Reactivity with electrophiles of imido groups supported on trinuclear titanium systems

Article abstract of DOI:10.1021/ic4018294

Several trinuclear titanium complexes bearing amido μ-NHR, imido μ-NR, and nitrido μ_n-N ligands have been prepared by reaction of [{Ti(η⁵-C₅Me₅)(μ-NH)} ₃(μ₃-N)] (1) with 1 equiv of electrophilic reagents ROTf (R = H, Me, SiMe₃; OTf = OSO₂CF₃). Treatment of 1 with triflic acid or methyl triflate in toluene at room temperature affords the precipitation of compounds [Ti₃(η ⁵-C₅Me₅)₃(μ₃-N) (μ-NH)₂(μ-NH₂)(OTf)] (2) or [Ti₃(η ⁵-C₅Me₅)₃(μ₃-N) (μ-NH)(μ-NH₂)(μ-NMe)(OTf)] (3). Complexes 2 and 3 exhibit a fluxional behavior in solution consisting of proton exchange between μ-NH₂ and μ-NH groups, assisted by the triflato ligand, as could be inferred from a dynamic NMR spectroscopy study. Monitoring by NMR spectroscopy the reaction course of 1 with MeOTf allows the characterization of the methylamido intermediate [Ti₃(η⁵-C ₅Me₅)₃(μ₃-N)(μ-NH) ₂(μ-NHMe)(OTf)] (4), which readily rearranges to give 3 by a proton migration from the NHMe amido group to the NH imido ligands. The treatment of 1 with 1 equiv of Me₃SiOTf produces the stable ionic complex [Ti₃(η⁵-C₅Me₅) ₃(μ₃-N)(μ-NH)₂(μ-NHSiMe ₃)][OTf] (5) with a disposition of the nitrogen ligands similar to that of 4. Complex 5 reacts with 1 equiv of [K{N(SiMe₃)₂}] at room temperature to give [Ti₃(η⁵-C ₅Me₅)₃(μ₃-N)(μ-N)(μ-NH) (μ-NHSiMe₃)] (6), which at 85 C rearranges to the trimethylsilylimido derivative [Ti₃(η⁵-C ₅Me₅)₃(μ₃-N)(μ-NH) ₂(μ-NSiMe₃)] (7). Treatment of 7 with [K{N(SiMe ₃)₂}] affords the potassium derivative [K{(μ₃-N)(μ₃-NH)(μ₃-NSiMe ₃)Ti₃(η⁵-C₅Me₅) ₃(μ₃-N)}] (8), which upon addition of 18-crown-6 leads to the ion pair [K(18-crown-6)][Ti₃(η⁵-C ₅Me₅)₃(μ₃-N)(μ-N)(μ-NH) (μ-NSiMe₃)] (9). The X-ray crystal structures of 2, 5, 6, and 8 have been determined.

Full text of DOI:10.1021/ic4018294

Article
pubs.acs.org/IC
Reactivity with Electrophiles of Imido Groups Supported on
Trinuclear Titanium Systems
Jorge Caballo, Mariano Gonzalez-Moreiras, Miguel Mena, Adrian Perez-Redondo, and Carlos Yelamos*
́ ́ ́ ́
́ ́ ́ ́
Departamento de Química Organica y Química Inorganica, Universidad de Alcala, 28871 Alcala de Henares-Madrid, Spain
*
S Supporting Information
ABSTRACT: Several trinuclear titanium complexes bearing
amido μ-NHR, imido μ-NR, and nitrido μ -N ligands have
n
5
been prepared by reaction of [{Ti(η -C Me )(μ-NH)} (μ -
5
5
3
3
N)] (1) with 1 equiv of electrophilic reagents ROTf (R = H,
Me, SiMe ; OTf = OSO CF ). Treatment of 1 with triflic acid
3
2
3
or methyl triflate in toluene at room temperature affords the
5
precipitation of compounds [Ti (η -C Me ) (μ -N)(μ-
3
5
5
3
3
5
NH) (μ-NH )(OTf)] (2) or [Ti (η -C Me ) (μ -N)(μ-
2
2
3
5
5
3
3
NH)(μ-NH )(μ-NMe)(OTf)] (3). Complexes 2 and 3 exhibit
2
a fluxional behavior in solution consisting of proton exchange between μ-NH and μ-NH groups, assisted by the triflato ligand, as
2
could be inferred from a dynamic NMR spectroscopy study. Monitoring by NMR spectroscopy the reaction course of 1 with
5
MeOTf allows the characterization of the methylamido intermediate [Ti (η -C Me ) (μ -N)(μ-NH) (μ-NHMe)(OTf)] (4),
3
5
5 3
3
2
which readily rearranges to give 3 by a proton migration from the NHMe amido group to the NH imido ligands. The treatment
5
of 1 with 1 equiv of Me SiOTf produces the stable ionic complex [Ti (η -C Me ) (μ -N)(μ-NH) (μ-NHSiMe )][OTf] (5) with
3
3
5
5 3
3
2
3
a disposition of the nitrogen ligands similar to that of 4. Complex 5 reacts with 1 equiv of [K{N(SiMe ) }] at room temperature
3
2
5
to give [Ti (η -C Me ) (μ -N)(μ-N)(μ-NH)(μ-NHSiMe )] (6), which at 85 °C rearranges to the trimethylsilylimido derivative
Ti (η -C Me ) (μ -N)(μ-NH) (μ-NSiMe )] (7). Treatment of 7 with [K{N(SiMe ) }] affords the potassium derivative
K{(μ -N)(μ -NH)(μ -NSiMe )Ti (η -C Me ) (μ -N)}] (8), which upon addition of 18-crown-6 leads to the ion pair [K(18-
crown-6)][Ti (η -C Me ) (μ -N)(μ-N)(μ-NH)(μ-NSiMe )] (9). The X-ray crystal structures of 2, 5, 6, and 8 have been
3
5
5 3
3
3
5
[
[
3 5 5 3 3 2 3 3 2
5
3
3
3
3
3
5
5 3
3
5
3
5
5 3
3
3
determined.
INTRODUCTION
bearing terminal imido or nitrido ligands, the reported
reactivity of polynuclear early transition metal (groups 4 and
■
An extensive chemistry has been developed with transition
metal complexes bearing imido or nitrido ligands as a terminal
5
) derivatives containing bridging (μ -NR or μ -N) ligands
n n
8
,9
1
remains comparatively scarce.
Representative examples
functionality, MNR or MN. The reactivity of terminal
include nucleophilic attack of a phosphine to a bridging nitrido
imido or nitrido ligands has been an area of interest because of
their potential to show either electrophilic or nucleophilic
character. The behavior of these nitrogen ligands depends on
the nature of the metal, its oxidation state, and the ancillary
ligands. These factors determine the π interaction between the
imido or nitrido moiety and the metal center, and
considerations based on frontier molecular orbitals have been
used to rationalize the increasing electrophilicity of the nitrogen
10
ligand in a dinuclear titanium derivative, or electrophilic
attacks on μ-N groups of dinuclear titanium, niobium, or
11
tantalum complexes generated by dinitrogen activation.
During the past decade, we have been involved in the study
of the reactivity of the trinuclear imido-nitrido titanium(IV)
5
12
derivative [{Ti(η -C Me )(μ-NH)} (μ -N)] (1). This mole-
5
5
3
3
cule contains two potentially reactive functionalities: the three
1
,2
μ-NH imido groups and the μ -N nitrido ligand. While complex
3
ligand upon going from early to later transition metals. This
experimental trend in the reactivity of nitrido complexes has
been recently interpreted in terms of the variation of the partial
charge on the nitrogen atom. The reaction of imido or nitrido
ligands with nucleophiles leads to a two-electron reduction of
the metal center and a reduced bond order between the metal
and the nitrogen atom. In contrast, when imido or nitrido
ligands react with electrophiles, the net result is a reduced bond
order between the metal and the nitrogen atom, to produce
amido and imido derivatives, respectively, with no change in the
1
is capable of acting as a Lewis base through the imido groups
toward many metal derivatives to give cube-type adducts
5
13
3
[L M{(μ -NH) Ti (η -C Me ) (μ -N)}], the Lewis base
n 3 3 3 5 5 3 3
behavior of the apical μ -N nitrido ligand has been only
3
14
documented with copper and silver MX Lewis acids. This
nitrido ligand is quite chemically unreactive, although we have
4
,5
reported the “apparent” nucleophilic attack of an acetylide
−
group [CCR] at this site to yield alkynylimido μ -NCCR
3
1
5
ligands with a two-electron reduction of the Ti core. Density
3
6
,7
oxidation state of the metal.
