Half-sandwich dichloro, alkyl chloro, dialkyl, alkyl methyl and amido methyl imido cyclopentadienyl niobium and tantalum(V) complexes. Dynamic behaviour of amido imido tantalum derivatives

Article abstract of DOI:10.1016/S0022-328X(98)01142-5

Reactions of NbCp′Cl₄ (Cp′=η⁵-C₅H₄SiMe₃) and TaCp*Cl₂Me₂ (Cp=η⁵-C₅Me₅) with two equivalents of NHR¹SiMe₃ and two equivalent

Full text of DOI:10.1016/S0022-328X(98)01142-5

Journal of Organometallic Chemistry 580 (1999) 161–168
Half-sandwich dichloro, alkyl chloro, dialkyl, alkyl methyl and amido
methyl imido cyclopentadienyl niobium and tantalum(V) complexes.
Dynamic behaviour of amido imido tantalum derivatives
Aurora Castro, Mikhail V. Galakhov, Manuel Go´mez *, Fernando Sa´nchez
Departamento de Qu´ımica Inorga´nica, Uni6ersidad de Alcala´ de Henares, Campus Uni6ersitario, E-28871 Alcala´ de Henares, Spain
Received 2 September 1998
Abstract
Reactions of NbCp%Cl4 (Cp%=h5-C5H4SiMe3) and TaCp*Cl2Me2 (Cp*=h5-C5Me5) with two equivalents of NHR1SiMe3 and
two equivalents of CNR1 (M=Nb, R1=2,6-Me2C6H3; 2,6-iPr2C6H3; SiMe3; M=Ta, R1=2,6-Me2C6H3), respectively, gave the
dichloro imido derivatives [MCpCl2(NR1)] (M=Nb, Cp=Cp%, R1=2,6-Me2C6H3, 1; 2,6-iPr2C6H3, 2; SiMe3, 3; M=Ta,
Cp=Cp*, 4). Alkyl chloro imido complexes [MCpClR(NR1)], (R1=2,6-Me2C6H3; M=Nb, Cp=Cp%, R=Me, 5; Cp*, 6;
M=Ta, Cp=Cp*, R=Me, 7; NMe2, 8; OtBu, 9) were obtained by treatment of dichloro derivatives 1 and 4 with the appropriate
amount of alkylating reagent (5, 7 one equivalent of ZnMe2; 6 four equivalents of LiCp*; 8, 9 one equivalent of LiR). When the
same reaction was carried out with two equivalents of MgClMe (10) or LiR(RꢀCH2SiMe3, 11; CH2CMe3, 12; NMe2, 13) and one
equivalent of Mg(CH2C6H5)2(THF)2 (14) or Li2[o-(NSiMe3)2C6H4] (15) the corresponding dialkyl imido derivatives
[MCpR2(NR1)], (R1=2,6-Me2C6H3; M=Nb, Cp=Cp%, R=Me, 10; CH2SiMe3, 11; CH2CMe3, 12; NMe2, 13; CH2C6H5, 14;
M=Ta, CpꢀCp*, R=o-(NSiMe3)2C6H4, 15) can be prepared. The methyl complexes 5, 7 reacted with one equivalent of LiR to
give mixed alkyl methyl and amido methyl imido derivatives [MCpMeR(NR1)] (R1=2,6-Me2C6H3; M=Nb, Cp=Cp%, R=
CH2SiMe3, 16; NMe2, 17; M=Ta, Cp=Cp*, R=NMe2, 18; NiPr2, 19; NHtBu, 20). All the new complexes were characterized
i
by usual IR and NMR spectroscopic methods. The rotation of 2,6-Me2C6H3 ring and NR2(R=Me, Pr) amido ligands around
C
ipso(aryl)–Nimido and Namido–Ta bonds, respectively, were observed for complexes 8, 18 and 19 and studied by 1H-DNMR
spectroscopy. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Niobium; Tantalum; Alkyl amido imido cyclopentadienyl complexes
Although the imide ligand has been employed as an
1. Introduction
ancillary group to support high-oxidation state metal
centres, an increasing number of complexes having
reactive imide ligands have been shown to participate in
C–H bond activation [5] and cycloaddition reactions
[6]. It is noteworthy that early transition metal–imido
complexes are widely represented and a great number
of established imido functional groups of d0 niobium
and tantalum has been reported during the last few
years [7].
We previously reported a first study of the behaviour
of pentamethylcyclopentadienyl arylimido tantalum
complexes [TaCp*XY{N(2,6-Me2C6H3)}] (X=Y=Cl;
X=Cl, Y=Me) towards alkylating reagents [8] and in
Complexes containing metal–ligand multiple bonds
constitute extremely interesting subjects of research in
organometallic chemistry, since they are implicated as
effective reagents or catalysts for oxidations, metathesis
reactions and ammoxidations [1]. In particular, transi-
tion metal–imido complexes have been studied as
molecular precursors to thin films [2], models of hydro-
denitrogenation [3] and probes of electronic structure
via their luminescent properties [4].
* Corresponding author. Tel.: +34-1-885-47-64; fax: +34-1-885-
46-83.
E-mail address: mgomez@inorg.alcala.es (M. Go´mez)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01142-5

162
A. Castro et al. / Journal of Organometallic Chemistry 580 (1999) 161–168
this article we report our efforts to complete the study,
and also the results obtained with similar half-sandwich
arylimido niobium complexes.
