Group 5 imido complexes supported by diamido-pyridine ligands: Aryloxide, amide, benzamidinate, alkyl, and cyclopentadienyl derivatives

Article abstract of DOI:10.1021/om0103153

A family of new organometallic and coordination group 5 compounds supported by an imido-diamido-pyridine ligand set are reported. Reaction of [M(NR)Cl(κ₃-N₂N_py)(py)] (M = Nb, R = ^tBu 1; M = Ta, R = ^tBu 2 or Ar 3, where N₂N_py = MeC(2-C₅H₄N)(CH₂NSiMe₃)₂ and Ar = 2,6-C₆H₃ⁱPr₂) with LiOAr or LiPhC(NSiMe₃)₂ afforded the five-coordinate aryloxide and benzamidinate complexes [M(NR)(OAr)(κ³-N₂N_py)] (R = ^tBu, M = Nb 4 or Ta 5; R = Ar, M = Ta 6) and [Nb(N^tBu){PhC(NSiMe₃)₂}(κ³- N₂N_py)] (7), respectively. Reaction of 1 or 2 with LiN(SiMe₃)₂ or LiCH(SiMe₃)₂ gave the four-coordinate compounds [M(N^tBu)(X)(κ²-N₂N_py)] (M = Nb, X = N(SiMe₃)₂ 8 or CH(SiMe₃)₂ 9; M = Ta, X = CH(SiMe₃)₂ 10). Reaction of 1 or 2 with the less sterically demanding alkyl reagent LiCH₂SiMe₃ gave, in the case of niobium, only the four-coordinate [M(N^tBu)(CH₂SiMe₃)(κ²- N₂N_py)] (11), but for tantalum an equilibrium mixture of four- (κ²-) and five- (κ³-) coordinate isomers was identified by NMR spectroscopy. Reaction of 1 with PhCH₂MgCl or LiC₅H₄Me gave the corresponding η¹- or η⁵-hydrocarbyl complexes [Nb(N^tBu)(CH₂Ph)(κ³-N₂ N_py)(py)] (13) and [Nb(N^tBu)(η⁵-C₅H₄Me)- (κ²-N₂N_py)] (14). The X-ray crystal structures of the compounds 4, 6, and 9 are reported.

Full text of DOI:10.1021/om0103153

Organometallics 2001, 20, 3531-3542
3531
Gr ou p 5 Im id o Com p lexes Su p p or ted by
Dia m id o-p yr id in e Liga n d s: Ar yloxid e, Am id e,
Ben za m id in a te, Alk yl, a n d Cyclop en ta d ien yl Der iva tives
Stephen M. Pugh,† Dominique J . M. Tro¨sch,‡ Michael E. G. Skinner,†
Lutz H. Gade,*,‡ and Philip Mountford*,†
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road,
Oxford OX1 3QR, U.K., and Laboratoire de Chimie Organome´tallique et de Catalyse
(CNRS UMR 7513), Institut Le Bel, Universite´ Louis Pasteur, 4 Rue Blaise Pascal,
67000 Strasbourg, France
Received April 17, 2001
A family of new organometallic and coordination group 5 compounds supported by an
imido-diamido-pyridine ligand set are reported. Reaction of [M(NR)Cl(κ3-N2Npy)(py)]
(M ) Nb, R ) tBu 1; M ) Ta, R ) tBu 2 or Ar 3, where N2Npy ) MeC(2-C5H4N)(CH2NSiMe3)2
i
and Ar ) 2,6-C6H3 Pr2) with LiOAr or LiPhC(NSiMe3)2 afforded the five-coordinate aryloxide
and benzamidinate complexes [M(NR)(OAr)(κ3-N2Npy)] (R ) tBu, M ) Nb 4 or Ta 5; R ) Ar,
M ) Ta 6) and [Nb(NtBu){PhC(NSiMe3)2}(κ3-N2Npy)] (7), respectively. Reaction of 1 or 2
with LiN(SiMe3)2 or LiCH(SiMe3)2 gave the four-coordinate compounds [M(NtBu)(X)(κ2-
N2Npy)] (M ) Nb, X ) N(SiMe3)2 8 or CH(SiMe3)2 9; M ) Ta, X ) CH(SiMe3)2 10). Reaction
of 1 or 2 with the less sterically demanding alkyl reagent LiCH2SiMe3 gave, in the case of
niobium, only the four-coordinate [M(NtBu)(CH2SiMe3)(κ2-N2Npy)] (11), but for tantalum an
equilibrium mixture of four- (κ2-) and five- (κ3-) coordinate isomers was identified by NMR
spectroscopy. Reaction of 1 with PhCH2MgCl or LiC5H4Me gave the corresponding η1- or
η5-hydrocarbyl complexes [Nb(NtBu)(CH2Ph)(κ3-N2Npy)(py)] (13) and [Nb(NtBu)(η5-C5H4Me)-
(κ2-N2Npy)] (14). The X-ray crystal structures of the compounds 4, 6, and 9 are reported.
In tr od u ction
into two areas: reactions at the MdNR linkage itself,
and reactions in which the imido ligand acts as a
supporting (spectator) ligand.1-8 Our approach8 to de-
veloping early transition metal imido chemistry has
The chemistry of transition metal imido complexes
(containing the NR ligand, where R is typically a
hydrocarbyl group) has continued to attract considerable
attention, particularly over the last 15 years.1-8 Apart
from theoretical aspects of such systems,9-17 interest
with regard to reactivity studies can broadly be divided
been to use the diamido-pyrdine ligand MeC(2-C5H4-
18-20
N)(CH2NSiMe3)2 (hereafter N2Npy
)
and related
systems21-23 as a potential flexible, robust, dianionic
supporting group. The new classes of groups 4-6
monoimido complexes of the N2Npy ligand are sum-
marized in Chart 1.20,24-26
* To whom correspondence should be addressed. E-mail: gade@
chimie.u-strasbg.fr. E-mail: philip.mountford@chemistry.oxford.ac.uk.
Fax: +44 1865 272690.
† University of Oxford.
(13) Green, J . C.; Green, M. L. H.; J ames, J . T.; Konidaris, P. C.;
Maunder, G. H.; Mountford, P. J . Chem. Soc., Chem. Commun. 1992,
1361.
(14) Brennan, J . G.; Green, J . C.; Redfern, C. M. Inorg. Chim. Acta
1987, 139, 331.
(15) Glueck, D. S.; Green, J . C.; Michelman, R. I.; Wright, I. N.
Organometallics 1992, 11, 4221.
‡ Universite´ Louis Pasteur.
(1) Nugent, W. A.; Mayer, J . M. Metal-Ligand Multiple Bonds;
Wiley-Interscience: New York, 1988.
(2) Chisholm, M. H.; Rothwell, I. P. Comprehensive Coordination
Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J . A., Eds.;
Pergamon Press: Oxford, 1987; Vol. 2.
(3) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239.
(4) Mountford, P. Chem. Commun. 1997, 2127.
(5) Sharp, P. R. Dalton Trans. 2000, 2647.
(16) Lin, Z.; Hall, M. B. Coord. Chem. Rev. 1993, 123, 149.
(17) For leading references on the electron and structural effects of
the trans infuence of multiply bonded ligands see: Kaltsoyannis, N.;
Mountford, P. J . Chem. Soc., Dalton Trans. 1999, 781.
(18) Friedrich, S.; Schubart, M.; Gade, L. H.; Scowen, I. J .; Edwards,
A. J .; McPartlin, M. Chem. Ber. 1997, 130, 1751.
(19) Galka, C. H.; Tro¨sch, D. J . M.; Schubart, M.; Gade, L. H.;
Radojevic, S.; Scowen, I. J .; McPartlin, M. Eur. J . Inorg. Chem. 2000,
2577.
(20) For a review of the chemistry of diamido-donor ligands in
general see: Gade, L. H. Chem. Commun. 2000, 173 (feature article).
(21) Pugh, S. M.; Clark, H. S. C.; Love, J . B.; Blake, A. J .; Cloke, F.
G. N.; Mountford, P. Inorg. Chem. 2000, 39, 2001.
(22) Skinner, M. E. G.; Cowhig, D. A.; Mountford, P. Chem. Com-
mun. 2000, 1167.
(6) For applications of imido complexes in olefin polymerization
catalysis see: Coles, M. P.; Dalby, C. I.; Gibson, V. C.; Clegg, W.;
Elsegood, M. R. J . J . Chem. Soc., Chem. Commun. 1995, 1709. Chan,
M. C. W.; Chew, K. C.; Dalby, C. I.; Gibson, V. C.; Kohlmann, A.; Little,
I. R.; Reed, W. Chem. Commun. 1998, 1673.
(7) For applications of imido complexes in ring-opening metathesis
polymerization see: Schrock, R. R. Acc. Chem. Res. 1990, 23, 158.
Gibson, V. C. Adv. Mater. 1994, 6, 37.
(8) For a review of imido complexes with diamido-donor ligands
see: Gade, L. H.; Mountford, P. Coord. Chem. Rev., 2001, in press
(9) For a general review and leading references see: Cundari, T. R.
Chem. Rev. 2000, 100, 807.
(10) Cundari, T. R. J . Am. Chem. Soc. 1992, 114, 7879.
(11) Mountford, P.; Swallow, D. J . Chem. Soc., Chem. Commun.
1995, 2357.
(12) Schofield, M. H.; Kee, T. P.; Anhaus, J . T.; Schrock, R. R.;
J ohnson, K. H.; Davis, W. M. Inorg. Chem. 1991, 30, 3595.
(23) Cloke, F. G. N.; Hitchcock, P. B.; Nixon, J . F.; Wilson, D. J .;
Mountford, P. Chem. Commun. 1999, 661.
(24) Blake, A. J .; Collier, P. E.; Gade, L. H.; Lloyd, J .; Mountford,
P.; Pugh, S. M.; Schubart, M.; Skinner, M. E. G.; Tro¨sch, D. J . M. Inorg.
Chem. 2001, 40, 870.
10.1021/om0103153 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 07/13/2001

