NMR (benzene-d6, 20 °C,
100.6 MHz): δ ) 158.8 (s, aryl ipso), 138.7 (s, m-Ar), 123.3 (s,
p-Ar), 122.0 (s, o-Ar), 75.2 (bs, N-CH2), 68.8 (s, THF), 36.6 (s,
C(CH3)3), 30.2 (s, C(CH3)3), 26.1 (s, THF), 22.2 (s, Ar-CH3) ppm.
FTIR (KBr windows, nujol mull): 1585(s), 1573(m), 1363(m),
1584(s), 1365(s), 1155(s), 995(s), 844(s), 687(s), 581(s) cm-1
.
Synthesis of (η2-AsPNMes*)Nb(N[Np]Ar)3, 4. To a stirring,
thawing Et2O solution (30 mL) of [Na(THF)][AstNb(N[Np]Ar)3]
([Na(THF)][3], 0.496 g, 0.595 mmol) was added dropwise a thawing
solution of Mes*NPCl (0.212 g, 0.651 mmol, 1.1 equiv) in Et2O
(3 mL). The dark red reaction mixture was allowed to stir while
warming to room temperature over 30 min and was then pumped
down to dryness. The remaining red residue was taken up in 25
mL of n-pentane and filtered through Celite to remove NaCl. A
31P{1H} NMR spectrum of the filtrate showed quantitative conver-
sion to the arsaphosphaazide complex (η2-AsPNMes*)-
Nb(N[Np]Ar)3, 4. The n-pentane filtrate was concentrated to a
volume of 2 mL and placed in the freezer for 16 h at -35 °C,
during which time a dark red-orange powder precipitated. The
powder was collected by filtration and washed with an additional
1 mL of cold n-pentane. Total yield: 0.313 g, 0.304 mmol, 51%.
Crystals suitable for X-ray diffraction were obtained by dissolving
the powder in a 50/50 Et2O/(Me3Si)2O mixture and placing the
resulting solution in the freezer for 16 h at -35 °C. Elem. Anal.
Calcd for C57H89N4PAsNb: C, 66.52; H, 8.72; N, 5.44. Found: C,
1292(m), 1152(m), 994(m), 840(m), 680(s), 675(s) cm-1
.
Synthesis of [Na(12-crown-4)2][AstNb(N[Np]Ar)3], [(Na(12-c-
4)2][3]. A solution of 0.084 g of 12-crown-4 (0.48 mmol, 2.7
equivalents) in 3 mL of n-pentane was added dropwise with stirring
to a solution of [Na(THF)][AstNb(N[Np]Ar)3] ([Na(THF)][3],
0.150 g, 0.180 mmol) in n-pentane (3 mL). The red color of the
solution faded and a yellow precipitate formed. The reaction mixture
was allowed to stir for 30 min and then placed in the freezer for
16 h at -35 °C to induce further precipitation. The yellow powder
was isolated by filtration and washed with cold n-pentane (3 mL).
Total yield: 0.153 g, 0.137 mmol, 76%. Crystals suitable for X-ray
diffraction were grown by adding 3 mL of n-pentane to the yellow
powder, followed by the dropwise addition of Et2O until all the
material dissolved. The n-pentane/Et2O mixture was placed in the
freezer for one week at -35 °C, during which time crystals
deposited. Elem. Anal. Calcd for C55H92N3O8NaAsNb: C, 59.29;
1
66.49; H, 8.68; N, 5.43. H NMR (benzene-d6, 20 °C, 500 MHz):
δ ) 7.76 (s, 2H, m-Mes*), 6.63 (s, 6H, o-Ar), 6.58 (s, 3H, p-Ar),
4.11 (s, 6H, N-CH2), 2.12 (s, 18H, Ar-CH3), 1.93 (s, 18H, o-C(CH3)3
Mes*), 1.48 (s, 9H, p-C(CH3)3 Mes*), 0.82 (s, 27H, CH2-C(CH3)3)
ppm. 13C{1H} NMR (benzene-d6, 20 °C, 125.8 MHz): δ ) 152.2
1
H, 8.32; N, 3.77. Found: C, 58.84; H, 8.10; N, 3.80. H NMR
(benzene-d6, 20 °C, 500 MHz): δ ) 7.02 (s, 6H, o-Ar), 6.42 (s,
3H, p-Ar), 4.98 (bs, 6H, N-CH2), 3.20 (s, 24H, crown), 2.25 (s,
18H, Ar-CH3), 1.46 (s, 27H, C(CH3)3) ppm. 13C{1H} NMR
