Preparation, characterization, and reactivity of dinitrogen molybdenum complexes with bis(diphenylphosphino)amine derivative ligands that form a unique 4-membered P-N-P chelate ring

Article abstract of DOI:10.1021/ic301577a

Five dinitrogen-molybdenum complexes bearing bis(diphenylphosphino)amine derivative ligands (L^R) that form a unique 4-membered P-N-P chelate ring, trans-[Mo(N₂)₂(L^R)₂] (2 ^R: R = Ph, Xy, p-MeOPh, 3,5-iPr₂Ph, iPr), were prepared for the purpose of binding a dinitrogen molecule. The corresponding two dichloride-molybdenum complexes, trans-[MoCl₂(L^R) ₂] (1^R: R = Ph, Xy), were also prepared as comparisons. FT-IR spectra of 2^R were measured and compared the ν(Ni? - N) values. Moreover, X-ray crystal structure determination of 1^R (R = Ph, Xy) and 2^R (R = Xy, 3,5-iPr₂Ph) is performed. These experimental results indicated that the coordinated dinitrogen molecule gets easily influenced by the N-substitutent of diphosphinoamine ligand. In addition, to investigate the effect of the properties of the diphosphinoamine ligand for the dinitrogen molybdenum complexes, we performed DFT calculations that focused on the difference of N-substituent, the dihedral angle between P-N-P plane and N-substituent aryl group, and the P-N-P bite angle. This calculation revealed that the competition between the back-donation from metal to dinitrogen and that from metal to ligand was affected by P-N-P bite angle and the dihedral angle of N-substituent of ligand. In order to examine the reactivity with respect to conversion of dinitrogen to ammonia, protonation and trimethylsilylation reactions of the coordinated dinitrogens were carried out for 2^R.

Full text of DOI:10.1021/ic301577a

Article
pubs.acs.org/IC
Preparation, Characterization, and Reactivity of Dinitrogen
Molybdenum Complexes with Bis(diphenylphosphino)amine
Derivative Ligands that Form a Unique 4‑Membered P−N−P Chelate
Ring
Takahiko Ogawa,† Yuji Kajita,† Yuko Wasada-Tsutsui,† Hiroaki Wasada,‡ and Hideki Masuda*,†
†Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology Gokiso, Showa, Nagoya
466-8555, Japan
‡Faculty of Regional Studies, Gifu University, Gifu 501-1193, Japan
S
* Supporting Information
ABSTRACT: Five dinitrogen−molybdenum complexes bearing bis-
(diphenylphosphino)amine derivative ligands (LR) that form a unique 4-
membered P−N−P chelate ring, trans-[Mo(N2)2(LR)2] (2R: R = Ph, Xy,
p-MeOPh, 3,5-iPr2Ph, iPr), were prepared for the purpose of binding a
dinitrogen molecule. The corresponding two dichloride-molybdenum
complexes, trans-[MoCl2(LR)2] (1R: R = Ph, Xy), were also prepared as
comparisons. FT-IR spectra of 2R were measured and compared the
ν(NN) values. Moreover, X-ray crystal structure determination of 1R
(R = Ph, Xy) and 2R (R = Xy, 3,5-iPr2Ph) is performed. These
experimental results indicated that the coordinated dinitrogen molecule
gets easily influenced by the N-substitutent of diphosphinoamine ligand.
In addition, to investigate the effect of the properties of the diphosphinoamine ligand for the dinitrogen molybdenum complexes,
we performed DFT calculations that focused on the difference of N-substituent, the dihedral angle between P−N−P plane and
N-substituent aryl group, and the P−N−P bite angle. This calculation revealed that the competition between the back-donation
from metal to dinitrogen and that from metal to ligand was affected by P−N−P bite angle and the dihedral angle of N-substituent
of ligand. In order to examine the reactivity with respect to conversion of dinitrogen to ammonia, protonation and
trimethylsilylation reactions of the coordinated dinitrogens were carried out for 2R.
tungsten-dinitrogen complexes, [M(N2)2(PR3)4] (M = Mo, W,
R = alkyl, aryl), and further investigated various transition metal
phosphorus dinitrogen complexes.6 In 1997, Fryzuk and co-
workers studied the reaction of a side-on bound dinitrogen
zirconium complex, {[PhP(CH2SiMe2NSiMe2CH2)2PPh]-
Zr}2(μ-η2-N2), with dihydrogen to obtain the complex
{[PhP(CH2SiMe2NSiMe2CH2)2PPh]Zr}2(μ-η2-N2H)(μ-H).7
For metal complexes without phosphine ligands, many
dinitrogen complexes with various metals and various ligands
have been synthesized and studied.8 In 2003, Schrock and
Yandulov succeeded in direct catalytic conversion of dinitrogen
to ammonia using a molybdenum−dinitrogen complex bearing
triamidoamine as the supporting ligand.9 In 2004, Chirik and
co-workers reported direct generation of ammonia from
dinitrogen and dihydrogen using a dinuclear zirconium
complex with a cyclopentadienyl derivative.10 Despite the
many studies of metal complexes with amido or cyclo-
pentadienyl ligands, there are few examples of catalytic
reduction of dinitrogen using molybdenum(0)−phosphine
complexes.11 Recently, Nishibayashi and co-workers reported
INTRODUCTION
■
It is well-known that nitrogen (N) is an essential atom in
biology. This atom also has important applications in
production of fertilizers, and energy sources, as well as
commercial chemical products and pharmaceuticals. Today,
we depend on the Haber−Bosch process as a source of
nitrogen. The Haber−Bosch process can reduce dinitrogen
(N2) to ammonia under high temperature and high pressure in
the presence of Fe/Ru catalysts. However, this process requires
a large amount of energy and dihydrogen generated from fossil
fuels.1 Development of dinitrogen fixation systems which
function effectively under ambient reaction conditions is one
of the most important research areas in chemical research and
development.2 Since the first example of a transition metal−
dinitrogen complex, [Ru(N2)(NH3)5]2+, was reported by Allen
and Senoff in 1965,3 various transition metal−dinitrogen
complexes have been synthesized to study its reaction
mechanism in efforts to develop dinitrogen fixation systems.4
In 1969, Hidai and co-workers reported on a group 6 metal
dinitrogen complex, trans-[Mo(N2)2(dppe)2] (dppe = 1,1′-
bis(diphenylphosphino)ethane).5 In 1975, Chatt and co-
workers succeeded in obtaining stoichiometric reduction of
dinitrogen to ammonia by protonation of molybdenum- and
Received: July 19, 2012
Published: December 11, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Article
infrared (FT-IR) spectra of solid compounds were taken as KBr pellets
using a JASCO FT/IR-410 spectrophotometer. Elemental analyses
were obtained with a Perkin-Elmer CHN-900 elemental analyzer. Gas
chromatography/mass spectrometry (GC/MS) measurements were
carried out on a SHIMADZU GCMS-QP2010 system.
high-performance catalytic reduction of dinitrogen to ammonia
using a molybdenum(0)−phosphine complex bearing PNP-
type pincer ligands.12 Extensive studies have been carried out
over the past 40 years. However, there have been few examples
of complexes for which the reduction mechanism has been
rationally demonstrated by isolating the fully characterized
intermediate stages. Therefore, molybdenum− and tungsten−
phosphine dinitrogen complexes remain the subject of
aggressive research with the objective of clarifying the reaction
mechanism and developing high-performance dinitrogen
reduction catalysts.13
X-ray Crystallography. Single crystals of 1Ph, 1Xy, 2Xy, and 23,5iPrPh
suitable for X-ray diffraction analyses were obtained from THF
solutions after standing for a few days under an N2 atmosphere at 238
K. A crystal was mounted on a glass fiber, and diffraction data were
collected on a Rigaku/MSC Mercury CCD using graphite
monochromated Mo Kα radiation at 173 K. The structures were
solved by a combination of direct methods (SIR 92 or SIR2004) and
Fourier techniques. All non-hydrogen atoms except for some of the
solvated molecules were refined anisotropically. Hydrogen atoms were
refined using the riding model. The Sheldrick weighting scheme was
employed. Plots of ∑w(|Fo| − |Fc|)2 versus |Fo|, reflection order in data
collection, sin θ/λ, and various classes of indices showed no unusual
trends. Neutral atomic scattering factors were obtained from
For the Chatt type Mo/W−N2 complexes, Tuczuk and co-
workers investigated these complexes using spectroscopic and
theoretical methods and evaluated the mechanism of dinitrogen
reduction.14 On the basis of these studies, we noted the bite
angle of phosphine ligands. Because bite angle is a very
important factor to determine the properties of metal
complexes, correlation between bite angle of diphosphine
ligands and reactivity of complexes was investigated a lot.15
Although there are many examples of trans-[M(N2)2(P4)] (P4 =
mono-, bi-, or tridentate phosphine ligand), these complexes
have five- or six-membered chelate ring and similar bite angle
(P−M−P angle is around 90°). This means that these
complexes have relatively flexible phosphorus atoms which
can locate a vertex of the regular octahedron. However,
examples of Mo/W−N2 complexes which have rigid
phosphorus atoms and small bite angles (structure completely
different from the regular octahedron) are very limited.16 We
decided to use a ligand system that has a small bite angle (P−
M−P angle) and 4-membered chelate ring for N2 chemistry.
We therefore focused on molybdenum diphosphinoamine
dinitrogen complexes. It was previously reported that Cr(III)
complexes with diphosphinoamine ligands have the potential to
catalyze more highly selective ethylene polymerizations relative
to similar complexes with diphenylphosphinoethane and N,N′-
diphenylphosphinohydrazine ligands.17 The phosphinoamine
ligands have been used widely because they can be prepared
easily, can exchange N-substituent easily, and can provide small
bite angle structure.18,19 Therefore, systematic study can be
performed.20 The molybdenum atom is occasionally employed
for dinitrogen fixation chemistry and plays a key role in the
active site of nitrogenase enzymes.21
International Tables for X-ray Crystallography Vol. IV.22 Anomalous
23
dispersion terms were included in Fcalc
,
and the values for Δf′ and
Δf″ were obtained from the International Tables for X-ray
Crystallography Vol. C.24 The values of the mass attenuation
coefficients are those of the International Tables for X-ray
Crystallography Vol. C.25 All calculations were performed using the
crystallographic software package, CrystalStructure.26 A summary of
crystallographic data is provided in the Supporting Information (Table
S1).
Materials. The molybdenum trichloride tris(tetrahydrofuran)
complex, [MoCl3(THF)3], was synthesized according to the literature
procedure.27 3,5-Diisopropylaniline was synthesized from 2,6-
diisopropylaniline according to the literature procedures.28 Other
reagents were employed in the commercially available forms. All
anhydrous solvents were purchased from Sigma-Aldrich. For
degassing, these solvents were bubbled with N2.
Syntheses of Diphenylphosphinoamine Ligands (LR: R = Ph, Xy,
p-MeOPh, iPr, p-CNPh, H). The diphenylphosphinoamine ligands
except L3,5iPrPh were prepared by a modification of literature
procedures.19b,29−31 A flask equipped with an additional funnel was
charged with a THF solution of 1 equiv of primary amine and 2 equiv
of triethylamine. The solution was cooled to 0 °C. The additional
funnel was charged with a THF solution of 2 equiv of
chlorodiphenylphosphine, and the resulting solution was slowly
added to the amine solution. The mixture was stirred at room
temperature overnight. The triethylammonium salt was then separated
by filtration, and the solvent was removed under vacuum. The residue
was dissolved in a small amount of dichloromethane and precipitated
with n-hexane, to produce a white powder. In the case of LH,
1,1,1,3,3,3-hexamethyldisilazane was used instead of primary amine.30
Herein, we report the synthesis and characterization of
molybdenum(0)−dinitrogen complexes supported by bis-
(diphenylphosphino)amine ligands that form a unique 4-
membered chelate ring, and discuss the dinitrogen activation
factor by comparing the influences on the coordinated
dinitrogen by N-substituents of diphosphinoamine ligands on
the basis of structural and IR spectroscopic studies as well as
from the viewpoint of theoretical calculations and reactions
involving protonation and silylation.
