Unusual electronic effects imparted by bridging dinitrogen: An experimental and theoretical investigation

Article abstract of DOI:10.1021/ic101528d

We describe the preparation, structural and magnetic characterizations, and electronic structure calculations for a redox-related family of dinitrogen-bridged chromium acetylide complexes containing the [RC ₂Cr(μ-N₂)CrC₂R]ⁿ⁺ (R = Ph-, ⁱPr₃Si-; n = 0, 1,2) backbone: [(dmpe)₄Cr ₂(C₂Ph)₂(μ-N₂)] (1), [(dmpe) ₄Cr₂(C₂SiⁱPr₃) ₂(μ-N₂)] (2), [(dmpe)₄Cr ₂(C2SiⁱPr₃)₂(μ-N ₂)]BAr^F₄ (3), and [(dmpe)₄Cr ₂(C₂SiⁱPr₃)₂(μ-N ₂)](BAr^F₄)₂ (4). Compounds 3 and 4 are synthesized via chemical oxidation of 2 with [Cp₂Co]⁺ and [Cp^*₂Fe]⁺, respectively. X-ray structural analyses show that the alteration of the formal Cr oxidation states does not appreciably change the Cr-N-N-Cr skeletal structures. Magnetic data collected for 2 and 4 are consistent with high-spin triplet and quintet ground states, respectively. The mixed-valent complex 3 exhibits temperature dependent magnetic behavior consistent with a quartet ? doublet twocenter spin equilibrium. Electronic structure calculations (B3LYP) performed on the full complexes in 2 and 4 suggest that the high-spin states arise from singly occupied orthogonal π^* orbitals coupled with a variable occupation of dδ orbitals. Significant N-N and Cr-N π-bonding pins the occupation of the π manifold, leading to variable occupation of the dδ space. In contrast, mixed-valent 3 is not well described by a B3LYP hybrid density functional model. A [9,11] CAS-SORCI study on a simplified model of 3 reproduces the observed Hund's rule violation for the S = 1/2 ground state and places the lowest quartet 1.45 kcal/mol above the doublet ground state.

Full text of DOI:10.1021/ic101528d

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Inorg. Chem. 2010, 49, 9497–9507 9497
DOI: 10.1021/ic101528d
Unusual Electronic Effects Imparted by Bridging Dinitrogen: an Experimental
and Theoretical Investigation
ꢀ
Wesley A. Hoffert, Anthony K. Rappe, and Matthew P. Shores*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Received July 29, 2010
We describe the preparation, structural and magnetic characterizations, and electronic structure calculations for
a redox-related family of dinitrogen-bridged chromium acetylide complexes containing the [RC2Cr(μ-N2)CrC2R]nþ
i
(R = Ph-, Pr3Si-; n = 0, 1, 2) backbone: [(dmpe)4Cr2(C2Ph)2(μ-N2)] (1), [(dmpe)4Cr2(C2SiiPr3)2(μ-N2)] (2),
[(dmpe)4Cr2(C2SiiPr3)2(μ-N2)]BArF4 (3), and [(dmpe)4Cr2(C2SiiPr3)2(μ-N2)](BArF ) (4). Compounds 3 and 4 are
4 2
synthesized via chemical oxidation of 2 with [Cp2Co]þ and [Cp*2Fe]þ, respectively. X-ray structural analyses show
that the alteration of the formal Cr oxidation states does not appreciably change the Cr-N-N-Cr skeletal structures.
Magnetic data collected for 2 and 4 are consistent with high-spin triplet and quintet ground states, respectively. The
mixed-valent complex 3 exhibits temperature dependent magnetic behavior consistent with a quartet h doublet two-
center spin equilibrium. Electronic structure calculations (B3LYP) performed on the full complexes in 2 and 4 suggest
that the high-spin states arise from singly occupied orthogonal π* orbitals coupled with a variable occupation of dδ
orbitals. Significant N-N and Cr-N π-bonding pins the occupation of the π manifold, leading to variable occupation of
the dδ space. In contrast, mixed-valent 3 is not well described by a B3LYP hybrid density functional model. A [9,11]
CAS-SORCI study on a simplified model of 3 reproduces the observed Hund’s rule violation for the S = 1/2 ground state
and places the lowest quartet 1.45 kcal/mol above the doublet ground state.
