Complexes obtained by electrophilic attack on a dinitrogen-derived terminal molybdenum nitride: Electronic structure analysis by solid state CP/MAS 15N NMR in combination with DFT calculations

Article abstract of DOI:10.1016/j.poly.2004.08.010

¹⁵N Solid state CP/MAS NMR spectroscopy has been used to study a dinitrogen-derived terminal nitride of molybdenum, ¹⁵NMo(N[ ^tBu]Ar)₃ (Ar = 3,5-C₆H₃(CH ₃)₂). Several derivatives were prepared by the reaction of ¹⁵NMo(N[^tBu]Ar)₃ with Lewis acids and potent electrophiles (e.g., CH₃I, Me₃Si-OTf, PhC(O)OTf). ¹⁵N solid state NMR spectroscopy and density functional calculations were used to probe the electronic structure of ¹⁵NMo(N[ ^tBu]Ar)₃ and its derivatives. ¹⁵N Solid state CP/MAS NMR spectroscopy has been used to study a dinitrogen-derived terminal nitride of molybdenum, ¹⁵NMo(N[^tBu]Ar)₃ (Ar = 3,5-(CH₃)₂C₆H₃). A number of Lewis acid adducts, including X₃E-NMo(N[^tBu]Ar)₃ (X = F, E = B; X = Cl, E = B, Al, Ga, In; X = Br, E = Al; X = I, E = Al) and Cl ₂E-NMo(N[^tBu]Ar)₃ (E = Ge, Sn), were prepared by the combination of ¹⁵NMo(N[^tBu]Ar)₃ with 1 equiv. of Lewis acid. A series of cationic imido complexes, [RNMo(N[ ^tBu]Ar)₃]X was prepared by the reaction of electrophiles, RX [R = CH₃, X = I; R = PhC(O) or Me₃Si, X = OTf (OTf = SO₃CF₃)], with NMo(N[^tBu]Ar)₃. Deprotonation of [CH₃NMo(N[^tBu]Ar)₃]I by LiN(SiMe₃)₂ afforded the ketimide complex H ₂CNMo(N[^tBu]Ar)₃, which has been shown to undergo a reaction with neat CH₃I to form [CH₃CH ₂NMo(N[^tBu]Ar)₃]I. ¹⁵N solid state CP/MAS NMR spectroscopy was employed in the characterization of each complex. Complementary density functional theory (DFT) studies of ¹⁵NMo(N[^tBu]Ar)₃ and derivatives enabled a detailed examination of the experimental solid state NMR parameters in terms of electronic structure at the labeled N-atom. Computational analysis demonstrated that significant paramagnetic contributions to the perpendicular components of the chemical shielding tensor (δ₁₁ and δ₂₂) were responsible for the huge span of the tensor measured for ¹⁵NMo(N[^tBu]Ar)₃ (Ω = 1187 ppm). Perturbation of the electronic structure in ¹⁵NMo(N[ ^tBu]Ar)₃ upon coordination of a Lewis acid or formation of a cationic imido complex was attributed to stabilization of a σ-symmetric orbital. An upfield shift in the perpendicular components of the chemical shift tensor results from the reduced paramagnetic contribution to these tensor components upon increasing the energy gap between the magnetically coupled occupied and virtual orbitals (e_occ - e_vir).

Full text of DOI:10.1016/j.poly.2004.08.010

Polyhedron 23 (2004) 2751–2768
www.elsevier.com/locate/poly
Complexes obtained by electrophilic attack on a
dinitrogen-derived terminal molybdenum nitride:
15
electronic structure analysis by solid state CP/MAS N NMR
in combination with DFT calculations
a
a
a,
*
Emma L. Sceats , Joshua S. Figueroa , Christopher C. Cummins
Nikolaus M. Loening a,b, Patrick Van der Wel a,b, Robert G. Griffin
,
a,b
a
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
b
Received 25 April 2004; accepted 25 August 2004
Available online 28 October 2004
Abstract
15N Solid state CP/MAS NMR spectroscopy has been used to study a dinitrogen-derived terminal nitride of molybdenum,
15NMo(N[tBu]Ar)3 (Ar = 3,5-(CH3)2C6H3). A number of Lewis acid adducts, including X3E–NMo(N[tBu]Ar)3 (X = F, E = B;
X = Cl, E = B, Al, Ga, In; X = Br, E = Al; X = I, E = Al) and Cl2E–NMo(N[tBu]Ar)3 (E = Ge, Sn), were prepared by the combi-
nation of 15NMo(N[tBu]Ar)3 with 1 equiv. of Lewis acid. A series of cationic imido complexes, [RNMo(N[tBu]Ar)3]X was prepared
by the reaction of electrophiles, RX [R = CH3, X = I; R = PhC(O) or Me3Si, X = OTf (OTf = SO3CF3)], with NMo(N[tBu]Ar)3.
Deprotonation of [CH3NMo(N[tBu]Ar)3]I by LiN(SiMe3)2 afforded the ketimide complex H2CNMo(N[tBu]Ar)3, which has been
shown to undergo a reaction with neat CH3I to form [CH3CH2NMo(N[tBu]Ar)3]I. 15N solid state CP/MAS NMR spectroscopy
was employed in the characterization of each complex. Complementary density functional theory (DFT) studies of 15NMo(N[t-
Bu]Ar)3 and derivatives enabled a detailed examination of the experimental solid state NMR parameters in terms of electronic struc-
ture at the labeled N-atom. Computational analysis demonstrated that significant paramagnetic contributions to the perpendicular
components of the chemical shielding tensor (d11 and d22) were responsible for the huge span of the tensor measured for 15NMo(N[t-
Bu]Ar)3 (X = 1187 ppm). Perturbation of the electronic structure in 15NMo(N[tBu]Ar)3 upon coordination of a Lewis acid or for-
mation of a cationic imido complex was attributed to stabilization of a r-symmetric orbital. An upfield shift in the perpendicular
components of the chemical shift tensor results from the reduced paramagnetic contribution to these tensor components upon
increasing the energy gap between the magnetically coupled occupied and virtual orbitals (eocc ꢀ evir).
ꢀ 2004 Elsevier Ltd. All rights reserved.
Keywords: 15N solid state NMR; DFT calculations; Molybdenum nitride; Imido complexes; Lewis acids
1. Introduction
logical systems and organic molecules [1–9]. In contrast,
solid state NMR spectroscopy of spin-active nuclei in
transition metal complexes has been underutilized [10–
16]. The isotropic chemical shift value determined in
solution NMR measurements is the most reported spect-
roscopic parameter, but a wealth of additional informa-
tion may be gained through the acquisition of solid state
NMR spectra [17,18]. The use of 31P solid state NMR
The use of isotopic labeling of compounds with 15N
to probe their structure and bonding by means of solid
state NMR spectroscopy is most commonly seen in bio-
*
Corresponding author. Tel.: +6174522517; fax: +6172585700.
E-mail address: ccummins@mit.edu (C.C. Cummins).
0277-5387/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2004.08.010

2752
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
spectroscopy to probe the structure and bonding in a
terminal molybdenum phosphide, PMo(N[tBu]Ar)3
(Ar = 3,5-(CH3)2C6H3) was reported previously [19].
The significant anisotropy of the 31P chemical shift ten-
sor and the enormous paramagnetic deshielding of the
31P nucleus when the applied field is oriented perpendic-
ular to the Mo–P triple bond were explained in terms of
the magnetic coupling of filled and vacant molecular
orbitals.
[29] were prepared according to the literature proce-
dures. Other chemicals were purified and dried by stand-
ard procedures [30] or were used as received. Celite,
˚
alumina and 4 A molecular sieves were dried in vacuo
for 36 h at ꢁ250 ꢁC. Infrared spectra were recorded
on a Bio-Rad 135 Series FT-IR spectrometer.
2.2. X-ray crystal structure determinations
Herein, we extend our study of structure and bonding
through the use of solid state NMR spectroscopy to the
analogous terminal nitride of molybdenum, NMo(N[t-
Bu]Ar)3 (1) [20]. The splitting of dinitrogen by a three-
coordinate molybdenum(III) complex Mo(N[tBu]Ar)3
was first reported in 1995 [21]. The 15N-labeled form
of 1, 15NMo(N[tBu]Ar)3 is readily prepared through
the use of 15N-isotopically-enriched dinitrogen gas,
thereby making this complex amenable to study by
15N NMR spectroscopy [22]. The potential of 1 to serve
as a platform in N-atom transfer chemistry is currently
under investigation in our laboratories. It is of interest
to determine what insight solid state NMR spectroscopy
can provide into the electronic structure for a molybde-
num complex which is activated toward productive re-
moval of the dinitrogen-derived N-atom into an
organic product. The synthesis of several derivatives of
1 and subsequent measurement of their 15N solid state
CP/MAS NMR spectra are described.
Complementary density functional theory analyses of
1 and its derivatives were completed utilizing a simpli-
fied ligand framework (NH2 ligands replacing
N[tBu]Ar). DFT permits a detailed analysis of experi-
mental solid state NMR parameters (d11, d22 and d33)
via calculation of the absolute shielding tensors [23–27]
together with correlation to calculated molecular orbit-
als. The validity of our analyses of electronic structure
may be assessed by comparison of the experimental ten-
sor values with those determined computationally.
The X-ray data collections were carried out on a Sie-
mens Platform three-circle diffractometer mounted with
a CCD or APEX CCD detector and outfitted with a
low-temperature, nitrogen-stream aperture. The struc-
tures were solved by direct methods, with the exception
of 1-BF3, which was solved using the Patterson Method,
in conjunction with standard difference Fourier tech-
niques and refined by full-matrix least-squares proce-
dures. A summary of crystallographic data is given in
Table 1, with full details found in the Supporting Infor-
mation. The systematic absences in the diffraction data
were uniquely consistent with the assigned space groups
of P21 for 3 and P213 for both [2b]OTf and [2d]I (Flack
parameters are 0.01(4), ꢀ0.01(4)and 0.00(3), respec-
tively). No symmetry higher than triclinic was indicated
in the diffraction data for 1-BF3. These choices led to
chemically sensible and computationally stable refine-
ments. All hydrogen atoms were placed in calculated
positions, with the exception of the ketimide protons
of 3, which were located in the electron density map
and refined isotropically. The Mo1–N2–C21 unit of
[2d]I was found to be coincident with a three-fold axis
of the P213 space group, thus imposing three-site posi-
tional disorder in the corresponding methyl group
(C22). This disorder was modeled and no hydrogen
atoms were generated for methylene carbon C21. Com-
plex [2c]OTf crystallized in the centrosymmetric space
group P21/n with two-site positional disorder found
for the triflate counter ion. This disorder was modeled
to 70:30 occupancy over the two sites as indicated by
the refinement statistics and resulted in chemically sensi-
ble geometries. Isomorphous 1-GeCl2 and 1-SnCl2 were
found to crystallize in the centrosymmetric space group
P21/c and both contained two-site positional disorder of
the aryl-ring and ECl2 residues. Both possible chiral,
three-blade propeller orientations of the aryl rings were
present and were modeled each with 50% occupancy as
indicated by the refinement statistics. Additionally, each
aryl-ring orientation corresponded to distinct ECl2 ori-
entation and was similarly modeled. Only one orienta-
tion of 1-GeCl2 and 1-SnCl2 is presented in the text.
No hydrogen atoms were generated for the aryl rings
of 1-GeCl2 and 1-SnCl2 due to the disorder. All software
for diffraction data processing and crystal-structure
solution and refinement are contained in the SHELXTL
(v6.14) program suite (G. Sheldrick, Bruker XRD, Mad-
ison, WI).
2. Experimental
2.1. General information
Unless stated otherwise, all operations were per-
formed in a vacuum atmospheres drybox under an
atmosphere of purified dinitrogen. Diethyl ether, n-pen-
tane and dichloromethane were dried and deoxygenated
using the method of Grubbs and coworkers [28]. THF
was distilled from purple Na/benzophenone ketyl and
collected under dinitrogen. C6D6 and CDCl3 were de-
gassed and dried over 4 A molecular sieves. 15N2 was
˚
purchased from Cambridge Isotope Laboratories
(CIL) in 0.1 mL break-seal glass vessels. CH3I was,
˚
freeze–pump–thaw degassed and stored over 4 A sieves
prior to use. Mo(N[tBu]Ar)3 [20] and benzoyl triflate