1
All compounds were characterized by H NMR, 31P{1H} NMR, and
FT-IR spectroscopies.
Synthesis of N,N-Bis(diphenylphosphino)-3,5-diisopropylaniline
(L3,5iPrPh). The novel ligand L3,5iPrPh was prepared from 3,5-
diisopropylaniline instead of primary amine,28 according to the same
procedure described above (white powder, 1.52 g, yield 82.2%)
1H NMR (600 MHz, δ/ppm from TMS in CDCl3): δ 0.87 [d, 12H,
CH(CH3)2], 2.51 [sep, 2H, CH(CH3)2], 6.17 [s, 2H, o-H-PhN], 6.64
[s, 1H, p-H-PhN], 7.25−7.37 [m, 20H, H-Ph2P]. 31P{1H} NMR (243
MHz, δ/ppm from H3PO4 in CDCl3): δ 69.5. FT-IR (KBr, cm−1): ν
3050 (aromatic C−H), 2956 (aliphatic C−H), 1309 (aromatic C−N),
881 (P−N), δ 1435, 1430 (aliphatic C−H), 748, 697 (aromatic C−H).
Anal. Calcd for C36H37NP2: C, 79.24; H, 6.83; N, 2.57. Found: C,
79.39; H, 6.94; N, 2.45.
EXPERIMENTAL SECTION
General Methods. All manipulations were carried out under an
atmosphere of purified argon or dinitrogen gas in a mBRAUN MB
■
1
150B-G glovebox or by standard Schlenk techniques. H NMR (300
MHz Varian, 600 MHz Bruker) and 31P{1H} NMR (121 MHz Varian,
243 MHz Bruker) spectra were measured on a Varian Mercury 300
Synthesis of trans-[MoCl2(LR)2] (1R: R = Ph (1Ph), Xy (1Xy)). A
mixture of 2 equiv of LR (R = Ph, Xy) and 1 equiv of [MoCl3(THF)3]
in toluene was added to 1 equiv of potassium graphite. The mixture
was stirred under an argon atmosphere at room temperature for 24 h.
After concentration in vacuo, the residue was extracted with THF.
Recrystallization at −35 °C gave red-orange crystals of 1R (R = Ph
(1Ph: yield 14.5%), Xy (1Xy: 39.2%)).
l
spectrometer or a Bruker Avance 600 spectrometer, and H chemical
shifts were estimated relative to TMS as an internal standard. 31P
chemical shifts are quoted relative to an external standard of 85%
H3PO4. 15N{1H} NMR (61 MHz) spectra were measured on a Bruker
Avance 600 spectrometer, and 15N chemical shifts are quoted relative
to an external standard of nitromethane (CH3NO2). Fourier transform
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Inorganic Chemistry
Article
Data for 1Ph follow. FT-IR (KBr, cm−1): ν 1313 (aromatic C−N),
862 (P−N). Anal. Calcd for C60H50Cl2MoN2P4: C, 66.13; H, 4.62; N,
2.57. Found: C, 65.85; H, 4.59; N, 2.58. Data for 1Xy follow. FT-IR
(KBr, cm−1): ν 1313 (aromatic C−N), 862 (P−N). Anal. Calcd for
C62H54Cl2MoN2P4·1.5THF: C, 67.04; H, 5.63; N, 2.23. Found: C,
66.42; H, 5.73; N, 2.23.
structure of the bisdinitrogen complex. We employed ρ(r, complex
without dinitrogen) of S = 1 state which has lower energy than that of
the S = 0 state.
Δρ condensed to atoms and bonds, Δρatom (eq 2) and ΔρBO (eq 3),
respectively, were computed by carrying out natural population
analysis.
Synthesis of trans-[Mo(N2)2(LR)2] (2R: R = Ph (2Ph), Xy (2Xy), p-
MeOPh (2MeOPh), iPr (2iPr), 3,5-iPr2Ph (23,5iPrPh)) and trans-[Mo-
(15N2)2(LR)2] (3R: R =Ph (3Ph), Xy (3Xy), 3,5-iPr2Ph (33,5iPrPh)). An excess
amount of magnesium turnings was added to a mixture of 2 equiv of
LR and 1 equiv of [MoCl3(THF)3] in toluene. The mixture was stirred
under dinitrogen (2R) or a 15N-enriched dinitrogen (3R) atmosphere
at room temperature for 24 h. Removal of unreacted magnesium and
filtration afforded a residue. Washing with methanol and n-hexane gave
a red-orange powder of 2R and 3R (yields: 2Ph, 34.5%; 2MeOPh, 23.9%;
Δρatom(A) = +ρatom(A, bisdinitrogen complex)
− {ρatom(A, complex without dinitrogen)
+ ρatom(A, bisdinitrogen)}
(2)
ΔρBO(A−B) = −ρBO( A− B, bisdinitrogen complex)
+ {ρBO( A− B, complex without dinitrogen)
2
iPr, 13.6%; 3Ph, 34.5%; 3Xy, 25.1%; 33,5iPrPh, 47.6%). The solubility of
2R is extremely low except for 2Xy and 23,5iPrPh. Therefore, it is difficult
+ ρBO( A− B, bisdinitrogen)}
(3)
to dissolve these compounds in a number of solvents. The complexes
2
Xy and 23,5iPrPh were dissolved in THF and were recrystallized at −35
Here −ρatom(A, X) and ρBO(AB, X) represent the natural atomic
charge of atom A and the bond order between atoms A and B,
respectively, for species X. An increase of Δρatom(A) or ΔρBO(AB)
indicates a concentration of electron density in atom A or bond AB,
respectively.
We evaluated the donor−acceptor interaction energy between a
lone-pair orbital and an antibonding orbital in the bisdinitrogen
complexes, i.e., electron donations from a 4d orbital of Mo to a π*
orbital of N2 and a σ* orbital of the P−C bond and those from a lone-
pair orbital of amine to a P−C σ* orbital, by a second-order
perturbation theory analysis of the Fock matrix in the natural bond
orbital (NBO) basis.34b,c Electron donation to a π orbital of an N-
phenyl group was estimated as electron donation from a lone pair of
an amine to a double bond containing the ipso-carbon in the Kekule
structure. In this theory, energy lowering of donor−acceptor
interaction is given by eq 4
°C to give red-orange crystals (yields: 2Xy, 12.0%; 23,5iPrPh, 49.7%).
FT-IR (KBr, cm−1): ν 1990 (2Ph), 2011 (2Xy), 1995 (2MeOPh), 1975
(2iPr), 2014 (23,5iPrPh) 1923 (3Ph), 1934 (3Xy), 1947 (33,5iPrPh
)
(asymmetric N2). Anal. Calcd for C64H58MoN6P4·THF: C, 67.88; H,
5.53; N, 6.99. Found: C, 66.88; H, 5.53; N, 6.82. (2Xy). Anal. Calcd for
C72H74MoN6P4·2THF: C, 69.25; H, 6.54; N, 6.06. Found: C, 69.45;
H, 5.23; N, 6.20. (23,5iPrPh). 1H NMR (300 MHz, δ/ppm from TMS in
THF-d8): δ 1.86 (s, 12H, CH3), 6.43 (s, 4H, o-H-PhN), 6.51 (s, 2H, p-
H-PhN), 7.10−7.27 (m, 40H, H-Ph2P) (2Xy), 0.84 (d, 24H,
CH(CH3)2), 2.51 (sep, 4H, CH(CH3)2), 6.57 (s, 2H, p-H-PhN),
6.62 (s, 4H, o-H-PhN), 7.15−7.36 (m, 40H, H-Ph2P). 15N{1H} NMR
(60.8 MHz, δ/ppm from CH3NO2 in THF-d8): δ −46.34 (15Nα),
−45.99 (15Nβ) (3Xy), −46.47 (15Nα), −46.03 (15Nβ) (33,5iPrPh).
Reaction with HOTf. The dinitrogen complex 23,5iPrPh (70.0 mg,
0.0563 mmol) was treated with HOTf (16.9 mg, 0.113 mmol) in THF
(10 mL). After 1 h, the solution was concentrated and measured by
FT-IR spectroscopy.
Reaction with H2SO4. To a THF (5 mL) suspension of a complex
was added H2SO4. After 24 h of stirring at room temperature,
ammonia was base distilled and quantified by the indophenol method.
Silylation of 2R. The dinitrogen complex 2R (10.0 mg) was treated
with trimethylsilyl chloride (856 mg, 7.88 mmol) and sodium metal
(50.0 mg, 2.17 mmol) in THF (5 mL). After 24 h, the solution was
quantified by GC/MS.
́
(0)
2
⟨ϕd(o0n)or F ϕ
⟩
̂
acceptor
(0)
ΔE(2) = −n
donor
(0)
(0)
ε
− ε
acceptor
donor
(4)
where ϕ(0)
and ϕ(0)
represent NBOs of the donor and
acceptor
donor
(0)
̂
acceptor, F represents the Fock operator, and ε
and ε(0)
acceptor
donor
represent NBO orbital energies of the donor and acceptor.
All electronic structure calculations were performed with the
Gaussian 09 package35 on the Fujitsu HX600 system at the Nagoya
University Information Technology Center.
COMPUTATIONAL DETAILS
■
Geometry optimizations of dinitrogen complexes, 2Ph, 2Xy, and
2
RESULTS AND DISCUSSION
Synthesis and Characterization of Some Diphosphi-
3,5iPrPh, and dinitrogen methyl-substituted complex 2(PMe2)Ph
■
where all the phenyl groups in diphenylphosphino groups of 2Ph
were substituted by methyl groups were performed at the B3LYP
level.32 We used Stuttgart/Dresden ECP and basis set on Mo33a
together with 6-31+G(d) on P and N,33b,c 6-31G(d)33d on N-
substituted aryl carbon and ipso-carbons attached to phosphorus, and
6-31G on the remaining atoms.33e The results of frequency
calculations at the same level confirmed that each of the optimized
structures are at a minimum on the potential energy surface.
