Amide-stabilized, diamagnetic chromium(II) nitrosyl complexes

Article abstract of DOI:10.1021/om980954m

Treatment of Cr(NO)(NⁱPr₂)₃ with 2 equiv of PhCO₂H affords Cr(NO)(NⁱPr₂)(O₂CPh)₂ (1) in good yields. Reaction of CpNa-DME with 1 produces CpCr(NO)(NⁱPr₂)(OC(O)Ph) (2), which subsequently yields a series of 18-valence-electron (18e) complexes, CpCr(NO)(NⁱPr₂)(R) (R = η¹-Cp (3), CH₂SiMe₃ (4), η¹-CH₂Ph (5), Ph (6), CPh=CH₂ (7), C≡CCMe₃ (8), C≡CPh (9)) when treated with additional CpNa-DME or 1 equiv of the appropriate R₂Mg·x(dioxane) reagent. 2-9 are low-spin (S = 0) compounds, and their electronic stability is attributed to the strong, synergic π-bonding interactions present in the [Cr(NO)(NⁱPr₂)]²⁺ core, as indicated both by the NMR and IR spectra of these complexes and by the solid-state molecular structures of 2-4. The strong-field nitrosyl and amide ligands also enforce a diamagnetic configuration on the 14e bis(hydrocarbyl) complexes Cr(NO)(NⁱPr₂)R₂ (R = CH₂SiMe₃ (10), CH₂Ph (11), o-tolyl (12)), which are obtained in good yields by reaction of 1 with 2 equiv of the corresponding R₂Mg·x(dioxane) reactants. The solid-state molecular structures of 10 and 11 have also been established by single-crystal X-ray crystallographic analyses. Variable-temperature NMR spectroscopy was employed to study the fluxional processes occurring in 1, 10, and 11.

Full text of DOI:10.1021/om980954m

1994
Organometallics 1999, 18, 1994-2004
Am id e-Sta bilized , Dia m a gn etic Ch r om iu m (II) Nitr osyl
Com p lexes1
Eric W. J andciu, J ane Kuzelka, Peter Legzdins,* Steven J . Rettig,† and
Kevin M. Smith
Department of Chemistry, The University of British Columbia,
Vancouver, British Columbia, Canada V6T 1Z1
Received November 30, 1998
Treatment of Cr(NO)(NiPr2)3 with 2 equiv of PhCO2H affords Cr(NO)(NiPr2)(O2CPh)2 (1)
in good yields. Reaction of CpNa‚DME with 1 produces CpCr(NO)(NiPr2)(OC(O)Ph) (2), which
subsequently yields a series of 18-valence-electron (18e) complexes, CpCr(NO)(NiPr2)(R) (R
) η1-Cp (3), CH2SiMe3 (4), η1-CH2Ph (5), Ph (6), CPhdCH2 (7), CtCCMe3 (8), CtCPh (9))
when treated with additional CpNa‚DME or 1 equiv of the appropriate R2Mg‚x(dioxane)
reagent. 2-9 are low-spin (S ) 0) compounds, and their electronic stability is attributed to
the strong, synergic π-bonding interactions present in the [Cr(NO)(NiPr2)]2+ core, as indicated
both by the NMR and IR spectra of these complexes and by the solid-state molecular
structures of 2-4. The “strong-field” nitrosyl and amide ligands also enforce a diamagnetic
configuration on the 14e bis(hydrocarbyl) complexes Cr(NO)(NiPr2)R2 (R ) CH2SiMe3 (10),
CH2Ph (11), o-tolyl (12)), which are obtained in good yields by reaction of 1 with 2 equiv of
the corresponding R2Mg‚x(dioxane) reactants. The solid-state molecular structures of 10 and
11 have also been established by single-crystal X-ray crystallographic analyses. Variable-
temperature NMR spectroscopy was employed to study the fluxional processes occurring in
1, 10, and 11.
In tr od u ction
reduced CpCr(NO)-containing species were unsuccess-
ful, we next turned our attention to an inorganic
precursor that already possesses a π-donor-stabilized
Cr(II) nitrosyl entity, namely Cr(NO)(NiPr2)3.5 This
approach proved to be successful, and so we now
describe in this paper the first well-characterized Cr(II)
nitrosyl hydrocarbyl compounds, which have been syn-
thesized via amine-elimination and salt-metathesis
reactions of the tris(amide) complex. Portions of this
work have been previously communicated.6
In synthetic organometallic chemistry even the most
reasonable and appealing target molecules remain
inaccessible in the absence of suitable synthetic precur-
sors. A particular case in point involves diamagnetic,
unsaturated, mid-valent hydrocarbyl compounds of
chromium, which are potentially useful complexes for
studying the chemistry of Cr-C σ bonds. Interestingly,
examples of such species are readily available for the
heavier group 6 congeners molybdenum and tungsten.
For example, a plethora of Cp′M(NO)R2 compounds (Cp′
) Cp, Cp*; M ) Mo, W; R ) hydrocarbyl ligand) can be
obtained by treatment of their dichloro precursors with
organomagnesium reagents.2 Unfortunately, the corre-
sponding Cp′Cr(NO)Cl2 precursor compounds are not
known, and attempts to prepare them via oxidation of
Cr(I) or Cr(0) nitrosyl complexes result in the dissocia-
tion of the NO ligand.3
Exp er im en ta l Section
Gen er a l Meth od s. All reactions and subsequent manipula-
tions were performed under anhydrous and anaerobic condi-
tions by employing conventional Schlenk, vacuum-line, and
inert-atmosphere glovebox techniques unless otherwise speci-
fied. General procedures routinely employed in these labora-
tories have been described in detail previously.7 Glassware was
first heated in an oven at 200 °C to remove any moisture and
then cooled to room temperature under vacuum prior to use.
Solvents were purchased from either Aldrich or BDH Chemi-
cals and were purified under a continuous flow of dinitrogen
by employing published procedures.8 Et2O and hexanes were
predried over CaH2 and then distilled from Na/benzophenone;
Our recent investigations into ligand-bonding proper-
ties and electronic configurations of CpCr(NO)-contain-
ing species suggested that π-donor ligands might sta-
bilize organometallic Cr(II) nitrosyl complexes.4 After
our attempts to synthesize such compounds from more
† Deceased October 27, 1998.
(1) π-Bonding and Reactivity in Transition Metal Nitrosyl Com-
plexes, 6. Part 5: Reference 4.
(2) (a) Legzdins, P.; Veltheer, J . E. Acc. Chem. Res. 1993, 26, 41.
(b) Veltheer, J . E.; Legzdins, P. In Synthetic Methods of Organometallic
and Inorganic Chemistry; Herrmann, W. A., Ed.; Thieme: Stuttgart,
Germany, 1997; Vol. 8; pp 79-85.
(3) Legzdins, P.; McNeil, W. S.; Rettig, S. J .; Smith, K. M. J . Am.
Chem. Soc. 1997, 119, 3513.
(5) (a) Bradley, D. C.; Newing, C. W. J . Chem. Soc., Chem. Commun.
1970, 219. (b) Bradley, D. C.; Newing, C. W.; Chisholm, M. H.; Kelly,
R. L.; Haitko, D. A.; Little, D.; Cotton, F. A.; Fanwick, P. E. Inorg.
Chem. 1980, 19, 3010. (c) Odom, A. L.; Cummins, C. C.; Protasiewicz,
J . D. J . Am. Chem. Soc. 1995, 117, 6613.
(6) Kuzelka, J .; Legzdins, P.; Rettig, S. J .; Smith, K. M. Organome-
tallics 1997, 16, 3569.
(7) Legzdins, P.; Rettig, S. J .; Ross, K. J .; Batchelor, R. J .; Einstein,
F. W. B. Organometallics 1995, 14, 5579.
(4) Legzdins, P.; McNeil, W. S.; Smith, K. M.; Poli, R. Organome-
tallics 1998, 17, 615.
(8) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 3rd ed.; Wiley-Interscience: New York, 1986.
10.1021/om980954m CCC: $18.00 © 1999 American Chemical Society
Publication on Web 04/25/1999

Amide-Stabilized Cr(II) Nitrosyl Complexes
Organometallics, Vol. 18, No. 10, 1999 1995
pentane was distilled from Na/benzophenone; CH2Cl2 was
distilled from CaH2; THF was predried over CaH2 and then
distilled from K metal. All solvents were deaerated with argon
or dinitrogen for 10-15 min prior to use or were vacuum-
transferred from the appropriate drying agent.
while its contents were being stirred. The initially green
mixture gradually became red over a 1 h period at room
temperature. The THF was removed from the final mixture
in vacuo, the remaining solid was dissolved in Et2O, and the
solution was filtered through Celite (0.5 × 2 cm) supported
on glass wool in a glass pipet. The volume of the filtrate was
diminished under reduced pressure, and an equal volume of
hexanes was then added. The volume of this new solution was
again reduced under vacuum, and it was then placed in a
freezer at -30 °C overnight to induce the deposition of dark
red crystals of 3, which were collected by filtration.
Nitric oxide was obtained from Linde Union Carbide Spe-
cialty Gas, and prior to use, higher oxides of nitrogen were
removed from it by passage of the gas through a column of
activated silica gel at -78 °C. Benzoic acid was purchased from
Fisher and was purified by sublimation prior to use. Anhy-
drous CrCl3 was also obtained from Fisher and was used as
received. Dicyclopentadiene was purchased from Aldrich and
was thermally cracked to produce C5H6. HCl was purchased
from Aldrich as a 1.0 M Et2O solution and was diluted with
additional Et2O to a concentration of 0.01 M prior to use.
Hexamethyldisiloxane was purchased from Aldrich and was
distilled from sodium metal prior to use. t-BuNC was pur-
chased from Aldrich and dried over 4 Å sieves. CpNa‚DME9
and R2Mg‚x(dioxane) (x ≈ 1)10 were prepared according to
literature procedures. Cr(NO)(NiPr2)3 was prepared by the
reaction of CrCl3(THF)3 with lithium diisopropylamide, and
the subsequent nitrosylation was effected with approximately
1.5 equiv of NO gas.5 All spectral and characterization data
for the new complexes isolated during this work were obtained
in the manner previously specified.7
P r ep a r a tion of Cr (NO)(NiP r 2)(O2CP h )2 (1). A solution
of red Cr(NO)(NiPr2)3 (1.74 g, 4.55 mmol) in a minimum
amount of Et2O (150 mL) was prepared in a Schlenk tube. In
a second Schlenk tube benzoic acid (1.11 g, 9.09 mmol) was
dissolved in Et2O (25 mL). The benzoic acid solution was then
slowly cannulated into the stirred Cr(NO)(NiPr2)3 solution.