3532 Organometallics, Vol. 20, No. 16, 2001
Pugh et al.
techniques under an atmosphere of argon or of dinitrogen.
Solvents were predried over activated 4 Å molecular sieves
and were refluxed over appropriate drying agents under a
dinitrogen atmosphere and collected by distillation. Deuterated
solvents were dried over appropriate drying agents, distilled
under reduced pressure, and stored under dinitrogen in Teflon
valve ampules. NMR samples were prepared under dinitrogen
in 5 mm Wilmad 507-PP tubes fitted with J . Young Teflon
valves. 1H, 13C{1H}, and 13C NMR spectra were recorded on
Bruker AM 300 and DPX 300 and Varian Unity Plus 500
Ch a r t 1
1
spectrometers. H and 13C assignments were confirmed when
necessary with the use of NOE, DEPT-135, DEPT-90, and two-
dimensional 1H-1H and 13C-1H NMR experiments. All spectra
were referenced internally to residual protio-solvent (1H) or
solvent (13C) resonances and are reported relative to tetra-
methylsilane (δ ) 0 ppm). Chemical shifts are quoted in δ
(ppm) and coupling constants in hertz. Infrared spectra were
prepared as Nujol mulls between CsBr or NaCl plates and
were recorded on Perkin-Elmer 1600 and 1710 series, Nicolet
Avatar 360, or Mattson Polaris FTIR spectrometers. Infrared
data are quoted in wavenumbers (cm-1). NMR assignments
assume the following numbering scheme for the pyridyl group
of N2Npy
:
We have recently reported on the reactions of the
group 4 imido systems I and II in which the MdNR
bond itself is the main reactive site, undergoing reac-
tions with a range of unsaturated substrates of the type
RNC, RCN, RCtCMe, tBuCtP, RCHdCdCH2, and
18
Liter a tu r e P r ep a r a tion s. H2N2Npy
,
Li2[N2Npy],19 [Nb-
(NtBu)Cl(κ3-N2Npy)(py)] (1),25 [Ta(NtBu)Cl(κ3-N2Npy)(py)] (2),25
and [Ta(NAr)Cl(κ3-N2Npy)(py)] (3)25 were prepared according
to methods we have described previously.
i
ArNCO (hereafter Ar ) 2,6-C6H3 Pr2).20,27-31 Group 5
and 6 compounds of the type III and IV have not yet
shown comparable reactivity of the MdNR bond. None-
theless, the presence of M-Cl group(s) in these systems
presents an opportunity for evaluating the supporting
ligand characteristics of the combined tetraanionic-
{(NR)(N2Npy)} imido-diamido-pyridine ligand set.32
Here we report a family of new group 5 organometallic
and coordination derivatives of the complexes [M(NR)-
Cl(κ3-N2Npy)(py)] (M ) Nb, R ) tBu 1; M ) Ta, R ) tBu
2 or Ar 3].25
[Nb(NtBu )(OAr )(K3-N2Np y)] (4). To a mixture of solid [Nb-
(NtBu)Cl(κ3-N2Npy)(py)] (249 mg, 0.42 mmol) and solid LiOAr
(78 mg, 0.42 mmol) was added cold (7 °C) benzene (10 mL).
The yellow solution was allowed to stir at room temperature
for 40 h, becoming very pale during this time. The solution
was filtered, and the volatiles were removed under reduced
pressure to provide [Nb(NtBu)(OAr)(κ3-N2Npy)] (4) as a cream-
colored solid. Yield: 203 mg (74%). Colorless crystals suitable
for X-ray diffraction were obtained from a saturated pentane
solution at 5 °C over a period of 48 h. 1H NMR (C6D6, 500.0
MHz, 298 K): 8.95 (1 H, d, 3J (H6H5) ) 5.0 Hz, H6), 7.26 (2 H,
Exp er im en ta l Section
3
i
3
d, J ) 7.5 Hz, m-C6H3 Pr2), 7.02 (1 H, t, J ) 7.5 Hz, p-C6H3-
iPr2), 6.99 (1 H, overlapping m, H4), 6.84 (1 H, d, J (H3H4) )
3
Gen er a l Meth od s a n d In str u m en ta tion . All manipula-
tions were carried out using standard Schlenk line or drybox
8.0 Hz, H3), 6.48 (1 H, apparent t, apparent J ) 5.5 Hz, H5),
3.85 (2 H, d, 2J ) 13.5 Hz, CH2), 3.60 (2 H, septet, 3J ) 7.0
2
(25) Pugh, S. M.; Blake, A. J . Inorg. Chem. 2001, submitted.
(26) Ward, B. J .; Gade, L. H.; Mountford, P. Manuscript in prepara-
tion.
(27) Blake, A. J .; Collier, P. E.; Gade, L. H.; McPartlin, M.;
Mountford, P.; Schubart, M.; Scowen, I. J . Chem. Commun. 1997, 1555.
(28) Bashall, A.; Collier, P. E.; Gade, L. H.; McPartlin, M.; Mount-
ford, P.; Tro¨sch, D. T. Chem. Commun. 1998, 2555.
(29) Pugh, S. M.; Tro¨sh, D. J . M.; Wilson, D. J .; Bashall, A.; Cloke,
F. G. N.; Gade, L. H.; Hitchcock, P. B.; McPartlin, M.; Nixon, J . F.;
Mountford, P. Organometallics 2000, 19, 3205.
(30) Bashall, A.; Gade, L. H.; McPartlin, M.; Mountford, P.; Pugh,
S. M.; Radojevic, S.; Schubart, M.; Scowen, I. J .; Tro¨sch, D. J .
M.Organometallics 2000, 19, 4784.
(31) Tro¨sch D. J . M.; Collier, P. E.; Bashall, A.; Gade, L. H.;
McPartlin, M.; Mountford, P.; Radojevic, S. Organometallics 2001,
submitted for publication
(32) For the chemistry of group 6 complexes supported by ancilliary
imido-diamido ligand sets see: Ortiz, C. G.; Abboud, K. A.; Boncella,
J . M. Organometallics 1999, 18, 4253. Wang, S.-S. W.; VanderLende,
D. D.; Abboud, K. A.; Boncella, J . M. Organometallics 1998, 17, 2628,
and references therein.
Hz, CHMe2), 3.49 (2 H, d, J ) 13.5 Hz, CH2), 1.35 (12 H, d,
3J ) 6.5 Hz, CHMe2), 1.35 (9 H, s, Bu), 1.08 (3 H, s, Me of
t
N2Npy), 0.39 (18 H, s, SiMe3). 13C{1H} NMR (C6D6, 125.7 MHz,
298 K): 164.6 (C2), 160.5 (ipso-Ar), 148.4 (C6), 138.0 (C4), 135.9
(o-Ar), 123.4 (m-Ar), 121.3 (C5), 119.5 (p-Ar), 119.0 (C3), 61.6
(CH2NSiMe3), 49.9 (C(CH2NSiMe3)2), 33.4 (NCMe3), 27.3
(CHMe2), 23.9 (CHMe2), 22.2 (Me of N2Npy), 2.4 (SiMe3) (the
signal for NCMe3 was not observed). IR (NaCl plates, Nujol,
cm-1): 1601 (m), 1589 (m), 1574 (w), 1358 (s), 1337 (s), 1304
(w), 1267 (s), 1245 (s), 1211 (s), 1159 (m), 1138 (w), 1122 (m),
1091 (m), 1070 (m), 1057 (m), 1037 (m), 1014 (m), 986 (m),
962 (s), 915 (s), 890 (s), 854 (s), 788 (m), 753 (s), 723 (m), 693
(m), 640 (w), 604 (m), 583 (m), 562 (w), 525 (w), 496 (w), 473
(w). Anal. Found (calcd for C31H55N4NbOSi2): C 57.9 (57.4),
H 8.7 (8.5), N 8.2 (8.6).
[Ta (NtBu )(OAr )(K3-N2Np y)] (5). To a mixture of solid [Ta-
(NtBu)Cl(κ3-N2Npy)(py)] (190 mg, 0.28 mmol) and solid LiOAr