While those extensive studies have been mainly carried out
Received: July 16, 2013
on mononuclear midtransition metal (groups 6−8) complexes
Published: September 23, 2013
©
2013 American Chemical Society
11519
dx.doi.org/10.1021/ic4018294 | Inorg. Chem. 2013, 52, 11519−11529

Inorganic Chemistry
Article
functional theory (DFT) calculations showed that this reaction
involves the formation of an alkynyl titanium intermediate
followed by migration of the alkynyl group to the apical nitrido
ligand. We were interested in studying the reactivity of 1
toward electrophiles, and here we report on the reaction with 1
equiv of ROTf to generate polynuclear complexes by selective
functionalization of the imido groups. The preliminary results
on the treatment of complex 1 with MeOTf have been recently
MHz, CDCl
3
, 20 °C): δ −77.9. Anal. Calcd for C34
H F N O SSiTi
57 3 4 3 3
(
M = 830.59): C, 49.17; H, 6.92; N, 6.75; S, 3.86. Found: C, 48.83;
w
H, 6.44; N, 6.72; S, 3.48.
5
Synthesis of [Ti (η -C Me ) (μ -N)(μ-N)(μ-NH)(μ-NHSiMe )]
6). A 100 mL Schlenk flask was charged with 5 (0.30 g, 0.36
3
5
5 3
3
3
(
mmol), [K{N(SiMe ) }] (0.080 g, 0.40 mmol), and toluene (20 mL).
3
2
The reaction mixture was stirred at room temperature for 30 min to
give a red solution and a white fine solid. After filtration, the volatile
components of the solution were removed under reduced pressure,
and the resultant dark red solid was vacuum-dried for 3 h to give 6
1
6
communicated.
−1
(
0.17 g, 69%). IR (KBr, cm ): ν
(s), 2721 (w), 1495 (w), 1435 (m), 1374 (m), 1261 (w), 1246 (m),
167 (w), 1066 (w), 1030 (m), 956 (w), 838 (s), 783 (s), 764 (s), 731
̃
3353 (w), 3273 (w), 2909 (s), 2857
EXPERIMENTAL SECTION
■
1
General Considerations. All manipulations were carried out
under argon atmosphere using Schlenk line or glovebox techniques.
Toluene and hexane were distilled from Na/K alloy just before use.
NMR solvents were dried with Na/K alloy (C D , C D ) or calcium
1
(
(
s), 703 (s), 609 (s), 516 (m), 482 (w), 466 (w), 422 (m). H NMR
300 MHz, C D , 20 °C): δ 12.84 (s br, 1H; NH), 3.72 (s br, 1H;
6
6
NHSiMe ), 2.14 (s, 15H; C Me ), 2.03 (s, 15H; C Me ), 1.94 (s, 15H;
3
5
5
5
5
6
6
7
8
13
1
C Me ), 0.25 (s, 9H; NHSiMe ). C{ H} NMR (75 MHz, C D , 20
hydride (CDCl , C D N) and vacuum-distilled. Dichloromethane-d
5
5
3
6
6
3
5
5
2
°
1
C): δ 118.3 (C Me ), 117.4 (C Me ), 117.1 (C Me ), 12.5 (C Me ),
2.4 (C Me ), 12.0 (C Me ), 5.7 (SiMe ). Anal. Calcd for
5 5 5 5 3
was dried over activated molecular sieves and stored under argon.
Oven-dried glassware was repeatedly evacuated with a pumping system
5 5 5 5 5 5 5 5
−
3
C H N SiTi (M = 680.51): C, 58.24; H, 8.29; N, 8.23. Found:
(
(
ca. 1 × 10 Torr) and subsequently filled with inert gas. ROSO CF3
33 56
4
3
w
2
R = H, Me, Me Si), [K{N(SiMe ) }], and 1,4,7,10,13,16-hexaox-
C, 57.97; H, 8.08; N, 7.59.
3
3 2
5
acyclooctadecane (18-crown-6) were purchased from Aldrich and used
Synthesis of [Ti
100 mL ampule (Teflon stopcock) was charged with 5 (0.30 g, 0.36
mmol), [K{N(SiMe }] (0.080 g, 0.40 mmol), and toluene (20 mL).
3 5 5 3 3 2 3
(η -C Me ) (μ -N)(μ-NH) (μ-NSiMe )] (7). A
5
as received. [{Ti(η -C Me )(μ-NH)} (μ -N)] (1) was prepared
5
5
3
3
12b
according to a published procedure. The syntheses and character-
ization of complexes 3 and 4 have been reported previously.
)
3 2
1
6
The reaction mixture was stirred at 85 °C for 16 h. After filtration, the
Samples for infrared spectroscopy were prepared as KBr pellets, and
the spectra were obtained using an FT-IR Perkin-Elmer SPECTRUM
volatile components of the solution were removed under reduced
pressure to give 7 as an orange solid (0.17 g, 69%). H NMR (300
1
1
13
1
19
2
000 spectrophotometer. H, C{ H}, and F NMR spectra were
MHz, C D , 20 °C): δ 14.22 (s br, 2H; NH), 2.09 (s, 30H; C Me ),
6
6
5
5
13
1
recorded on Varian Unity-300, Mercury-300, and/or Unity-500 Plus
1.88 (s, 15H; C Me ), 0.15 (s, 9H; NSiMe ). C{ H} NMR (75
5
5
3
1
13
1
spectrometers. Chemical shifts (δ, ppm) in the H and C{ H} NMR
spectra are given relative to residual protons or to carbon of the
solvent. Chemical shifts (δ, ppm) in the 19F NMR spectra are given
MHz, C D , 20 °C): δ 118.2 (C Me ), 117.8 (C Me ), 12.5 (C Me ),
6 6 5 5 5 5 5 5
11.8 (C Me ), 7.0 (SiMe ). Compound 7 has been previously prepared
5
5
3
17
in 59% yield by a different procedure.
5
relative to CFCl as external reference. Microanalyses (C, H, N, S)
Synthesis of [K{(μ -N)(μ -NH)(μ -NSiMe )Ti (η -C Me ) (μ -
3
3 3 3 3 3 5 5 3 3
were performed in a Leco CHNS-932 microanalyzer.
N)}] (8). A 100 mL ampule (Teflon stopcock) was charged with 5
5
Synthesis of [Ti (η -C Me ) (μ -N)(μ-NH) (μ-NH )(OSO CF )]
(0.30 g, 0.36 mmol), [K{N(SiMe ) }] (0.16 g, 0.80 mmol), and
3
5
5 3
3
2
2
2
3
3 2
(
2). A 100 mL Schlenk flask was charged with 1 (0.30 g, 0.49
toluene (30 mL). The reaction mixture was stirred at 85 °C for 24 h.