Shortly after completion of the addition of the benzoic acid
solution, the reaction mixture changed from red to orange, and
an orange precipitate was deposited. After being stirred for
approximately 1 h, during which time no further color change
was evident, the supernatant solution was removed from the
mixture via cannula. The solid remaining in the flask was
washed with Et2O (2 × 30 mL), and the washings were added
to the supernatant solution and placed in a freezer at -30 °C
overnight to obtain additional precipitate. The collected orange
solid, Cr(NO)(NiPr2)(O2CPh)2 (1), was dried under vacuum at
ambient temperatures for 1 h.
Alt er n a t ive P r ep a r a t ion of Cp Cr (NO)(NiP r 2)(η1-Cp )
(3). A Schlenk tube was charged with Cr(NO)(NiPr2)(O2CPh)2
(2; 0.200 g, 0.47 mmol) and CpLi (0.068 g, 0.94 mmol). THF
(30 mL) was vacuum-transferred onto the two solids, and the
frozen solution was then warmed slowly to room temperature
while being stirred. The initially pale orange solution gradually
became dark red over a 1 h period. The solvent was removed
from the final mixture in vacuo, and the residue was dissolved
in Et2O and filtered through Celite (1 × 1 cm) supported on a
medium-porosity glass frit. The same procedure for crystal-
lization as described in the preceding paragraph was employed
to obtain analytically pure, dark red crystals of 3.
P r ep a r a tion of Cp Cr (NO)(NiP r 2)(CH2SiMe3) (4). A 25
mL Erlenmeyer flask was charged with CpCr(NO)(NiPr2)(O2-
CPh) (2; 0.096 g, 0.26 mmol) and (CH2SiMe3)2Mg‚x(dioxane)
(x ≈ 1; 0.041 g, 0.26 mmol) and was cooled to -196 °C in a
cold well. THF (15 mL), previously cooled to -30 °C, was slowly
run down the walls of the flask in a dropwise fashion, thereby
allowing the THF to freeze on contact with the cold solids. The
flask was then placed on a rotary evaporator and slowly
rotated in order to melt the frozen THF. Upon melting, the
initially green mixture gradually became red over a 30 min
period as it was warmed to room temperature. Stirring was
continued for another 1 h, after which time the THF was
removed under vacuum. The remaining solid was dissolved
in pentane and filtered through Celite (1 × 1 cm) supported
on a medium-porosity glass frit. The volume of the red filtrate
was reduced under vacuum before being placed in a freezer
at -30 °C to induce crystallization. Red crystals of analytically
pure CpCr(NO)(NiPr2)(CH2SiMe3) (4) were collected by filtra-
tion after 24 h at -30 °C.
Tr ea tm en t of Cp Cr (NO)(NiP r 2)(CH2SiMe3) (4) w ith
Dilu te HCl. A solution of CpCr(NO)(NiPr2)(CH2SiMe3) (0.010
g, 0.030 mmol) was prepared in Et2O (10 mL). To this was
added a dilute solution of HCl in Et2O (3 mL, 0.01 M) via
cannula, whereupon an immediate color change from red to
dark red-brown occurred. Stirring of the mixture for 15 min
resulted in the deposition of a small amount of blue-green
precipitate on the walls of the reaction Schlenk. Solution IR
spectroscopy identified no new nitrosyl-containing products,
and further stirring overnight resulted in no visible or
spectroscopic changes. Subsequent addition of excess dilute
HCl afforded additional blue-green precipitate, which was
isolated and identified as [CpCrCl2]2 by mass spectrometry.11
P r ep a r a tion of Cp Cr (NO)(NiP r 2)(η1-CH2P h ) (5), Cp Cr -
(NO)(NiP r 2)(P h ) (6), Cp Cr (NO)(NiP r 2)(CP h dCH 2) (7),
Cp Cr (NO)(NiP r 2)(CtCCMe3) (8), a n d Cp Cr (NO)(NiP r 2)-
(CtCP h ) (9). These five complexes were synthesized in a
P r ep a r a tion of Cp Cr (NO)(NiP r 2)(O2CP h ) (2). A 50 mL
Erlenmeyer flask was charged with Cr(NO)(NiPr2)(O2CPh)2 (1;
0.334 g, 0.79 mmol). The orange powder was dissolved in THF
(10 mL) to obtain a red solution, which was then frozen solid
at -196 °C in a cold well. Crystalline CpNa‚DME (0.140 g,
0.79 mmol) was added to the frozen solution, and the mixture
was then warmed to room temperature while being stirred.
Once the frozen solution had melted, an almost immediate
color change from red to dark green occurred. After being
stirred for 2 h, the mixture was filtered through Celite (1 × 1
cm) supported on a medium-porosity glass frit. The THF
solvent was removed from the filtrate under vacuum, and the
residue was then redissolved in Et2O and cooled overnight at
-30 °C to induce the deposition of dark green crystals of CpCr-
(NO)(NiPr2)(O2CPh) (2), which were collected by filtration,
washed with hexanes, and dried under vacuum at ambient
temperatures.
P r ep a r a tion of Cp Cr (NO)(NiP r 2)(η1-Cp ) (3). A 25 mL
Erlenmeyer flask was charged with CpCr(NO)(NiPr2)(O2CPh)
(2; 0.050 g, 0.14 mmol) and crystalline CpNa‚DME (0.024 g,
0.14 mmol) and was cooled to -196 °C in a cold well. THF (15
mL) was slowly run down the walls of the vial in a dropwise
fashion, thereby allowing the THF to freeze upon contact with
the cold solids. The vial was then warmed to room temperature
manner identical with that described for compound
4
by employing (PhCH2)2Mg‚x(dioxane), Ph2Mg‚x(dioxane),
(CH2dCPh)2Mg‚x(dioxane), Me3CCtCLi, and (PhCtC)2Li,
respectively, in place of (CH2SiMe3)2Mg‚x(dioxane). Like 4,
complexes 5, 6, and 8 were isolated by crystallization from
pentane at -30 °C, whereas analytically pure complexes 7 and
9 were obtained by crystallization from Et2O at the same
temperature.
P r ep a r a tion of Cr (NO)(NiP r 2)(CH2SiMe3)2 (10). THF (25
mL) was vacuum-transferred onto bis(benzoate) 1 (0.500 g,
(9) Smart, J . C.; Curtis, C. J . Inorg. Chem. 1977, 16, 1788.
(10) (a) Andersen, R. A.; Wilkinson, G. J . Chem. Soc., Dalton Trans.
1977, 809. (b) Dryden, N. H.; Legzdins, P.; Trotter, J .; Yee, V. C.
Organometallics 1991, 10, 2857.
(11) Kohler, F. H.; de Cao, R.; Ackermann, K.; Sedlmair, J . Z. Z.
Naturforsch., B 1983, 38, 1406.

1996 Organometallics, Vol. 18, No. 10, 1999
J andciu et al.
1.17 mmol) and (Me3SiCH2)2Mg‚x(dioxane) (x ≈ 1; 0.363 g, 2.34
mmol). The mixture changed from an orange suspension to a
clear, red solution as it was warmed slowly to room temper-
ature over 30 min. After being stirred for an additional 1 h at
ambient temperatures, the solvent was then removed from the
final mixture under vacuum, and the red residue was extracted
with hexanes (5 × 10 mL). The extracts were filtered through
Celite (2 × 5 cm) supported on a medium-porosity glass frit.
The solvent was removed from the filtrate under vacuum to
obtain a red powder. Dark red needles of 10 were obtained by
recrystallization of this material from (Me3Si)2O.
The hydrogen atoms of interest in 11 (i.e. H(1,2,15)) were also
refined isotropically. Secondary extinction corrections were not
necessary. Neutral-atom scattering factors and anomalous
dispersion corrections were taken from ref 13. Selected bond
lengths and angles for the five compounds appear in Tables
2-6. Atomic coordinates, anisotropic thermal parameters, all
bond lengths and angles, torsion angles, intermolecular con-
tacts, and least-squares planes are provided as Supporting
Information.
P r ep a r a tion of Cr (NO)(NiP r 2)(CH2P h )2 (11) a n d Cr -
(NO)(NiP r 2)(o-tolyl)2 (12). These two complexes were syn-
thesized in a manner identical with that described in the
preceding paragraph by employing (CH2Ph)2Mg‚x(dioxane) and
(o-tolyl)2Mg‚x(dioxane), respectively, in place of (Me3SiCH2)2-
Mg‚x(dioxane) and with the entire reaction mixture being kept
at or below 0 °C at all times during the procedure. Also, Et2O
was used as the solvent in order to facilitate its removal. Both
complexes 11 and 12 were isolated by crystallization from
hexanes.
Resu lts a n d Discu ssion
Analytical data and a numbering scheme for all new
complexes isolated during this work are presented in
Table 7, and their mass spectroscopic and IR spectral
data are collected in Table 8. 1H and 13C{1H} NMR
spectroscopic data for complexes 1-13 are given in
Table 9.
Am in e-Elim in a tion Rea ction s. The organometallic
chromium(II) nitrosyl compounds described in this
paper are all derived from Cr(NO)(NiPr2)3 via selective
substitution of two of the amide ligands. Similar amine-
elimination reactions have been developed for the
synthesis of olefin-polymerization-catalyst precursors
from tetrakis(amido)titanium, -zirconium, and -hafnium
complexes. For example, protonolysis of two amide
ligands from group 4 M(NR2)4 species with the proto-
nated form of the appropriate incoming ligand provides
ansa-metallocene,14,15 amide-functionalized cyclopenta-
dienyl,16-20 or chelating diamide21-23 compounds. In
addition, recent work by Cummins and co-workers has
demonstrated that highly selective amine-elimination
reactions are possible. By using MeI, alcohols, and
amine hydrohalides as “deprotecting” agents, they have
crafted elaborate ligand sets from Ti(IV) mixed-tetrakis-
(amide)24 and Cr(VI) tris(amide) nitride25 precursors.
