A ligand composed of dinitrogen and methyldiphenylphosphane in a cationic molybdenum complex

Article abstract of DOI:10.1002/anie.200463057

(Chemical Equation Presented) Dinitrogen connects molybdenum and phosphorus: The diazo(triorgano)-phosphorane ligand Ph₂MePN ₂ is multiply bonded through its N-terminus to molybdenum in the depicted cationic complex. The Ph₂MePN₂ ligand was synthesized within molybdenum's coordination sphere by rational assembly from a precursor N₂ complex. Computational studies reveal the N-N bond to be highly activated in the complex, which nonetheless readily fragments (with N₂ loss) upon one-electron reduction.

Full text of DOI:10.1002/anie.200463057

Communications
N,P Ligands
denum center.[3] The X-ray structural analysis, as well as
topological charge density analysis based on density func-
tional calculations, indicate that the diazophosphorane ligand
is stabilized effectively by equal contributions from the two
following limiting resonance structures: (triorgano-
A Ligand Composed of Dinitrogen and
Methyldiphenylphosphane in a Cationic
Molybdenum Complex**
Tetsuro Murahashi, Christopher R. Clough,
Joshua S. Figueroa, and
Christopher C. Cummins*
It is well recognized that a reactive chemical
fragment, which in its free state may be fleeting,
not isolable, and difficult to observe, may some-
times be stabilized or trapped in an isolable form
by coordination to a transition-metal center.
Herein we report a unique ligand made of N2 and
PPh2Me, which is stabilized in a cationic molyb-
denum complex. It does not seem likely that such
a “diazophosphorane” NNPR3 ligand[1] would be
obtained simply by addition of PR3 to a terminal
metal–N2 complex, because the b-N (terminal)
atom of coordinated N2 typically is at least
moderately nucleophilic. Furthermore, the
NNPR3 moiety is expected to be thermodynami-
cally unstable in view of the great stability of N2
and PR3. To the best of our knowledge, the
diazophosphorane NNPR3 unit has not been
identified previously as a ligand in spite of the
prevalent use of tertiary phosphines as ligands in
N2 coordination chemistry. However, the NNPR3
moiety is found as a chemical fragment in some
known phosphorus–nitrogen compounds such as
+
= À =
À = À
À =
R2C N N PR3, [R N N PR3] , CN N PR3,
and [NNNPR3]+.[1,2]
As hinted at by the structures of these known
compounds, we suggest that the NNPR3 moiety
has the potential for stabilization by binding with
a metal center at its N-terminus. This study
presents a rational construction of the surprising
NNPR3 ligand by stepwise functionalization of
dinitrogen coordinated to a formally d2 molyb-
Scheme 1. Synthesis of [1-NNPPh2Me][OTf], resonance structures, and reactivity.
R=tBu, Ar=3,5-dimethylphenyl.
[*] Dr. T. Murahashi,[+] C. R. Clough, J. S. Figueroa,
Prof. Dr. C. C. Cummins
+ = À =
phosphoranylidene)hydrazido(2À) [Mo] ( N N PR3) and
+
= À
(triorganophosphonio)diazenido [Mo](N N PR3); these
Department of Chemistry
are shown as A and B, respectively, in Scheme 1.