We calculated natural atomic charges and atom−atom overlap-
weighted natural atomic orbital (NAO) bond orders34a,b of the
dinitrogen complexes and the methyl-substituted complex. We also
calculated differential density Δρ(r) (eq 1) in order to investigate the
change in electron density accompanied by coordination of dinitrogen
molecules to the molybdenum complex.
noamine Ligands. Bis(diphenylphosphino)amine ligands
i
(LR: R = Ph, Xy, p-MeOPh, 3,5-iPr2Ph, p-CNPh, Pr, and H)
were synthesized easily from chlorodiphenylphosphine and
corresponding amines29,31 (anilines and primary amines)
except for LH which was synthesized from 1,1,1,3,3,3-
hexamethyldisilazane.30 Table 1 provides 31P{1H} NMR
spectral data of diphosphinoamine derivative ligands. 31P{1H}
NMR chemical shift values of LPh, LXy, LMeOPh, LCNPh, and
L3,5iPrPh were detected in a lower magnetic field region
compared with that of LH, and that of LiPr was higher than
LPh.31 These results indicate that the electron withdrawing aryl
groups cause a decrease in the electron density of the
phosphorus atoms through the diphosphinoamine nitrogen
atom, and the electron donating alkyl groups cause an increase
in the electron density of the phosphorus atoms. On the other
hand, δ values of LPh, LXy, LMeOPh, LCNPh, and L3,5iPrPh are very
similar to each other. Therefore, we can conclude that no
correlation exists among these groups. This indicates that the
aryl substituent groups do not exert an electronic effect on the
phosphorus atoms. Similar behavior has been identified in the
Δρ(r) = ρ(r, dinitrogen complex)
− {ρ(r, complex without dinitrogen)
+ ρ(r, dinitrogen)}
(1)
Here ρ(r, X) represents electron density of species X at a point r. The
values of ρ(r, complex without dinitrogen) and ρ(r, bisdinitrogen)
were calculated using atomic coordinates fixed to the optimized
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Inorganic Chemistry
Article
found to shift to a lower energy region relative to those of LR
(873 and 872 cm−1 for R = Ph and Xy, respectively). These
small shift values indicate that LR ligands bind to the metal. The
molecular structures of 1Ph and 1Xy were clarified by X-ray
crystallography. ORTEP views of 1Ph and 1Xy are shown in
Figures 1 and 2. Selected interatomic distances and angles for
1Ph and 1Xy are listed in Table 2. For both 1Ph and 1Xy, the
molybdenum centers are octahedrally coordinated, and two
chloride anions are coordinated in with trans geometry. Mo−
Cl(1) and Mo−Cl(2) bond lengths are 2.3922(8) and
2.4018(8) Å for 1Ph and 2.4285(5) and 2.4019(5) Å for 1Xy,
respectively. The Mo−P bond lengths of 1Ph are 2.4832(8) Å
for Mo−P(1), 2.5035(7) Å for Mo−P(2), 2.4776(8) Å for
Mo−P(3), and 2.4806(7) Å for Mo−P(4), respectively. The
Mo−P bond distances of 1Xy are 2.4888(6) Å for Mo−P(1),
2.4724(6) for Mo−P(2) Å, 2.4660(6) Å for Mo−P(3), and
2.4849 Å for Mo−P(4), respectively. The interatomic distances
of Mo···N bonds are 3.138(2) and 3.152(2) Å for 1Ph and
3.1485(18) and 3.1476(18) Å for 1Xy, and the P−N−P angles
are 104.70(11)° and 103.10(11)° for 1Ph and 103.22(10)° and
103.12(10)° for 1Xy, respectively. These findings indicate that
the phosphinoamine nitrogen atoms do not bind to the metal
ions. In addition, the average P(1)···P(4) and P(2)···P(3)
distances are 4.1733(13) Å and 4.1487(12) Å for 1Ph, and
4.1835(8) Å and 4.1079(8) Å for 1Xy.
Table 1. 31P{1H} NMR Data of LR in CDCl3 (δ/ppm from
85% H3PO4)
R
δ/ppm
ref
p-MeOPh
3,5-iPr2Ph
Ph
70.8
69.5
69.1
68.5
67.2
48.8
43.6
a
b
c
p-CNPh
Xy
d
e
iPr
f
H
g
a
b
c
d
e
Reference 29a. This work. Reference 31a. Reference 31b. This
f
g
work. Reference 20. Reference 30.
31P{1H} NMR spectral data of trans-[RuCl2(LR)] (R = H, Me,
Et, nPr, iPr, nBu, and Ph) reported previously.29a
Synthesis and Characterization of trans-[MoCl2(LR)2]
Complexes. Treatment of [MoCl3(THF)3] with 2 equiv of
diphosphinoamine derivative ligand (LPh, LXy) and 1 equiv of
KC8 in THF under an argon atmosphere at room temperature
for 12 h gave trans-[MoCl2(LR)2] (R = Ph: 1Ph, Xy: 1Xy)
(Scheme 1). The P−N stretching vibration frequencies ν(P−
The P−N bond lengths (∼1.73 Å) obtained for 1Ph and 1Xy
lie in a usual P−N single bond (1.7−1.8 Å).18c,d,36 Although the
nitrogen atom of the diphosphinoamine ligand is a tertiary
amine, the nitrogen atoms are located within the plane defined
by two phosphorus and ipso carbon atoms. The nitrogen atom
is sp2-hybridized. Such hybridization is occasionally found in
compounds with a P−N bond.37 Additionally, the aromatic ring
attached to the nitrogen atom is largely twisted against the P−
N−P plane (ca. 60−90°) as shown also in X-ray crystal
structures. The twisting motion that might be attributed to
steric interactions with the nitrogen atom has been identified in
Scheme 1
N) of 1Ph and 1Xy, as measured by FT-IR spectroscopy, were
observed at 862 and 869 cm−1, respectively. These values were
Figure 1. ORTEP view of trans-[MoCl2(LPh)2] (1Ph) with 50% thermal ellipsoids. Hydrogen atoms and THF molecules are omitted for clarity.
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Figure 2. ORTEP view of trans-[MoCl2(LXy)2] (1Xy) with 50% thermal ellipsoids. Hydrogen atoms and THF molecules are omitted for clarity.
Table 2. Interatomic Distances (Å) and Angles (deg) of 1Ph
and 1Xy
Scheme 2
complex
1Ph
Interatomic Distances (Å)
1Xy
Mo(1)−Cl(1)
Mo(1)−Cl(2)
Mo(1)−P(1)
Mo(1)−P(2)
Mo(1)−P(3)
Mo(1)−P(4)
Mo(1)···N(1)
Mo(1)···N(2)
P(1)···P(2)
2.3922(8)
2.4018(8)
2.4832(8)
2.5035(7)
2.4776(8)
2.4806(7)
3.138(2)
3.152(2)
2.7289(9)
2.7142(9)
1.722(2)
1.724(2)
1.735(2)
1.731(2)
2.4285(5)
2.4019(5)
2.4888(6)
2.4724(6)
2.4660(6)
2.4849(6)
3.1485(18)
3.1476(18)
2.7121(8)
2.7180(8)
1.7294(17)
1.7308(19)
1.7365(18)
1.7335(19)
enriched dinitrogen molybdenum complexes, trans-[Mo-
(15N2)2(LR)2] (3Ph, 3Xy, and 33,5iPrPh), were prepared by a
procedure similar to the procedure used to prepare 2R. In the
case of molybdenum complexes 2R with R = p-CNPh and H,
the corresponding dinitrogen molybdenum complexes were not
obtained by synthetic methods similar to the method used in
synthesis of 2R. The solubility of dinitrogen molybdenum
complexes 2R obtained here is very low, and it is therefore
difficult to characterize these complexes in the solution state.
Fortunately, single crystals of 2Xy and 23,5iPrPh were obtained
from THF solution at 238 K. The crystal structures of 2Xy and
23,5iPrPh were explicitly clarified by X-ray crystallography.
ORTEP views of 2Xy and 23,5iPrPh are shown in Figures 3 and
4. The selected bond lengths and angles are listed in Table 3 for
2Xy and 23,5iPrPh. In addition, a comparison of crystal parameters
of several MoN2 complexes is listed in Table 4. The crystal of
2Xy contains four molybdenum complexes and four disordered
THF molecules in the unit cell. The crystal of 23,5iPrPh contains
two slightly different independent molecules of 23,5iPrPh and
four highly disordered THF molecules in the unit cell. In both
cases, the molybdenum center is octahedrally six-coordinated
with two diphosphinoamine ligands in the equatorial plane and
two dinitrogen molecules in trans mode in the axial positions.
The structures are similar to those of the dichloride
molybdenum complexes 1Ph and 1Xy. The aromatic rings of
3,5-dimethylbenzene and 3,5-diisopropylbenzene moieties
P(3)···P(4)
P(1)−N(1)
P(2)−N(1)
P(3)−N(2)
P(4)−N(2)
Bond Angles (deg)
P(1)−N(1)−P(2)
P(3)−N(2)−P(4)
P(1)−Mo(1)−P(2)
P(3)−Mo(1)−P(4)
104.70(11)
103.10(11)
66.35(2)
103.22(10)
103.12(10)
66.28(2)
66.38(2)
66.59(2)
Dihedral Angles (deg)
plane[P(1)−N(1)−P(2)]−plane[aryl1]
plane[P(3)−N(2)−P(4)]−plane[aryl2]
67.90
89.15
64.88
77.27
diphosphinoamine ligands without a metal as well as in
intramolecular charge-transfer compounds such as p-dimethy-
laminobenzonitrile.31b,38,39
Synthesis and Characterization of Dinitrogen Molyb-
denum Complexes, trans-[Mo(N2)2(LR)2] (R = Ph, Xy, iPr,
p-MeO-Ph, 3,5-iPr2Ph). Treatment of [MoCl3(THF)3] with 2
equiv of diphosphinoamine ligand (LR) in the presence of an
excess amount of magnesium in toluene under a dinitrogen
atmosphere at room temperature for 24 h produces trans-
[Mo(N2)2(LR)2] (R = Ph, 2Ph; Xy, 2Xy; iPr, 2iPr; p-MeOPh,
2MeOPh; 3,5-iPr2Ph, 23,5iPrPh) (Scheme 2). Additionally, the 15N-
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Figure 3. ORTEP view of trans-[Mo(N2)2(LXy)2] (2Xy) with 50% thermal ellipsoids. Hydrogen atoms and THF molecules are omitted for clarity.
Figure 4. ORTEP view of trans-[Mo(N2)2(L3,5iPrPh)2] (23,5iPrPh) with 30% thermal ellipsoids. Hydrogen atoms and THF molecules are omitted for
clarity.
attached at the diphosphinoamine nitrogen are only slightly
twisted (ca. 3−6°) against the P−N−P plane, unlike the
twisting observed for 1Ph and 1Xy (ca. 60−90°).