2. Results and discussion
The starting point for these studies are the pseudo-te-
trahedral dichloro imido complexes [NbCp%Cl2(NR1)],
(R1=2,6-Me2C6H3, 1; 2,6-iPr2C6H3, 2; SiMe3, 3) whose
syntheses can be easily carried out when
dichloromethane solutions of NbCp%Cl4, (Cp%=h5-
C5H4SiMe3) are treated at room temperature (r.t.) with
aryl trimethylsilyl amine NHR1SiMe3 (R1=2,6-
Me2C6H3; 2,6-iPr2C6H3; SiMe3) in a 1:2 molar ratio, as
shown in Scheme 1. Complex 4 was prepared as de-
scribed previously [9].
Reactions of 1 or 4 with the stoichiometric amounts
of the alkylating reagent gave solutions from which the
alkyl chloro imido [MCpClR{N(2,6-Me2C6H3)}], (M=
Nb, Cp=Cp%, R=Me, 5; Cp*, 6; M=Ta, Cp=Cp*,
R=Me, 7; NMe2, 8; OtBu, 9) and dialkyl imido
[MCpR2{N(2,6-Me2C6H3)}] (M=Nb, Cp=Cp%, R=
Me, 10; CH2SiMe3, 11; CH2CMe3, 12; NMe2, 13;
CH2C6H5, 14; M=Ta, Cp=Cp*, R=o-(NSi-
Me3)2C6H4, 15) were isolated in good yields (see
Scheme 2).
Furthermore, the reaction between chloro methyl
imido complexes 5 or 7 and LiR at r.t in n-hexane gives
the mixed alkyl methyl and amido methyl imido com-
plexes [MCpMeR{N(2,6-Me2C6H3)}] (M=Nb, Cp=
Cp%, R=CH2SiMe3, 16; NMe2, 17; M=Ta, Cp=Cp*,
R=NMe2, 18; NiPr2, 19; NHtBu, 20), as shown in
Scheme 3.
Scheme 2. Synthesis of alkyl chloro and dialkyl imido complexes.
Reagents and conditions: i–5, 7 one equivalent of ZnMe2, n-hexane,
12 h, r.t.; 6, four equivalents of LiCp*: 8 and 9, one equivalent of
LiR, toluene, 12 h, r.t. ii–10, two equivalents of MgClMe; 11–13,
two equivalents of LiR; 14, one equivalent of Mg(CH2C6H5)2(THF)2;
15, one equivalent of Li2[o-(NSiMe3)2C6H4], toluene, 12 h, r.t.
using 0.5 equivalents of dimethyl zinc. Several attempts
to prepare the pentamethylcyclopentadienyl niobium
derivative 6 using a stoichiometric amount of LiCp*
were unsuccessful, but the compound was obtained
employed a large excess of lithium reagent. On the
other hand [NbCp%(NMe2)2{N(2,6-Me2C6H3)}], 13, can
be easily prepared via the treatment of 1 with LiNMe2
in a molar ratio 1:2, however, a related reaction of
[TaCp*Cl2{N(2,6-Me2C6H3)}], 4, with this reagent af-
forded a mixture of the mono dimethylamido derivative
8 and an excess of the alkylating reagent.
Treatment of the starting dichloro imido niobium
and tantalum complexes with one equivalent of ZnMe2
gave the chloro methyl derivatives 5 and 7, as the only
products, but unidentified mixtures were obtained by
All of the complexes 1–20 are soluble in most or-
ganic solvents including saturated hydrocarbons. They
are extremely air- and moisture-sensitive, and rigor-
ously dried solvents and handling under dry inert atmo-
sphere were found to be imperative for successful
preparations.
Scheme 1. Synthesis of dichloro imido complexes. Reagents and
conditions: i–two equivalents of NHR1SiMe3, dichloromethane, 12 h,
r.t. ii–Two equivalents of CNR1, toluene, 24 h, r.t.
Scheme 3. Synthesis of alkyl methyl and amido methyl imido com-
plexes.

A. Castro et al. / Journal of Organometallic Chemistry 580 (1999) 161–168
163
Table 1
IRa data of the new complexes
wMꢀN
wls(Me Si) wC–C
wC–H
wM–C
wM–Cl
Others
(Cp*)
(Cp%)
3
1
2
3
5
6
8
1300(vs)
1329(m)
1311(vs)
1304(vs)
1299(vs)
1322(vs)
1254(vs)
1252(vs)
1249(vs)
1252(vs)
840(vs)
841(vs)
840(vs)
836(vs)
368(s)
379(s)
1400(m), 1170(m), 1092(w), 1042(s), 902(m), 690(w), 632(m)
1170(m), 1048(s), 903(m), 690(w), 633(m)
387(m) 1170(m), 1103(w), 1049(s), 903(m), 697(w), 631(m)
634(m) 364(m) 1410(m), 1172(m), 1092(w), 1042(s), 902(m), 690(w), 480(m)
629(m) 342(m) 1405(m), 1096(w), 1040(s), 907(m)
1247(vs)
1020(m) 835(vs)
1027(m)
351(s)
1621(m), 1586(m), 1249(s), 1141(m), 1095(m), 962(vs), 798(vs), 763(m),
577(m), 395(m)
9
1331(vs)
1025(m)
834(vs)
839(vs)
837(vs)
837(vs)
833(vs)
1027(m)
342(s)
1589(w), 1234(m), 1177(s), 1096(m), 985(vs), 789(vs), 391(m)
1420(m), 1174(m), 1096(w), 1040(s), 910(m), 690(w), 494(m),
1174(m), 1044(s), 700(m)
1411(m), 1175(m), 1097(w), 1045(s), 906(w), 697(w)
1410(m), 1176(m), 1096(w), 1045(s), 906(m), 680(w), 632(m), 545(s)
1177(m), 1094(w), 1032(s), 900(m), 692(w), 494(s)
1588(w), 1098(m), 941(s), 905(s), 839(vs), 790(m), 749(m), 723(m),
684(w), 500(w), 344(m), 288(w)
10 1308(vs)
11 1306(vs)
12 1301(vs)
13 1291(vs)
14 1288(vs)
15 1320(s)
1252(vs)
1248(vs)
1250(vs)
1245(vs)
1245(vs)
1247(s)
632(m)
633(m)
632(m)
16 1306(vs)
17 1307(vs)
18 1330(vs)
1248(vs)
1248(vs)
838(vs)
838(vs)
633(m)
1411(m), 1174(m), 1096(w), 1043(s), 903(m), 705(w), 485(m)
1413(m), 1173(m), 1096(w), 1043(s), 903(m), 705(w), 485(m)
1634(w), 1589(m), 1159(m), 1141(m), 1095(m), 983(m), 964(m), 758(s),
489(m), 392(m), 349(s)
1026(m)
19 1323(vs)
20 1331(vs)
1025(m)
1025(m)
1589(w), 1158(s), 1101(s), 982(m), 937(m), 818(m), 760(s), 723(m),
486(w), 344(m)
3299(m), 1892(w), 1756(w), 1681(m), 1588(s), 1209(vs), 1094(s), 979(s),
942(m), 759(vs), 605(m), 575(m), 491 (m), 349(s), 272(m)
a Nujol mull, w in cm−1
.