Massachusetts Institute of Technology
77 Massachusetts Avenue, Room 2-227
Cambridge, MA 02139-4307 (USA)
Fax: (+1)617-258-5700
The anionic end-on dinitrogen complex [(N2)Mo{N-
(tBu)Ar}3]1À [1-N2]À (Ar= 3,5-Me2C6H3), which is conve-
niently obtained as its sodium salt by the reaction of
[Mo{N(tBu)Ar}3] (1) with excess Na/Hg under 1 atm N2,[4]
was derivatized smoothly by reaction with Ph2PCl in THF to
afford the diphenylphosphinodiazenido complex 1-
NNPPh2.[5] The 31P{1H} NMR signal of 1-NNPPh2 appears at
d = 67.3 ppm as a singlet in C6D6. Monitoring of the reaction
in C6D6 or [D8]THF showed two major by-products: [Mo{N-
(tBu)Ar}3] (1) and Ph2PPPh2, with additional formation of
[ClMo{N(tBu)Ar}3][5] in small quantities. The molar ratio of 1-
NNPPh2/Ph2PPPh2 in C6D6 after 1 h at 238C was 78/22. The
E-mail: ccummins@mit.edu
[+] Permanent address:
Department of Applied Chemistry
Graduate School of Engineering
Osaka University
Suita, Osaka, 565-0871 (Japan)
[**] This work was supported by the United States National Science
Foundation (CHE-0316823). Funding from the Japan Society for the
Promotion of Science (JSPS) for Research Abroad (to T.M.) is also
acknowledged.
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Chemie
orange complex 1-NNPPh2 was isolated in 10% yield after
repeated recrystallization from pentane at À358C. The low
yield of isolated 1-NNPPh2 was due in part to the spontaneous
decomposition of 1-NNPPh2 to [Mo{N(tBu)Ar}3],[6]
Ph2PPPh2, and N2 during the recrystallization. The structure
of 1-NNPPh2 was determined by X-ray structure analysis
the PPh2(CH3) moiety in [1-NNPPh2Me][OTf] appears at d =
3.03 ppm as a doublet (2JPH = 13 Hz). The 31P{1H} NMR
spectrum of [1-NNPPh2Me][OTf] shows a singlet at d =
20.0 ppm. The structure of [1-NNPPh2Me][OTf] was deter-
mined in a single-crystal X-ray diffraction study (Figure 2).
À
À
(Figure 1). The N4 N5 (1.261(4) ꢀ) and N5 P1 (1.736(4) ꢀ)
Figure 2. Molecular structure of cation [1-NNPPh2Me]+ with thermal
ellipsoids drawn at the 50% probability level. The triflate counteranion
is not shown.
Figure 1. Molecular structure of 1-NNPPh2 with thermal ellipsoids
drawn at the 50% probability level.
The Mo1-N4-N5 angle (166.7(2)8) and N4-N5-P1 angle
(129.2(2)8) in [1-NNPPh2Me][OTf] differ only slightly from
corresponding angles in 1-NNPPh2 (Figure 1). The system is
thus still close to linear at the a-N atom and rather bent at the
=
À
lengths are consistent with a N N double bond and N P
single bond, respectively. Complex 1-NNPPh2 can thus be
À =
formulated as the phosphinodiazenido derivative [(Ph2P N
N)Mo{N(tBu)Ar}3].[7] The Mo1-N4-N5-P1 skeleton is bent at
b-N atom. However, the N5 P1 interatomic distance
À
À
the N5 atom (N4-N5-P1 122.6(3)8). The Mo1 N4 length
(1.753(3) ꢀ) and Mo1-N4-N5 angle (171.1(3)8) are similar to
(1.626(3) ꢀ) is 0.11 ꢀ shorter than the corresponding distance
in 1-NNPPh2 (1.736(4) ꢀ), and is similar to that found in a
[9]
=
those found for the related trimethylsilyldiazenido complex
typical iminophosphorane (e.g. 1.602 ꢀ for Ph-N PPh3).
[4]
=
À
[(Me3Si-N N)Mo{N(tBu)Ar}3]. Strong multiple bonding
Note also that the N4 N5 (1.299(3) ꢀ) bond length is
between the a-N atom and Mo in these systems is indicated
by the short bond length, the linear bond angle, and the
isolobal relationship with nitrosyl complexes such as
[(ON)Mo{N(tBu)Ar}3].[8] Because of the isolobal relationship
between the quartet ground state Mo{N(tBu)Ar}3 fragment[6]
and a ground state N atom, we recognize that nitrosyl
[(ON)Mo{N(tBu)Ar}3] is isolobal with the triatomic N2O
molecule: both systems share a stable (p0)4(p1)4 configuration
across linear MoNO and NNO moieties, respectively.
increased relative to 1-NNPPh2; in other words, greater
activation of the N2 unit has been triggered by the methyl-
À
ation reaction, although the N4 N5 distance in [1-
NNPPh2Me][OTf] is still considerably shorter than that
observed for phosphoranylidene hydrazone systems such as
[10]
= À =
À
Ph3P N N CPh2 (N N = 1.388 ꢀ).