N(3)−(4) (1.058(6) Å) bond lengths are shorter than the
bond length of the free dinitrogen molecule. Such short NN
bond lengths of coordinated dinitrogen have been reported
previously.41 These NN bond lengths suggest that the
coordinated dinitrogen molecule has been hardly activated
compared with previous cases (1.118(8) Å for trans-[Mo-
(N2)2(dppe)2],40b 1.133(7) and 1.154(7) Å for trans-[Mo-
(N2)2(PMePh2)4],40c and 1.122(2) and 1.124(2) Å for trans-
[Mo(N2)2(depf)2]40d). The P−N−P angle for 2Xy is 99.32(9)°
for P(1)−N(3)−P(2), and for 23,5iPrPh the angles are
99.16(18)° for P(1)−N(5)−P(2) and 98.58(17)° for P(3)−
N(6)−P(4), respectively. These values are 4−5° less than those
of 1Ph and 1Xy. These smaller angles are attributed to the
twisting of the aryl groups which occurs because the dihedral
angles in dichloride molybdenum complexes with a larger P−
N−P angle, 1Ph (104.70(11) and 103.10(11)°) and 1Xy
(103.22(10) and 103.12(10)°), are larger than those of
dinitrogen molybdenum complexes with a smaller P−N−P
The Mo−N2 bond of 2Xy is 2.0378(18) Å for Mo(1)−N(1),
and for 23,5iPrPh, the bond lengths are 2.064(4) and 2.074(4) Å
for Mo(1)−N(1) and Mo(2)−N(3), respectively. The Mo−N
bond length of the former is shorter than the Mo−N bond
lengths of the latter, although these lengths are longer than the
Mo−N bond lengths of several previously reported neutral
dinitrogen molybdenum complexes with phosphorus ligands:
2.014(5) Å for trans-[Mo(N2)2(dppe)2] (dppe =1,2-bis-
(diphenylphosphino)ethane),40b 2.000(5) and 1.989(5) Å for
trans-[Mo(N2)2(PMePh2)4],40c and 2.0097(16) and
2.0211(15) Å for trans-[Mo(N2)2(depf)2] (depf = 1,1′-
bis(diethylphosphino)ferrocene.40d The NN bond length of
the coordinated dinitrogen for 2Xy is 1.110(3) Å, which is
slightly longer than that of the free dinitrogen molecule (1.095
Å).2 In the case of 23,5iPrPh, the N(1)−N(2) (1.078(6) Å) and
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(2.46−2.50 Å).42 This shorter Mo−P bond may have been
caused by unique property of diphosphinoamine ligand unlike
diphosphinomethane ligand.16
Table 3. Interatomic Distances (Å) and Angles (deg) of 2Xy
and 23,5iPrPh
23,5iPrPh
The dihedral angles between the P−N−P plane and N-
substituted phenyl ring, as also described above, are 4.7° for
2Xy, and 3.4° and 6.3° for 23,5iPrPh. The substituent phenyl rings
in these complexes exhibit very little twisting against the P−N−
P plane, in contrast with those of 1R. Although the coplanarity
may also be affected by the crystal packing, such examples are
quite rare among the metal complexes with N-arylated
diphosphinoamine ligands reported previously.14a,18g,29c,31b,42
Careful observation of these two crystal structures has revealed
an interesting structural feature. As shown in Figure 5, we
complex
2Xy
Interatomic Distances (Å)
structure (1) structure (2)
Mo−N2
Mo−P
2.0378(18)
2.4069(6)
2.4130(6)
1.7462(16)
1.7432(16)
1.110(3)
2.064(4)
2.074(4)
2.4086(11)
2.3997(12)
1.747(4)
2.4026(11)
2.4186(13)
1.736(3)
P−N
1.758(4)
1.752(4)
N−N
Mo···N
P···P
1.078(6)
1.058(6)
3.1391(17)
2.6598(7)
3.132(4)
3.151(4)
2.6516(16)
4.0110(19)
2.6606(17)
4.0207(18)
a
b
c
P···P′
4.0027(8)
Bond Angles (deg)
99.32(9)
P−N−P
P−Mo−P
99.16(18)
66.93(4)
98.58(17)
66.99(4)
66.985(17)
Dihedral Angles (deg)
plane[P−N−P]−plane[aryl] 4.667 3.448
6.289
a
The primed and nonprimed atom are related to each other by the
b
operation −x + 2, y, −z + 1/2 + 1. The primed and nonprimed atom
c
are related to each other by the operation −x, −y + 2, −z + 1. The
double primed and nonprimed atom are related to each other by the
operation −x + 1, −y + 2, −z + 2.
angle, such as 2Xy (99.32(9)°) and 23,5iPrPh (99.16(18) and
98.58(17)°). The aryl groups with a small twisting angle push
against the phenyl groups of diphenylphosphines. This results
in narrowing of the P−N−P angles. A narrow P−N−P angle
causes smaller P···P separations compared with those of 1Ph and
1Xy; the lengths for 2Xy are 4.0027(8) Å for P(1)···P(1)′and
4.0585(8) Å for P(2)···P(2)′, and the lengths for 23,5iPrPh are
4.0110(19) Å for P(1)−P(2)′, and 4.0207(18) Å for P(3)′−
P(4). In addition, the P−Mo−P angle for 2Xy is 66.99(2)° for
P(1)−Mo(1)−P(2), and for 23,5iPrPh the angles are 66.93(4)°
for P(1)−Mo(1)−P(2) and 66.99(4)° for P(3)−Mo(2)−P(4),
respectively. These values are quite small compared with the
previous case (79.40(15)° for trans-[Mo(N2)2(dppe)2],40b
90.5(2)° for trans-[Mo(N2)2(PMePh2)4],40c and 95.56(2)°
and 84.88(2)° for trans-[Mo(N2)2(depf)2]40d).
Figure 5. ORTEP view of 23,5iPrPh with CH−π interactions.
identified a CH−π interaction between xylyl methyl groups and
the phenyl rings of diphenylphosphine of 2Xy and between the
isopropyl methyl groups and the phenyl rings of diphenyl-
phosphine of 23,5iPrPh; the distances between methyl carbon
atoms (C(31) and C(32) for 2Xy and C(32), C(35), C(68), and
C(72) for 23,5iPrPh) from the mean plane of phenyl rings are
∼4.63 Å for 2Xy and 4.00−4.12 Å for 23,5iPrPh, respectively. This
CH−π interaction was also identified from the shift of the ring
1
current in H NMR spectra of 2Xy and 23,5iPrPh, the methyl
The Mo−P bond lengths for 2Xy are 2.4069(6) Å for
Mo(1)−P(1) and 2.4130(6) Å for Mo(1)−P(2). The Mo−P
bond lengths for 23,5iPrPh are 2.4086(11) Å for Mo(1)−P(1),
2.3997(12) Å for Mo(1)−P(2), 2.4026(11) Å for Mo(2)−
P(3), and 2.4186(13) Å for Mo(2)−P(4). These Mo−P bond
lengths are shorter than those of 1Ph and 1Xy, although the
charges (0) on the central metal of 2Xy and of 23,5iPrPh are less
than the +2 charge on the central metal of 1Ph and 1Xy. This
result indicates an increase in the covalency of Mo−P bond due
to changing in the oxidation states from Mo(+2) to Mo(0).
The Mo−P bond lengths observed for 2Xy and 23,5iPrPh lie in a
significantly shorter range than those of the trans-dinitrogen
tetraphosphine molybdenum complexes reported previously
protons are shifted to a higher field region relative to those of
the diphenylphosphinoamine derivative ligands, and the 3- and
5-methyl protons and 4-protons of 2Xy and the 3- and 5-
isopropylmethyl protons and 4-protons of 23,5iPrPh ligands are
shifted to an upper field region by 0.01−0.20 ppm relative to
those of metal free ligands. The narrow P−N−P angle (∼99°)
described above may have been induced by the push effect of
the methyl groups against the phenyl rings of diphenylphos-
phine through the CH−π interactions.
In order to clarify the coordination of the dinitrogen
molecule to the metal in solution, we synthesized a series of
15N-enriched dinitrogen molybdenum complexes, 3R (R = Ph,
Xy, 3,5-iPr2Ph). 15N{1H} NMR spectra of 3Xy and 33,5iPrPh in
Table 4. Comparison of trans-[Mo(N2)2(P4)] Complexes
complex
ν(NN) (cm−1
)
N−N (Å)
1.110(3)
av Mo−P (Å)
P−Mo−P (deg)
[Mo(N2)2(LXy)2] 2Xy
[Mo(N2)2(L3,5iPrPh)2] 23,5iPrPh
[Mo(N2)2(dppe)2]
2011
2014
1975
1922
1907
2.410(4)
2.407(8)
2.454(10)
2.496(3)
2.493(9)
66.99(2)
1.078(6), 1.058(6)
1.118(8)
66.93(4), 66.99(4)
79.40(15)
[Mo(N2)2(PMePh2)4]
[Mo(N2)2(depf)2]
1.133(7), 1.154(7)
1.122(2), 1.124(2)
90.5(2)
95.56(2), 84.88(2)
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THF solution exhibit two peaks arising from the internal and
terminal nitrogens at −46.34 and −45.99 ppm for 3Xy and
−46.47 and −46.03 ppm for 33,5iPrPh, respectively (with respect
to nitromethane nitrogen as a standard) (Figure S3). These
peaks are detected in the lower field region relative to the
metal-free dinitrogen molecule (−75.3 ppm) and are in the
usual range (−35 to −50 ppm) of the dinitrogen molybdenum
complexes reported previously, showing that the dinitrogen
molecule binds to molybdenum atoms.43 The chemical shift
values and the spin−spin coupling constants of 3Xy and 33,5iPrPh
are similar to those of a “trans” isomer of dinitrogen
molybdenum complexes (e.g., trans-[Mo(N2)2(dppe)2] dppe
=1,2-bis(diphenylphosphino)ethane).40b Therefore, it is sug-
gested that the structures of dinitrogen−molybdenum
diphosphinoamine complexes 2R in solution are “trans” isomers
as shown in the crystal structures.
H, CH3, CH3O),45 Rankin and co-workers reported on a
relationship between ν(NN) values and Hammett parame-
ters (σ+).46 They showed that the ν(NN) values (1931
cm−1) of the metal complexes with an electron donating p-
methoxyphenyl group are smaller than those of metal
complexes with a phenyl group (1945 cm−1). However, such
a relationship between the ν(NN) values and the electron
donating characteristics of LR was not identified. As described
in the 31P NMR section above, it is considered that the electron
densities on the phosphorus atoms of the phosphinoamine
ligands are not affected by the N-substituent aryl groups. For
example, the ν(NN) value of 2MeOPh (1995 cm−1) which has
the electron donating p-methoxyphenyl group is higher than
that of 2Ph (1990 cm−1) which has a phenyl group. Similarly,
the ν(NN) values of 2Xy (2011 cm−1) and 23,5iPrPh (2014
cm−1) are higher than that of 2Ph. To clarify these unusual and
interesting findings, we perform theoretical study described
below.
Solid state FT-IR spectra of 2R show a strong ν(NN) band
at 1990 cm−1 for 2Ph, 2011 cm−1 for 2Xy, 1995 cm−1 for 2MeOPh
,
The ν(NN) value (2011 cm−1) for 2Xy is smaller than that
of 23,5iPrPh (2014 cm−1), although the difference is quite small.
Likewise, the average Mo−P bond length of 2Xy (2.410(4) Å) is
quite similar to that of 23,5iPrPh (2.407(8) Å). In previous
reports, it was established that the dinitrogen molybdenum
complexes with longer Mo−P bonds tend to tend to show
lower ν(NN) values. We identified a correlation between the
ν(NN) values and the Mo−P bond lengths, as shown in
Figure 6. It was previously found that certain trans isomers of
2014 cm−1 for 23,5iPrPh, and 1975 cm−1 for 2iPr. These values are
clearly shifted to a lower energy region relative to the
corresponding value of the metal-free dinitrogen molecule
(2231 cm−1). Additionally, 15N-enriched dinitrogen complexes
3R exhibit an isotope shift of the ν(NN) band at 1923 cm−1
for 3Ph (Δ 67 cm−1), 1934 cm−1 for 3Xy (Δ 77 cm−1), and 1947
cm−1 for 33,5iPrPh (Δ 67 cm−1). These values appear to be
reasonable in considering the isotope effect of the coordinated
dinitrogen molecules. These spectral patterns of this observa-
tion region are characteristic of the trans isomer of dinitrogen
molybdenum complexes. The ν(NN) values are listed in
Table S1 together with those of previously reported dinitrogen
tetraphosphine molybdenum complexes. The ν(NN) values
of 2R are also similar to that of the previously reported “trans”
isomer of tetraphosphine molybdenum complex trans-[Mo-
(N2)2(Ph2PCH2PPh2)2] which has a 4-membered chelate ring
(1995 cm−1).4c,44
It is noteworthy that the ν(NN) values of 2R are
significantly affected by N-substituent groups of diphosphino-
amine ligands; the ν(NN) values decrease according to the
following order, 23,5iPrPh (2014 cm−1) > 2Xy (2011 cm−1) >
2MeOPh (1995 cm−1) > 2Ph (1990 cm−1) > 2iPr (1975 cm−1).