(benzene-d6, 20 °C, 125.8 MHz): δ ) 157.6 (s, aryl ipso), 137.4
(s, m-Ar), 121.3 (s, p-Ar), 120.9 (s, o-Ar), 76.9 (bs, N-CH2), 66.0
(s, crown), 36.9 (s, C(CH3)3), 31.0 (s, C(CH3)3), 22.4 (s, Ar-CH3)
ppm. FTIR (KBr windows, nujol mull): 1582(m), 1365(m),
(s, aryl ipso), 151.3 (d, JCP ) 36 Hz, Mes* ipso), 142.8 (d, JCP
)
5 Hz, m-Mes*), 139.4 (d, JCP ) 12 Hz, o-Mes*), 138.6 (s, m-Ar),
127.6 (s, p-Ar), 124.3 (s, o-Ar), 122.7 (d, JCP ) 5 Hz, p-Mes*),
73.1 (d, JCP ) 7 Hz, N-CH2), 37.3 (d, JCP ) 2 Hz, o-C(CH3)3 Mes*),
37.1 (s, CH2-C(CH3)3), 35.2 (d, JCP ) 1 Hz, p-C(CH3)3 Mes*), 33.6
(s, o-C(CH3)3 Mes*), 32.5 (s, p-C(CH3)3 Mes*), 30.2 (s, CH2-
C(CH3)3), 21.9 (s, Ar-CH3) ppm. 31P{1H} NMR (benzene-d6, 20
°C, 121.5 MHz): δ ) 349 (s) ppm. FTIR (KBr windows, nujol
mull): 1602(m), 1587(s), 1416(s), 1365(s), 1311(s), 1293(s), 996(m),
1289(m), 1135(s), 1095(s), 1021(s), 916(m), 674(m) cm-1
.
Synthesis of [Na][(OC)5WAstNb(N[Np]Ar)3], [Na][3-W(CO)5].
A 0.015 M yellow solution of W(CO)5(THF) was prepared by
irradiating a solution of W(CO)6 (0.650 g, 1.85 mmol) in 50 mL
of THF in a Rayonet photoreactor equipped with six RPR-4190
(emission maximum at 419 nm) and ten RPR-2540 (emission
maximum at 254 nm) lamps for 24 h, with periodic degassing to
remove CO. To this solution (42 mL, 0.630 mmol of W(CO)5(THF))
was added 0.525 g of [Na(THF)][AstNb(N[Np]Ar)3] ([Na-
(THF)][3], 0.630 mmol), and the dark-red reaction mixture was
stirred for 2 h and then pumped down to dryness to yield a dark-
red residue. The residue was taken up in 50 mL of n-pentane and
filtered through Celite to remove excess W(CO)6. The red filtrate
was pumped down to dryness and a 1H NMR spectrum of the crude
material showed quantitative conversion of [Na(THF)][3] to the
W(CO)5-capped analogue [Na(THF)][3-W(CO)5]. To obtain ana-
lytically pure material, the red residue was taken up in 8 mL of
Et2O and placed in the freezer for 16 h at -35 °C. During this
time, white crystals of W(CO)6 deposited. The red supernatant
848(m), 692(m), 684(m) cm-1
.
Synthesis of (OC)5W(cyclo-AsP2)Mo(N[iPr]Ar)3, 7-W(CO)5.
To a thawing solution of [Na(THF)][(OC)5WAstNb(N[Np]Ar)3]
([Na(THF)][3-W(CO)5], 0.300 g, 0.259 mmol) in Et2O (10 mL)
was added a thawing Et2O (3 mL) solution of Mes*NPCl (0.085
g, 0.261 mmol, 1 equiv) dropwise. After 30 s of stirring, a yellow
Et2O (4 mL) solution of P≡Mo(N[iPr]Ar)3 (6, 0.159 g, 0.259 mmol,
1 equiv) was quickly added. There was no apparent color change
to the dark red reaction mixture. After 3 h of stirring, the reaction
mixture was pumped down to dryness, taken up in 30 mL of toluene
and filtered through Celite to remove NaCl. The red filtrate was
evaporated to dryness, leaving behind a red solid. The solid was
slurried in 15 mL of n-hexane, and this mixture was placed in the
freezer for 16 h at -35 °C. The red precipitate (0.285 g) was
isolated by filtration and washed with 3 mL of n-hexane. Analysis
1
by H NMR spectroscopy of this material revealed that it was
9
J. AM. CHEM. SOC. VOL. 131, NO. 44, 2009 16241

A R T I C L E S
Spinney et al.