The IR spectra (see Table 1) of all complexes show
the characteristic absorptions for the pentamethylcy-
clopentadienyl (wC–C ca. 1025 cm−1) [10], trimethylsilyl-
cyclopentadienyl (wC–H ca. 837 cm−1) [11] rings and for
2.1. Dynamic beha6iour in solution of the amido imido
complexes 8, 18 and 19
1
At r.t. the H-NMR spectra of the complexes 8, 13
the trimethylsilyl substituent (wlsCH3 ca. 1249 cm−1
)
and 18 show very broad signals for the resonances
corresponding to the dimethylamido ligand. The 1H-
NMR spectrum of complex 19 also shows one broad
resonance at l 3.4 for two equivalent methyne protons
and two broad signals at l 1.2 and 1.1 for the methyl
groups of the isopropylamido ligand, while in its
13C{1H} spectrum at the same temperature, the signals
of this ligand are not observed. At 233 K the 13C{1H}
spectrum of 19 shows very good resolved signals for
Me–Ta at l 24.08, for the Cp* ring at l 114.4 and
11.03, for the methyl substituents of 2,6-Me2C6H3 ring
at l 19.72 and 18.97, for the aromatic carbons of the
C6H3Me2 ring at l 153.6(Cipso), 136.2, 131.7(C2, C6),
127.05, 126.8(C3, C5) and 120.1(C4), and also two sig-
nals at l 52.1 and 48.6 for the inequivalent secondary
carbon of the CHMe2 group and four resonances at l
30.0, 27.8, 26.8 and 20.9 for the inequivalent and
diasterotopic methyl carbons of the CHMe2 group in
[11d, 12]. Absorptions due to the MꢀN, M–C and
M–Cl stretching vibrations are observed at ca.
1304(Nb), 1326(Ta) [7i,m], 633(Nb), 517(Ta) [13] and
368(Nb), 346(Ta) [11d, 14] cm−1, respectively.
At r.t. the chiral complexes 6 and 19 show the
expected spectral 1H-NMR (see Table 2) behaviour
with both inequivalent o-methyl(phenyl) groups, how-
ever, for the analogous complexes 5, 8, 9, 16–18 and
20, such groups are equivalent. This difference is prob-
ably due to different rates of Berry pseudorotation
processes [15] as we have observed for similar tantalum
complexes [8]. The absence of local symmetry in the
trimethylsilylcyclopentadienyl ring of the complexes 5,
6, 16 and 17 is consistent with the chiral character of
the niobium atom.
1
The H-NMR spectra of the complexes 11, 12 and 14
show an AB and AA%BB% spin system for the a-CH2
groups and the trimethylsilylcyclopentadienyl ring, re-
spectively, in accordance with the prochiral character of
the metal center. Furthermore, complex 15 shows an
AA%BB% spin system for the phenyl ring protons. All the
data are consistent with a Cs symmetry for complexes
10–15 and with a metal center in a characteristic three-
legged piano-stool environment.