Introduction
Metal complexes of another common bridging ligand,
dinitrogen, are most often studied for their reactivity. While
understanding the electronic structure of transition metal
dinitrogen complexes is important for elucidating mecha-
nistic aspects of N2 activation, it is also critical to probing
metal-metal electronic communication and its impact on
magnetic communication and electron transfer. Dating from
the 1970s,15,16 physical measurements on metal dinitrogen
complexes have contributed significantly to our understand-
ing of mixed valency as well as bridge-mediated/inner-sphere
electron transfer processes.
A central focus of complex coordination chemistry is to
ascertain the role(s) of bridging ligands in mediating metal-
metal interactions. Advances in the areas of electron transfer,
magnetic exchange, and small molecule activation can be
attributed in large part to the results of fundamental studies
carried out on model M-L-M systems.1-3 For example, the
judicious choice of ligands to link paramagnetic metal centers
has profound implications for the ability of a given complex
to exist in a well-defined spin ground state.4 Thus, learning
how simple ligands like oxo5-8 and cyanide9-14 manage
exchange interactions has been critical to the development
of molecular magnetism.
The intramolecular magnetic interactions of paramagnetic
M2(μ2-η1:η1-N2) complexes including a tetrahedral VIII
-
2
(μ-N2) complex17 and an octahedral CrI (μ-N2) complex18
2
*To whom correspondence should be addressed. E-mail: shores@
lamar.colostate.edu.
(1) Richardson, D. E.; Taube, H. Coord. Chem. Rev. 1984, 60, 107–129.
(2) Gunay, A.; Theopold, K. H. Chem. Rev. 2010, 110, 1060–1081.
(3) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. Rev. 2001, 101,
2655–2686.
(10) Toma, L. M.; Lescouezec, R.; Pasan, J.; Ruiz-Perez, C.; Vaissermann, J.;
Cano, J.; Carrasco, R.; Wernsdorfer, W.; Lloret, F.; Julve, M. J. Am. Chem. Soc.
2006, 128, 4842–4853.
(11) Sokol, J. J.; Hee, A. G.; Long, J. R. J. Am. Chem. Soc. 2002, 124,
7656–7657.
(12) Entley, W. R.; Treadway, C. R.; Girolami, G. S. Mol. Cryst. Liq.
Cryst. Sci. Tech. 1995, 273, 153–166.
(4) Single-Molecule Magnets and Related Phenomena; Winpenny, R., Ed.;
Springer: Berlin, 2006; Vol. 129.
~
ꢀ
ꢀ
(13) Berlinguette, C. P.; Vaughn, D.; Canada-Vilalta, C.; Galan-Mascaros,
(5) Goodenough, J. B. Phys. Rev. 1955, 100, 564.
(6) Weihe, H.; Gudel, H. U. Inorg. Chem. 1997, 36, 3632–3639.
(7) Kanamori, J. J. Phys. Chem. Solids 1959, 10, 87–98.
(8) Goldberg, D. P.; Caneschi, A.; Delfs, C. D.; Sessoli, R.; Lippard, S. J.
J. Am. Chem. Soc. 1995, 117, 5789–5800.
J. R.; Dunbar, K. R. Angew. Chem., Int. Ed. 2003, 42, 1523–1526.
ꢀ
(14) Li, D.; Parkin, S.; Wang, G.; Yee, G. T.; Clerac, R.; Wernsdorfer, W.;
Holmes, S. M. J. Am. Chem. Soc. 2006, 128, 4214–4215.
(15) Magnuson, R. H.; Taube, H. J. Am. Chem. Soc. 1972, 94, 7213–7214.
(16) Richardson, D. E.; Sen, J. P.; Buhr, J. D.; Taube, H. Inorg. Chem.
1982, 21, 3136–3140.
ꢀ
(9) Miyasaka, H.; Julve, M.; Yamashita, M.; Clerac, R. Inorg. Chem.