composed of (OC)5W(cyclo-AsP2)Mo(N[iPr]Ar)3 (7-W(CO)5, 68%)
and (Mes*N)Nb(N[Np]Ar)3 (5, 32%). The red solid was slurried
in 5 mL of n-pentane and toluene was added dropwise until all the
solids dissolved. The toluene/pentane mixture was placed in the
freezer for 16 h at -35 °C, during which time dark-red, block-
shaped, X-ray quality crystals of 7-W(CO)5 grew. The red crystals
were isolated by filtration and washed with 15 mL of cold n-pentane
to remove small, yellow cocrystals of 5. Total yield: 0.082 g, 0.079
mmol, 31%. Elem. Anal. Calcd for C38H48N3O5P2AsMoW: C,
of acetonitrile was added, resulting in precipitation of a yellow solid.
The yellow solid was isolated by filtration and characterized as the
niobium imido complex, (Mes*N)Nb(N[Np]Ar)3 (5, 0.159 g, 0.172
mmol, 99% yield). The red-brown filtrate was evaporated to dryness
and slurried in 2 mL of n-pentane. Diethyl ether was added dropwise
to the slurry until all solids present dissolved and the mixture was
placed in the freezer for 48 h at -35 °C. Colorless, X-ray quality
crystals of 9-W(CO)5 were isolated by filtration and washed with
1 mL of cold n-pentane. Total yield: 0.018 g, 0.030 mmol, 17%.
Note: Monitoring formation of 9-W(CO)5 Via 31P NMR (PPh3 as
internal standard) gave an overall yield of 53%. Elem. Anal. Calcd
for C17H20O5PAsW: C, 34.37; H, 3.39. Found: C, 33.92; H 3.32.
1H NMR (benzene-d6, 20 °C, 400 MHz): δ ) 2.25 (m, 2H), 2.05
(d, 2H), 1.70-1.57 (m, 4H), 1.55 (bs, 6H, CH3), 1.38 (dd, J ) 5
Hz and 1 Hz, 6H, CH3) ppm. 13C{1H} NMR (benzene-d6, 20 °C,
100.6 MHz): δ ) 199.9 (d, JCP ) 23 Hz, trans CO), 197.7 (d, JCP
) 6.5 Hz, JCW ) 125 Hz, cis CO), 124.3 (d, JCP ) 8 Hz, CCH3),
33.8 (bs), 26.6 (d, JCP ) 6 Hz), 21.4 (d, JCP ) 2 Hz), 20.8 (d, JCP
) 3 Hz) ppm. 31P{1H} NMR (benzene-d6, 20 °C, 121.5 MHz): δ
) -12 (s, JWP ) 225 Hz) ppm. FTIR (KBr windows, thin film):
2916(w), 2857(w), 2354(w), 2065(s), 1975(s), 1950(s), 1898(vs),
1645(w), 1436(w), 1408(w), 1383(w), 1048(w), 1018(w), 830(m),
819(m), 808(w), 780(w), 597(s), 575(vs) cm-1. HRMS-EI:
Calcd for M+: 593.978. Found: 593.980. Also present: 565.984
(W(CO)4(AsP)(C6H10)2+), 483.906 (W(CO)4(AsP)(C6H10)+), 429.822
(W(CO)5(AsP)+), 401.827 (W(CO)4(AsP)+), and others.
Kinetics of the Fragmentation of 4. Stock solutions containing
(η2-AsPNMes*)Nb(N[Np]Ar)3 (4, 151 mg) and ONb(N[Np]Ar)3
(46 mg, used as an internal standard) were prepared in C6D6 (4.1
g) and stored at -35 °C between uses. An aliquot of this solution
was transferred to a sealable (J. Young) NMR tube, which was
then inserted into a prewarmed NMR probe (60 °C). The temper-
ature of the probe was verified by an ethylene glycol NMR
thermometer before each run. Four-scan 1H NMR spectra (relaxation
delay ) 16 s) were collected every three minutes over a period of
three half-lives. The integral of the methylene residue of 4 as a
function of time, corrected versus the internal standard, was fit to
the first-order rate equation, I(t) ) Ae-kt + b, using the automated
routine of Origin 6.48 The experiment was repeated three times.
