Transition-metal imido-boroxide complexes: A structural and spectroscopic investigation of the influence of boron

Article abstract of DOI:10.1039/b207277g

The four coordinate compounds Mo(NR)₂[OB(mes)₂]₂ (1, R = ^tBu; 2, R = Ar = 2,6-ⁱPr₂C₆H₃) have been obtained from the reaction of [(mes)₂BOLi(Et₂O)_n]_x with the corresponding Mo(NR)₂Cl₂(dme) starting material; for structural comparison, Mo(N^tBu)₂[OCH(mes)₂]₂ (3) was also synthesised. The related five-coordinate complexes Ti(N^tBu)-[OB(mes)₂]₂(py)₂ (4) and Ti(N^tBu)[OCH(mes)₂]₂(py)₂ (5) were made using analogous procedures. X-Ray structural investigations of 1-5 were performed to assess the effect that the boron atom has on the metal-oxygen and metal-nitrogen(imido) bond lengths and angles. On average, both classes of compound displayed longer metal-oxygen bonds and shorter metal-nitrogen(imido) bonds for the boroxide derivatives. Spectroscopic investigation of the Δδ values for the tert-butylimido derivatives revealed that complexes incorporating the [OB(mes)₂]^-anion exhibit a reduction in the electron density at the imido nitrogen atom, in agreement with the observations from the solid state structures.

Full text of DOI:10.1039/b207277g

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Transition-metal imido-boroxide complexes: a structural and
spectroscopic investigation of the influence of boron
Sarah C. Cole, Martyn P. Coles* and Peter B. Hitchcock
Department of Chemistry, University of Sussex, Falmer, Brighton, UK BN1 9QJ.
E-mail: m.p.coles@sussex.ac.uk.
Received 24th July 2002, Accepted 10th September 2002
First published as an Advance Article on the web 21st October 2002
The four coordinate compounds Mo(NR)2[OB(mes)2]2 (1, R = tBu; 2, R = Ar = 2,6-iPr2C6H3) have been obtained
from the reaction of [(mes)2BOLi(Et2O)n]x with the corresponding Mo(NR)2Cl2(dme) starting material; for structural
comparison, Mo(NtBu)2[OCH(mes)2]2 (3) was also synthesised. The related five-coordinate complexes Ti(NtBu)-
[OB(mes)2]2(py)2 (4) and Ti(NtBu)[OCH(mes)2]2(py)2 (5) were made using analogous procedures. X-Ray structural
investigations of 1–5 were performed to assess the effect that the boron atom has on the metal–oxygen and metal–
nitrogen(imido) bond lengths and angles. On average, both classes of compound displayed longer metal–oxygen
bonds and shorter metal–nitrogen(imido) bonds for the boroxide derivatives. Spectroscopic investigation of the
∆δ values for the tert-butylimido derivatives revealed that complexes incorporating the [OB(mes)2]Ϫ anion exhibit
a reduction in the electron density at the imido nitrogen atom, in agreement with the observations from the solid
state structures.
fluorinated-mesityl derivative, 2,4,6-(CF3)3C6H2 (Fmes) at
1
Introduction
boron, and concluded that a greater localisation of electron
density was present in the B–O bond, from a structural com-
parison of [(Fmes)2BOLi(THF)]2 with the non-fluorinated
mesityl analogue.6 We have initiated a study into the application
of these, and related boron–oxygen ligands in early transition-
metal chemistry,7 and present in this publication a structural
and spectroscopic study comparing the bonding of [(mes)2BO]Ϫ
with the sterically similar, ‘conventional’ alkoxide [(mes)2CHO]Ϫ
in molybdenum- and titanium-imido complexes.
Alkoxide and aryloxide ligands have played a prominent role in
coordination chemistry during the past 30 years.1 The proper-
ties which these ligands impart to the complex can be varied
through derivatisation of the alkyl and aryl substituents, lead-
ing to their application in a range of catalytic processes. Fre-
quently, sterically bulky groups are employed in these systems
in order to limit the aggregation to molecular species that are
more amenable to study. Each ligand type is capable of
coordinating to a transition-metal centre as a [1σ, 2π] donor
group, able to contribute up to five electrons to the metal
centre.2 The extent of π-donation that occurs from the oxygen
lone-pairs can be influenced by the nature of the substituents,
which can have a profound effect on the chemistry observed
at the metal centre. For example, the introduction of electron
withdrawing (fluorinated) substituents in the molybdenum
alkylidene complexes Mo(NAr)(CHR)(ORЈ)2 results in dramat-
ically different reactivity with respect to catalytic olefin
metathesis.3
An alternative approach to controlling the extent of
π-donation from alkoxide-type ligands is by the incorporation
of an atom with an empty π-acceptor orbital adjacent to the
O-atom. This has previously been the subject of a brief study
for boron-substituted alkoxide (boroxide) ligands, where the
empty 2p-orbital on boron is accessible for donation from the
oxygen lone pairs. Power and co-workers reported a series of
late transition-metal compounds, showing that ligands of the
type [R2BO]Ϫ (R = mes = 2,4,6-Me3C6H2; 2,4,6-iPr3C6H2) are
able to adopt either a bridging or terminal coordination mode
(Fig. 1).4,5 Additional work by Gibson and co-workers sought
to enhance this effect through the introduction of the
2
Experimental
General experimental procedures
All manipulations were carried out under dry nitrogen using
standard Schlenk and cannula techniques, or in a conventional
nitrogen-filled glovebox. Solvents were dried over the appropri-
ate drying agent and degassed prior to use. The compounds
(mes)2BOH,8 (mes)2CHOH,9 Mo(NR)2Cl2(dme), (R = tBu,
Ar = 2,6-iPr2C6H3)10 and Ti(NtBu)Cl2(py)3,11 were synthesised
according to literature procedures.