2009, 48, 3420–3437.
r
2010 American Chemical Society
Published on Web 09/15/2010
pubs.acs.org/IC

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9498 Inorganic Chemistry, Vol. 49, No. 20, 2010
Hoffert et al.
have been interpreted in terms of weak antiferromagnetic
coupling. In light of the similarity of dinitrogen to the iso-
electronic cyanide bridging ligand, this model makes intuitive
sense.12 However, recent investigations involving N2-bridged
transition metal complexes indicate that non-diamagnetic,
high spin electronic ground states exist for tetrahedral MoIII
-
2
(μ-N2)19 and trigonal planar CoI2(μ-N2) complexes.20 To corre-
spond with the antiferromagnetic terminology employed by
others, we loosely refer to these non-diamagnetic ground states
as “ferromagnetically coupled”; however, a more accurate
description would invoke an electronic structure similar to the
triplet ground state of dioxygen. The electronic structure that
accounts for high spin magnetic behavior in a formally FeI2-
(μ-N2) complex has been described in terms of strong direct
antiferromagnetic coupling between FeII and a diazenido
(N22-) bridging ligand.21-23 Recent theoretical calculations
involving a NiI2(μ-N2) complex predict a triplet ground state,
although magnetic measurements made in solution are con-
sistent with the presence of two weakly- or non-coupled S =
1/2 NiI ions.24 There, stepwise potassium metal reduction of the
Ni2 complex occurs at the dinitrogen ligand, and metal-ligand
antiferromagnetic coupling is invoked to account for the
diamagnetism observed in the di-reduced K2Ni2(μ-N2) species.
It is clear from the available reports that interpretations of
the measured magnetic properties of dinitrogen-bridged
complexes are varied. One source of difficulty is that the
few structurally characterized paramagnetic M-N-N-M
complexes display myriad coordination geometries as well as
a variety of valence bond structures and electron counts,
which surely have profound implications for magnetic com-
munication. On a case-by-case basis, magneto-structural
correlations drawn from comparisons of metal dinitrogen
complexes have been consistent with the theory of the
day.17,25-27 However, a more detailed magneto-structural
and theoretical survey of multiple metal dinitrogen com-
plexes with comparable structures but with different electron
counts should allow for the drawing of a more complete
picture of dinitrogen-mediated magnetic properties, and
ultimately lead to a deeper understanding of metal dinitrogen
complex electronic structures.
Figure 1. Structure of 1 rendered with 40% ellipsoids. Green, violet,
dark blue and gray ellipsoids represent Cr, P, N, and C atoms, respec-
tively. Hydrogen atoms are omitted for clarity.
Our synthetic entry is more recent: in an effort to synthesize
paramagnetic transition metal ethynylbenzene complexes33
for single-molecule magnet investigations,4,34,35 our explora-
tions of low valent chromium acetylide chemistry led to the
isolation of a dinitrogen-bridged dinuclear CrI acetylide
complex (1, Figure 1).
The (trimethylsilyl)acetylide analogue of this complex was
reported recently by Berben and Kozimor.18,36 There, the
magnetic susceptibility data for that complex were fit to a
model where S=1/2 centers weakly couple antiferro-
magnetically. Cyclic voltammograms obtained for the Me3Si-
containing complex indicated that the oxidized product(s)
were stable on the electrochemical time scale. Further,
density functional calculations performed on a neutral model
complex offered a forecast of a weakened dinitrogen bond
upon oxidation of the CrI centers.18
Intrigued by the potential for redox changes to influence
dinitrogen activation, we have set out to isolate and study the
[RC2Cr(μ-N2)CrC2R]nþ species in all its chemically available
oxidation states. More generally, the work is motivated by
the opportunity to establish magneto-structural and electronic
correlations in transition metal complexes bridged by small
ligands, which in turn can benchmark theoretical modeling for
species with tailored magnetic and electronic properties. Herein,
we report the syntheses, characterizations, and initial com-
putational investigations of a structurally related family of
[RC2Cr(μ-N2)CrC2R]nþ (R = Ph, iPr3Si; n = 0, 1, 2) species,
where redox tuning gives rise to significant changes in magne-
tism, but negligible alteration of the dinitrogen moiety. As will
be shown below, experimental data from all three compounds is
necessary to begin to understand the changes in electronic
structure brought about by redox events.
From a theoretical standpoint, we have been interested for
some time in the electronic structures underpinning dinitrogen
activation28,29 as well as bridge-mediated magnetism.30-32
(17) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.; Re, N.;
Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1997, 119, 10104–10115.
(18) Berben, L. A.; Kozimor, S. A. Inorg. Chem. 2008, 47, 4639–4647.
