Dinitrogen and acetylide complexes of low-valent chromium

Article abstract of DOI:10.1021/ic702275g

Reaction of trans-(dmpe)₂CrCl₂ (dmpe = 1,2-bis(dimethylphosphino)ethane) with one equivalent of LiCCSiMe₃ and one equivalent of nBuLi in THF under a dinitrogen atmosphere affords dark orange trans,trans-[(Me₃SiCC)(dmpe)₂Cr] ₂(μ-N₂)·hexane (1). Under similar conditions but in the absence of acteylide ligand, the reaction of trans-(dmpe) ₂CrCl₂ with 2 equivalents of nBuLi yields the previously characterized complex trans-(dmpe)₂Cr(N₂)₂ (2), while the reaction of trans-(dmpe)₂CrCl₂ with 2 equivalents of LiCCSiMe₃ in THF yields trans-(dmpe) ₂Cr(CCSiMe₃)₂ (3). Compound 3 can also be synthesized by irradiating a mixture of trans-(dmpe)₂CrMe₂ and HCCSiMe₃ or by reduction of HCCSiMe₃ with compound 2. The magnetic properties, electrochemistry, and crystal structure of trans,trans-[(Me₃SiCC)(dmpe)₂Cr]₂(μ-N ₂) are consistent with the complex containing two Cr^I ions bridged by a neutral N₂ moiety, with a 1.178(10) A N≡N bond distance. For complex 1 redox processes centered at E_1/2 = -1.69 V (ΔE_p = 185 mV) and -1.43 V (ΔE_p = 182 mV) versus Fe(Cp)₂/Fe(Cp)₂⁺ are assigned to the Cr^ICr^I/Cr^ICr^II and Cr ^ICr^II/Cr^IICr^II couples, respectively. For trans-(dmpe)₂Cr(CCSiMe₃)₂ a reversible couple assigned as the Cr^II/III couple was observed at -1.59 V (ΔE_p = 242 mV) versus Fe(Cp)₂/Fe(Cp) ₂⁺. The dinuclear Cr(I)-dinitrogen complex 1 has a room temperature magnetic moment of 2.77 μ_B while compound 3 displays a moment of 2.55 μ_B. Density-functional theory calculations performed on a model compound of 1, namely, trans,trans-[(HCC)(dpe) ₂Cr]₂(μ-N₂) (dpe = diphospinoethane), indicate that oxidation of the molecule should result in weakening of the dinitrogen triple bond.

Full text of DOI:10.1021/ic702275g

Inorg. Chem. 2008, 47, 4639-4647
Dinitrogen and Acetylide Complexes of Low-Valent Chromium
Louise A. Berben* and Stosh A. Kozimor
Department of Chemistry, UniVersity of California, Berkeley, California 94720–1460
Received November 20, 2007
Reaction of trans-(dmpe)2CrCl2 (dmpe ) 1,2-bis(dimethylphosphino)ethane) with one equivalent of LiCCSiMe3 and
one equivalent of nBuLi in THF under a dinitrogen atmosphere affords dark orange trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-
N2) · hexane (1). Under similar conditions but in the absence of acteylide ligand, the reaction of trans-(dmpe)2CrCl2
with 2 equivalents of nBuLi yields the previously characterized complex trans-(dmpe)2Cr(N2)2 (2), while the reaction
of trans-(dmpe)2CrCl2 with 2 equivalents of LiCCSiMe3 in THF yields trans-(dmpe)2Cr(CCSiMe3)2 (3). Compound 3
can also be synthesized by irradiating a mixture of trans-(dmpe)2CrMe2 and HCCSiMe3 or by reduction of HCCSiMe3
with compound 2. The magnetic properties, electrochemistry, and crystal structure of trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-
N2) are consistent with the complex containing two CrI ions bridged by a neutral N2 moiety, with a 1.178(10) Å
Nt N bond distance. For complex 1 redox processes centered at E1/2 ) –1.69 V (∆Ep ) 185 mV) and –1.43 V
(∆Ep ) 182 mV) versus Fe(Cp)2/Fe(Cp)2+ are assigned to the CrICrI/CrICrII and CrICrII/CrIICrII couples, respectively.
For trans-(dmpe)2Cr(CCSiMe3)2 a reversible couple assigned as the CrII/III couple was observed at –1.59 V (∆Ep
) 242 mV) versus Fe(Cp)2/Fe(Cp)2+. The dinuclear Cr(I)-dinitrogen complex 1 has a room temperature magnetic
moment of 2.77 µB while compound 3 displays a moment of 2.55 µB. Density-functional theory calculations performed
on a model compound of 1, namely, trans,trans-[(HCC)(dpe)2Cr]2(µ-N2) (dpe ) diphospinoethane), indicate that
oxidation of the molecule should result in weakening of the dinitrogen triple bond.
Introduction
catalytic conversion of dinitrogen into ammonia.11 Given the
exciting N2 chemistry available to low valent molybdenum,
it seems possible that the lighter congener chromium, in
The activation of dinitrogen by end-on, side-on, or bridging
coordination to transition metal complexes has been of long-
standing interest to chemists.1 Early transition metal com-
plexes demonstrate rich reactivity with N2, for example, with
Y,2 La,2 Ti,3 Zr,4 Hf,5 V,6 Nb,7 Ta,8 Mo,9 and W.10 In
particular, many recent advances in dinitrogen chemistry have
been achieved with molybdenum complexes including the
cleavage of N2 to form metal nitrido complexes9a and the
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* To whom correspondence should be addressed. E-mail: lberben@
mit.edu.