Group 5 Imido Complexes
Organometallics, Vol. 20, No. 16, 2001 3533
(52 mg, 0.28 mmol) was added cold (7 °C) benzene (10 mL).
The yellow solution was allowed to stir at room temperature
for 65 h, becoming very pale during this time. The solution
was filtered, and the volatiles were removed under reduced
pressure to provide [Ta(NtBu)(OAr)(κ3-N2Npy)] (5) as a cream-
colored solid. Yield: 120 mg (58%). 1H NMR (C6D6, 500.0 MHz,
298 K): 9.03 (1 H, d, 3J (H6H5) ) 4.5 Hz, H6), 7.27 (2 H, d,
(w), 914 (s), 894 (s), 873 (s), 834 (s), 782 (m), 752 (s), 741 (s),
702 (m), 677 (w), 641 (w), 626 (w), 603 (s), 587 (m), 559 (w),
551 (w), 495 (w), 464 (w), 441 (w), 428 (w). Anal. Found (calcd
for C39H63N4OSi2Ta): C 55.3 (55.7), H 7.2 (7.6), N 6.5 (6.7).
[Nb(NtBu ){P h C(NSiMe3)2}(K3-N2Np y)] (7). To a mixture
of solid [Nb(NtBu)Cl(κ3-N2Npy)(py)] (263 mg, 0.44 mmol) and
solid LiPhC(NSiMe3)2 (121 mg, 0.44 mmol) was added cold (7
°C) benzene (20 mL). The yellow solution was stirred at room
temperature for 40 h, turning paler during this time. The
solution was filtered, and the volatiles were removed under
reduced pressure to yield an orange powder. This powder was
extracted into pentane (5 mL) and filtered, and the solvent
was stripped under reduced pressure to give [Nb(NtBu){PhC-
(NSiMe3)2}(κ3-N2Npy)] (7) as a pale orange powder. Yield: 204
i
3J ) 7.5 Hz, m-C6H3 Pr2), 7.02 (1 H, t, 3J ) 7.5 Hz, p-C6H3-
iPr2), 6.99 (1 H, dd, 3J (H4H5) ) 8.0 Hz, 3J (H4H3) ) 8.0 Hz,
3
H4), 6.79 (1 H, d, J (H3H4) ) 8.0 Hz, H3), 6.48 (1 H, apparent
2
t, apparent J ) 5.5 Hz, H5), 3.85 (2 H, d, J ) 13.0 Hz, CH2),
3.64 (2 H, septet, 3J ) 7.0 Hz, CHMe2), 3.58 (2 H, d, 2J ) 13.0
Hz, CH2), 1.36 (9 H, s, tBu), 1.35 (12 H, d, 3J ) 7.0 Hz, CHMe2),
1.03 (3 H, s, Me of N2Npy), 0.40 (18 H, s, SiMe3).13C{1H} NMR
(C6D6, 125.7 MHz, 298 K): 164.6 (C2), 160.6 (ipso-Ar), 148.1
(C6), 138.4 (C4), 136.2 (o-Ar), 123.4 (m-Ar), 121.5 (C5), 120.0
(p-Ar), 119.0 (C3), 65.7 (NCMe3), 61.4 (CH2NSiMe3), 49.2
(C(CH2NSiMe3)2), 34.8 (NCMe3), 27.1 (CHMe2), 24.0 (CHMe2),
22.3 (Me of N2Npy), 2.5 (SiMe3). IR (NaCl plates, Nujol, cm-1):
1602 (s), 1452 (s), 1432 (m), 1358 (s), 1341 (m), 1335 (m), 1257
(s), 1210 (s), 1159 (m), 1137 (m), 1114 (m), 1092 (s), 1073 (m),
1060 (m), 1036 (s), 1013 (w), 968 (s), 933 (m), 925 (w), 891
(m), 862 (s), 834 (m), 807 (w), 787 (w), 755 (m), 740 (w), 604
(m), 593 (m), 583 (m). Anal. Found (calcd for C31H55N4OSi2-
Ta): C 50.7 (50.5), H 7.2 (7.5), N 6.8 (7.6).
1
mg (62%). H NMR (C6D6, 500.0 MHz, 298 K): 8.70 (1 H, m,
3
H6), 7.42 (2 H, d, J ) 6.5 Hz, o-C6H5), 7.06 (1 H, overlapping
m, m-C6H5), 7.06 (1 H, overlapping m, p-C6H5), 7.06 (1 H,
overlapping m, H4), 7.03 (1 H, overlapping d, H3), 6.62 (1 H,
2
apparent t, apparent J ) 5.5 Hz, H5), 4.13 (2 H, d, J ) 15.0
2
t
Hz, CH2), 3.51 (2 H, d, J ) 15.0 Hz, CH2), 1.64 (9 H, s, Bu),
1.25 (3 H, s, Me of N2Npy), 0.50 (18 H, s, CH2NSiMe3), -0.06
(18 H, s, C(NSiMe3)2). 13C{1H} NMR (C6D6, 125.7 MHz, 298
K): 182.8 (C(NSiMe3)2), 168.2 (C2), 148.8 (C6), 142.6 (ipso-
C6H5), 136.8 (C4), 128.6, 128.1 (m-C6H5 and p-C6H5), 127.0 (o-
C6H5), 120.4 (C5), 120.0 (C3), 67.7 (NCMe3), 59.7 (CH2NSiMe3),
44.5 (C(CH2NSiMe3)2), 33.3 (NCMe3), 22.9 (Me of N2Npy), 3.8,
3.1 (CH2NSiMe3 and C(NSiMe3)2). IR (NaCl plates, Nujol,
cm-1): 1663 (w), 1602 (m), 1590 (w), 1574 (w), 1435 (s), 1355
(m), 1289 (w), 1244 (s), 1225 (s), 1160 (w), 1138 (w), 1117 (m),
1090 (w), 1075 (w), 1063 (w), 1038 (m), 1020 (m), 1008 (m),
990 (m), 976 (m), 965 (m), 953 (w), 911 (s), 844 (s), 784 (m),
754 (s), 724 (m), 701 (m), 670 (w), 638 (w), 600 (m), 553 (w),
529 (w), 488 (m), 478 (w). Anal. Found (calcd for C32H61N6-
NbSi4): C 52.1 (52.3), H 8.1 (8.4), N 11.3 (11.4).
[Ta (NAr )(OAr )(K3-N2Np y)] (6). To a mixture of solid [Ta-
(NAr)Cl(κ3-N2Npy)(py)] (190 mg, 0.24 mmol) and solid LiOAr
(45 mg, 0.24 mmol) was added cold (7 °C) benzene (10 mL).
The yellow-green solution was stirred at room temperature
for 72 h. The solution was filtered, and the volatiles were
removed under reduced pressure to provide [Ta(NAr)(OAr)-
(κ3-N2Npy)] (6) as a yellow solid. Yield: 124 mg (60%). Yellow
crystals suitable for X-ray diffraction were prepared by re-
crystallization of the product from
a 1:1 hexane/toluene
[Nb(NtBu ){N(SiMe3)2}(K2-N2Np y)] (8). To a mixture of
solid [Nb(NtBu)Cl(κ3-N2Npy)(py)] (225 mg, 0.38 mmol) and solid
LiN(SiMe3)2 (64 mg, 0.38 mmol) was added cold (7 °C) benzene
(10 mL). The solution was allowed to warm to room temper-
ature, then stirred at 50 °C for 65 h. The resulting pale yellow
solution was filtered, and the volatiles were removed under
reduced pressure to provide a waxy yellow solid. This solid
was extracted with hexane (10 mL) and filtered, and the
volatiles were again removed under reduced pressure. [Nb-
(NtBu){N(SiMe3)2}(κ2-N2Npy)] (8) was obtained as an orange
solid. Yield: 145 mg (61%). Compound 8 was spectroscopically
>90% pure, but trace impurites could not be removed by
washing with hexanes, and the compound could not be
successfully recrystallized. 1H NMR (C6D6, 500.0 MHz, 298
K): 8.47 (1 H, d, 3J (H6H5) ) 4.0 Hz, H6), 7.08 (1 H, overlapping
m, H4), 7.08 (1 H, overlapping d, H3), 6.57 (1 H, apparent t,
apparent J ) 5.5 Hz, H5), 4.64 (2 H, d, 2J ) 15.5 Hz, CH2),
1
solution at 5 °C over 48 h. H NMR (CD2Cl2, 500.0 MHz, 298
K): 8.82 (1 H, d, 3J (H6H5) ) 5.0 Hz, H6), 7.90 (1 H, dd,
3
3
3J (H4H5) ) 8.0 Hz, J (H4H3) ) 8.0 Hz, H4), 7.54 (1 H, d, J
(H3H4) ) 8.0 Hz, H3), 7.22 (1 H, apparent t, apparent J ) 6.0
i
Hz, H5), 7.14 (2 H, d, 3J ) 7.5 Hz, m-O-C6H3 Pr2), 6.95 (2 H, d,
3J ) 7.5 Hz, m-N-C6H3 Pr2), 6.89 (1 H, t, 3J ) 7.5 Hz, p-O-
i
i
3
i
C6H3 Pr2), 6.67 (1 H, t, J ) 7.5 Hz, p-N-C6H3 Pr2), 3.89 (2 H,
2
d, J ) 12.5 Hz, CH2), 3.85 (2 H, br overlapping s, CHMe2 of
N-C6H3 Pr2), 3.82 (2 H, d, 2J ) 12.5 Hz, CH2), 3.36 (2 H, septet,
i
3J ) 7.0 Hz, CHMe2 of O-C6H3 Pr2), 1.57 (3 H, s, Me of N2Npy),
i
3
i
1.35 (12 H, d, J ) 6.0 Hz, CHMe2 of O-C6H3 Pr2), 0.98 (12 H,
br s, CHMe2 of N-C6H3 Pr2), 0.03 (18 H, s, SiMe3). 1H NMR
i
(CD2Cl2, 500.0 MHz, 218 K): 8.80 (1 H, d, 3J (H6H5) ) 4.0 Hz,
H6), 7.91 (1 H, dd, 3J (H4H5) ) 8.0 Hz, 3J (H4H3) ) 8.0 Hz,
H4), 7.53 (1 H, d, J (H3H4) ) 8.0 Hz, H3), 7.23 (1 H, apparent
3
t, apparent J ) 6.0 Hz, H5), 7.11 (2 H, d, 3J ) 7.5 Hz, m-O-
C6H3 Pr2), 6.98 (1 H, d, 3J ) 6.5 Hz, m-N-C6H3 Pr2), 6.85 (1 H,
i
i
2
t
3
i
3.62 (2 H, d, J ) 15.5 Hz, CH2), 1.55 (9 H, s, Bu), 1.14 (3 H,
s, Me of N2Npy), 0.41 (18 H, s, N(SiMe3)2), 0.36 (18 H, s, CH2-
NSiMe3). 13C{1H} NMR (C6D6, 125.7 MHz, 298 K): 166.1 (C2),
149.4 (C6), 136.0 (C4), 121.1 (C5), 120.3 (C3), 56.7 (CH2NSiMe3),
45.7 (C(CH2NSiMe3)2), 33.6 (NCMe3), 19.6 (Me of N2Npy), 6.2
(N(SiMe3)2), 3.5 (CH2NSiMe3 (signal for NCMe3not observed).
IR (NaCl plates, Nujol, cm-1): 1588 (m), 1571 (m), 1434 (s),
1402 (m), 1356 (m), 1311 (w), 1247 (s), 1232 (s), 1214 (m), 1157
(w), 1133 (m), 1083 (m), 1029 (s), 987 (m), 960 (m), 938 (m),
895 (s), 839 (s), 786 (s), 711 (m), 667 (m), 636 (w), 620 (w), 586
(w), 552 (w), 501 (w). Anal. Found (calcd for C25H56N5NbSi4):
C 45.9 (47.5), H 8.5 (8.9), N 10.3 (11.1).
overlapping t, J ) 7.5 Hz, p-O-C6H3 Pr2), 6.84 (1 H, overlap-
i
ping d, m-N-C6H3 Pr2), 6.63 (1 H, t, 3J ) 7.5 Hz, p-N-C6H3-
iPr2), 4.23 (1 H, br s, CHMe2 of N-C6H3 Pr2), 3.83 (2 H, d, 2J )
i
2
12.5 Hz, CH2), 3.73 (2 H, d, J ) 12.5 Hz, CH2), 3.38 (1 H, br
i
s, CHMe2 of N-C6H3 Pr2), 3.30 (2 H, septet, 3J ) 7.0 Hz, CHMe2
i
3
of O-C6H3 Pr2), 1.54 (3 H, s, Me of N2Npy), 1.26 (6 H, d, J )
6.5 Hz, CHMeMe of N-C6H3 Pr2), 1.21 (6 H, d, 3J ) 6.5 Hz,
i
i
3
CHMeMe of O-C6H3 Pr2), 0.97 (6 H, d, J ) 6.5 Hz, CHMeMe
of O-C6H3 Pr2), 0.53 (6 H, d, 3J ) 6.5 Hz, CHMeMe of N-C6H3-
i
iPr2), -0.04 (18 H, s, SiMe3). 13C{1H} NMR (CD2Cl2, 125.7 MHz,
218 K): 163.9 (C2), 159.0 (ipso-OAr), 153.0 (ipso-NAr), 147.5
(C6), 139.8 (C4), 136.3 (o-NAr), 128.8 (o-OAr), 123.5 (m-OAr),
122.4 (C5), 121.8 (m-NAr), 120.4 (p-NAr), 119.5, 119.4 (p-OAr
& C3), 62.6 (CH2NSiMe3), 49.6 (C(CH2NSiMe3)2), 27.4 (CHMe2
of NAr), 27.0 (CHMe2 of OAr), 26.0 (CHMeMe of OAr), 24.8
(CHMe2 of NAr), 24.4 (CHMe2 of NAr), 23.4 (Me of N2Npy), 22.3
(CHMeMe of OAr), 0.7 (SiMe3). IR (NaCl plates, Nujol, cm-1):
1602 (s), 1589 (m), 1577 (m), 1456 (s), 1353 (s), 1328 (s), 1296
(s), 1285 (s), 1262 (s), 1209 (s), 1163 (m), 1140 (m), 1110 (m),
1091 (m), 1062 (m), 1029 (s), 1013 (m), 993 (w), 981 (m), 952
[Nb(NtBu ){CH(SiMe3)2}(K2-N2Np y)] (9). To a mixture of
solid [Nb(NtBu)Cl(κ3-N2Npy)(py)] (200 mg, 0.34 mmol) and solid
LiCH(SiMe3)2 (57 mg, 0.34 mmol) was added cold (7 °C)
benzene (10 mL). The resulting solution was stirred for 5 h at
room temperature, becoming red during this time. The solution
was then filtered, and the volatiles were removed under
reduced pressure to leave [Nb(NtBu){CH(SiMe3)2}(κ2-N2Npy)]
(9) as an orange-white solid. Yield: 126 mg (59%). Colorless