Experimental Section
Preparation of Compounds. Manipulations were performed
either inside a dinitrogen-filled glovebox (MBRAUN Labmaster
130) or via Schlenk techniques on dinitrogen manifolds. Pentane
was distilled over sodium metal and subjected to three freeze-
pump-thaw cycles. Other solvents were sparged with dinitrogen,
€
(19) Curley, J. J.; Cook, T. R.; Reece, S. Y.; Muller, P.; Cummins, C. C.
J. Am. Chem. Soc. 2008, 130, 9394–9405.
(20) Ding, K.; Pierpont, A. W.; Brennessel, W. W.; Lukat-Rodgers, G.;
Rodgers, K. R.; Cundari, T. R.; Bill, E.; Holland, P. L. J. Am. Chem. Soc.
2009, 131, 9471–9472.
(21) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.;
Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.; Holland,
P. L. J. Am. Chem. Soc. 2006, 128, 756–769.
ꢀ
(31) Hart, J. R.; Rappe, A. K.; Gorun, S. M.; Upton, T. H. J. Phys. Chem.
(22) Stoian, S. A.; Vela, J.; Smith, J. M.; Sadique, A. R.; Holland, P. L.;
1992, 96, 6255–6263.
€
Munck, E.; Bominaar, E. L. J. Am. Chem. Soc. 2006, 128, 10181–10192.
ꢀ
(32) Hart, J. R.; Rappe, A. K.; Gorun, S. M.; Upton, T. H. J. Phys. Chem.
(23) Holland, P. L. Acc. Chem. Res. 2008, 41, 905–914.
1992, 96, 6264–6269.
(33) Weyland, T.; Costuas, K.; Mari, A.; Halet, J. F.; Lapinte, C.
Organometallics 1998, 17, 5569–5579.
(34) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993,
365, 141–143.
(35) Milios, C. J.; Vinslava, A.; Wernsdorfer, W.; Moggach, S.; Parsons,
S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129,
2754–2755.
€
(24) Pfirrmann, S.; Limberg, C.; Herwig, C.; Stosser, R.; Ziemer, B.
Angew. Chem., Int. Ed. 2009, 48, 3357–3361.
(25) Chatt, J.; Fay, R. C.; Richards, R. L. J. Chem. Soc. A. 1971, 702–704.
(26) Treitel, I. M.; Flood, M. T.; Marsh, R. E.; Gray, H. B. J. Am. Chem.
Soc. 1969, 91, 6512–6513.
(27) Sellmann, D. Angew. Chem., Int. Ed. Engl. 1974, 13, 639–649.
ꢀ
(28) Rappe, A. K. Inorg. Chem. 1984, 23, 995–996.
ꢀ
(29) Rappe, A. K. Inorg. Chem. 1986, 25, 4686–4691.
ꢀ
(30) Hart, J. R.; Rappe, A. K.; Gorun, S. M.; Upton, T. H. Inorg. Chem.
(36) Berben, L. A. Ph.D. Thesis, University of California, Berkeley, CA,
2005.
1992, 31, 5254–5259.

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Article
Inorganic Chemistry, Vol. 49, No. 20, 2010 9499
Table 1. Crystallographic Dataa for Compounds [(dmpe)4Cr2(PhC2)2(μ-N2)] C6H12 (1 C6H12), [(dmpe)4Cr2(iPr3SiC2)2(μ-N2)] (2), [(dmpe)4Cr2(iPr3SiC2)2(μ-N2)]BArF 1.5
4
3
3
3
Et2O (3 1.5Et2O), [(dmpe)4Cr2(iPr3SiC2)2(μ-N2)]BArF (3), and [(dmpe)4Cr2(iPr3SiC2)2(μ-N2)](BArF
)
3.5 THF (4 3.5THF)
4
4
2
3
3
3
1 C6H12
3
2
3 1.5Et2O (100 K)
3
3 (296 K)
4 3.5THF
3
formula
Mr
T (K)
space group
a (A)
C46H86Cr2N2P8 C46H104Cr2N2P8Si2 C84H133BCr2F24N2P8Si2O1.5 C78H118BCr2F24N2P8Si2 C124H158B2Cr2F48N2P8Si2O3.5
1018.93
100(2)
P21/c
1093.25
100(2)
Pbca
2069.67
100(2)
P21/c
1958.49
296(2)
P21/c
3072.98
100(2)
P1
12.3602(2)
12.7197(2)
34.7651(6)
90
93.298(1)
90
18.0902(5)
19.6306(5)
34.351(1)
90
90
90
14.5070(4)
29.6682(9)
24.1919(7)
90
96.531(2)
90
14.6116(6)
30.4203(13)
24.9312(10)
90
96.848(2)
90
14.2386(5)
17.1753(6)
31.8445(11)
88.101(2)
86.562(2)
69.587(2)
7248.8(4)
2
b (A)
c (A)
R (deg)
β (deg)
γ (deg)
V (A3)
Z
5457.0(2)
4
0.075
12198.8(6)
8
0.094
10344.5(5)
4
0.063
11002.6(8)
4
0.060
Rint
0.068
0.093
0.277
R1b (I > 2σ(I)) 0.046
0.062
0.180
0.065
0.209
0.077
0.220
wR2c (all data) 0.113
P
P
P
a Obtained with graphite-monochromated Mo KR (λ = 0.71073 A) radiation. b R1 = ||Fo| - |Fc||/ |Fo|. c wR2 = { w(Fo - Fc2)2/
2
P
w(Fo2)2}0.5
.