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10.1021/ic702275g CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 11, 2008 4639
Published on Web 05/06/2008

Berben and Kozimor
conjunction with electron-donating ancillary ligands, might
also display unique reactivity with dinitrogen. However, few
reports have been made focusing on the reactivity between
N2 and Cr,12–17 and the majority of the previous studies
suggest that chromium typically forms highly unstable
coordination complexes with N2.13–15
dinitrogen ligand is bound side-on to the Cr(I) centers.
Several other Cr(0) compounds containing π-bonded cyclic
hydrocarbon ligands, [(CO)2(η6-Ar)Cr]2(µ-N2) (where Ar )
C6H6, C6Me6, or C6H3Me3), were also reported but have not
yet been structurally characterized.13
Although few stable Cr/N2 complexes exist, those that have
been reported recently are stabilized by their anionic or
π-accepting nitrogen based ancillary ligand sets; diiminepy-
ridine,16 and ꢀ-diketiminate.17 It has been demonstrated that
acetylide ligands possess strong π-donor character with early
first row transition metals and that the bond is of a highly
covalent nature.18 We have previously shown that (Ct CR)1-
ligands are capable of providing highly stable mononuclear
chromium compounds, and19 given the experimental obser-
vations reported herein, we believe that the ancillary acetylide
ligand is important to the stabilization of trans,trans-
[(Me3SiCC)(dmpe)2Cr]2(µ-N2). We also demonstrate that the
synthetic approach that afforded 1 can be extended to provide
an alternate, mercury-free method for synthesis of the known
compound trans-(dmpe)2Cr(N2)2 (2).12 To our knowledge,
these complexes, along with trans-(dmpe)2Cr(CCSiMe3)2 (3),
represent the first examples of end-bound acetylide ligands
with monovalent and divalent octahedral chromium.20
A handful of chromium dinitrogen complexes exist: of the
mononuclear complexes, only trans-(dmpe)2Cr(N2)2 (dmpe
) 1,2-bis(dimethylphosphino)ethane), in which two dinitro-
gen moieties are terminally trans-bound to a Cr(0) center,
is stable at room temperature and has been structurally
characterized.12 In this case, the complex is generated by
reducing trans-(dmpe)2CrCl2 with Na/Hg. Analogues of this
compound, namely, trans-(Me3P)4Cr(N2)2 and trans-(dppe)2-
Cr(N2)214 (dppe ) 1,2-bis(diphenylphosphino)ethane, are less
stable and lose dinitrogen and phosphine ligands when
warmed to room temperature, and hence, they have not been
structurally characterized. Dinuclear complexes of chromium
have also been reported, and of these, three have been
structurally characterized: [(CO)2(η6-C6Et6)Cr]2(µ-N2),15 which
is unstable to recrystallization above 0 °C, {[{2,6-[2,6-(i-
Pr)2PhN)C(CH3)]2(C5H3N)}Cr(THF)]2(µ-N2)}·THF,16 which
canactivateandreducedinitrogen,and[(i-Pr2Ph)2nacnacCr]2(µ-
N2)17 (nacnac ) ꢀ-diketiminate) in which the bridging
Experimental Section
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Preparation of Compounds. All manipulations were performed
under a nitrogen atmosphere using standard Schlenk line and
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(dmpe)2CrMe2 were carried out as reported previously.21 Diethyl
ether and toluene were passed over alumina, tetrahydrofuran was
distilled from Na/benzophenone, and hexanes were purchased in
Sure/Seal bottles. All solvents were degassed prior to use and stored
over 3 Å molecular sieves. Trimethylsilylethyne was degassed by
three freeze–pump–thaw cycles and stored over 3 Å molecular
sieves. All other reagents were used without further purification.
trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-N2)·Hexane (1). To a
40 mL tetrahydrofuran (THF) solution of trans-(dmpe)2CrCl2 (1.0
g, 2.4 mmol) and Me3SiCCH (0.29 g, 3.0 mmol) chilled at -78
°C, nBuLi (2.4 mL, 5.9 mmol, 2.5 M hexanes solution) was added
via syringe, and the mixture was stirred at -78 °C for 3 h. Upon
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1996, 35, 1621. (f) Butts, M. D.; Bryan, J. C.; Luo, X.-L.; Kubas,
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chromium acetylide complexes include Schlientz, W. J.; Ruff, J. K.
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L. G.; Vinogradova, V. N.; Korneva, V. N.; Ustynyuk, N. A. J.
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4640 Inorganic Chemistry, Vol. 47, No. 11, 2008

Dinitrogen and Acetylide Complexes of Low-Valent Chromium
Table 1. Crystallographic Dataa for the Complexes trans,trans-
[(Me3SiCC)(dmpe)2Cr]2(µ-N2)·Hexane (1) and trans-(dmpe)2Cr-
(CCSiMe3)2 (3)
warming to room temperature with vigorous stirring, a dark orange
solution formed, which was stirred for an additional 2 h before
removal of the solvent in vacuo. The orange-brown product was
extracted into hexanes (ca. 10 mL), filtered to remove LiCl, and
stored at -25 °C. After 2 days, 0.25 g of orange plate-shaped
crystals of product were collected by filtration and dried under N2.
