Insertion routes to tetrasubstituted guanidinate complexes of Ta(V) and Nb(V)

Article abstract of DOI:10.1021/ic980709v

The preparation and characterization of guanidinate-containing complexes of Nb and Ta is described. The direct reactions of M(NMe₂)₅ with either dicyclohexylcarbodiimide (CyN=C=NCy) and diisopropylcarbodiimide (ⁱPrN=C=NⁱPr) proceeded smoothly at room temperature under nitrogen to yield [RNC(NMe₂)NR]M(NMe₂)₄ (M = Ta, Mb; R = Cy, ⁱPr). The spectroscopic characterization of these materials is consistent with a symmetrical chelating bidentate guanidinate anion bonded to a pseudo-octahedral metal center. Confirmation of these details was provided by a single-crystal X-ray diffraction study in the case of [CyNC(NMe₂)NCy]Ta(NMe₂)₄ (1). Delocalization of the lone pair of electrons on the guanidinate NMe₂ group into the ligand N-C-N π system does not appear to be significant in these species. 1999 American Chemical Society.

Full text of DOI:10.1021/ic980709v

998
Inorg. Chem. 1999, 38, 998-1001
Insertion Routes to Tetrasubstituted Guanidinate Complexes of Ta(V) and Nb(V)
Ma Khin Tan Tin, Glenn P. A. Yap, and Darrin S. Richeson*
Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
ReceiVed June 22, 1998
The preparation and characterization of guanidinate-containing complexes of Nb and Ta is described. The direct
reactions of M(NMe2)5 with either dicyclohexylcarbodiimide (CyNdCdNCy) and diisopropylcarbodiimide
(iPrNdCdNiPr) proceeded smoothly at room temperature under nitrogen to yield [RNC(NMe2)NR]M(NMe2)4
(M ) Ta, Nb; R ) Cy, iPr). The spectroscopic characterization of these materials is consistent with a symmetrical
chelating bidentate guanidinate anion bonded to a pseudo-octahedral metal center. Confirmation of these details
was provided by a single-crystal X-ray diffraction study in the case of [CyNC(NMe2)NCy]Ta(NMe2)4 (1).
Delocalization of the lone pair of electrons on the guanidinate NMe2 group into the ligand N-C-N π system
does not appear to be significant in these species.
Introduction
Chart 1
Amidinates and triazinates are well established as versatile
and flexible ligand systems for a variety of transition metal and
main group metal centers (Chart 1).1 Among the important
features of these species are the donor ability of the nitrogen
centers and the potential to explore both the steric and electronic
effects induced by the variation of organic substituents on the
ligand framework. This is particularly true and has been heavily
exploited in the amidinate ligand system, for which a wide
variety of organic substituents on both nitrogen and the central
carbon atoms have been reported.2
A key difference between these ligands is the presence of an
additional nitrogen center for the guanidinate. The resonance
contributors for N,N,N′,N′′-tetrasubstituted guanidinate anions
are summarized in Scheme 1. The lone pair of electrons on the
NR′2 function allows the zwitterionic resonance structure A. In
order for this particular contribution to be important, the N center
must be planar, sp2 hybridized and there must be a small dihedral
angle between the planes defined by the CNR′2 group and that
defined by the conjugated NCN moiety. Although the ability
to stabilize the positive charge on nitrogen is likely to be superior
for the disubstituted NR′2 group, the attendant steric congestion
caused by the two organic substituents would probably encour-
age a larger dihedral angle thus hindering the appropriate