Table 1
Crystallographic data
Compound
1-BF3
1-GeCl2
1-SnCl2
[2b]OTf
[2c]OTf
[2d]I
3
Crystal data
Empirical formula
Formula weight
T (K)
C37H56N4MoBCl2F3 C36H54N4MoGeCl2 C36H54N4MoSnCl2 C40H63N4MoF3SO3Si C48H67F3N4MoO5S C39H57N4MoCl2I C37H56N4Mo
791.51
193(2)
triclinic
781.54
193(2)
828.36
193(2)
861.03
193(2)
cubic
P213
965.06
193(2)
875.63
193(2)
cubic
P213
652.80
193(2)
monoclinic
P21
Crystal system
Space group
Unit cell dimensions
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/n
ꢀ
P1
˚
a (A)
10.5757(6)
12.1291(7)
16.6647(10)
93.3220(10)
107.3170(10)
94.7260(10)
2026.2(2)
2
14.803(3)
13.711(3)
19.132(4)
90
14.803(3)
13.711(3)
19.132(4)
90
16.5578(5)
16.5578(5)
12.3168(8)
26.7028(17)
14.7598(10)
90
16.3118(6)
16.3118(6)
11.2145(10)
11.0462(10)
14.7723(13)
90
˚
b (A)
˚
c (A)
16.5578(5)
90
16.3118(6)
90
a (ꢁ)
b (ꢁ)
c (ꢁ)
90.12(3)
90
90.12(3)
90
90
90
90.4400(10)
90
90
90
94.367(2)
90
3
˚
Unit cell volume (A )
3883.0(13)
4
3883.0(13)
4
4539.5(2)
4
4854.3(6)
4
4340.2(3)
4
1824.6(3)
2
Z
Data collection
˚
k (Mo Ka) (A)
0.71073
1.297
0.499
0.71073
1.292
1.261
0.71073
1.370
1.131
0.71073
1.260
0.71073
1.321
0.373
0.71073
1.340
1.166
0.71073
1.188
qcalc (g cmꢀ3
)
l (mmꢀ1
Reflections collected
)
0.412
19 536
0.388
6978
8791
16 135
5070 (0.0387)
none
9171
20 454
6353 (0.0489)
none
18 731
1907 (0.0620)
none
Independent collections (Rint
)
5282 (0.0355)
empirical
Maximum and minimum transmission 0.3804 and 0.3026
6358 (0.0308)
none
na
1988 (0.0601)
empirical
0.2741 and 0.2191
4511 (0.0592)
empirical
0.3092 and 0.2137
Absorption correction
na
na
na
Structure refinement
Refinement method
Observed reflections [I > 2r(I)]
Number of parameters
Number of restraints
Goodness-of-fit on F2
R[I > 2r(I)]
full-matrix least squares on F2 was used for all complexes
5282
433
5070
552
6358
552
1988
160
6353
572
1907
151
4511
387
0
0
0
0
0
0
1
1.076
0.0488
0.1264
1.237
0.0461
0.1176
1.217
0.0620
0.1602
1.074
0.0269
0.0658
1.044
0.0464
0.1153
1.068
0.0314
0.0744
1.053
0.0406
0.1063
wR2

2754
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
2.3. NMR measurements
2.5. Computational details
Solution state 1H, 13C and 19F NMR spectra were re-
corded on a Varian Mercury-300 spectrometer operating
Theoretical calculations were carried out using the
Amsterdam Density Functional package (version
ADF2002.02) [35–38]. The Slater-type orbital (STO) ba-
sis sets were of triple-f quality augmented with two
polarization functions (ADF basis TZ2P). Full elec-
tronic configuration was used for all atoms. Relativistic
effects were included by virtue of the zero order regular
approximation (ZORA) [39–41]. The local density
approximation (LDA) by Vosko, Wilk and Nusair
(VWN) [42] was used together with the exchange corre-
lation corrections of Becke [43] and Perdew [44] (BP-86).
The 15N NMR chemical shielding calculations were
performed using a multi-step procedure. First, the
geometry of the compound of interest was optimized
using X-ray parameters as a starting point. The optim-
ized geometry was then subjected to a single-point calcu-
lation incorporating spin–orbit effects. The output of
this calculation was used as the input for the ADF
NMR utility [45].
1
at 300 MHz for H. Solution state 27Al, 11B and 119Sn
NMR spectra were recorded on a Varian INOVA-500
spectrometer operating at 500 MHz for 1H. 15N Solution
state NMR spectra were acquired on a Bruker DRX600
spectrometer operating at 600 MHz for 1H (60 MHz for
15N) and equipped with a triple resonance (1H/13C/15N)
probe. All solution NMR chemical shifts are reported
in parts per million (ppm) and coupling constants (J)
1
in Hertz (Hz). H and 13C chemical shifts are reported
with respect to the internal solvent (C6D6, 7.16 and
128.39; THF-d8, 3.58 and 1.73, 67.57 and 25.37; CDCl3,
7.27 and 77.0). 15N chemical shifts are referenced to
external neat CH3NO2 (d = 380.2 ppm with respect to
neat liquid NH3 (0.0 ppm)) [22]. Other nuclei were refer-
enced using an external standard, as follows: 19F spectra
were referenced with respect to CFCl3 (0.0 ppm); 27Al
3+
spectra were referenced to Al(D2O)6 (1.0 M AlCl3 in
D2O; 0.0 ppm); 11B spectra were referenced to neat
BF3 Æ OEt2 (0.0 ppm); 119Sn spectra were referenced to
0.5 M Sn(CH3)4 in CH2Cl2 (0.0 ppm). Solid state 15N
NMR spectra were acquired using a custom-designed
Values of the absolute shielding tensor, calculated
using density functional methods, were converted to ref-
erenced chemical shifts (d ppm) using the following
equation [46]
1
spectrometer operating at 501 MHz for H (50.8 MHz
for 15N). All spectra were acquired using a triple-reso-
nance (1H/13C/15N) magic-angle spinning (MAS) probe
from Chemagnetics (Fort Collins, CO) configured for
4.0 mm zirconium rotors. Spinning frequencies of 3–6.5
kHz were used. The identity of the isotropic peak (diso)
was confirmed by measurement of the spectra at sev-
eral different spinning speeds. Proton–nitrogen cross-
polarization (CP) under the Hartmann–Hahn match
was used to enhance the sensitivity of all 15N NMR spec-
tra [31]. Samples were referenced indirectly to the 13C
CP/MAS spectrum of adamantane or directly to the
15N CP/MAS spectrum of NH4Cl (acquired prior to each
sample acquisition). Both solution and solid state NMR
spectra were acquired at 20 ꢁC. For further details of
parameters used in the acquisition of solid state NMR
spectra refer to the Supporting Information.
dðS; calcÞ ¼ rðN2; calcÞ ꢀ rðS; calcÞ þ dðN2; refÞ; ð1Þ
where d(S, calc) is the calculated chemical shift (ppm) of
the compound of interest (S), r(N2, calc) is the calcu-
lated absolute shielding tensor for the reference com-
pound (N2 gas; r(N2, calc) = ꢀ80.012 ppm), r(S, calc)
is the calculated absolute shielding tensor of S and
d(N2, ref) is the experimental chemical shift (ppm) of
the reference compound (N2 gas). The NMR shielding
(r) of N2 (with reference to neat nitromethane) = + 74.2
ppm, therefore d (N2, ref) = 380.2ꢀ74.2 = 306 ppm (ref-
erenced to liquid NH3 at 0 ppm) [47,48].
2.6. Synthesis of 15NMo(N[tBu]Ar)3 (1)
A 300-mL 3-neck flask fitted with a 0.1 L break-seal
flask containing 15N2 was charged with KH (3.52 g, 88
mmol). THF (50 mL) was added and the slurry was stir-
red while sparging with argon for 15 min. Stirring was
paused and the headspace of the flask was evacuated.
Under a static vacuum the break-seal was opened and
the slurry was stirred vigorously as the headspace filled
with 15N2. In a second flask, Mo(N[tBu]Ar)3 (5.5 g, 8.8
mmol) was dissolved in THF (35 mL) and the solution
was stirred while sparging with argon for 10 min. Addi-
tion of the Mo(N[tBu]Ar)3 solution via syringe to the
KH/THF slurry afforded a dark orange mixture. Stir-
ring was continued for 24 h after which time the solution
was filtered through Celite. Solvent removal in vacuo
gave an orange powder, which upon dissolution in pen-
2.4. Simulation and calculation of solid state NMR
spectra
The principal components of the chemical shift ten-
sors were determined experimentally by fitting simulated
spectra to the experimental data [32,33] using Simpson
[34] (a general simulation program for NMR spectros-
copy). The residuals between the simulated spectrum
and the experimental spectrum could, in most cases, be
reduced to <5%. The Simpson program was also used
to calculate spectra based on values of the chemical shift
tensor (in ppm) that were calculated using ADF. See the
Supporting Information. for further details.