3534 Organometallics, Vol. 20, No. 16, 2001
Pugh et al.
crystals suitable for X-ray diffraction were obtained from a
[Ta (NtBu )(CH2SiMe3)(K2,3-N2Np y)] (12a ,b). To a mixture
of solid [Ta(NtBu)Cl(κ3-N2Npy)(py)] (195 mg, 0.29 mmol) and
solid LiCH2SiMe3 (27 mg, 0.29 mmol) was added cold (7 °C)
benzene (10 mL). The resulting solution was stirred at 50 °C
for 20 h, turning orange during this time. The solution was
filtered, and the volatiles were removed under reduced pres-
sure to leave an orange oil. The oil was extracted with hexane
(10 mL) and filtered, and the volatiles were again removed
saturated solution of [Nb(NtBu){CH(SiMe3)2}(κ2-N2Npy)] in
1
hexane at 5 °C over 2 weeks. H NMR (C6D6, 300.1 MHz, 298
K): 8.45 (1 H, dt, 3J (H6H5) ) 4.8 Hz, 4J (H6H4) ) 1.3 Hz, H6),
7.07 (1 H, overlapping m, H4), 7.07 (1 H, overlapping d, H3),
6.55 (1 H, apparent t, apparent J ) 6.0 Hz, H5), 4.81 (2 H, d,
2
2J ) 15.1 Hz, CH2), 3.67 (2 H, d, J ) 15.6 Hz, CH2), 1.63 (9
H, s, tBu), 1.10 (3 H, s, Me of N2Npy), 0.61 (1 H, s, CH(SiMe3)2),
0.36 (18 H, s, CH(SiMe3)2), 0.33 (18 H, s, NSiMe3). 13C{1H}
NMR (C6D6, 75.5 MHz, 298 K): 166.0 (C2), 149.5 (C6), 136.2
(C4), 121.3 (C5), 120.3 (C3), 69.9 (NCMe3), 58.1 (CH(SiMe3)2),
56.7 (CH2NSiMe3), 47.3 (C(CH2NSiMe3)2), 34.2 (NCMe3), 19.5
(Me of N2Npy), 5.1 (CH(SiMe3)2), 3.5 (NSiMe3). IR (CsBr plates,
Nujol, cm-1): 1735 (w), 1590 (m), 1508 (w), 1248 (s), 1154 (w),
1051 (m), 965 (w), 904 (s), 843 (s), 787 (m), 689 (m), 660 (w).
Anal. Found (calcd for C26H57N4NbSi4): C 48.9 (49.5), H 9.2
(9.1), N 8.9 (8.9).
under reduced pressure to provide [Ta(NtBu)(CH2SiMe3)(κ2,3
-
N2Npy)] (12) as an orange viscous oil, which could not be
crystallized. Yield: 159 mg (85%). 1H NMR (C6D6, 500.0 MHz,
298 K) (signals associated with the five-coordinate isomer (12b)
are denoted with a prime (′); H′ and H are in an approximate
3
ratio 1 H′:2 H): 9.02 (1 H′, d, J (H4H5) ) 4.5 Hz, H6′), 8.50 (1
3
3
H, d, J (H4H5) ) 4.5 Hz, H6), 7.07 (1 H, dd, J (H4H5) ) 8.0
Hz, 3J (H4H3) ) 8.0 Hz, H4), 7.01 (1 H, overlapping d, H3), 7.01
(1 H′, overlapping m, H4′), 6.69 (1 H′, d, J (H3′H4′) ) 8.0 Hz,
3
H3′), 6.59 (1 H, apparent t, apparent J ) 6.0 Hz, H5), 6.57 (1
[Ta (NtBu ){CH(SiMe3)2}(K2-N2Np y)] (10). To a mixture of
solid [Ta(NtBu)Cl(κ3-N2Npy)(py)] (218 mg, 0.32 mmol) and solid
LiCH(SiMe3)2 (54 mg, 0.32 mmol) was added cold (7 °C)
benzene (10 mL). The solution was stirred for 17 h at room
temperature, becoming orange during this time. The solution
was then filtered, and the volatiles were removed under
reduced pressure to leave the compound [Ta(NtBu){CH-
(SiMe3)2}(κ2-N2Npy)] (10) as an orange-white solid. Yield: 111
H′, apparent t, apparent J ) 6.0 Hz, H5′), 4.57 (2 H, d, J )
2
2
14.5 Hz, CH2), 4.05 (2 H′, d, J ) 13.0 Hz, CH2′), 3.63 (2 H, d,
2J ) 15.0 Hz, CH2′), 3.38 (2 H′, d, J ) 13.5 Hz, CH2), 1.65 (9
2
t
t
H′, s, Bu′), 1.62 (9 H, s, Bu), 1.17 (3 H, s, Me of N2Npy), 0.95
(3 H′, s, Me′ of N2Npy), 0.47 (2 H, s, CH2SiMe3), 0.45 (2 H′, s,
CH′2SiMe3), 0.44 (9 H′, s, CH2SiMe′3), 0.30 (18 H, s,NSiMe3),
0.30 (9 H, s, CH2SiMe3), -0.01 (18 H′, s, NSiMe′3). 13C NMR
(C6D6, 125.7 MHz, 298 K) (signals associated with the five-
coordinate complex (12b) are denoted with a prime [′]): 165.5
(C2), 160.3 (C2′), 149.1 (C6), 147.9 (C6′), 137.2 (C4′), 136.3 (C4),
121.4 (C5), 121.0 (C5′), 120.7 (C3′), 120.1 (C3), 67.0 (NCMe3),
66.3 (NC′Me3), 62.1 (C′H2NSiMe3), 57.4 (CH2NSiMe3), 49.8
(C′H2SiMe3), 49.7 (C(CH2NSiMe3)2), 45.9 (C′(CH2NSiMe3)2),
42.3 (CH2SiMe3), 35.2 (NCMe3), 34.9 (NCMe′3), 23.5 (Me′ of
N2Npy), 19.2 (Me of N2Npy), 3.6 (t, 1J (CH) ) 117 Hz, CH2SiMe′3),
3.2 (t, 1J (CH) ) 104 Hz, CH2SiMe3), 2.9 (NSiMe3), 0.9 (NSiMe′3).
IR (NaCl plates, thin film, cm-1): 1602 (s), 1588 (s), 1572 (s),
1469 (s), 1432 (s), 1402 (w), 1378 (w), 1354 (s), 1311 (w), 1245
(s), 1212 (s), 1157 (w), 1139 (m), 1089 (m), 1039 (s), 1000 (s),
945 (s), 683 (m), 668 (m), 631 (w), 620 (w), 594 (m), 554 (m),
520 (w), 496 (m), 450 (m). Anal. Found (calcd for C23H49N4Si3-
Ta): C 42.1 (42.7), H 7.4 (7.6), N 8.5 (8.6).
1
mg (48%). H NMR (C6D6, 500.0 MHz, 298 K): 8.43 (1 H, dt,
3J (H6H5) ) 4.5 Hz, 4J (H6H4) ) 1.5 Hz, H6), 7.06 (1 H,
overlapping m, H4), 7.06 (1 H, overlapping m, H3), 6.54 (1 H,
2
apparent t, apparent J ) 6.0 Hz, H5), 4.64 (2 H, d, J ) 15.0
Hz, CH2), 3.60 (2 H, d, 2J ) 15.0 Hz, CH2), 1.76 (1 H, s,
CH(SiMe3)2), 1.63 (9 H, s, tBu), 1.16 (3 H, s, Me of N2Npy), 0.37
(18 H, s, CH(SiMe3)2), 0.35 (18 H, s, NSiMe3). 13C NMR (C6D6,
125.7 MHz, 298 K): 165.7 (C2), 149.5 (C6), 136.3 (C4), 121.4
(C5), 120.4 (C3), 67.9 (NCMe3), 63.2 (CH(SiMe3)2), 56.3 (CH2-
NSiMe3), 47.1 (C(CH2NSiMe3)2), 35.4 (NCMe3), 19.7 (Me of
1
N2Npy), 5.3 (d, J (CH) ) 94 HzCH(SiMe3)2), 3.7 (NSiMe3). IR
(NaCl plates, Nujol, cm-1): 1651 (w), 1604 (m), 1588 (m), 1572
(m), 1401 (w), 1356 (m), 1250 (s), 1214 (m), 1157 (m), 1092
(w), 1048 (m), 1015 (s), 990 (s), 945 (m), 920 (s), 901 (s), 839
(s), 793 (s), 755 (s), 703 (w), 663 (s), 632 (w), 618 (m), 600 (w),
555 (w), 532 (w). Anal. Found (calcd for C26H57N4Si4Ta): C 42.3
(43.4), H 7.1 (7.8), N 7.7 (8.0).
[Nb(NtBu )(CH2P h )(K3-N2Np y)(p y)] (13). To a cooled (10
°C) solution of [Nb(NtBu)Cl(κ3-N2Npy)(py)] (204 mg, 0.35 mmol)
in benzene (15 mL) was added PhCH2MgCl (1.0 M solution in
diethyl ether, 0.37 mL, 0.35 mmol). The resulting solution was
stirred at room temperature for 17 h, during which time an
orange color developed. 1,4-Dioxane (1 mL) was added to the
solution in order to precipitate MgCl2 as MgCl2‚dioxane, and
the solution was filtered. The volatiles were removed under
reduced pressure, and the orange residues were washed with
hexane (3 × 10 mL). [Nb(NtBu)(CH2Ph)(κ3-N2Npy)(py)] (13) was
obtained as an orange powder, but attempted recrystallization
led to decomposition. Yield: 85 mg (38%). 1H NMR (C6D6, 300.1
MHz, 298 K): 8.81 (1 H, d, 3J (H6H5) ) 5.0 Hz, H6), 7.87 (2 H,
d, 3J (o-C6H5m-C6H5) ) 7.5 Hz, o-C6H5), 7.58 (2 H, br s,
o-NC5H5), 7.25 (2 H, t, 3J ) 7.7 Hz, m-C6H5), 7.04 (1 H,
overlapping m, H4), 6.99 (1 H, overlapping m, H3), 6.99 (1 H,
overlapping m, p-C6H5), 6.70 (1 H, br s, p-NC5H5), 6.51 (1 H,
apparent t, apparent J ) 6.5 Hz, H5), 6.44 (2 H, br s,
m-NC5H5), 3.67 (1 H, d, 2J ) 11.8 Hz, Me3SiNCH2), 3.18 (1 H,
[Nb(NtBu )(CH2SiMe3)(K2-N2Np y)] (11). To a mixture of
solid [Nb(NtBu)Cl(κ3-N2Npy)(py)] (270 mg, 0.46 mmol) and solid
LiCH2SiMe3 (43 mg, 0.46 mmol) was added cold (7 °C) benzene
(10 mL). The resulting yellow solution was stirred at room
temperature for 17 h, turning red during this time. The
solution was filtered, and the volatiles were removed under
reduced pressure to leave a red oil. The oil was extracted with
hexane (3 mL) and filtered, and the volatiles were again
removed under reduced pressure to provide [Nb(NtBu)(CH2-
SiMe3)(κ2-N2Npy)] (11) as a red viscous oil, which could not be
crystallized. Yield: 180 mg (70%). 1H NMR (C6D6, 500.0 MHz,
3
4
298 K): 8.52 (1 H, d, J (H6H5) ) 4.5 Hz, J (H6H4) ) 1.0 Hz,
H6), 7.06 (1 H, dd, 3J (H4H5) ) 7.5 Hz, 3J (H4H3) ) 7.5 Hz,
H4), 7.01 (1 H, d, J (H3H4) ) 7.5 Hz, H3), 6.58 (1 H, apparent
3
t, apparent J ) 6.0 Hz, H5), 4.62 (2 H, d, J ) 14.5 Hz, CH2),
2
2
t
3.59 (2 H, d, J ) 14.0 Hz, CH2), 1.59 (9 H, s, Bu), 1.09 (3 H,
s, Me of N2Npy), 0.79 (2 H, s, CH2SiMe3), 0.29 (9 H, s, CH2-
SiMe3), 0.26 (18 H, s, NSiMe3). 13C{1H} NMR (C6D6, 125.7
MHz, 298 K): 165.8 (C2), 149.1 (C6), 136.4 (C4), 121.3 (C5),
120.1 (C3), 62.8 (CH2SiMe3), 58.1 (CH2NSiMe3), 50.2 (C(CH2-
NSiMe3)2), 33.9 (NCMe3), 19.3 (Me of N2Npy), 3.1 (CH2SiMe3),
2.7 (NSiMe3) (the signal for NCMe3 was not observed. IR (NaCl
plates, thin film, cm-1): 1600 (w), 1588 (m), 1572 (w), 1456
(w), 1432 (m), 1402 (w), 1355 (m), 1311 (w), 1244 (s), 1213 (m),
1157 (w), 1138 (m), 1089 (w), 1051 (m), 1030 (s), 1000 (m),
990 (m), 942 (m), 916 (s), 892 (s), 839 (s), 792 (s), 711 (m), 683
(w), 666 (m), 633 (w), 619 (w), 596 (w), 576 (w), 554 (w), 498
(w), 453 (w), 403 (w). Anal. Found (calcd for C23H49N4NbSi3):
C 49.4 (48.6), H 8.8 (8.2), N 10.0 (10.0).
2
d, 2J ) 11.4 Hz, Me3SiNCH2), 3.00 (1 H, d, J ) 12.5 Hz, Me3-
2
SiNCH2), 2.83 (1 H, d, J ) 12.5 Hz, Me3SiNCH2), 1.43 (3 H,
t
s, Me of N2Npy), 1.16 (9 H, s, Bu), 0.08 (9 H, s, SiMe3), -0.14
(9 H, s,SiMe3) (the signals for PhCH2 were not observed). 13C-
{1H} NMR (C6D6, 75.5 MHz, 298 K): 170.5 (C2), 153.4 (ipso-
C6H5), 148.4 (C6), 147.9 (o-NC5H5), 139.0 (C4), 137.7 (p-NC5H5),
130.5 (o-C6H5), ca. 128 (obscured by solvent signal, m-C6H5,
p-C6H5), 124.0 (m-NC5H5), 121.4 (C5), 119.5 (C3), 65.0 (NCMe3),
60.9 (CH2NSiMe3), 46.9 (CH2NSiMe3), 32.2 (NCMe3), 26.7 (Me
of N2Npy), 1.9 (SiMe3), 0.1 (SiMe3) (signals for C(CH2NSiMe3)2
and PhCH2 were not observed). IR (CsBr plates, Nujol, cm-1):
1735 (w), 1630 (w), 1602 (m), 1572 (m), 1523 (w), 1508 (w),