The average rate constant for the decay of 4 at 60 °C is 2.9(1) ×
10-4 s-1. As shown in Figure S22 (Supporting Information),
conversion of 4 to the niobium imido complex, (Mes*N)Nb-
(N[Np]Ar)3 (5), is not quantitative and other unidentified anilide-
containing products are also formed.
1
43.74; H, 4.64; N, 4.03. Found: C, 43.91; H, 4.81; N, 4.02. H
NMR (benzene-d6, 20 °C, 500 MHz): δ ) 6.68 (s, 3H, p-Ar), 6.27
(bs, 6H, o-Ar), 4.90 (bs, 3H, CH(CH3)2), 2.10 (s, 18H, Ar-CH3),
0.91 (bs, 18H, CH(CH3)2) ppm. 13C{1H} NMR (benzene-d6, 20 °C,
125.8 MHz): δ ) 197.9 (s, cis CO), 148.6 (bs, aryl ipso), 137.9 (s,
p-Ar), 137.5 (s, m-Ar), 129.4 (s, o-Ar), 64.8 (bs, N-CH(CH3)2),
22.6 (bs, CH(CH3)2), 21.7 (s, Ar-CH3) ppm. Note: The trans CO
resonance was not located in the 13C{1H} spectrum. 31P{1H} NMR
(benzene-d6, 20 °C, 121.5 MHz): δ ) -169 (bs) ppm. A small
peak at δ ) -139 ppm is also always observed and has been
attributed to a small amount of (cyclo-AsP2)Mo(N[iPr]Ar)3 (7)
without the W(CO)5 unit appended. FTIR (KBr windows, nujol
mull): 2214(w), 2067(vs), 1980(vs), 1940(vs), 1599(s), 1583(m),
1357(s), 1152(s), 1107(s), 987(s), 933(s), 859(s), 687(vs), 593(vs),
573(vs) cm-1
.
Synthesis of (OC)5W(AsP)(C6H8)2, 8-W(CO)5. To a thawing
solution of [Na(THF)][(OC)5WAstNb(N[Np]Ar)3] ([Na(THF)][3-
W(CO)5], 0.200 g, 0.173 mmol) in Et2O (7 mL) was added dropwise
a thawing Et2O (5 mL) solution of Mes*NPCl (0.057 g, 0.175
mmol, 1 equiv). After 30 s of stirring, 175 µL (1.78 mmol, 10.3
equiv) of 1,3-cyclohexadiene was added Via syringe. The dark red
reaction mixture was stirred for 1 h and then pumped down to
dryness, leaving behind a brown residue. The residue was extracted
with 10 mL of toluene and filtered through Celite to remove NaCl.
The red filtrate was evaporated to dryness and 5 mL of acetonitrile
was added, resulting in precipitation of a yellow solid. The yellow
solid was isolated by filtration, and characterized as the niobium
imido complex, (Mes*N)Nb(N[Np]Ar)3 (5, 0.1324 g, 0.143 mmol,
83% yield). The orange filtrate was evaporated to dryness and
slurried in 2 mL of n-pentane. Diethyl ether was added dropwise
to the slurry until all solids present dissolved and the mixture was
placed in the freezer for 48 h at -35 °C. Colorless, X-ray quality
crystals of 8-W(CO)5 were isolated by filtration and washed with
1 mL of cold n-pentane. Total yield: 0.015 g, 0.025 mmol, 14%.
Note: Monitoring formation of 8-W(CO)5 Via 31P NMR (PPh3 as
internal standard) gave an overall yield of 46%. 1H NMR (benzene-
d6, 20 °C, 500 MHz): δ ) 5.18 (td, J ) 8 and 4 Hz, 2H), 4.75 (td,
J ) 8 and 3.5 Hz, 2H), 2.55 (m, 4H), 1.70 (q, J ) 12 Hz, 2H),
1.23-1.17 (m, 2H), 1.13-1.06 (m, 2H), 0.93-0.78 (m, 2H) ppm.