Concentration of the solution to about 3 mL and cooling to -25
°C yielded further crops, to give 1 (0.38 g) in 29% crystalline yield.
Absorption spectrum (MeCN): λmax (εM) 264 (sh, 15390), 291
(16740), 352 (15320), 426 (58860), 942 (25340 L mol-1cm-1) nm.
IR(solid, ATR): λCC 2011 (w), λCSi 836 (s) cm-1. µeff ) 2.77 µB at
295 K. ES+-MS (MeCN/THF): m/z 546 ([1]+). Anal. Calcd for
C40H96Cr2N2P8Si2: C, 47.42; H, 9.55; N, 2.77. Found: C, 47.50; H,
9.82; N, 2.61. 1H-HMR (C6D6): δ 5.10 (br, 18H, SiMe3), 2.18 (br,
8H, PCH2), –2.85 (br, 24H, PCH3), –4.35 (br, 8H, PCH2), –20.17
(br, 24H, PCH3).
trans-(dmpe)2Cr(N2)2 (2). A green solution of trans-(dmpe)2Cr-
Cl2 (0.55 g, 1.3 mmol) in THF (10 mL) was cooled to –73 °C, and
nBuLi (1.0 mL, 2.5 mmol, 2.5 M hexanes solution) was added.
The reaction was allowed to warm to room temperature, and the
color of the solution changed to dark red. After stirring for 10 h,
the solvent was removed in vacuo, toluene (10 mL) was added,
and a yellow solid was separated from a red solution by filtration
through celite. The toluene was removed in vacuo to afford 2 as a
red solid (0.45 g, 85%) which was crystallized from a saturated
solution of toluene (5 mL) to give 0.25 g (47%) of product. The
unit cell parameters and 1H NMR and IR spectra of this compound
were consistent with previous reports.12
trans-(dmpe)2Cr(CCSiMe3)2 (3) from trans-(dmpe)2CrCl2. A
solution of Me3SiCCH (45 mg, 0.46 mmol) in THF (5 mL) was
cooled to –78 °C, and nBuLi (0.22 mL, 0.55 mmol) was added.
After stirring the colorless solution for 1 h at room temperature, a
green solution of trans-(dmpe)2CrCl2 (99 mg, 0.23 mmol) in THF
(5 mL) was added dropwise, inducing an immediate color change
to red. The solution was stirred for 12 h, and the solvent was
removed under reduced pressure. Toluene was added, and a white
precipitate was removed by filtration. The solvent was removed
under reduced pressure to give 120 mg (94%) of 3 as a dark orange
powder. X-ray quality crystals of 3 were obtained from concentrated
solutions of toluene cooled to -25 °C. Absorption spectrum (THF):
1
3
formula
fw
T, K
space group
Z
C40H96Cr2N2P8Si2
1013.11
124
C2/c
8
C16H50CrP4Si2
546.68
165
j
P1
1
a, Å
b, Å
c, Å
37.302(8)
20.407(4)
16.971(3)
9.349(3)
9.985(3)
10.209(4)
70.405(5)
64.705(5)
72.222(5)
797.0(5)
1.139
R, deg
ꢀ, deg
γ, deg
V, Å3
dcalc, g/cm3
116.85(3)
11526(4)
1.673
9.45 (27.88)
R1(wR2),b
%
4.58 (11.11)
a Obtained with graphite-monochromated Mo KR (λ ) 0.71073 Å)
radiation. b R1 ) Σ4Fo| - Fc4/Σ|Fo|, wR2 ) {Σ[w(Fo - Fc )2]/
Σ[w(Fo2)2]}1/2
2
2
.
X-ray Structure Determinations. Single crystal X-ray structure
determinations were performed for compounds 1 and 3 (see Table
1).22 Crystals were quickly coated in Paratone-N oil under a nitrogen
atmosphere, mounted on Kaptan loops, transferred to a Siemens
APEX diffractometer, and cooled in a nitrogen stream. Initial lattice
parameters were obtained from a least-squares analysis of more
than 30 centered reflections; these parameters were later refined
against all data. A full hemisphere of data was collected for all
compounds. Data were integrated and corrected for Lorentz
polarization effects using SAINT22b and were corrected for absorp-
tion effects using SADABS 2.3.22c
Space group assignments were based upon systematic absences,
E statistics, and successful refinement of the structures. Structures
were solved by direct methods with the aid of successive difference
Fourier maps and were refined against all data using the SHELXTL
5.0 software package.22d Thermal parameters for all non-hydrogen
atoms were refined anisotropically, except for the disordered dmpe
ligands and hexane solvate molecules in 1. Refinement of the N
atoms in 1 as either C or O atoms resulted in unreasonable thermal
parameters. Hydrogen atoms, where added, were assigned to ideal
positions and refined using a riding model with an isotropic thermal
parameter 1.2 times that of the attached carbon atom (1.5 times
for methyl hydrogens).