1
the bis-isopropylamido ligand. At 233 K the H-NMR
spectrum of this complex displays, besides other signals,
two septuplets at l 3.51 and 3.26 for the isopropylic
proton and four doublets at l 1.24, 1.21, 1.14 and 0.95,
respectively, for the methyl groups of the bis-isopropy-
lamido ligand. All these data are consistent with the
absence of any symmetry into 19 at low temperatures

164
A. Castro et al. / Journal of Organometallic Chemistry 580 (1999) 161–168
Table 2
1H and 13C{1H}-NMR data of the new complexes
1H
13C{1H}
1
2
a6.80 (t, 1H, 3JH–H=7.2 Hz), 6.65 (d, 2H, 3JH–H=7.2 Hz,
H3C6Me2N), 6.24 (m, 2H), 5.90 (m, 2H, H4C5SiMe3), 2.40 (s, 6H, C6H3Me2N), 125.94 (Cm, C6H3Me2N), 125.77 (Ci, C5H4SiMe3),
Me2C6H3N), 0.19 (s, 9H, Me3SiC5H4)
a153.54 (Ci, C6H3Me2N), 134.99 (Co, C6H3Me2N), 126.7 (Cp,
122.95 (C2,5, C5H4SiMe3), 113.07 (C3,4, C5H4SiMe3), 19.22
(Me2C6H3N), −0.76 (Me3SiC5H4)
a7.01 (d, 2H, 3JH–H=7.7 Hz), 6.89 (t, 1H, 3JH–H=7.7 Hz,
a144.75 (Ci, C6H3i Pr2N), 126 (Ci, C5H4SiMe3), 125.19 (Co,
H3C6i Pr2N), 6.27 (m, 2H), 6.01 (m, 2H, H4C5SiMe3), 3.74 [m, 2H, C6Hi3Pr2N), 122.7 (Cm, C6Hi3Pr2N), 120.93 (Cp, C6H3i Pr2N), 114.3
(HCMe2)2C6H3N]. 1.28 [d, 2H, 2JH–H=6.6 Hz,
(Me2CH)2C6H3N], 0.20 (s, 9H, Me3SiC5H4)
(C2,5, C5H4SiMe3), 108.7 (C3,4, C5H4SiMe3), 28.5
[(CHMe2)2C6H3N], 24.25, 24 [(HCMe2)2C6H3N], −0.59
(Me3SiC5H4)
3
5
a6.34 (m, 2H), 6.10 (m, 2H, H4C5SiMe3), 0.21 (s, 9H,
Me3SiC5H4), 0.06 (s, 9H, Me3SiN)
a125.6 (Ci, C5H4SiMe3), 122 (C2,5, C5H4SiMe3), 113.5 (C3,4
C5H4SiMe3), 0.37 (Me3SiN), −0.5 (Me3SiC5H4)
,
a6.90 (d, 2H, 3JH–H=7.8 Hz), 6.76 (t, 1H, 3JH–H=7 8 Hz,
H3C6Me2N), 6.14 (m, 1H), 5.87 (m, 1H), 5.70 (m, 1H), 5.59 (m,
a140.3, 135.5, 134.4, 124.4, 121.8 (several phenyl, C6H3Me2N),
121.8, 120.2, 114.8, 114.6, 109.4 (Ci,2,3,4,5, C5H4SiMe3), 69.5 (Me–
1H, H4C5SiMe3), 2.42 (s, 6H, Me2C6H3N), 1.37 (s, 3H, Me–Nb), Nb), 19.6 (Me2C6H3N), −0 62 (Me3SiC5H4)
0.16 (s, 9H, Me3SiC5H4)
6
a6.96 (d, 1H, 3JH–H=7.5 Hz), 6.88 (d, 1H, 3JH–H=7.5 Hz), 6.65 a156.1 (Ci, C6H3Me2N), 140.3 (Ci, C5H4SiMe3), 130.1, 127.2, 125.6,
(t, 1H, 3JH–H=7.5 Hz, H3C6Me2N), 6.34 (m, 2H), 5.79 (m, 1H), 124.9, 122.2 (C2,3,4,5,6, C6H3Me2N), 121.4, 120.9, 110.6, 107.4
5.40 (m, 1H, H4C5SiMe3), 2.41 (s, 3H), 2.27 (s, 3H, Me2C6H3N), (C5H4SiMe3), 120.5 (C5Me5), 20.5, 19.2 (Me2C6H3N), 12.1 (C5Me5),
1.67 (s, 15H, C5Me5), 0.26 (s, 9H, Me3SiC5H4)
0.1 (Me3SiC5H4)
8
9
b6.93 (d, 2H, 3JH–H=7.3 Hz), 6.65 (t, 1H, 3JH–H=7.3 Hz,
b152.5, 130.3, 127, 121.6 (Ci, Co, Cm, Cp, C6H3Me2N), 117.5
H3C6Me2N), 3.53 (br, 6H, Me2N), 2.32 (s, 6H, Me2C6H3N), 2.04 (C5Me5), 51.8 (br, Me2N), 19 (Me2C6H3N), 11 (C5Me5)
(s, 15H, C5Me5)
a7.05 (d, 2H, 3JH–H=7.5 Hz), 6.73 (t, 1H, 3JH–H=7.5 Hz,
H3C6Me2N), 2.53 (s, 6H, Me2C6H3N), 1.9 (s, 15H, C5Me5), 1.27
(s, 9H, Me3C)
a153, 128.3, 127.5, 122 (Ci, Co, Cm, Cp, C6H3Me2N) 119.1 (C5Me5),
81 (CMe3), 31.9 (Me3C), 19.8 (Me2C6H3N), 11.1 (C5Me5)
10 a7.01 (d, 2H, 3JH–H=7.3 Hz), 6.8 (t, 1H, 3JH–H=7.3 Hz,
a149.5, 140, 134.6, 123.1 (Ci, Co, Cp, Cm, C6H3Me2N), 117 (Ci,
H3C6Me2N), 5.93 (m, 2H), 5.80 (m, 2H, H4C5SiMe3), 2.51 (s, 6H, C5H4SiMe3), 114.4 (C2,5, C5H4SiMe3), 111.9 (C3,4, C5H4SiMe3),
Me2C6H3N), 0.77 (s, 6H, Me2–Nb), 0.05 (s, 9H, Me3SiC5H4)
72.1 (Me2–Nb), 19.9 (Me2C6H3N), 0.52 (Me3SiC5H4)
11 a7.02 (d, 2H, 3JH–H=7.6 Hz), 6.83 (t, 1H, 3JH–H=7.6 Hz,
a140.5, 133.5, 128.3, 122.7 (Ci, Co, Cp, Cm, C6H3Me2N), 123 (Ci.