Elemental analyses were performed by S. Boyer at the
University of North London. NMR spectra were recorded
using a Bruker Avance DPX 300 MHz spectrometer. Coupling
constants are quoted in Hz.
Mo(NtBu)2[OB(mes)2]2 (1)
nBuLi (1.6 mL of a 2.5 M solution in hexane, 4.00 mmol)
was added via syringe to a solution of (mes)2BOH (1.00 g,
3.76 mmol) in Et2O (25 mL) at 0 ЊC. The resultant mixture was
allowed to warm to ambient temperature and was stirred for
1 h, during which time a fine white precipitate formed. The
slurry was added to a solution of Mo(NtBu)2Cl2(dme) (0.75 g,
1.88 mmol) in Et2O (40 mL) that had been cooled to Ϫ78 ЊC.
Upon warming, a dark green solution and an off-white pre-
cipitate formed, which was stirred at room temperature for 14 h.
The volatiles were removed under reduced pressure and the
resultant green solid extracted from LiCl with hexane. Concen-
tration and storage at Ϫ30 ЊC yielded 1 as yellow-green crystals.
Yield 0.76 g (53%).
Fig. 1
4168
J. Chem. Soc., Dalton Trans., 2002, 4168–4174
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b207277g

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Anal. calc. for C44H62N2B2MoO2: C, 68.76; H, 8.13; N, 3.64.
Found: C, 68.92; H, 8.23; N, 3.52%. H NMR (C6D6, 298 K):
δ 6.73 (s, 8H, C6H2), 2.45 (s, 24H, 2,6-Me2), 2.14 (s, 12H, 4-Me),
1.08 (s, 18H, NCMe3). 13C NMR (C6D6, 298 K): δ 141.1 (CH),
138.1 (C), 128.7 (C), 128.6 (C), 71.0 (NCMe3), 31.6 (NCMe3),
23.2 (2,6-Me2) 21.2 (4-Me). MS (EIϩ, m/z) 769 [M]ϩ.
NCMe3). 13C NMR (CDCl3, 298 K): δ 150.4 (CH), 142.1 (br,
1
C), 140.2 (C), 136.9 (C), 135.9 (CH), 127.4 (CH), 123.0 (CH),
69.6 (C), 31.6 (CH3), 22.6 (CH3), 21.1 (CH3). MS (EIϩ, m/z):
1178 [dimer Ϫ (mes)]ϩ, 914 [dimer Ϫ [OB(mes)2] Ϫ (mes)]ϩ, 842
[dimer Ϫ (NtBu) Ϫ [OB(mes)2] Ϫ (mes)]ϩ (where dimer =
{Ti(NtBu)[OB(mes)2]2}2, believed to form by combination of
molecular fragments upon loss of pyridine).
Mo(NAr)2[OB(mes)2]2 (2)
Ti(NtBu)[OCH(mes)2]2(py)2 (5)
nBuLi (0.9 mL of a 2.5 M solution in hexane, 2.25 mmol) was
added via syringe to a solution of (mes)2BOH (0.60 g, 2.25
mmol) in Et2O (20 mL) at 0 ЊC. The resultant mixture was
allowed to warm to ambient temperature and was stirred for
1 h, during which time a fine white precipitate formed. The
slurry was added to a solution of Mo(NAr)2Cl2(dme) (0.68 g,
1.12 mmol) in Et2O (30 mL) that had been cooled to Ϫ78 ЊC,
affording a red solution and a yellow precipitate, which was
stirred at room temperature for 14 h. The volatiles were
removed under reduced pressure, and the resultant red solid
was extracted from LiCl with pentane. Concentration and
storage at Ϫ30 ЊC yielded 2 as red crystals. Yield 0.71 g (65%).
Anal. calc. for C60H78N2B2MoO2: C, 74.60; H, 8.14; N, 2.90.
nBuLi (0.6 mL of a 2.5 M solution in hexane, 1.50 mmol) was
added via syringe to a solution of (mes)2CHOH (0.40 g, 1.49
mmol) in Et2O (25 mL) at 0 ЊC. The resultant mixture was
allowed to warm to ambient temperature and was stirred for
1 h, during which time a fine white precipitate formed. The
slurry was added to a solution of Ti(NtBu)Cl2(py)3 (0.32 g,
0.75 mmol) in Et2O (25 mL) that had been cooled to Ϫ78 ЊC,
affording a yellow solution which was stirred at room temper-
ature for 14 h. The volatiles were removed under reduced
pressure, and the resultant yellow solid was extracted with
hexane and filtered. Concentration and storage at Ϫ4 ЊC
yielded 5 as yellow crystals. Yield 0.42 g (70%).
Anal. calc. for C50H63N3B2O2Ti: C, 76.92; H, 8.08; N, 5.18.