The resulting halides or alkoxides can then be alkylated
to form high-valent, d0 organometallic species. We have
successfully employed a similar protocol with our system
for the selective replacement of two amide ligands by
carboxylate groups, as shown in eq 1. This transforma-
tion is specific for the Cr(NO)(NiPr2)3 reactant shown;
the analogous Cr(NO)(N(SiMe3)2)3 simply does not react
P r ep a r a tion of Cr (NO)(NiP r 2)(CH2SiMe3)(C(dNtBu )-
CH2SiMe3) (13). A solution of Cr(NO)(NiPr2)(CH2SiMe3)2
(0.150 g, 0.42 mmol) was prepared in Et2O (10 mL) and cooled
t
to -196 °C in a cold well. BuNC (48 µL, 0.42 mmol) was
diluted in Et2O (3 mL) and cooled to -30 °C in a freezer. The
former solution was allowed to melt and stirred as the cooled
tBuNC solution was added in a dropwise manner. A nearly
immediate color change from deep red to pale orange occurred.
The reaction mixture was stirred for an additional 10 min
before the solvent was removed in vacuo. The remaining solid
was dissolved in hexamethyldisiloxane, and the solution was
filtered through Celite (0.5 × 2 cm) supported on glass wool
in a glass pipet. The volume of the filtrate was reduced, and
it was then placed in a freezer at -30 °C to induce the
formation of orange, microcrystalline 13.
X-r a y Cr ysta llogr a p h ic An a lyses of Com p lexes 2-4,
10, a n d 11. Crystallographic data are collected in Table 1. The
final unit cell parameters were based on 25 reflections with
2θ ) 16.3-25.4° for 2, 22 161 reflections with 2θ ) 4.0-60.1°
for 3, 7654 reflections with 2θ ) 4.0-63.6° for 4, 17 441
reflections with 2θ ) 4.0-61.3° for 10, and 5847 reflections
with 2θ ) 4.0-61.1° for 11. The data were processed12 and
corrected for Lorentz and polarization effects and for absorp-
tion (empirical, based on azimuthal scans for 2 and on three-
dimensional analyses of symmetry-equivalent data using
fourth-order spherical harmonics for 3, 4, 10, and 11). A linear
decay correction was applied for 2.
The structures of 2-4 were solved by the Patterson method
and those of 10 and 11 by direct methods. The analyses of 2,
4, 10, and 11 were initiated in the centrosymmetric space
group P1h on the basis of the E statistics, these choices being
confirmed by subsequent calculations. The asymmetric units
of 4, 10, and 11 contain two independent molecules. All non-
hydrogen atoms were refined with anisotropic thermal pa-
rameters. Hydrogen atoms were fixed in calculated positions
(15) Herzog, T. A.; Zubris, D. L.; Bercaw, J . E. J . Am. Chem. Soc.
1996, 118, 11988.
(16) (a) Hughes, A. K.; Meetsma, A.; Teuben, J . H. Organometallics
1993, 15, 1936. (b) Hughes, A. K.; Marsh, S. M. B.; Howard, J . A. K.;
Ford, P. S. J . Organomet. Chem. 1997, 528, 195.
(17) (a) Herrmann, W. A.; Morawietz, M. J . A.; Priermeir, T. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1946. (b) Herrmann, W. A.; Morawietz,
M. J . A. J . Organomet. Chem. 1994, 482, 169.
(18) Mu, Y.; Piers, W. E.; MacGillivray, L. R.; Zaworotko, M. J .
Polyhedron 1995, 14, 1.
with C-H ) 0.98 Å and BH ) 1.2Bbonded
for 2, 4, 10, and
atom
(19) Carpenetti, D. W.; Kloppenburg, L.; Kupec, J . T.; Petersen, J .
L. Organometallics 1996, 15, 1572.
11 and were refined with isotropic thermal parameters for 3.
(20) McKnight, A. L.; Masood, M. A.; Waymouth, R. M.; Strauss, D.
A. Organometallics 1997, 16, 2879.
(21) (a) Gue´rin, F.; McConville, D. H.; Vittal, J . J . Organometallics
1996, 15, 5586. (b) Scollard, J . D.; McConville, D. H.; Vittal, J . J .
Organometallics 1997, 16, 4415.
(12) (a) teXsan, Crystal Structure Analysis Package, Version 1.8;
Molecular Structure Corp., The Woodlands, TX, 1997. (b) d*TREK,
Area Detector Software; Molecular Structure Corp., The Woodlands,
TX, 1997.
(13) (a) International Tables for X-ray Crystallography; Kynoch
Press: Birmingham, U.K. (present distributor Kluwer Academic:
Boston, MA), 1974; Vol. IV, pp 99-102. (b) International Tables for
Crystallography; Kluwer Academic: Boston, MA, 1992; Vol. C, pp 200-
206.
(14) (a) Diamond, G. M.; J ordan, R. F.; Petersen, J . L. Organome-
tallics 1996, 15, 4030. (b) Christopher, J . N.; Diamond, G. M.; J ordan,
R. F.; Petersen, J . L. Organometallics 1996, 15, 4038. (c) Diamond, G.
M.; J ordan, R. F.; Petersen, J . L. Organometallics 1996, 15, 4045. (d)
Diamond, G. M.; J ordan, R. F.; Petersen, J . L. J . Am. Chem. Soc. 1996,
118, 8024.
(22) (a) Baumann, R.; Davis, W. M.; Schrock, R. R. J . Am. Chem.
Soc. 1997, 119, 3830. (b) Aizenberg, M.; Turculet, L.; Davis, W. M.
Schattenmann, F.; Schrock, R. R. Organometallics 1998, 17, 4795. (c)
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J .; Howard, P. Chem. Commun. 1998, 313.
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Amide-Stabilized Cr(II) Nitrosyl Complexes
Organometallics, Vol. 18, No. 10, 1999 1997
Ta ble 1. Cr ysta llogr a p h ic Da ta for Com p lexes 2-4, 10, a n d 11
2a
3b
4b
10b
11b
formula
fw
C18H24CrN2O3
368.40
C16H24CrN2O
312.37
C15H30CrN2OSi
334.50
C14H36CrN2OSi2
356.62
C20H28CrN2O
364.45
color, habit
cryst size, mm
cryst system
space group
a, Å
brown, plate
0.05 × 0.25 × 0.35
triclinic
red, prism
0.10 × 0.25 × 0.35
orthorhombic
Pbca (No. 61)
10.093(2)
14.5436(3)
21.8340(9)
90
red, irregular
0.25 × 0.30 × 0.45
triclinic
red, plate
0.20 × 0.45 × 0.60
triclinic
red, prism
0.45 × 0.30 × 0.20
triclinic
P1h (No. 2)
7.4431(10)
8.682(2)
15.453(3)
95.805(4)
98.073(3)
95.757(3)
977.0(3)
2
P1h (No. 2)
9.6028(7)
11.292(1)
8.954(1)
95.786(10)
96.597(8)
105.207(7)
922.0(2)
2
P1h (No. 2)
10.568(2)
13.584(3)
14.158(3)
89.945(6)
85.289(3)
71.2344(12)
1917.3(6)
4
P1h (No. 2)
11.2004(12)
12.8210(9)
16.979(2)
91.035(7)
91.743(3)
113.6640(8)
2231.0(3)
4
b, Å
c, Å
R, deg
â, deg
90
90
γ, deg
V, Å3
3205.0(4)
8
Z
T, °C
21(1)
1.327
388
6.20
-93(1)
-93(1)
-93(1)
-93(1)
F
calcd, g cm-3
1.295
1. 159
1.062
1.239
F(000)
1328
720
776
388
µ(Mo KR), cm-1
transmissn
factors
7.12
6.58
6.20
5.94
0.88-1.00
0.85-1.00c
0.85-1.00c
0.63-1.00c
0.84-1.00c
scan type
ω-2θ
1.10 + 0.35 tan θ
16 (up to 9 scans)
0.5° oscilln
0.5° oscilln
0.5° oscilln
0.5° oscilln
scan range, deg
scan speed, deg
min-1
no. of data
images
460 (70 s)
460 (35 s)
460 (60 s)
462 (10 s)
φ oscilln range (ø
- 90), deg
ω oscilln range (ø
- 90), deg
2θmax, deg
data collected
crystal decay, %
total no. of rflns
no. of unique
rflns
0.0-190.0
0.0-190.0
0.0-190.0
0.0-190.0
-22.0 to +18.0
-22.0 to +18.0
-22.0 to +18.0
-23.0 to +18.0
55.0
h, (k, (l
1.73
4488
4233
60.1
full sphere
60.0
full sphere
60.0
full sphere
61.1
full sphere
30 062
4396
17 805 (2θmax ) 63.8°)
8354 (2θmax ) 60°)
20 644
8207
8979
4457
Rmerge
no. of rflns with I
G 3σ(I)
no. of variables
R(F) (I g 3σ(I))
Rw
0.026
2289
0.067
2450
0.056
3839
0.040
4520
0.047
2480
217
277
361
361
229
0.035
0.032
1.68
0.035
0.066
1.68
0.042
0.073
1.88
0.056
0.100
2.25
0.042
0.039
1.23
GOF
max ∆/σ (last
cycle)
0.002
0.002
0.0003
0.0007
0.0009
residual density,
e Å-3
-0.23, +0.18
-0.74, +0.70
-0.74, +0.73
-0.61, +0.54
-0.69, +0.63
a
Conditions and definitions: Rigaku AFC6S diffractometer, Mo KR radiation (λ ) 0.710 69 Å), graphite monochromator, takeoff angle
6.0°, aperture 6.0 × 6.0 mm at a distance of 285 mm from the crystal, stationary background counts at each end of the scan (scan/
background time ratio 2:1), σ2(F2) ) [S2(C + 4B)]/Lp2 (S ) scan rate, C ) scan count, B ) normalized background count), function minimized
∑w(|Fo| - |Fc|)2, where w ) 4Fo2/σ2(Fo2), R ) ∑||Fo| - |Fc||/∑|Fo|, Rw ) (∑w(|Fo| - |Fc|)2/∑w|Fo| )1/2, and GOF ) [∑w(|Fo| - |Fc|)2/(m - n)]1/2
.
2
b
Values given for R, Rw, and GOF are based on those reflections with I g 3σ(I). Conditions and definitions: Rigaku AFC7/ADSC Quantum
1 CCD diffractometer, Mo KR radiation (λ ) 0.710 69 Å), graphite monochromator, takeoff angle 6.0°, aperture 94 × 94 mm at a distance
of 39.2 mm from the crystal, oscillation width 0.5°, σ2(F2) ) 3[(C + B)1/2/Lp] (C ) scan count, B ) background count), function minimized
2
2
2
2
2 2
2
2
∑w(|Fo - Fc |)2 where w ) 1/σ2(Fo2), R ) ∑||Fo| - |Fc||/∑|Fo|, Rw ) (∑w(|Fo - Fc |)2/∑w|Fo | )1/2, and GOF ) [∑w(|Fo | - |Fc |)2/(m - n)]1/2
.