H3C6Me2N), 6.66 (m, 2H), 5.78 (m, 2H, H4C5SiMe3), 2.53 (s, 6H, C5H4SiMe3), 118.7 (C2,5, C5H4SiMe3), 105.6 (C3,4, C5H4SiMe3),
Me2C6H3N), 1.54, 0.77 (AB, 2H, 2JH–H=9.36 Hz, H2CSiMe3),
0.24 (s, 9H, Me3SiC5H4), 0.09 (s, 9H, Me3SiCH2)
56.5 (br, CH2SiMe3), 20.5 (Me2C6H3N), 2.85 (Me3SiCH2), −0.02
(Me3SiC5H4)
12 a6.99 (d, 2H, 3JH–H=7.8 Hz), 6.79 (t, 1H, 3JH–H=7.8 Hz,
a155, 138, 134.5, 123.25 (Ci, Co, Cp, Cm, C6H3Me2N), 117.9 (Ci,
H3C6Me2N), 6.60 (m, 2H), 5.89 (m, 2H, H4C5SiMe3), 2.56 (s, 6H, C5H4SiMe3), 116.7 (C2,5, C 5H4SiMe3), 109.3 (C3,4, C5H4SiMe3),
Me2C6H3N), 2.17. 0.79 (AB, 2H, 2JH–H=6.54 Hz, H2CCMe3),
1.11 (s, 18H, Me3CCH2), 0.12 (s, 9H, Me3SiC5H4)
85.5 (br, CH2CMe3), 37.4 (Me3CCH2), 35.8 (Me3CCH2), 20.8
(Me2C6H3N), 0.6 (Me3SiC5H4)
13 a7.00 (d, 2H, 3JH–H=7.3 Hz), 6.78 (t, 1H, 3JH–H=7.3 Hz,
H3C6Me2N), 6.16 (m, 2H), 6.07 (m, 2H, H4C5SiMe3), 3.22 (s,
12H, Me2N), 2.52 (s, 6H, Me2C6H3N), 0.14 (s, 9H, Me3SiC5H4)
a152.4, 140.5, 132.2, 122.4 (Ci, Co, Cp, Cm, C6H3Me2N), 117.6 (Ci,
C5H4SiMe3), 115.6 (C2,5, C5H4SiMe3), 112.8 (C3,4, C5H4SiMe3),
52.9 (Me2N), 20.2 (Me2C6H3N), −0.02 (Me3SiC5H4)
14 a7.03–6.73 (several phenyl, H3C6Me2N, H5C6CH2), 6 85 (m, 2H), a155.4, 140.6, 134.6, 130, 129.1, 128.2, 125.6, 123.1 (several phenyl,
5.67 (m, 2H, H4C5SiMe3), 2.35 (s, 6H, Me2C6H3N), 2.17, 1.86
C6H3Me2N, C6H5CH2), 115.6 (C2,5, C5H4SiMe3), 113.9 (Ci,
C5H4SiMe3). 112.8 (C3,4, C5H4SiMe3), 47 (CH2C6H5), 20.44
(Me2C6H3N), −0.16 (Me3SiC5H4)
(AB, 2H, 2JH–H=8.7 Hz, H2CC6H5), 0.07 (s, 9H, Me3SiC5H4)
15 a7.12 (d, 2H, 3JH–H=7.5 Hz), 6.76 (t, 1H, 3JH–H=7 5 Hz,
H3C6Me2N), 7.00 [m, 4H, H4C6(NSiMe3)2], 2.58 (s, 6H,
Me2C6H3N) 1.66 (s, 15H, C5Me5), 0.28 [s, 18H, (Me3SiN)2C6H4]
16 a, 7.01 (d, 2H, 3JH–H=7.5 Hz), 6.8 (t, 1H, 3JH–H=7.5 Hz,
H3C6Me2N), 6.29 (m, 1H), 5.91 (m, 1H), 5.83 (m, 1H), 5.79 (m,
1H, H4C5SiMe3), 2.53 (s, 6H, Me2C6H3N), 1.47, 0.45 (AB, 2H,
2JH–H=10.5 Hz, H2CSiMe3), 0.74 (s. 3H, Me–Nb), 0.14 (s, 9H,
Me3SiC5H4), 0.07 (s, 9H, Me3SiCH2)
a155.4, 131.9, 127.9, 121.2 (Ci, Co, Cm, Cp, C6H3Me2N), 135.4,
128.3, 125.6, 122.9 [C6H4(NSiMe3)2], 115.9 (C5Me5), 20.9
(Me2C6H3N), 10.8 (C5Me5), 3.37 [(Me3SiN)2C6H4]
a, 154.8, 140.3, 134.2, 123 (Ci, Co, Cp, Cm, C6H3Me2N), 118.4,
118.7, 112.3, 112.1, 110.4 (Ci,2,3,4,5, C5H4SiMe3), 60.4 (br,
CH2SiMe3), 36.3 (br, Me–Nb), 20.3 (Me2C6H3N), 2.33
(Me3SiCH2), −0.4 (Me3SiC5H4)
17 a7.01 (d, 2H, 3JH–H=7.5 Hz), 6.77 (t, 1H, 3JH–H=7.5 Hz,
H3C6Me2N), 6.15 (m, 1H), 6 (m, 1H), 5.9 (m, 1H), 5.74 (m, 1H,
H4C5SiMe3), 3.13 (s, br, 6H, Me2N), 2.52 (s, 6H, Me2C6H3N),
0.75 (s, 3H, Me–Nb), 0.08 (s, 9H, Me3SiC5H4)
a155.3, 140.2,133.6, 122.5 (Ci, Co, Cp, Cm, C6H3Me2N), 117.1,
117.7, 112.9, 111.8, 110.6 (Ci,2,3,4,5, C5H4SiMe3), 52.7 (s, Me2N),
38.5 (Me–Nb), 19.9 (Me2C6H3N), −0.3 (Me3SiC5H4)
18 a7.11 (d, 2H, 3JH–H=7.5 Hz), 6.79 (t, 1H, 3JH–H=7.5 Hz,
a154.5, 128.3, 127.5, 121 (Ci, Co, Cm, Cp, C6H3Me2N), 114.7
H3C6Me2N), 3.16 (br, 6H, Me2N), 2.48 (s, 6H, Me2C6H3N), 1.76 (C5Me5), 50.9 (br, Me2N), 25.7 (Me–Ta), 19.4 (Me2C6H3N), 10.7
(s, 15H, C5Me5), 0.50 (s, 3H, Me–Ta)
(C5Me5)
19 b6.97 (d, 1H, 3JH–H=7.2 Hz), 6.9 (d, 1H, 3JH–H=7.2 Hz), 6.61
(t, 1H, 3JH–H=7.2 Hz, H3C6Me2N), 3.41 (br, 2H, HCMe2), 2.5
b153.7, 136.1, 131.1, 126.9, 126.8, 120.2 (Ci, C2,3,4,5,6, C6H3Me2N),
114.6 (C5Me5), 50.3 (vbr, CHMe2), 28 (Me–Ta), 24.3 (vbr,
(s, 3H), 2.12 (s, 3H, Me2C6H3N), 1.95 (s, 15H, C5Me5), 1.18 (br, Me2CH), 20.6, 19.8 (Me2C6H3N), 11 (C5Me5)
12H, Me2CH), 0.14 (s, 3H, Me–Ta)