E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
2755
tane and storage at ꢀ35 ꢁC gave amber crystals of the
desired terminal nitride complex, 15NMo(N[tBu]Ar)3
(4.72 g, 7.40 mmol, 84%). This procedure is a modifica-
tion of previously reported syntheses for complex 1
[20,50,51].
CD2Cl2, 20 ꢁC) d: 89.30 (s, m1/2 = 10 Hz). 15N-1-AlCl3:
27Al NMR (130.1 MHz, CD2Cl2, 20 ꢁC) d: 87.76 (m).
Isolated yield of bright yellow 1-AlBr3 (from 0.070 g
1): 0.083 g, 0.092 mmol, 84%. Anal. Calc. for
C36H54N4MoAlBr3: C, 47.75; H, 6.01; N, 6.19. Found:
C, 47.63; H, 5.88; N, 6.12%. 1H NMR (300 MHz,
CD2Cl2, 20 ꢁC) d: 6.95 (s, 3 H, para), 5.70 (br s, 6 H,
ortho), 2.16 (s, 18 H, ArCH3), 1.35 (s, 27 H, NC(CH3)3).
13C NMR (75.0 MHz, CD2Cl2, 20 ꢁC) d: 148.2 (ipso),
138.3 (meta), 130.1 (ortho), 129.0 (para), 66.66
(NC(CH3)3), 32.50 (NC(CH3)3), 21.62 (ArCH3). pre-
sup27Al NMR (130.1 MHz, CH2Cl2, 20 ꢁC) d: 80.17
(br s, m1/2 = 85 Hz). 15N-1-AlBr3: 27Al NMR (130.1
MHz, CH2Cl2, 20 ꢁC) d: 80.18 (br s, m1/2 = 88 Hz).
Isolated yield of yellow 1-AlI3 (from 0.100 g 1): 0.111
g, 0.106 mmol, 68%. Anal. Calc. for C36H54N4MoAlI3:
C, 41.16; H, 5.18; N, 5.71. Found: C, 41.16; H, 5.23;
2.7. Synthesis of lewis acid adducts 1-LA
A solution of NMo(N[tBu]Ar)3 in n-pentane (5 mL)
was prepared in a 20-mL scintillation vial and chilled
to ꢀ35 ꢁC. In a second vial a solution of the Lewis acid
(1 equiv. in 2 mL pentane) was prepared and chilled to
ꢀ35 ꢁC. The Lewis acid solution was added to the
NMo(N[tBu]Ar)3 solution and stirred at room tempera-
ture for 1 h. Upon addition of the Lewis acid a yellow
solid precipitated from solution. Filtration of the sus-
pension, washing with pentane and subsequent drying
under a dynamic vacuum afforded the desired product.
Recrystallization from a concentrated CH2Cl2 solution
layered with pentane at ꢀ35 ꢁC afforded yellow crystals
of X3E–NMo(N[tBu]Ar)3.
Isolated yield of yellow 1-BF3 Æ CH2Cl2 (from 0.115 g
of 1): 0.092 g, 0.130 mmol, 72%. Anal. Calc. for
C36H54N4MoBF3: C, 61.19; H, 7.70; N, 7.93. Found:
C, 60.94; H, 7.76; N, 7.86%. 1H NMR (300 MHz,
CD2Cl2, 20 ꢁC) d: 6.92 (s, 3 H, para), 5.71 (br s, 6 H,
ortho), 2.15 (s, 18 H, ArCH3), 1.31 (s, 27 H, NC(CH3)3).
13C NMR (75.0 MHz, CD2Cl2, 20 ꢁC) d: 147.8 (ipso),
138.4 (meta), 129.9 (ortho), 129.1 (para), 65.95
(NC(CH3)3), 32.14 (NC(CH3)3), 21.60 (ArCH3). 19F
NMR (282 MHz, CD2Cl2, 20 ꢁC) d: 143.7 (m, BF3).
11B NMR (160 MHz, CD2Cl2, 20 ꢁC) d: ꢀ2.83 (m,
BF3). 15N-1-BF3: 11B NMR (160 MHz, CDCl3, 20 ꢁC)
d: ꢀ1.72 (br s, m1/2 = 49 Hz). 15N NMR (60 MHz,
CDCl3, 20 ꢁC) d: 591.9 (s).
Isolated yield of yellow 1-BCl3 (from 0.110 g of 1):
0.104 g, 0.138 mmol, 80%. Anal. Calc. for
C36H54N4MoBCl3: C, 57.20; H, 7.20; N, 7.41. Found:
C, 57.11; H, 7.14; N, 7.45%. 1H NMR (300 MHz,
CD2Cl2, 20 ꢁC) d: 6.98 (s, 3 H, para), 5.7 (br s, 6 H,
ortho), 2.19 (s, 18 H, ArCH3), 1.35 (s, 27 H, NC(CH3)3).
13C NMR (75.0 MHz, CD2Cl2, 20 ꢁC) d: 148.0 (ipso),
138.2 (meta), 130.8 (ortho), 128.6 (para), 67.56
(NC(CH3)3), 31.74 (NC(CH3)3), 21.65 (ArCH3). 11B
NMR (160 MHz, CD2Cl2, 20 ꢁC) d: 3.91 (s, m1/2 = 16.5
Hz). 15N-1-BCl3: 11B NMR (160 MHz, CD2Cl2, 20 ꢁC)
d: 4.02 (br s, m1/2 = 36.6 Hz).
Isolated yield of yellow 1-AlCl3 (from 0.075 g 1):
0.081 g, 0.104 mmol, 89%. Anal. Calc. for
C36H54N4MoAlCl3: C, 55.69; H, 7.01; N, 7.73. Found:
C, 55.84; H, 7.11; N, 7.22. 1H NMR (300 MHz, CD2Cl2,
20 ꢁC) d: 6.95 (s, 3 H, para), ꢁ5.6 (br s, 6 H, ortho), 2.16
(s, 18 H, ArCH3), 1.34 (s, 27 H, NC(CH3)3). 13C NMR
(75.0 MHz, CD2Cl2, 20 ꢁC) d: 148.2 (ipso), 138.4 (meta),
130.1 (ortho), 128.9 (para), 66.38 (NC(CH3)3), 32.40
(NC(CH3)3), 21.61 (ArCH3). 27Al NMR (130.1 MHz,
1
N, 5.26%. H NMR (300 MHz, CDCl3, 20 ꢁC) d: 6.46
(s, 3 H, para), 5.47 (br s, 6 H, ortho), 1.83 (s, 18 H,
ArCH3), 1.14 (s, 27 H, NC(CH3)3). 13C NMR (75.0
MHz, CDCl3, 20 ꢁC) d: 136.4 (meta), 128.1 (para),
32.36 (NC(CH3)3), 20.70 (ArCH3). 27Al NMR (130.1
MHz, CDCl3, 20 ꢁC) d: 26.03 (s, m1/2 = 226 Hz). 15N-1-
AlI3: 27Al NMR (130.1 MHz, THF, 20 ꢁC) d: 24.34
(br s, m1/2 = 357 Hz).
Isolated yield of bright yellow 1-GaCl3 (from 0.150 g
1): 0.174 g, 0.214 mmol, 91%. Anal. Calc. for
C36H54N4MoGaCl3: C, 53.06; H, 6.68; N, 6.88. Found:
C, 53.20; H, 6.62; N, 6.95%. 1H NMR (300 MHz,
CD2Cl2, 20 ꢁC) d: 6.95 (s, 3 H, para), 5.62 (br s, 6 H,
ortho), 2.16 (br s, 18 H, ArCH3), 1.33 (s, 27 H,
NC(CH3)3). 13C NMR (75.0 MHz, CD2Cl2, 20 ꢁC) d:
148.0 (ipso), 138.4 (meta), 130.1 (ortho), 128.9 (para),
66.26 (NC(CH3)3), 32.42 (NC(CH3)3), 21.61 (ArCH3).
Isolated yield of yellow 1-InCl3 (from 0.175 g 1):
0.198 g, 0.230 mmol, 84%. Anal. Calc. for
C36H54N4MoInCl3: C, 50.28; H, 6.33; N, 6.52. Found:
C, 50.41; H, 6.38; N, 6.67%. 1H NMR (300 MHz,
CDCl3/THF, 20 ꢁC) d: 6.59 (s, 3 H, para), ꢁ5.6 (br s,
6 H, ortho), 1.95 (s, 18 H, ArCH3), 1.26 (s, 27 H,
NC(CH3)3). 13C NMR (75.0 MHz, CDCl3/THF, 20
ꢁC) d: 136.7 (meta), 128.4 (ortho), 32.62 (NC(CH3)3),
21.05 (ArCH3).
Isolated yield of yellow 1-GeCl2 (from 0.075 g 1 and
0.027 g GeCl2.dioxane): 0.069 g, 0.079 mmol, 68%. Anal.
Calc. for C36H54N4MoGeCl2: C, 54.99; H, 6.92; N, 7.63.
Found: C, 55.21; H, 6.91; N, 7.12. 1H NMR (300 MHz,
CDCl3, 20 ꢁC) d: 6.88 (s, 3 H, para), 5.66 (br s, 6 H,
ortho), 2.15 (s, 18 H, ArCH3), 1.37 (s, 27 H, NC(CH3)3).
13C NMR (75.0 MHz, CDCl3, 20 ꢁC) d: 148.4 (ipso),
137.7 (meta), 129.2 (ortho), 128.8 (para), 65.11
(NC(CH3)3), 32.66 (NC(CH3)3), 21.56 (ArCH3).
Isolated yield of yellow 1-SnCl2 (from 0.165 g 1):
0.160 g, 0.193 mmol, 75%. Anal. Calc. for
C36H54N4MoSnCl2: C, 51.95; H, 6.54; N, 7.21. Found:
C, 52.28; H, 6.53; N, 6.69%. 1H NMR (300 MHz,

2756
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
CD2Cl2, 20 ꢁC) d: 6.89 (s, 3 H, para), 5.68 (br s, 6 H,
ortho), 2.14 (s, 18 H, ArCH3), 1.35 (s, 27 H, NC(CH3)3).
13C NMR (75.0 MHz, CD2Cl2, 20 ꢁC) d: 144.6 (ipso),
138.1 (meta), 129.4 (ortho), 129.1 (para), 64.60
(NC(CH3)3), 32.93 (NC(CH3)3), 21.59 (ArCH3). 119Sn
NMR (186 MHz, THF, 20 ꢁC) d: 333 (s, m1/2 = 31 Hz).
15N-1-SnCl2: 119Sn NMR (186 MHz, THF, 20 ꢁC) d:
333 (s, m1/2 = 57 Hz).
1.2 equiv.) in CH2Cl2 was prepared and similarly chilled
to ꢀ35 ꢁC. The PhC(O)OTf solution was added to the stir-
red solution of NMo(N[tBu]Ar)3 and an immediate dark-
ening of the solution to a brown-orange color was noted.
The solution was stirred at 25 ꢁC for 1 h before concentra-
tion of the solution (to ꢁ0.5 mL) and addition of cold
Et2O caused precipitation of a red powder. The solution
was filtered and the precipitate washed with pentane to af-
ford a red-orange powder in 72% yield (0.198 g, 0.225
mmol). Material suitable for elemental analysis and crys-
tallographic characterization was prepared by recrystalli-
zation from a concentrated THF solution layered with
pentane. Anal. Calc.: C, 58.63; H, 6.75; N, 6.36. Found:
C, 59.04; H, 6.73; N, 6.20%. IR (KBr plates, THF);
2.8. Synthesis of [CH3NMo(N[tBu]Ar)3]I [2a]I
Crystalline 1 (0.120 g, 0.188 mmol) was dissolved in
neat CH3I (0.8 mL, 12.8 mmol) in a 20 mL vial. The
solution was stirred at 25 ꢁC for 20 h after which time
removal of excess CH3I in vacuo and a pentane wash af-
forded a bright yellow powder in 92% yield (0.137 g,
m
(CO) 1668 cmꢀ1. 1H NMR (500 MHz, CDCl3, 20 ꢁC) d:
8.32 (m, 2 H, meta Ph), 7.83 (m, 1 H, para Ph), 7.77 (m,
2 H, ortho Ph), 7.10 (s, 3 H, para), 5.74 (br s, 6 H, ortho),
2.26 (s, 18 H, ArCH3), 1.34 (s, 27 H, NC(CH3)3). 13C
NMR (125 MHz, CDCl3, 20 ꢁC) d: 147.7 (ipso), 139.1
(meta), 136.3 (Ph), 131.5 (Ph), 131.42 (Ph), 130.0 (para),
126.8 (ortho), 72.0 (NC(CH3)3), 31.8 (NC(CH3)3), 21.7
(ArCH3). 19F NMR (282.23 MHz, CDCl3, 20 ꢁC) d:
ꢀ78.3.
1
0.173 mmol). H NMR (300 MHz, CDCl3, 20 ꢁC) d:
6.99 (s, 3 H, para), 5.67 (br s, 6 H, ortho), 5.16 (s, 3
H, NCH3), 2.19 (s, 18 H, ArCH3), 1.29 (s, 27 H,
NC(CH3)3). 13C NMR (75.0 MHz, CDCl3, 20 ꢁC) d:
145.3 (ipso), 138.6 (meta), 130.8 (para), 128.4 (ortho),
68.50 (NC(CH3)3), 65.60 (NCH3), 32.30 (NC(CH3)3),
21.70 (ArCH3). NMR data for 15N-[2a]I: 1H NMR
2
(300 MHz, CDCl3, 20 ꢁC) d: 5.11 (d, JN–H 3.3 Hz, 3
H, 15N–CH3). 15N NMR (60 MHz, CH3I/CDCl3, 20
ꢁC) d: 462.6. Anal. Calc. for C37H39D18N4MoI: [49] C,
55.63; H, 7.19; N, 7.01. Found: C, 56.13; H, 7.64; N,
6.53%.
2.11. Synthesis of H2CNMo(N[tBu]Ar)3 (3)
To a chilled suspension of [CH3NMo(N[tBu]Ar)3]I
(0.098 g, 0.126 mmol) in pentane (5 mL) was added a
chilled solution of 1.1 equiv. LiN[(Si(CH3)3]2 (0.023 g,
0.138 mmol) in pentane (4 mL). Within a few minutes
of adding base the yellow suspension turned bright
red. The suspension was stirred for 12 h at 25 ꢁC after
which time the solution had turned dark purple and
no suspended solids were observed. The solution was fil-
tered through Celite and solvent removed in vacuo to
yield a dark red-purple solid. Recrystallization from
Et2O yielded dark red crystals 0.051 g (0.077 mmol,
62%). Anal. Calc.: C, 68.017; H, 8.579; N, 8.583. Found:
C, 68.22; H, 8.51; N, 8.68%. m.p.: 124 ꢀ135 ꢁC (dec). IR
2.9. Synthesis of [(CH3)3 Si–NMo(N[tBu]Ar)3][SO3-
CF3] [2b]OTf
A 20-mL scintillation vial was charged with 1 (0.10 g,
0.157 mmol) and Et2O (2 mL). In a second vial a solution
of trimethylsilyl trifluoromethanesulfonate [CF3SO3-
Si(CH3)3] (0.174 g, 5 equiv., 0.783 mmol) in Et2O (1
mL) was prepared. The contents of the two vials were
combined and the solution stirred at 25 ꢁC for 10 h. Sol-
vent removal resulted in the isolation of a bright yellow-
orange powder; yield 89% (0.120 g, 0.139 mmol). Anal.
Calc.: C, 55.75; H, 7.32; N, 6.50. Found: C, 55.54; H,
7.75; N, 6.60%. 1H (300 MHz, CDCl3, 20 ꢁC) d: 6.91 (s,
3 H, para), 5.62 (br s, 6 H, ortho), 2.17 (s, 18 H, ArCH3),
1.34 (s, 27 H, N(CH3)3), 0.69 (s, 9 H, Si(CH3)3). 13C NMR
(75.0 MHz, CDCl3, 20 ꢁC) d: 147.7 (ipso), 137.5 (meta),
129.5 (ortho), 128.0 (para), 66.04 (NC (CH3)3), 32.44
(NC(CH3)3), 21.65 (ArCH3), 1.94 (Si(CH3)3. 19F NMR
(282 MHz, CDCl3, 20 ꢁC) d: ꢀ78.44 (s, SO3CF3). 15N
NMR (60 MHz, CDCl3, 20 ꢁC) d: 537.
(Et2O solution, cmꢀ1): 1600 (m, tNC), 1587 (s, tAr–CH ).
3
1H NMR (300 MHz, C6D6, 20 ꢁC) d: 6.69 (s, 3 H, para),
6.289 (s, 3 H, ortho), 6.287 (s, 3 H, ortho), 5.91 (s, 2 H,
NCH2), 2.16 (s, 18 H, ArCH3), 1.41 (s, 27 H,
NC(CH3)3). 13C NMR (75.0 MHz, C6D6, 20 ꢁC); d:
150.2 (ipso), 138.9 (NCH2), 137.4 (meta), 129.9 (ortho),
127.7 (para), 63.1 (NC(CH3)3), 32.8 (NC(CH3)3), 22.1
1
(ArCH3). 3-15N: H NMR (300 MHz, C6D6, 20 ꢁC) d:
2
5.90 (d JN–H 1.37 Hz, 2 H, 15NCH2). 13C NMR (75.0
1
MHz, C6D6, 20 ꢁC) d: 138.9 (d JN–C 5.78 Hz,
15NCH2). 15N NMR (60 MHz, C6D6, 20 ꢁC) d: 454.6.
2.12. Synthesis of [CH3CH2NMo(N[tBu]Ar)3]I [2d]I
2.10. Synthesis of [PhC(O)NMo(N[tBu]Ar)3]SO3CF3
[2c]OTf
Crystalline 1 (0.200 g, 0.31 mmol) was dissolved in
CH2Cl2 (2 mL) in a 20-mL scintillation vial and chilled
to ꢀ35 ꢁC. A solution of PhC(O)OTf (0.89 g, 0.37 mmol,
Crystalline H2CNMo(N[tBu]Ar)3 (3, 0.074 g, 0.113
mmol) was dissolved in neat CH3I (0.8 mL) in a 20-
mL scintillation vial. The solution was stirred at 25 ꢁC