Values for Rw and GOF are based on all unique data. c Single-step correction for absorption, decay, and scaling based on symmetry analysis
of redundant data.
under identical experimental conditions.
deposits the less soluble Cr(NO)(NiPr2)(O2CPh)2 (1). If
less than 2 equiv of benzoic acid is used, the final
reaction mixture contains the bis(benzoate) 1 and un-
reacted Cr(NO)(NiPr2)3 (1H NMR, C6D6). Reaction with
a third equivalent of benzoic acid appears to occur at a
slower rate. IR spectroscopic monitoring of the reaction
of bis(benzoate) 1 with PhCO2H in CH2Cl2 reveals that
the ν(NO) band due to 1 gradually diminishes in
intensity and that the final green solution is devoid of
IR bands in the nitrosyl-stretching region. Related
protonolysis reactions have previously been reported for
Cr(NO)(NiPr2)3 and Cr(N)(NiPr2)3 with alcohols.5,25
Nevertheless, favorable solubility properties and rela-
tive rates of amine elimination combine to make conver-
sion 1 a particularly convenient route to the Cr(II)
nitrosyl mono(amide) precursor required for the subse-
quent preparation of alkyl complexes. Thus, treatment
of a red, Et2O solution of Cr(NO)(NiPr2)3 with 2 equiv
of PhCO2H leads to an orange solution that readily
In the absence of an X-ray crystallographic analysis,
the molecular structure of 1 must be deduced from its

1998 Organometallics, Vol. 18, No. 10, 1999
J andciu et al.
Ta ble 2. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) for Com p lex 2a
Ta ble 4. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) for Com p lex 4a
Cr(1)-O(2)
Cr(1)-N(2)
Cr(1)-C(2)
Cr(1)-C(4)
Cr(1)-CP
O(2)-C(6)
N(2)-C(13)
C(1)-C(2)
C(2)-C(3)
C(4)-C(5)
1.975(2)
1.840(2)
2.233(3)
2.249(3)
1.95
1.295(3)
1.485(3)
1.388(4)
1.397(4)
1.383(4)
Cr(1)-N(1)
Cr(1)-C(1)
Cr(1)-C(3)
Cr(1)-C(5)
O(1)-N(1)
O(3)-C(6)
N(2)-C(14)
C(1)-C(5)
C(3)-C(4)
C(6)-C(7)
1.670(2)
2.272(3)
2.185(3)
2.292(3)
1.194(3)
1.220(3)
1.484(3)
1.415(4)
1.396(4)
1.502(4)
Cr(1)-N(1)
Cr(1)-N(2)
Cr(1)-C(1)
Cr(1)-C(2)
Cr(1)-C(3)
Cr(1)-C(4)
Cr(1)-C(5)
Cr(1)-C(6)
Cr(1)-CP(1)
O(1)-N(1)
N(2)-C(10)
N(2)-C(13)
C(1)-C(2)
C(1)-C(5)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
1.653(2)
1.832(2)
2.238(3)
2.207(3)
2.275(2)
2.294(3)
2.276(3)
2.111(2)
1.92
1.211(2)
1.486(3)
1.489(3)
1.396(4)
1.398(4)
1.397(4)
1.380(4)
1.413(4)
Cr(2)-N(3)
Cr(2)-N(4)
Cr(2)-C(16)
Cr(2)-C(17)
Cr(2)-C(18)
Cr(2)-C(19)
Cr(2)-C(20)
Cr(2)-C(21)
Cr(2)-CP(2)
O(2)-N(3)
N(4)-C(25)
N(4)-C(28)
C(16)-C(17)
C(16)-C(20)
C(17)-C(18)
C(18)-C(19)
C(19)-C(20)
1.657(2)
1.834(2)
2.216(3)
2.273(3)
2.305(3)
2.281(3)
2.245(3)
2.103(2)
1.93
1.204(2)
1.473(4)
1.493(3)
1.404(4)
1.392(4)
1.387(4)
1.383(4)
1.397(4)
O(2)-Cr(1)-N(1)
O(2)-Cr(1)-CP
N(1)-Cr(1)-CP
Cr(1)-O(2)-C(6)
Cr(1)-N(2)-C(13) 126.7(2)
C(13)-N(2)-C(14) 112.3(2)
C(1)-C(2)-C(3)
C(3)-C(4)-C(5)
O(2)-C(6)-O(3)
94.19(10) O(2)-Cr(1)-N(2)
116.3
116.6
124.7(2)
101.95(8)
99.5(1)
123.2
N(1)-Cr(1)-N(2)
N(2)-Cr(1)-CP
Cr(1)-N(1)-O(1)
165.7(2)
Cr(1)-N(2)-C(14) 121.0(2)
C(2)-C(1)-C(5)
C(2)-C(3)-C(4)
C(1)-C(5)-C(4)
O(2)-C(6)-C(7)
108.5(3)
108.0(3)
107.0(3)
114.4(3)
107.7(3)
108.9(3)
125.5(3)
N(1)-Cr(1)-N(2) 100.12(10) N(3)-Cr(2)-N(4)
N(1)-Cr(1)-C(6) 97.45(10) N(3)-Cr(2)-C(21)
N(1)-Cr(1)-CP(1) 119.4
N(2)-Cr(1)-C(6)
99.42(11)
95.67(10)
N(3)-Cr(2)-CP(2) 120.5
98.61(10) N(4)-Cr(2)-C(21) 99.79(10)
N(4)-Cr(2)-CP(2) 124.3
N(1)-Cr(1)-N(2)-C(13)
-0.9(2)
N(2)-Cr(1)-CP(1) 124.2
C(6)-Cr(1)-CP(1) 112.1
Cr(1)-N(1)-O(1) 168.0(2)
Cr(1)-N(2)-C(10) 127.2(2)
Cr(1)-N(2)-C(13) 120.7(2)
C(10)-N(2)-C(13) 112.1(2)
C(2)-C(1)-C(5)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-C(5)
C(1)-C(5)-C(4)
a
CP refers to the unweighted centroid of the C(1-5) ring.
C(21)-Cr(2)-CP(2) 111.8
Cr(2)-N(3)-O(2)
169.6(2)
Ta ble 3. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) for Com p lex 3a
Cr(2)-N(4)-C(25) 127.2(2)
Cr(2)-N(4)-C(28) 120.2(2)
C(25)-N(4)-C(28) 112.6(2)
C(17)-C(16)-C(20) 107.3(3)
C(16)-C(17)-C(18) 108.8(3)
C(17)-C(18)-C(19) 107.2(3)
C(18)-C(19)-C(20) 109.2(3)
C(16)-C(20)-C(19) 107.5(3)
Cr(1)-N(1)
Cr(1)-C(1)
Cr(1)-C(3)
Cr(1)-C(5)
Cr(1)-CP
N(2)-C(11)
C(1)-C(2)
C(2)-C(3)
C(4)-C(5)
C(6)-C(10)
C(8)-C(9)
1.660(2)
2.245(2)
2.276(2)
2.264(2)
1.918
1.488(2)
1.411(3)
1.414(3)
1.412(3)
1.453(3)
1.447(3)
Cr(1)-N(2)
Cr(1)-C(2)
Cr(1)-C(4)
Cr(1)-C(6)
O(1)-N(1)
N(2)-C(12)
C(1)-C(5)
C(3)-C(4)
C(6)-C(7)
C(7)-C(8)
C(9)-C(10)
1.845(2)
2.215(2)
2.295(2)
2.195(2)
1.213(2)
1.485(2)
1.394(3)
1.385(3)
1.461(3)
1.349(3)
1.354(3)
108.5(3)
107.8(3)
108.2(3)
108.6(3)
106.8(3)
Cr(1)-C(6)-Si(1) 122.87(13) Cr(2)-C(21)-Si(2) 121.27(12)
N(1)-Cr(1)-N(2)-C(10) -0.6(2)
a
CP(1,2) refer to the unweighted centroids of the C(1-5, 16-
20) rings, respectively.
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-CP
N(2)-Cr(1)-CP
Cr(1)-N(1)-O(1)
99.87(7) N(1)-Cr(1)-C(6)
91.66(7)
99.24(7)
115.2
Ta ble 5. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) for Com p lex 10
120.5
N(2)-Cr(1)-C(6)
C(6)-Cr(1)-CP
124.1
170.1(1)
Cr(1)-N(2)-C(12) 127.2(1)
Cr(1)-N(2)-C(11) 120.8(1)
C(11)-N(2)-C(12) 112.0(1)
Cr(1)-N(1)
Cr(1)-N(2)
Cr(1)-C(1)
Cr(1)-C(2)
O(1)-N(1)
N(2)-C(9)
N(2)-C(12)
1.616(2)
1.763(2)
2.026(3)
1.986(3)
1.221(3)
1.464(4)
1.491(3)
Cr(2)-N(3)
Cr(2)-N(4)
Cr(2)-C(15)
Cr(2)-C(16)
O(2)-N(3)
N(4)-C(23)
N(4)-C(26)
1.637(3)
1.770(2)
1.992(3)
2.022(3)
1.207(3)
1.461(4)
1.477(4)
C(2)-C(1)-C(5)
C(2)-C(3)-C(4)
C(1)-C(5)-C(4)
Cr(1)-C(6)-C(10) 110.5(1)
C(6)-C(7)-C(8)
C(8)-C(9)-C(10)
107.4(2)
108.1(2)
108.6(2)
C(1)-C(2)-C(3)
C(3)-C(4)-C(5)
Cr(1)-C(6)-C(7)
C(7)-C(6)-C(10)
C(7)-C(8)-C(9)
C(6)-C(10)-C(9)
107.9(2)
107.9(2)
112.1(1)
104.1(2)
108.5(2)
109.7(2)
109.5(2)
108.2(2)
N(1)-Cr(1)-N(2) 102.91(12) N(3)-Cr(2)-N(4)
102.60(11)
N(1)-Cr(1)-N(2)-C(11)
-175.9(1)
N(1)-Cr(1)-C(1) 102.85(12) N(3)-Cr(2)-C(15) 102.47(14)
N(1)-Cr(1)-C(2) 103.46(12) N(3)-Cr(2)-C(16) 103.73(13)
N(2)-Cr(1)-C(1) 113.72(11) N(4)-Cr(2)-C(15) 116.33(12)
N(2)-Cr(1)-C(2) 115.68(11) N(4)-Cr(2)-C(16) 112.57(13)
a
CP refers to the unweighted centroid of the C(1-5) ring.