20 a7.13 (d, 2H, 3JH–H=7.5 Hz), 6.79 (t, 1H, 3JH–H=7.5 Hz,
H3C6Me2N), 5.04 (br, 1H, HNtBu), 2.56 (s, 6H, Me2C6H3N),
1.74 (s, 15H, C5Me5), 1.23 (s, 9H, Me3CNH), 0.46 (s, 3H, Me–
Ta)
a154.7, 128.3, 127.6, 120.9 (Ci, Co, Cm, Cp, C6H3Me2N), 114.4
(C5Me5), 56.34 (CMe3NH), 33.8 (Me3CNH), 24.7 (Me–Ta), 19.9
(br, Me2C6H3N), 10.8 (C5Me5)
a Chemical shifts in l: benzene-d6.
b Chemical shifts in l: chloroform-d.

A. Castro et al. / Journal of Organometallic Chemistry 580 (1999) 161–168
165
1
At 288 K the H-NMR spectrum of 19 allows the
fields [18]. Therefore, the absence of rotation around
the Nimido–Cipso(aryl) bond is not due to the positive
charge delocalization by p-interaction in the ring but
to the steric hindrance of the Z and Cp* groups.
The kinetic parameters for the rotation of the
NMe2 ligand around the Namido–Ta bond clearly de-
pend of the electronic and steric properties of the
substituent X=Cl, 8; Me, 18 and it can be consid-
ered as a diamido–amido imido exchange process
(Scheme 4), in agreement with the negligible DSc
values.
Further studies will be carried out to probe the
similarities and differences in the reactivity of these
alkyl imido cyclopentadienyl niobium and tantalum
systems with carbon monoxide and isocyanides and
the intramolecular rearrangements of the insertion
products.
detection of the coalescence of the H CMe2 proton
isopropylic resonances (DGc288 K=13.9 kcal mol−1
)
corresponding to the slow rotation around the Ta–
NiPr2 bond and due to the multiple character of the
mentioned bond.
This result moved us to study the fluxional be-
haviour of the dimethylamido derivatives 8 and 18,
by variable temperature 1H-NMR spectroscopy. At
193 K the NMR spectra of both complexes showed
sharp resonances for the methyl groups of the
dimethylamido ligand at l 3.92, 3.04 (8) and 3.72,
2.82 (18) and also for the o-methyl(phenyl) group of
the imido ligand at l 2.34, 2.08 (8) and 2.45, 2.05
(18). At higher temperatures typical dynamic be-
haviour involving the exchange of two equally popu-
lated signals occur [16] and it is possible to observe
the collapse point of the 2,6-Me2C6H3 methyl proton
resonances for 8 (DGc238 K=11.7 kcal mol−1) and
18 (DGc233 K=12.0 kcal mol−1) and also of the
3. Experimental
NMe2 methyl proton resonances for 8 (AGc278 K
=
3.1. General considerations
14.2 kcal mol−1) and 18 (DGc251 K=12.6 kcal
mol−1).
All reactions and manipulations were carried out
under an argon atmosphere using standard Schlenk-
tube and cannula techniques or in a conventional ar-
gon-filled glove-box. Solvents were refluxed over an
appropriate drying agent and distilled and degassed
prior to use: dichloromethane (P4O10), n-hexane (Na/
K alloy) and toluene (Na). Literature methods were
employed for the synthesis of starting materials
NbCp%Cl4 [19] and [TaCp*Cl2{N(2,6-Me2C6H3)}] [9],
alkyl lithium LiR (R=C5Me5 [20], CH2SiMe3 [21],
CH2CMe3 [21], NiPr2 [20], NHtBu [20], OtBu [20]),
Li2[o-(NSiMe3)2C6H4] [22] and dibenzyl magnesium
Mg(CH2C6H5)2(THF)2 [23]. The reagents NHR1SiMe3
(R1=2,6-Me2C6H3; 2,6-iPr2C6H3) were prepared in
the usual way by treatment of amines R1NH2 with
LinBu followed by SiClMe3. All commercial products
were purchased from Aldrich and were used without
further purification as follows: LiNMe2, MgClMe 3
M in diethyl ether, ZnMe2 2 M in toluene, R1NH2
(R1=2,6-Me2C6H3; 2,6-iPr2C6H3) and NH(SiMe3)2.