Group 5 Imido Complexes
Organometallics, Vol. 20, No. 16, 2001 3535
Ta ble 1. X-r a y Da ta Collection a n d P r ocessin g P a r a m eter s for [Nb(NtBu )(O-2,6-C6H3 P r 2)(N2Np y)] (4),
i
i
i
[Ta (N-2,6-C6H3 P r 2)(O-2,6-C6H3 P r 2)(N2Np y)] (6), a n d [Nb(NtBu ){CH(SiMe3)2}(N2Np y)] (9)
4
6
9
molecular formula
fw
C
31H55N4NbOSi2
C39H63N4OSi2Ta
841.08
C26H57N4NbSi4
631.03
648.89
temperature/°C
cryst syst
space group
unit cell dimens: a/Å
b/Å
-98
-123
-103
monoclinic
P21/n
triclinic
triclinic
P1h
P1h
15.1000(6)
13.0490(5)
18.6330(4)
96.504(2)
97.705(2)
92.121(2)
3638.3
11.5100(3)
11.9300(2)
14.9600(4)
92.37(5)
11.296(6)
17.556(7)
18.07(1)
c/Å
R/deg
â/deg
98.040(1)
92.05(4)
2018.0
93.16(5)
γ/deg
volume/Å3
3572.9
Z
4
1.18
2
1.38
2
1.17
density(calcd)/Mg m-3
radiation (λ/Å)
abs coeff/mm-1
F(000)
Mo KR (0.71073)
0.41
1384
Mo KR (0.71073)
2.78
868
Cu KR (1.54180)
4.26
1352
cryst description
cryst size/mm
θ range for data collection/deg
scan type
decay correction/%
index ranges
colorless block
0.40 × 0.30 × 0.30
1.4 to 26.5
ω scans
not measured
0 e h e 18, 0 e k e 16,
-21 e l e 20
20 879
yellow block
0.35 × 0.25 × 0.25
0.5 to 27
colorless block
0.80 × 0.65 × 0.50
2.45 to 74.90
ω-2θ scans
9.69
-14 e h e 14, -21 e k e 21,
-22 e l e 0
14 970
ω scans
not measured
0 e h e 14, -14 e k e 14,
-18 e l e 18
12 281
no. of reflns collected
no. of ind reflns
7344
7866
14 493
no. of obsd reflns [I>3σ(I)]
R(merge)
4637
0.030
7025
0.035
11 933
0.030
abs corr
multiscan
Tmin ) 0.848, Tmax ) 0.884
4637
0
352
Chebychev polynomial
none required
R1 ) 0.041
Rw ) 0.038
1.126
0.001
0.52 and -0.98
multiscan
Tmin ) 0.378, Tmax ) 0.488
7025
0
425
Chebchev polynomial
43(6)
R1 ) 0.0264
Rw ) 0.0295
1.0539
0.0038
0.95 and -1.27
Ψ-scans
max. and min. transmn
no. of data used in refinement
no. of restraints applied
no. of params refined
weighting scheme
Tmin ) 0.71, Tmax ) 1.00
11 933
0
633
Chebychev polynomial
83(8)
R1 ) 0.0531
Rw ) 0.0615
0.9920
0.064
0.97 and -2.20
extinction coeff
final R indicesa [I>3σ(I)]
goodness-of-fit (on F)
final (∆/σ)max
largest residual peaks/e Å-3
a
2
R1 ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) x{∑w (|Fo| - |Fc|)2/∑(w|Fo| }.
1247 (s), 1211 (m), 1156 (w), 1070 (w), 945 (m), 838 (s), 770
(m), 752 (s), 701 (m), 634 (w). A satisfactory elemental analysis
could not be obtained for this compound.
Cr ysta l Str u ctu r e Deter m in a tion s of [Nb(NtBu )(OAr )-
(K3-N2Np y)] (4), [Ta (NAr )(OAr )(K3-N2Np y)] (6), a n d [Nb-
(NtBu ){CH(SiMe3)2}(K2-N2Np y)] (9). Crystal data collection
and processing parameters are given in Table 1. Crystals were
immersed in a film of perfluoropolyether oil on a glass fiber
and transferred to an Enraf-Nonius DIP2000 image plate
diffractometer or CAD4 four-circle diffractometer equipped
with an Oxford Cryosystems low-temperature device.33 Data
were collected at low temperature using Mo KR (4 and 6) or
Cu KR (for 9) radiation; equivalent reflections were merged
and for 4 and 6 the images were processed with the DENZO
and SCALEPACK programs.34 Corrections for Lorentz-polar-
ization effects and absorption were performed, and the struc-
tures were solved by direct methods using SIR92.35 Subsequent
difference Fourier syntheses revealed the positions of all other
non-hydrogen atoms. Hydrogen atoms were placed geometri-
cally and refined in a riding model. Extinction corrections were
applied as required.36 Crystallographic calculations were
performed using SIR9235 and CRYSTALS.36 A full listing of
atomic coordinates, bond lengths and angles, and displacement
parameters for 4, 6, and 9 have been deposited at the
[Nb(NtBu )(η5-C5H4Me)(K2-N2Np y)] (14). To a mixture of
solid [Nb(NtBu)Cl(κ3-N2Npy)(py)] (187 mg, 0.32 mmol) and solid
LiC5H4Me (28 mg, 0.32 mmol) was added cold (7 °C) benzene
(10 mL). The solution was heated to 60 °C and stirred for 72
h, becoming orange during this time. It was then filtered, and
the volatiles were removed under reduced pressure, to leave
[Nb(NtBu)(η5-C5H4Me)(κ2-N2Npy)] (14) as an orange solid.
Yield: 115 mg (65%). 1H NMR (C6D6, 500.0 MHz, 298 K): 8.57
(1 H, d, 3J (H6H5) ) 4.0 Hz, H6), 7.10 (1 H, overlapping m,
H4), 7.09 (1 H, overlapping d, H3), 6.58 (1 H, apparent t,
apparent J ) 6.0 Hz, H5), 6.22 (2 H, apparent s, C5H4Me), 5.86
2
(2 H, apparent s, C5H4Me), 4.16 (2 H, d, J ) 15.0 Hz, CH2),
3.48 (2 H, d, 2J ) 15.0 Hz, CH2), 1.89 (3 H, s, C5H4Me), 1.37 (9
t
H, s, Bu), 1.01 (3 H, s, Me of N2Npy), 0.27 (18 H, s, SiMe3).
13C{1H} NMR (C6D6, 125.7 MHz, 298 K): 167.6 (C2), 149.4 (C6),
135.9 (C4), 124.0 (1-C5H4Me), 120.8, 120.7 (C5 and C3), 105.6,
104.2 (2 × ring CH of C5H4Me), 61.4 (CH2NSiMe3), 44.5
(C(CH2NSiMe3)2), 33.3 (NCMe3), 20.3 (Me of N2Npy), 14.2
(C5H4Me), 3.6 (SiMe3) (the signal for NCMe3 was not observed).
IR (NaCl plates, Nujol, cm-1): 1587 (s), 1569 (m), 1496 (w),
1435 (m), 1356 (m), 1342 (w), 1314 (w), 1262 (m), 1242 (s),
1222 (s), 1153 (m), 1119 (s), 1091 (w), 1055 (m), 1036 (s), 994
(w), 981 (m), 938 (m), 918 (s), 839 (s), 800 (m), 787 (s), 681
(w), 666 (s), 630 (w), 620 (m), 590 (w), 557 (s), 531 (w), 496
(w), 481 (w), 453 (m), 429 (w). Anal. Found (calculated for
C25H45N4NbSi2): C 54.6 (54.5), H 7.8 (8.2), N 9.8 (10.2).
(33) Cosier, J .; Glazer, A. M. J . Appl. Crystallogr. 1986, 19, 105.
(34) Gewirth, D. The HKL Manual, written with the cooperation of
the program authors, Otwinowski, Z., and Minor, W., Yale University,
1995.
(35) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J . Appl. Crystallogr. 1994, 27,
435.
(36) Larson, A. C. Acta Crystallogr. 1967, 12, 664.

3536 Organometallics, Vol. 20, No. 16, 2001
Pugh et al.
Sch em e 1. Ar yloxid e, Am id e, a n d Ben za m id in a te Com p lexes w ith Im id o-Dia m id o-P yr id in e Liga n d Setsa
a
(i) Li-OAr, benzene, 7 °C then rt, 40-72 h, 58-74%; (ii) LiN(SiMe3)2, benzene, 7 °C then 50 °C, 65 h, 61%; (iii)
LiPhNC(NSiMe3)2, benzene, 7 °C then rt, 40 h, 62%.
Cambridge Crystallographic Data Center. See Notice to
Authors, Issue No. 1.
Resu lts a n d Discu ssion
The compounds [M(NR)Cl(κ3-N2Npy)(py)] (M ) Nb,
t
t
R ) Bu 1; M ) Ta, R ) Bu 2 or Ar 3, where N2Npy
)
i
MeC(2-C5H4N)(CH2NSiMe3)2 and Ar ) 2,6-C6H3 Pr2)
were prepared according to methods we have reported
previously.25 The syntheses and proposed structures of
the new complexes with O- or N-donor ligands are
summarized in Scheme 1.
Ar yloxid e Der iva tives. Cooled benzene was added
to a mixture of solid [Nb(NtBu)Cl(κ3-N2Npy)(py)] (1) and
solid LiOAr. The yellow solution was stirred at ambient
temperature for 40 h to give analytically pure [Nb-
(NtBu)(OAr)(κ3-N2Npy)] as a cream-colored solid in 74%
yield. Analogous methods were used to obtain the
tantalum tert-butylimido and arylimido analogues [Ta-
(NtBu)(OAr)(κ3-N2Npy)] (5) and [Ta(NAr)(OAr)(κ3-N2Npy)]
(6) as cream and yellow solids, respectively. The reac-
tions for tantalum required longer reaction times than
for niobium and gave slightly lower yields (5, 58%; 6,
60%). The new compounds 4-6 were characterized by
IR and NMR spectroscopy and also by elemental analy-
sis. Compounds 4 and 6 were, in addition, characterized
by X-ray structural analysis.
F igu r e 1. Displacement ellipsoid plot (20% probablility)
of [Nb(NtBu)(OAr)(κ3-N2Npy)] (4) with hydrogen atoms
omitted.
structures of 4 and 6 are distorted trigonal bipyramidal
about a pentavalent metal center. They contain a
facially coordinated N2Npy ligand with the pyridyl group
occupying one of the axial positions of the trigonal
bipyramid. The other axial site is occupied by the imido
group, with the aryloxide ligand in the remaining
equatorial position. The low-temperature 1H and 13C
NMR spectra of 4-6 are consistent with the solid-state
structures shown, with evidence of restricted rotation
Colorless single crystals of [Nb(NtBu)(OAr)(κ3-N2Npy)]
(4) were grown from a saturated pentane solution, and
crystals of 6 were grown from a 1:1 (v/v) hexane/toluene
solution, at 5 °C over 48 h. The molecular structures
are shown in Figures 1 and 2, respectively; selected bond
distances and angles are compared in Table 2. The