After filtration, the volatile components of the solution were removed
mmol), HOSO CF (0.089 g, 0.59 mmol), and toluene (10 mL). The
2
3
reaction mixture was stirred at room temperature for 24 h to give a
yellow solid and an orange solution. The solid was isolated by filtration
onto a glass frit and vacuum-dried to afford 2 as a yellow powder (0.31
under reduced pressure to give 8 as a red solid (0.23 g, 89%). IR (KBr,
−1
cm ): ν
1
̃
3325 (w), 2905 (s), 2856 (s), 2720 (w), 1496 (w), 1437 (m),
374 (m), 1257 (m), 1244 (s), 1095 (w), 1065 (w), 1023 (m), 956
−1
g, 84%). IR (KBr, cm ): ν
914 (s), 2860 (m), 2727 (w), 1578 (w), 1490 (w), 1436 (m), 1377
s), 1315 (vs), 1279 (w), 1234 (vs), 1207 (vs), 1199 (vs), 1167 (vs),
̃
3374 (w), 3362 (w), 3353 (m), 3250 (m),
(
vs), 822 (s), 731 (vs), 702 (s), 662 (s), 629 (s), 593 (w), 548 (w),
2
(
1
5
1
2
10 (w), 473 (m), 415 (w). H NMR (300 MHz, C D , 20 °C): δ
6 6
3.34 (s br, 1H; NH), 2.30 (s, 15H; C Me ), 2.13 (s, 15H; C Me ),
5
5
5
5
1
6
068 (w), 1017 (vs), 779 (vs), 775 (vs), 752 (vs), 708 (m), 678 (s),
34 (s), 548 (m), 508 (m), 485 (w), 461 (w), 420 (w). H NMR (300
1
.11 (s, 15H; C Me ), 0.11 (s, 9H; NSiMe ). H NMR (300 MHz,
1
5
5
3
C D N, 20 °C): δ 13.92 (s br, 1H; NH), 2.28 (s, 15H; C Me ), 2.15
5
5
5
5
MHz, CDCl , 20 °C): δ 12.50 (s br, 2H; NH), 3.28 (s br, 1H; NHH),
2
not detected. H NMR (500 MHz, CD Cl , 20 °C): δ 13.65 (s br, 2H;
NH), 4.23 (m br, 1H; NHH), 3.32 (m br, 1H; NHH), 2.09 (s br, 45H;
C Me ). H NMR (500 MHz, CD Cl , −50 °C): δ 13.69 (s br, 2H;
NH), 4.37 (d, J(H,H) = 9.5 Hz, 1H; NHH), 3.20 (d, J(H,H) = 9.5
Hz, 1H; NHH), 2.04 (s, 30H; C Me ), 1.92 (s, 15H; C Me ). C{ H}
3
(
s, 15H; C Me ), 2.05 (s, 15H; C Me ), 0.30 (s, 9H; NSiMe ).
5 5 5 5 3
.01 (s br, 45H; C Me ), one resonance signal for the NH ligand was
13
1
5
5
2
C{ H} NMR (75 MHz, C D , 20 °C): δ 116.6 (C Me ), 115.9
1
6
6
5
5
2
2
(
C Me ), 113.9 (C Me ), 13.0 (C Me ), 12.8 (C Me ), 12.5 (C Me ),
5 5 5 5 5 5 5 5 5 5
13
1
1
9.0 (SiMe
(C Me ), 114.4 (C
12.0 (C Me ), 8.4 (SiMe
3
). C{ H} NMR (75 MHz, C
Me ), 113.3 (C Me ), 12.7 (C
). Anal. Calcd for C33
5 5
D N, 20 °C): δ 115.4
5
5
2
2
2
2
5
5
5
5
5
5
5
Me
5
), 12.5 (C
SiTi (M
w
5
Me
5
),
=
13
1
5
5
3
H55KN
4
3
5
5
5
5
7
18.60): C, 55.16; H, 7.71; N, 7.80. Found: C, 54.89; H, 7.90; N, 7.76.
NMR (75 MHz, CDCl , 20 °C): δ 121.4 (s br, C Me ), 11.9 (C Me ),
3
5
5
5
5
5
19
Synthesis of [K(18-crown-6)][Ti (η -C Me ) (μ -N)(μ-N)(μ-
3
5
5 3
3
the CF carbon atom resonance was not detected. F NMR (282
3
NH)(μ-NSiMe )] (9). A toluene solution (10 mL) of 18-crown-6
3
MHz, CDCl , 20 °C): δ −78.3. Anal. Calcd for C H F N O STi
3
31 49
3
4
3
3
(0.074 g, 0.28 mmol) was added to a solution of 8 (0.20 g, 0.28 mmol)
(
M = 758.41): C, 49.09; H, 6.51; N, 7.39; S, 4.23. Found: C, 49.88;
w
in toluene (15 mL). The reaction mixture was stirred at room
temperature for 5 min to give an abundant yellow solid. The solid was
isolated by filtration onto a glass frit and vacuum-dried to afford 9 as a
H, 6.24; N, 7.33; S, 4.26.
5
Synthesis of [Ti (η -C Me ) (μ -N)(μ-NH) (μ-NHSiMe )]-
3
5
5 3
3
2
3
[
O SCF ] (5). In a fashion similar to the preparation of 2, the
3 3
−
1
yellow powder (0.22 g, 80%). IR (KBr, cm ): ν
747 (m), 2715 (m), 1496 (w), 1472 (m), 1452 (m), 1372 (m), 1352
s), 1286 (w), 1249 (s), 1230 (s), 1105 (vs), 960 (s), 939 (s), 826 (s),
̃
3348 (w), 2901 (vs),
treatment of 1 (0.60 g, 0.98 mmol) with Me SiOSO CF (0.22 g, 0.99
3
2
3
2
(
mmol) in toluene (10 mL) for 24 h afforded 5 as an orange powder
−
1
(
(
0.72 g, 88%). IR (KBr, cm ): ν
̃
3271 (m, broad), 2952 (m), 2914
1
7
44 (vs), 686 (s), 623 (s), 514 (m), 487 (m), 441 (w). H NMR (300
s), 1489 (w), 1428 (m), 1380 (s), 1252 (vs), 1222 (s), 1200 (m),
1
6
149 (vs), 1031 (vs), 951 (w), 840 (vs), 761 (vs), 736 (vs), 711 (vs),
66 (vs), 637 (vs), 571 (m), 517 (m), 461 (m), 422 (w). H NMR
MHz, C
2.44 (s, 15H; C
0.48 (s, 9H; NSiMe
113.9 (C Me ), 113.0 (C
(C Me ), 12.6 (C Me ), 12.3 (C
C H KN O SiTi (M = 982.91): C, 54.99; H, 8.10; N, 5.70. Found:
5
D
5
N, 20 °C): δ 14.09 (s br, 1H; NH), 3.41 (s, 24H; OCH
Me ), 2.31 (s, 15H; C Me ), 2.26 (s, 15H; C Me
). C{ H} NMR (75 MHz, C
Me ), 112.6 (C Me ), 70.0 (OCH
Me ), 8.3 (SiMe ). Anal. Calcd for
2
),
1
),
5
5
5
5
5
5
13
1
(
300 MHz, CDCl , 20 °C): δ 14.10 (s br, 2H; NH), 4.92 (s, 1H;
3
5
D
5
N, 20 °C): δ
), 12.8
3
NHSiMe ), 2.12 (s, 30H; C Me ), 2.01 (s, 15H; C Me ), −0.17 (s,
5
5
5
5
5
5
2
3
5
5
5
5
13
1
9
(
H; NHSiMe ). C{ H} NMR (75 MHz, CDCl , 20 °C): δ 123.9
5
5
5
5
5
5
3
3
3
C Me ), 123.3 (C Me ), 12.7 (C Me ), 11.9 (C Me ), 4.8 (SiMe ),
5 5 5 5 5 5 5 5 3
45 79
4
6
3
w
19
the CF carbon atom resonance was not detected. F NMR (282
C, 55.17; H, 7.91; N, 5.30.