E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
2757
for 14 h after which time removal of excess CH3I in va-
cuo and a pentane wash afforded a bright yellow powder
in 96% yield (0.086 g, 0.109 mmol). Recrystallization
from a concentrated CH2Cl2 solution layered with
Et2O afforded yellow crystals of [2d]I Æ CH2Cl2. 1H
NMR (300 MHz, CDCl3, 20 ꢁC) d: 6.98 (s, 3 H, para),
combination of 1 with 1 equiv. of Lewis acid (Fig. 2).
Yields were generally quite high and in the range of
59–91%. These adducts were typically bright yellow, or
sometimes pale orange, exhibiting limited solubility in
pentane, benzene and Et2O. The decomposition of these
adducts to starting materials was observed in 0.5–3 h in
THF solution at 25 ꢁC.
3
5.60 (br s, 6 H, ortho), 5.38 (q, 2 H, JHH 7.2 Hz,
3
NCH2CH3), 2.17 (s, 18 H, ArCH3), 1.95 (t JHH 7.2
Infrared spectroscopy yielded little information with
regard to the strength of the Mo–N or N–LA (LA = Le-
wis acid) interactions as a result of strong absorptions
(attributed to the amide ligands) in the relevant regions
of the spectrum.
Hz, 3 H, NCH2CH3), 1.27 (s, 27 H, NC(CH3)3). 13C
NMR (75.0 MHz, CDCl3, 20 ꢁC) d: 145.9 (ipso), 138.6
(meta), 130.7 (para), 128.3 (ortho), 77.6 (NCH2CH3),
68.1 (NC(CH3)3), 32.4 (NC(CH3)3), 21.7 (ArCH3),
17.5 (NCH2CH3). Anal. Calc.: C, 57.43; H, 7.48; N,
7.55. Found: C, 57.52; H, 7.44; N, 7.89%.
1
A feature observed in the H NMR spectrum of all
complexes 1-LA was significant broadening of the
ꢁ5.6 ppm resonance assigned to the six ortho protons
of the N[tBu]Ar ligands. This broadening is attributed
to hindered rotation about the Mo–Namide bond due
to increased steric congestion at the metal center.
3. Results and discussion
3.1. Synthesis and characterization
As expected, the 11B isotropic chemical shift for 1-
BF3 (ꢀ2.8 ppm) and 1-BCl3 (3.9 ppm) showed an upfield
shift of each resonance with respect to the uncoordi-
nated Lewis acid (BF3 = 10.0 ppm, BCl3 = 46.5
The first reported synthesis of NMo(N[tBu]Ar)3 (1)
from Mo(N[tBu]Ar)3 required a 3 d incubation period
at ꢀ35 ꢁC during which time N2 uptake takes place
[20]. Subsequent work focusing on the redox-catalyzed
binding and splitting of dinitrogen by Mo(N[tBu]Ar)3
has led to the development of a more efficient route to
the terminal nitride 1 [50]. Most recently, Lewis bases
were found to accelerate the uptake of N2 by Mo(N[-
tBu]Ar)3 enabling its quantitative conversion to 1 within
24 h. Potassium hydride (10 equiv. in THF) was found
to be the most efficient base for effecting this conversion
(Fig. 1) [51].
Furthermore, the ease of separation of KH from the
pentane soluble product (1) by filtration makes this
route particularly attractive. Complete characterization
for complex 1 was reported previously. We will however
make note of the 15N solution NMR shift of 1 (840 ppm)
for comparison with values reported in the following
discussions. While complex 1 has eluded characteriza-
tion by single crystal X-ray diffraction, the structure of
a related compound, NMo(N[tBu]Ph)3 [20], is pseudo
tetrahedral with the Mo„N bond aligned with a crystal-
lographic C3 axis.
1
ppm). A similar upfield shift of the 27Al resonance,
with respect to the uncoordinated Lewis acid, was ob-
served for the complexes 1-AlCl3, 1-AlBr3 and 1-AlI3
[52].
The 119Sn NMR spectrum of 1-SnCl2 displayed a
broad singlet at 333 ppm, which is shifted downfield of
the resonance for SnCl2 in THF solution (236 ppm)
[53]. The 119Sn resonance for 1-SnCl2 compares with
other SnCl2 Æ N-donor complexes such as SnCl2 Æ pyri-
dine in which the 119Sn resonance occurs at 295 ppm
[54].
In the 27Al, 11B and 119Sn NMR spectra of 15N-1-
BX3, 15N-1-AlX3 and 15N-1-SnCl2, coupling of the two
spin active nuclei (e.g., 15N–27Al) could not be resolved.
In each case the resonance for the spin-active Lewis acid
center was broadened upon coordination to 15N-1 (e.g.
11B NMR: 14N-1-BCl3 1/2 = 16.5 Hz; 15N-1-BCl3 m1/
m
2 = 36.6 Hz). Likewise, the 15N solution NMR spectrum
measured for 15N-1-BF3 displayed only a broad singlet
at 592 ppm. This value is shifted upfield quite consider-
ably (248 ppm) with respect to 15N-1.
A number of neutral Lewis acid adducts of formula
X3E–NMo(N[tBu]Ar)3 (X = F, E = B: X = Cl, E = B,
Al, Ga, In; X = Br, E = Al; X = I, E = Al) and Cl2E–
NMo(N[tBu]Ar)3 (E = Ge, Sn), were prepared by simple
Crystallographic characterization of 1-BF3 confirmed
the formation of a r-bond through donation of the ter-
minal nitride N-lone pair into the vacant p-orbital on
the Lewis acidic boron center. 2 The N–B bond distance
˚
of 1.609(7) A is similar to those reported in the literature
for other Lewis acid adducts of nitridometal complexes
N
[55–57]. Complex 1-BF3 crystallized in the space group
ꢀ
N2 (1 atm)
NtBuAr
ArtBuN Mo
Mo
P1 with the B–F bonds of the Lewis acid staggered with
NtBuAr
NtBuAr
ArtBuN
NtBuAr
10 eq KH
THF / 24 h
1
NMR shifts reported in methylcyclohexane solution; referenced to
1
BF3 Æ OEt2 at 0.0 ppm.
See Supporting Information for X-ray crystallographic details and
an ORTEP plot of 1-BF3.
2
Fig. 1. Formation of complex 1 via the base-catalyzed cleavage of
dinitrogen.

2758
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
LA
R
LA
[2a]I: R = CH3
[2b]OTf: R = Me3Si X = OTf 89%
[2c]OTf: R = PhC(O) X = OTf 72%
X = I
92%
RX
N
N
X
1
NtBuAr
NtBuAr
Mo
Mo
NtBuAr
NtBuAr
Et2O/pentane
Yield = 59-91%
ArtBuN
ArtBuN
1-LA
2
Fig. 2. Syntheses of 1-LA and the cationic imido complexes [2a]I, [2b]OTf and [2c]OTf.
Fig. 3. 35% thermal ellipsoid (ORTEP) representations of 1-GeCl2 (left) and 1-SnCl2 (right).
respect to the Mo–Namide bonds. The Mo„N bond
length (1.678(4) A) in 1-BF3 was identical (to within
3r) to the Mo„N bond in NMo(N[tBu]Ph)3 and, as ex-
pected, the Mo–N–B bond angle was nearly linear
(177.6(4)ꢁ).
Complexes 1-GeCl2 and 1-SnCl2 were found (by X-
ray crystallography) to be isomorphous (Fig. 3). To
the best of our knowledge, complexes 1-GeCl2 and 1-
SnCl2 represent the first examples of coordination at
the terminally bound nitrogen in a nitridometal complex
by a divalent germanium or tin halide [58,59]. Upon
addition of GeCl2 Æ dioxane or SnCl2 to 1, simple Lewis
acid adducts analogous to those of the Group 13 1-LA
complexes were formed. Complexes 1-GeCl2 and 1-
SnCl2 were characterized by Mo„N bond lengths of
˚
˚
ꢁ1.70 A and N–Ge (or N–Sn) bond lengths typical of
single bonds.
Transition-metal imido complexes containing an
Mo„NR bonding motif are well documented in the lit-
erature [60,61]. Reaction of 1 with electrophiles, RX
C21
Si1
O1
C41
N4
N4
Mo1
Mo1
N1
N1
Fig. 4. 50% thermal ellipsoid (ORTEP) representations of cations [2b] (left) and [2c] (right).