Group 5 Imido Complexes
Organometallics, Vol. 20, No. 16, 2001 3537
F igu r e 2. Displacement ellipsoid plot (20% probablility)
of [Ta(NAr)(OAr)(κ3-N2Npy)] (6) with hydrogen atoms omit-
ted.
F igu r e 3. Qualitative analysis of Nb 4dπ-(Ν,Ã) 2ππ
bonding interactions and N2Npy ligand π-nonbonding orbit-
als for [M(NR)(OAr)(N2Npy)] (4-6), showing the Cartesian
coordinate system chosen.
Ta ble 2. Com p a r ison of Bon d Len gth s (Å) a n d
An gles (d eg) for [Nb(NtBu )(OAr )(K3-N2Np y)] (4) a n d
[Ta (NAr )(OAr )(K3-N2Np y)] (6)
4
6
distance
distance
the unusually long Nb-O linkage [2.022(2) Å]; such
linkages more typically lie in the range 1.82-1.92 Å
(average 1.882 Å for six examples),39,40 and the highly
bent Nb-O-C linkage (Nb-O-C(20) ) 135.8(2)°, cf. the
previously known range: 139.3(4)-176.7(3)°, average
172.4°, for the above examples). The Nb-O-C linkage
bends such that the aryl group is oriented “downward”
toward the imido ligand, with the atoms Nb(1), O(1),
and C(20) lying in the approximate molecular mirror
plane that also contains N(1), N(4), and C(10). It seems
likely that the origin of this bend is electronic, serving
to remove any competition between the aryloxide and
the imido π-donor ligands for the same metal π-acceptor
orbital, as discussed below with reference to Figure 3.
The reason that the aryloxide is oriented toward the
imide, rather than toward the pyridyl group, probably
lies in the trans influence of the imido ligand,17 which
will tend to increase the N(imide)-Nb-O bond angle to
greater than 90° [N(1)-Nb-O ) 96.9(1)°]. An “upward”
(toward pyridyl) bend of the Nb-O-Ar linkage would
then place the aryl group close to the N2Npy pyridyl
group and particularly close to the pyridyl H6 atom. To
relieve steric pressure, a bend away from the pyridyl
group is therefore expected. Despite the relatively long
Nb-O bond length, the angle of the Nb-O-C(20) bend
suggests that the oxygen atom may still be capable of
π-donation to the metal by way of a 2p π-donor orbital
lying in the equatorial plane (see below).
Nb(1)-N(1)
1.782(3)
2.010(3)
2.022(3)
2.475(3)
2.022(2)
1.464(4)
Ta(1)-N(1)
1.808(2)
1.990(3)
2.000(2)
2.473(2)
1.954(2)
1.384(4)
Nb(1)-N(2)
Nb(1)-N(3)
Nb(1)-N(4)
Nb(1)-O(1)
N(1)-C(1)
Ta(1)-N(2)
Ta(1)-N(3)
Ta(1)-N(4)
Ta(1)-O(1)
N(1)-C(1)
angle
angle
Nb(1)-N(1)-C(1)
Nb(1)-O(1)-C(20)
N(1)-Nb(1)-N(2)
N(1) - Nb(1)-N(3)
N(1)-Nb(1)-N(4)
N(1)-Nb(1)-O(1)
N(2)-Nb(1)-N(3)
172.1(2)
135.8(2)
104.8(1)
104.2(1)
169.7(1)
96.9(1)
Ta(1)-N(1)-C(1)
Ta(1)-O(1)-C(20)
N(1)-Ta(1)-N(2)
N(1)-Ta(1)-N(3)
N(1)-Ta(1)-N(4)
N(1)-Ta(1)-O(1)
N(2)-Ta(1)-N(3)
176.1(2)
158.5(2)
103.5(1)
102.3(1)
173.0(1)
100.9(1)
100.8(1)
99.1(1)
about the N-Ar and O-Ar bonds in the more sterically
crowded 6. Complete NMR assignments (suported by
NOE and correlation experiments) are given in the
Experimental Section for these and all the new com-
plexes described herein.
Compound 4 is only the second structurally charac-
terized example of an aryloxide-supported niobium
imido complex, the first being [Nb(NMe)(O-2,6-C6H3-
Ph2)3(NHMe2)] (4.2).38 At 1.782(3) Å, the NbdN(imide)
bond length is effectively identical to that in the
octahedral starting complex [Nb(NtBu)Cl(κ3-N2Npy)(py)]
(1). The Nb-N(amide) bonds are both marginally shorter
[by 0.030(7) and 0.022(7) Å] relative to those in 1,25
while the Nb-N(pyridyl) bond is longer by 0.047(6) Å. The
most significant features of this structure, however, are
The sum of the angles about the amide nitrogen atoms
N(2) and N(3) (357.7(4)° and 356.4(4)°, respectively)
(37) Watkin, D. J .; Prout, C. K.; Carruthers, J . R.; Betteridge, P.
W. CRYSTALS Issue 10; Chemical Crystallography Laboratory, Uni-
versity of Oxford, 1996.
(38) Schrock, R. R.; Wesolek, M.; Liu, A. H.; Wallace, K. C.; Dewan,
J . C. Inorg. Chem. 1988, 27, 2050.
(39) Chesnut, R. W.; Fanwick, P. E.; Rothwell, I. P. Inorg. Chem.
1988, 27, 752.
(40) Clark, J . R.; Puvirenti, A. L.; Fanwick, P. E.; Sigalas, M.;
Eisenstein, O.; Rothwell, I. P. Inorg. Chem. 1997, 36, 3623.

3538 Organometallics, Vol. 20, No. 16, 2001
Pugh et al.
shows both to be effectively planar, so that the atoms
are sp2-hybridized and capable of π-donation to the
metal. Comparison of the angles between the amide
planes containing {C(11), N(2), Si(1)} and {C(12), N(3),
Si(2)} with that of the best fit equatorial plane {N(2)-
N(3)-Nb-O(1)} shows that they are oriented at 57.0°
and 49.1° with respect to the latter and hence that the
π-donor 2p orbitals on N(2) and N(3) should achieve
some overlap with the niobium 4d π-acceptor orbitals
in the equatorial plane as discussed below. The orienta-
tion of the amido SiMe3 groups “down” toward the imido
ligand and displacement of the aryloxide substituent in
the same direction help to orient the OAr isopropyl
groups further away from the SiMe3 groups.
The most obvious difference between the structure of
[Ta(NAr)(OAr)(N2Npy)] (6, Figure 2) and that of its
niobium tert-butylimido analogue 4 is the much more
sterically crowded environment of the tantalum com-
plex. The aryloxide M-O-C linkage is much more
linear in 6 [158.5(2)°] than in 4 [135.8(2)°], due presum-
ably to the steric pressure exerted by the arylimide
isopropyl groups (note that the bending of the aryloxide
Ta-O-C(28) linkage remains “upward” toward the
imido group). Bond lengths and angles in this compound
are, on the whole, unremarkable, with little change from
the starting complex [Ta(NAr)Cl(N2Npy)(py)] (3) in metal
imide and amide bond lengths. Only the Ta-O bond
length [1.990(3) Å] lies outside typical ranges (range
1.875(3)-1.945(5) Å, average 1.909 Å, for 10 exam-
ples),38,41-45 just as for the niobium complex 4. It is
notable, however, that with two amide donors in addi-
tion to the imide and aryloxide ligands, complexes 4 and
6 are both considerably more heavily “π-loaded”46 than
most other structurally characterized niobium or tan-
talum aryloxides, which may well reduce the π-donor
ability and thus the effective bond order of the aryloxide
ligand in these two cases. Note that despite the more
linear Ta-O-Ar angle in 6, the oxygen is still unlikely
to be able sigificantly to donate π-electron density to
the appropriate metal π-acceptor orbital (dyz in Figure
3, see discussion of π-bonding framework below) from
its developing second 2p π-donor orbital because of
competition with the dominant imido 2py π-donor
orbital.
appear to be another result of the increased steric
pressure from the arylimide isopropyl groups forcing the
trimethylsilyl amide substituents away from the aryl-
imido group.
Figure 3 presents a qualitative analysis of the metal-
ligand π-bonding framework in the complexes 4-6.
Taking the coordination as trigonal bipyramidal and the
Cartesian axes as shown, there are four nd π-acceptor
orbitals available after forming the metal-ligand σ-bond
framework.47 The near-linear imido linkages (M-
N(imide)-R ) 172.1(2)° and 176.1(2)°) reveal that these
nitrogen atoms are sp-hybridized and offer two 2pπ-
donor orbitals (2px and 2py in the chosen coordinate
system). These will interact strongly with the metal ndxz
and ndyz atomic orbitals as shown in A and B, respec-
tively in Figure 3. Taking the amido nitrogen 2p π-donor
orbitals to lie mainly in the equatorial (xy) plane gives
two symmetry-adapted linear combinations in the ef-
fective Cs point group of 4-6. One of these will find a
2
2
match with the dx -y orbital of the metal (C) while the
other (D) is metal-ligand nonbonding since its nodal
plane passes through the metal. Therefore there is one
remaining metal π-acceptor orbital available, namely
the ndxy level. This matches with the 2px π-donor lone
pair of OAr (E). The other lone pair of oxygen is
nonbonding either because of the oxygen’s hybridization
(for 4, Nb-O-Ar ) 135.8(2)° so sp2) or (for 6) because
the only suitable metal acceptor orbital (dyz) is used in
the comparatively stronger M-N(imide) π-bonding. This
analysis accounts for the bending of the M-O-Ar
linkages in 4 and 6 which removes the oxygen 2px lone
pair from the bonding picture. It is also clear that
(formally) the imido ligands in 4-6 acts as four-electron
donors, the OAr as a three-electron donor, and each of
the amido nitogens as two-electron donors, with a
ligand-based lone pair localized on the amido nitrogens.
Ben zam idin ato an d Bis(tr im eth ylsilyl)am ido De-
r iva tives Given the similarities of the reactions of the
niobium and tantalum starting complexes 1-3 with
LiOAr as described above, subsequent studies focused
mainly on the tert-butylimido niobium homologue 1, as
it can be most readily prepared in synthetically useful
quantities.25
Reactions of 1 with LiPhC(NSiMe3)2 and LiN(SiMe3)2
are summarized in Scheme 1. In both cases cold benzene
was added to a mixture of solid [Nb(NtBu)Cl(κ3-N2Npy)-
(py)] (1) and the lithiated benzamidinate or amide
reagent. Subsequent workup after stirring at room
temperature or heating (for LiN(SiMe3)2) for 40-65 h
gave [Nb(NtBu){PhC(NSiMe3)2}(κ3-N2Npy)] (7) and [Nb-
(NtBu){N(SiMe3)2}(κ2-N2Npy)] (8) as pale orange solids
in 61-62% isolated yield. The compound 8 was spec-
troscopically >90% pure, but trace impurites could not
be removed by washing with hexanes, and the com-
pound could not be successfully recrystallized. We now
discuss the characterization of 7 and 8 in turn.
The bond angles about the amide groups in 6 show
that both are, as in 4, effectively planar and thus in
principle capable of π-donation to the metal (sums of
angles about N(2) and N(3), 360.0(3)° and 359.7(3)°,
respectively). The planes defined by {C(18), N(2), Si(1)}
and by {C(20), N(3), Si(2)} make angles of 49.9° and
39.5°, respectively, with the best fit equatorial plane
{N(2), N(3), Ta, O(1)}, suggesting that the amide 2p
π-donor orbitals are twisted slightly further out of the
equatorial plane than in complex 4. This effect would
(41) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J . C. J . Chem.
Soc., Chem. Commun. 1986, 1203.
(42) Chamberlain, L. R.; Steffey, B. D.; Rothwell, I. P.; Huffman, J .
C. Polyhedron 1989, 8, 341.
(43) Chao, Y.-W.; Wexler, P. A.; Wigley, D. E. Inorg. Chem. 1989,
28, 3860.
(44) Gray, S. D.; Weller, K. J .; Bruck, M. A.; Briggs, P. M.; Wigley,
D. E. J . Am. Chem. Soc. 1995, 117, 10678.
1
The H and 13C NMR spectra of 7 are consistent with
the presence of a molecular mirror plane in solution.
For example, the 1H NMR spectrum features two SiMe3
environments (each integrating as 18H) and one pair
of mutually coupled doublets (total integral 4H) for the
CH2 groups of N2Npy. Resonances for the apical Me of
(45) Weller, K. J .; Gray, S. D.; Briggs, P. M.; Wigley, D. E.
Organometallics 1995, 14, 5588.
(46) For a treatment of π-loaded complexes see: Benson, M. T.;
Bryan, J . C.; Burrell, A. K.; Cundari, T. R. Inorg. Chem. 1995, 34, 2348,
and references therein.
(47) Albright, T. A.; Burdett, J . K.; Whangbo, M.-H. Orbital Interac-
tions in Chemistry; Wiley-Interscience: New York, 1985.