Other Physical Measurements. Absorption spectra were mea-
sured with a Hewlett-Packard 8453 spectrophotometer. Infrared
spectra were recorded on a Nicolet Avatar 360 FTIR spectrometer
equipped with a horizontal attenuated total reflectance (ATR)
accessory. Cyclic voltammetry was performed in a 0.1 M solution
of (Bu4N)ClO4 in THF using a Bioanalytical systems CV-50W
voltammograph, a platinum disk working electrode, a platinum wire
supporting electrode, and a silver wire reference electrode. Reported
potentials are all referenced to the [Fe(Cp)2]0/+ couple and were
determined using decamethylferrocene as an internal standard
(-0.58 V versus [Fe(Cp)2]0/+ for this system). Magnetic susceptibil-
ity data were measured on a Quantum Design MPMS-XL SQUID
magnetometer for compound 1 with the sample sealed in a
polycarbonate capsule and a heat-sealable plastic bag. Diamagnetic
λ
max (εM) 255 (11300), 269 (sh, 7560), 281 (sh, 6220), 351 (4790),
365 (5030), 395 (3510 L mol-1 cm-1) nm. IR (solid, ATR): λCC
1973 (m), 1932 (w), λCSi 836 (s) cm-1. µeff ) 2.55 µB at 295 K.
ES+-MS (MeCN/THF): m/z 546 ([3]+). Anal. Calcd for
1
C22H50P4Si2Cr: C, 48.37; H, 9.15. Found: C, 48.56; H, 9.27. H
NMR (C6D6): δ –27.6 (s, 24H, PCH3), –3.9 (s, 8H, PCH2) 9.05 (s,
18H, SiMe3).
trans-(dmpe)2Cr(CCSiMe3)2 (3) from trans-(dmpe)2Cr(N2)2.
To a red solution of trans-(dmpe)2Cr(N2)2 (27 mg, 0.066 mmol) in
toluene (5 mL) was added Me3SiCCH (16 mg, 0.16 mmol). The
reaction mixture was heated for 12 h at 80 °C and then filtered
through Celite. The solvent was removed under reduced pressure
to afford 36 mg (97%) of 3 as an orange solid. When this reaction
was carried out in a sealed NMR tube, a 1H resonance at 4.47 ppm
attributable to H2 was observed.
trans-(dmpe)2Cr(CCSiMe3)2 (3) from trans-(dmpe)2CrMe2. A
solution of trans-(dmpe)2CrMe2 (20 mg, 0.052 mmol) and
Me3SiCCH (21 mg, 0.21 mmol) in toluene-d8 was sealed in a quartz
NMR tube fitted with a J. Young cap and then irradiated with 320
nm radiation in a Rayonet apparatus. After 18 h, the solution had
(22) (a) SMART Software Users Guide, Version 5.1; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 1999. (b) SAINT Software Users
Guide, Version 7.0; Bruker Analytical X-Ray Systems, Inc.: Madison,
WI, 1999. (c) Sheldrick, G. M. SADABS, Version 2.03; Bruker
Analytical X-Ray Systems, Inc.: Madison, WI, 2000. (d) Sheldrick,
G. M. SHELXTL, Version 6.12; Bruker Analytical X-Ray Systems,
Inc.: Madison, WI, 1999. (e) Internation Tables for X-Ray Crystal-
lography; Kluwer Academic Publishers: Dordrecht, 1992; Vol. C.
1
become dark orange, and the H NMR spectrum indicated that
complete conversion to 3 had occurred with no formation of side
products.
Inorganic Chemistry, Vol. 47, No. 11, 2008 4641

Berben and Kozimor
Scheme 1
corrections were obtained from Pascal’s constants and for the
sample holder by subtraction of the measured susceptibility of an
empty holder. The Evans method was used for determining the room
temperature susceptibility of compound 3.23 Mass spectrometric
measurements were performed on VG Quattro (Micromass) spec-
trometer equipped with an analytical electrospray ion source
instrument. NMR spectra were measured with a Bruker AVB 400
MHz instrument. Elemental analyses were performed by the UC
Berkeley, Department of Chemistry Analytical Facility.
Electronic Structure Calculations. Density-functional theory
(DFT) calculations were performed using Jaguar version 6.524 with
a spin-unrestricted formalism for compound 1; a small amount of
spin contamination was observed (12%). Electronic structure
calculations were performed on trans,trans-[(HCC)(dpe)2Cr]2(µ-
N2) (dpe ) diphosphinoethane) as a model for complex 1. Complex
geometries were taken from the crystal structure of 1 and optimized
using the B3LYP functional.25 The bond lengths and angles for
the optimized structure deviated from those observed in the crystal
structure by less than 4%. Effective core potentials were employed
for Cr and Si (LACVP**) together with the corresponding Gaussian
basis sets.26 The 6–31+G* basis set was used for C, P, N, and
H.27
Results and Discussion
Synthesis of trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-N2)]
(1). Our entry into chromium dinitrogen chemistry arose
when the reaction of LiCCSiMe3 with trans-(dmpe)2CrCl2
yielded a small amount of crystalline trans,trans-[(Me3-
SiCC)(dmpe)2Cr]2(µ-N2) (1) in addition to the targeted
product, trans-(dmpe)2Cr(CCSiMe3)2 (3). Optimization of the
stoichiometry and the reaction times enabled compounds 1
and 3 to be exclusively synthesized (Scheme 1). In the first
step of the optimized synthesis of 1, two equivalents of nBuLi
were added to a 1:1 solution of trans-(dmpe)2CrCl2 and
HCCSiMe3 in THF held at –78 °C. Stirring this solution for
3 h at low temperature enabled all of the HCCSiMe3 to be
converted to LiCCSiMe3; the solution remained bright green
during this time. Upon removal of the cooling bath and with
rapid stirring of the reaction mixture to ensure saturation with
N2, the color of the solution quickly turned to dark orange.