3
1
1520
dx.doi.org/10.1021/ic4018294 | Inorg. Chem. 2013, 52, 11519−11529

Inorganic Chemistry
Article
Table 1. Experimental Data for the X-ray Diffraction Studies on Compounds 2, 5, 6, and 8
2
5
6
8
formula
C H F N O STi
C H F N O SSiTi
C H N SiTi
C H KN SiTi
3
3
1
49
3
4
3
3
34 57
3
4
3
3
33 56
4
3
33 55
4
Mr
758.50
200(2)
0.71073
830.69
680.61
718.70
T [K]
200(2)
200(2)
0.71073
triclinic
200(2)
λ [Å]
0.71073
0.71073
crystal system
space group
a [Å]; α [deg]
b [Å]; β [deg]
c [Å]; γ [deg]
V [Å3]
monoclinic
orthorhombic
Pbca
monoclinic
P2 /c
1
P1
̅
P2 /c
1
11.864(2)
19.579(14)
20.000(6)
21.224(1)
8311(6)
8
11.293(6); 95.11(3)
12.691(1); 100.16(3)
12.788(7); 98.30(3)
1773(1)
10.918(9)
19.143(4); 117.84(1)
18.043(3)
3264(1)
16.303(8); 115.38(4)
22.856(11)
3676(4)
Z
4
2
4
ρcalcd [g cm−
μMoKα [mm−
F(000)
3
]
]
1.390
1.328
1.275
1.299
1
0.754
0.691
0.720
0.809
1584
3488
724
1520
3
crystal size [mm ]
θ range (deg)
index ranges
0.22 × 0.20 × 0.17
3.19−27.51
−15 to 15
0.16 × 0.15 × 0.14
3.01−26.17
−24 to 24
−24 to 24
−26 to 26
0.27 × 0.23 × 0.20
3.26−27.50
−14 to 14
−15 to 16
−16 to 16
0.25 × 0.13 × 0.12
3.06−27.51
−14 to 14
−21 to 21
−29 to 14
73 443
−
−
24 to 24
15 to 23
reflns collected
unique data
86 270
225 003
71 530
8304 [R(int) = 0.051]
5593
8226 [R(int) = 0.197]
5083
8151 [R(int) = 0.073]
5849
8449 [R(int) = 0.097]
4692
obsd data [I > 2σ(I)]
GOF on F2
1.035
1.038
1.055
1.015
a
final R indices [I > 2σ(I)]
R1 = 0.049
wR2 = 0.110
R1 = 0.090
wR2 = 0.124
0.349 and −0.623
R1 = 0.073
wR2 = 0.172
R1 = 0.131
wR2 = 0.219
0.787 and −0.597
R1 = 0.052
wR2 = 0.136
R1 = 0.083
wR2 = 0.150
0.504 and −0.565
R1 = 0.068
wR2 = 0.133
R1 = 0.144
wR2 = 0.155
0.991 and −0.496
a
R
indices (all data)
largest diff. peak/hole [e Å−3]
R1 = ∑||F | − |F ||/[∑|F |]; wR2 = {[∑w(F − F ) ]/[∑w(F ) ]}1/2.
a
2
2
c
2
2 2
0
c
0
0
0
X-ray Structure Determination of 2, 5, 6, and 8. Crystals of
final values of occupancy were 62.0% and 38.0%. All non-hydrogen
atoms were anisotropically refined. The amido group hydrogen atom,
H(12), was located in the difference Fourier map and refined
isotropically, whereas the rest of the hydrogen atoms were positioned
geometrically and refined using a riding model. The imido hydrogen
atom was distributed over the nitrogen atoms N(13) and N(23) with
complexes 2 and 6 were grown from toluene solutions at −35 °C.
Crystals of complexes 5 and 8 were obtained from toluene solutions at
room temperature. The crystals were removed from the Schlenk flasks
and covered with a layer of a viscous perfluoropolyether (FomblinY).
A suitable crystal was selected with the aid of a microscope, mounted
on a cryoloop, and immediately placed in the low temperature
nitrogen stream of the diffractometer. The intensity data sets were
collected at 200 K on a Bruker-Nonius KappaCCD diffractometer
equipped with an Oxford Cryostream 700 unit. Crystallographic data
for all of the complexes are presented in Table 1.
5
0% of occupancy for each position. The nondisordered pentam-
ethylcyclopentadienyl groups, linked to Ti(2) and Ti(3), were
restrained with DELU instructions.
Finally, the solid-state structure of compound 8 revealed a one-
dimensional polymeric network. All of the non-hydrogen atoms were
anisotropically refined, whereas all of the hydrogen atoms were
positioned geometrically and refined by using a riding model, except
that of the imido group (H(23)), which was located in the difference
Fourier map and refined isotropically.
1
8
The structures were solved, using the WINGX package, by direc2t
methods (SHELXS-97) and refined by least-squares against F
19
(
SHELXL-97). In the crystallographic study of compound 2, all
non-hydrogen atoms were anisotropically refined. All of the hydrogen
atoms were positioned geometrically and refined by using a riding
model, except those of the imido (H(12) and H(23)) and amido
RESULTS AND DISCUSSION
■
5
(
H(13a) and H(13b)) groups, which were located in the difference
The treatment of [{Ti(η -C Me )(μ-NH)} (μ -N)] (1) with 1
5
5
3
3
Fourier map and refined isotropically. Moreover, SADI restraints were
employed for the lengths N(12)−H(12) and N(23)−H(23).
equiv of triflic acid or methyl triflate in toluene at room
temperature afforded the precipitation of complexes [Ti (η -
5
−
3
Anion CF SO in complex 5 presented disorder, which was treated
5
3
3
C Me ) (μ -N)(μ-NH) (μ-NH )(OSO CF )] (2) or [Ti (η -
5
5 3
3
2
2
2
3
3
conventionally by using the PART tool of the SHELXL-97 program
and allowing free refinement of the occupancy factors with the FVAR
command. The final values of occupancy were 55% and 45%. All non-
hydrogen atoms were anisotropically refined, except fluorine and
oxygen atoms for the triflate group (F(1), F(2), F(3), F(1)′, F(2)′,
F(3)′, O(1), O(2), O(3), O(1)′, O(2)′, and O(3)′), which were
refined isotropically. The hydrogen atoms were positioned geometri-
cally and refined using a riding model. Furthermore, DFIX restraints
were used for the triflate anion.
C Me ) (μ -N)(μ-NH)(μ-NH )(μ-NMe)(OSO CF )] (3)
5
5
3
3
2
2
3
Scheme 1. Reaction of 1 with 1 equiv of ROTf (R = H, Me)
Crystals of the complex 6 showed disorder for the carbon atoms
C(11)−C(20) of the pentamethylcyclopentadienyl ligand linked to
Ti(1). This disorder was also treated using the PART tool, and the
1
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Inorganic Chemistry
Article
Scheme 2. Schematic Representation of the Bonding
Situation in Complexes 2 and 3
Figure 1. Perspective view of complex 2 (thermal ellipsoids at the 50%
probability level). The methyl groups of the pentamethylcyclopenta-
dienyl ligands are omitted for clarity.
(
(
Scheme 1). Compounds 2 and 3 were isolated in good yields
84% and 78%, respectively) as yellow or orange solids, which
are very soluble in halogenated solvents, moderately soluble in
toluene or benzene, and insoluble in hexane.
Complexes 2 and 3 were characterized by analytical and
spectroscopic methods, as well as by X-ray crystal structure
determinations. The solid-state structure of 2 is shown in
Figure 1, which is almost identical to that reported for complex
3
with the substitution of the methylimido group for a μ-NH
16
ligand. Selected distances and angles for both structures are
compared in Table 2, showing very similar values for the two
compounds. The crystal structures show six-membered Ti N3
3
rings in chair conformation with the three titanium atoms also
bridged by a further nitrogen atom. Two of the titanium atoms,
Ti(1) and Ti(3), have classical three-legged piano-stool
arrangements, where the legs are occupied by one μ-NH2
Figure 2. Simplified view of the intra- and intermolecular hydrogen-
bonding interactions present in complexes 2 and 3. Symmetry code:
(i) −x, 2 − y, −z.