spectroscopic properties (Tables 8 and 9). Its ν(NO) band
at 1710 cm-1 (Nujol) is higher in energy than the value
of 1641 cm-1 reported for the tris(amide) precursor,5
indicative of a decrease in electron density at Cr. Other,
weaker IR bands observed at 1599, 1518, 1493, and 1420
cm-1 are attributable to the benzoate ligands. However,
the modes of attachment (η1 or η2) of the benzoate
ligands to the metal center cannot be unambiguously
C(1)-Cr(1)-C(2)
115.72(12) C(15)-Cr(2)-C(16) 116.52(13)
Cr(1)-N(1)-O(1) 179.0(3)
Cr(1)-N(2)-C(9) 138.4(2)
Cr(1)-N(2)-C(12) 103.2(2)
C(9)-N(2)-C(12) 118.3(2)
Cr(2)-N(3)-O(2)
179.5(2)
Cr(2)-N(4)-C(23) 136.8(2)
Cr(2)-N(4)-C(26) 103.8(2)
C(23)-N(4)-C(26) 119.1(2)
Cr(1)-C(1)-Si(1) 117.39(14) Cr(2)-C(15)-Si(3) 128.8(2)
Cr(1)-C(2)-Si(2) 130.6(2)
Cr(2)-C(16)-Si(4) 120.2(2)
N(1)-Cr(1)-N(2)-C(9)
-1.2(3)
(26) When the difference between the IR bands for the symmetric
and asymmetric stretches of a carboxylate complex is greater than the
difference observed in the free carboxylate, it is indicative of η1
coordination, while a lower difference suggests η2 coordination. In the
assigned on the basis of these bands due to their
intermediate values and the possibility that one or more
of them may well correspond to aromatic C-C bond
stretches.26
case of compound
2 this difference is clearly greater and is in
agreement with the molecular structure showing the benzoate ligand
coordinated in an η1 fashion. Compound 1 is not as clearly defined,
since two benzoate ligands are present, although when the difference
between the highest and lowest carboxylate bands is used, the value
still lies below that of the free benzoate difference and thus implies η2
coordination. Nakamoto, K. Infrared and Raman Spectra of Inorganic
and Coordination Compounds, 5th ed.; Wiley: New York, 1997; Part
B, pp 59-60. See also: Cotton, F. A.; Wilkinson, G. Advanced Inorganic
Chemistry, 5th ed.; Wiley-Interscience: New York, 1988; pp 483-484.
1
Variable-temperature H NMR spectroscopy proved
to be very useful in delineating the fluxional processes
present in 1. At -30 °C, the spectrum in toluene-d8
displays four sharp doublets for the amide methyl
groups, two multiplets for the methine moieties, and two
distinct resonances for the ortho protons of the O2CPh

Amide-Stabilized Cr(II) Nitrosyl Complexes
Organometallics, Vol. 18, No. 10, 1999 1999
Ta ble 6. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) for Com p lex 11
Ph) (2), as illustrated in eq 3. The presence of a ν(CO)
Cr(1)-N(1)
Cr(1)-C(1)
Cr(1)-C(8)
N(2)-C(15)
C(1)-C(2)
C(2)-C(7)
C(8)-C(9)
1.629(2)
2.063(3)
2.085(3)
1.484(3)
1.450(4)
1.417(3)
1.493(3)
Cr(1)-N(2)
Cr(1)-C(2)
O(1)-N(1)
N(2)-C(18)
C(2)-C(3)
C(3)-C(4)
C(9)-C(10)
1.772(2)
2.352(3)
1.219(2)
1.464(3)
1.412(4)
1.374(4)
1.384(3)
band at 1629 cm-1 in the Nujol-mull IR spectrum of 2
suggests that the remaining benzoate ligand adopts an
η1-coordination mode.26 The room-temperature 1H NMR
spectrum of 2 displays distinct doublets for each of the
four amide methyl groups, thereby indicating that any
rotation about the Cr-N linkage is slow on the NMR
time scale. Consistently, the solid-state molecular struc-
ture of 2 (Figure 1 and Table 2) provides evidence for
the presence of a strong Cr-amide π-bonding interac-
tion. Specifically, the Cr-N bond is short,27 and the Cr-
NC2 unit is planar. Significantly, the amide plane is
aligned with the Cr-nitrosyl bond, a conformation that
permits π-donation from the filled amide nitrogen p
orbital into the empty Cr d orbital that lies in the plane
perpendicular to the Cr-NO axis.28 This electronically
preferred conformation is observed even though it places
one of the amide iPr groups close to the Cp ligand. These
distinctive structural features are also evident in the
solid-state molecular structures of the two other CpCr-
(NO)(NiPr2)X complexes we have characterized crystal-
lographically (vide infra).
Our previous analysis of π-bonding in cyclopentadi-
enylchromium complexes indicated that the presence of
the Cr-N π-bond increases the HOMO-LUMO gap of
CpCr(NO)(NR2)X complexes, which in turn improves the
relative stability of the low-spin, diamagnetic electronic
configuration.3 On the other hand, CpCr(NO)Cl2, which
lacks this significant π-bonding interaction, is calculated
to have a triplet ground state;4 the consequent weaken-
ing of the Cr-NO bond renders this compound unstable
with respect to loss of the nitrosyl ligand.3 Finally, it
may be noted that reactions analogous to that shown
in eq 3 using Cp* or HB(pyrazolyl)3 salts instead of
CpNa‚DME in our hands afforded no tractable products.
One possible rationale for the difference is that, with
these larger ligands, the preferred Cr-amide conforma-
tion cannot be achieved due to increased intramolecular
steric repulsion, thereby leading to the disruption of the
Cr-N π-bond and subsequent decomposition via NO
dissociation.
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-C(2)
N(2)-Cr(1)-C(1)
N(2)-Cr(1)-C(8)
Cr(1)-N(1)-O(1)
102.63(9) N(1)-Cr(1)-C(1)
103.93(10) N(1)-Cr(1)-C(8)
109.42(11) N(2)-Cr(1)-C(2)
108.28(10) C(1)-Cr(1)-C(2)
99.37(12)
99.80(10)
140.61(13)
37.62(10)
178.1(2)
Cr(1)-N(2)-C(15) 105.32(15)
Cr(1)-N(2)-C(18) 137.36(15) C(15)-N(2)-C(18) 117.2(2)
Cr(1)-C(1)-C(2)
Cr(1)-C(8)-C(9)
N(2)-C(15)-C(16) 111.7(2)
C(16)-C(15)-C(17) 112.1(2)
82.1(2)
Cr(1)-C(2)-C(1)
60.31(14)
121.8(2)
114.80(15) C(8)-C(9)-C(10)
N(2)-C(15)-C(17) 111.7(2)
N(2)-C(18)-C(19) 110.3(2)
N(1)-Cr(1)-N(2)-C(18)
-0.8(3)
ligands. These are features of a compound possessing
C1 symmetry. At ambient temperatures two broad
singlets are observed for the amide Me groups, along
with two additional broad singlets for the amide CH
groups, and only a single sharp resonance corresponding
to the ipso carbons of the benzoate groups. A species
approaching Cs symmetry results upon warming to 76
°C, as indicated by the methyl signals collapsing to a
single broad resonance, the methine signals shifting
toward each other, and the single ipso carbon resonance
sharpening further.
To explain these observations, it is necessary to
invoke two different fluxional processes. The first is
amide rotation about the Cr-N bond, as indicated by
the variation of the amide resonances with temperature.
The second is a geometric shift at the metal center
converting enantiomers A and B, as depicted in eq 2.
At low temperature both of these processes are slow,
resulting in four different methyl environments and two
different benzoate groups. Because the complex is chiral,
the amide ligand methyl groups are diastereotopic and
thus each has a unique chemical shift. At room tem-
perature the geometric shift has quickened such that
only two methyl environments exist in the molecule,
since any influence from the benzoate ligands will have
become an averaged effect from both ligands. At high
temperature, when amide rotation is also occurring
rapidly, only one methyl group environment is present
and gives rise to the observed singlet.
E lect r on ica lly Sa t u r a t ed H yd r oca r b yl Com -
p lexes. The reaction of 2 with a second equivalent of
(27) Cr-N bond lengths range from 1.77 to 2.12 Å for nonbridging
Cr(II) amides; see: (a) Bradley, D. C.; Hursthouse, M. B.; Newing, C.
W.; Welch, A. J . J . Chem. Soc., Chem. Commun. 1972, 567. (b) Sim,
G. A.; Woodhouse, D. I.; Knox, G. R. J . Chem. Soc., Dalton Trans. 1979,
83. (c) Edema, J . J . H.; Gambarotta, S.; Meetsma, A.; Spek, A. L.;
Smeets, W. J . J .; Chiang, M. Y. J . Chem. Soc., Dalton Trans. 1993,
789. (d) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. R. Organometallics
1995, 14, 5193.
While these data are consistent with several of the
possible geometric isomers of Cr(NO)(NiPr2)(O2CPh)2,
a pseudo-octahedral arrangement with bidentate car-
boxylates and the amide and nitrosyl ligands occupying
mutually cis positions is a reasonable candidate for the
low-temperature limiting structure of 1.
(28) (a) Ashby, M. T.; Enemark, J . H. J . Am. Chem. Soc. 1986, 108,
730. (b) Hubbard, J . L.; McVicar, W. K. Inorg. Chem. 1992, 31, 910.
(c) Legzdins, P.; Ross, K. J .; Sayers, S. F.; Rettig, S. J . Organometallics
1997, 16, 190.
(29) (a) Piper, T. S.; Wilkinson, G. J . Inorg. Nucl. Chem. 1956, 3,
104. (b) Cotton, F. A.; Musco, A.; Yagupsky, G. J . Am. Chem. Soc. 1967,
89, 6136. (c) Cotton, F. A.; Legzdins, P. J . Am. Chem. Soc. 1968, 90,
6232. (d) Calderon, J . L.; Cotton, F. A.; Legzdins, P. J . Am. Chem. Soc.
1969, 91, 2528. (e) Hames, B. W.; Legzdins, P.; Martin, D. T. Inorg.