13C{1H} NMR (benzene-d6, 20 °C, 125.8 MHz): δ ) 199.2 (d, JCP
) 24 Hz, trans CO), 197.9 (d, JCP ) 6.5 Hz, JCW ) 125 Hz, cis
CO), 125.1 (d, JCP ) 8 Hz), 124.5 (d, JCP ) 10 Hz), 33.8 (d, JCP
) 2 Hz), 32.2 (d, JCP ) 4 Hz), 25.4 (d, JCP ) 8 Hz), 23.0 (d, JCP
) 5 Hz) ppm. 31P{1H} NMR (benzene-d6, 20 °C, 161.9 MHz): δ
) -22 (s, JWP ) 219 Hz) ppm. FTIR (KBr windows, thin film):
2171(m), 2064(s), 1971(m), 1918(vs), 1453(w), 1019(w), 725(w),
644(w), 616(w), 600(m), 573(s) cm-1. HRMS-EI: Calcd. for M+:
589.946. Found: 589.945. Also present: 509.882 (W(CO)5(AsP)-
(C6H8)+), 429.820 (W(CO)5(AsP)+), 401.825 (W(CO)4(AsP)+), and
others.
Kinetics of the Fragmentation of 4-W(CO)5. A thawing Et2O
solution (1 mL) of Mes*NPCl (17 mg, 0.052 mmol, 1 equiv) was
added to a thawing Et2O solution (2 mL) of [Na(THF)][(OC)5-
WAstNb(N[Np]Ar)3] ([Na(THF)][3-W(CO)5], 58 mg, 0.050 mmol).
This solution was stirred for 30 s before the solvent was removed
in Vacuo. The residue was extracted with a toluene-d8 (1.0 g)
solution of ferrocene (7 mg, 0.04 mmol, used as an internal
standard). The resulting extract was filtered cold through Celite
into a sealable (J. Young) NMR tube, which was then frozen for
transport to an NMR probe precooled to 10 °C. The temperature
of the probe was verified by an ethylene glycol NMR thermometer
1
before each run. Four-scan H NMR spectra (relaxation delay )
16 s) were collected every three minutes over a period of four half-
lives. The integral of the methylene residue of 4-W(CO)5 as a
function of time, corrected versus the internal standard, was fit to
the first-order rate equation, I(t) ) Ae-kt + b, using the automated
routine of Origin 6.48 The experiment was repeated three times.49
The average rate constant for the decay of 4-W(CO)5 at 10 °C is
5.5(3) × 10-4 s-1. As shown in Figure S23 (Supporting Informa-
Synthesis of (OC)5W(AsP)(C6H10)2, 9-W(CO)5. To a thawing
solution of [Na(THF)][(OC)5WAstNb(N[Np]Ar)3] ([Na(THF)][3-
W(CO)5], 0.200 g, 0.173 mmol) in Et2O (7 mL) was added dropwise
a thawing Et2O (5 mL) solution of Mes*NPCl (0.057 g, 0.175
mmol, 1 equiv). After 30 s of stirring, 200 µL (1.77 mmol, 10.2
equiv) of 2,3-dimethyl-1,3-butadiene was added Via syringe. The
dark red reaction mixture was stirred for 1 h and then pumped down
to dryness, leaving behind a dark red residue. The residue was
extracted with 10 mL of toluene and filtered through Celite to
remove NaCl. The red filtrate was evaporated to dryness and 5 mL
(48) http://microcal.com, Microcal(TM) Origin Version 6.0, Copyright
1991-1999, Microcal Software, Inc.
(49) It should be noted that by the time data collection started on the
samples, approximately one third of the 4-W(CO)5 present in solution
had already converted to the niobium imido 5.
9
16242 J. AM. CHEM. SOC. VOL. 131, NO. 44, 2009

Triple-Bond Reactivity of an AsP Complex Intermediate
A R T I C L E S
tion), conversion of 4-W(CO)5 to the niobium imido complex,
(Mes*N)Nb(N[Np]Ar)3 (5), is nearly quantitative.
functional of Vosko, Wilk, and Nusair (VWN)57 together with the
GGA functional known as OLYP which is a combination of Handry
and Cohen’s OPTX functional for exchange and Lee, Yang, and
Parr’s nonlocal correlation functional.58 The zero-order regular
approximation (ZORA) was used for inclusion of relativistic
effects.59 The basis sets were QZ4P (quadruple-ꢀ with four
polarization functions) for As, P, Nb, TZ2P for C, N, W, and O
and DZP for H atoms as supplied with ADF, and frozen core
approximations were made for carbons on the anilide ligands (1s).