orientation for π conjugation. Clearly, the role of substituents
from both an electronic and a steric standpoint will be important
in analyzing the bonding in guanidinate anions.
Guanidinate anions also fall into this general class of ligands
and, in contrast to the aforementioned species, have received
limited attention in coordination and organometallic chemistry.3,4
(1) (a) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219. (b)
Moore, D. S.; Robinson, S. D. AdV. Inorg. Chem. Radiochem. 1986,
30, 1.
(2) Littke, A.; Sleiman, N.; Bensimon, C.; Richeson, D.; Yap, G. P. A.;
Brown, S. Organometallics 1998, 17, 446 and references therein.
Foley, S. R.; Bensimon, C.; Richeson, D. S. J. Am. Chem. Soc. 1997,
119, 10359 and references therein. Coles, M. P.; Jordan, R. F. J. Am.
Chem. Soc. 1997, 119, 8125. Coles, M. P.; Swenson, D. C.; Jordan,
R. F.; Young, V. G. Organometallics 1997, 16, 5183. Berno, P.; Hao,
S.; Minhas, R.; Gambarotta, S. J. Am. Chem. Soc. 1994, 116, 7417
and references therein.
We were attracted to N-substituted guanidinate anions,
[RNC(NR′2)NR]-, as bulky supporting ligands for transition
metal complexes, and herein we report the synthesis and
characterization of a series of guanidinate complexes of Nb and
Ta. These species were derived by the insertion of carbodiimides
into the metal-amido linkage of M(NMe2)5 to yield [RNC-
(NMe2)NR]M(NMe2)4 (M ) Nb, Ta; R ) cyclohexyl, isopro-
pyl). These materials represent the first report of the use of
guanidinate ligands for the metals of group 5.
(3) For examples of transition metal complexes of guanidinate mono-
anions, see: (a) Chandra, G.; Jenkins, A. D.; Lappert, M. F.;
Srivastava, R. C. J. Chem. Soc. A 1970, 2550. (b) Lappert, M. F.;
Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal and Metalloid
Amides; Ellis Horwood-John Wiley: Chichester, U.K., 1980; Chapter
10 and references therein. (c) Yip, H.-K.; Che, C.-M.; Zhou, Z.-Y.;
Mak, T. C. W. J. Chem. Soc., Chem. Commun. 1992, 1369. (d) Bailey,
P. J.; Mitchell, L. A.; Parsons, S. J. Chem. Soc., Dalton Trans. 1996,
2839. (e) da S. Maia, J. R.; Gazard, P. A.; Kilner, M.; Batsanova, A.
S.; Howard, J. A. K. J. Chem. Soc., Dalton Trans. 1997, 4625. (f)
Robinson, S. D.; Sahajpal, A. J. Chem. Soc., Dalton Trans. 1997, 3349.
(g) Bailey, P. J.; Bone, S. F.; Mitchell, L. A.; Parsons, S.; Taylor, K.
J.; Yellowless, L. J. Inorg. Chem. 1997, 36, 867.
(4) For examples of main group metal complexes of guanidinate mono-
anions, see: (a) Snaith, R.; Wade, K.; Wyatt, B. K. J. Chem. Soc. A
1970, 380. (b) Clegg, W.; Snaith, R.; Shearer, H. M. M.; Wade, K.;
Whitehead, G. J. Chem. Soc., Dalton Trans. 1983, 1309. (c) Pattison,
I.; Wade, K.; Wyatt, B. K. J. Chem. Soc. A 1968, 837. (d) Bailey, P.
J.; Gould, R. O.; Harmer, C. N.; Pace, S.; Steiner, A.; Wright, D. S.
Chem. Commun. 1997, 1161. (e) Chivers, T.; Parvez, M.; Schatte, G.
J. Organomet. Chem. 1998, 550, 213.
Experimental Section
General Considerations. All manipulations were carried out either
in a nitrogen-filled drybox or under nitrogen using standard Schlenk-
line techniques. Solvents were distilled under nitrogen from Na/K alloy.
Deuterated benzene and deuterated pyridine were dried by vacuum
transfer from potassium. Diisopropylcarbodiimide, dicyclohexylcarbo-
diimide, and bis(trimethylsilyl)carbodiimide were purchased from
10.1021/ic980709v CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/18/1999