Group 5 Imido Complexes
Organometallics, Vol. 20, No. 16, 2001 3539
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
N2Npy and for the pyridyl and phenyl groups can also
be assigned. The κ3-coordination mode (i.e., with the
pyridyl nitrogen bound to Nb) is established by the shift
of the pyridyl H6 (ortho to N) resonance which appears
at 8.70 ppm. In the “free” (protonated) ligand H2N2Npy
in the same solvent, this resonance appears at 8.47
ppm.18 To verify these assignments, NOE difference
spectra were recorded. Irradiation at the benamidinate
phenyl ortho-protons showed an NOE enhancement of
both the tert-butyl signal and the higher field SiMe3
resonance (confirming this as corresponding to the
benzamidinate group) and also of the N2Npy pyridyl H6
proton. This final observation could only be made in the
case of a coordinated pyridyl group. The orientation of
the benamidinate ligand as being mutually cis to NtBu
and the pyridyl moiety (i.e., in the equatorial plane as
shown in Scheme 1) is confirmed by the NOE experi-
ments and rules out an alternative isomer with PhC-
(NSiMe3)2 trans to pyridyl and NtBu in the equatorial
plane of a pseudo-trigonal bipyramidal structure. Ex-
amples of amidinate-supported imido complexes of the
heavier group 5 metals have been reported pre-
viously.48-51
(d eg) for [Nb(NtBu ){CH(SiMe3)2}(K2-N2Np y)] (9)a
distances
Nb(1)-N(1)
Nb(1)-N(2)
Nb(1)-N(3)
Nb(1)-C(20)
N(1)-C(1)
1.772(3)
2.003(3)
1.986(3)
2.210(3)
1.458(4)
[1.768(3)]
[2.006(3)]
[1.978(3)]
[2.219(3)]
[1.467(4)]
angles
Nb(1)-N(1)-C(1)
N(1)-Nb(1)-N(2)
N(1)-Nb(1)-N(3)
N(1)-Nb(1)-C(20)
N(2)-Nb(1)-N(3)
N(2)-Nb(1)-C(20)
N(3)-Nb(1)-C(20)
176.4(3)
[177.9(2)]
[112.9(1)]
[110.7(1)]
[111.2(1)]
[100.6(1)]
[112.5(1)]
[108.3(1)]
111.6(1)
110.3(1)
112.6(1)
99.6(1)
112.9(1)
109.0(1)
a
The corresponding values for the other crystallographically
independent molecule in the asymmetric unit are given in
brackets.
alkyl complexes incorporating diamido ligands have also
been prepared,20 but none have yet been described
incorporating both an imido ligand and a diamido
ligand. The syntheses and proposed structures of the
new organometallic compounds derived from 1 and 2
are summarized in Scheme 2.
In contrast to the situation found for 7, the NMR
spectra for [Nb(NtBu){N(SiMe3)2}(κ2-N2Npy)], 8, are
consistent with the presence of a κ2-bound N2Npy ligand.
The 1H and 13C NMR spectra of 8 in C6D6 show the
expected resonances for all of the groups present and
are consistent with Cs symmetry in solution. Interest-
ingly the H6 signal appears at 8.47 ppm and clearly
implies that the N2Npy pyridyl group is now pendant.
This shift can be compared with the corresponding
values (in C6D6) for the κ2-coordinated N2Npy ligand in
the complexes [Ti{N(tBu)PC(tBu)}(κ2-N2Npy)] (δ ) 8.50)
and [Ti2{µ-η2-NC(Me)NtBu}2(κ2-N2Npy)2] (δ ) 8.50).29 No
NOE enhancement of any other resonance (other than
those of N2Npy itself) was found on irradiation at the
H6 proton, consistent with a noncoordinated pyridyl
group. The proposed κ2-bound structure is related to
those of some of the organometallic derivatives of 1,
which are presented in the next section. Compound 8
can be viewed as a tris(amido)-imido-niobium complex;
compounds of the type [M(NR)(NR′2)3] have been re-
ported and structurally characterized previously.3,52
Bis(tr im eth ylsilyl)m eth yl, Tr im eth ylsilylm eth yl,
Ben zyl, a n d Cyclop en a d ien yl Der iva tives. Group
5 metal alkyl complexes incorporating imido coligands
are well established.3,53-57 Several nonimido group 5
The compounds [M(NtBu){CH(R)SiMe3}(κn-N2Npy)]
(R ) SiMe3, M ) Nb 9 or Ta 10; R ) H, M ) Nb 11 or
Ta 12a ,b; n ) 2 or 3) were prepared from solid 1 or 2
and LiCH(R)SiMe3 followed by addition of cold benzene
and stirring for 5-20 h at room temperature or 50 °C
(for 12a ,b). Subsequent workups gave [M(NtBu){CH-
(SiMe3)2}(κ2-N2Npy)] in 59% (M ) Nb, 9) or 48% (M )
Ta, 10) yields as white solids. The less sterically
demanding alkyl group CH2SiMe3 gave the correspond-
ing products 11 and 12a ,b as viscous red oils in 70 and
85% yield, respectively. These could not be crystallized.
Colorless crystals of [Nb(NtBu){CH(SiMe3)2}(κ2-N2Npy)]
(9) suitable for X-ray diffraction were obtained from a
saturated hexane solution after 2 weeks at 5 °C. The
crystals of 9 contain two independent molecules in the
asymmetric unit. Selected bond lengths and angles are
listed in Table 3, and a view of one of the molecules is
given in Figure 4. Since the bond distances and angles
for the two molecules are effectively identical, discussion
will focus on only that shown in Figure 4.
Compound 9 contains a monomeric niobium(V) center
coordinated by one carbon and three nitrogen atoms in
a pseudo-tetrahedral manner. The N2Npy ligand is
coordinated in a bidentate fashion, with the pyridyl
group acting as a pendant arm. The structure is
analogous to that proposed on the basis of NMR data
for [Nb(NtBu){N(SiMe3)2}(κ2-N2Npy)] (8) above. The κ2
coordination mode is presumably the result of steric
crowding at the metal center and the good donor
abilities of the alkyl, amido, and imido ligands. The six-
membered chelate ring {(Nb(1),N(2),C(11),C(10),C(12),-
N(3)} adopts a stable chair-type conformation, which
places the pyridyl group as far as possible away from
the metal. Even with full lone pair donation from all of
the remaining nitrogen donor atoms, compound 9 can
achieve a valence electron count of only 16 at niobium.
The Nb-N(imide)-C angle of 176.4(3)° in 9 shows
that the imido group is effectively linear, and the
NbdN(imide) bond length of 1.772(3) Å is well within
known ranges.1,3,60 The Nb-C distance of 2.210(3) Å is
also typical for such linkages.60 Bond distances and
(48) Dawson, D. Y.; Arnold, J . Organometallics 1997, 16, 1111.
(49) Cotton, F. A.; Daniels, L. M.; Matonic, J . H.; Wang, X.; Murillo,
C. A. Polyhedron 1997, 16, 1177.
(50) Stewart, P. J .; Blake, A. J .; Mountford, P. Inorg. Chem. 1997,
36, 1982.
(51) Chen, C. T.; Doerrer, L. H.; Williams, V. C.; Green, M. L. H. J .
Chem. Soc., Dalton Trans. 2000, 967.
(52) Nugent, W. A.; Harlow, R. L. J . Chem. Soc., Chem. Commun.
1978, 579. Herrmann, W. A.; Baratta, W.; Herdtweck, E. J . Organomet.
Chem. 1997, 541, 445. Song, J .-H.; Gambarotta, S. Chem. Eur. J . 1996,
2, 1258.
(53) Gibson, V. C.; Poole, A. D.; Siemeling, U.; Williams, D. N.; Clegg,
W.; Hockless, D. C. R. J . Organomet. Chem. 1993, 462, C12.
(54) Buijink, J .-K. F.; Meetsma, A.; Teuben, J . H.; Kooijman, H.;
Spek, A. L. J . Organomet. Chem. 1995, 497, 161.
(55) Schmidt, S.; Sundermeyer, J . J . Organomet. Chem. 1994, 472,
127.
(56) Antin˜olo, A.; Otero, A.; Prashar, S.; Rodriguez, A. M. Organo-
metallics 1998, 17, 5454.
(57) Chan, M. C. W.; Cole, J . M.; Gibson, V. C.; Howard, J . A. K.;
Lehmann, C.; Poole, A. D.; Siemeling, U. J . Chem. Soc., Dalton Trans.
1998, 103.

3540 Organometallics, Vol. 20, No. 16, 2001
Pugh et al.
Sch em e 2. Alk yl, Ben zyl, a n d Cyclop en ta d ien yl Com p lexes w ith Im id o-Dia m id o-P yr id in e Liga n d Setsa
a
(i) PhCH2MgCl (1.0 M solution in diethyl ether), benzene, rt, 17 h, 38%; (ii) LiC5H4Me, benzene, 7 °C then 60 °C, 72 h, 65%;
(iii) LiCHR(SiMe3) (R ) H or SiMe3), benzene, 7 °C then rt, 5-20 h, 48-85%.
angles about the niobium center and the attached
carbon atom C(20) give no direct indication of any
R-agostic interactions, the Nb-C(20)-Si(3), Nb-C(20)-
Si(4), and Si(3)-C(20)-Si(4) bond angles (115.7(2)°,
117.5(2)°, and 114.0(2)°, respectively) all being slightly
greater than the ideal tetrahedral angle of ca. 109.5°
presumably due to the bulk of the trimethylsilyl sub-
stituents, and showing no unexpected distortions. The
hydrogen atom attached to C(20) could not be located
and was placed in a calculated position based on the
geometry at C(20). The placed H atom lies ca. 2.75 Å
(2.79 Å in the second molecule) from Nb(1), a distance
significantly greater than that observed in authentic
R-agostic imido niobium systems (e.g., Nb‚‚‚H ) 2.321-
t
(29) and 2.405(33) Å in [Nb(NAr)(η5-C5H5)(CH2 Bu)2]).61
The possibility of R-agostic interactions has been further
examined for the tantalum analogue [Ta(NtBu){CH-
(SiMe3)2}(N2Npy)] (10) by spectroscopic means (see
below).
Compounds 9-12 were characterized by 1H and
13C{1H} NMR spectroscopy and by IR spectroscopy and
elemental analysis. The spectra support the Cs-sym-
metric structures shown in Scheme 2. The NMR data
for 9 in solution are consistent with the solid-state
structure; in particular the H6 proton resonance appears
at 8.45 ppm, consistent with a noncoordinated pyridyl
group. A singlet at 0.61 ppm of relative intensity 1H
corresponds to the CH(SiMe3)2 methine proton. The Nb-
bound carbon atom of CH(SiMe3)2 was not directly
observed, presumably due to strong coupling to the
(58) Mashima, K.; Matsuo, Y.; Tani, K. Organometallics 1999, 18,
1471.
(59) Aoyagi, K.; Gantzel, P. K.; Kalai, K.; Tilley, T. D. Organome-
tallics 1996, 15, 923.
(60) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. The United
Kingdom Chemical Database Service. J . Chem. Inf. Comput. Sci. 1996,
36, 746. Allen, F. H.; Kennard, O. Chem. Des. Automat. News 1993, 8,
1 and 31.
(61) Poole, A. D.; Williams, D. N.; Kenwright, A. M.; Gibson, V. C.;
Clegg, W.; Hockless, D. C. R.; O’Neil, P. A. Organometallics 1993, 12,
2549.