Found: C, 77.02; H, 8.14; N, 5.19%. 1H NMR (CDCl3, 298 K):
δ 8.47 (d, br, 2JHH = 4.5, 4H, pyridine-Hortho), 7.53 (t, 2JHH = 7.6,
2H, pyridine-Hpara), 7.27 (s, 2H, CHO), 7.02 (t, 2JHH = 6.6, 4H,
pyridine-Hmeta), 6.57 (s, 8H, C6H2), 2.16 (s, 12H, 4-Me), 2.13
(s, 24H, 2,6-Me2), 0.77 (s, 9H, NCMe3). 13C NMR (CDCl3, 298
K): δ 150.3 (CH), 140.4 (C), 136.3 (C), 136.1 (CH), 134.1 (C),
129.6 (CH), 122.8 (CH), 81.4 (CH), 67.1 (C), 32.1 (CH3), 21.2
(CH3), 20.4 (CH3).
1
Found: C, 74.59; H, 8.25; N, 2.83%. H NMR (C6D6, 298 K):
δ 6.89 (mult, 6H, C6H3), 6.66 (s, 8H, C6H2), 3.62 (sept, JHH
3
=
6.9, 4H, CHMe2), 2.41 (s, 24H, 2,6-Me2), 2.13 (s, 12H 4-Me),
3
0.97 (d, JHH = 6.9, 24H CHMe2). 13C NMR (C6D6, 298 K):
δ 154.2 (C), 143.1 (C), 141.0 (C), 139.4 (br, C), 138.4 (C), 128.8
(CH), 127.3 (CH), 123.0 (CH), 28.7 (CHMe2), 23.7 (CH3), 23.1
(CH3), 21.3 (CH3). MS (EIϩ, m/z) 978 [M]ϩ.
Mo(NtBu)2[OCH(mes)2]2 (3)
nBuLi (0.6 mL of a 2.5 M solution in hexane, 1.50 mmol) was
added via syringe to a solution of (mes)2CHOH (0.40 g, 1.49
mmol) in Et2O (20 mL) at 0 ЊC. The resultant mixture was
allowed to warm to ambient temperature and was stirred for
1 h, during which time a fine white precipitate formed. The
slurry was added to a solution of Mo(NtBu)2Cl2(dme) (0.30 g,
0.75 mmol) in Et2O (25 mL) that had been cooled to Ϫ78 ЊC,
affording a brown solution and a white precipitate which was
stirred at room temperature for 14 h. The volatiles were
removed under reduced pressure, and the resultant brown solid
was extracted from LiCl with hexane. Concentration and
cooling to Ϫ30 ЊC yielded 3 as colourless crystals. Yield 0.34 g
(61%).
Crystallography
Details of the crystal data, intensity collection and refinement
for complexes 1, 2 and 3 are listed in Table 1, and for complexes
4 and 5 in Table 2. Crystals were covered in oil and suitable
single crystals were selected under a microscope and mounted
on a Kappa CCD diffractometer. The structures were refined
with SHELXL-97.12 Additional features are described below.
CCDC reference numbers 190539–190543.
See http://www.rsc.org/suppdata/dt/b2/b207277g/ for crystal-
lographic data in CIF or other electronic format.
Mo(NtBu)2[OB(mes)2]2 (1). The molecule lies on a 2-fold
rotation axis.
Mo(NAr)2[OB(mes)2]2 (2). Two independent molecules are
present in the unit cell with slight differences in bond lengths
and angles.
Anal. calc. for C46H64N2MoO2: C, 71.48; H, 8.35; N, 3.62.
1
Found: C, 71.45; H, 8.41; N, 3.54%. H NMR (C6D6, 298 K):
δ 7.33 (s, 2H, CHO), 6.70 (s, 8H, C6H2), 2.36 (s, 24H, 2,6-Me2),
2.13 (s, 12H, 4-Me), 1.20 (s, 18H, NCMe3). 13C NMR (C6D6,
298 K): δ 138.3 (CH), 137.1(C), 136.0 (C), 130.9 (C), 88.2
(CHO), 69.6 (NCMe3), 32.4 (NCMe3), 22.0 (CH3), 20.9 (CH3).
Ti(NtBu)[OB(mes)2]2(py)2 (4). The complex crystallises with
half a molecule of hexane in the unit cell.
Ti(NtBu)[OCH(mes)2]2(py)2 (5). Two independent molecules
are present in the unit cell with slight differences in bond
lengths and angles.
Ti(NtBu)[OB(mes)2]2(py)2 (4)
nBuLi (0.8 mL of a 2.5 M solution in hexane, 2.00 mmol) was
added via syringe to a solution of (mes)2BOH (0.50 g, 1.88
mmol) in Et2O (25 mL) at 0 ЊC. The resultant mixture was
allowed to warm to ambient temperature and was stirred for
1 h, during which time a fine white precipitate formed. The
slurry was added to a solution of Ti(NtBu)Cl2(py)3 (0.40 g,
0.94 mmol) in Et2O (25 mL) that had been cooled to Ϫ78 ЊC,
affording a yellow solution which was stirred at room temper-
ature for 14 h. The volatiles were removed under reduced
pressure, and the resultant brown solid was extracted with
hexane and filtered. Concentration and storage at Ϫ4 ЊC
yielded 3 as yellow crystals. Yield 0.41 g (53%).