E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
2759
(R = CH3, X = I and R = Me3Si, X = SO3CF3 = OTf),
afforded the imido salts [2a]I [49] and [2b]OTf as bright
yellow, pentane insoluble powders in high yield (92%
and 89%, respectively, Fig. 2). In the 1H NMR spectrum
of 15N-[2a]I a two-bond coupling (2JN–H) of 3.3 Hz was
resolved for the methyl protons (15N–CH3). Solution
15N{1H} NMR spectra for [2a]I and [2b]OTf displayed
resonances at 463 and 537 ppm, respectively.
phile [29,63,64]. There are similarly few reports of
transition metal complexes containing a benzoylimido
moiety i.e., PhC(O)N„M [65–68]. X-ray crystallo-
graphic characterization of [2c]OTf showed that the
Mo„N bond is lengthened slightly with respect to the
Mo„N bond in complex 1 (Fig. 4). The N–C bond of
˚
1.422(5) A is in the range typical for an N–C single bond
suggesting that resonance forms involving N–C multiple
bond character do not provide added stabilization in
this complex.
Dehydrohalogenation of [2a]I was achieved using
lithium hexamethyldisilazide in pentane (Fig. 5) and re-
sulted in isolation of the purple ketimide complex
H2CNMo(N[tBu]Ar)3 (3). Complex 3 provides a rare
example of terminal –NCH2 complexation [69–72], and
represents the first example of such ligation to be struc-
turally characterized (Fig. 6). The ketimide N–C bond
The imido complex [2b]OTf crystallized in the cubic
space group P213 (Fig. 4). The Mo–N–Si bond angle
is linear as mandated by its alignment with the crystallo-
graphic C3 axis. The Mo„N bond distance of 1.715(6)
˚
˚
A was lengthened 0.057 A with respect to the Mo„N
bond distance in 1 while the N–Si bond distance of
˚
˚
1.795(6) A was slightly longer (ꢁ0.08 A) than might
be expected for a single N–Si bond [62]. While complex
[2a]I has not been structurally characterized, the cation
[CH3NMo(N[tBu]Ar)3]+ [2a] is presumably a 4-coordi-
nate monomer related to the trimethylsilylimido cation
[(CH3)3Si–NMo(N[tBu]Ar)3]+ [2b].
˚
distance of 1.300(7) A is consistent with the presence
of an N–C double bond. The Mo–N bond distance of
˚
1.777(4) A is in the range between that of a Mo–N dou-
Reaction of benzoyl triflate (PhC(O)OTf) with 1 in
CH2Cl2 enabled isolation of the red-orange benzoylim-
ido complex [PhC(O)NMo(N[tBu]Ar)3]OTf [2c]OTf in
72% yield. There have been very few reports of the use
of benzoyl triflate in synthetic inorganic chemistry de-
spite this reagentꢀs obvious utility as a potent electro-
ble bond and that of a Mo–N single bond. This multiple
bond character is the result of p-donation from a filled
Mo d-orbital into the ketimide p*(N–C) molecular orbi-
tal and in the perpendicular plane, p-donation from the
nitrogen lone pair of electrons into a vacant Mo d-
orbital. Complex 3 has a distorted tetrahedral geometry
H
CH
3
H
CH
3
H C
2
neat CH I
3
+ Li[N(SiMe ) ]
3 2
14 h
N
I
t
I
N
N
Mo
Mo
t
Mo
- HN(SiMe )
3 2
- LiI
N BuAr
N BuAr
t
t
t
N BuAr
Ar BuN
Ar BuN
t
t
t
Ar BuN
N BuAr
t
N BuAr
N BuAr
[2d]I (96%)
[2a]I
3 (62%)
Fig. 5. Dehydrohalogenation of [2a]I in the formation of 3 and subsequent reaction with CH3I to form [2d]I.
H41a
H41b
C22
C41
C21
N4
N2
Mo1
Mo1
N1
N1
N2
Fig. 6. 50% thermal ellipsoid (ORTEP) representation of 3 (left) and cation [2d] (right).

2760
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
Table 2
Experimental and calculated 15N chemical shielding tensors for 1 and derivatives thereof. diso is the isotropic shift, j is the span defined as d11 ꢀ d33; j
P
is the skew of the shielding tensor defined as [3(d22 ꢀ diso)/(d11 ꢀ d33)]; v2 = ((obs ꢀ calc)2)/calc
Cplx
Model
Principal components of shielding tensor (ppm)
v2
Experimental
Calculated
diso
d11
d22
d33
X
j
diso
d11
d22
d33
X
j
1
1m
1m-BF3
1m-BCl3
833
593
558
555
602
599
598
595
591
590
608
603
643
666
457
453
441
436
541
467
460
446
1229
836
773
779
864
835
828
810
809
848
879
846
918
955
614
611
594
576
744
701
613
591
1229
836
747
733
823
835
828
810
809
813
874
846
918
955
614
611
594
576
716
493
613
591
42
107
152
154
119
126
139
165
156
110
72
1187
729
621
625
745
709
689
645
653
738
807
729
825
866
471
475
457
420
581
494
460
436
1
795
630
562
1137
878
1137
875
113
137
102
1024
741
1
1-BF3
1-BCl3
1
0.99
1
12
31
0.92
0.85
0.89
1
792
791
690
1-AlCl3
1-AlBr3
1-AlI3
1m-AlCl3
624
873
872
128
745
1
4
1
1
1
1-GaCl3
1-InCl3
1m-GaCl3
1m-InCl3
0.90
0.99
1
635
645
886
902
885
902
133
131
753
771
1
1
15
30
117
93
1-GeCl2
1-SnCl2
[2a]I
1m-GeCl2
1m-SnCl2
2a-m
1
660
669
463
930
944
606
928
938
603
123
125
182
807
819
424
1
8
11
9
89
1
0.99
0.99
143
136
137
156
163
207
153
155
1
1
1
1
[2b]OTf
[2c]OTf
[2d]I
2b-m
2c-m
0.91
0.16
1
556
507
486
458
448
758
695
653
600
570
758
642
637
563
457
153
183
168
212
318
605
512
485
388
252
1
4
41
6
0.79
0.93
0.81
0.11
2d-m
3m-C3
3m-Cs
3
1
18
125
(average N–Mo–Namide bond angle 101.7ꢁ) with pseudo-
C3 point symmetry courtesy of the nearly linear
(178.0(4)ꢁ) Mo–N–C bond axis of the ketimide moiety
(which lies along the pseudo-C3 axis).
The overall sequence leading to complex 3 can be
viewed as formal carbene (CH2) addition to 1. Addition
of the Group 14 halides GeCl2 and SnCl2 to 1 did not give
products analogous to 3 (i.e., oxidation Ge/SnII ! Ge/
SnIV with concomitant formation of a molybdenum(IV)
complex Cl2E@NMo(N[tBu]Ar)3). This observation is
consistent with the increased stability of divalent Group
14 species as the group is descended [73].
Synthesis of the ethylimido complex [CH3CH2NMo-
(N[tBu]Ar)3]I [2d]I, was achieved by reaction of 3 with
CH3I (Fig. 5). This reaction demonstrates the nucleophi-
licity of the ketimide carbon in 3 and is in direct contrast
with organic ketimines (RN@CH2) which are electrophi-
lic at carbon [74]. Interestingly, we were unable to gen-
erate [2d] via the reaction of 1 with a suitable source
of [CH3CH2]+ (e.g., EtI, EtOTf, [Et3O]BF4). The inabil-
ity to generate [2d] in this way is consistent with the dra-
matic decrease in rate observed for other nucleophilic
substitution reactions (SN2) upon increasing the steric
bulk at the a-carbon of the alkyl electrophile
(methyl ꢂ ethyl > propyl) [75]. Crystallographic charac-
terization of [2d]I showed that an N–C single bond
3.2. 15N Solid state CP/MAS NMR Spectroscopy of
15NMo(N[tBu]Ar)3 (1)
Measurement of the 15N solid state CP/MAS NMR
spectrum of 15NMo(N[tBu]Ar)3 (1) was carried out in
order to probe experimentally the electronic structure
at nitrogen. The principal components of the chemical
shift tensor can be extracted from the experimental data
by simulation of the experimental spectrum or by the
analysis of the relative intensities of the spinning side-
bands [32,33]. The experimentally measured values of
the shift tensor components for 1 and each of its deriv-
atives are presented in Table 2.
The experimental, simulated and DFT calculated 15
N
solid state CP/MAS NMR spectra of 1 (calculated spec-
trum of complex 1m) are presented in Fig. 7. In the ab-
sence of unusual averaging effects, the shift parameter
measured in solution for a given complex is equal to
the isotropic chemical shift, which is calculated as the
average of the principal components of the chemical
shift tensor [diso = 1/3(d11 + d22 + d33)] [76]. Indeed, the
isotropic shift (diso) of 833 ppm determined for complex
1 from the solid state NMR spectrum is in close agree-
ment with the 15N solution NMR shift of 840 ppm.
The axial symmetry of complex 1 constrains the orienta-
tion of the principal components of the 15N chemical
shift tensor such that the unique axis is aligned along
the Mo„N bond. Our experimental results confirm
˚
˚
(1.457(11) A) and an Mo–N triple bond (1.708(9) A)
are formed upon the reaction of complex 3 with CH3I.

E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
2761
*
1000
800
600
400
200
0
15
Fig. 7. Experimental (—) and simulated (---) 15N solid state CP/MAS
NMR spectra for complex 1 and DFT calculated (---) NMR spectrum
for complex 1m. The simulated and calculated spectra are offset with
respect to the experimental spectrum for clarity. The isotropic peak
(diso) is indicated by an asterisk.
N Chemical Shift (ppm)
Fig. 8. Experimental 15N solid state CP/MAS NMR spectrum of 1-
BCl3. The expanded portion of the spectrum shows the multiplicity of
one of the spinning sidebands. The isotropic peak is marked with an
asterisk.
the axial nature of the chemical shift tensor
(d11 = d22 = 1229 ppm).
The span (X) of the 15N chemical shift tensor was re-
duced upon coordination of a Lewis acid, from 1187
ppm in 1 to between 866 ppm (1-SnCl2) and 621 ppm
(1-BCl3). While the magnitude of the most shielded com-
ponent d33 varied over ca. 90 ppm for 1-LA, the perpen-
dicular components of the shift tensor, d11 and d22
(referred to as d^), varied over ca. 180 ppm. Thus, the
variation in the span may be attributed primarily to
changes in the perpendicular component of the 15N
chemical shielding tensor (d^). Again, this observation
can be explained by analysis of the density functional
calculations (Section 4.2).
A feature of the spectrum that is immediately obvious
is the enormous span (X = d11–d33) of the chemical shift
anisotropy (CSA) tensor (X = 1187 ppm). An under-
standing of the origin of this significant paramagnetic
shift at nitrogen, when the external magnetic field is ori-
ented perpendicular to the Mo„N bond, was ascer-
tained by analysis of density functional calculations
(see Section 4.1) performed on the model complex
NMo(NH2)3 (1m).
3.3. 15N solid state CP/MAS NMR spectroscopy of the
Lewis acid adducts of 1 (1-LA)
The 15N solid state NMR spectrum of 1-BCl3 dis-
played multiplicity in the isotropic peak and each of
the associated spinning sidebands (Fig. 8).
The isotropic shift of 1-BF3 (diso = 593 ppm) deter-
mined from the experimental 15N solid state NMR spec-
trum is in excellent agreement with diso (591 ppm)
measured in solution. Complex 1-BF3 exhibited an axi-
ally symmetric chemical shift tensor wherein
Two unique sets of data, which differed in the magni-
tude of the isotropic shift (558 and 555 ppm) and the
span of the chemical shift tensor (621 and 625 ppm),
were obtained on simulation of the experimental spec-
trum. Similar multiplicity was observed in the spectra
acquired for 1-AlI3 and 1-InCl3. This feature could be
attributed either to coupling of 15N with the adjacent
spin-active nucleus or structural polymorphism [77–79].
d
11 = d22 = 836 ppm. Solid state 15N NMR spectra ob-
tained for complexes 1-BCl3, 1-AlCl3, 1-GaCl3 and 1-
InCl3 indicate lower symmetry for the geometries of
these complexes (j < 1). The isotropic shift for the Lewis
acid adducts (1-LA) studied was shifted upfield by 167
ppm (1-SnCl2) to 278 ppm (1-BCl3) with respect to the
terminal nitride (1). This upfield shift in the magnitude
of diso can be explained by an increase in the energy
gap between occupied frontier molecular orbitals and
low-lying virtual molecular orbitals upon coordination
of a Lewis acid to the terminally bound N-atom. This
information can be extracted from density functional
calculations performed on model complexes (1m-LA)
and is discussed in Section 4.2.
3.4. 15N solid state CP/MAS NMR spectroscopy of the
imido complexes [2a]I, [2b]OTf, [2c]OTf and [2d]I
The axial symmetry of cation [2a] was apparent from
the tensor components determined by simulation of the
15N solid state NMR spectrum (d11 = d22). The isotropic
shift obtained from 15N solid state NMR measurements
(457 ppm) was in excellent agreement with diso obtained
in solution (463 ppm). The spectrum acquired for the