Group 5 Imido Complexes
Organometallics, Vol. 20, No. 16, 2001 3541
(trimethylsilyl)methyl ligand rather than of agostic
interactions. Furthermore, in a study of tantalum
alkylidene complexes supported by facially coordinating
tridentate N-donor ligands, Boncella and co-workers
noted that R-agostic interactions tend to compete with
π-donation.64 With three π-donor ligands present in 9
and 10, it seems unlikely that an R-agostic interaction
will be observed. If such an interaction were to take
place, then the crystal structure of 9 suggests that the
R-agostic hydrogen would bond trans to the imido group
and would thus find itself competing for metal d orbitals
with the strongly trans-labilizing imido ligand, again
making an R-agostic interaction seem unlikely. The IR
spectra of both 9 and 10 also give no indications of
agostic interactions, with no absorptions visible in the
range 2250-2800 cm-1, which could be attributable to
ν(C-H‚‚‚M) bands.62
The NMR spectra of [Nb(NtBu){CH2SiMe3}(κ2-N2Npy)]
(11) are analogous to those of 9. The 1H spectrum
suggests molecular Cs symmetry. The pyridyl H6 reso-
nance at 8.52 ppm is indicative of a noncoordinated
pyridyl group. The CH2SiMe3 methylene H atoms ap-
pear as a singlet at 0.79 ppm, and the corresponding
1
13C resonance was at 62.8 ppm. However, the H NMR
F igu r e 4. Displacement ellipsoid plot (20% probablility)
of one of the two crystallographically independent mol-
ecules of [Nb(NtBu){CH(SiMe3)2}(κ2-N2Npy)] (9) with hy-
drogen atoms omitted.
spectrum of [Ta(NtBu)(CH2SiMe3)(κ2,3-N2Npy)] (12a ,b)
is more complicated than those of 9-11. Thus, in
addition to resonances corresponding to the four-
coordinate tantalum species 12a (with H6 appearing at
8.50 ppm) a second product (12b) is also present in an
approximately 1(12b):2(12a ) ratio by integration. The
κ3-coordinated N2Npy ligand in 12b is characterized by
an H6 resonance at 9.02 ppm, indicative of a coordinated
quadrupolar metal nucleus (93Nb, 100% abundance,
I ) 9/2), but was indirectly located at 56.7 ppm from an
HMQC 13C-1H correlation spectrum. The spectra of the
tanalum analogue 10 are very similar to those of 9, the
main difference being that the carbon atom of CH-
(SiMe3)2 was now directly observable in the 13C NMR
spectra at 63.2 ppm (the corresponding CH(SiMe3)2
methine proton appears at 1.76 ppm).
1
pyridyl group. Other resonances in the H NMR spec-
trum are broadly similar to those discussed above; the
13C{1H} NMR spectrum shows two distinct sets of
resonances attributed to the κ2- and κ3-isomers 12a and
12b, respectively. Since the atomic radii of Nb and Ta
are effectively identical,65a the apparent tendency to
increase coordination number from M ) Nb (for 11) to
Ta (for 12b) is consistent with the well-known increase
in metal-ligand bond strengths down a transition metal
triad.65b
To exclude the possibility of an R-agostic interaction
in 9 and 10 (and to support conclusions based on the
1
X-ray structure of 9), we wished to measure J (CH) for
the metal-bound CH(SiMe3)2 carbon.62 This was not
possible for 9 even at low temperature (where “thermal
decoupling” due to shorter T1 nuclear relaxation times
might assist), and so measurements focused on the
tantalum system 10. Measurement of the gated-de-
We were interested to probe for R-agostic interactions
in the four- and five-coordinate complexes, and again it
was necessary to focus on the tantalum systems 12a ,b.
Measurement of the gated decoupled 13C NMR spectrum
of [Ta(NtBu)(CH2SiMe3)(N2Npy)] (12) produced averaged
1J (CH) coupling constants for the metal-bound methylene
group of 117 Hz for 12b and 104 Hz for 12a . The value
1
coupled 13C NMR spectrum of 10 yielded a J (CH) value
of 94 Hz for the CH(SiMe3)2 methine carbon. This value
is lower than might be expected for a typical sp3-
hybridized carbon atom (1J (CH) ≈ 120-130 Hz), although
is higher than the values normally expected for carbons
bearing R-agostic hydrogens (1J (CH) ≈ 60-90 Hz).62
1
of J (CH) ) 104 Hz for 12a is somewhat less than the
1
Thus although the J (CH) value for 10 may indicate an
1
typical J (CH) value for an sp3-hybridized carbon atom
R-agostic interaction, it is equally possible that it is
caused by the presence of the two large, electropositive
trimethylsilyl substituents on the carbon atom. The
titanium imido complexes [Ti(NR){CH(SiMe3)2}{PhC-
(120-130 Hz). As before, this may simply reflect the
steric bulk of the trimethylsilyl group (note that the
alkyl ligand in 12a ,b is less bulky than that in [Ta-
1
(NtBu){CH(SiMe3)2}(N2Npy)] (10), for which J (CH) ) 94
t
1
(NSiMe3)2}(py)] (R ) Bu, 2,6-C6H3Me2) feature J (CH)
values of 93 and 91 Hz for the bis(trimethylsilyl)methyl
methine carbon atoms.63 These 14 valence electron
complexes were shown by X-ray crystallography not to
Hz). The value of 1J (CH) ) 117 Hz for five-coordinate 12b
1
is nearer to the commonly encountered J (CH) range for
nonagostic sp3 carbon atoms of 120-130 Hz, so is less
likely to suggest any R-agostic interactions. Similar
1
possess R-agostic interactions, and the low J (CH) values
1
values and trends for J (CH) were reported recently for
were therefore deemed to be a characteristic of the bis-
(64) Boncella, J . M.; Cajigal, M. L.; Abboud, K. A. Organometallics
1996, 15, 1905.
(65) (a) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
Pergamon Press: Oxford, 1994; p 1141. (b) Mingos, D. M. P. Essential
Trends in Inorganic Chemistry; Oxford University Press: Oxford, 1998.
(62) For a review see: Brookhart, M.; Green, M. L. H.; Wong, L.-L.
Prog. Inorg. Chem. 1988, 36, 1.
(63) Stewart, P. J .; Blake, A. J .; Mountford, P. Organometallics 1998,
17, 3271.

3542 Organometallics, Vol. 20, No. 16, 2001
Pugh et al.
two crystallographically characterized titanium imido
derivatives with CH2SiMe3 ligands.63,66 It was demon-
strated in these cases that a J (CH) value of 104 Hz for
the methylene carbon atom of a CH2SiMe3 ligand did
not necessarily indicate the presence of R-agostic inter-
actions. The IR spectra of [Nb(NtBu)(CH2SiMe3)(N2Npy)]
(11) and [Ta(NtBu)(CH2SiMe3)(N2Npy)] (12) do not pro-
vide any evidence of agostic interactions, since no ν(C-
H‚‚‚M) absorption bands were visible in the 2250-2800
cm-1 range.
heated to 60 °C and stirred for 72 h, yielding [Nb(NtBu)-
(η5-C5H4Me)(κ2-N2Npy )] (14) as an orange solid in 65%
yield. An analogous reaction between [Nb(NtBu)Cl-
(N2Npy)(py)] (1) and LiC5Me5 yielded only complex
mixtures presumably because of the bulkier nature of
C5Me5.
1
The 1H NMR spectrum of [Nb(NtBu)(η5-C5H4Me)-
(N2Npy)] (14) features characteristic N2Npy ligand sig-
nals, with the pyridyl H6 resonance appearing at 8.57
ppm, indicative of an uncoordinated pyridyl group.
Signals for the cyclopentadienyl ring protons are at 5.86
and 6.22 ppm, and the ring methyl group is a singlet at
1.89 ppm. The C6D6 13C{1H} NMR spectrum of 14 is
entirely as expected for the proposed structure.
The proposed structure of [Nb(NtBu)(η5-C5H4Me)-
(N2Npy )] (14) is analogous to those of [Nb(NAr)(η5-C5H4-
Me)(NMe2)2]67 and the complexes [Nb(NR)(η5-C5H5)-
(NHR)2] (R ) tBu, 2,6-C6H3Me2).68 All of these compounds
possess an imido-cyclopentadienyl-bis(amido) coordi-
nation sphere similar to that of 14, which has an 18
valence electron count.
Attempts to prepare methyl analogues of 9-13 from
1 or 2 and methyllithium or methyl Grignard reagents
were unsuccessful. The benzyl derivative [Nb(NtBu)-
(CH2Ph)(κ3-N2Npy)(py)] (13, Scheme 2) was prepared
from a cooled solution of [Nb(NtBu)Cl(N2Npy)(py)] (1) in
benzene and 1 equiv of PhCH2MgCl as solution in
diethyl ether. Standard workup gave 13 as an orange
powder in 38% yield. The compound 13 was fairly
unstable in solution, and attempts to obtain analytically
pure samples by recrystallization were unsuccessful. It
was therefore characterized by IR and NMR spectros-
copy only. The compound is notable in being the first
example of a derivative of [Nb(NtBu)Cl(N2Npy)(py)] (1)
to retain the pyridine ligand after chloride substitution.
Con clu sion s
We have reported a range of new organometallic and
coordination complexes supported by an imido-di-
amido-pyridine ligand set. The flexible nature of the
diamido-pyridine ligand is revealed by the crystal
structure of the κ2-bound compound 9 and the formation
of the compounds 8, 10, 11, 12a ,b, and 14.
1
In the H NMR spectrum of 13 the N2Npy trimethyl-
silyl resonances appear as two separate singlets at
-0.14 and 0.08 ppm, consistent with a six-coordinate
complex in which there is no plane of symmetry and no
fast exchange of pyridine and benzyl ligand coordination
sites on the NMR time scale. The H6 atom appears at
8.81 ppm, consistent with a coordinated pyridyl group.
Resonances corresponding to a coordinated pyridine
ligand are also visible, but all of these signals are broad.
This broadening may possibly be related to fluxionality
involving dissociation/association of the ligand similar
to that observed in the parent complex [Nb(NtBu)Cl-
(N2Npy)(py)] (1).25 Unusually, the benzyl group CH2
resonances were observed in neither the 1H nor 13C
spectra.
Ack n ow led gm en t . We thank the Deutsche
Forschungsgemeinschaft, the Engineering and Physical
Sciences Research Council, the Leverhulme Trust, the
Fonds der Chemischen Industrie, the Royal Society, the
DAAD, and the British Council for financial support.
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic files in CIF and PDF format for the structure
determinations of 4, 6, and 9. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM0103153
Treatment of an equimolar mixture of solid [Nb-
(NtBu)Cl(κ3-N2Npy)(py)] (1) and solid LiC5H4Me with
cooled benzene provided a yellow solution, which was
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