3
Results and discussion
The lithium salt of dimesitylborinic acid has previously been
isolated as the mono-THF solvate, [(mes)2BOLi(THF)]2, in
near quantitative yields.4 For the purposes of our investigations
we found it convenient to generate the salt in situ as a fine, white
powder, that could be easily added as a suspension (in diethyl
ether) to the appropriate metal-chloride salt. Thus, 2 equiv-
alents of [(mes)2BOLi(Et2O)n]x, were reacted with Mo(NtBu)2-
Cl2(dme) at low temperature to afford Mo(NtBu)2[OB(mes)2]2
(1) as green-yellow crystals (Scheme 1). The bis(arylimido)
derivative, Mo(NAr)2[OB(mes)2]2 (2, Ar = 2,6-iPr2C6H3), was
isolated as red crystals from the analogous reaction using the
appropriate starting material. In both cases, NMR, mass
spectral and elemental analytical data were consistent with the
molybdenum bis(imido)bis(boroxide) complex. In order to help
to determine the influence of the boron substituent on the
Anal. calc. for C50H63N3B2O2Ti: C, 74.37; H, 7.86; N, 5.20.
Found: C, 74.38; H, 7.73; N, 5.30%. 1H NMR (CDCl3, 298 K):
2
δ 8.62 (d, br, JHH = 4.8, 4H, pyridine-Hortho), 7.49 (t, br, 2H,
pyridine-Hpara), 6.83 (t, br, 4H, pyridine-Hmeta), 6.56 (2, 8H,
C6H2), 2.20 (s, 12H, 4-Me), 2.13 (s, 24H, 2,6-Me2), 0.77 (s, 9H,
J. Chem. Soc., Dalton Trans., 2002, 4168–4174
4169

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Table 1 Crystal structure and refinement data for 1, 2 and 3
1
2
3
Formula
C44H62B2MoN2O2
768.52
173(2)
0.71073
C60H78B2MoN2O2
976.80
173(2)
C46H64MoN2O2
772.93
173(2)
Formula weight
Temperature/K
Wavelength/Å
0.71073
0.71073
Crystal size/mm
Crystal system
Space group
a/Å
b/Å
0.4 × 0.3 × 0.3
Orthorhombic
Aba2 (no. 41)
19.0505(6)
27.3517(7)
8.4738(2)
90
4415.4(2)
4
1.16
0.2 × 0.2 × 0.1
Orthorhombic
Pbc21 (no. 29)
13.2134(4)
19.0999(5)
44.5065(13)
90
11232.3(6)
8
1.16
0.3 × 0.3 × 0.2
Monoclinic
P21/n (no. 14)
19.4544(3)
11.2123(2)
19.7023(3)
94.658(2)
4283.44(10)
4
c/Å
β/Њ
V/Å3
Z
Dc/Mg mϪ3
1.199
Absorption coefficient/mmϪ1
θ Range for data collection/Њ
Reflections collected
Independent reflections
Reflections with I > 2σ(I)
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole/e ÅϪ3
0.33
3.92–25.96
14970
3706 (Rint = 0.072)
2900
3706/1/235
1.034
R1 = 0.039, wR2 = 0.080
R1 = 0.061, wR2 = 0.091
0.31 and Ϫ0.43
0.28
3.71–21.97
48346
12794 (Rint = 0.157)
6842
12794/721/1223
0.980
R1 = 0.068, wR2 = 0.127
R1 = 0.161, wR2 = 0.158
0.39 and Ϫ0.51
0.343
3.71–25.73
35268
7830 (Rint = 0.048)
6626
7830/0/468
0.993
R1 = 0.036, wR2 = 0.086
R1 = 0.048, wR2 = 0.092
0.33 and Ϫ0.52
Table 2 Crystal structure and refinement data for 4 and 5
4
5
Formula
C50H63B2N3O2Ti (½C6H14)
850.64
173(2)
C52H65N3O2Ti
811.97
173(2)
Formula weight
Temperature/K
Wavelength/Å
Crystal size/mm
Crystal system
Space group
0.71073
0.71073
0.3 × 0.25 × 0.25
Triclinic
0.4 × 0.4 × 0.15
Triclinic
¯
¯
P1 (no. 2)
P1 (no. 2)
a/Å
b/Å
c/Å
α/Њ
11.7856(6)
11.9638(4)
17.5685(9)
93.029(3)
10.5789(1)
19.5500(3)
22.5063(3)
82.370(1)
β/Њ
98.130(2)
91.726(3)
2447.1(2)
89.957(1)
83.821(1)
4586.3(1)
γ/Њ
V/Å3
Z
2
1.15
4
1.18
Dc/Mg mϪ3
Absorption coefficient/mmϪ1
θ Range for data collection/Њ
Reflections collected
Independent reflections
Reflections with I > 2σ(I)
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole/e ÅϪ3
0.22
4.12–25.01
14981
8545 (Rint = 0.043)
6318
8545/4/565
1.018
R1 = 0.062, wR2 = 0.146
R1 = 0.090, wR2 = 0.162
0.77 and Ϫ0.58
0.23
3.75–24.76
60894
15458 (Rint = 0.071)
11334
15458/0/1061
1.102
R1 = 0.065, wR2 = 0.136
R1 = 0.098, wR2 = 0.148
0.47 and Ϫ0.36
Table
3
Selected bond lengths (Å) and angles (Њ) for
Mo(NtBu)2[OB(mes)2]2 (1)
Mo–N
B–O
1.726(3)
1.353(5)
Mo–O
1.914(2)
N–Mo–NЈ
O–Mo–OЈ
Mo–N–C(1)
110.0(3)
110.33(17)
162.2(4)
N–Mo–O
N–Mo–OЈ
Mo–O–B
111.83(14)
106.48(14)
155.5(3)
Scheme 1
Symmetry elements: Ј Ϫx, Ϫy, z.