2762
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
ethylimido complex [2d]I was very similar to that of the
methylimido complex [2a]I in terms of the isotropic
shift, span and skew of the chemical shift tensor.
(d11 = d22 = 591 ppm). This symmetry is expected due
to the Mo–N multiple bonding in 3 which is attributed
to the p-donor/acceptor character of the ketimide
(NCH2) ligand. The isotropic peak in the 15N solid state
NMR spectrum was in close agreement with that ob-
tained in solution. Complex 3 has the most shielded va-
lue of d11 with respect to all other complexes measured
in this study and a narrow span of the shift tensor
(X = 436 ppm).
Complex [2b]OTf showed unexpected asymmetry in
the values of the chemical shift tensor (i.e., d11 ¼ d22).
Interestingly, the spectrum of [2b]OTf was more reminis-
cent of those acquired for the Lewis acid adducts (e.g. 1-
BCl3) in terms of the magnitude of the chemical shift
tensors (dnn) and the span of the spectrum (X) than
either of the imido complexes [2a]I or [2d]I. Again, diso
obtained for complex [2b]OTf from solid state NMR
experiments (541 ppm) was in close agreement with
the isotropic shift measured in solution (537 ppm).
The acylimido complex [2c]OTf is unique in the series
of imido complexes that are presented in this study for
two reasons: (i) significant asymmetry is present in
[2c]OTf due to the coordination of the [PhC(O)]+ electr-
ophile to the terminal nitrogen; (ii) the presence of a car-
bonyl functional group on the fragment coordinated to
N provides an opportunity for resonance stabilization of
this imido complex. The asymmetric geometry of com-
plex [2c]OTf was confirmed by the 15N solid state
NMR spectrum for which a rhombic chemical shift ten-
sor (d11 ¼ d22 ¼ d33) was observed (d11 = 701 ppm,
4. Chemical shielding: experimental measurement and
computational analysis [76]
The measurement of solid state NMR spectra allows
for the experimental determination of the principal com-
ponents (d11, d22 and d33) of the chemical shift tensors.
To complement our experimental measurements and
enable a more complete analysis of these compounds
at the electronic level, DFT calculations were performed
on model complexes in which the bulky amido ligands
N[tBu]Ar were replaced by three NH2 ligands (for com-
putational expediency). In most cases, the calculated
tensors compared favorably with the experimental
4
d
22 = 493 ppm and d33 = 207 ppm).
data thus discussion of the electronic structure based
The span (X) of the 15N chemical shift tensors for the
on DFT calculations can be assumed to be valid. A sum-
mary of the analysis of the chemical shielding tensors,
together with equations relevant to this analysis, are pre-
sented here in brief.
The absolute shielding tensor r (calculated using
DFT), is related to the chemical shift tensor d
four imido complexes was in the range of 420–581 ppm,
which is smaller than for both 1 (X = 1187 ppm) and the
LA adducts (1-LA X = 866–621 ppm). This reduction in
the span could be attributed (in all four complexes) to
the reduced magnitude of the most deshielded compo-
nent of the chemical shift tensor, d11. Again, analysis
of density functional calculations performed on model
complexes enabled a rationalization of this effect in
terms of perturbation of the electronic structure of 1
upon coordination of an electrophile (Section 4.3).
Multiplicity in the isotropic peak and the associated
spinning sidebands was observed in the 15N solid state
d ¼ rref ꢀ r;
ð2Þ
where rref is the calculated absolute shielding tensor
of the reference compound. NMR calculations based
on the density functional method have been used to
identify the individual contributions to the total shield-
ing tensor made by the diamagnetic (rdia), paramagnetic
(rpara) and spin orbit (rso) components
3
NMR spectrum of [2a]I. Four isotropic peaks were
identified and the span of the chemical shift tensor var-
ied between 420 and 475 ppm. Unlike the Lewis acid ad-
ducts (1-LA) for which this multiplicity might be
attributed to the adjacent spin-active nucleus, complex
[2a]I is thought to display this multiplicity due to struc-
tural polymorphism [77–79].
rtotal ¼ rdia þ rpara þ rso:
ð3Þ
For complexes presented in this study, spin–orbit
contributions to the total shielding tensor were included
[80,81] but in most cases this contribution was found to
be negligible. 5 The diamagnetic contribution is depend-
ent upon the core electron density and acts to reduce the
applied magnetic field at the nucleus under observation.
In the presence of an applied field rdia will generally
show insignificant variation in its contribution to the to-
tal shielding tensor. The paramagnetic contribution
3.5. 15N NMR spectroscopy of H2CNMo(N[tBu]Ar)3
(3)
The experimental 15N solid state NMR spectrum of 3
displays a number of noteworthy features. First, the
principal components of the chemical shift tensor sug-
4
Supporting Information: Section 2.15. Plot of the experimental
gest that complex
3
is axially symmetric
15N chemical shift tensor (d11, ppm) vs. Calculated 15N chemical shift
tensor (d11, ppm).
For most of the complexes studied the spin–orbit contribution to
3
5
See Supporting Information for experimental, simulated and
calculated 15N solid state CP/MAS NMR spectra for [2a]I.
the principal components of the chemical shift tensor was <5 ppm.

E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
2763
reinforces the applied field and is determined primarily
by magnetic perturbation of the frontier molecular
orbitals. Overall, rpara shows the most significant varia-
tion in its contribution to the total shielding tensor due
to the sensitivity of this term to changes in the electronic
environment [82,83]. The paramagnetic contribution to
the shielding tensor (rpara) is proportional to the mag-
netic coupling of high-lying occupied molecular orbitals
with low-lying virtual molecular orbitals (Mk) and inver-
sely proportional to the energy difference between these
orbitals (eocc ꢀ evir). The gauge including atomic orbitals
(GIAO) formalism makes it possible to analyze the con-
tributions to the shielding in terms of orbital contribu-
tions [84–89]. A molecular orbital analysis of these
contributions is contained within the NMR output file
symmetry with a relatively low-lying doubly degenerate
E set of p-symmetry.
For 1 and the derivatives presented in this article,
there is a much less significant variation in the magni-
tude of the most shielded component (d33) compared
to either d11 or d22. Where as d11(expt) varies over 653
ppm (1229–576 ppm), d33(expt) varies over a much smal-
ler range of 165 ppm (from 207 to 42 ppm). This can be
understood if one considers the chemical shift for a lin-
ear molecule. The shift in C symmetric molecules is
1
dominated by the diamagnetic contribution; the para-
magnetic contribution disappears completely along the
molecular axis. In 1, a paramagnetic component to the
shift tensor is introduced when the C symmetry of
1
the Mo„N axis is broken by the addition of ligands
to the metal. This paramagnetic component of d33 is
fairly constant for all of the complexes presented herein.
6
of a density functional calculation. This analysis pro-
vides a list of the leading contributions to the kth compo-
nent (k = 1, 2, 3) of the magnetic field and includes
information about the individual pairs of molecular
orbitals (which mix in the applied field), the energy dif-
ference between these orbitals (eocc ꢀ evir) and the mag-
nitude of the coupling (Mk). Occasionally, one pair of
magnetically coupled molecular orbitals is found to pro-
vide the major contribution to the paramagnetic
deshielding for each component of the magnetic field
(k1, k2 and k3).
4.2. Theoretical studies on the Lewis acids adducts of 1
(1-LA)
DFT calculations performed on the model complexes
1m-LA predicted the Lewis acid adducts to be axially
symmetric, as evidenced by the calculated skew of the
tensor (j = 1 or 0.99 in all cases). Experimental 15N solid
state NMR spectra for 1-BCl3, 1-AlCl3 and 1-GaCl3,
indicate that the geometry of these complexes is lower
than the symmetry to which our DFT calculations con-
verged (j < 1). The optimized symmetry of 1m-BF3 pre-
dicted similar Mo–N and N–B bond lengths to those
measured for 1-BF3, however, in the model complex
the B–F bonds of the Lewis acid were eclipsed with re-
spect to the Mo–Namide bonds. Calculations were per-
formed on model complexes in which either the
eclipsed or staggered geometries were enforced in order
to gauge the effect of rotation about the N–LA bond on
the 15N chemical shift. In general, calculations for the
eclipsed and staggered geometries predicted very similar
values of the principal components of the 15N chemical
shift tensor.
4.1. Theoretical studies on 15NMo(N[tBu]Ar)3 (1)
The NMR shift calculation for 1m predicted an axi-
ally symmetric chemical shift tensor in which
d
11(calc) = d22(calc) = 1137 ppm. The primary contribu-
tions to d11(calc) and d22(calc) resulted from rotational
mixing in the presence of an applied field of HOMO–
2
3, r(N pz–Mo dz ) with LUMO, p* (N py–Mo dyz)
and LUMO + 1, p* (Npx–Mo dxz). There is a relatively
small energy gap (eocc ꢀ evir) between the filled N pz or-
bital (of A symmetry in the point group C3), which con-
tains the nitrogen lone pair, and the empty p*(Mo–N)
orbitals (a degenerate E set in C3). Since the paramag-
netic contribution to the chemical shielding tensor is
proportional to the coupling between these virtual and
occupied orbitals and inversely proportional to the en-
ergy gap between them, it follows that the components
of the shift tensor (d11 and d22) that are aligned perpen-
dicular to the Mo„N bond will exhibit extreme para-
magnetic shifts.
The large paramagnetic contribution to the perpen-
dicular components of the chemical shift tensor (d^) is
reminiscent of that observed for related terminal phos-
phide and carbide complexes [19,90]. In all three exam-
ples, the principal contributions to d^ are the result of
mixing in the applied field of a high-lying orbital of r-
In each of the model Lewis acid adducts (1m-LA) the
principal contributions to d^ resulted from rotational
mixing in the applied field of an occupied r-orbital with
a vacant p*(Mo–N) orbital. The occupied orbital is
characterized by significant N pz character, involved in
donation of the N-lone pair into the vacant orbital on
the Lewis acidꢀs central atom. For example, analysis
of the DFT calculation performed on 1m-BCl3 shows
that the principal contributions to d11 and d22 are pro-
vided by mixing in the applied field of HOMO ꢀ 4,
r(N pz–B pz) with LUMO, p*(Npy–Mo dyz) and
7
LUMO + 1, p*(N px–Mo dxz), respectively.
6
7
Supporting Information: Section 2.13. Representative input and
output files of an NMR calculation.
See Supporting Information 2.17 for selected (calculated) orbitals
of 1m-BCl3.