amido, one μ -N nitrido, and a μ-NR imido ligand. The Ti(2)
3
Table 2. Selected Lengths (Å) and Angles (deg) for Complexes 2 and 3
lengths
2
3
angles
2
3
Ti(1)−N(1)
Ti(1)−N(12)
Ti(1)−N(13)
Ti(2)−N(1)
Ti(2)−N(12)
Ti(2)−N(23)
Ti(2)−O(1)
Ti(3)−N(1)
Ti(3)−N(13)
Ti(3)−N(23)
Ti(1)···Ti(2)
Ti(1)···Ti(3)
Ti(2)···Ti(3)
N(23)−C(1)
1.866(2)
1.858(3)
2.107(3)
2.137(2)
1.986(2)
1.975(2)
2.172(2)
1.877(2)
2.103(2)
1.844(3)
2.874(1)
2.910(1)
2.895(1)
1.868(3)
1.831(3)
2.102(3)
2.084(3)
1.983(3)
2.000(3)
2.157(3)
1.868(3)
2.099(3)
1.856(3)
2.866(1)
2.905(1)
2.853(1)
1.467(5)
N(1)−Ti(1)−N(12)
N(1)−Ti(1)−N(13)
N(12)−Ti(1)−N(13)
N(1)−Ti(2)−N(12)
N(1)−Ti(2)−N(23)
N(12)−Ti(2)−N(23)
N(1)−Ti(2)−O(1)
N(12)−Ti(2)−O(1)
N(23)−Ti(2)−O(1)
N(1)−Ti(3)−N(13)
N(1)−Ti(3)−N(23)
N(13)−Ti(3)−N(23)
Ti(1)−N(1)−Ti(2)
Ti(1)−N(1)−Ti(3)
Ti(2)−N(1)−Ti(3)
Ti(1)−N(12)−Ti(2)
Ti(1)−N(13)−Ti(3)
Ti(2)−N(23)−Ti(3)
Ti(2)−N(23)−C(1)
Ti(3)−N(23)−C(1)
90.3(1)
85.3(1)
101.0(1)
79.6(1)
78.4(1)
118.9(1)
145.0(1)
84.2(1)
82.9(1)
85.1(1)
88.7(1)
101.5(1)
91.5(1)
102.1(1)
92.1(1)
96.8(1)
87.5(1)
98.5(1)
88.7(1)
85.1(1)
102.2(1)
78.9(1)
80.5(1)
118.8(1)
145.0(1)
82.1(1)
83.3(1)
85.2(1)
90.3(1)
101.2(1)
92.8(1)
102.1(1)
92.3(1)
97.4(1)
87.5(1)
95.4(1)
124.4(3)
140.2(3)
1
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Inorganic Chemistry
Table 3. Relevant Hydrogen Bonds for Compounds 2 and 3
Article
a
−1
one absorption at 1578 and 1590 cm , respectively, assignable
to the NH bending mode. Also, the IR spectra show several
2
3
2
D−H···A
D···A/Å
H···A/Å D−H···A/deg
−1
strong absorptions in the range 1315−1011 cm for the
2
3
N(12)−H(12)···O(3)
N(13)−H(13a)···O(2)
N(12)−H(12)···O(2)
N(13)−H(13a)···O(3)
N(13)−H(13b)···O(3)i
3.574(5)
3.198(4)
3.130(4)
3.417(6)
3.576(6)
3.15(4)
2.45(4)
118(3)
164(4)
24
1
13
1
coordinated triflato ligands. Surprisingly, the H and C{ H}
NMR spectra of solutions of crystals of complexes 2 and 3 in
benzene-d or chloroform-d at room temperature are different
6
1
2.54(5)
2.81(4)
159(4)
146(3)
from those expected from their solid-state structures. Thus, the
H NMR spectra of 2 show only one broad singlet for the η -
C Me ligands, instead of the two 2:1 signals expected from the
b
1
5
a
b
A = acceptor; D = donor. Symmetry code: (i) −x, 2 − y, −z.
5
5
nearly C symmetry in the solid state. Furthermore, while the
s
1
H NMR spectra of 3 reveal the resonance signals predicted for
atoms exhibit four-legged piano-stool arrangements, in which
the legs are occupied by one triflato, one nitrido, and two imido
ligands.
the C symmetry determined in the crystal structure, the
1
5
resonances for two η -C Me ligands and those assigned to the
NH and NH groups are broad. These NMR data suggest a
dynamic exchange process in solution, and H NMR spectra of
5
5
16
2
The NH2 amido group bridges the titanium(1) and
titanium(3) atoms in a symmetric fashion with Ti−N(13)
bond lengths, 2.099(3)−2.107(3) Å, which are about 0.25 Å
longer than the distances between those titanium atoms and the
imido ligands, Ti(1)−N(12) and Ti(3)−N(23), of 1.831(3)−
1
dichloromethane-d solutions were taken at low temperatures
2
1
in a 500 MHz spectrometer. Indeed, the H NMR spectrum of
2
at −50 °C is consistent with a C symmetric structure in
s
solution, while the spectrum of 3 at −30 °C reveals sharp
resonances for the η -C Me and μ-NH ligands.
1.858(3) Å. The nitrido group N(1) also bridges symmetrically
5
5
5
2
the Ti(1) and Ti(3) atoms with values of titanium−nitrogen
bond lengths (1.866(2)−1.877(2) Å) in the upper limit of
those found in titanium dinuclear complexes containing Ti
The dynamic behavior may be the result of the proton
transfer from the NH group to the hydrogen-bonded triflato
2
1
1d,12b,20
ligand and generation of HOTf, which then delivers that proton
to the NH imido group with concomitant coordination of the
triflato ligand at the opposite titanium atom. Such rapid
exchange would create time-averaged C symmetry for 2 and
NTi fragments (1.763(5)−1.878(7) Å),
and sub-
stantially shorter than the Ti(2)−N(1) separations (2.137(2) Å
in 2 and 2.084(3) Å for 3). A plausible interpretation of the
bonding in complexes 2 and 3 is illustrated in Scheme 2. The
bonding system can be described by two resonance forms in
valence bond theory terms involving one titanium−nitrogen(1)
double bond within the flat ring containing the Ti(1), Ti(3),
N(1), and N(13) atoms. The first of the contributing structure
represents a double bond between the Ti(3) and N(1) atoms
with concomitant dative N(13)→Ti(3) bond, whereas the
second structure comprises Ti(1)N(1) and N(13)→Ti(1)
units.
3
v
1
C symmetry for 3 by H NMR spectroscopy at high
s
temperatures. Complete dissociation of HOTf can be ruled
out because addition of small amounts of 1 to solutions of
compounds 2 or 3 does not affect the exchange process, and
the activation parameters remain unaltered. A plausible
mechanism for the dynamic event is shown in Scheme 3,
which is similar to that studied in detail by Limbach and co-
workers for the proton exchange process in diaryltriazenes
25
catalyzed by the presence of bases in the medium. In our
complexes, the function of the basic catalyst could be played by
the triflato ligand, which is prone to dissociate from the metal
The triflato group is linked to titanium(2) with Ti(2)−O(1)
bond lengths of 2.172(2) Å in 2 and 2.157(3) Å for 3, which
are longer than the Ti−O distances found in other titanium
derivatives with terminal triflato ligands (1.938(1)−2.142(3)
26
centers.
2
1
The kinetic parameters (Table 4) of the observed process
Å). The remaining oxygen atoms of the trifluoromethane-
sulfonato groups, O(2) and O(3), are involved in N−H···O
intramolecular hydrogen bonds with the endo hydrogen of the
1
were calculated on the basis of dynamic H NMR spectroscopy
5
data with line shape analysis of the η -C Me resonances using
5
5
μ-NH amido and the μ-NH imido ligands (Figure 2, Table 3).
the gNMR program (see Tables S1−S4 and Figures S1−S8 in
2
27
Those interactions can be classified as weak according to the
the Supporting Information). The results disagree with those
expected for an intramolecular nondissociative mechanism
2
2
criteria on the donor−acceptor distances. In addition,
complex 3 displays intermolecular hydrogen-bonding inter-
actions between the O(3) atom and the exo hydrogen of the μ-
⧧
because the log A (8.5−9.8) and ΔS (between −7.7 and −21.3
eu) values differ from the usual parameters for that type of
⧧
28
NH amido ligand to result in molecules linked in pairs across a
process (log A = 12−14 and ΔS ≈ 0 eu). However, in
2
⧧
crystallographic symmetry center (Figure 2).
accord with the highly negative values of ΔS , the reduction of
the log A in several units could be related to the organization of
The IR spectra (KBr) of compounds 2 and 3 show several
−1
bands in the νNH region, between 3374 and 3250 cm , for the
the solvent molecules due to the participation of zwitterionic
25b,29
NH and NH ligands. In addition, the spectra of 2 and 3 reveal
species in the exchange process,
as shown in Scheme 3.
2
Scheme 3. Proposed Fluxional Process for Complexes 2 and 3
1
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Inorganic Chemistry
Article
5
Table 4. Activation Parameters for the Exchange of η -C Me Resonances in Complexes 2 and 3
5
5
compound
log A
Ea [kcal mol−1]
ΔH⧧ [kcal mol−1
]
ΔS⧧ [cal mol−1 K−1
]
ΔG⧧298K [kcal mol−1
]
a
2
8.5 ± 1.2
9.8 ± 0.8
8.5 ± 2.1
8.9 ± 1.7
b
8.4 ± 0.3
9.7 ± 0.2
8.4 ± 0.5
9.5 ± 0.4
7.9 ± 0.3
9.2 ± 0.2
11.9 ± 0.5
8.9 ± 0.4
−2
−21.3 ± 1.0
−15.6 ± 0.7
−7.7 ± 1.8
−19.7 ± 1.5
14.2
13.9
14.2
14.8
b
2
c
2
a
3
a
−2
−3
c
CD Cl , ∼1.6 × 10 M. CD Cl , ∼5.3 × 10 M. C D , ∼1.1 × 10 M.
