Synthesis and structure of a (NacNac)rhodium terminal dinitrogen complex

Article abstract of DOI:10.1139/v05-012

Reaction of the ligand salt CH[C(Me)(N-i-Pr₂C₆H ₃)]₂Li(Et₂O) with [Rh(COE)(μ-Cl)] ₂ proceeds to give the brown crystalline product CH[C(Me)(N-i-Pr ₂C₆H₃)]₂Rh(N₂)(COE) (1) in 38% yield. X-ray crystallographic study of 1 confirmed the formulation and revealed that the terminally bound dinitrogen molecule exhibits a Rh-N distance of 1.943(4) A and an N-N distance of 1.091(6) A. Comparison to other Rh-N₂ complexes shows that the N-N distances reflect a complex admixture of the trans influence and the net electron density at the metal center.

Full text of DOI:10.1139/v05-012

324
Synthesis and structure of a (NacNac)rhodium
terminal dinitrogen complex1
Jason D. Masuda and Douglas W. Stephan
Abstract: Reaction of the ligand salt CH[C(Me)(N-i-Pr2C6H3)]2Li(Et2O) with [Rh(COE)(µ-Cl)]2 proceeds to give the
brown crystalline product CH[C(Me)(N-i-Pr2C6H3)]2Rh(N2)(COE) (1) in 38% yield. X-ray crystallographic study of 1
confirmed the formulation and revealed that the terminally bound dinitrogen molecule exhibits a Rh—N distance of
1.943(4) Å and an N—N distance of 1.091(6) Å. Comparison to other Rh–N2 complexes shows that the N—N dis-
tances reflect a complex admixture of the trans influence and the net electron density at the metal center.
Key words: rhodium, dinitrogen, diimine, structure.
Résumé : La réaction du sel ligand CH[C(Me)(N-i-Pr2C6H3)]2Li(Et2O) avec le [Rh(COE)(µ-Cl)]2 conduit à la forma-
tion d’un produit cristallin de couleur brune, le CH[C(Me)(N-i-Pr2C6H3)]2Rh(N2)(CEO) (1), avec un rendement de
38 %. Une étude du composé 1 par diffraction des rayons X a permis de confirmer sa formule et a révélé que, pour la
molécule de diazote liée en position terminale, la distance Rh—N est égale à 1,943(4) Å et la distance N—N est égale
à 1,091(6) Å. Une comparaison avec d’autres complexes Rh–N2 montre que les distances N—N sont un reflet d’un
mélange complexe de l’influence trans et d’une densité électronique nette au niveau du centre métallique.
Mots clés : rhodium, diazote, diimine, structure.
[Traduit par la Rédaction] Masuda and Stephan 327
Introduction
(phosphine, alkyl, aryl) is thought to strengthen the Rh to N2
interaction via π backbonding (Scheme 2).
The 40th anniversary of the discovery of dinitrogen com-
plexes is being commemorated with this issue of the Cana-
dian Journal of Chemistry. During the time since the initial
discovery of a Ru–N2 complex by Allen and Senoff (1), a
variety of other metals have been shown to interact with N2
in either a terminal or bridging fashion. Moreover, the reac-
tivity of such species, have led to significant breakthroughs
in the understanding of N2 chemistry. This work has been re-
viewed (2–6). In continuing studies towards N2 reduction,
metals capable of both N2 and H2 activation have been tar-
geted. While Rh complexes have a well-known ability to
react with hydrogen, few Rh–N2 complexes are known. Sev-
eral examples of bridging bimetallic species have been
described including [Me2C6H(CH2P-i-Pr2)2Rh]2(µ-N2) (7),
[Me2C6H(CH2P-t-Bu2)(CH2NEt2)Rh]2(µ-N2) (8), [MeC6H2(CH2P-
t-Bu2)2ORh]2(µ-N2) (9), [(i-Pr3P)2RhH]2(µ-N2) (10), and
[CH2(P-t-Bu2)2RhCH2-t-Bu]2(µ-N2) (11) (Scheme 1). In
addition, terminally bound N2 complexes that have been
characterized include (Ph-t-Bu2P)2RhH(N2) (12), (i-Pr3P)2-
RhCl(N2) (13), and MeCH(CH2CH2P-t-Bu2)2Rh(N2) (14). In
these complexes, the presence of strong σ-donor ligands
In this manuscript, we report the synthesis and X-ray
structural study of the terminal N2–Rh complex, CH[C(Me)(N-
i-Pr2C6H3)]2Rh(N2)(COE) (COE = cyclooctene), containing
a bulky diimine ligand (NacNac). In a very recent communi-
cation, Chirik and co-workers (15) reported the analogous Ir
complex.
Experimental
General data
All preparations were done under an atmosphere of dry,
O2-free N2 in a Vacuum Atmospheres inert atmosphere
glovebox. Solvents were purified employing a Grubbs’ type
solvent purification system manufactured by Innovative
Technology. Deuterated solvents were purified using the ap-
propriate techniques. All organic reagents were purified by
1
conventional methods. H and 13C NMR spectra were re-
corded on Bruker Avance-300 and -500 spectrometers. Trace
amounts of protonated solvents were used as references and
chemical shifts are reported relative to SiMe4. Combustion
analyses were done in house employing a PerkinElmer CHN
analyzer. CH[C(Me)(N-i-Pr2C6H3)]2Li(Et2O) was prepared
following the literature procedure (16). [Rh(COE)2(µ-Cl)]2
was purchased from the Strem Chemical Co.
Received 12 October 2004. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 17 March 2005.
Synthesis of CH[C(Me)(N-i-Pr2C6H3)]2Rh(N2)(COE) (1)
To a solution of CH[C(Me)(N-i-Pr2C6H3)]2Li(Et2O) (0.500 g,
1.00 mmol) in 10 mL Et2O was added [Rh(COE)2(µ-Cl)]2
(0.249 g, 0.50 mmol). The mixture was allowed to stir over-
night whereupon the solution became dark brown-black. The
mixture was filtered through Celite® and solvent was re-
moved to give a dark semisolid. This material was dissolved
J.D. Masuda and D.W. Stephan.2 Department of Chemistry
and Biochemistry, University of Windsor, Windsor, ON
N9B3P4 Canada.
1This article is part of a Special Issue dedicated to Dinitrogen
Chemistry.
2Corresponding author (e-mail: stephan@uwindsor.ca).
Can. J. Chem. 83: 324–327 (2005)
doi: 10.1139/V05-012
© 2005 NRC Canada