Of further interest:
À
the Mo1 N4 (1.739(2) ꢀ) bond length in [1-NNPPh2Me]-
[OTf] is slightly shorter than that found for precursor 1-
NNPPh2.
Treatment of 1-NNPPh2 with MeOTf (1 equiv) in C6D6
rapidly provided the triflate salt [(Ph2MePNN)Mo{N-
(tBu)Ar}3][OTf] [1-NNPPh2Me][OTf] of the desired cation,
in essentially quantitative fashion at ambient temperature. No
N-methylated product was observed, even when excess
MeOTf was employed. In contrast, it has been reported that
the reaction of Na[1-N2] with MeOTf (2 equiv) afforded the
doubly N-methylated product [(Me2NN)Mo{N(tBu)Ar}3]-
[OTf].[4] The yellow-orange complex [1-NNPPh2Me][OTf]
was isolated in 41% yield from Na[1-N2] without purification
The isolated salt [1-NNPPh2Me][OTf] is stable in C6D6
solution in the dark for several days at ambient temperature
and even at 508C for one day. Interestingly, degradation of the
NNPPh2Me ligand was observed when [1-NNPPh2Me][OTf]
was photoirradiated (Rayonet RPR-2537 ꢀ lamps, Pyrex
tube) in C6D6. After irradiation for 1 h at ca. 408C,
approximately 50% of [1-NNPPh2Me][OTf] was converted
into the triflate complex [Mo{N(tBu)Ar}3(OTf)] (1-OTf) and
PPh2Me (ca. 90% based on the consumption of [1-
NNPPh2Me][OTf]) and presumably N2. The purple triflate
complex 1-OTf was identified by an independent synthesis,
1
of the intermediate 1-NNPPh2. The H NMR resonance for
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Communications
[1-NNPPh2Me][OTf]:
A
solution of Na[1-N2] (600 mg,
that is, oxidation of [Mo{N(tBu)Ar}3] with AgOTf or [Cp2Fe]-
0.888 mmol) in THF (6 mL) was combined with a solution of
Ph2PCl (159 mL, 0.888 mmol) in THF (2 mL) at À358C. After the
reaction mixture was stirred for 5 min, THF solvent was removed in
vacuo. Then pentane (10 mL) was added, the mixture was filtered
through Celite, and pentane was removed in vacuo to give a crude
material containing 1-NNPPh2 (387 mg). To a pentane solution of the
crude product, MeOTf (55 mL, 0.49 mmol) was added, and the
mixture was stirred for 5 min. Yellow powder was immediately
generated. The liquid was decanted off, and the yellow powder was
washed with pentane twice, and dried in vacuo. Recrystallization from
CH2Cl2/Et2O/n-hexane gave yellow-orange microcrystals of [1-
NNPPh2Me][OTf] (363 mg, 41% yield from Na[1-N2]). 1H NMR
(C6D6): d = 8.19 (dd, J = 13.1 Hz, J = 8.4 Hz, 4H), 7.32 (m, 4H), 7.15
(overlap with solvent residuals, 2H), 6.62 (s, 3H), 5.77 (brs, 6H), 3.03
(d, J = 13.2 Hz, 3H), 1.97 (s, 18H), 1.16 ppm (s, 27H); 31P{1H} NMR
[OTf]. The reaction of [1-NNPPh2Me][OTf] with [nBu4N]
[CN] in C6D6 upon mixing afforded [(NC)Mo{N(tBu)Ar}3] (1-
CN)[11] and PPh2Me + N2 almost quantitatively (Scheme 1).