These results indicate that the greatest π-back-donation from
molybdenum to the dinitrogen molecule is observed for 2iPr.
This indicates that 2iPr has the highest electron density on the
metal ion. This is quite reasonable in light of the fact that the
N-isopropyl group has greater electron donating ability and the
N-aryl groups are electron-withdrawing groups. This result may
explain the dynamics of the 31P{1H} NMR chemical shift values
for the supporting ligands LiPr. The 31P{1H} chemical shift
value of LiPr is observed in the higher magnetic field region
relative to the 31P chemical shift value of LPh (Table 1). This
indicates that the phosphinoamine phosphorus atom of LiPr has
the greatest electron density. We can therefore rationalize that
the greater electron density on the phosphinoamine phospho-
rus atom may enhance the extent of electron donation to the
molybdenum atom and weaken the extent of π-back-donation
from the metal atom to the phosphorus atoms, resulting in
enhancement of π-back-donation from molybdenum to the
coordinated dinitrogen molecule.
Figure 6. Correlation between the average Mo−P bond lengths (Å)
and the ν(N2) values (cm−1) in the trans-[Mo(N2)2P4] (P = tertiary
phosphine ligand): (A) 23,5iPrPh, (B) 2Xy, (C) trans-[Mo(N2)2(dppe)2],
(D) trans-[Mo(N2)2(PMePh2)4], (E) trans-[Mo(N2)2(depf)2].
tetraphosphine dinitrogen molybdenum complexes which can
activate dinitrogen have longer Mo−P bond lengths as follows:
trans-[Mo(N2)2(dppe)2] (2.454(10) Å),40b trans-[Mo-
(N2)2(PMePh2)4] (2.496(3) Å),40c and trans-[Mo-
(N2)2(depf)2] (2.493(9) Å).40d A similar relationship is also
seen in the substituent effect of diphosphinoamine derivative
ligands, which influences Mo−P bond lengths and ν(NN)
stretching vibration values of 2Xy and 23,5iPrPh. This effect,
however, is very small. In previously reported tetracarbonyl
molybdenum complexes with phosphine derivative ligands,
such a substituent effect has not been identified in the
coordination behavior of the carbonyl molecule which is well-
known to exhibit coordination behavior similar to that of the
dinitrogen molecule. The ν(CO) values of the tetracarbonyl
Next, we examine the dynamics of ν(NN) values of 2R
with aryl groups. Previously, for the tetraphosphine molybde-
num complexes, trans-[Mo(N2)(p-XC6H4CN)(dppe)2] (X =
CH3CO, Cl, H, CH3, NH2), and trans-[Mo(N2)2{(p-
XC6H4)2PCH2CH2P(p-XC6H4)2}2] (M = Mo, X = CF3, Cl,
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d i p h e n y l p h o s p h i n o a m i n e c o m p l e x e s [ M o -
(CO)4{(PhO)2PNRP(OPh)2}] (R = Me, Ph) were essentially
the same for the respective substituents.29d In addition, the
ν(CO) stretching vibrations of C-substituted diphenylphosphi-
nomethane complexes [Mo(CO)4{Ph2PC(R1R2)PPh2}] (R =
H, alkyl, allyl) have essentially the same values.47 In these cases,
the ν(CO) values of the coordinated carbonyl groups were not
affected by the substituent groups. However, in our case, the
characteristics of the coordinated dinitrogen molecules are
significantly reflected by the ν(NN) values which are
influenced by the N-substituent groups; the ν(NN) value
ranges between 1975 to 2014 cm−1. This may indicate that
equatorial coordination of two diphosphinoamine derivative
ligands to the molybdenum ion is favorable for axial
coordination of dinitrogen molecules as a result of a through-
bond interaction of the P−Mo−N2 moiety. It is therefore quite
informative that the N-substituent groups of the diphenylphos-
phinoamine ligand can indirectly control the electronic
properties of metal centers. We have succeeded in controlling
the ν(NN) by using the dinitrogen molybdenum complexes
with N-substituted diphenylphosphinoamine ligands.
based on the complex detected by GC/MS, (Me3Si)3N, are as
follows: 3.95 (2Ph), 0.28 (2Xy), 1.06 (2iPr), 1.03 (2MeOPh), 2.95
(23,5iPrPh); for (Me3Si)2NH, 0.25 (2Ph), 0.09 (2Xy), 0.35 (2iPr),
0.35 (2MeOPh), 0.48 (23,5iPrPh). We could not distinguish
whether the obtained silylamines were made from dinitrogen
or diphosphinoamine ligand. However, any decomposed
products from phosphinoamine ligand were not detected in
GC/MS chromatogram.] While the silylation of 2R using
trimethylsilyl chloride proceeded, the protonation of 2R with
acids did not. This may indicate that the trimethylsilyl group
(as the species reacting with dinitrogen) is sterically too large to
successfully attack the sterically hindered nitrogen atom of the
diphosphinoamine ligand. Decomposition of ligands by the
reagents was prevented, and then the trimethylsilyl group
reacted with dinitrogen preferentially. Since the solubilities of
these complexes vary and tend to be quite low, we could not
identify a correlation between the reactivities and ν(NN)
values of the coordinated dinitrogen molybdenum complexes.
We have succeeded in reduction of dinitrogen using
molybdenum complexes with diphosphinoamine derivative
ligands.
Theoretical Study: Full Optimization of Dinitrogen
Molybdenum Complexes by DFT Calculations. The
experimental results described above have suggested that the
influence of the N-substituent groups of the supporting P−N−
P ligands is an important factor in the ν(NN) value. To
improve our understanding of the steric and electronic effects
of N-substituents, we performed a theoretical study of
dinitrogen molybdenum complexes with a particular focus on
the dihedral angle between the P−N−P plane and the N-
substituted phenyl ring because the values of ν(NN) have
been found to be affected by the steric bulk of N-substituted
aryl groups. We expect that variations in the dihedral angle may
be responsible for this effect. Interestingly, the crystal structures
of dichloride−molybdenum complexes indicate large dihedral
angles of 67.9° and 89.1° for 1Ph and 64.9° and 77.3° for 1Xy.
The dihedral angles of dinitrogen−molybdenum complexes
have small dihedral angles of 4.7° for 2Xy and of 3.45° and 6.29°
for 23,5iPrPh, respectively. Although this may simply be caused
by differences in crystal packing, we focused on these
meaningful dihedral angles in our theoretical approach to
understand the correlation between the dihedral angle and the
behavior of coordinated dinitrogen using geometry optimiza-
tion and natural population analysis, especially with respect to
the differences in electron densities.
Reactivity of Dinitrogen Diphosphinoamine Molyb-
denum Complexes. Efforts To Protonate Dinitrogen
Molybdenum Complexes. To develop dinitrogen reduction
catalysts, we first attempted to prepare a hydrazide complex
according to a previously reported method using various
dinitrogen molybdenum complexes.6c,d,48 Complex 23,5iPrPh was
treated with 2 equiv of trifluoromethanesulfonic acid (HOTf)
in THF at room temperature for 24 h. The resulting solution
was concentrated to dryness, and the precipitate was measured
by FT-IR spectroscopy. No peaks assignable to ν(NN) of
dinitrogen complex and ν(NH) of hydrazide complex were
observed. In addition, we attempted to generate ammonia by
treating complex 23,5iPrPh with an excess amount of sulfuric acid
(H2SO4) in THF at room temperature for 24 h.6a,b,49 However,
formation of ammonia was not detected as measured by using
the indophenol method.50 In 31P{1H} NMR spectra of the
resulting residues in benzene-d6, there were no peaks
corresponding to the diphenylphosphinoamine ligand. This
suggests that the supporting ligand of 23,5iPrPh reacts with acid
and decomposes. Burg and Slota have previously reported that
the P−N bond of the dimethylaminodimethylphosphine is
cleaved by acids such as water and hydrogen chloride.51 For
2
3,5iPrPh, it is possible that the P−N bond of the
diphenylphosphinoamine ligand has been cleaved by HOTf
or H2SO4. Therefore, the coordinated dinitrogen was not
reduced to ammonia. Furthermore, this reaction was
unsuccessful for all of the other dinitrogen molybdenum
complexes with diphosphinoamine ligands. These results
indicate that it is difficult for 2R to reduce dinitrogen by
protonolysis.
The DFT calculations were performed for 2Ph, 2Xy, and
23,5iPrPh with appropriate dihedral angles (0° and 90°) between
the P−N−P plane and the N-substituted phenyl ring plane.
Calculations were also carried out for 2(PMe2)Ph with dihedral
angles of 0° and 90°, where 2(PMe2)Ph represents a complex
with the diphenylphosphine of 2Ph replaced by dimethylphos-
phine. For 2(PMe2)Ph, calculations were also performed for the
complex without dinitrogen. For the DFT calculations, the Mo
atom was positioned on the origin, the z-axis of d-orbital of Mo
atom is placed parallel to the Mo−N(N2) bonds, and the x-axis
is directed toward the diphosphinoamine N atoms. These
model complexes were fully optimized using the crystal
structures obtained from single crystals of 2Xy and 23,5iPrPh as
their initial bond parameters. The bond parameters for the
Conversion of Dinitrogen to Silylamine Using
Dinitrogen Molybdenum Complexes. Next, we studied
the reactivity of the dinitrogen molybdenum complexes 2R
toward silylation, because Hidai and co-workers reported
catalytic dinitrogen reduction to silylamines using molybde-
num− and tungsten−dinitrogen complexes (TON = 24.3).4c,52
Treatment of 2R with excess trimethylsilyl chloride and sodium
metal in THF under a dinitrogen atmosphere at room
temperature for 24 h produced tris(trimethylsilyl)amine and
hexamethyldisilazane. Fortunately, the silylation of the
coordinated dinitrogen molecules proceeded, although the
yields of silylamines were very low. [The turnover numbers
metal atoms in the structures optimized for 2Ph, 2Xy, 23,5iPrPh
,
and 2(PMe2)Ph with dihedral angles of 0° and 90° are shown in
Table S3 together with the estimated ν(NN) stretching
vibration values. The whole structures and bond parameters for
190
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Inorganic Chemistry
Article
the optimized dinitrogen molybdenum complexes are similar to
those obtained from the crystal structures within experimental
error. However, elongations were identified in the Mo−P bond
lengths of the optimized structures.
with an N-aryl group in the perpendicular form is negatively
charged. This is favorable for attack of external protons.
Theoretical Study: Change in Electronic Structures of
the Molybdenum Complex by Coordination of the
Dinitrogen Molecule. The calculation of 2′(PMe2)Ph without
dinitrogen molecules was carried out using the four-coordinate
diphosphinoamine molybdenum model complex from which
dinitrogen molecules were removed 2(PMe2)Ph. The calcu-
lation of the metal-free dinitrogen molecule was carried out
using the structure that was removed from the four-coordinate
square-planar molybdenum complex from 2(PMe2)Ph. The
atomic charges and bond orders of these complexes are listed in
Table S5. As shown in Table S5, for both 2(PMe2)Ph and
2′(PMe2)Ph complexes with and without dinitrogen molecules,
the total energy is lower with the perpendicular form than with
the coplanar form. The atomic charges on molybdenum and
dinitrogen were found to increase upon coordination of
dinitrogen. The atomic charge on the phosphorus atoms was
found to decrease. The order of the NN bond was found to
decrease upon coordination to the metal from 1.804/1.805 to
1.609/1.612 for perpendicular and coplanar forms, respectively.
