Synthesis and structure of a terminal dinitrogen complex of nickel

Article abstract of DOI:10.1139/v05-011

Reaction of petroleum ether solutions of [(dtbpe)Ni]₂(η ²,μ-C₆H₆) (1, dtbpe = 1,2-bis(di-tert- butylphosphino)ethane) with triphenylphosphine under a dinitrogen atmosphere gives the Ni(0) dinitrogen adduct (dtbpe)Ni(N₂)(PPh₃) (2), which can be isolated as dark red crystals in 87% yield. The X-ray crystal structure of 2 reveals pseudotetrahedral geometry about Ni and a terminal dinitrogen ligand with Ni-N(1) = 1.830(2) A, N(1)-N(2) = 1.112(2) A, and Ni-N(1)-N(2) = 177.5(2)°.

Full text of DOI:10.1139/v05-011

328
Synthesis and structure of a terminal dinitrogen
complex of nickel1
Rory Waterman and Gregory L. Hillhouse
Abstract: Reaction of petroleum ether solutions of [(dtbpe)Ni]2(η2,µ-C6H6) (1, dtbpe = 1,2-bis(di-tert-butylphosphino)ethane)
with triphenylphosphine under a dinitrogen atmosphere gives the Ni(0) dinitrogen adduct (dtbpe)Ni(N2)(PPh3) (2),
which can be isolated as dark red crystals in 87% yield. The X-ray crystal structure of 2 reveals pseudotetrahedral ge-
ometry about Ni and a terminal dinitrogen ligand with Ni—N(1) = 1.830(2) Å, N(1)—N(2) = 1.112(2) Å, and Ni-N(1)-
N(2) = 177.5(2)°.
Key words: dinitrogen, nickel, X-ray.
Résumé : La réaction du [(dtbpe)Ni]2(η2,µ-C6H6) (1, dtbpe = 1,2-bis(di-tert-butylphosphino)éthane) en solution dans
l’éther de pétrole avec de la triphénylphosphine, sous une atmosphère de diazote, conduit à la formation de l’adduit
Ni(0) diazote (dtbpe)Ni(N2)(PPh3) (2) que l’on peut isoler sous la forme de cristaux rouges avec un rendement de
87 %. Une étude du composé 2 par diffraction des rayons X révèle une géométrie pseudotétraédrique autour du Ni et
la présence d’un ligand diazote terminal avec des distances Ni—N(1) = 1,830(2) Å, N(1)—N(2) = 1,112(2) Å et un
angle Ni-N(1)-N(2) = 177,5(2)°.
Mots clés : diazote, nickel, rayons X.
[Traduit par la Rédaction] Waterman and Hillhouse 331
Introduction
Most Ni–N2 species have been dimeric or polynuclear (7–9).
The dinitrogen-bridged complex [(PCy3)2Ni]2(µ-N2) has
been reported and structurally characterized (9), and an un-
usual side-bound N2 complex, [(C6H5Li)6Ni2(N2)(Et2O)2]2,
has been reported (7). Surprisingly, no structural data exist
for simple end-on N2 complexes of nickel, even though this
is by far its most common coordination mode. Herein we re-
port the preparation and structural characterization of the
first example of an isolable nickel complex containing a ter-
minal, end-on bound N2 ligand.
The 1965 report from University of Toronto chemistry
professor Bert Allen and his graduate student Caesar Senoff
of the isolation of stable dinitrogen complex salts of the for-
mulation [(NH3)5Ru(N2)2+][X–]2 from the reaction of ruthe-
nium trichloride with hydrazine hydrate in aqueous solution
rocked the scientific community (1, 2). The crystallographic
determination of the solid-state structure of the Allen and
Senoff complex definitively corroborated the proposed bind-
ing mode of the N2 ligand as end-on bound to Ru in an ap-
proximately linear fashion (3). In the intervening 40 years
since that landmark discovery, nearly every transition metal
has been reported to utilize dinitrogen as a ligand, and many
binding modes, including bridging and side-bound, have
been observed for N2 ligands (4). Because the dinitrogen
ligand is often quite labile, N2 complexes can serve as useful
starting materials for a variety of synthetic purposes. Over
the years much research effort has focused on designing sys-
tems containing ligated N2 in which coordination to a metal
sufficiently activates the N—N triple bond that it can be sub-
sequently functionalized (or “fixed”), such as in processes
that cleave the N—N triple bond (5) or model enzymatic
systems that perform catalytic N2 reduction (6).
Experimental
General considerations
Unless otherwise stated, all operations were performed in
a M. Braun Lab Master dry box under an atmosphere of
purified nitrogen. Petroleum ether was dried by passage
through activated alumina and Q-5 columns. C6D6 was pur-
chased form Cambridge Isotope Laboratory (CIL), degassed,
and dried over CaH2 or activated 4 Å molecular sieves.
Celite®, alumina, and 4 Å molecular sieves were activated
under vacuum overnight at a temperature above 180 °C.
Anhydrous solvents such as benzene and diethylether were
purchased from Acros or Fischer, stirred over sodium metal,
and filtered through activated alumina. [(dtbpe)Ni]2(η2,µ-
C6H6) (1, dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) was
Only a few examples of isolated nickel complexes pos-
sessing a dinitrogen ligand have appeared in the literature.
Received 9 September 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 22 March 2005.
R. Waterman2 and G.L. Hillhouse.3 Department of Chemistry, The University of Chicago, 5735 S. Ellis Avenue, Chicago, IL
60637, USA.
1This article is part of a Special Issue dedicated to Dinitrogen Chemistry.
2Present address: Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720, USA.
3Corresponding author (e-mail: g-hillhouse@uchicago.edu).
Can. J. Chem. 83: 328–331 (2005)
doi: 10.1139/V05-011
© 2005 NRC Canada