Treatment of [1-NNPPh2Me][OTf] with cobaltocene
(1 equiv) in C6D6 gave [Mo{N(tBu)Ar}3] (1) and PPh2Me +
N2 again in almost quantitative yield. Thus, as expected, the
fragile NNPPh2Me ligand in [1-NNPPh2Me][OTf] is subject to
facile fragmentation to its PPh2Me and N2 components under
exposure to some chemical or physical stimuli (Scheme 1).
To gain insight into the electronic structure of [1-
NNPPh2Me][OTf] we performed density functional calcula-
tions (ADF 2004.01, BP86/TZ2P) on the model compounds
[(H2PNN)Mo(NH2)3] (1’-NNPH2) and [(H3PNN)Mo(NH2)3]+
([1’-NNPH3]+).[12] A topological charge density analysis[13] was
carried out with aid of the Xaim program,[14] based on the
calculated charge density distributions. The electronic charge
(C6D6): d = 20.0 ppm (s); FTIR (C6D6, KBr): n˜NN = 1586 cmÀ1
Elemental analysis calcd C 59.93, H 6.74, N 6.99; found: C 59.96, H
6.78, 6.92. Crystallographic data for [1-NNPPh2Me][OTf]:
.
N
C50H67N5PSO3F3Mo, M = 1002.07, space group P21/n, a =
21.3981(17), b = 10.8084(8), c = 22.1557(17) ꢀ; b = 95.1550(10)8, V=
5103.4(7) ꢀ3, Z = 4, F(000) = 2104, 1cald = 1.304 gcmÀ3, 577 parame-
ters refined with 11891 reflections with I > 2s(I) to R = 0.0505.
CCDC-258978 (1-NNPPh2), CCDC-258979 ([1-NNPPh2Me]-
[OTf]) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
1-OTf: Mo(N[tBu]Ar)3 (1, 273 mg, 0.437 mmol) was dissolved in
pentane (10 mL), and AgOTf (112 mg, 0.437 mmol) was added at
room temperature. The mixture was stirred vigorously for 4 h. The
color of the mixture changed to deep purple. Then the mixture was
filtered through Celite, and the filtrate was concentrated. Crystal-
lization from pentane at À358C gave deep purple microcrystals of 1-
OTf (43.8 mg, 13% yield). 1H NMR (C6D6): d = 6.06 (s, 3H), 5.94
(brs, 6H), 2.11 (s, 27H), 1.86 ppm (s, 18H); Elemental analysis calcd
C 57.43, H 7.03, N 5.43; found: C 57.59, H 7.12, N 5.38. An NMR
monitoring experiment of the reaction of [Mo{N(tBu)Ar}3] with
[Cp2Fe][OTf] (1 equiv) in C6D6/[D8]THF (v/v = 9/1) showed quanti-
tative formation of 1-OTf.
À
density value found at the (3,À1) critical point (1b) of the N
N bond in [1’-NNPH3]+ (0.4016 ea0À3) was lower than that in
1’-NNPH2 (0.4586 ea0À3). The calculated bond order (n) is 1.6
for [1’-NNPH3]+ (cf. 1.9 for 1’-NNPH2); this estimation is
derived from Baderꢁs linear correlation between 1b values and
[13]
À
À
N N bond order (n). These data indicate that the N N
bond in the cationic [Mo](NNPR3) complex is intermediate
between a single and a double bond.[15] The information
obtained from computational analysis, combined with the
experimental structural data, reveal that the bonding in cation
[1-NNPPh2Me]+ is characterized by resonance between the
hydrazido(2-) form A and the diazenido form B (Scheme 1).