The electron density on the coordinated dinitrogen molecule
was found to increase with twisting of the aryl groups from the
coplanar form.
In the calculations, the substituent effects of N-aryl groups
were not observed, although there are significant differences in
the Mo−P bond lengths and P−N−P angles between
dinitrogen molybdenum complexes with dihedral angles of 0°
(coplanar form) and 90° (perpendicular form). The Mo−P
bond lengths for the complexes with the coplanar form are
shorter than those with the perpendicular form. This indicates
that for dinitrogen molybdenum complexes with the
perpendicular form, the π-back-donation from the Mo ion to
the P atoms is smaller than the π-back-donation of the
complexes with the coplanar form. The P−N−P angles (∼99°)
of the dinitrogen molybdenum complexes with the coplanar
form were found to be narrower than the P−N−P angles of the
dinitrogen molybdenum complexes with the perpendicular
form (∼103°). This may be explained by the steric repulsion
which occurs between the two phenyl groups of the
diphenylphosphine and the N-aryl substituents with the
coplanar form. The greater repulsion narrows the P−N−P
angle, while the perpendicular form has less repulsion and does
not enforce an acute angle. Such behavior is also seen in the
crystal structures of 1Ph (av 103.9°)/1Xy (av 103.2°) and 2Xy
(99.32°)/23,5iPrPh (av 98.87°).
In the estimated ν(NN) stretching vibration values, the
dinitrogen molybdenum complexes with the perpendicular
form (2040 cm−1) have lower frequencies than with the
coplanar form (2060 cm−1), although such behavior was found
to not be affected by the substituent effect of the N-aryl groups.
To simplify the DFT calculations, we also analyzed
dinitrogen molybdenum complexes with diphenylphosphine
replaced by dimethylphosphine (2(PMe2)Ph). The entire
structure and parameters of the bonds to the metal atoms are
essentially identical to those calculated for complexes 2Ph, 2Xy,
and 23,5iPrPh described above (see also Table S3). The most
significant change is within 0.01 Å for the Mo−P bonds. The
estimated ν(NN) bands also exhibit the same tendency.
Theoretical Study: Natural Population Analysis of the
Model Dinitrogen Molybdenum Complexes. In order to
examine the natural atomic charges and atom−atom overlap-
weighted NAO bond orders of the model dinitrogen
molybdenum complexes, we performed a natural population
analysis of 2Ph, 2Xy, 23,5iPrPh, and 2(PMe2)Ph (Table S4). The
orientations of the N-aryl groups (perpendicular and coplanar
forms) exert a significant effect on the natural atomic charges
and NAO bond order, but the substituents of the N-aryl groups
do not.
The energy levels of the d-orbitals of molybdenum atoms for
the complexes with coplanar and perpendicular forms are listed
in Table S6. It can be seen that the d-orbitals of the six-
coordinate dinitrogen molybdenum complex are doubly
2
2
occupied in all of the dzx, dx ‑y , dyz orbitals in this order. The
d-orbitals of the four-coordinate molybdenum complex were
2
2
2
found to be singly occupied in the dx ‑y , dyz, dzx, dz orbitals for
2
2
α-orbitals and in the dx ‑y , dzx orbitals for the β-orbitals. The α-
and β-orbital energies of the four-coordinate molybdenum
complexes are higher for the perpendicular form than those for
the coplanar form. This indicates that the d-orbitals of the
molybdenum complex with the perpendicular form interact
more readily with the π*-orbitals of dinitrogen molecules and
induce π-back-donation more effectively than those with the
coplanar form. Indeed, the dzx and dyz orbitals for the
molybdenum complexes with vertical and coplanar forms are
largely stabilized by dinitrogen coordination compared with the
other d-orbitals. The dzx orbital energy is changed between
perpendicular and coplanar forms. This behavior has also been
identified in the occupation numbers of the d-orbitals of the
Mo atom and the p-orbital of the diphosphinoamine N atom.
The occupation numbers of both the α- and the β-orbitals are
larger in the perpendicular form in than the coplanar form, and
the occupation numbers of the α- and β-dzx orbitals are
especially larger, as shown in Table S7.
As shown in Table S8, the extent of electron donation from
the Mo d orbitals to the N2 π* orbitals is competitive with the
extent of electron donation to the σ* orbitals of the P−C
bonds. This electron donation to the σ* orbitals of P−C bonds
is also competitive with the electron donation from the p-type
lone-pair of the diphosphinoamine nitrogen which is in so-
called hyperconjugation. The competitive electron donation
involving the σ* orbitals of the P−C bonds is demonstrated in
Scheme 3, where the thickness of the arrows shows the
magnitude of donation. The significant electron donation
originating from the lone-pair of the diphosphinoamine
nitrogen inhibits electron donation from the Mo d orbitals to
the P−C σ* orbitals. This inhibition of electron donation
(Table S8) is larger in the dinitrogen molybdenum complex
The electron density localized on the diphosphinoamine
nitrogen flows into the outer nitrogen of the coordinated
dinitrogen molecules through the NPMoNN bond
when the phenyl groups twist around the NC bond from the
coplanar form. The NAO bond orders of NPMoNN
bonds alternate between an increase and a decrease relative to
the coplanar form. The bond order alternates by twisting of the
coplanar form to the perpendicular form. For example, in the
case of 2Xy, it decreases for NC (0.845 → 0.813), increases
for NP (0.630 → 0.634), decreases for MoP (0.830 →
0.821), increases for MoN (0.841 → 0.856), and decreases
for NN (1.568 → 1.565). These findings suggest that the
dinitrogen molecule of the molybdenum dinitrogen complex
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Inorganic Chemistry
Article
Interpretation of the Difference Between Experimen-
tal and Theoretical Results. In the present work we
synthesized new dinitrogen molybdenum complexes with
diphosphinoamine derivative ligands which form a 4-membered
chelate ring and investigated the ability of these complexes. As
considered from the ν(NN) stretching vibration values of
the dinitrogen molybdenum complexes, the dinitrogen
molybdenum complexes with an alkyl group are more effective
at decreasing the bond strength of the coordinated dinitrogen
molecule than the complexes that have an aryl group. Among
the complexes with aryl groups, the ν(NN) values of
dinitrogen molybdenum complexes were different from each
other. Therefore, in order to explain the effect for the bond
strength of dinitrogen, we performed full geometry optimiza-
tion of the dinitrogen molybdeum complexes with the twisted
aryl group (∼90°) (perpendicular form) and with the
nontwisted aryl group (∼0°) (coplanar form) at the DFT
level. Interestingly, the DFT study has suggested that the
perpendicular form would be more effective at decreasing the
bond strength of dinitrogen molecule than the coplanar form.
This behavior has been elucidated as follows: The twisting of
aryl groups in the vertical form inhibits π-back-donation from
the metal to phosphinoamine. The electron density is pooled
on the metal and flows to the coordinated dinitrogen molecule
by π-back-donation. This theoretical result can explain the
experimental result of the ν(NN) value of dinitrogen
molybdenum complexes. 2iPr has smallest value of ν(NN),
because the alkyl substituent group of 2iPr that does not have π-
conjugation system inhibits π-back-donation from the metal to
phosphinoamine ligand. Therefore, the order of ν(NN) is
2Ph > 2iPr. On the other hand, 2Xy and 23,5iPrPh have the largest
value of ν(NN). Interestingly the 3,5-dimethyl groups of the
xylyl substituent and the 3,5-diisopropyl groups of the 3,5-
diisopropylphenyl substituent interact with the phenyl groups
of the diphenylphosphine substituents via a CH−π interaction,
as described above in the crystal structure section. Therefore,
these complexes have small dihedral angles and small bite
angles effect, and the substituent of 23,5iPrPh is sterically larger
than that of 2Xy. However, there are no CH−π interactions in
2Ph and 2MeOPh; thus, these complexes have large dihedral
angles. Also, 2MeOPh has p-methoxy group which has an
inductive withdrawing effect. Thus, the order of ν(NN) is
23,5iPrPh > 2Xy > 2MeOPh > 2Ph. Consequently, the ν(NN)
values decrease according to the following order: 23,5iPrPh > 2Xy
> 2MeOPh > 2Ph > 2iPr. The ν(NN) values of 2R are
significantly affected by N-substituent groups of diphosphino-
amine ligands from the viewpoint of both experimental and
theoretical study.
Scheme 3. Comparison of Hyperconjugation Schemes of the
Dinitrogen Molybdenum Complex with the Perpendicular
Form (Top) and the Coplanar Form (Bottom)
a
a
The Mo atom was positioned on the origin, the z-axis of d-orbital of
Mo atom is placed parallel to the Mo−N(N2) bonds, and the x-axis is
directed toward the diphosphinoamine N atoms.
with the perpendicular form relative to that of the N2
molybdenum complex with the coplanar form. In the
dinitrogen molybdenum complex with perpendicular phenyl
groups, the occupation numbers of the lone-pair electrons on
the diphosphinoamine nitrogen are greater. The extent of
electron donation from the lone-pairs to the σ* orbitals of P−C
bonds is also greater, and the extent of electron donation from
the 4dzx orbital to the σ* orbitals of P−C bonds is less than that
of the coplanar form.
The electron densities of the lone-pairs of the diphosphino-
amine groups are delocalized to a phenyl group parallel to the
P−N−P plane, and the basicity of the diphosphinoamine
nitrogen decreases in a manner similar to that of the amino
group of aniline. In contrast, the electron density within the
dinitrogen molybdenum complex with vertical phenyl groups
remains high and is transferred to the σ* orbitals of P−C
bonds. Electron donation from the Mo atom to σ* orbitals of
P−C bonds is inhibited, and this increases the d-electron
donation from Mo to π* orbitals of the N2 ligands. The
interaction between the dzx orbital of Mo and the σ* orbital of
P−C bond gives an unusual property due to the small bite
angle of the diphosphinoamine ligand. In the larger P−Mo−P
angle structure reported previously, however, the phosphorus
atoms are located near along the diagonal lines of the regular
octahedron. Therefore, it is difficult for the σ* orbital of P−C
bond to interact with dzx orbital of Mo ion.
CONCLUSION
■
We have prepared novel molybdenum dinitrogen complexes
with P−N−P type diphenylphosphinoamine ligands which
form a 4-membered chelate ring as an auxiliary ligand. The
ν(NN) values are not clearly reflected in N−N bond
distances but indicate that these ligands can control the
property of the coordinated dinitrogen through the changes in
the structure of metal center according to the properties of the
N-substituent groups on the P−N−P ligand. The steric
bulkiness of the substituent groups on diphenylphosphino-
amine nitrogens contributes to the bond order of the
coordinated dinitrogen molecules. This behavior has been
elucidated theoretically by DFT calculations. The coplanarity of
the N-substituent aryl groups of the P−N−P plane regulates
These theoretical results indicate that the nitrogen lone-pair
of the diphosphinoamine ligand controls the electron density at
the metal center (including phosphorus and dinitrogen). This
effect is sometimes seen in molybdenum complexes with
diphosphinoamine ligands, and not in the case of metal
complexes with carbon-bridged bidentate phosphine ligands
such as diphenylphosphinomethane and diphenylphosphinoe-
thane.19b,47
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Inorganic Chemistry
Article
Chem. Rev. 2004, 104, 527−559. (c) Lee, S. C.; Holm, R. H. Chem.