Waterman and Hillhouse
329
prepared according to the literature procedure (10). All other
chemicals were used as received. IR data (Fluorolube S-20
mulls, CaF2 plates) were measured with a Nicolet 670 FT-IR
instrument. Elemental analyses were performed by Desert
ware and sources of scattering factors are contained in the
SHELXTL (version 5.1) program library (G. Sheldrick,
Bruker Analytical X-ray Systems, Madison, Wisconsin). De-
tails of the data set and the refinement are given in Table 1.
Table 2 lists selected metrical parameters of the structure of
2.4
1
Analytics (Tucson, Arizona). H, 13C, and 31P NMR spectra
were recorded on Bruker 500 and 400 MHz NMR spectrom-
1
eters. H and 13C NMR data are reported with reference to
solvent resonances (residual C6D5H in C6D6, 7.16 ppm;
C6D6, 128.0 ppm). 31P NMR spectra are reported with re-
spect to external 85% H3PO4 (0 ppm).
Results and discussion
Reaction of petroleum ether solutions of the benzene com-
plex [(dtbpe)Ni]2(η2,µ-C6H6) (1) with PPh3 under a nitrogen
atmosphere gives a dark red solution from which the
dinitrogen complex (dtbpe)Ni(N2)(PPh3) (2) can be isolated
as analytically pure dark red crystals in 87% yield (eq. [1]).
Preparation of (dtbpe)Ni(N2)(PPh3) (2)
In a nitrogen-filled, inert-atmosphere glovebox, a 25 mL
round-bottomed flask was charged with 72 mg (0.087 mmol)
of [(dtbpe)Ni]2(η2,µ-C6H6) (1) and 45 mg (0.174 mmol) of
PPh3 followed by 14 mL of petroleum ether. The solution
immediately became blood red in color and was filtered after
stirring for 90 min. The volume of the solution was reduced
to ca. 5 mL, and upon cooling to –35 °C, dark red crystals
were deposited, which were collected by filtration and dried
to give 2 (100 mg, 0.149 mmol, 87% yield). IR (Fluorolube
mull, CaF2, cm–1): 3050–2850 (vs br), 2072 (νNN, vs), 1585
(m), 1479 (s), 1434 (s), 1392 (m), 1360 (m). 1H NMR
(22 °C, 500 MHz, C6D6) δ: 7.94 (t, C6H5, 6H), 7.15 (d,
C6H5, 6H), 7.07 (t, C6H5, 3H), 1.55 (m, CH2, 4H), 1.17
(mult, CH3, 36H). 13C NMR (22 °C, 125.8 MHz, C6D6) δ:
133.8 (d, JPC = 14.5 Hz, C6H5), 129.8 (s, C6H5), 127.9 (s,
C6H5), 127.7 (s, C6H5), 34.6 (m, JPC = 4.4 Hz, C(CH3)3),
30.7 (br t, JPC = 3.6 Hz, CH3), 23.4 (m, CH2). 31P NMR
(22 °C, 202.4 MHz, C6D6) δ: 73.5 (d, JPP = 95.7 Hz, PtBu2),
28.7 (t, JPP = 95.7 Hz, PPh3). Anal. calcd for C36H55N2NiP3:
C 64.78, H 8.31, N 4.20; found: C 65.11, H 8.28, N 3.31.
Even in the presence of excess PPh3, only 2 is formed and
no tetrakisphosphine species is observed. Characterization of
2 followed from its NMR (1H, 13C, and 31P) and IR spectra,
and a single crystal X-ray diffraction study. The 31P NMR
spectrum of 2 exhibits a single resonance for the dtbpe
ligand (δ 73.5, doublet) and a single resonance for the PPh3
2
ligand (δ 28.7, triplet) with JPP = 95.7 Hz for both, indicat-
ing pseudotetrahedral geometry about nickel. The solid-state
IR spectrum of 2 shows a strong νNN at 2072 cm–1.
Crystallographic data collection, structure solution, and
refinement
Single crystals of 2 were grown by slowly cooling con-
centrated hexane solutions of the complex. A perspective
view of the molecular structure of 2 is shown in Fig. 1 with
select metrical parameters given in Table 2. The complex
features a pseudotetrahedral nickel center with a terminal N2
ligand with a short N—N bond of 1.112(2) Å. The nitrogen
ligand is approximately linear with Ni-N(1)-N(2) = 177.5(2)°
and Ni—N(1) = 1.830(2) Å. These values can be compared
with those found in the structure of the bridged dinitrogen
complex [(PCy3)2Ni]2(µ-N2) (9) which exhibits Ni—N bond
lengths of 1.77 and 1.79 Å, and N—N = 1.12 Å (standard
errors were not given in the report).
A dark red block of 2 was selected under a stereomicro-
scope while immersed in mineral oil to avoid possible reac-
tion with air. The crystal was removed from the oil using a
tapered fiber that also served to hold the crystal for data col-
lection, and mounted and centered on a Bruker SMART
APEX system in a nitrogen cold stream. Rotation and still
images showed diffractions to be sharp, while frames sepa-
rated in reciprocal space were obtained and provided an
orientation matrix and initial cell parameters. Final cell para-
meters were obtained from the full data set. A “hemisphere”
data set was obtained, which samples approximately 1.2
hemispheres of reciprocal space to a resolution of 0.84 Å us-
ing 0.3° steps in ω using 10 s integration times for each
frame. The space group was determined as P-1 based on sys-
tematic absences and intensity statistics. Direct methods
were used to locate non-hydrogen atoms from the electron
difference map. All heavy atoms were converted to and re-
fined anisotropically. Hydrogen atoms were refined isotro-
pically and fixed at calculated positions. A semiempirical
absorption correction was applied from ψ-scans. All soft-
Complex 2 is, to the best of our knowledge, the only ex-
ample of an isolated nickel complex with a terminal N2
ligand. It is noteworthy that a family of nickel(II) complexes
of the formula L2NiX2 (X = Cl or Br; L = PEt3, PEt2Ph, or
P-n-Bu3) can be reduced in the presence of phosphine to
generate L3Ni(N2) species that have not been isolated, but
lose N2 to give tetrakisphosphine nickel(0) complexes L4Ni
upon precipitation (11). Solution IR, 31P NMR, and elec-
tronic spectra for these L3Ni(N2) complexes led to their for-
mulation as pseudotetrahedral at Ni with terminal N2 ligands
4 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml for information on ordering electronically).
CCDC 261287 contains the crystallographic data for this manuscript. These data can be obtained, free of charge, via
ww.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2005 NRC Canada