bonding parameters in 1 and 2, in particular the bonding within
the Mo–B–O linkage, X-ray structural analyses were per-
formed. The molecular structures are illustrated in Figs. 2 and 3,
crystal data are summarised in Table 1 and selected bond
lengths and angles are collected in Tables 3 and 4.
arrangement of ligands around the molybdenum centre [inter-
ligand angles in the range 106.48(14)–111.83(14)Њ]. The Mo–N
bond length [1.726(3) Å] lies towards the lower end of the range
typically found in d0-bis(imido) molybdenum complexes,13
which may reflect an increase in π-electron donation to a more
electron deficient metal centre (vide infra). The Nimido angle
Compound 1 crystallises from hexane as the monomeric
species, Mo(NtBu)2[OB(mes)2]2, with a pseudo-tetrahedral
4170
J. Chem. Soc., Dalton Trans., 2002, 4168–4174

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Table 4 Selected bond lengths (Å) and angles (Њ) for Mo(NAr)2-
[OB(mes)2]2 (2).
Mo–N(1)
1.745(6)
1.714(7)
1.870(5)
1.887(5)
1.338(11)
1.358(11)
Mo–N(2)
Mo–O(2)
O(2)–B(2)
1.737(6)
1.746(7)
1.883(5)
1.899(6)
1.361(11)
1.352(12)
Mo–O(1)
O(1)–B(1)
N(1)–Mo–N(2)
O(1)–Mo–O(2)
N(2)–Mo–O(1)
Mo–N(1)–C(37)
Mo–N(2)–C(49)
106.8(3)
108.7(3)
113.1(2)
114.1(2)
109.6(3)
109.2(3)
154.7(6)
155.3(6)
163.4(6)
160.7(7)
N(1)–Mo–O(1)
N(1)–Mo–O(2)
N(2)–Mo–O(2)
Mo–O(1)–B(1)
Mo–O(2)–B(2)
108.2(3)
107.1(3)
108.5(3)
107.7(3)
110.5(3)
110.0(3)
174.5(6)
170.4(6)
158.4(6)
158.0(6)
Figures (one below the other) correspond to two crystallographically
independent molecules.
The bis(arylimido) analogue (2) crystallises with two
independent molecules in the unit cell, each of which adopt
distorted tetrahedral geometry at the metal centre, with inter-
ligand angles in the range 106.8(3)–113.1(2)Њ {107.1(3)–
114.1(2)Њ} (values in { } refer to the second independent
molecule). The Mo–N bond lengths of 1.737(6) and 1.745(6) Å
{1.714(7) and 1.746(7) Å} are on average longer than in 1, a
likely consequence of π-delocalisation into the aromatic ring of
the aryl substituent, while the Mo–N–C angles of 154.7(6) and
163.4(6)Њ {155.3(6) and 160.7(7)Њ} are again within the range
associated with ‘linear’ imido ligands.13 The Mo–O bond
lengths of 1.883(5) and 1.870(5) Å {1.899(6) and 1.887(5) Å}
are notably shorter than in 1, implying an increase in
π-donation from the oxygen lone-pairs, which may compensate
for the reduced electron donation from the arylimido substitu-
ents. One of the boroxide oxygens in each molecule is consider-
ably more bent 158.4(6)Њ {158.0(6)Њ} than the other 174.5(6)Њ
{170.4(6)Њ}, although this has little effect on the corresponding
Mo–O bond length. In addition the B–O distances within the
‘bent’ 1.361(11) Å {1.352(12) Å} and ‘linear’ 1.338(11) Å
{1.358(11) Å} boroxide ligands do not appear to follow any
predictable trend, suggesting that steric factors are also
contributing to the overall molecular structure.
Fig. 2 Molecular structure of Mo(NtBu)2[OB(mes)2]2 (1) with thermal
ellipsoids drawn at the 50% probability level. Hydrogen atoms omitted
for clarity.
In an attempt to evaluate the impact of the boron atom on
the metal–oxygen bonding in 1 and 2, we sought other molyb-
denum bis(imido)bis(alkoxide) complexes with which to com-
pare bond parameters. However, the relative dearth of structur-
ally characterised examples became apparent, and hence we
embarked on the synthesis of compounds of general formula
Mo(NR)2(ORЈ)2. To maximise the validity of any comparison
in terms of the electronic effect of the boron atom, we targeted
an alkoxide incorporating similarly sized substituents to the
[(mes)2BO]Ϫ ligand used in 1 and 2. Accordingly dimesityl-
methanol, (mes)2CHOH, was prepared according to literature
procedures,9 and investigated as an alkoxide ligand precursor in
molybdenum bis(imido) chemistry.
Fig. 3 Molecular structure of Mo(NAr)2[OB(mes)2]2 (2) with thermal
ellipsoids drawn at the 50% probability level. Hydrogen atoms omitted
for clarity.
Deprotonation of the alcohol proceeds in Et2O at 0 ЊC using
nBuLi, to afford a white precipitate, presumed to be the ether
adduct [(mes)2CHOLi(Et2O)n]x (Scheme 2). We again found it
[162.2(4) Å] is, however, unremarkable, being in the range
typical of terminal, 4-electron donor imido ligands.13 The
Mo–O bond length [1.914(2) Å] is longer than in the related
four-coordinate bis(tert-butylimido)molybdenum complex,
[Mo(NtBu)2{O(Ph2SiO)2}]2,14 [Mo–O = 1.899(6) and 1.873(6)
Å] suggestive of a lower Mo–O bond order, predicted if
π-electron density is being donated to the boron atom. How-
ever, the B–O bond length in 1 [1.353(5) Å] is not significantly
shortened when compared with the parent borinic acid
[1.367(6) Å].4
Scheme 2
J. Chem. Soc., Dalton Trans., 2002, 4168–4174
4171

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Fig.