passed over activated alumina, and degassed prior to use. The
preparation of [(dmpe)2CrCl2] (dmpe =1,2-bis(dimethylphosphino)-
pentane at -40 ꢀC for 3 days. Absorption spectrum (toluene):
λmax (εM) 427 (44900), 940 nm (30300 M-1 cm-1). IR (solid,
3
ethane) has been described elsewhere.37 The reagent [CoCp2]BArF
mineral oil): νCC 1944, νCSi 833 cm-1. Raman (solution in C6H6):
4
(BArF4 = tetrakis(3,5-bis(trifluoromethylphenyl)borate)) was made
by PF6- anion metathesis in a manner analogous to the preparation
1
νNN 1680 cm-1. H NMR (C6D6) δ 2.16 (br, 50H, -PCH2 and
-Si(C3H7)3), -3.71 (br, 24H, -PCH3), -4.60 (br sh, 8H, -PCH2),
-19.95 ppm (br, 24H, -PCH3). Anal. Calcd. for C46H106N2-
Cr2P8Si2: C, 50.44; H, 9.75; N, 2.56. Found: C, 50.28; H, 9.68;
N, 2.28.
of [Cp*2Fe]BArF .38 All other reagents were purchased commercially
and were used without further purification.
4
[(dmpe)4Cr2(C2Ph)2(μ-N2)] (1). A solution of [(dmpe)2CrCl2]
(100 mg, 0.236 mmol) in 5 mL of tetrahydrofuran (THF) at -78 ꢀC
was added to a mixture of phenylacetylene (26 μL, 0.24 mmol) and
n-BuLi (0.33 mL of a 1.59 M solution in hexanes, 0.53 mmol) in
20 mL of pentane at -78 ꢀC. The green mixture was warmed to
293 K, resulting in a color change to brown. After stirring for 1 h,
the solvent was removed in vacuo, and the residue was extracted
with 25 mL of a 10% (v/v) methylcyclopentane/hexanes solution.
The brown solution was concentrated to about 15 mL and was
placed in a -40 ꢀC freezer for 1 day, resulting in the precipitation of
dark brown needle crystals suitable for X-ray analysis. The crystals
were isolated by filtration, washed with cold (-40 ꢀC) pentane (3 ꢀ
2 mL), and dried under dinitrogen to afford the final product
(13 mg, 0.014 mmol, 12%). Absorption spectrum (toluene): λmax (εM)
[(dmpe)4Cr2(C2SiiPr3)2(μ-N2)]BArF4 (3). To a stirred solution
of 2 (30 mg, 0.027 mmol) in 3 mL of diethyl ether was added a
solution of [Cp2Co]BArF (28.8 mg, 0.027 mmol) in 3 mL of
4
diethyl ether. The solution immediately turned dark green. After
stirring for 5 min, the solution was filtered through a plug of
Celite. Pentane (10 mL) was added, and the solution was cooled to
-40 ꢀC. After 1 day, dark green needle crystals formed. The crystals
were isolated by filtration, washed with pentane (2 ꢀ 10 mL), and
dried under dinitrogen to afford 38 mg (0.019 mmol, 71%) of
product. X-ray quality parallelepiped crystals of 3 1.5 Et2O were
3
grown by layering pentane over a concentrated solution of 3 in
diethyl ether in a -40 ꢀC freezer for 2 days. Absorption spectrum
(diethyl ether): λmax (εM) 343 (10410), 361 (10590), 408 (8850), 607
317 (12300), 441 (10400), 543 (sh, 4100), 955 nm (6800 M-1 cm-1).