The kinetic parameters of this dynamic process (see
Tables 3 and 4) which were calculated on the basis of
1H-DNMR data by NMR line shape analysis and
DNMR5 program [16] are in good agreement with an
intramolecular process (log A ca. 12.97; 13.75) and
takes place without any change of the entropy factor
(DSc ca. 0).
In the ground state, the complexes 8, 18 and 19 do
not present rotation around the Ta–Nimido and
Nimido–Cipso(aryl) bonds, probably due to the positive
charge delocalization into the Nimido–Ta–Namido moi-
ety (Scheme 4), as has been reported for bis-dimethy-
lamido titanium(IV) derivatives [17]. The DGc values
for the rotation of the 2,6-Me2C6H3 ring around the
Nimido–Cipso(aryl) bond are similar for 8 and 18 and
therefore independent of the substituent X (X=Cl, 8;
Me, 18). However, in the case of complex 19 with
bulkier substituents (Z=iPr) such effect is not ob-
served at higher temperatures or at minor magnetic
Table 3
Kinetic parameters of rotation of 2,6-Me2C6H3 ring around Nimido–Cipso(aryl) bond
Log A
Ea (kcal mol−1
)
DH" (kcal mol−1
)
DS" (e.u.)
DG"298 K (kcal
mol−1
)
8
13.5590.17
r=0.998
12.490.1
r=0.999
11.890.2
10.190.1
11.390.2
r=0.998
9.690.1
r=0.999
1.991.0
10.7
18
−3.5910.4
10.6

166
A. Castro et al. / Journal of Organometallic Chemistry 580 (1999) 161–168
Scheme 4.
IR spectra were recorded on a Perkin-Elmer 583
2: Yield 5.69 g (89%). Anal. Found: C, 50.15; H,
6.33; N, 2.95. C20H30Cl2NSiNb. Calc.: C, 50.24; H,
6.35; N, 2.94%.
3: Yield 4.64 g (89%). Repeated attempts to obtain
satisfactory microanalysis on this material were
unsuccessful.
spectrophotometer (4000–200 cm−1) as Nujol mulls
1
between CsI or polyethylene pellets. H and 13C{1H}-
NMR spectra were recorded on Varian Unity 300 and
Varian Unity 500 Plus spectrometers and chemical
1
shifts were measured relative to residual H and 13C
resonances in the deuterated solvents C6D6 (l 7.15),
CDCl3 (l 7.24), C6D6 (l 128) and CDCl3 (l 77),
respectively. C, H and N analyses were carried out with
a Perkin-Elmer 240C microanalyzer.
3.3. Synthesis of [MCpClR{N(2,6-Me2C6H3)}],
(M=Nb, Cp=Cp%, R=Me, 5; Cp*, 6; M=Ta,
Cp=Cp*, R=NMe2, 8; OtBu, 9)
3.2. Synthesis of [NbCp%Cl2(NR1)],
(R1=2,6-Me2C6H3, 1; 2,6-iPr2C6H3, 2; SiMe3, 3)
A 2 M solution of ZnMe2 in toluene (0.60 ml, 1.19
mmol) was added at r.t. to a red solution of
NbCp%Cl2(NR1) (0.50 g, 1.19 mmol) in n-hexane (50 ml)
and the mixture was stirred for 12 h. The solution was
filtered and the solvent reduced to dryness to give a
brown residue which was extracted with n-hexane (2×
20 ml). Subsequently, the solution was concentrated to
ca. 5 ml and cooled to −40°C overnight to afford 5 as
an extremely air-sensitive brown microcrystalline solid.
5: Yield 0.38 g (80%). Satisfactory elemental analysis
could not be obtained for this compound due to its
extreme air and moisture sensitivity.
A solution of NHR1SiMe3 (26.90 mmol; R1=2,6-
Me2C6H3, 5.74 ml; 2,6-iPr2C6H3, 7.14 ml; SiMe3, 5.62
ml) in dichloromethane (10 ml) was added at r.t. to a
red solution of NbCp%Cl4 (5.00 g, 13.45 mmol) in
dichloromethane (150 ml) and the mixture was stirred
for 12 h. The supernatant solution was filtered from the
[NH2R1SiMe3]Cl residue. The solvent removed in vacuo
and the oily red solid residue obtained was heated at
100°C and at reduced pressure during the 4 h to
eliminate volatiles. Extraction of the residue into n-hex-
ane (2×50 ml) afforded a red solution which was
filtered, concentrated to ca. 25 ml and cooled to −
40°C to give 1–3 as oily red solids.
3.3.1. Synthesis of 6, 8 and 9
1: Yield 5.02 g (89%). Anal. Found: C, 45.66; H,
5.52; N, 3.44. C16H22Cl2NSiNb. Calc.: C, 45.73; H,
5.28; N, 3.33%.
LiR (R=Cp*, 8 mmol, 1.13 g; 2 mmol, R=NMe2,
0.10 g; OtBu, 0.19 g) was added under rigorously
anhydrous conditions to a solution of MCpCl2(NR1) (2
Table 4
i
Kinetic parameters of rotation of NZ2 (Z=Me, 8, 18; Pr, 19) ligand around the Namido–Ta bond
Log A
Ea (kcal mol−1
)
DH" (kcal mol−1
)
DS" (e.u.)
DG"298 K (kcal
mol−1
)
8
18
19
14.190.2
r=0.998
13.490.25
r=0.998
–
13.890.25
11.790.25
–
13.390.25
r=0.998
11.290.25
r=0.998
–
4.491.0
1.491.0
–
12
10.8
13.9a
a Value determinated at 288 K in the collapse point of HCMe2 signals.