4
Molecular structure of Mo(NtBu)2[OCH(mes)2]2 (3) with
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms,
except for CH(mes)2, omitted for clarity.
Fig. 5 Space filling representation of [(mes)2BO]Ϫ and [(mes)2CHO]Ϫ.
Crystal data taken from compounds 1 and 3 respectively.
Table 5 Selected bond lengths (Å) and angles (Њ) for Mo(NtBu)2-
[OCH(mes)2]2 (3)
denum centre. The Mo–N–C angles in 3 [160.41(18) and
162.78(18)Њ], however, do not exhibit any significant bending
compared with the corresponding angle in 1 [162.2(4)Њ]. The
Mo–O distances [1.8930(16) and 1.9089(16) Å] are shorter than
in 1, consonant with greater π-donation from the oxygen to
the Mo-centre. The angles at the O-atoms [144.99(15) and
131.69(14)Њ] are considerably more bent than in the boroxide
ligands of 1, which may indicate that the π-electrons are
distributed across the three atoms of the ‘Mo–O–B’ moiety in 1,
while in 3 they are localised to a greater extent in the Mo–O
bond.
Mo–N(1)
Mo–O(1)
1.742(2)
1.8930(16)
Mo–N(2)
Mo–O(2)
1.740(2)
1.9089(16)
N(1)–Mo–N(2)
O(1)–Mo–O(2)
N(2)–Mo–O(1)
Mo–N(1)–C(1)
Mo–N(2)–C(5)
110.27(10)
113.34(7)
105.86(8)
160.41(18)
162.78(18)
N(1)–Mo–O(1)
N(1)–Mo–O(2)
N(2)–Mo–O(2)
Mo–O(1)–C(9)
Mo–O(2)–C(28)
112.62(8)
106.62(8)
108.07(8)
144.99(15)
131.69(14)
most convenient to proceed without isolation of the Li-salt, and
hence addition of a slurry of two equivalents of the Li-salt to
a cooled (Ϫ78 ЊC) solution of Mo(NtBu)2Cl2(dme) afforded,
after the appropriate work-up, the compound Mo(NtBu)2-
[OCH(mes)2]2 (3). Cooling a saturated hexane solution of 3
to Ϫ30 ЊC afforded colourless crystals suitable for an X-ray
analysis. The molecular structure is illustrated in Fig. 4, crystal
data are summarised in Table 1 and selected bond lengths and
angles are collected in Table 5.
Compound 3 also exists as a monomeric, distorted tetra-
hedral complex, with bond lengths and angles in the range
105.86(8)–113.34(7)Њ. Whilst a greater distortion from ideal
tetrahedral geometry exists when compared with 1, the differ-
ences are deemed sufficiently small to permit reasonable com-
parisons to be made between the corresponding bond lengths
and angles. It is also noted that although the two complexes
crystallise in different space groups (1, orthorhombic; 3, mono-
clinic), no intermolecular contacts are present in either struc-
ture. While detailed cone angle measurements have not been
conducted, space filling models generated from the crystal data
for 1 and 3 illustrate the correlation that exists between the
steric demands of the [OB(mes)2]Ϫ and [OCH(mes)2]Ϫ ligands
(Fig. 5). However, it is noted that such considerations should be
treated as a first approximation, as a slightly larger O–Mo–O
angle is observed in 3 [113.34(7) Њ] than in 1 [110.33(17)Њ], which
reflects the different projection of the mesityl substituents that
results from changing a trigonal planar boron atom (Σangles in 1
and 2 = 360 0.2Њ) to a tetrahedral carbon in 3.
In summary, the molybdenum–nitrogen and molybdenum–
oxygen bond lengths show the changes predicted if the boroxide
ligand is acting as a less effective π-donor to the metal centre;
comparison of the analogous compounds 1 and 3 therefore
reveals shorter imido and longer alkoxide bonds in the boron-
containing complex. However, the relative angles at nitrogen
and oxygen do not clearly support these observations, highlight-
ing the ‘soft’ nature of the metal–imido and metal–alkoxide
bonds. We conclude that, although care has been taken to
maximise the structural similarities between the complexes,
steric interactions may still play an important role that needs to
be considered when comparing bond lengths and angles.
To elucidate further the role that sterics play in the arrange-
ment of the ligand substituents, we initiated a study of the
related tert-butylimido complexes of titanium that, on the basis
of previously reported aryloxide derivatives,15 were predicted to
display a different coordination geometry at the metal centre.
Thus, reaction between two equivalents of the lithium salt of
(mes)2BOH or (mes)2CHOH, prepared as described above, and
the ubiquitous titanium imido starting material, [Ti(NtBu)-
Cl2(py)3],11 afforded the compounds Ti(NtBu)[OB(mes)2]2(py)2
(4) and Ti(NtBu)[OCH(mes)2]2(py)2 (5) respectively (Scheme 3).