Removal of the solvent under vacuum after no more than
1 h, followed by extraction of the product into hexane, and
cooling of the solution to –25 °C for over 1 week allowed
1 to be isolated in high purity as small, plate-shaped crystals
in 29% crystalline yield.
Compound 1 is very soluble in common organic solvents
and was analyzed by a variety of spectroscopic techniques.
The UV–vis absorption spectrum of 1 is dominated by
intense metal-to-ligand charge-transfer (MLCT) and ligand-
to-metal charge transfer (LMCT) bands at high energy, which
obscure any d-d transitions that may otherwise have been
observed. The nature of the intense low energy band at 942
nm (25340 L mol-1 cm-1) is unknown; however, it is most
likely because of a d-d transition to which the highly
covalent bonding nature imparts significant ligand character
or a LMCT absorption.28 Although 1 is paramagnetic, with
(23) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Beccoonsal, J. K. Mol.
Phys. 1968, 15, 129.
(24) Jaguar, Version 6.5; Schrodinger, LLC: New York, NY, 2005.
(25) Zheng, K. C.; Wang, J. P.; Peng, W. L.; Liu, X. W.; Yun, F. C. J.
Phys. Chem. A 2001, 105, 10899.
(26) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299. (d) Dunning, T. H., Jr.; Hay,
P. J. In Modern Theoretical Chemistry; Schaefer, H. F., Ed.; Plenum:
New York, 1976; Vol. 3, p 1. (e) Check, C. E.; Faust, T. O.; Bailey,
J. M.; Wright, B. J.; Gilbert, T. M.; Sunderlin, L. S. J. Phys. Chem.
A 2001, 105, 8111.
(27) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,
56, 2257. (c) Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1972, 56,
4233. (d) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28,
213. (e) Binkley, J. S.; Pople, J. A. J. Chem. Phys. 1977, 66, 879. (f)
Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; DeFrees,
D. J.; Pople, J. A.; Gordon, M. S. J. Chem. Phys. 1982, 77, 3654.
1
a magnetic moment of 2.77 µB at 298 K, the H NMR
(28) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier:
Amsterdam, 1984.
4642 Inorganic Chemistry, Vol. 47, No. 11, 2008

Dinitrogen and Acetylide Complexes of Low-Valent Chromium
Table 2. Selected Average Interatomic Distances (Å) and Angles (deg)
for the Complexes trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-N2)·Hexane (1)
and trans-(dmpe)2Cr(CCSiMe3)2 (3)
spectrum is informative and contains five paramagnetically
shifted resonances that have peak areas in a ratio of 12:12:
9:4:4. Resonances attributable to the methyl (-2.84 and
–20.2 ppm) and methylene (2.18 and –4.37 ppm) protons of
the dmpe ligands were resolved, and the resonance for the
SiMe3 group is shifted downfield to 5.10 ppm. The IR
spectrum displays just one peak in the region 1800–2100
cm-1 that is attributed to the Ct C stretching mode (2011
cm-1). As for the [(CO)2(η6-C6Et6)Cr]2(µ-N2) complex,15
there is no peak in the IR spectrum of 1 that is attributable
to a Nt N stretching mode, which is consistent with the
centrosymmetric geometry.
1
3
CrsP
CrsN
CrsC
Nt Na
2.315(4)
1.870(10)
2.007(10)
1.177(10)
1.24(1)
2.334(6)
2.043(3)
Ct
C
1.207(4)
1.815(3)
CsSi
1.810(10)
PsCrsP
PsCrsN
PsCrsC
CsCrsN
81.9(2), 96.9(2), 167.7(2)
96.1(3)
84.9(4)
83.07(4), 96.92(4), 180
90.0(3)
177.8(4)
CrsNt
CrsCt
Ct CsSi
N
C
177.1(8)
177.4(1)
178.5(8)
176.4(2)
173.5(3)
Compound 1 is remarkably stable when compared to many
of the known mono- and dinuclear chromium dinitrogen
complexes. Heating a toluene-d8 solution of 1 at 90 °C under
vacuum, under a dinitrogen atmosphere, or in the presence
of HCCSiMe3 for 48 h yielded no appreciable change to the
1H NMR spectrum. Additionally, when compound 1 was
photolysized with 320 nm radiation in a Rayonet apparatus
a bond-length from one measurement only, not an average.
solution of nBuLi, and longer reaction times (12 h) than
employed in the synthesis of 1 were required to obtain a
pure product. The Ct C stretching frequency in the IR
spectrum of 3 occurs at a lower frequency (1969 cm-1) than
in the case of 1, and the identity of this product was
confirmed by single crystal X-ray diffraction. As in the case
of compound 1, the 1H NMR spectrum of the paramagnetic
compound 3 is informative and contains three paramagneti-
cally shifted resonances that have peak areas in a ratio of
12:9:4. There is one distinct chemical shift for each of the
methyl and methylene resonances of dmpe at –27.6 and –3.9
ppm, respectively, indicating that the complex favors the
trans configuration in solution.