Chem. 1978, 17, 3644. (f) Hunt, M. H.; Kita, W. G.; Mann, B. E.;
McCleverty, J . A. J . Chem. Soc., Dalton Trans. 1978, 467.
In tr od u ction of a Cp Liga n d . Treatment of 1 with
1 equiv of CpNa‚DME affords CpCr(NO)(NiPr2)(OC(O)-

2000 Organometallics, Vol. 18, No. 10, 1999
J andciu et al.
Ta ble 7. Nu m ber in g Sch em e, Yield , a n d An a lytica l Da ta for Com p lexes 1-13
anal. found (calcd)
H
complex
no.
color
(isolated yield, %)
complex
C
N
Cr(NO)(NiPr2)(O2CPh)2
1
2
3
4
5
6
7
8
9
10
11
12
13
orange (75)
56.58 (56.60)
58.83 (58.70)
61.61 (61.54)
53.91 (53.89)
63.63 (63.91)
63.13 (62.96)
65.05 (65.14)
62.42 (62.20)
65.69 (65.52)
46.77 (47.15)
65.46 (65.91)a
65.13 (65.91)a
51.25 (51.94)a
5.52 (5.70)
6.50 (6.52)
7.55 (7.69)
8.97 (8.98)
7.87 (7.69)
7.26 (7.41)
7.44 (7.43)
8.72 (8.54)
6.85 (6.90)
9.84 (10.17)
7.41(7.74)
7.72 (7.74)
10.18 (10.25)
6.78 (6.60)
7.56 (7.61)
8.80 (8.97)
8.32 (8.38)
8.39 (8.28)
8.52 (8.64)
7.57 (8.00)
8.38 (8.54)
7.94 (8.05)
7.66 (7.86)
7.45(7.69)
7.47 (7.69)
9.26 (9.57)
CpCr(NO)(NiPr2)(O2CPh)
CpCr(NO)(NiPr2)(η1-Cp)
dark green (43)
dark red (47)
red (69)
red (45)
red (42)
CpCr(NO)(NiPr2)(CH2SiMe3)
CpCr(NO)(NiPr2)(η1-CH2Ph)
CpCr(NO)(NiPr2)(Ph)
CpCr(NO)(NiPr2)(CPhdCH2)
CpCr(NO)(NiPr2)(CtCCMe3)
CpCr(NO)(NiPr2)(CtCPh)
Cr(NO)(NiPr2)(CH2SiMe3)2
Cr(NO)(NiPr2)(CH2Ph)2
red (38)
dark red-green (31)
dark red-green (54)
dark red (85)
red-orange (47)
dark red (39)
dark red (53)
Cr(NO)(NiPr2)(o-tolyl)2
Cr(NO)(NiPr2)(CH2SiMe3)(C(dNtBu)CH2SiMe3)
a
The carbon determination was repeatedly low despite numerous attempts with a variety of oxidizing agents.
compounds containing σ-bonded hydrocarbyl ligands.30
The hydrocarbyl ligands span a broad range of sizes,
from the large CH2SiMe3 ligand to the less sterically
demanding phenyl and alkynyl derivatives. The differ-
ent electronic properties of these ligands are reflected
in the varying values of ν(NO) for compounds 2-9
collected in Table 8. These bands suggest that the
relative electron-donating power of the ligands to the
CpCr(NO)(NiPr2) fragment follows the order alkenyl <
Ta ble 8. Ma ss Sp ectr oscop ic a n d IR Sp ectr a l Da ta
for Com p lexes 1-13
complex
no.
probe
temp (°C)
MSa (m/z)
IR (Nujol, cm-1
)
1
394 (P+ - NO)
150
1710 (s, νNO), 1599 (w),
1518 (w), 1493 (w),
1420 (w)
1668 (s, νNO), 1629 (m),
1574 (w), 1334 (m)
2
368 (P+)
120
3
4
5
6
7
8
9
10
11
12
13
312 (P+)
334 (P+)
338 (P+)
324 (P+)
350 (P+)
328 (P+)
348 (P+)
356 (P+)
364 (P+)
364 (P+)
439 (P+)
150
150
150
150
120
150
120
150
150
150
150
1670 (s, νNO
1660 (s, νNO
)
)
η1-Cp ≈ benzoate ≈ alkynyl < benzyl ≈ CH2SiMe3
<
phenyl. The 1H NMR spectra for these compounds
(Table 9) reveal that in each case all four amide Me
groups are inequivalent, again implying that any rota-
tion about the Cr-N bonds is slow on the NMR time
scale at room temperature. That the specific orientation
of this “locked” conformation is the planar ON-Cr-NC2
alignment dictated by the respective π-bonding require-
ments of the nitrosyl and amide ligands is confirmed
by the solid-state molecular structure of 4 (Figure 3).
Attempts to prepare other CpCr(NO)(NiPr2)R com-
pounds via the methodology shown in eq 5 were unsuc-
cessful for R ) Me or CH2CMe3. A possible reason for
this failure is suggested by the byproducts of the
thermolytic decomposition of complex 4. When CpCr-
(NO)(NiPr2)(CH2SiMe3) is heated in C6D6 at 50 °C, the
1661 (w, νNO)
1644 (s, νNO
1676 (s, νNO
1667 (s, νNO
)
)
)
1670 (m, νNO)
1670 (s, νNO
1634 (s, νNO
1620 (s, νNO
1609 (s, νNO
)
)
)
)
a
Values for the highest intensity peak of the calculated isotopic
cluster (52Cr).
Cp anion produces CpCr(NO)(NiPr2)(η1-Cp) (3) (eq 4).
1
characteristic H NMR spectroscopic features of 4 are
diminished and resonances attributable to Me4Si and
Me2CdNiPr are observable. These observations indicate
that â-H abstraction from the amide methine group by
the alkyl ligand may well be an important decomposi-
tion pathway for CpCr(NO)(NiPr2)R species, as has been
previously suggested for Cr(NiPr2) alkyl nitride com-
plexes.25
Electr on ica lly Un sa tu r a ted Hyd r oca r byl Com -
p lexes. Coordinatively and electronically unsaturated
chromium hydrocarbyl compounds of intermediate oxi-
dation state (II-IV) are of interest as potential model
complexes for heterogeneous, Cr-based olefin polymer-
ization catalysts.31 These mid-valent, open-shell orga-
nochromium species typically possess unpaired elec-
trons, a property that reduces the utility of NMR
The 18e bis(cyclopentadienyl) complex 3 can also be
generated directly from bis(benzoate) 1 by treatment
with 2 equiv of Cp anion. Complex 3 contains one η5-
and one η1-Cp ligand, as is clearly evident in its solid-
state molecular structure (Figure 2 and Table 3).
Similar structural features have been observed previ-
ously for CpCr(NO)2(η1-Cp), Cp2Mo(NO)(η1-Cp), and
CpMo(NO)(S2CNR2)(η1-Cp).29
Other 18e hydrocarbyl complexes analogous to com-
plex 3 can be conveniently prepared by the hydrocar-
bylation reactions of the Cp-benzoate complex 2 sum-
marized in eq 5. The electronically saturated CpCr(NO)-
(30) Cr(NO)(CH(SiMe3)2)3 has previously been proposed as the
product of the reaction of Cr(CH(SiMe3)2)3 with NO gas. However, this
species was reported to be too unstable to be isolated and was
characterized solely by solution IR spectroscopy: (a) Barker, G. K.;
Lappert, M. F. J . Organomet. Chem. 1974, 76, C45. (b) Barker, G. K.;
Lappert, M. F.; Howard, J . A. K. J . Chem. Soc., Dalton Trans. 1978,
734.
(NiPr2)R complexes (R ) CH2SiMe3 (4), η1-CH2Ph (5),
Ph (6), CPhdCH2 (7), CtCCMe3 (8), CtCPh (9)) are
the first well-characterized examples of Cr(II) nitrosyl
(31) (a) Theopold, K. H. CHEMTECH 1997, 27(10), 26. (b) Theopold,
K. H. Eur. J . Inorg. Chem. 1998, 1, 15.

Amide-Stabilized Cr(II) Nitrosyl Complexes
Organometallics, Vol. 18, No. 10, 1999 2001
Ta ble 9. 1H a n d 13C{1H} NMR Sp ectr oscop ic Da ta for Com p lexes 1-13
complex
no.
13C{1H} NMRa
δ (ppm)
complex
no.