2
2
2
2
7
8
Scheme 4. Reaction of 1 with 1 equiv of ROTf (R = Me,
SiMe3)
is ca. 25% after 2 h, and 100% after 24 h). Thus, complex 4
could not be obtained in a pure form and was only
1
13
1
19
characterized by H, C{ H}, and F NMR spectroscopy.
The NMR data are consistent with a C symmetric structure
s
with the methylamido and triflato ligands in the mirror plane of
the molecule. NOESY-1D experiments on a solution of 4 in
benzene-d provided evidence for the endo position of the
6
methyl group for the μ-NHMe ligand. The rotation of the
amido ligand in complex 4 could place the methyl fragment in
the exo position, while the new endo hydrogen interacts with
the triflato ligand. From this undetected intermediate, the
migration of the proton from the NHMe amido group to the
NH imido ligands, assisted by the triflato moiety, would lead to
compound 3.
To isolate a stable compound with a structure similar to 4,
5
we studied the reaction of [{Ti(η -C Me )(μ-NH)} (μ -N)]
5
5
3
3
(
1) with an electrophilic reagent ROTf bearing a bulkier R
fragment. Thus, the treatment of 1 with 1 equiv of trimethylsilyl
triflate in toluene at room temperature afforded the
5
precipitation of the ionic complex [Ti (η -C Me ) (μ -N)(μ-
3
5
5 3
3
NH) (μ-NHSiMe )][O SCF ] (5) (Scheme 4). This com-
2
3
3
3
pound was obtained in 88% yield as an orange solid, which is
poorly soluble in toluene or benzene but exhibits a good
solubility in halogenated solvents. Solutions of 5 in chloroform-
d1 slowly decompose within several days at room temperature
to give free trimethylsilylamine and several unidentified
1
products as determined by H NMR spectroscopy. The IR
spectrum (KBr) of 5 shows only one broad band centered at
1
3
271 cm− for the νNH vibrations and several strong absorptions
−1
24
in the range 1252−1031 cm for the triflate group. In
particular, the observed ν (SO ) band at 1252 cm is typical
for the free trifluoromethanesulfonate anion. The H NMR
−1
as
3
24b
1
spectrum in chloroform-d at room temperature reveals two
1
5
resonances for η -C Me ligands in a 2:1 ratio, two broad
5
5
signals for the μ-NH and μ-NHSiMe protons, and a high-field
singlet for the trimethylsilyl group. These data agree with a Cs
3
Figure 3. Perspective view of the cationic fragment of complex 5
(
thermal ellipsoids at the 50% probability level). Hydrogen atoms are
symmetric structure in solution, which is also consistent with
omitted for clarity.
13
1
the C{ H} NMR spectrum. NOESY-1D experiments
confirmed the endo disposition of the trimethylsilyl group in
a fashion similar to the methyl fragment in 4, but the greater
Indeed, when the kinetic parameters of complex 2 are obtained
in toluene-d , the entropy of activation falls to −7.7 eu in
steric bulk of the SiMe group precludes the coordination of the
3
8
agreement with that expected for a less polar solvent.
triflate to yield 5 as an ion pair, at least in the solid state.
The X-ray crystal structure of 5 shows each trifluorometha-
nesulfonate anion linking through hydrogen-bonding inter-
actions two trinuclear titanium cations to give polymeric zigzag
chains (see Figure S9 in the Supporting Information). Thus,
each triflate anion establishes a hydrogen bond between the
O(1) atom and the NH ligand of one cation (N(13)···O(1) =
3.322(1) Å), while O(3) interacts with the NH group of a
contiguous cation (N(23)b···O(3) = 3.311(1) Å). The cationic
fragment of compound 5 is presented in Figure 3, while
selected distances and angles are given in Table 5.
⧧
Importantly, the ΔG values calculated in both solvents are
comparable, suggesting that the exchange process is the same
and the entropy differences are compensated by the enthalpy
2
9
associated.
An analogue proton migration within the trinuclear titanium
system could explain the formation of complex 3 in the
treatment of 1 with 1 equiv of MeOTf (Scheme 4). Monitoring
by NMR spectroscopy the reaction course in benzene-d6
showed that the alkylation occurs at the NH imido ligands of
5
1
to produce the methylamido derivative [Ti (η -C Me ) (μ -
3
5
5 3
3
16
N)(μ-NH) (μ-NHMe)(OTf)] (4). Compound 4 readily
The cation of 5 contains a [Ti N ] six-membered ring that
2
3
3
12a
rearranges at room temperature to form complex 3 (conversion
adopts a chair conformation similar to that of 1, and those
1
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Inorganic Chemistry
Article
Table 5. Selected Lengths (Å) and Angles (deg) for Complexes 5 and 6
lengths
5
6
angles
5
6
Ti(1)−N(1)
Ti(1)−N(12)
Ti(1)−N(13)
Ti(2)−N(1)
Ti(2)−N(12)
Ti(2)−N(23)
Ti(3)−N(1)
Ti(3)−N(13)
Ti(3)−N(23)
N(12)−Si(1)
Ti(1)···Ti(2)
Ti(1)···Ti(3)
Ti(2)···Ti(3)
1.921(4)
2.121(5)
1.866(4)
1.917(5)
2.111(5)
1.868(4)
1.938(4)
1.971(5)
1.978(5)
1.808(5)
2.935(2)
2.818(1)
2.804(1)
1.878(2)
2.105(2)
1.817(2)
1.914(2)
2.103(2)
1.793(3)
1.957(2)
1.923(2)
1.892(2)
1.761(2)
2.896(1)
2.753(2)
2.746(1)
N(1)−Ti(1)−N(12)
N(1)−Ti(1)−N(13)
N(12)−Ti(1)−N(13)
N(1)−Ti(2)−N(12)
N(1)−Ti(2)−N(23)
N(12)−Ti(2)−N(23)
N(1)−Ti(3)−N(13)
N(1)−Ti(3)−N(23)
N(13)−Ti(3)−N(23)
Ti(1)−N(1)−Ti(2)
Ti(1)−N(1)−Ti(3)
Ti(2)−N(1)−Ti(3)
Ti(1)−N(12)−Ti(2)
Ti(1)−N(12)−Si(1)
Ti(2)−N(12)−Si(1)
Ti(1)−N(13)−Ti(3)
Ti(2)−N(23)−Ti(3)
85.9(2)
87.3(2)
103.3(2)
86.3(2)
88.0(2)
105.2(2)
84.0(2)
84.4(2)
110.4(2)
99.8(2)
93.8(2)
93.3(2)
87.8(2)
124.0(2)
120.6(2)
94.5(2)
93.5(2)
86.8(1)
89.1(1)
103.6(1)
85.9(1)
88.4(1)
103.8(1)
83.8(1)
84.4(1)
109.9(1)
99.6(1)
91.7(1)
90.4(1)
87.0(1)
124.5(1)
121.0(1)
94.7(1)
96.3(1)
Scheme 5. Schematic Representation of the Bonding
Situation in the Cation of 5 and Complex 6
Scheme 6. Reactions with [K{N(SiMe ) }]
3 2
described above for complexes 2 and 3. In contrast to
compounds 2 and 3, the cationic fragment of 5 shows the
nitrido atom N(1) bridging the titanium atoms with the three
Ti−N(1) distances within a narrow range, 1.917(5)−1.938(4)
Å, which compares well with that determined in complex 1
(
Ti−N(1) 1.912(1) Å). The NHSiMe amido group in 5
3
bridges the Ti(1) and Ti(2) atoms in a symmetric fashion with
Ti−N(12) bond lengths, 2.111(5) and 2.121(5) Å, very similar
to the Ti−N separations found for the NH amido ligand in
2
complexes 2 and 3. Each NH imido ligand of 5 bridges
asymmetrically two titanium atoms with Ti(1)−N(13) and
Ti(2)−N(23) bond lengths, 1.866(4) and 1.868(4) Å, clearly
shorter than those found with the Ti(3) atom, Ti(3)−N(13)
5
the molecular complex [Ti (η -C Me ) (μ -N)(μ-N)(μ-
3
5
5
3
3
NH)(μ-NHSiMe )] (6), NH(SiMe ) , and KOTf (Scheme
3
3 2
6). Compound 6 was isolated as a red solid in 69% yield if the
reaction and subsequent workup were performed within 1 h.