All molecules were geometry-optimized without symmetry con-
straints to default convergence criteria and energies are uncorrected
for zero-point energies. Bond multiplicities were calculated using
the method of Nalewajski and Mrozek within ADF.60 Electron
density topologies and orbital contours were analyzed using the
packages DGrid and Basin by Kohout.61
Details of X-ray Structure Determinations. Crystals were
mounted in hydrocarbon oil on a nylon loop or a glass fiber. Low-
temperature (100 K) data were collected on a Siemens Platform
three-circle diffractometer coupled to a Bruker-AXS Smart Apex
CCD detector with graphite-monochromated Mo KR radiation (λ
) 0.71073 Å) performing φ- and ω-scans. A semiempirical
absorption correction was applied to the diffraction data using
SADABS.50 All structures were solved by direct or Patterson
methods using SHELXS51,52 and refined against F2 on all data by
full-matrix least-squares with SHELXL-97.52,53 All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were
included in the model at geometrically calculated positions and
refined using a riding model. The isotropic displacement parameters
of all hydrogen atoms were fixed to 1.2 times the U value of the
atoms to which they are linked (1.5 times for methyl groups). In
structures where disorders were present, the disorders were refined
within SHELXL with the help of rigid bond restraints as well as
similarity restraints on the anisotropic displacement parameters for
neighboring atoms and on 1,2- and 1,3-distances throughout the
disordered components. The relative occupancies of disordered
components were refined freely within SHELXL.54 Further details
on all structures are provided in Tables S1 and S2, and in the form
of cif files available as part of the Supporting Information or from
the CCDC under deposition numbers 710856-710861.55
Acknowledgment. We gratefully acknowledge the US National
Science Foundation for support of this research through grant CHE-
719157 and through a predoctoral fellowship to N.A.P. This work
was also supported by the Natural Sciences and Engineering
Research Council (NSERC) of Canada through a postdoctoral
fellowship to H.A.S. We also thank Prof. Dr. Karsten Meyer for a
generous gift of the As4-generating apparatus.
Supporting Information Available: Additional experimental
details and figures showing NMR spectra, variable-temperature
NMR data, mass spectra, kinetic plots, and X-ray crystallography
structures; tables of crystal data; lists of optimized geometry
coordinates; and crystallographic information files. This material
is available free of charge via the Internet at http://pubs.acs.org.
Computational Details. Quantum chemical calculations were
carried out to probe the electronic structure of the As-As and As-P
bonds contained within the new product molecules, to be compared
with some reference systems analyzed by the same methods.
Geometry optimizations were carried out using the Amdsterdam
Density Functional (ADF) code version 2006.01,56 using the LDA
JA906550H
(50) Sheldrick, G. M. SADABS; Bruker AXS Inc.: 2005.
(51) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990,
46, 467–473.
(52) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008,
64, 112–122.
(57) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200–
1211.
(53) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: 1997.
(54) Mu¨ller, P.; Herbst-Irmer, R.; Spek, A. L.; Schneider, T. R.; Sawaya,
M. R. Crystal structure refinement: A crystallographer’s guide to
SHELXL; Oxford University Press: Oxford, 2006.
(55) This material is available free of charge at http://www.ccdc.cam.ac.uk/
data_ request/cif.
(56) (a) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.;
van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem.
2001, 22, 931–967. (b) http://www.scm.com, ADF 2006.01, SCM,
Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands.
(58) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(b) Handry, N. C.; Co-hen, A. J. Mol. Phys. 2001, 99, 403–412. (c)
Baker, J.; Pulay, P. J. Comput. Chem. 2003, 24, 1184–1191.
(59) (a) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys.
1993, 99, 4597–4610. (b) van Lenthe, E.; Baerends, E. J.; Snijders,
J. G. J. Chem. Phys. 1994, 101, 9783–9792. (c) van Lenthe, E.; Ehlers,
A.; Baerends, E.-J. J. Chem. Phys. 1999, 110, 8943–8953.
(60) Michalak, A.; DeKock, R. L.; Ziegler, T. J. Phys. Chem. A 2008, 112,
7256–7263.
(61) Kohout, M. DGrid, version 4.3, 2008.
9
J. AM. CHEM. SOC. VOL. 131, NO. 44, 2009 16243