13C{1H} NMRa
δ (ppm)
1H NMRa δ (ppm)
1.30 (br s, 6H, CH3)
1.88 (br s, 6H, CH3)
5.08 (br s, 1H, CH)
1H NMRa δ (ppm)
1b
21.9 (CH3)
28.5 (CH3)
69.0 (CH)
70.6 (CH)
129.1 (CHo/CHm
130.0 (CHo/CHm
131.5 (CCO2)
134.1 (CHp)
2
0.62 (d, 3H, CH3, J ) 7 Hz)
0.98 (d, 3H, CH3, J ) 7 Hz)
1.30 (d, 3H, CH3, J ) 7 Hz)
1.69 (d, 3H, CH3, J ) 7 Hz)
3.97 (m, 1H, CH)
5.40 (s, 5H, C5H5)
5.51 (m, 1H, CH)
20.3 (CH3)
22.2 (CH3)
26.7 (CH3)
29.5 (CH3)
65.0 (CH)
75.7 (CH)
6.62 (br s, 1H, CH)
7.43 (vt, 4H, Hm, J app ) 7.8 Hz)
7.57 (vt, 2H, Hp, J app ) 7.5 Hz)
8.03 (d, 4H, Ho, J ) 7.2 Hz)
)
)
105.2 (C5H5)
128.1 (CHo/CHm
130.3 (CHo/CHm
130.7 (CHp)
136.7 (CCO2)
171.2 (CO2)
3.0 (Si(CH3))
15.0 (CH2)
7.15 (m, 3H, C6H5)
8.40 (m, 2H, C6H5)
)
)
182.5 (CO2)
3
5
0.60 (d, 3H, CH3, J ) 6.2 Hz)
0.78 (d, 3H, CH3, J ) 6.5 Hz)
1.22 (d, 3H, CH3, J ) 6.5 Hz)
1.70 (d, 3H, CH3, J ) 6.5 Hz)
3.70 (m, 1H, CH)
19.9 (CH3)
20.5 (CH3)
25.4 (CH3)
30.7 (CH3)
62.0 (CH)
4
0.37 (s, 9H, Si(CH3)3)
0.60 (d, 1H, CHH′, J ) 11 Hz)
0.65 (vt, 6H, CH3, J app ) 7 Hz)
0.82 (d, 1H, CHH′, J ) 11 Hz)
1.38 (d, 3H, CH3, J ) 7 Hz)
1.62 (d, 3H, CH3, J ) 7 Hz)
3.65 (m, 1H, CH)
4.25 (m, 1H, CH)
5.17 (s, 5H, C5H5)
0.91 (d, 3H, CH3, J ) 7 Hz)
1.23 (vt, 6H, CH3, J app ) 7 Hz)
1.45 (d, 3H, CH3, J ) 7 Hz)
4.10 (m, 1H, CH)
17.6 (CH3)
20.3 (CH3)
25.8 (CH3)
30.0 (CH3)
60.8 (CH)
71.0 (CH)
102.2 (C5H5)
18.9 (CH3)
21.5 (CH3)
26.1 (CH3)
30.8 (CH3)
62.2 (CH)
73.0 (CH)
4.40 (m, 1H, CH)
72.2 (CH)
4.59 (s, 5H, η5-C5H5)
105.0 (η5-C5H5)
6.39 (s, 5H, η1-C5H5)
116.2 (br, η1-C5H5)
0.63 (d, 3H, CH3, J ) 7 Hz)
0.70 (d, 3H, CH3, J ) 7 Hz)
1.40 (d, 3H, CH3, J ) 7 Hz)
1.70 (d, 3H, CH3, J ) 7 Hz)
2.68 (d, 1H, CHH′, J ) 11 Hz)
3.70 (m, 1H, CH)
18.3 (CH3)c
20.6 (CH3)
25.3 (CH3)
30.9 (CH3)
37.6 (CH2)
61.0 (CH)
6d
5.07 (m, 1H, CH)
5.56 (s, 5H, C5H5)
3.70 (d, 1H, CHH′, J ) 11 Hz)
71.3 (CH)
103.1 (C5H5)
123.0 (CHp)
6.92 (t, 1H, CHp, J ) 5 Hz)
7.01 (vt, 2H, CHm, J app ) 5 Hz)
7.51 (d, 2H, CHo, J ) 5 Hz)
103.8 (C5H5)
124.5 (CHp)
127.4 (CHo/CHm
138.7 (CHo/CHm
4.30 (m, 1H, CH)
4.95 (s, 5H, C5H5)
6.98 (m, 1H, Hp)
)
)
153.3 (Cipso)
7.22 (m, 2H, Ho/Hm
7.50 (m, 2H, Ho/Hm
0.41 (d, 3H, CH3, J ) 6.4 Hz)
0.67 (d, 3H, CH3, J ) 6.5 Hz)
1.36 (d, 3H, CH3, J ) 6.5 Hz)
1.55 (d, 3H, CH3, J ) 6.5 Hz)
3.65 (m, 1H, CH)
4.31 (m, 1H, CH)
5.21 (s, 5H, C5H5)
5.36 (d, 1H, CHH′, J ) 1.7 Hz)
6.08 (d, 1H, CHH′, J ) 1.7 Hz)
7.04 (m, 1H, CHp)
7.20 (m, 2H, CHo/CHm
7.22 (m, 2H, CHo/CHm
)
)
171.0 (Cipso)
7e
18.5 (CH3)f
12.0 (CH3)
26.5 (CH3)
61.9 (CH)
73.0 (CH)
103.2 (C5H5)
120.2 (CdCH2)
125.0 (CHp)
8g
0.61 (d, 3H, CH3, J ) 7 Hz)
1.18 (d, 3H, CH3, J ) 7 Hz)
1.30 (d, 3H, CH3, J ) 7 Hz)
1.40 (s, 9H, C(CH3)3)
1.95 (d, 3H, CH3, J ) 7 Hz)
3.90 (m, 1H, CH)
18.9 (CH3)
19.6 (CH3)
26.0 (CH3)
29.8 (CH3)
32.4 (C(CH3)3)
62.7 (C(CH3)3)
62.9 (CH)
4.50 (m, 1H, CH)
5.25 (s, 5H, C5H5)
73.1 (CH)
102.5 (C5H5)
127.1 (CHo/CHm
128.2 (CHo/CHm
155.9 (Cipso)
)
)
106.7 (CtCC(CH3)3)
133.1 (CtCC(CH3)3)
)
)
180.7 (CdCH2)
9g
0.58 (d, 3H, CH3, J ) 7 Hz)
1.10 (d, 3H, CH3, J ) 7 Hz)
1.38 (d, 3H, CH3, J ) 7 Hz)
1.90 (d, 3H, CH3, J ) 7 Hz)
3.90 (m, 1H, CH)
19.9 (CH3)h
10
0.20 (d, 2H, CHH′, J ) 6.6 Hz)
0.32 (s, 18H, Si(CH3)3)
0.92 (br s, 6H, CH3)
1.14 (br s, 6H, CH3)
1.99 (d, 2H, CHH′, J ) 6.6 Hz)
3.28 (v br s, 1H, CH)
2.54 (Si(CH3)3)
21.3 (br, CH3)
28.1 (br, CH3)
68.1 (CH2)
49.0 (br, CH)
55.9 (br, CH)
20.4 (CH3)
27.2 (CH3)
30.2 (CH3)
63.7 (CH)
4.50 (m, 1H, CH)
74.3 (CH)
5.22 (s, 5H, C5H5)
7.01 (m, 1H, CHp)
7.10 (m, 2H, CHo/CHm
7.60 (m, 2H, CHo/CHm
0.72 (br s, 6H, CH3)
1.19 (br s, 6H, CH3)
102.8 (C5H5)
125.5 (CHp)
127.9 (CHo/CHm
130.5 (CHo/CHm
20.6 (br, CH3)
27.3 (br, CH3)
42.8 (br, CH)
53.6 (CH2)
4.36 (v br s, 1H, CH)
)
)
)
)
11
12
1.04 (d, 3H, CH3, J ) 6.0 Hz)
1.19 (d, 3H, CH3, J ) 6.1 Hz)
1.29 (d, 6H, CH3, J ) 5.8 Hz)
2.40 (s, 3H, PhCH3)
18.9 (CH3)i
24.2 (CH3)
25.1 (CH3)
25.4 (CH3)
25.7 (CH3)
27.7 (CH3)
61.7 (CH)
64.5 (CH)
124.5 (Caromatic
126.0 (Caromatic
127.0 (Caromatic
127.7 (Caromatic
129.0 (Caromatic
1.72 (d, 2H, CHH′, J ) 7.0 Hz)
2.20 (v br s, 1H, CH)
2.85 (d, 2H, CHH′, J ) 7.0 Hz)
3.22 (v br s, 1H, CH)
6.81 (m, 4H, C6H5)
56.4 (br, CH)
125.8 (CHp)
129.1 (CHo/CHm
130.5 (CHo/CHm
2.77 (s, 3H, PhCH3)
4.49 (m, 1H, CH)
4.68 (m, 1H, CH)
6.73-7.17 (m, 6H, Hm/Hp)
8.00 (d, 1H, Ho, J ) 7.6 Hz)
8.13 (d, 1H, Ho, J ) 6.9 Hz)
)
)
7.00 (m, 6H, C6H5)
138.1 (Cipso
)
)
)
)
)
)
2 × 130.0 (Caromatic
)
141.3 (Caromatic
143.0 (Caromatic
161.6 (Caromatic
177.4 (Caromatic
30.3 (C(CH3)3)
)
)
)
)
13j
0.03 (s, 9H, Si(CH3)3)
0.22 (s, 9H, Si(CH3)3)
0.68 (d, 1H, CHH′, J ) 9.9 Hz)
0.70 (d, 3H, CH3, J ) 6.3 Hz)
0.91 (d, 3H, CH3, J ) 6.3 Hz)
1.50 (s, 9H, C(CH3)3)
0.3 (Si(CH3)3)
3.0 (Si(CH3)3)
19.2 (CH3)
22.2 (CH3)
23.7 (CH2)
28.3 (CH3)
13j
1.82 (d, 1H, CHH′, J ) 9.9 Hz)
2.93 (d, 1H, -CCHH′Si(Me3), J ) 11.4 Hz) 31.1 (CH2)
3.00 (m, 1H, CH)
3.30 (d, 1H, CCHH′Si(Me3), J ) 11.1 Hz)
3.56 (m, 1H, CH)
53.2 (CH)
56.2 (CH)
62.1 (C(CH3)3)
220.8 (CdN)
1.56 (vt, 6H, CH3, J app ) 6.0 Hz) 29.5 (CH3)
a
b
1H and 13C{1H} NMR spectra recorded in C6D6 at room temperature unless otherwise noted. Spectra recorded in THF-d8. c The CHo
and CHm signals were obstructed by the solvent signal. Spectra recorded in CD3CN. e 13C{1H} spectrum recorded in (CD3)2CO. f The
d
fourth methyl signal was obstructed by the solvent signal. g 13C{1H} spectrum recorded in CDCl3. The Cipso and two quaternary acetylide
h
carbon signals were not observed in the spectrum. One aromatic signal was obstructed by the solvent signal. j Spectra recorded in CDCl3.
i

2002 Organometallics, Vol. 18, No. 10, 1999
J andciu et al.
F igu r e 4. Solid-state molecular structure of complex 10
(50% probability thermal ellipsoids are shown).
F igu r e 1. Solid-state molecular structure of complex 2
(33% probability thermal ellipsoids are shown).
halide or hydrocarbyl ligand. Although theoretical stud-
ies suggest that compounds such as CpCr(NO)(R)Cl and
CpCr(NO)(NR2)Cl should be less prone to loss of nitric
oxide than is CpCr(NO)Cl2,4 such complexes have yet
to be isolated or even spectroscopically detected in any
of the reactions that we have attempted to date.
Comparatively mild reagents fail to react with the
amide alkyl species, and more harsh reagents such as
anhydrous HCl in Et2O lead to decomposition to non-
nitrosyl-containing Cr(III) species.6
Fortunately, unsaturated hydrocarbyl compounds are
obtainable by the direct hydrocarbylation of bis(ben-
zoate) complex 1 with organomagnesium reagents (eq
6). The novel Cr(NO)(NiPr2)R2 products (R ) CH2SiMe3
F igu r e 2. Solid-state molecular structure of complex 3
(50% probability thermal ellipsoids are shown).