Masuda and Stephan
325
Scheme 1. Bridged Rh–N2 complexes.
Scheme 2. Terminal Rh–N2 complexes.
collected (4.5° < 2θ < 45–50.0°). The intensities of reflec-
tions within these frames showed no statistically significant
change over the duration of the data collections. The data
were processed using the SAINT and XPREP processing
packages. An empirical absorption correction based on re-
dundant data was applied to each data set. Subsequent solu-
tion and refinement was performed using the SHELXTL
solution package.
in pentane (5 mL) and again filtered through Celite®. Cooling
the pentane solution to –35 °C gave 263 mg of a dark brown
crystalline solid after work-up. Yield: 38%. IR (KBr
pellet, cm–1): 2175 (νNN), 1535 (νNC). 1H NMR (0 °C): 6.98–
7.13 (m, 6H, m, p-Ar, C7D7H), 5.12 (s, 1H, CH), 3.73 (br
Structure solution and refinement
Non-hydrogen atomic scattering factors were taken from
the literature tabulations (17, 18). The heavy atom positions
were determined using direct-methods employing the
SHELXTL direct-methods routine. The remaining non-
hydrogen atoms were located from successive difference
Fourier map calculations. The refinements were carried out
by using full-matrix least-squares techniques on F, minimiz-
ing the function ω (Fo – Fc)2 where the weight ω is defined
sept, 2H, CH, JH-H = 7 Hz), 3.46 (br sept, 2H, CH, JH-H
7 Hz), 3.27 (br d, 2H, HC=CH, JH-H = 10 Hz), 1.59 (6H,
Me), 1.58 (d, 6H, i-Pr, JH-H = 7 Hz), 1.47 (d, 6H, i-Pr, JH-H
=
=
7 Hz), 1.41 (br m, 4H, CH2), 1.20 (d, 6H, i-Pr, JH-H = 7 Hz),
1.14–1.29 (m, 8H, CH2), 1.02 (d, 6H, i-Pr, JH-H = 7 Hz). 13C
NMR (25 °C, partial): 157.8, 152.0, 143.1, 142.7, 141.6,
130.3, 127.8–129.0 (m, obscured by toluene-d8) 126.2,
125.4, 123.9, 123.7, 123.3, 97.3 (C-H, 74.9, (broad s, H-
C=C-H), 30.7, 29.5, 28.9, 28.1, 26.5, 25.8, 25.0, 24.7, 24.4,
23.6. Anal. calcd. for C37H55N4Rh: C 67.46, H 8.42, N 8.50;
found: C 67.46, H 8.49, N 8.30.
2
2
as 4Fo /2σ(Fo ), and Fo and Fc are the observed and calcu-
lated structure factor amplitudes. In the final cycles of each
refinement, all non-hydrogen atoms were assigned aniso-
tropic temperature factors in the absence of disorder or
insufficient data. In the latter cases, atoms were treated
isotropically. C-H atom positions were calculated and al-
lowed to ride on the carbon to which they are bonded assum-
ing a C—H bond length of 0.95 Å. H atom temperature
factors were fixed at 1.10 times the isotropic temperature
factor of the C atom to which they are bonded. The H-atom
contributions were calculated, but not refined. The locations
of the largest peaks in the final difference Fourier map cal-
culation as well as the magnitude of the residual electron
densities in each case were of no chemical significance. Ad-
ditional details are provided in the supplementary data.3
X-ray data collection and reduction
Crystals were manipulated and mounted in capillaries in a
glovebox, thus maintaining a dry, O2-free environment for
each crystal. Diffraction experiments were performed on a
Siemens SMART System CCD diffractometer. The data
were collected in a hemisphere of data in 1448 frames with
10 s exposure times. The observed extinctions were consis-
tent with the space groups in each case. The data sets were
3 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml for information on ordering electronically).
CCDC 252431 contains the crystallographic data for this manuscript. These data can be obtained, free of charge, via
ww.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2005 NRC Canada