Rev. 2004, 104, 1135−1158. (d) Mori, H.; Seino, H.; Hidai, M.;
Mizobe, Y. Angew. Chem., Int. Ed. 2007, 46, 5431−5434. (e) Laplaza,
C. E.; Cummins, C. C. Science 1995, 268, 861−863. (f) 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. (g) Bowman, A. C.; Bart, S. C.;
Heinemann, F. W.; Meyer, K.; Chirik, P. J. Inorg. Chem. 2009, 48,
5587−5589.
(9) (a) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76−78.
(b) Schrock, R. R. Acc. Chem. Res. 2005, 38, 955−962.
(10) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527−
530.
the flow of electrons from the lone-pair on the nitrogen atom of
the P−N−P ligands to the coordinated dinitrogen through the
π*(N2)-dxz,dyx(Mo)−σ*(P−C) interaction of the N2−Mo−P
bond. In order to examine the factors controlling the
conversion of the molybdenum complexes from dinitrogen to
ammonia, we investigated the reduction of dinitrogen by
sialylation. Reduction was observed, but the yields were quite
low. Unfortunately, a correlation between N-substituents and
yields of silylamines was not identified because of the low
solubility of the molybdenum complexes 2R. Here we have
investigated the structural effect for dinitrogen coordination by
using novel molybdenum complexes with the P−N−P type
diphenylphosphinoamine ligands which form a 4-membered
chelate ring, and found that the location of phosphorus atoms
relative to molybdenum atom (M−P bond lengths and P−M−
P bite angles) is very important for controlling the property of
coordinated dinitrogen.
(11) (a) Komori, K.; Oshita, H.; Mizobe, Y.; Hidai, M. J. Am. Chem.
Soc. 1989, 111, 1939−1940. (b) Tanaka, H.; Sasada, A.; Kouno, T.;
Yuki, M.; Miyake, Y.; Nakanishi, H.; Nishibayashi, Y.; Yoshizawa, K. J.
Am. Chem. Soc. 2011, 133, 3498−3506.
(12) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2011, 3,
120−125.
(13) (a) Romer, R.; Gradert, C.; Bannwarth, A.; Peters, G.; Nather,
̈
̈
ASSOCIATED CONTENT
* Supporting Information
■
C.; Tuczek, F. Dalton Trans. 2011, 40, 3229−3236. (b) Stephan, G. C.;
S
Nather, C.; Sivasankar, C.; Tuczek, F. Inorg. Chim. Acta 2008, 361,
̈
1008−1019. (c) Yuki, M.; Midorikawa, T.; Miyake, Y.; Nishibayashi, Y.
Organometallics 2009, 28, 4741−4746. (d) Nishibayashi, Y.; Iwai, S.;
Hidai, M. Science 1998, 279, 540−542. (e) Harvey, B. G.; Arif, A. M.;
Ernst, R. D. Inorg. Chim. Acta 2010, 363, 221−224. (f) Watanabe, D.;
Gondo, S.; Seino, H.; Mizobe, Y. Organometallics 2007, 26, 4909−
4920.
Crystallographic data in CIF format. Additional figures, tables,
and details. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: masuda.hideki@nitech.ac.jp.
■
(14) (a) Tuczek, F.; Lehnert, N. Angew. Chem., Int. Ed. 1998, 37,
2636−2638. (b) Lehnert, N.; Tuczek, F. Inorg. Chem. 1999, 38, 1659−
1670. (c) Lehnert, N.; Tuczek, F. Inorg. Chem. 1999, 38, 1671−1682.
(d) Horn, K. H.; Lehnert, N.; Tuczek, F. Inorg. Chem. 2003, 42, 1076−
Notes
The authors declare no competing financial interest.
1086. (e) Horn, K. H.; Bo
C.; Peters, G.; Tuczek, F. Inorg. Chem. 2005, 44, 3016−3030.
̆ ̆
res, N.; Lehnert, N.; Mersmann, K.; Nather,
ACKNOWLEDGMENTS
■
(15) (a) Tolman, C. A. Chem. Rev. 1977, 77, 313−348. (b) van
Leeuwen, P. W. M. N.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P.
Chem. Rev. 2000, 100, 2741−2769. (c) Freixa, Z.; van Leeuwen, P. W.
N. M. Dalton Trans. 2003, 1890−1901.
We gratefully acknowledge support for this work provided by a
Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology and the
Knowledge Cluster Project. We also thank Yachiyo Taniyama
and Hiroko Amano for their helpful support.
(16) Klatt, K.; Nather, C.; Tuczek, F. Acta Crystallogr. 2008, E64,
̈
m1382.
(17) (a) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.;
Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto,
S.; Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J.
Am. Chem. Soc. 2004, 126, 14712−14713. (b) McGuinness, D. S.;
Overett, M.; Tooze, R. P.; Blann, K.; Dixon, J. T.; Slawin, A. M. Z.
Organometallics 2007, 26, 1108−1111. (c) McGuinness, D. S. Chem.
Rev. 2011, 111, 2321−2341.
REFERENCES
■
(1) Ertl, G. The Nobel Prize in Chemistry; The Royal Swedish
Academy of Sciences: Stockholm, 2007.
(2) Bazhenova, T. A.; Shilov, A. E. Coord. Chem. Rev. 1995, 144, 69−
145.
(3) Allen, A. D.; Senoff, C. V. J. Chem. Soc., Chem. Commun. 1965,
621−622.
(4) (a) Allen, A. D.; Harris, R. O.; Loescher, B. R.; Stevens, J. R.;
Whiteley, R. N. Chem. Rev. 1973, 73, 11−20. (b) Chatt, J.; Dilworth, J.
R.; Richards, R. L. Chem. Rev. 1978, 78, 589−625. (c) Hidai, M.;
Mizobe, Y. Chem. Rev. 1995, 95, 1115−1133. (d) Hidai, M. Coord.
Chem. Rev. 1999, 185−186, 99−108. (e) Fryzuk, M. D.; Johnson, S. A.
Coord. Chem. Rev. 2000, 200−202, 379−409. (f) Barriere, F. Coord.
̀
Chem. Rev. 2003, 236, 71−89. (g) MacKay, B. A.; Fryzuk, M. D. Chem.
Rev. 2004, 104, 385−401.
(5) (a) Hidai, M.; Tominari, K.; Uchida, Y.; Misono, A. J. Chem. Soc.
D 1969, 814. (b) Hidai, M.; Tominari, K.; Uchida, Y.; Misono, A. J.
Chem. Soc. D 1969, 1392. (c) Hidai, M.; Tominari, K.; Uchida, Y. J.
Am. Chem. Soc. 1972, 94, 110−114.
(6) (a) Chatt, J.; Pearman, A. J.; Richards, R. L. Nature 1975, 253,
39−40. (b) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc.,
Dalton Trans. 1977, 1852−1860. (c) Chatt, J.; Dilworth, J. R. Chem.
Commun. 1975, 983−984. (d) George, T. M.; Tisdale, C. J. Am. Chem.
Soc. 1985, 107, 5157−5159.
(7) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997,
275, 1445−1447.
(8) (a) Yoshida, T.; Adachi, T.; Kaminaka, M.; Ueda, T.; Higuchi, T.
J. Am. Chem. Soc. 1988, 110, 4872−4873. (b) Rao, P. V.; Holm, R. H.
(18) (a) Balakrishna, M. S.; Reddy, V. S.; Krishnamurthy, S. S.;
Nixon, J. F.; St. Laurent, J. C. T. R. B. Coord. Chem. Rev. 1994, 129, 1−
90. (b) Witt, M.; Roesky, H. W. Chem. Rev. 1994, 94, 1163−1181.
(c) Appleby, T.; Woollins, J. D. Coord. Chem. Rev. 2002, 235, 121−
140. (d) Fei, Z.; Dyson, P. J. Coord. Chem. Rev. 2005, 249, 2056−2074.
(e) Benito-Garagorri, D.; Kirchner, K. Acc. Chem. Res. 2008, 41, 201−
213. (f) Brown, G. M.; Finholt, J. E.; King, R. B.; Lee, T. W. J. Am.
Chem. Soc. 1981, 103, 5249−5250. (g) Tarassoli, A.; Chen, H.;
Thompson, M. L.; Allured, V. S.; Haltiwanger, R. C.; Norman, A. D.
Inorg. Chem. 1986, 25, 4152−4157. (h) Brown, G. M.; Finholt, J. E.;
King, R. B.; Lee, T. W. J. Am. Chem. Soc. 1981, 103, 5249−5250.
(i) Kubo, K.; Akimoto, T.; Mizuta, T.; Miyoshi, K. Chem. Lett. 2008,
37, 166−167. (j) Ashby, M. T.; Li, Z. Inorg. Chem. 1992, 31, 1321−
1322. (k) Fei, Z.; Scopelliti, R.; Dyson, P. J. Inorg. Chem. 2003, 42,
2145−2130. (l) Trinquier, G.; Ashby, M. T. Inorg. Chem. 1994, 33,
1306−1313. (m) Poetschke, N.; Nieger, M.; Khan, M. A.; Niecke, E.;
Ashby, M. T. Inorg. Chem. 1997, 36, 4087−4093. (n) Priya, S.;
Balakrishna, M. S.; Mague, J. T. J. Organomet. Chem. 2003, 679, 116−
124. (o) Fedotova, Y. V.; Kornev, A. N.; Sushev, V. V.; Kursky, Y. A.;
Mushtina, T. G.; Makarenko, N. P.; Fukin, G. K.; Abakumov, G. A.;
Zakharov, L. N.; Rheingold, A. L. J. Organomet. Chem. 2004, 689,
3060−3074. (p) Kremer, T.; Hampel, F.; Knoch, F. A.; Bauer, W.;
193
dx.doi.org/10.1021/ic301577a | Inorg. Chem. 2013, 52, 182−195

Inorganic Chemistry
Article
Schmidt, A.; Gabold, P.; Schtz, M.; Ellermann, J.; Schleyer, P. V. R.
Organometallics 1996, 15, 4776−4782. (q) Kubo, K.; Nakazawa, H.;
Inagaki, H.; Miyoshi, K. Organometallics 2002, 21, 1942−1948.
(r) Blaurock, O. K. S.; Sieler, J.; Hey-Hawkins, E. Polyhedron 2001,
20, 111−117. (s) Fenske, D.; Maczek, B.; Maczek, K. Z. Anorg. Allg.
Chem. 1997, 623, 1113−1120. (t) Gray, G.; Kraihanzel, C. S. J.
(34) (a) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys.
1985, 83, 735−746. (b) Weinhold, F.; Landis, C. R. Valency and
Bonding: A Natural Bond Orbital Donor-Acceptor Perspective; Cam-
bridge University Press: Cambridge, U.K., 2005. (c) Reed, A. E.;
Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899−926.
(35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A. Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009.
(36) (a) Schmidpeter, A.; Burget, G. Angew. Chem., Int. Ed. Engl.
1985, 24, 580−581. (b) Braunstein, P.; Hasselbring, R.; Tripicchio, A.;
Ugozzoli, F. J. Chem. Soc., Chem. Commun. 1995, 37−38.
(37) (a) Albright, T. A.; Burdett, J. K. Problems in Molecular Orbital
Theory; Oxford University Press; New York, 1992; pp 103−104.