Compound 3 can also be synthesized from trans-(dmpe)2-
Cr(N2)2 (2) by reduction of HCCSiMe3 to (CCSiMe3)1- and
H2 (Scheme 1). In contrast to 1, which showed no reactivity
with HCCSiMe3, the reaction between 2 and HCCSiMe3
proceeds in an NMR tube that was flame sealed under
vacuum, and the formation of H2 was observed by 1H NMR
spectroscopy. This reaction also proceeds under dinitrogen
atmospheres when heated at 60 °C for 12 h. In contrast, it
has been shown previously that the reaction of 2 with
PhCCPh under thermal conditions leads to formation of cis-
(dmpe)2Cr(PhCCPh)2, and it has also been reported that
complex 2 does not react with HCCPh to form an analogue
of 3 under photolysis conditions; this reaction was not
investigated at elevated temperatures.12
1
for 48 h, no decomposition was apparent by H NMR
spectroscopy. In contrast to the (dmpe)2Cr(N2)2 (2) complex,
which reacts with HCCSiMe3 (vide infra), the Cr-N2-Cr
bonds remain intact after heating 1 as high as 90 °C in the
presence of HCCSiMe3 which suggests that the stability of
1 might be related to the presence of the π-donating acetylide
ligands. In addition, it appears that 1 represents a rare
example of an octahedral chromium(I) complex which is not
of the form (CO)5CrX (where X ) a monoanionic ligand).29
Synthesis of trans-(dmpe)2Cr(N2)2 (2). In the reaction to
form compound 1, reduction of the chromium center is
occurring to yield a Cr(I) species from the Cr(II) starting
material. It seemed possible that this synthetic procedure
should be extendable to other systems and provide a
convenient approach for the synthesis of other Cr/N2
compounds. In an effort to further probe this idea, reactions
between nBuLi and trans-(dmpe)2CrCl2 were investigated.
Previously, it has been established that trans-(dmpe)2CrCl2
reacted with nBuLi under H2 or Ar atmospheres to form
(dmpe)2Cr(H)4.12 However, when one equivalent of nBuLi
was reacted with trans-(dmpe)2CrCl2 under an atmosphere
1
of N2, the crude reaction mixture analyzed by H NMR
spectroscopy to contain both trans-(dmpe)2CrCl2 and trans-
(dmpe)2Cr(N2)2 (2) as the major components of a mixture
of products. Compound 2 was cleanly synthesized by reacting
two equivalents of nBuLi with (dmpe)2CrCl2 under an
atmosphere of N2 (Scheme 1), and characterized by com-
A third method for the synthesis of 3 was also developed.
When HCCSiMe3 was added to a solution of trans-
(dmpe)2CrMe2 in THF and the mixture was photolyzed (320
1
nm) for 18 h, the H NMR spectrum indicated complete
1
parison of the H NMR and IR spectra and the unit cell
consumption of the reactants and quantitative formation of
3 (Scheme 1). Photolysis has previously been employed in
the synthesis of similar compounds such as trans-(dmpe)2-
Fe(CCPh)2 from (dmpe)2FeMe2 and HCCPh.30
Crystal Structures. Orange plate-shaped crystals of 1
suitable for X-ray analysis were obtained by cooling a concen-
trated hexane solution at -25 °C for several days (see Tables
1 and 2). The diffraction data for 1 was of high enough quality
to determine connectivity. However, factors such as loss of
parameters with the previously reported data.12 This synthesis
of 2 represents a useful alternative to the previously reported
procedure12 because it avoids the use of mercury yet provides
synthetically useful quantities of trans-(dmpe)2Cr(N2)2.
Synthesis of trans-(dmpe)2Cr(CCSiMe3)2 (3). The syn-
thesis of 3 could be achieved by reacting trans-(dmpe)2CrCl2
with two equivalents of LiCCSiMe3. Precise control over
reaction stoichiometry, achieved by titrating the hexane
(29) A search of the Cambridge Structural database revealed no octahedral
chromium(I) complexes, other than those of the form (CO)5 CrX
(where X ) monoanionic ligand).
(30) Field, L. D.; Turbull, A. J.; Turner, P. J. Am. Chem. Soc. 2002, 124,
3692.
Inorganic Chemistry, Vol. 47, No. 11, 2008 4643

Berben and Kozimor
Figure 1. Structure of the dinitrogen complex trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-N2) (1). Red, green, gray, pink, and blue ellipsoids represent Cr, Si,
C, P, and N atoms, respectively; ellipsoids are shown at the 30% probability level, and H atoms are omitted for clarity.
solvate hexane molecules and the extremely narrow width
of the plate-shaped crystal contributed to low resolution of
the diffraction data, and large errors are associated with the
distances and angles listed in Table 2. As shown in Figure
1, the ligand geometry around the chromium atoms is
approximately octahedral. The dmpe ligands are slightly
splayed out from the center of the molecule most likely
because of steric interactions with the dmpe ligands on the
opposite Cr center. The entire Si-Ct CsCrsNt NsCrs
Ct CsSi backbone of compound 1 is quite linear, as
illustrated by the angles of the Ct CsSi, CrsCt C,
CsCrsN, and CrsNt N units which all deviate from 180°
by only 3° at most. The 1.178(3) Å Nt N distance is slightly
longer than the 1.0987 Å distance in free dinitrogen31 suggesting
that some small reduction of the Nt N triple bond has
occurred.32 Given the quality of the X-ray crystallographic data,
caution must be taken when drawing conclusions based on the
Nt N bond distance, however; the additional physical measure-
ments described below are consistent with the formulation that
1 contains two Cr(I) ions bridged by neutral N2.