Reactions carried out at room temperature for longer periods of
time afforded mixtures of compound 6 and the trimethylsily-
limido derivative [Ti (η -C Me ) (μ -N)(μ-NH) (μ-NSiMe )]
(7). Indeed, complex 7 was isolated as an orange solid in 69%
yield if the reaction mixture of 5 and [K{N(SiMe ) }] in
toluene was heated at 85 °C for 24 h. Compound 7 has been
previously obtained by treatment of [{Li(μ -N)(μ -NH) Ti -
(η -C Me ) (μ -N)} ] with [SiClMe ] in toluene at room
5 5 3 3 2 3
17
1
.971(5) and Ti(3)−N(23) 1.978(5) Å. Therefore, the solid-
state structure of the cation of 5 is very close to the Cs
symmetry determined in solution by NMR spectroscopy. A
plausible interpretation of the bonding in the trinuclear cation
is illustrated on the left side of Scheme 5. The bonding system
can be described by two resonance forms in valence bond
theory terms containing the positive charge on titanium(1) or
titanium(2) atoms.
5
3
5
5 3
3
2
3
3
2
4
3
2
3
5
The treatment of 5 with 1 equiv of potassium bis-
trimethylsilyl)amide in toluene at room temperature leads to
(
temperature.
1
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Inorganic Chemistry
Article
Figure 4. Perspective view of complex 6 (thermal ellipsoids at the 50%
probability level). Methyl groups of the pentamethylcyclopentadienyl
ligands and hydrogen atoms of the imido groups are omitted for
clarity.
Compound 6 was characterized by spectroscopic and
analytical methods, as well as by an X-ray crystal structure
determination. The IR spectrum (KBr) of 6 shows two ν
NH
−
1
vibrations at 3353 and 3273 cm for the NH and NHSiMe3
1
ligands. The H NMR spectrum of 6 in benzene-d at room
6
5
temperature reveals three resonance signals for the η -C Me
5
5
Figure 6. Perspective view of a portion of the polymeric chain of
compound 8 (thermal ellipsoids at the 50% probability level).
Hydrogen atoms of the pentamethylcyclopentadienyl groups are
omitted for clarity. Symmetry code: (i) −x, 1/2 + y, −1/2 − z; (ii) −x,
−1/2 + y, −1/2 − z.
ligands in a 1:1:1 ratio, one broad signal for one μ-NH imido
group, and those assigned to the NHSiMe trimethylsilylamido
3
ligand. The NMR data of 6 are consistent with a C symmetric
1
structure in solution, in contrast to the C symmetry observed
s
17
in the NMR spectra of 7. The solid-state structure of 6 is
presented in Figure 4, while selected distances and angles are
given in Table 5. The [Ti N ] core of 6 is similar to that
structure of 6 determined by X-ray crystallography, is the
existence of an equimolecular mixture of the two enantiomers
originated from proton abstraction of complex 5 as shown in
the right side of Scheme 5.
3
4
described above for the cation of complex 5. Thus, the nitrido
ligand N(1) bridges the three titanium centers with Ti−N(1)
bond lengths in the range 1.878(2)−1.957(2) Å, and the amido
NHSiMe ligand is bonded to the Ti(1) and Ti(2) atoms with
The conversion of 6 to complex 7 in solution at room
3
1
distances Ti(1)−N(12) and Ti(2)−N(12) of 2.105(2) and
temperature was monitored by H NMR spectroscopy. Once 6
2.103(2) Å, respectively. While these bond lengths are very
is isolated in a pure form, the rearrangement is slow in benzene-
close to those observed in 5, complex 6 shows a shortening of
the Ti−N distances associated with the N(13) and N(23)
atoms as expected for the deprotonation of one NH imido
group to yield a μ-N nitrido ligand. The Ti(1)−N(13) and
Ti(2)−N(23) bond lengths, 1.817(2) and 1.793(2) Å,
respectively, are about 0.1 Å shorter than the titanium−
nitrogen distances associated to the titanium(3) atom, Ti(3)−
N(13) 1.923 Å and Ti(3)−N(23) 1.892(2) Å. A plausible
d6 solution (ca. 10% after 3 days). While addition of
tetrahydrofuran (4 equiv) to a benzene-d solution of 6 does
6
not affect the reaction rate, the presence of residual free
NH(SiMe ) in the solution produces a significant enhance-
3
2
ment of the transformation. Furthermore, upon dissolving 6 in
chloroform-d , there is an immediate color change from red to
1
1
orange, and the H NMR spectrum shows resonances assigned
to 7. Complete consumption of 6 in chloroform-d occurs
1
interpretation of these distances, and the nearly C symmetric
within 24 h to give complex 7, with partial incorporation of
s
Figure 5. Perspective view of the polymeric chain of 8. Hydrogen atoms are omitted for clarity.
1
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Inorganic Chemistry
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a
Table 6. Selected Lengths (Å) and Angles (deg) for Compound 8
K(1)−N(12)
K(1)−N(23)
3.108(4)
2.776(4)
3.168(4)
3.000(4)
2.882
K(1)−N(13)
2.772(4)
3.258(4)
3.006(4)
3.171(4)
1.980(3)
1.916(3)
1.961(3)
1.905(3)
1.883(3)
1.957(3)
1.713(3)
2.762(2)
i
K(1)−C(11)
K(1)−C(13)
K(1)−C(15)
i
i
i
K(1)−C(12)
K(1)−C(14)
i
i
K(1)−Cm(1)
Ti(1)−N(12)
Ti(1)−N(1)
Ti(2)−N(12)
Ti(2)−N(1)
Ti(3)−N(13)
Ti(3)−N(1)
Si(1)−N(12)
Ti(1)···Ti(3)
Ti(1)−N(13)
Ti(1)−Cm(1)
Ti(2)−N(23)
Ti(2)−Cm(2)
Ti(3)−N(23)
Ti(3)−Cm(3)
Ti(1)···Ti(2)
Ti(2)···Ti(3)
N(12)−K(1)−N(13)
N(13)−K(1)−N(23)
N(13)−K(1)−Cm(1)
N(12)−Ti(1)−N(13)
N(13)−Ti(1)−N(1)
N(12)−Ti(2)−N(1)
N(13)−Ti(3)−N(23)
N(23)−Ti(3)−N(1)
Ti(1)−N(12)−K(1)
Ti(1)−N(12)−Si(1)
K(1)−N(12)−Si(1)
K(1)−N(13)−Ti(1)
Ti(2)−N(23)−Ti(3)
K(1)−N(23)−Ti(3)
Ti(1)−N(1)−Ti(3)
1.860(3)
2.147
b
b
1.913(4)
2.105
1.929(4)
2.126
b
2.805(2)
2.804(3)
62.0(1)
65.2(1)
152.9
N(12)−K(1)−N(23)
N(12)−K(1)−Cm(1)
N(23)−K(1)−Cm(1)
N(12)−Ti(1)−N(1)
61.9(1)
143.2
i
i
i
128.9
104.7(2)
87.7(1)
87.4(1)
103.3(2)
85.4(2)
89.1(1)
133.6(2)
85.1(1)
102.5(1)
93.7(2)
91.8(1)
91.0(1)
86.5(1)
103.4(1)
87.3(2)
85.9(1)
90.7(1)
90.8(1)
135.3(2)
95.1(2)
92.9(1)
102.6(1)
94.5(1)
93.1(2)
N(12)−Ti(2)−N(23)
N(23)−Ti(2)−N(1)
N(13)−Ti(3)−N(1)
Ti(1)−N(12)−Ti(2)
Ti(2)−N(12)−K(1)
Ti(2)−N(12)−Si(1)
Ti(1)−N(13)−Ti(3)
K(1)−N(13)−Ti(3)
K(1)−N(23)−Ti(2)
Ti(1)−N(1)−Ti(2)
Ti(2)−N(1)−Ti(3)
a
b
5
Symmetry code: (i) −x, 1/2 + y, −1/2 − z. Cm = centroid of the η -C Me groups.