326
Can. J. Chem. Vol. 83, 2005
Scheme 3. CH[C(Me)(N-i-Pr2C6H3)]2Rh(N2)(COE).
Fig. 1. ORTEP drawing of 1; 30% thermal ellipsoids are shown.
Hydrogen atoms are omitted for clarity. Distances (Å) and angles
(°): Rh(1)—N(1) 2.076(3), Rh(1)—N(2) 2.074(3), Rh(1)—N(3)
1.943(4), Rh(1)—C(30) 2.167(5), Rh(1)—C(37) 2.179(5), N(1)—
C(1) 1.320(6), N(2)—C(4) 1.332(6), N(3)—N(4) 1.091(6),
C(37)—C(30) 1.382(6), N(3)-Rh(1)-N(2) 179.04(16), N(3)-Rh(1)-
N(1) 88.93(15), N(2)-Rh(1)-N(1) 90.11(13), N(3)-Rh(1)-C(30)
87.94(17), N(2)-Rh(1)-C(30) 92.99(15), N(1)-Rh(1)-C(30)
161.49(16), N(3)-Rh(1)-C(37) 86.29(17), N(2)-Rh(1)-C(37)
94.59(15), N(1)-Rh(1)-C(37) 160.58(16), C(30)-Rh(1)-C(37)
37.08(17), N(4)-N(3)-Rh(1) 179.6(6).
Table 1. Crystallographic data.
1
Formula
C37H55N4Rh
Molecular weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
630.72
Monoclinic
P21/c
8.9010(6)
17.2004(12)
23.4064(16)
92.926(2)
3578.9(4)
4
1.167
0.502
17 436
5 150
β (°)
V (Å3)
Z
D(calcd.) (gcm–1)
Abs. coeff. (µ, mm–1)
Data collected
2
2
Data Fo > 3σ(Fo )
N2 fragment forms angles of 179.04(16)° and 88.93(15)°
with the N atoms of the diimine ligand consistent with the
pseudo-square-planar coordination geometry (Fig. 1).
The diimine ligand is N-bound to Rh with Rh—N dis-
tances of 2.076(3) and 2.074(3) Å. The “bite-angle” (N-Rh-
N) for the diimine ligand is 90.11(13)°. These compare with
the Rh—N distance and bite angle of 1.955(2) Å and
91.3(1)° observed in the three coordinate Rh–olefin complex
CH[C(Me)(NMe2C6H3)]2Rh(COE) (19). The imine C—N
bond distances are typical at 1.320(6) and 1.332(6) Å, re-
spectively.
The olefinic fragment of a COE ligand is π bound to Rh
with Rh—C distances of 2.167(5) and 2.179(5) Å, and a cor-
responding C—C distance of 1.382(6) Å. In comparison, the
coordinated olefinic C—C bond distance of CH[C(Me)(N-
Me2C6H3)]2Rh(COE) (19) gives rise to Rh—C distances
of 2.072(5) and 2.066(6) Å, with a C—C distance of
1.398(7) Å. The longer Rh—C distances in 1 are consistent
with the lesser Lewis acidity of the Rh center as a result of
its four-coordinate nature.
The terminally bound dinitrogen molecule exhibits a Rh—N
distance of 1.943(4) Å. This is shorter than the Rh—N bond
distance in (Ph-t-Bu2P)2RhH(N2) (1.970(4) Å) (12) and yet
much longer than that in (i-Pr3P)2RhCl(N2) (1.885(4) Å)
(13) (Table 2). The Rh-N-N angle is essentially linear with
an angle of 179.6(6)° and the N—N distance is 1.091(6) Å.
In general, the N—N distance in 1 is shorter than those ob-
served for the bridging Rh2(µ-N2) complexes, consistent
with interaction with only one metal center. The N—N dis-
tance in 1 is also longer than the N—N distances seen in (i-
Pr3P)2RhCl(N2), MeCH(CH2CH2P-t-Bu2)2Rh(N2), and the Ir
Variables
R
Rw
384
0.0450
0.1138
1.035
GOF
Results and discussion
Reaction of the ligand salt CH[C(Me)(N-i-Pr2C6H3)]2-
Li(Et2O) with [Rh(COE)(µ-Cl)]2 proceeds with stirring over-
night to give a dark brown-black solution. Work-up of this
reaction afforded the dark brown crystalline product 1 in
38% yield. The NMR data were consistent with a disym-
metric environment about Rh as two sets of resonances at-
tributable to the isopropyl groups on the aryl rings of the
ligand. NMR data were also consistent with the presence of
a COE ring bound to Rh. The nature of the fourth ligand on
Rh was unclear although IR spectrometry revealed an ab-
sorption at 2175 cm–1. This compares with the absorption at
2126 cm–1 observed for the Ir analog of 1 suggesting the
possibility of coordinated N2. The higher N–N stretching
frequency further suggests a shorter N—N bond distance in
1. All of these data are consistent with the formulation of 1
as CH[C(Me)(N-i-Pr2C6H3)]2Rh(N2)(COE) (Scheme 3).
X-ray crystallographic study of 1 confirmed the formula-
tion and revealed the pseudo-square-planar coordination
geometry about Rh (Table 1). The coordination sphere is
comprised of the two nitrogen atoms of the diimine ligand, a
side-bound olefinic fragment of COE, and a terminally
bound dinitrogen molecule. The metal-bound nitrogen of the
© 2005 NRC Canada