(780), 914 nm (3180 M-1 cm-1). IR (solid, mineral oil): νCC 1963,
3
3
IR (solid, mineral oil): νCC 2027 cm-1. Raman (solution in C6H6):
νCSi 838 cm-1 Anal. Calcd. for C82H128N2Cr2BF24P8Si2O
(3 Et2O): C, 48.45; H, 6.35; N, 1.38. Found: C, 48.34; H, 6.41; N,
.
ν
NN 1685 cm-1. 1H NMR (C6D6) δ 20.50 (br, 10H, Ar-H), -3.05
3
(br, 24H, -PCH3), -4.56 (br, 8H, -PCH2), -19.23 (br, 24H,
-PCH3), -22.51 ppm (br, 24H, -PCH2). Anal. calcd. for
C40H74N2Cr2P8: C, 51.38; H, 7.98; N, 3.00. Found: C, 51.70; H,
7.85; N, 2.67.
1.23. Crystals maintained at room temperature release varying
amounts of Et2O; samples used for magnetic studies were ground
finely, releasing all solvate molecules.
[(dmpe)4Cr2(C2SiiPr3)2(μ-N2)](BArF4)2 (4). To a stirred solu-
tion of 2 (40 mg, 0.037 mmol) in 10 mL of diethyl ether was
[(dmpe)4Cr2(C2SiiPr3)2(μ-N2)] (2). A solution of [(dmpe)2-
CrCl2] (515 mg, 1.22 mmol) in 10 mL of THF at -78 ꢀC was
added to a mixture of triisopropylsilylacetylene (0.27 mL, 1.22 mmol)
and n-BuLi (1.77 mL of a 1.59 M solution on hexanes, 2.79 mmol) in
20 mL of pentane at -78 ꢀC. As the green mixture was slowly
warmed to 293 K, the color changed to dark red-orange. After
stirring for 1 h, the solvent was removed in vacuo overnight, and
the residue was extracted with 20 mL of pentane and filtered
through Celite. The red-orange solution was concentrated to
about 10 mL and was placed in a -40 ꢀC freezer for crystal-
lization. After 1 day, dark red needle crystals formed. The
crystals were isolated by filtration, washed with cold (-40 ꢀC)
pentane (2 ꢀ 2 mL), and dried under dinitrogen to afford 110 mg
of the final product (0.100 mmol, 17%). Crystals suitable for
X-ray analysis were obtained by cooling a dilute solution of 2 in
added a solution of [Cp*2Fe]BArF (86.9 mg, 0.073 mmol) in
4
5 mL of diethyl ether. The solution first turned green, then dark
brown. After stirring for 5 min, the solution was filtered through
Celite. Pentane (ca. 20 mL) was added to precipitate a brown
solid. The solid was isolated by filtration and washed with
pentane (2 ꢀ 10 mL) to remove any trace of [Cp*2Fe], yielding
75 mg (0.027 mmol, 73%) of product. X-ray quality parallele-
piped crystals of 4 3.5THF were grown by layering pentane
over a concentrated solution of 4 in THF in a -40 ꢀC freezer
for 2 days. Absorption spectrum (diethyl ether): λmax (εM) 349
3
(10440), 446 (5280), 904 nm (7000 M-1 cm-1). IR (solid,
3
mineral oil): νCC 1982, νCSi 838 cm-1 Raman (THF): νNN
.
1710 cm-1 Anal. Calcd. for C110H130N2B2Cr2F48P8Si2: C,
.
46.80; H, 4.65; N, 0.99. Found: C, 46.67; H, 4.74; N, 0.90.
X-ray Structure Determinations. All compounds reported
herein were characterized by single crystal X-ray analysis (Table 1).