NMR data and combustion analysis were consistent with a
monomeric, five-coordinate mono(imido)bis(alkoxide) struc-
ture retaining two equivalents of pyridine per titanium. Mass
spectral analysis of complex 4, however, was consistent with a
higher nuclearity cluster, conceivably occurring through either
bridging imido or alkoxide groups. To determine the bonding
present in the solid state, and compare the bond parameters of
the ligands, X-ray structural analyses were performed on 4 and
5. The molecular structures are illustrated in Figs. 6 and 7,
The Mo–N bond distances in 3 [1.742(2) and 1.740(2) Å] are
notably longer than in the analogous tert-butylimido com-
pound 1, suggestive of a reduction in π-donation to the molyb-
4172
J. Chem. Soc., Dalton Trans., 2002, 4168–4174

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Fig. 7 Molecular structure of Ti(NtBu)[OCH(mes)2]2(py)2 (5) with
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms,
except for CH(mes)2, omitted for clarity.
Fig.
6
Molecular structure of Ti(NtBu)[OB(mes)2]2(py)2 (4) with
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
omitted.
Table 6 Selected bond lengths (Å) and angles (Њ) for Ti(NtBu)-
[OB(mes)2]2(py)2 (4)
Ti–N(3)
Ti–N(1)
Ti–N(2)
1.706(3)
2.246(2)
2.236(3)
Ti–O(1)
Ti–O(2)
O(1)–B(1)
O(2)–B(2)
1.925(2)
1.934(2)
1.332(4)
1.331(4)
N(1)–Ti–N(2)
N(1)–Ti–N(3)
N(2)–Ti–N(3)
O(1)–Ti–N(1)
O(1)–Ti–N(2)
O(1)–Ti–N(3)
Ti–N(3)–C(47)
167.95(10)
91.86(10)
100.14(11)
87.55(8)
86.66(9)
113.19(11)
169.4(2)
O(1)–Ti–O(2)
O(2)–Ti–N(1)
O(2)–Ti–N(2)
O(2)–Ti–N(3)
Ti–O(1)–B(1)
Ti–O(2)–B(2)
133.50(9)
88.92(8)
87.42(9)
113.26(11)
159.91(19)
158.92(19)
Table 7 Selected bond lengths (Å) and angles (Њ) for Ti(NtBu)-
[OCH(mes)2]2(py)2 (5)
Ti–N(3)
Ti–O(1)
Ti–O(2)
Ti–N(1)
Ti–N(2)
O(1)–C(15)
1.715(3)
1.716(3)
1.887(2)
1.889(2)
1.872(2)
1.875(2)
2.271(3)
2.258(3)
2.247(3)
2.243(3)
1.403(3)
1.404(4)
O(2)–C(34)
C(15)–C(16)
C(15)–C(25)
C(34)–C(35)
C(34)–C(44)
N(3)–C(11)
1.402(4)
1.405(4)
1.539(4)
1.541(4)
1.540(4)
1.541(4)
1.544(4)
1.537(4)
1.545(5)
1.542(5)
1.458(4)
1.453(4)
Scheme 3
crystal data are summarised in Table 2 and selected bond
lengths and angles are collected in Tables 6 and 7.
Both complexes 4 and 5 exist as monomeric species in the
solid state, each with a terminal imido, two alkoxide/boroxide
ligands and two donor pyridine molecules. This suggests that
the higher peaks in the mass spectra are an artifact generated
from combination of molecular fragments that most likely
occur upon loss of pyridine. Compound 5 is present as two
independent molecules in the unit cell (values in { } refer to the
second molecule). The five-coordinate Ti centre in 4 is virtually
midway between a trigonal bipyramid and square-based
pyramid, as defined by the τ value of 0.57.16 In contrast, 5 is
much closer to true trigonal bipyramidal geometry with τ = 0.78
{τ = 0.79}, arising primarily from a significant reduction in
the O–Ti–O bond angle from 133.50(9)Њ in 4 to 123.24(10)Њ
N(1)–Ti–N(2)
N(3)–Ti–O(1)
N(3)–Ti–O(2)
O(1)–Ti–O(2)
N(3)–Ti–N(1)
170.32(10)
170.42(10)
118.21(11)
117.39(11)
118.29(11)
119.08(11)
123.24(10)
123.25(10)
90.58(11)
Ti–O(1)–C(15)
Ti–O(2)–C(34)
Ti–N(3)–C(11)
O(1)–C(15)–C(16)
O(2)–C(34)–C(35)
149.44(19)
147.27(19)
151.8(2)
152.6(2)
167.8(2)
167.7(2)
114.7(2)
114.9(2)
114.6(3)
114.5(3)
{123.25(10)Њ} in 5, with a commensurate increase in the Nimido
–
90.37(11)
Ti–O angles.
The Ti᎐N
distance for 4 [1.706(3) Å] is unchanged from
᎐
imido
Figures (one below the other) correspond to two crystallographically
independent molecules.
the starting material,11 with a Ti–N–C angle [169.4(2)Њ] typical
for a linear, 4-electron donor group. Although no significant
J. Chem. Soc., Dalton Trans., 2002, 4168–4174
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Table 8 ∆δ Values for complexes of general formula Mo(NtBu)2X2.
Spectra recorded in C6D6 unless otherwise stated
Table 9 ∆δ Values for complexes of general formula Ti(NtBu)X2-
(donor)n. Spectra recorded in CDCl3 unless otherwise stated
X
α
β
∆δ
Ref.
X
(donor)n
α
β
∆δ
Ref.