A. Castro et al. / Journal of Organometallic Chemistry 580 (1999) 161–168
167
mmol; M=Nb, 0.84 g; Ta, 1.00 g) in toluene (50 ml)
3.5. Synthesis of [MCpMeR{N(2,6-Me2C6H3)}]
(M=Nb, Cp=Cp%, R=CH2SiMe3, 16; NMe2, 17;
and the reaction mixture was stirred for 48 (6) or 12 (8
and 9) h. The resulting suspension was filtered, the
solvent reduced to dryness and the residue extracted
into n-hexane (2×10 ml). The solution was concen-
trated to half volume and cooling to −40°C afforded
6, 8 and 9 as red–brown (6) and brown (8 and 9)
microcrystalline solids.
6: Yield 0.41 g (40%). Anal. Found: C, 60.20; H,
7.32; N, 2.67. C26H37ClNSiNb. Calc.: C, 60.07; H, 7.12;
N, 2.69%.
8: Yield 0.77 g (75%). Anal. Found: C, 46.74; H,
5.81; N, 5.63. C20H30ClN2Ta. Calc.: C, 46.66; H, 5.87;
N, 5.44%.
9: Yield 0.81 g (75%). Anal. Found: C, 48.52; H,
6.47; N, 2.40. C22H33ClONTa. Calc.: C, 48.58; H, 6.41;
N, 2.57%.
M=Ta, Cp=Cp*, R=NMe2, 18; NiPr2, 19; NHtBu,
20)
At r.t., n-hexane (50 ml) was added to a mixture of
the compound [MCpClMe{N(2,6-Me2C6H3)}] (1.25
mmol; M=Nb, Cp=Cp%, 0.50 g; M=Ta, Cp=Cp*,
0.61 g) and LiR (1.25 mmol; R=CH2SiMe3, 0.12 g;
NMe2, 0.06 g; NiPr2, 0.13 g; NHtBu, 0.10 g). The
mixture was stirred for 12 h, during which time a
brown solution and a pale gelatinous precipitate of
LiCl were formed. Filtration of the supernatant solu-
tion from the solid, followed by concentration to ca. 10
ml and cooling to −40°C, afforded 16–18 as brown,
19 yellow and 20 oily brown microcrystalline solids.
16: Yield 0.39 g (70%). Anal. Found: C, 55.72; H,
7.97:N, 3.15. C21H36NSi2Nb. Calc.: C, 55.85; H, 8.03;
N, 3.10%.
3.4. Synthesis of [MCpR2{N(2,6-Me2C6H3)}] (M=Nb,
Cp=Cp%, R=Me, 10; CH2SiMe3, 11; CH2CMe3, 12;
NMe2, 13; CH2C6H5, 14; M=Ta, Cp=Cp*, R=o-
(NSiMe3)2C6H4, 15)
17: Yield 0.35 g (70%). Anal. Found: C, 55.80; H,
7.55; N, 6.68. C19H31N2SiNb. Calc.: C, 55.87; H, 7.65;
N, 6.86%.
18: Yield 0.43 g (70%). Anal. Found: C, 50.80; H,
6.43; N, 5.60. C21H33N2Ta. Calc.: C, 51.00; H, 6.73; N,
5.66%.
A 3 M solution of MgClMe in diethyl ether (0.8 ml,
2.38 mmol) or LiR (2.38 mmol; R=CH2SiMe3, 0.22 g;
19: Yield 0.44 g (65%). Anal. Found: C, 54.47; H,
7.20; N, 4.90. C25H41N2Ta. Calc.: C, 54.54; H, 7.50; N,
5.09%.
20: Yield 0.42 g (65%). Anal. Found: C, 53.29; H,
7.24; N, 5.15. C23H37N2Ta. Calc.: C, 52.87; H, 7.14; N,
5.36%.
CH2CMe3,
0.19
g;
NMe2,
0.12
g)
or
Mg(CH2C6H5)2(THF)2 (0.42 g, 1.19 mmol) or Li2[o-
(NSiMe3)2C6H4] (0.31 g, 1.19 mmol) was added to a
toluene (30 ml) solution of [MCpCl2{N(2,6-Me2C6H3)}]
(1.19 mmol; M=Nb, Cp=Cp%, 0.50 g; M=Ta, Cp=
Cp*, 0.60 g) under rigorously anhydrous conditions
and the reaction mixture stirred for 12 h. The resulting
suspension was filtered, the solvent evaporated to dry-
ness and the residue extracted into n-hexane (2×10
ml). The solution was concentrated to half volume and
cooled to −40°C to yield 10–15 as brown (10–14) or
yellow (15) microcrystalline solids.
Acknowledgements
We are grateful to DGICYT (Project PB-97-0761)
and to Universidad de Alcala´ (Project E009/98) for
financial support of this research.
10: Yield 0.36 g (80%). Anal. Found: C, 56.81; H,
7.40; N, 3.51. C18H28NSiNb. Calc.: C, 56.98; H, 7.44;
N, 3.69%.
11: Yield 0.42 g (80%). Anal. Found: C, 54.76; H,
8.36; N, 2.52. C24H44NSi3Nb. Calc.: C, 55.03; H, 8.47;
N, 2.67%.
12: Yield 0.46 g (80%). Anal. Found: C, 63.45; H,
8.97; N, 2.67. C26H44NSiNb. Calc.: C, 63.52; H, 9.02;
N, 2.85%.
13: Yield 0.40 g (80%). Anal. Found: C, 54.85; H,
7.76; N, 9.50. C20H34N3SiNb. Calc.: C, 54.91; H, 7.83;
N, 9.60%.
14: Yield 0.50 g (80%). Anal. Found: C, 67.58; H,
6.87; N, 2.53. C30H36NSiNb. Calc.: C, 67.78; H, 6.82;
N, 2.63%.
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