Compound 3 readily crystallizes from solutions of hexanes
cooled to –25 °C as orange block-shaped crystals. As
observed for 1, the ligands adopt an octahedral geometry
around the chromium center (Figure 2). The 2.043(3) Å
Cr-C distance for the divalent 3 is comparable to the
Figure 2. Structure of trans-(dmpe)2Cr(CCSiMe3)2 (3). Red, green, pink,
analogous 2.057(3) and 2.077(3) distances observed in
and gray spheres represent Cr, Si, P and C atoms, respectively; ellipsoids
are shown at the 30% probability level, and H atoms are omitted for clarity.
trivalent [(Me3tacn)Cr(CCH)3]19 (Me3tacn ) N, N′, N′′-
trimethyl-1,4,7-triazacyclononane) and Li3[Cr(CCSiMe3)6]·
6THF complexes,18e respectively. Consistent with the ex-
Magnetic Measurements. Consistent with the X-ray
pected triple bond character, the 1.207(4) Å acetylide Ct C
structural characterization and elemental analysis data which
distance is comparable to the 1.2033(2) Å distance in
formulate 1 as two S ) 1/2 CrI ions bridged by a neutral N2
acetylene.34 The Ct CsSi and CrsCt C angles deviate only
ligand, values for ꢁMT of 0.96 cm3 K/mol per dimer and a g
slightly from 180° at 173.5(3)° and 176.4(3)°, respectively.
value of 2.25 at 300 K were obtained (Figure 3). These values
indicate that the chromium centers are essentially magneti-
cally isolated.35 The slight drop in ꢁMT at low temperature
can be attributed to inter- or intramolecular antiferromagnetic
(31) Wilkinson, P. G.; Houk, N. B. J. Chem. Phys. 1956, 24, 528.
(32) Assignment of this molecule as a Cr(II)-diimide suggested would
require an N-N single bond. Although the crystal structure is not of
high quality, the N-N distance is clearly not a single bond. (NN triple
bond ) 1.0999 Å,31 NN double bond ) 1.22 Å, NN single bond )
1.45 Å,33 observed here ) 1.177(10) Å).
(35) (a) Kanamori, K Coord. Chem. ReV. 2003, 237, 147. (b) Lukens,
W. W., Jr.; Andersen, R. A. Inorg. Chem. 1995, 34, 3440. (c) Ren,
Q.; Chen, Z.; Ren, J.; Wei, H.; Feng, W.; Zhang, L. J. Phys. Chem.
A 2002, 106, 6161.
(33) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butter-
worth Heinemann: Boston, 1997.
(34) Fast, H.; Welsh, H. L. J. Mol. Spectrosc. 1972, 41, 203.
4644 Inorganic Chemistry, Vol. 47, No. 11, 2008

Dinitrogen and Acetylide Complexes of Low-Valent Chromium
Scheme 2
Figure 3. Magnetic behavior of trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-N2)
(1) (circles) in an applied field of 10 kG. The solid line represents a
calculated fit to the data.
–0.91 cm-1.36 The small magnitude of this magnetic coupling
is in agreement with that observed in other dinitrogen bridged
complexes of early first-row transition metals such as
[{K(diglyme)3(µ-mesitylene)2(3,5-Me2C6H3)V}2(µ-N2)] at J
6d
-1
) –0.65 cm
.
Magnetic susceptibility measurements
performed on compound 3 revealed a magnetic moment of
2.55 µB. This value is in close agreement with those reported
for trans-(dmpe)2CrCl2 and trans-(dmpe)2CrMe2 at 2.76 and
2.70 µB, respectively.21
Electrochemical Measurements. Evidence for the exist-
ence of electron delocalization between the Cr(I) centers of
1 is seen in the cyclic voltammogram (Figure 4). The
experiment was performed in a 1 mM solution of 1 in THF
with 0.1 M Bu4NClO4 as the supporting electrolyte. Two
distinct oxidation events are evident at E1/2 ) –1.69 V (∆Ep
) 185 mV) and –1.43 V (∆Ep ) 182 mV) versus Fe(Cp)2/
Figure 4. Cyclic voltammogram of trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-
N2) (1) in THF. The measurements were performed on a platinum electrode
at a scan rate of 100 mV/s with 0.1 M (Bu4N)ClO4 as the supporting
+
electrolyte. Potentials are referenced to Fe(Cp)2/Fe(Cp)2
.
+
Fe(Cp)2 . The oxidation waves are each one-electron in
nature and are assigned to the CrI2/CrICrII and CrICrII/CrII
2
couples, respectively. The separation between these two
couples in the cyclic voltammogram of 1 indicates a
comproportionation constant of Kc ) 104.5 associated with
the following equilibrium (eq 1).
2+
I
II +
CrI + CrII
T 2 Cr Cr
(1)
[
]
[
]
[
]
2
2
This value is comparable to that usually associated with class
II mixed-valent compounds37 and is significantly smaller than
the value of Kc ) 109.9 calculated for the mixed-valent state
of [K(diglyme)3][(3,5-Me2C6H3)3V]2(µ-N2) in which the
dinitrogen bridge has been significantly reduced.6d The
voltammogram for 3 displays one reversible wave at –1.59
+
V (∆Ep ) 242 mV) versus Fe(Cp)2/Fe(Cp)2 , which is
assigned to the CrII/CrIII couple (Figure 5).