A similar pair of resonance structures was proposed in
connection with the particular diazenylphosphonium salt [4-
À
Et2NC6H4NNPPh3][BF4] and its short P N bond length
(l.648 ꢀ).[16]
In summary, this work demonstrates that the remarkable
ligand NNPR3 composed of dinitrogen and methyldiphenyl-
phosphane can be obtained and is understandable in the
context of a cationic molybdenum system. Because one-
electron reduction of cation [1-NNPPh2Me]+ leads to frag-
mentation to the three known molecules PPh2Me, N2, and
Received: December 24, 2004
Published online: March 22, 2005
Keywords: diazenido ligands · dinitrogen ligands ·
[Mo{N(tBu)Ar} ] (1), it may be said that this remarkable
.
3
molybdenum · N,P ligands · phosphine ligands
three-component system is bound together entirely by the
absence of a single electron.
[1] The abbreviation R which appears in some general formulas in
this paper is not necessarily identical to the R = tBu as in 1 and
complexes derived therefrom.
= À =
Experimental Section
[2] Selected examples; a) H2C N N PPh3: G. Wittig, W. Haag,
=
1-NNPPh2: A solution of Na[1-N2] (500 mg, 0.740 mmol) in THF
(6 mL) was combined with a solution of Ph2PCl (133 mL, 0.740 mmol)
in THF (2 mL) at À358C. After the reaction mixture had been stirred
for 5 min, THF solvent was removed in vacuo. Then pentane (10 mL)
was added, the mixture was filtered through Celite, and pentane was
removed in vacuo. Repeated recrystallization from pentane gave
Chem. Ber. 1955, 88, 1654; b) [Ph-N N-PPh3][BF4]: R. Yama-
shita, K. Kikukawa, F. Wada, T. Matsuda, J. Organomet. Chem.
1980, 201, 463; c) CN N PPh3: B. Weinberger, W. P. Fehlham-
mer, Chem. Ber. 1985, 118, 42; d) [NNNPPh3][SbCl6]: W. Buder,
A. Schmidt, Chem. Ber. 1973, 106, 3812.
À =
[3] It is known that some diazo compounds are stabilized by
coordination to a d2 metal center. For stabilization of terminal
organoazido (NNNR) ligands; a) G. Proulx, R. G. Bergman, J.
Am. Chem. Soc. 1995, 117, 6382; b) M. G. Fickes, W. M. Davis,
C. C. Cummins, J. Am. Chem. Soc. 1995, 117, 6384; c) G. Proulx,
R. G. Bergman, Organometallics 1996, 15, 684; d) G. Guillemot,
E. Solari, C. Floriani, C. Rizzoli, Organometallics 2001, 20, 607.
For stabilization of diazoalkane (NNCR2) ligands; e) M. Darti-
guenave, M. J. Menu, E. Deydier, Y. Dartiguenave, H. Siebald,
Coord. Chem. Rev. 1998, 178–180, 623, and references therein.
1
analytically pure 1-NNPPh2 (62.8 mg, 10% yield). H NMR (C6D6):
d = 8.01 (t, J = 7.2 Hz, 4H), 7.25 (t, J = 8.4 Hz, 4H), 7.10 (t, J = 7.2 Hz,
2H), 6.64 (s, 3H), 6.03 (s, 6H), 2.04 (s, 18H), 1.53 (s, 27H); 31P{1H}
NMR (C6D6): d = 67.3 (s); Elemental analysis calcd C 68.80, H 7.70, N
8.36: found: C 69.03, H 8.19, N 7.96. Crystallographic data for 1-
NNPPh2: C48H64N5PMo, M = 837.97, space group P21/c, a =
22.6095(19), b = 10.5661(9), c = 19.4732(16) ꢀ; b = 94.183(2)8, V=
4639.6(7) ꢀ3, Z = 4, F(000) = 1776, 1calcd = 1.20 gcmÀ3, 542 parameters
refined with 7280 reflections with I > 2s(I) to R = 0.0516.