(10), CH2Ph (11), o-tolyl (12)) are isolable as red,
diamagnetic solids that are sensitive to both air and
moisture. The solid-state molecular structures of 10
(Figure 4 and Table 5) and 11 (Figure 5 and Table 6)
have been established by single-crystal X-ray crystal-
lography, and they exhibit some interesting features.
For instance, the metrical parameters of both structures
provide evidence for the presence of strong Cr-N(amide)
π-bonding interactions. Indicative of the strong π-bonds
present in all of the molecular structures reported in
this paper are the N(nitrosyl)-Cr-N(amide)-C torsion
angles, which fall within 4° of 0 and 180°.6 The Cr-NO
bonds in 10 and 11 remain in the planes defined by the
Cr-NC2 units, and the Cr-N amide bond lengths of
1.763(2) Å in 10 and 1.772(2) Å in 11 are even shorter
than those extant in complexes 2-4.27 Furthermore, one
benzyl ligand in 11 is attached in a η1 fashion to the
metal center and functions as a one-electron donor,
whereas the other is attached in an η2 mode and
provides three electrons to the central chromium.32
F igu r e 3. Solid-state molecular structure of complex 4
(50% probability thermal ellipsoids are shown).
spectroscopy, the primary reaction-monitoring tech-
nique employed in homogeneous organometallic chem-
istry. However, it is not unreasonable to expect that if
the spin-pairing and orbital-splitting energies of such
compounds were properly manipulated by their ancil-
lary ligands, diamagnetic unsaturated chromium alkyl
complexes might then be attainable.
Such species could in principle be generated from the
CpCr(NO)(NiPr2)R compounds described in the preced-
ing section if the amide group could be replaced with a
(32) For related bis(benzyl) complexes of molybdenum and tungsten,
see: (a) Legzdins, P.; J ones, R. H.; Phillips, E. C.; Yee, V. C.; Trotter,
J .; Einstein, F. W. B. Organometallics 1991, 10, 986. (b) Dryden, N.
H.; Legzdins, P.; Trotter, J .; Yee, V. C. Organometallics 1991, 10, 2857
and references therein. (c) Reference 27b.

Amide-Stabilized Cr(II) Nitrosyl Complexes
Organometallics, Vol. 18, No. 10, 1999 2003
separate and sharpen moderately upon cooling of the
sample to -60 °C, although coupling to the methyl
protons is not visible. Calculation of ∆Gq322 K using these
resonances gives the nearly identical result of 14.2(1)
kcal mol-1
.
Another interesting feature is the significant variance
of the spectra with solvent. For example, in CD2Cl2 the
methyl signals have already gone past coalescence at
room temperature, suggesting that in polar solvents
rotation of the amide ligand is faster. This observation
can be rationalized by considering that a polar solvent
is able to draw electron density from the Cr-amide
bond, thus weakening the interaction and facilitating
the fluxional process.
Similarly, a variable-temperature 1H NMR study was
undertaken for compound 11. This showed that the
amide fluxional processes occurring in this compound
are analogous to those in 10. However, in addition to
amide ligand rotation, there is also the added element
of observing the two benzyl ligand interactions. In
contrast to the solid-state molecular structure of 11, the
η2-benzyl interaction is not detected in solution either
at room temperature or at -63 °C. Clearly in solution
the two benzyl ligands are “switching” from η1 to η2 at
a rate more rapid than the NMR time scale. At low
temperature slight broadening of the aromatic signals
occurs, thereby suggesting that these signals are begin-
ning to decoalesce.32
The agostic interaction is also absent in solution, as
evidenced by gated 13C{1H} NMR spectroscopy. At -63
°C the coupling constant for one of the methine carbon
signals is 144 Hz while the other is reduced to 118 Hz.
The latter value is not small enough to indicate an
agostic interaction conclusively, but it does demonstrate
that the two methine environments are indeed different.
Much sharper amide resonances and distinct aromatic
signals appear in the 1H NMR spectrum of the aryl
derivative 12, presumably due to the increased steric
demands of the ortho-substituted ligand. The net effect
is a slowing of the rotation of the amide moiety and an
inequivalent orientation of the o-tolyl ligands.
F igu r e 5. Solid-state molecular structure of complex 11
(50% probability thermal ellipsoids are shown).
There is also an agostic interaction involving the
chromium and the C(15)-H(15) bond, as indicated by
the refined Cr(1)-H(15) distance of 2.12(2) Å and the
Cr(1)-N(2)-C(15) angle of 105.32(15)°, which is much
smaller than the expected 120°. Consistently, the
Cr(1)-N(2)-C(18) angle has expanded to 137.36(15)°.
The solid-state structure of 10 also exhibits metrical
parameters indicative of a similar agostic interaction,
even though the relevant hydrogen atom was not located
and refined in this case. Nevertheless, the distortions
of the angles at N(2) (i.e. Cr(1)-N(2)-C(12) ) 103.2-
(2)° and Cr(1)-N(2)-C(9) ) 138.4(2)°) are even more
pronounced than in 11. Overall 10 has a flattened,
pseudotetrahedral geometry about the chromium center.
The spectroscopic properties of 10 and 11 (presented
in Tables 8 and 9) indicate that these complexes are
stereochemically nonrigid in solution. The room-tem-
1
perature H NMR spectrum of 10 in toluene-d8 exhibits
two broad singlets corresponding to the amide ligand
methyl groups. This is an indication that a single
fluxional process is occurring in this compound, namely
rotation about the Cr-amide bond. When the NMR
sample is warmed, these two signals move closer
together and coalesce at 32 °C with a chemical shift of
1.06 ppm. Further warming to 60 °C results in equili-
bration of the four methyl groups and thus one sharp
signal in the spectrum exhibiting 7 Hz coupling to the
methine protons. Cooling of the sample to -60 °C
produces a limiting spectrum with the two signals
further separated and methine proton coupling only
slightly visible. Using this variable-temperature NMR
data, it is possible to calculate ∆Gq305 K, the barrier to
this fluxional process, to be 14.1(1) kcal mol-1, which
falls in the expected range for this type of process.25b,33
It is also possible to study the methine protons and
obtain a measure of the barrier to rotation using their
1H NMR signals. In this case, two very broad singlets
are present in the room-temperature spectrum, which
shift toward each other upon warming and coalesce at
49 °C with a chemical shift of 3.85 ppm. The signals
As would be expected for coordinatively and electroni-
cally unsaturated compounds, 10-12 exhibit reactivity
toward a variety of small molecules. For example,
preliminary investigations indicate that t-BuNC can be
inserted into the bis(alkyl) complex 10 to generate 13,
as shown in eq 7.25b Further reactivity studies are
currently in progress.
Ep ilogu e
Understanding precisely why Cr(NO)(NiPr2)R2 com-
plexes are diamagnetic is an issue with potentially
important ramifications. While great advances have
recently been made in olefin polymerization catalyzed
by homogeneous, organometallic compounds of Ti and
Zr,34 the paramagnetic nature of unsaturated Cr alkyl
species has reduced their effectiveness as model com-
pounds for providing analogous insights into the de-
(33) Chisholm, M. H.; Folting, K.; Huffman, J . C.; Rothwell, I. P.
Organometallics 1982, 1, 251.

2004 Organometallics, Vol. 18, No. 10, 1999
J andciu et al.
tailed mechanism of commercially applicable, hetero-
geneous, Cr-based polymerization systems.31 Similar
obstacles will undoubtedly beset investigation of Fe and
Co model compounds in the wake of the recent develop-
ment of paramagnetic polymerization catalysts based
on these later first-row metals.35 Diamagnetic, mid-
valent, unsaturated alkyl compounds of these metals
are desirable both for practical reasons (increased utility
of NMR techniques) and for more fundamental concerns
(potential comparisons to gauge the role of spin state).36
The characteristic reactivities of Cr(NO)(NiPr2)R2 com-
pounds, as well as an evaluation of the factors which
contribute to their spin-paired electronic configuration,
will be the subjects of a later publication.37
containing σ-hydrocarbyl ligands. Selective amine elimi-
nation of two NiPr2 groups occurs upon treatment with
benzoic acid, and the resulting O2CPh ligands are
readily exchanged via salt metatheses. The CpCr(NO)-
(NiPr2) fragment readily supports a wide variety of R
ligands, although some of the alkyl derivatives are
thermally sensitive. Routes to Cp′Cr(NO)R2 compounds
remain to be devised, since this class of Cr(II) alkyl
compounds is not accessible from CpCr(NO)(NiPr2)X
precursors. However, unsaturated organochromium(II)
compounds with an even greater degree of unsaturation
can be obtained via the dialkylation of Cr(NO)(NiPr2)(O2-
CPh)2.
Ack n ow led gm en t. We are grateful to the Natural
Sciences and Engineering Research Council of Canada
for support of this work in the form of grants to P.L.
We also thank Dr. Sean Lumb for technical assistance
and many helpful discussions.
Con clu sion s
Cr(NO)(NiPr2)3 has proven to be a useful synthetic
precursor to diamagnetic Cr(II) nitrosyl compounds
(34) (a) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (b)
Obora, Y.; Stern, C. L.; Marks, T. J .; Nickias, P. N. Organometallics
1997, 16, 2503. (c) Halterman, R. L.; Tretyakov, A.; Kahn, M. A. J .
Organomet. Chem. 1998, 568, 41. (d) Bravakis, A. M.; Bailey, C. E.;
Pigeon, M.; Collins, S. Macromolecules 1998, 31, 1000.
(35) (a) Britovsek, G. J . P.; Gibson, V. C.; Kimberley, B. S.; Maddox,
P. J .; McTavish, S. J .; Solan, G. A.; White, A. J . P.; Williams, D. J .
Chem. Commun. 1998, 849. (b) Small, B. L.; Brookhart, M.; Bennett,
M. A. J . Am. Chem. Soc. 1998, 120, 4049. (c) Small, B. L.; Brookhart,
M. J . Am. Chem. Soc. 1998, 120, 7143.
Su p p or tin g In for m a tion Ava ila ble: Full details of the
crystallographic analyses, including tables of crystallographic
data, atomic coordinates, bond lengths, bond angles, hydrogen
atom coordinates, anisotropic displacement parameters, tor-
sion angles, and selected nonbonded contacts for complexes
2-4, 10, and 11. This material is available free of charge via
the Internet at http://pubs.acs.org.
(36) Poli, R. Chem. Rev. 1996, 96, 2135.
(37) J andciu, E. W.; Legzdins, P. Work in progress.
OM980954M