Electronic Structure Considerations. Electronic structure
calculations were performed on the model complex tran-
Figure 5. Cyclic voltammogram of trans-(dmpe)2Cr(CCSiMe3)2 (3) in THF.
The measurements were performed on a platinum electrode at a scan rate
(36) Schmitt, E. A., Ph. D. Thesis, University of Illinois at Urbana-
Champaign, 1995.
of 100 mV/s with 0.1 M (Bu4N)ClO4 as the supporting electrolyte. Potentials
+
are referenced to Fe(Cp)2/Fe(Cp)2
.
(37) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(38) (a) Chatt, J.; Fay, R. C.; Richards, R. L. J. Chem. Soc. A 1971, 2399.
(b) Treitel, J. M.; Flood, M. T.; March, R. E.; Gray, H. B. J. Am.
Chem. Soc. 1969, 91, 6512. (c) Treitel, J. M.; Flood, M. T.; March,
R. E.; Gray, H. B. Angew. Chem., Int. Ed. Engl. 1974, 13, 639.
interactions. The data for 1 were fit using an exchange
ˆ
ˆ
ˆ
Hamiltonian of the form H ) –2JSCr(1) · SCr(2), to give J )
Inorganic Chemistry, Vol. 47, No. 11, 2008 4645

Berben and Kozimor
Figure 6. Frontier portion of the energy level diagram calculated for trans,trans-[(HCC)(dpe)2Cr]2(µ-N2). One each of the degenerate R HOMO and R
HOMO-1 molecular orbitals are shown.
s,trans-[(HCC)(dpe)2Cr]2(µ-N2). As expected for a complex
consisting of two dinitrogen-bridged octahedral metal centers,
the frontier molecular orbitals comprise a series of four
doubly degenerate energy levels whose energies increase with
increasing nodal number (Scheme 2).38,6d The nature of the
metal nitrogen and metal acetylide interactions in the highest
occupied molecular orbital (HOMO) are similar, both are
antibonding with respect to the Cr-N bond because of
contributions from the dinitrogen and acetylide π-bonding
orbitals (Figure 6). One observation regards the apparent
stability of the Cr(I) dimer reported here as compared with
the Cr(0) dimers that have been reported previously. In the
d6 Cr(0) compounds an extra pair of d electrons would be
placed into the ꢀ molecular orbitals resulting in greater N-N
bond strength but lower Cr-N bond strength. Experimental
results agree with this statement in that many known Cr(0)
dinitrogen-bridged dimers decompose above room temper-
ature liberating dinitrogen,15,13 while compound 1 is remark-
ably stable even at elevated temperatures.
compound could represent a method for cleaving the bound
dinitrogen ligand. Although the Cr-N bond would be
strengthened, molecular orbital considerations imply that
formation of an octahedral chromium-nitride species would
be unfavorable because of the trans influence of the π-donat-
ing Me3SiCt C- ligand and the relatively high d-electron
count of the chromium center. Oxidative cleavage of
dinitrogen was documented for the molybdenum-trisanilide
system in which the C3V symmetry imparts large stabilization
to, and hence facilitates the isolation of, the terminal nitride
complex {N[C(CD3)2CH3][3,5-C6H3Me2]}3MoN.9a,b
Outlook
The foregoing results demonstrate that nBuLi can be used
as a convenient reagent for the formation of low-valent
chromium dinitrogen complexes such as trans,trans-
[(Me3SiCC)(dmpe)2Cr]2(µ-N2) and trans-(dmpe)2Cr(N2)2. In
contrast to many of the previously reported Cr-N2 com-
plexes, these results indicate that compound 1 is unusually
stable even at elevated temperatures, demonstrating a syn-
thetic application of the covalent and π-donating nature of
the (Ct CSiMe3)1- ancillary ligand positioned trans to the
N2 ligand. Molecular orbital considerations suggest that
oxidation of the dimer should weaken the Nt N bond and
activate the N2 ligand toward further reduction. Given that
The most striking insight available from the molecular
orbital depiction is that, upon removal of the two unpaired
electrons from the HOMO (which has N-N π and Cr-N
π* character), the bond strength of the dinitrogen bridge
should be reduced concomitant with the strengthening of the
metal-nitrogen bond. This suggests that oxidation of the
4646 Inorganic Chemistry, Vol. 47, No. 11, 2008

Dinitrogen and Acetylide Complexes of Low-Valent Chromium
the electrochemical studies imply that oxidation of compound
1 will lead to stable Cr(II) containing complexes, continuing
studies should focus on the use of chemical oxidants to
activatetheN2unitofthetrans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-
N2) complex and extend the synthetic approaches imple-
mented in this study toward the formation of more reactive
chromium dinitrogen complexes.
Grant CHE-0617063, Dr. F. J. Hollander for assistance with
the structure of compound 1, and Prof. K. P. C. Vollhardt
for the use of his Rayonet apparatus for photolysis experi-
ments.
Supporting Information Available: X-ray crystallographic files
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We would like to thank Prof. Jeffrey
R. Long for providing funding for this work through NSF
IC702275G
Inorganic Chemistry, Vol. 47, No. 11, 2008 4647