Cl
74.1
71.0
69.6
68.8
67.9
67.2
30.1
31.6
32.4
41.6
39.4
37.2
36.6
35.6
34.4
19
Cl
Cl
(py)2
(py)3
—
(py)2
(py)2
(py)2
(py)2
(pyЈ)2
(py)2
73.1
71.5
73.3
69.6
68.8
68.7
72.1
68.0
67.1
30.3
30.0
33.1
31.6
31.7
31.7
35.4
32.3
32.1
42.8
41.5
40.2
38.0
37.1
37.0
36.7
35.7
35.0
11
11
15
OB(mes)2
OCH(mes)2
This work
This work
18
20
20
OAr a
OB(mes)2
OArЈ
OAr
a
OSiMe3
32.2
This work
15
15
15
17b
This work
OtBu
b
NHtBu
32.8
OArЉ
OArЈ
OCH(mes)2
a Spectrum recorded in d8-toluene. b The two peaks for the β-carbons
(δ 32.4 and 32.2) corresponding to the OtBu and NtBu were unassigned
– the ∆δ was therefore calculated from an average value.
a Complex is dimeric in the solid state, with bridging tert-butylimido
ligands. Ar = 2,6-Me2C6H3; ArЈ = 2,6-iPr2C6H3; OArЉ = 2,6-tBu2C6H3;
pyЈ = 4-pyrrolidinopyridine.
reduction in the Ti᎐Nimido distance is observed on replacing the
᎐
boroxide ligands with ‘conventional’ hydrocarbon alkoxide
groups in 5 [1.715(3) Å {1.716(3) Å}], a slight decrease in the
Ti–N–C angle to 167.8(2)Њ {167.7(2)Њ} is noted, consistent with
an increase in the π-electron density at the nitrogen atom (vide
infra for spectroscopic analysis). The Ti–O bonds in 4 [1.925(2)
and 1.934(2) Å] are longer than in 5 [1.872(2) and 1.887(2) Å;
{1.889(2) and 1.875(2) Å}], and notably larger than in other,
related titanium(imido) aryloxide complexes.15,17 This is again
consistent with a reduction in π-donation to Ti from the
boroxide ligand, also supported by a reduction in the B–O bond
lengths 1.332(4) and 1.331(4) Å. In addition, the angles at
the oxygen atom in 4 [159.91(19) and 158.92(19)Њ] are signif-
icantly closer to linearity than the corresponding angles in 5
[149.44(19) and 151.8(2)Њ; {147.27(19) and 152.6(2)Њ}], suggest-
ing possible delocalisation across the Ti–O–B fragment.
Previous work on d0-tert-butylimido complexes have estab-
lished a correlation between the chemical shift difference of the
α and β-carbon atoms (∆δ), and the electron density of the
imido nitrogen atom.18 Table 8 displays ∆δ values for 1 and 3
along with data taken from related complexes of general
formula Mo(NtBu)2X2. Generally we observe that the lower the
electron donating ability of the X-ligand, the larger the ∆δ
value corresponding to a reduction in electron density at the
Nimido atom. Thus the value for 1 (∆δ = 39.4) is in agreement
with the boroxide ligand behaving as a relatively poor electron
donor to the molybdenum centre, with a value similar to that
observed for in the dichloride complex (∆δ = 41.6). The value
for 3 (∆δ = 37.2) is in the range observed for related siloxide and
alkoxide complexes, with the slightly higher value a possible
reflection of the electron withdrawing nature of the mesityl
substituents.
Acknowledgements
We thank the EPSRC for a research studentship (S. C. C.)
and University of Sussex for additional financial support. We
would like to thank Professor V. C. Gibson and Dr E. L.
Marshall for providing the 13C spectroscopic data for
Mo(NtBu)2(OtBu)2 and Mo(NtBu)2(NHtBu)2. We also wish
to acknowledge the use of the EPSRC’s Chemical Database
Service at Daresbury.
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Comparing the ∆δ values for 4 and 5 with a range of d0-
titanium mono(imido) complexes we see a similar trend to that
observed for the molybdenum bis(imido) species (Table 9).
Thus, as the electron density at titanium increases through
introduction of more donor groups to the metal centre, the
corresponding ∆δ value decreases, as illustrated by the relative
values of Ti(NtBu)Cl2(py)2 (∆δ = 42.8) vs. Ti(NtBu)Cl2(py)3
(∆δ = 41.5), and [Ti(NtBu)(OAr)2]2 (∆δ = 40.2) vs. Ti(NtBu)-
(OAr)2(py)2 (∆δ = 37.0) (Ar = 2,6-Me2C6H3). The value for
4 (∆δ = 38.0) is slightly greater than that for related aryloxide
complexes with two donor pyridine molecules (∆δ = 37.0–35.7),
suggesting similar donor properties. As predicted, compound
5 has a lower value (∆δ = 35.0) implying a higher electron
density at the imido nitrogen that arises from the alkoxide
ligand behaving as a better π-donor to the d0-metal centre. The
data taken from both systems is therefore in agreement with the
original proposal that the boroxide ligand behaves electron-
ically as an ‘electron-deficient’ alkoxide. We are currently
investigating the differences in the chemistry associated with the
boron-substituted ligand in comparison with the hydrocarbon
analogues.
20 M. Jolly, Ph.D. Thesis, Durham University, Durham, UK, 1994.
4174
J. Chem. Soc., Dalton Trans., 2002, 4168–4174