2764
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
The energy gap (eocc ꢀ evir) between the occupied
tions to d11 and d22 (d11(calc) = 606 ppm, d22(calc) = 603
ppm) are provided by mixing in the applied field of
HOMO–2, p(Mo dxz–N px) and HOMOꢀ3, p(Mo
r(NꢀLA) bonding orbital and the vacant p(Mo–N)
orbitals was found to depend principally on the extent
to which Lewis acid coordination to the terminal N-
atom lowers the energy of the r(N pz –LA sp) orbital.
In 1, the occupied (N pz) orbital contains a non-bonding
lone pair of electrons and is at a relatively high energy.
Formation of a r-bond to the Lewis acid lowers the en-
ergy of this orbital thereby increasing the energy gap be-
tween the magnetically coupled virtual and occupied
orbitals (eocc ꢀ evir). This in turn reduces the paramag-
netic contribution to the perpendicular component of
the chemical shift tensor resulting in more upfield shifted
values of d11 and d22.
d –N p ) with LUMO + 2, r (N p –C p ).
*
yz
y
z
z
Experimental data acquired for complex [2b]OTf dis-
played a rhombic 15N chemical shift tensor wherein the
skew of the shielding tensor j = 0.91. Since calculations
performed on cation [2b-m] provided the expected axi-
ally symmetric spectrum it is difficult to identify the elec-
tronic features of complex [2b]OTf that contribute to the
asymmetry of the experimental spectrum.
The lack of an axis of symmetry in the optimized
geometry for model cation [2c-m] is immediately appar-
ent from the calculated values of the chemical shift tensor
for [2c-m] (d11(calc) = 695 ppm, d22(calc) = 642 ppm and
Complex 1m-BCl3 has a smaller energy gap (eocc
ꢀ
evir = 3.65 eV) compared to 1m-BF3 (eocc ꢀ evir = 5.06
eV) but also a reduced paramagnetic contribution to
the chemical shift, as evidenced by the more upfield shift
of d11(calc). This apparently anomalous result can be ex-
plained if one accounts for the proportional relationship
between the paramagnetic contribution to the chemical
shift and the magnitude of the coupling (Mk) of occu-
pied and virtual molecular orbitals in the presence of
an applied magnetic field. Analysis of the DFT calcula-
tions for these two complexes shows that Mk in 1m-BF3
(0.438) is more than twice that calculated for 1m-BCl3
(0.206). This suggests that a much larger contribution
to the paramagnetic term is made by N-based orbitals
in 1m-BF3 compared to 1m-BCl3.
d33(calc) = 183 ppm). Our experimental results for com-
plex [2c]OTf displayed greater asymmetry (j = 0.16)
compared to the calculated spectrum (j = 0.79). Analysis
of DFT calculations performed on cation [2c-m] shows
that the principal contribution to d11(calc) is provided by
mixing in the applied field of HOMO ꢀ 3, r(N pz–C pz)
with LUMO + 1, p*(Mo dxz–N px). The principal contri-
bution to d22(calc) is provided by mixing in the applied
field of HOMO ꢀ 9, p(Mo dxz–N px) with LUMO+2,
r (N p –C p ). For [2c-m] the r(N p –C p ) orbital is
*
z
z
z
z
much higher in energy than the r-orbital comprising
the N–C bond in [2a-m] and [2d-m]. Indeed, the energy
ordering of the frontier orbitals in [2c-m] is more reminis-
cent of that seen in the Lewis acid adducts (1-LA) than
the other imido complexes. This implies that a weaker
bonding interaction exists between the terminal N-atom
and the electrophilic C-atom of the benzoyl fragment
(compared to the interaction in other imido complexes)
suggesting that resonance forms do not provide signifi-
cant stabilization in complex [2c]OTf. These facts are
consistent with the crystal structure of [2c]OTf in which
the N–C single bond does not appear to be shortened
through additional p-interactions.
4.3. Theoretical studies on the imido cations [2a-m], [2b-
m], [2c-m] and [2d-m]
Calculations performed on the model cation [2a-m]
predicted an axially symmetric spectrum (d11(calc)
ꢃ
d
22(calc)), which was in close agreement with the data ob-
tained experimentally for this complex. Upon formation
of an N–C single bond in cation [2a-m] there is a more
significant lowering in energy of the r(N–C) molecular
orbital compared to that observed for the Lewis acid ad-
ducts (1-LA) discussed previously. This can be attrib-
uted to the build up of positive charge on the complex
and the increased bond strength of the r(N–C) bond
compared to the r(N–LA) bond. All four of the imido
4.4. Theoretical studies on H2CNMo(N[tBu]Ar)3 (3)
The overall sequence leading to ketimide 3 embodies
formal carbene (CH2) addition to complex 1, a process
that encompasses C@N bond formation along with the
reduction of molybdenum from the +6 to the +4 formal
oxidation state. 8 Carbene addition to 1 is recognized as
a potentially valuable means for the activation of com-
plex 1 in the context of nitrogen atom transfer from
N2 into organic molecules [91]. It was therefore of inter-
est to determine what insight solid state NMR spectros-
copy could provide, in conjunction with DFT analysis
of the experimental shift tensors into the electronic
complexes exhibit a significant energy gap (eocc ꢀ evir
)
between the filled r(N–E) orbital to the lowest lying
unoccupied orbitals of p-symmetry. The stabilization
of this r(N–E) bonding orbital prohibits its participa-
tion in mixing interactions that supplement the para-
magnetic contribution to the chemical shift tensor. For
the model cation [2a-m] the principal contributions to
d^ result from rotational mixing in the applied field of
two occupied orbitals of p-symmetry (a doubly degener-
ate E set) with a vacant orbital of r-symmetry (A sym-
metry in C3). Analysis of the DFT calculation
performed on [2a-m] shows that the principal contribu-
8
DHRXN of CH2 addition to NMo (NH2)3 is calculated to be ꢀ93
kcal molꢀ1
.

E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
2765
3
H41b
H41a
*
C41
N4
Mo1
700 600 500 400 300 200 100
0
N1
15
N Chemical shift (ppm)
3m-C
3
*
H41b
H41a
C41
700 600 500 400 300 200 100
0
15
N4
N Chemical shift (ppm)
*
Mo1
3m-C
s
N1
700 600 500 400 300 200 100
0
15
N Chemical shift (ppm)
Fig. 9. Optimized geometries for the complexes 3m-C3 (top) and 3m-Cs (bottom) and solid state 15N spectra for complex 3: experimental (top),
calculated 3m-C3 (middle) and calculated 3m-Cs (bottom). The isotropic peak is indicated by an asterisk.
structure at this N-atom which is activated toward pro-
ductive removal from the molybdenum complex into an
organic product.
sitive. Analyses of the NMR calculation performed on
3m-C3 reveals that the principal contributions to the
most deshielded components of the tensor, d11(calc)
and d22(calc) (600 and 563 ppm, respectively), result
from mixing in the applied field of HOMO, p(N–C)
with LUMO, p*(Mo dyz–N py) and LUMO + 1,
p*(Mo dxz–N px). The same orbitals are responsible
for the paramagnetic contributions to d11(calc) and
d22(calc) in 3m-Cs (d11 = 570 ppm, d22 = 457 ppm) how-
ever, the energy gap (eocc ꢀ evir) between the HOMO
and LUMO orbitals in 3m-C3 is appreciably smaller
than in 3m-Cs (1.498 eV versus 1.696 eV). This in-
creased energetic separation of the orbitals in 3m-Cs re-
duces the degree of magnetic coupling and results in a
more shielded value of d11.
Interestingly, attempts to use DFT to reproduce the
observed 15N solid state CP/MAS NMR spectrum of 3
using H2CNMo(NH2)3, (the structure of which con-
verged to near Cs symmetry) as the computational
model were not entirely satisfactory. We probed
further and found that the larger model
H2CNMo(N[CH3]Ph)3 (3m) converged either to a pseu-
do-Cs (3m-Cs) or to a pseudo-C3 (3m-C3) conformation
depending upon the initial geometry employed. The
15N spectrum calculated for 3m-C3 was a good match
to the observed spectrum of 3 (v2 = 18), while that cal-
culated for 3m-Cs deviated noticeably (v2 = 125)
(Fig. 9). The two local minima for 3m differ with re-
spect to rotation about the Mo–Namide bonds, leading
to differing Mo–Namide dpꢀpp overlap, to which the
ketimide nitrogen electronic environment is clearly sen-
In addition, the calculated molecular orbitals for 3m-
C3 show significant p orbital contribution from the
ketimide carbon to the HOMO. This indicates that the
ketimide carbon has been rendered nucleophilic upon

2766
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
dehydrohalogenation of the methylimido complex [2a]I.
Experimental evidence for this is demonstrated by the
reaction of 3 with CH3I to generate the ethylimido com-
plex [2d]I.
of Health for support via a National Research Service
Award post-doctoral fellowship (F32 NS42425-01).
The authors acknowledge the significant contribu-
tions made to inorganic and organometallic chemistry
by Prof. Malcolm L.H. Green and the inspiration his
work has provided to many of his colleagues during
the course of his distinguished career.
5. Conclusions
The electronic structure of a dinitrogen-derived ter-
minal molybdenum nitride complex NMo(N[tBu]Ar)3
(1) has been studied by the combined use of 15N solid
state CP/MAS NMR spectroscopy and DFT calcula-
tions. Parallels may be drawn between the bonding ob-
served in this terminal nitride complex and the
analogous terminal phosphide and carbide complexes.
Perturbation of the electronic structure in the terminal
molybdenum nitride complex (1) upon coordination of
a Lewis acid can be attributed to stabilization of a r-
symmetric orbital as a result of N–LA bond formation.
Upon formation of a Lewis acid adduct of 1, the in-
creased energy gap (eocc ꢀ evir) between magnetically
coupled occupied and virtual orbitals, is seen to correlate
with a reduction in the magnitude of the perpendicular
components of the chemical shift tensor (d11 and d22).
The reaction of 1 with electrophiles RX, results in
further stabilization of the r-symmetric orbital contain-
ing the N-lone pair of electrons. The paramagnetic con-
tribution to the chemical shift tensor in this series of
cationic imido complexes results from rotational mixing
in the applied field of high-lying p(Mo–N) orbitals with
a low-lying r(N–E) (E = C, Si) orbital.
Finally, the strength of this combined approach to
studying the electronic structure of spin-active nuclei
in transition metal complexes is highlighted by our
investigations into a ketimide complex, H2CNMo(N[t-
Bu]Ar)3 (3). Computational studies performed on the
model complex H2CNMo(N[CH3]Ph)3 (3m) identified
two geometries to which this complex could converge.
The variation in Mo–N dp–pp overlap in the two struc-
tures is reflected in the calculated chemical shift tensors.
A quantitative comparison of the experimental spectrum
with the calculated spectra indicates that a closer match
is provided by the pseudo-C3 symmetric model, the
optimized geometry of which is in good agreement with
X-ray crystallographic data for complex 3.
Appendix A. Supplementary material
Complete crystallographic data for complexes 1-BF3
(CCDC-236925), 1-GeCl2 (CCDC-236927), 1-SnCl2
(CCDC-236930), [2b]OTf (CCDC-236926), [2c]OTf
(CCDC-236928), [2d]I (CCDC-236929) and 3 (CCDC-
236924) have been deposited in the Cambridge Structural
Database and can be obtained free of charge via
www.ccdc.cam.ak/conts/retrieving.htm (or from the
Cambridge Crystallographic Data Center, 12 Union
Road, Cambridge CB21EZ, UK; (fax: +44-1223-336-
033; or deposit@ccdc.cam.ac.uk). Supplementary Infor-
mation for this article includes the following materials:
ORTEP plots, anisotropic displacement parameters,
atomic coordinates, bond lengths and angles for com-
plexes 1-BF3, 1-GeCl2, 1-SnCl2, [2b]OTf, [2c]OTf, [2d]I
and 3; details of DFT calculations including optimized
geometries of all model complexes and selected (calcu-
lated) orbitals for complexes 1m, 1m-BCl3, [2b-m], [2c-m]
and 3m-C3; experimental information for the acquisition
of solid state spectra; representative experimental, simu-
lated and calculated solid state 15N CP/MAS spectra. Sup-
plementary data associated with this article can be found,
in the online version at doi:10.1016/j.poly.2004.08.010.
References
[1] D.D. Laws, H.-M.L. Bitter, A. Jerschow, Angew. Chem., Int. Ed.
41 (2002) 3096.
[2] R.G. Griffin, Nat. Struct. Biol. 5 (1998) 508.
[3] S.J. Opella, K.M. Morden, in: S.E. Harding, A.J. Rowe (Eds.),
Dynamic Properties of Biomolecular Assemblies, The Royal
Society of Chemistry, Cambridge, 1989, p. 196.
[4] E.Y. Chekmenev, Q. Zhang, K.W. Waddell, M.S. Mashuta, R.J.
Wittebort, J. Am. Chem. Soc. 126 (2004) 379.
[5] S. Luca, H. Heise, M. Baldus, Acc. Chem. Res. 36 (2003) 858.
[6] A. Shoji, S. Ando, S. Kuroki, I. Ando, G.A. Webb, Structural
studies of peptides and polypeptides in the solid state by nitrogen-
15 NMR, Ann. Rep. NMR Spectrosc. 26 (1993) 55.
[7] C.P. Jaroniec, J. Lansing, B. Tounge, M. Belenky, J. Herzfeld,
R.G. Griffin, J. Am. Chem. Soc. 123 (2001) 12929.
[8] A.E. McDermott, T. Polenova, A. Bockmann, K.W. Zilm, G.T.
Montellione, J. Biomol. NMR 16 (1999) 209219.
Acknowledgements
For support of this work, we are grateful to the US
National Science Foundation (CHE 0316823) and the
National Institutes of Health (EB002026). Christopher
Clough is thanked for his assistance with crystallogra-
phy. We thank Dr. Christopher D. Incarvito for his crys-
tallographic expertise in the structural refinement of
complex 1-SnCl2. N.M.L. thanks the National Institutes
[9] F. Castellani, B. van Rossum, A. Diehl, M. Schubert, K. Rehbein,
H. Oschkinat, Nature 420 (2002) 98.
[10] D.L. Bryce, R.E. Wasylishen, Phys. Chem. Chem. Phys. 4 (2002)
3591.
[11] Y. Huang, D.F.R. Gilson, I.S. Butler, J. Chem. Soc., Dalton
Trans. (1992) 2881.
[12] R. Salzmann, M. Wojdelski, M. McMahon, R.H. Havlin, E.
Oldfield, J. Am. Chem. Soc. 120 (1998) 1349.