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[4] J. C. Peters, J.-P. F. Cherry, C. Thomas, L. M. Baraldo, D. J.
Mindiola, W. M. Davis, C. C. Cummins, J. Am. Chem. Soc. 1999,
121, 10053.
[5] A. Fꢂrstner, C. Mathes, C. W. Lehmann, J. Am. Chem. Soc. 1999,
121, 9453.
[6] C. E. Laplaza, M. J. A. Johnson, J. C. Peters, A. L. Odom, E.
Kim, C. C. Cummins, G. N. George, I. J. Pickering, J. Am. Chem.
Soc. 1996, 118, 8623.
[7] A number of diazenido transition metal complexes are known;
a) J. P. Collman, L. S. Hegedus, J. R. Norton, R. G. Finke,
Principles and Applications of Organotransition Metal Chemis-
try, University Science Book, New York, 1987; b) D. Sutton,
Chem. Soc. Rev. 1975, 3, 443, and references therein. For
synthesis from dinitrogen complex see c) J. Chatt, A. A.
Diamantis, G. A. Heath, N. E. Hooper, G. J. Leigh, J. Chem.
Soc. Dalton Trans. 1977, 688; d) M. Hidai, Y. Mizobe, Chem. Rev.
1995, 95, 1115, and references therein.
[8] C. E. Laplaza, A. L. Odom, W. M. Davis, C. C. Cummins, J. Am.
Chem. Soc. 1995, 117, 4999.
[9] E. Bꢃhm, K. Dehnicke, J. Beck, W. Hiller, J. Strꢄhle, A. Maurer,
D. Fenske, Z. Naturforsch. B 1988, 43, 138.
[10] D. Bethell, M. P. Brown, M. M. Harding, C. A. Herbert, M. M.
Khodaei, M. I. Rios, K. Woolstencroft, Acta Crystallogr. Sect. B
1992, 48, 683.
[11] J. C. Peters, L. M. Baraldo, T. A. Baker, A. R. Johnson, C. C.
Cummins, J. Organomet. Chem. 1999, 591, 24.
[12] ADF Program Package (release 2004.1) was used. The opti-
mized structures showed good agreement with the metrical
parameters found in the crystal structure studies of 1-NNPPh2
and [1-NNPPh2Me][OTf], except for the N-N-P angle in [1’-
NNPH3]+ which appears marginally smaller compared to that
found experimentally for [1-NNPPh2Me][OTf]. Selected struc-
tural parameters of optimized 1’-NNPPH2 and [1’-NNPH3]+: for
À
À
À
1’-NNPH2: Mo N 1.757, N N 1.243, N P 1.770 ꢀ; Mo-N-N
+
À
À
174.6, N-N-P 120.948; for [1’-NNPH3] : Mo N 1.735, N N 1.295,
À
N P 1.627 ꢀ; Mo-N-N 172.8, N-N-P 121.68.
[13] R. F. W. Bader, Atoms in Molecules, a Quantum Theory, Oxford
University Press, New York, 1990. For calibration of linear 1b
versus n relationship, we performed similar calculations on
hydrazine, trans-diazene, and dinitrogen, the values of 1b(NN)
for which are taken to represent n = 1, 2, and 3, respectively.
[14] J. C. Ortiz Alba, C. Bo Jane, Xaim, Universitat Rovira I Virgili,
Tarragona, Spain.
À
[15] Analysis of the P N bond revealed a substantially higher 1b
value (0.1984 ea0À3) in [1’-NNPH3]+ than that in 1’-NNPH2
(0.1506 ea0À3). This is consistent with considerable iminophos-
phorane character in [1’-NNPH3]+. For the bonding on the
=
iminophosphorane N P bond see J. Koketsu, Y. Ninomiya, Y.
Suzuki, N. Koga, Inorg. Chem. 1997, 36, 694, and references
therein.
[16] F. W. B. Einstein, D. Sutton, P. L. Vogel, Can. J. Chem. 1978, 56,
891.
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