5
5
deuterium at the μ-NH ligands, along with minor amounts
A portion of the polymeric chain of compound 8 is shown in
(
∼7%) of compound 1. In view of these results, it appears that
Figure 6, while selected distances and angles are given in Table
the rearrangement is catalyzed by a Brønsted acid HA, such as
NH(SiMe ) or CHCl , which transfers the proton to the μ-N
ligand of 6, and subsequently its conjugate base A accepts the
proton from the μ-NHSiMe amido ligand to generate 7.
6. The crystal structure contains distorted [KTi
cores with the metalloligands [(μ -N)(μ -NH)(μ
Ti (η -C Me ) (μ -N)] coordinating in a tripodal fashion to
3
N
4
] cube-type
-NSiMe )-
3
2
3
3
3
3
3
5
−
3
5
5 3
3
the potassium centers. In addition, the alkali metal atoms are
3
5
bonded to the five carbon atoms of the η -C Me ligand linked
The reaction of 5 with 2 equiv of potassium bis-
5
5
to the titanium(1) atoms of a contiguous molecule. If the
centroid Cm(1) of these pentamethylcyclopentadienyl ligands
is considered, the coordination sphere about the potassium
atoms may be described as distorted tetrahedral with N−K(1)−
N and N−K(1)−Cm(1) ranging from 61.9(1)−65.2(1)° and
(
7
trimethylsilyl)amide in toluene at 85 °C or the treatment of
with 1 equiv of [K{N(SiMe ) }] at room temperature
3
2
afforded the potassium derivative [K{(μ -N)(μ -NH)(μ -
NSiMe )Ti (η -C Me ) (μ -N)}] (8) (Scheme 6). Compound
8
in benzene or toluene, suggesting a molecular structure in
solution. H and C{ H} NMR spectra in benzene-d or
pyridine-d show resonances for three different η -C Me
ligands and are consistent with a C symmetric structure in
solution. The NMR data could also agree with an edge-linked
double-cube structure similar to those documented in alkali
metal derivatives [{M(μ -N)(μ -NH) Ti (η -C Me ) (μ -
3
3
3
5
3
3
5
5 3
3
was isolated in 89% yield as a red solid, which is very soluble
1
28.9−152.9°, respectively. While the potassium−nitrogen
1
13
1
bond lengths for the nitrido and NH imido groups are almost
identical (K(1)−N(13) 2.772(4) and K(1)−N(23) 2.776(4) Å,
respectively), those associated with the bulkier trimethylsilyli-
mido ligands are significantly longer (K(1)−N(12) 3.108(4)
6
5
5
5
5
1
Å). The steric repulsion of the SiMe group might also be
3
responsible of the differences in the potassium−carbon(C Me )
5
5
5
4
3
2
3
5
5
3
3
bond lengths (range 3.000(4)−3.258(4) Å), the shortest
distances being those with the C(13) and C(14) carbon
atoms located on the opposite side of the trimethylsilylimido
ligand.
1
7,30
N)} ] (M = Li, Na, K, Rb, Cs).
However, in contrast to
2
those species that are only soluble in pyridine, the high
solubility of 8 in aromatic hydrocarbon solvents suggests a
different structure. The solid-state structure of 8 determined by
X-ray diffraction consists of polymeric zigzag chains made up of
cube-type [K{(μ -N)(μ -NH)(μ -NSiMe )Ti (η -C Me ) (μ -
Addition of 1 equiv of 18-crown-6 to a toluene solution of 8
led to the immediate precipitation of the well-separated ion pair
5
5
3
3
3
3
3
5
5 3
3
[K(18-crown-6)][Ti (η -C Me ) (μ -N)(μ-N)(μ-NH)(μ-
3
5
5
3
3
N)}] units with the potassium atom linked to the contiguous
molecule through a η -C Me ligand (Figure 5). Compound 8
NSiMe )] (9). This compound was isolated in 80% yield as a
3
5
5
5
yellow powder, which is only soluble in pyridine-d , while it
5
crystallized in the centrosymmetric P2 /c space group, and the
crystal lattice shows a regular alternation of the two
enantiomers in neighboring chains.
reacts immediately with chloroform-d to give complex 7 and
1
1
1
free 18-crown-6 according to NMR spectroscopy. H and
13
1
C{ H} NMR spectra of 9 in pyridine-d show resonances for
5
1
1527
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Inorganic Chemistry
Article
5
three different η -C Me ligands in accord with a C symmetry
ACKNOWLEDGMENTS
5
5
1
■
1
for the trinuclear titanium anion. In addition, the H NMR
spectrum of 9 shows a sharp singlet for the methylene groups
of one crown ether ligand per anionic fragment. The three
resonances for the ipso carbon of the η -C Me groups (δ =
14.2, 113.3, and 113.0) in the C{ H} NMR spectrum of 9
are slightly shifted upfield with respect to those found in the
spectra of 8 in pyridine-d (δ = 115.4, 114.4, and 113.3) or
We are grateful to the Spanish MICINN (CTQ2008-00061/
BQU), Comunidad de Madrid and the Universidad de Alcala
CCG10-UAH/PPQ-5935), and the Factoria de Cristalizacion
CONSOLIDER-INGENIO 2010 CSD2006-00015) for finan-
cial support of this research. J.C. and M.G.-M. thank the MEC
and UAH for doctoral fellowships.
́
(
(
́
5
5
5
1
3
1
1
5
benzene-d (δ = 116.6, 115.9, and 113.9). Most likely, the
structure of compound 8 in those solutions contains a cube-
6
REFERENCES
■
type [KTi N ] core with solvent molecules coordinated to the
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3
4
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(
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CONCLUSION
■
(
Catledge, F. A. Inorg. Chim. Acta 1982, 65, L91−L93. (b) Romo, S.;
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(
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(
(
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group and assists the proton exchange between amido NHR
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3
5
complex [Ti (η -C Me ) (μ -N)(μ-NH) (μ-NHSiMe )][OTf]
3
5
5 3
3
2
3
with the triflate anion not coordinated to the metals.
Deprotonation of this ionic compound with [K{N(SiMe ) }]
3
2
(5) For selected examples of the reaction of imido complexes with
occurs at the imido NH ligands rather than involving the
5
nucleophiles, see: (a) Shapley, P. A.; Shusta, J. M.; Hunt, J. L.
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3
3
C Me ) (μ -N)(μ-N)(μ-NH)(μ-NHSiMe )] readily rearranges
5
5 3
3
3
5
to the trimethylsilylimido derivative [Ti (η -C Me ) (μ -N)(μ-
3
5
5 3
3
1
(
1248−11249.
NH) (μ-NSiMe )]. The latter can be further deprotonated with
2
3
6) For selected examples of the reaction of nitrido complexes with
[
K{N(SiMe ) }] to give a potassium complex [K{(μ -N)(μ -
3
2
3
3
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5
NH)(μ -NSiMe )Ti (η -C Me ) (μ -N)}] and upon addition
3
3
3
5
5 3
3
of 18-crown-6 leads to the well-separated ion-pair [K(18-
5
crown-6)][Ti (η -C Me ) (μ -N)(μ-N)(μ-NH)(μ-NSiMe )].
3
5
5
3
3
3
The treatment of 1 with 1 equiv of electrophiles and
subsequent reactions on the resultant products described here
have shown the possibility of preparing a series of cationic,
neutral, or anionic trinuclear titanium complexes bearing
bridging amido, imido, and nitrido functionalities. Several of
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2
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ASSOCIATED CONTENT
■
(
*
S
Supporting Information
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Kinetic data and analyses for the dynamic NMR spectroscopy
study for compounds 2 and 3, and the crystal structure of
complex 5 showing the hydrogen bonds between ions. X-ray
crystallographic files in CIF format for complexes 2, 5, 6, and 8.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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(
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̈
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AUTHOR INFORMATION
■
*
(b) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239−482.
(9) For reviews on polynuclear nitrido complexes, see: (a) Dehnicke,
Corresponding Author
Fax: (+34) 91-8854683. E-mail: carlos.yelamos@uah.es.
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(
9
̈
Notes
̈
The authors declare no competing financial interest.
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1
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