E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
2767
[13] G.M. Bernard, R.E. Wasylishen, Phys. Organomet. Chem. 3
(2002) 165.
Chapter 9, 27Al NMR (reference [Al(D2O)6]3+, 0.0 ppm) Al2Cl6 (2
M, Et2O) d105 ppm; Al2Br6 (1.5 M, Et2O) d95 ppm; Al2I6 (1.5 M,
Et2O) d40 ppm.
[14] R. Salzmann, C.J. Ziegler, N. Godbout, M.T. McMahon, K.S.
Suslick, E. Oldfield, J. Am. Chem. Soc. 120 (1998) 11323.
[15] N. Godbout, L.K. Sanders, R. Salzmann, R.H. Havlin, M.
Wojdelski, E. Oldfield, J. Am. Chem. Soc. 121 (1999) 3829.
[16] R. Gobetto, Mater. Chem. Phys. 29 (1991) 221.
[17] M. Mehring, Principals of High Resolution NMR in Solids, 2nd
ed., Springer-Verlag, Berlin, 1983.
[53] P.C. Lauterbur, J.J. Burke, J. Am. Chem. Soc. 83 (1961) 326.
[54] C.C. Hsu, R.A. Geanangel, Inorg. Chem. 19 (1980) 110.
[55] L.H. Doerrer, A.J. Graham, M.L.H. Green, J. Chem. Soc.,
Dalton Trans. (1998) 3941.
[56] R. Dantona, E. Schweda, J. Stra¨hle, Z. Naturforsch. B 39 (1984)
733.
[57] U. Abram, B. Schmidt-Brucken, A. Hagenbach, M. Hecht, R.
¨
[18] M.J. Duer (Ed.), Solid-state NMR: Principals and Applications,
Blackwell Science, Oxford, 2002.
Kirmse, A. Voigt, Z. Anorg. Allg. Chem. 629 (2003) 838.
[58] Tin halide adducts with N-donor molecules: J.D. Donaldson,
D.G. Nicholson, Inorg. Nuc. Chem. Lett. 6 (1970) 151.
[59] W.W. Du Mont, H.J. Kroth, Naturforsch. B 35 (1980) 700.
[60] W.A. Nugent, J.M. Mayer, Metal–Ligand Multiple Bonds, Wiley,
New York, 1988.
[19] G. Wu, D. Rovnyak, M.J.A. Johnson, N.C. Zanetti, D.G.
Musaev, K. Morokuma, R.R. Schrock, R.G. Griffin, C.C.
Cummins, J. Am. Chem. Soc. 118 (1996) 10654.
[20] C.E. Laplaza, M.J.A. Johnson, J.C. Peters, A.L. Odom, E. Kim,
C.C. Cummins, G.N. George, I.J. Pickering, J. Am. Chem. Soc.
118 (1996) 8623.
[61] R.A. Eikey, M.M. Abu-Omar, Coord. Chem. Rev. 243 (2003) 83,
and references therein.
˚
[62] N–Si single bond distances are typically ca. 1.711 A: D.R. Lide
[21] C.E. Laplaza, C.C. Cummins, Science 268 (1995) 861.
[22] W. von Philipsborn, R. Muller, Angew. Chem., Int. Ed. 25 (1986)
¨
383.
(Ed.), CRC Handbook of Chemistry and Physics, 86th ed., CRC
Press, London, 2003, p. 9.
[23] The ADF package utilized in this study allows the calculation of
NMR shielding tensors: G. Schreckenbach, T. Ziegler, J. Phys.
Chem. 99 (1995) 606.
[63] F. Effenberger, E. Sohn, G. Epple, Chem. Ber. 116 (1983) 1195.
[64] Dissociation tendency of benzoyl triflate: F. Effenberger, G. Epple,
J.K. Eberhard, U. Buehler, E. Sohn, Chem. Ber. 116 (1983) 1183.
[65] D.E. Wigley, Prog. Inorg. Chem. 42 (1994) 239.
[66] A.J. Nielson, P.A. Hunt, C.E.F. Rickard, P. Schwerdtfeger, J.
Chem. Soc., Dalton Trans. (1997) 3311.
[24] G. Schreckenbach, T. Ziegler, Int. J. Quantum Chem. 60 (1996)
753.
[25] G. Schreckenbach, T. Ziegler, Int. J. Quantum Chem. 61 (1997)
899.
[26] S.K. Wolff, T. Ziegler, J. Chem. Phys. 109 (1998) 895.
[27] G. Schreckenbach, T. Ziegler, Theor. Chem. Acc. 99 (1998) 71.
[28] A.B. Pangborn, M.A. Giardello, R.H. Grubbs, R.K. Rosen, F.J.
Timmers, Organometallics 15 (1996) 1518.
[67] C. Tong, L.A. Bottomley, J. Porphyrins Phthalocyanines 2 (1998)
261.
[68] C.R. Clough, J.B. Greco, J.S. Figueroa, P.L. Diaconescu, W.M.
Davis, C.C. Cummins, J. Am. Chem. Soc. 126 (2004) 7742.
[69] N.C. Mo¨sch-Zanetti, R.R. Schrock, W.M. Davis, K. Wanninger,
W.S. Seidel, M.B. OꢀDonoghue, J. Am. Chem. Soc. 119 (1997)
11037.
[29] L. Brown, M. Koreeda, J. Org. Chem. 49 (1984) 3875.
[30] D.D. Perrin, W.L.F. Armarego, Purification of Laboratory
Chemicals, 3rd ed., Pergamon Press, Oxford, 1988.
[31] Theoretical basis of the cross polarization experiment: S.R.
Hartmann, E.L. Hahn, Phys. Rev. 128 (1962) 2042.
[32] M.M. Maricq, J.S. Waugh, J. Chem. Phys. 70 (1979) 3300.
[33] J. Herzfeld, A.E. Berger, J. Chem. Phys. 73 (1980) 6021.
[34] M. Bak, J.T. Rasmussen, N.C. Nielsen, J. Magn. Reson. 147
(2000) 296.
[70] P.A. Shapley, J.M. Shusta, J.L. Hunt, Organometallics 15 (1996)
1622.
[71] A. Hills, D.L. Hughes, C.J. Macdonald, M.Y. Mohammed, C.J.
Pickett, J. Chem. Soc., Dalton Trans. (1991) 121.
[72] J. Chatt, R.J. Dosser, G.J. Leigh, Chem. Commun. (1972) 1243.
[73] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd
ed., Butterworth-Heinemann, Oxford, 1998, p. 375.
[74] Ketimines undergo reactions with organolithium reagents (R0Li)
to form R(Li)N–C(R0)H2 via addition of nucleophilic [R0]ꢀ to the
electrophilic carbon of the ketimine: J. Huet, Bull. Soc. Chim. Fr.
(1964) 952, p. 960, 967 and 973.
[35] E.J. Baerends, D.E. Ellis, P. Ros, Chem. Phys. 2 (1973) 41.
[36] L. Versluis, T. Ziegler, J. Chem. Phys. 322 (1988) 88.
[37] G. te Velde, E.J. Baerends, J. Comput. Phys. 99 (1992) 84.
[38] C. Fonseca Guerra, J.G. Snijders, G. te Velde, E.J. Baerends,
Theor. Chem. Acc. 99 (1998) 391.
[39] J.G. Snijders, E.J. Baerends, P. Ros, Mol. Phys. 38 (1979) 1909.
[40] T. Ziegler, V. Tschinke, E.J. Baerends, J.G. Snijders, W.J.
Ravenek, Phys. Chem. 93 (1989) 3050.
[75] M.B. Smith, J. March, Marchꢀs Advanced Organic Chemistry, 5th
ed., Wiley, New York, 2001, p. 431.
[76] Conventions followed in this paper are defined in the
following article: J. Mason, Solid State Nucl. Magn. Reson.
2 (1993) 285.
[41] E. van Lenthe, E.J. Baerends, J.G. Snijders, J. Chem. Phys. 99
(1993) 4597.
[42] S.H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 58 (1980) 1200.
[43] A.D. Becke, Phys. Rev. A 38 (1988) 3098.
[44] J.P. Perdew, Phys. Rev. B 33 (1986) 8822.
[77] Polymorphism and characterization by solid state NMR spectr-
oscopy: M. Strohmeier, A.M. Orendt, D.W. Alderman, D.M.
Grant, J. Am. Chem. Soc. 123 (2001) 1713.
[45] G. Schreckenbach, T. Ziegler, J. Phys. Chem. 99 (1995) 606.
[46] C. van Wullen, Phys. Chem. Chem. Phys. 2 (2000) 2137.
¨
[78] M.T. Zell, B.E. Padden, D.J. Grant, M.-C. Chapeau, I. Prakash,
E.J. Munson, J. Am. Chem. Soc. 121 (1999) 1372.
[79] J. Smith, E. MacNamara, D. Raferty, T. Borchardt, S. Byrn, J.
Am. Chem. Soc. 120 (1998) 11710.
[47] W. Witanowski, W. Sicinska, G.A. Webb, Spectroscopy 9 (1991)
55.
[48] C.J. Jameson, A.K. Jameson, D. Oppusunggu, S. Wille, P.M.
Burrell, J. Mason, J. Chem. Phys. 74 (1981) 81.
[80] Q. Cui, D.G. Musaev, M. Svensson, S. Sieber, K. Morokuma, J.
Am. Chem. Soc. 117 (1995) 12366.
[49] Deuterated isotopomer synthesized by A.J. Johnson, Ph.D. Thesis,
Massachusetts Institute of Technology, Cambridge, MA, 1997.
[50] J.C. Peters, J.-P.F. Cherry, J.C. Thomas, L. Baraldo, D.J.
Mindiola, W.M. Davis, C.C. Cummins, J. Am. Chem. Soc. 121
(1999) 10053.
[81] K.M. Neyman, V.A. Nasluzov, J. Hahn, C.R. Landis, N. Ro¨sch,
Organometallics 16 (1997) 995.
[82] C.J. Jameson, J. Mason, in: J. Mason (Ed.), Multinuclear NMR,
Plenum Press, New York, 1987, p. 51.
[83] G. Schreckenback, J. Chem. Phys. 110 (1999) 11936.
[84] Y. Ruiz-Morales, T. Ziegler, J. Phys. Chem. A 102 (1998) 3970.
[85] Y. Ruiz-Morales, G. Schreckenbach, T. Ziegler, J. Phys. Chem. A
101 (1997) 4121.
[51] Y.-C. Tsai, C.C. Cummins, Inorg. Chim. Acta. 345 (2003) 63.
[52] J.F. Hinton, R.W. Briggs, in: B.E. Mann, R.K. Harris (Eds.),
NMR and the Periodic Table, Academic Press, London, 1978,

2768
E.L. Sceats et al. / Polyhedron 23 (2004) 2751–2768
[86] Y. Ruiz-Morales, G. Schreckenbach, T. Ziegler, Organometallics
15 (1996) 3920.
[89] A.W. Ehlers, Y. Ruiz-Morales, E.J. Baerends, T. Ziegler, Inorg.
Chem. 36 (1997) 5031.
[87] Y. Ruiz-Morales, G. Schreckenbach, T. Ziegler, J. Phys. Chem.
100 (1996) 3359.
[90] J.B. Greco, J.C. Peters, T.A. Baker, W.M. Davis, C.C. Cummins,
G. Wu, J. Am. Chem. Soc. 123 (2001) 5003.
[88] G. Schreckenbach, Y. Ruiz-Morales, T. Ziegler, J. Chem. Phys.
104 (1996) 8605.
[91] H. Henderickx, G. Kwakkenbos, A. Peters, J. van der Spoel, K.
de Vries, Chem. Commun. 16 (2003) 2050.