330
Can. J. Chem. Vol. 83, 2005
Table 1. Crystal and structure refinement for (dtbpe)Ni(N2)(PPh3) (2).
Empirical formula
C36H55N2NiP3
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
667.44
100
0.710 73
Triclinic
P-1
Unit cell dimensions
a (Å)
b (Å)
9.7911(7)
13.7423(10)
13.8687(10)
81.6860(10)
76.5680(10)
80.1250(10)
1777.4(2)
2
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mm–1)
F(000)
1.247
0.707
716
Crystal size, description
θ Range for data collection
Index ranges
210 µm × 200 µm × 200 µm, dark red cube
2.02–28.30
–12 ≤ h ≤ 12, –18 ≤ k ≤ 18, –18 ≤ l ≤18
19 902
8 305 (Rint = 0.0336)
94.0%
Semiempirical from ψ scans
0.8714 and 0.8656
Full-matrix least-squares on F2
8 305/0/379
Reflections collected
Independent reflections
Completeness to θ = 28.30
Absorption correction
Max and min transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices (I > 2σ(I))
R indices (all data)
Largest diff. peak and hole (e Å–3)
1.058
R1 = 0.0403, wR2 = 0.0968
R1 = 0.0461, wR2 = 0.0999
1.188 and –0.365
Fig. 1. A perspective view of the structure of (dtbpe)Ni(N2)(PPh3)
(2). Hydrogen atoms have been omitted for clarity.
Table 2. Selected bond lengths and angles for
(dtbpe)Ni(N2)(PPh3) (2).
Bond lengths (Å)
Ni—N(1)
Ni—P(3)
Ni—P(1)
Ni—P(2)
N(1)—N(2)
1.830(2)
2.2048(6)
2.2302(6)
2.2328(5)
1.112(2)
Bond angles (°)
N(2)-N(1)-Ni
N(1)-Ni-P(3)
N(1)-Ni-P(1)
P(3)-Ni-P(1)
N(1)-Ni-P(2)
P(3)-Ni-P(2)
P(1)-Ni-P(2)
177.5(2)
96.12(5)
112.07(5)
123.66(2)
109.03(5)
122.60(2)
93.62(2)
(11a). These transient complexes have N–N stretching fre-
quencies in the range of 2050–2100 cm–1, which can be
compared to the corresponding stretch at 2072 cm–1 in 2 and
2028 cm–1 in [(PCy3)2Ni]2(µ-N2), although the latter value,
recorded in solution, has been suggested to be for mono-
© 2005 NRC Canada