(b) Browning, C. S.; Farrar, D. H.; Frankel, D. C. Acta Crystallogr.
1992, C48, 806−811.
(38) (a) Grabowski, Z. R.; Rotkiewicz, K. Chem. Rev. 2003, 103,
3899−4031. (b) Druzhinin, S. I.; Dubbaka, S. R.; Knochel, P.;
Kovalenko, S. A.; Mayer, P.; Senyushkina, T.; Zachariasse, K. A. J. Phys.
Chem. A 2008, 112, 2449−2761.
(39) Ganesamoorthy, C.; Balakrishna, M. S.; Mague, J. T.;
Tuononen, H. M. Inorg. Chem. 2008, 47, 7035−7047.
Organomet. Chem. 1978, 146, 23−37. (u) Durap, F.; Biricik, N.;
̈
Gumgum, B.; Ozkar, S.; Ang, W. H.; Fei, Z.; Scopelliti, R. Polyhedron
̈
̈
2008, 27, 196−202.
(19) (a) Ahuja, R.; Samuelson, A. G. J. Organomet. Chem. 2009, 694,
1153−1160. (b) Jiang, T.; Zhang, S.; Jiang, X.; Yang, C.; Niu, B.; Ning,
Y. J. Mol. Catal. A: Chem. 2008, 279, 90−93. (c) Dulai, A.; McMullin,
C. L.; Tenza, K.; Wass, D. F. Organometallics 2011, 30, 935−941.
(20) Cross, R. J.; Green, T. H.; Keat, R. J. Chem. Soc., Dalton Trans.
1976, 1424−1428.
(21) (a) Burgess, B. K. Chem. Rev. 1990, 90, 1377−1406. (b) Smith,
B. E.; Durrant, M. C.; Fairhurst, S. A.; Gormal, C. A.; Gronberg, K. L.
̈
C.; Henderson, R. A.; Ibrahim, S. K.; Le Gall, T.; Pickett, C. J. Coord.
Chem. Rev. 1999, 185−186, 669−687. (c) Dos Santos, P. C.; Dean, D.
R.; Hu, Y.; Ribbe, M. W. Chem. Rev. 2004, 104, 1159−1173.
(d) Einsle, O.; Tezcan, F. A.; Andrade, S. L.; Schmid, B.; Yoshida, M.;
Howard, J. B.; Rees, D. C. Science 2002, 297, 1696−1700. (e) Spatzal,
T.; Aksoyoglu, M.; Zhang, L.; Andrade, S. L. A.; Schleicher, E.; Weber,
S.; Rees, D. C.; Einsle, O. Science 2011, 334, 940.
(22) Cromer, D. T.; Waber, J. T. International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, U.K.,1974; Vol. IV,
Table 2.2 A.
(23) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781.
(24) Creagh, D. C.; McAuley, W. J. International Tables for
Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers:
Boston, 1992; Vol. C, Table 4.2.6.8, pp 219−222.
(25) Creagh, D. C.; Hubbell, J. H. International Tables for
Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers:
Boston, 1992; Vol. C, Table 4.2.4.3, pp 200−206.
(26) (a) CrystalStructure 4.0: Crystal Structure Analysis Package;
Rigaku Corporation: Tokyo, 2000−2010; pp 196−8666. (b) Watkin,
D. J.; Prout, C. K. C.; Carruthers, J. R.; Betteridge, P. W. CRYSTALS
Issue 10; Chemical Crystallography Laboratory: Oxford, 1996.
(27) Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001,
2001, 2699−2701.
(40) (a) Uchida, T.; Uchida, Y.; Hidai, M.; Kodama, T. Bull. Chem.
Soc. Jpn. 1971, 44, 2883. (b) Uchida, T.; Uchida, Y.; Hidai, M.;
Kodama, T. Acta Crystallogr. 1975, B31, 1197−1199. (c) Morris, R. H.;
Ressner, J. M.; Sawyer, J. F. Acta Crystallogr. 1985, C41, 1017−1019.
(d) Yuki, M.; Miyake, Y.; Nishibayashi, Y.; Wakiji, I.; Hidai, M.
Organometallics 2008, 27, 3947−3953. (e) Yuki, M.; Miyake, Y.;
Nishibayashi, Y. Organometallics 2009, 28, 5821−5827.
(41) (a) Morris, R. H.; Ressner, J. M.; Sawyer, J. F.; Shiralian, M. J.
Am. Chem. Soc. 1984, 106, 3683−3684. (b) Mori, H.; Seino, H.; Hidai,
M.; Mizobe, Y. Angew. Chem., Int. Ed. 2007, 46, 5431−5434.
(42) Chen, H.; Tarassoli, A.; Haltiwanger, R. C.; Allured, V. S.;
Norman, A. D. Inorg. Chim. Acta 1982, 65, L69−L70.
(28) (a) Carver, F. J.; Hunter, C. A.; Livingstone, D. J.; MaCabe, J.
F.; Seward, E. M. Chem.Eur. J. 2002, 8, 2847−2859. (b) Malkov, A.
̌ ̌
́
V.; Figlus, M.; Stoncius, S.; Kocovsky, P. J. Org. Chem. 2007, 72,
1315−1325.
(29) (a) Balakrishna, M. S.; Panda, R.; Mague, J. T. Polyhedron 2003,
22, 587−593. (b) Killian, E.; Blann, K.; Bollmann, A.; Dixon, J. T.;
Kuhlmann, S.; Maumela, M. C.; Maumela, H.; Morgan, D. H.;
(43) (a) Chatt, J.; Fakley, M. E.; Richards, R. L.; Mason, J.;
Stenhouse, I. A. J. Chem. Res., Miniprint 1979, 0876−0889.
(b) Dilworth, J. R.; Donovan-Mtunzi, S.; Kan, C. T.; Richards, R. L.
Inorg. Chim. Acta 1981, 53, L161−L162. (c) Donovan-Mtunzi, S.;
Richards, R. L.; Mason, J. J. Chem. Soc., Dalton Trans. 1984, 469−474.
(d) Donovan-Mtunzi, S.; Richards, R. L.; Mason, J. J. Chem. Soc.,
Dalton Trans. 1984, 2429−2433. (e) Donovan-Mtunzi, S.; Richards, R.
L.; Mason, J. J. Chem. Soc., Dalton Trans. 1984, 2729−2731.
(f) Donovan-Mtunzi, S.; MacDonald, C. J.; Richards, R. L.; Hawkes,
G. E.; Mason, J. J. Chem. Soc., Dalton Trans. 1985, 2473−2477.
(g) Anderson, S. N.; Richards, R. L.; Hughes, D. L. J. Chem. Soc.,
Dalton Trans. 1986, 245−252.
Nongodlwana, P.; Overett, M. J.; Pretorius, M.; Hofener, K.;
̈
Wasserscheid, P. J. Mol. Catal. A: Chem. 2007, 270, 214−218.
(c) Balakrishna, M. S.; Prakasha, T. K.; Krishnamurthy, S. S.;
Siriwardane, U.; Hosmane, N. S. J. Organomet. Chem. 1990, 390,
203−216. (d) Biricik, N.; Durap, F.; Kayan, C.; Gumgum, B. Heteroat.
̈
̈
Chem. 2007, 18, 613−616.
(30) Noth, V. H.; Meinel, L. Z. Anorg. Allg. Chem. 1967, 349, 225−
̈
240.
(31) (a) Balakrishna, M. S.; Panda, R.; Smith, D. C., Jr.; Klaman, A.;
Nolan, S. P. J. Organomet. Chem. 2000, 599, 159−165. (b) Fei, Z.;
Scopelliti, R.; Dyson, P. J. J. Chem. Soc., Dalton Trans. 2003, 2772−
2779.
(44) Anker, M. W.; Chatt, J.; Leigh, G. J.; Wedd, A. G. J. Chem. Soc.,
Dalton Trans. 1975, 2639−2645.
(32) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
(33) (a) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
̈
(45) (a) Tatsumi, T.; Hidai, M.; Uchida, Y. Inorg. Chem. 1975, 14,
2530−2534. (b) Chatt, J.; Leigh, G. J.; Neukomm, H.; Pickett, C. J.;
Stanley, D. R. J. Chem. Soc., Dalton Trans. 1980, 121−127. (c) Hussain,
W.; Leigh, G. J.; Ali, H. M.; Pickett, C. J.; Rankin, D. A. J. Chem. Soc.,
Dalton Trans. 1984, 1703−1708.
Theor. Chim. Acta 1990, 77, 123−141. (b) Clark, T.; Chandrasekhar,
J.; Spitznagel, G. W.; Schleyer, P. V. R. J. Comput. Chem. 1983, 4, 294−
301. (c) 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,
3654−3665. (d) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973,
28, 213−222. (e) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem.
Phys. 1972, 56, 2257−2261.
(46) Hammet, L. P. J. Am. Chem. Soc. 1937, 59, 96−103.
(47) Hogarth, G.; Kilmartin, J. J. Organomet. Chem. 2007, 692, 5655−
5670.
194
dx.doi.org/10.1021/ic301577a | Inorg. Chem. 2013, 52, 182−195

Inorganic Chemistry
Article
(48) (a) Chatt, J.; Heath, G. A.; Richards, R. L. J. Chem. Soc., Dalton
Trans. 1974, 2074−2082. (b) Takahashi, T.; Mizobe, Y.; Sato, M.;
Uchida, Y.; Hidai, M. J. Am. Chem. Soc. 1979, 101, 3405−3407.
(c) Baumann, J. A.; George, T. A. J. Am. Chem. Soc. 1980, 102, 6153−
6154. (d) Takahashi, T.; Mizobe, Y.; Sato, M.; Uchida, Y.; Hidai, M. J.
Am. Chem. Soc. 1980, 102, 7461−7467. (e) Anderson, S. N.; Fakley,
M. E.; Richards, R. L. J. Chem. Soc., Dalton Trans. 1981, 1973−1980.
(f) Hussain, W.; Leigh, G. J.; Pickett, C. J. J. Chem. Soc., Chem.
Commun. 1982, 747−748. (g) Bossard, G. E.; George, T. A.; Howell,
D. B.; Koczon, L. M.; Lester, R. K. Inorg. Chem. 1983, 22, 1968−1970.
(49) (a) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc., Dalton
Trans. 1977, 1852−1860. (b) Hidai, M.; Mizobe, Y.; Takahashi, T.;
Uchida, Y. Chem. Lett. 1978, 1187−1188. (c) Takahashi, T.; Mizobe,
Y.; Sato, M.; Uchida, Y.; Hidai, M. J. Am. Chem. Soc. 1979, 101, 3405−
3407.
(50) Weatherburn, M. W. Anal. Chem. 1967, 39, 971−974.
(51) Burg, A.; Slota, P. J., Jr. J. Am. Chem. Soc. 1958, 80, 1107−1109.
(52) (a) Shiina, K. J. Am. Chem. Soc. 1972, 94, 9266−9267.
(b) Komori, K.; Oshita, H.; Mizobe, Y.; Hidai, M. J. Am. Chem. Soc.
1989, 111, 1939−1940. (c) Tanaka, H.; Sasada, A.; Kouno, T.; Yuki,
M.; Miyake, Y.; Nakanishi, H.; Nishibayashi, Y.; Yoshizawa, K. J. Am.
Chem. Soc. 2011, 133, 3498−3506.
195
dx.doi.org/10.1021/ic301577a | Inorg. Chem. 2013, 52, 182−195