Single crystals were coated in Paratone oil prior to removal from
the glovebox. Crystals to be investigated at low temperature
were supported on Cryoloops, then mounted on a Bruker
(37) Girolami, G. S.; Wilkinson, G.; Galas, A. M. R.; Thorntonpett, M.;
Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1985, 1339–1348.
ꢀ
(38) Chavez, I.; Alvarez-Carena, A.; Molins, E.; Roig, A.; Maniukiewicz,
´
W.; Arancibia, A.; Arancibia, V.; Brand, H.; Manrıquez, J. M. J. Organomet.
Chem. 2000, 601, 126–132.

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9500 Inorganic Chemistry, Vol. 49, No. 20, 2010
Hoffert et al.
Table 2. Selected Measured and Calculated (comp) Interatomic Distances (A) and Angles (deg) for the Structures of [(dmpe)4Cr2(iPr3SiC2)2(μ-N2)] (2),
[(dmpe)4Cr2(iPr3SiC2)2(μ-N2)](BArF4) 1.5Et2O (3 1.5Et2O), 3, and [(dmpe)4Cr2(iPr3SiC2)2(μ-N2)](BArF
) 3.5THF (4 3.5THF)
4 2
3
3
3
3
2
2 (comp)
3 1.5Et2O (100 K)
3
3 (296 K)
3 (comp)
4 3.5THF
3
4 (comp)
Cr-N
NtN
Cr-C
1.881[2]b
1.187[5]
2.053[2]
4.946[3]
1.231[4]
1.815[3]
2.32[3]
96.1[8]
84[1]
178.2[5]
179.0[4]
178[2]
176[1]
97.8[5]
97.9[5]
1.929
1.183
2.090
5.038
1.244
1.842
2.32a
95.90
84.11
179.28
179.34
179.13
178.15
1.857[7]
1.195[5]
2.051[8]
4.9083[10]
1.220[4]
1.85[1]
2.35[1]
97[2]
83[2]
178.15[8]
178.0[3]
179.0[5]
175.8[4]
99.2[4]
1.887[5]
1.164[3]
2.041[6]
4.9362[8]
1.223[6]
1.82[2]
2.37[2]
96[2]
2.050
1.156
2.044
5.256
1.242
1.838
2.35a
94.99
85.01
179.22
179.66
178.91
178.54
1.88[1]
1.181[8]
2.054[3]
4.9313[15]
1.216[4]
1.842[1]
2.42[2]
96[2]
84[2]
178.8[9]
178[1]
178.3[7]
178[1]
1.925
1.185
2.054
5.034
1.240
1.875
2.42a
96.58
83.42
179.12
179.27
178.78
178.61
Cr Cr
3 3 3
CtC
C-Si
Cr-P
P-Cr-N
P-Cr-C
C-Cr-N
Cr-NtN
Cr-CtC
CtC-Si
dmpe-Cr(1)-N
dmpe-Cr(2)-N
84[1]
177[1]
177.5[2]
178.9[8]
178.7[3]
98.2[5]
99[1]
98.5[4]
98.5[4]
99.8[4]
a The Cr-P distances were constrained to be the same as those from the X-ray structure. b Square brackets [ ] represent esds for averaged metric parameters.
Kappa Apex 2 CCD diffractometer under a stream of cold
dinitrogen. For the room-temperature data set for 3, the crystal
was encased in epoxy and supported on a glass fiber before being
mounted on the diffractometer under a stream of dinitrogen
maintained at 296 K. All data collections were performed with
Mo KR radiation and a graphite monochromator. Initial lattice
parameters were determined from a minimum of 189 reflections
harvested from 36 frames, and data sets were collected with
complete coverage and 4-fold redundancy. Data were integrated
and corrected for absorption effects with the Apex 2 software
package.39 Structures were solved by direct methods and refined
against |F2| with the SHELXTL software package.40 Thermal
parameters for all non-H atoms were refined anisotropically
with the exception of disordered C, P, and F atoms as noted in
the respective cif files. Hydrogen atoms were added at the ideal
positions and were refined using a riding model where the
thermal parameters were set at 1.2 times those of the attached
carbon atom (1.5 times for methyl carbons).