Waterman and Hillhouse
331
3. F. Bottomley and S.C. Nyberg. Acta Crystallogr. B24, 1289
(1968).
meric (PCy3)2Ni(N2) that results from dissociation of the
dimer in toluene (9). In the light of these previous reports,
the stability of 2 is undoubtedly the result of the steric bulk
of the dtbpe ligand, which prevents expulsion of N2 and co-
ordination of a second PPh3 ligand to give the hypothetical
(dtbpe)Ni(PPh3)2. Finally, an unusual “Ni(I)” complex,
(PEt3)2Ni(H)(N2), has been formulated on the basis of UV–
vis and IR data (12), but the absence of magnetic suscepti-
4. (a) J. Chatt, J.R. Dilworth, and R.L. Richards. Chem. Rev. 78,
589 (1978); (b) M.D. Fryzuk and S.A. Johnson. Coord. Chem.
Rev. 200, 379 (2000); (c) F. Barriere. Coord. Chem. Rev. 236,
71 (2003); (d) B.A. MacKay and M.D. Fryzuk. Chem. Rev.
104, 385 (2004).
5. C.E. Laplaza and C.C. Cummins. Science (Washington, D.C.),
268, 861 (1995).
1
bility or H NMR information and the similarity of the IR
and UV–vis spectra to (PEt3)3Ni(N2) (vide supra) casts doubts
on the identity of this compound (11a).
6. (a) D.V. Yandulov and R.R. Schrock. Science, 301, 76 (2003);
(b) R.R. Schrock. Chem. Commun. 2389 (2003).
7. C. Krüger and T.-Y. Tsay. Angew. Chem. Int. Ed. Engl. 12,
998 (1973).
8. K. Jonas, D.J. Brauer, C. Krüger, P.J. Roberts, and Y.-H. Tsay.
Acknowledgments
J. Am. Chem. Soc. 98, 74 (1976).
9. P.W. Jolly, K. Jonas, C. Krüger, and Y.-H. Tsay. J. Organomet.
Chem. 33, 109 (1971).
We are grateful to the National Science Foundation for
financial support of this research through Grant CHE-
0244239 to G.L.H., and to the Department of Education for
a GAANN Fellowship to R.W.
10. I. Bach, R. Goddard, C. Kopiske, K. Seevogel, and K.R.
Pörschke. Organometallics, 18, 10 (1999).
11. (a) G.A. Tolman, D.H. Gerlach, J.P. Jesson, and R.A. Schunn.
J. Organomet. Chem. 65, C23 (1974); (b) M. Aresta, G.F.
Nobile, and A. Sacco. Inorg. Chim. Acta, 12, 167 (1975).
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(1969).
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© 2005 NRC Canada