Masuda and Stephan
Table 2. Metric parameters for selected Rh/Ir–N2 complexes.
327
Compound
M–N
N–N
ν (cm–1)
Ref.
7
[Me2C6H(CH2P-i-Pr2)2Rh]2(µ-N2)
[Me2C6H(CH2P-t-Bu2)(CH2NEt2)Rh]2(µ-N2)
[MeC6H2(CH2P-t-Bu2)2ORh]2(µ-N2)
[(i-Pr3)2RhH]2(µ-N2)
1.9687(17)
2.021
1.108(3)
1.130
2165
n/a
8
1.912(2)
1.981(5)
1.129(4)
1.134(5)
2095
2140
9
10
1.973(5)
1.973(3)
[CH2(P-t-Bu2)2RhCH2-t-Bu]2(µ-N2)
(Ph-t-Bu2P)2RhH(N2)
1.106(6)
1.074(7)
0.958(5)
0.963(14
1.107(4)
1.091(6)
n/a
n/a
11
1.970(4)
1.885(4)
2.032(1)
1.903(3)
1.943(4)
n/a
2155
2100
2110
2126
2175
2172
12
(i-Pr3P)2RhCl(N2)
13
MeCH(CH2CH2P-t-Bu2)2Rh(N2)
CH[C(Me)(N-i-Pr2C6H3)]2Ir(N2)(COE)
CH[C(Me)(N-i-Pr2C6H3)]2Rh(N2)(COE)
CH[C(Me)(NMe2C6H3)]2Rh(N2)(COE)
14
15
Herein
19
Note: n/a = not available.
6. M. Hidai and Y. Mizobe. Chem. Rev. 95, 1115 (1995).
analog of 1. While the Rh—N distance in the terminal-N2
complexes correlates the expected trend based on the trans
influence of the ligand opposite dinitrogen, there is no direct
correlation between the Rh—N and the N—N distances in
the compiled list of Rh–N2 complexes (Table 2). The N—N
distance appears to reflect a complex mixture of the trans in-
fluence and the net electron density at the metal center. It is
these features together that impact on the π donation to N2,
and thus the resulting N—N bond length.
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Acknowledgments
Financial support from the Natural Sciences and Engi-
neering Research Council of Canada (NSERC) is gratefully
acknowledged. JDM is grateful for the award of an Ontario
Graduate Scholarship and a Bayer Inc. Award from the
Macromolecular Science and Engineering Division of the
Canadian Institute for Chemistry.
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© 2005 NRC Canada