Other Physical Measurements. UV-vis absorption spectra
were obtained with a Hewlett-Packard 8453 spectrophotometer
in airfree cuvettes. Vis-Near IR spectra were recorded using a
Cary 500 spectrophotometer. IR spectra were measured with a
Nicolet 380 FT-IR using mineral oil mulls sandwiched between
1
NaCl plates. H NMR spectra were recorded using a Varian
INOVA instrument operating at 300 MHz. EPR spectra were
obtained using a continuous wave X-band Bruker EMX 200U
instrument outfitted with a liquid nitrogen cryostat. Raman
spectra were acquired with a Nicolet 760 spectrometer equipped
with an FT-Raman module using an incident laser wavelength
of 1064 nm. Cyclic voltammetry was done in 0.1 M solutions
of (Bu4N)PF6 in THF unless otherwise noted. The volta-
mmograms were recorded with a CH Instruments potentiostat
(either model 1230A or 660C) using a 0.25 mm Pt disk working
electrode, Ag/Agþ reference electrode, and a Pt mesh auxiliary
electrode. All voltammograms shown were measured with a
scan rate of 0.1 V/s. Reported potentials are referenced to the
[Cp2Fe]þ/[Cp2Fe] redox couple and were determined by adding
ferrocene as an internal standard at the conclusion of each
electrochemical experiment. Elemental analyses for compounds
1, 2, and 4 were performed by Robertson Microlit Laboratories
in Madison, NJ. The elemental analysis for 3 was performed by
the microanalytical laboratory at the University of California at
Berkeley.
Magnetic Susceptibility Measurements. Magnetic susceptibil-
ity data were collected with a Quantum Design MPMS-XL
SQUID magnetometer. Samples were loaded into gelatin cap-
sules and inserted into straws prior to analysis. The straws were
sealed in plastic bags prior to removal from the glovebox, and
were quickly loaded into the instrument to minimize exposure to
air. Diamagnetic corrections were applied by using Pascal’s
constants and by subtracting the diamagnetic susceptibility
from an empty sample holder. Susceptibility data were fit with
theoretical models using a relative error minimization routine
(julX 1.41).41 Zero field splitting parameters obtained with julX
are based on the spin Hamiltonian
Electronic Structure Calculations. Restricted and unrestricted
B3LYP hybrid density functional studies43 were carried out in
the G03 suite of electronic structure codes.44 Geometry opti-
mized structures for the triplet, quartet, and quintet states of 2, 3,
and 4 utilized the X-ray coordinates for 2, 3 1.5 Et2O, and
3
4 3.5 THF as initial input (sp3 C-H bond distances adjusted
ns
3
X
to 1.096 A and sp2 C-H bonds adjusted to 1.090 A). Metric
parameters are collected in Table 2 and calculated coordinates
are provided as Supporting Information. The LANL245 basis
sets and effective core potentials were used for Si, P, and Cr; H,
C, and N were described with a 6-31 g* model.46-49 ORCA50
¼
Di½Sz2,i - 1=3SiðSi þ 1Þ þ Ei=DiðSx2,i - Sy2,iÞꢁ
^
H
i ¼ 1
ns
X
Fits of the magnetization data were obtained with the
F
F
þ
gβSi B
3
i ¼ 1
ANISOFIT42 program and are based on the spin Hamiltonian
H = DSz þ E(Sxþ Sy) þ gisoβSB BB.
(43) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(44) Frisch, M. J. et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004.
(45) Hay, P. J.; Dunning, T. H. J. Chem. Phys. 1976, 64, 5077.
(46) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724.
(47) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257.
2
2
2
^
^
^
^
3
(39) APEX 2; Bruker Analytical X-Ray Systems, Inc.: Madison, WI, 2008.
(40) Sheldrick, G. M. SHELXTL; Bruker Analytical X-Ray Systems, Inc.:
Madison, WI, 1999.
(48) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980,
102, 939.
(41) Bill, E. julX, 1.41; Max Planck Institute for Bioinorganic Chemistry:
M€ulheim an der Ruhr, Germany, 2008 (http://ewww.mpi-muelheim.mpg.de/bac/
logins/bill/julX_en.php).
(42) Shores, M. P.; Sokol, J. J.; Long, J. R. J. Am. Chem. Soc. 2002, 124,
2279–2292.
(49) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
(50) ORCA is a modern electronic structure program package written by
F. Neese, with contributions from Becker, U.; Ganiouchine, D.; Kossmann,
S.; Petrenko, T.; Riplinger, C.; Wennmohs, F.; .