Guanidinate Complexes of Ta(V) and Nb(V)
Inorganic Chemistry, Vol. 38, No. 5, 1999 999
Results and Discussion
Scheme 1
While the insertion reactions of carbodiimides is rather well-
known for a variety of M-H6 and M-R7,8 species where M is
either a main group or transition metal, the analogous reactions
with M-Cl9 and M-N3,10,11 are less well developed especially
for the transition metals. The only reported examples of
carbodiimide insertion into transition metal amido functions is
the reaction between M(NMe2)4 (M ) Ti, Zr, Hf) and di(p-
tolyl)carbodiimide or dicyclohexylcarbodiimide to yield a bis-
(dimethylamido)bis(guanidinate)M series (eq 1).3a,b In the case
of zirconium, this reaction has been generalized to include
insertion of carbodiimides into Zr-P and Zr-As bonds.12
Aldrich and used without further purification. Preparation of Ta(NMe2)5
and Nb(NMe2)5 was carried out according to literature procedures.5 1
H
NMR spectra were run on a Gemini 200 MHz spectrometer with
deuterated benzene or pyridine as a solvent and internal standard. All
elemental analyses were run on a Perkin-Elmer PE CHN 4000 elemental
analysis system.
Ta(NMe2)4[CyNC(NMe2)NCy] (1). A Schlenk flask was charged
with CyNCNCy (0.103 g, 0.498 mmol) in 20 mL of hexane. TaN-
(Me2)5 (0.200 g, 0.498 mmol) was added slowly to this solution. The
reaction mixture was stirred for 24 h at room temperature followed by
removal of solvent under oil pump vacuum. The product was
subsequently recrystallized from toluene at -30 °C to yield 0.245 g
(80%) of 1: 1H NMR (C6D6, ppm): 3.52, (s, 12H, NMe2), 3.39 (s,
12H, NMe2), 3.10 (br, 2H, C6H11), 2.48 (s, 6H, NMe2), 1.1-1.9 (m,
20H, C6H11). 13C NMR (C6D6, ppm): 167.7 (NC(NMe2)N), 56.9 (NCH),
48.7 (TaNCH3), 48.2 (TaNCH3), 40.84 (CNCH3), 35.4, 27.2, 26.4 (3s,
C6H11). Anal. Calcd for C23H52N7Ta: C, 45.46; H, 8.63; N, 16.14.
Found: C, 45.77; H, 9.05; N, 16.64.
The direct reactions of pentakis(dimethylamido)M (M ) Ta,
Nb) complexes with both dicyclohexylcarbodiimide and diiso-
propylcarbodiimide proceeded smoothly at room temperature
under nitrogen to provide good yields of complexes 1-4 (eq
2). Only in the case of the Nb reactions was a dramatic color
change noted during reaction. The origin of this phenomenon
is not clear, but given the fact that all of the products are pale
in color, this red material is likely a minor product or impurity.
These new guanidinate-containing complexes were characterized
by spectroscopic methods and in the case of 1 by a single-crystal
X-ray diffraction study.
Nb(NMe2)4[CyNC(NMe2)NCy] (2). A Schlenk flask was charged
with CyNCNCy (0.264 g, 1.28 mmol) in 20 mL of hexane. To this
solution was added 0.400 g of Nb(NMe2)5 (1.28 mmol) to produce a
blood red solution mixture, which was stirred overnight at room
temperature. The solvent was removed under oil pump vacuum, and
the 0.458 g of residue was extracted with diethyl ether, concentrated
to 30 mL, and cooled to -30 °C to give 0.240 g of pale orange 2
1
(70% yield). H NMR (C6D6, ppm): 3.38 (s, 12H, NbNMe2), 3.22 (s,
12H, NbNMe2), 3.20 (br, 2H, C6H11), 2.53 (s, 6H, CNMe2), 1.15-
1.90 (m, 20H, C6H11). 13C NMR (C6D6, ppm): 167.23 (NC(NMe2)N),
57.18 (NCH), 50.13 (NbNCH3), 49.57 (NbNCH3), 40.95 (CNCH3),
35.67, 27.19, 26.50 (3s, C6H11). Anal. Calcd for C23H52N7Nb: C, 53.16;
H, 10.09; N, 18.87. Found: C, 53.51; H, 10.46; N, 18.48.
(6) For some examples, see: (a) Dionne, M.; Hao, S.; Gambarotta, S.
Can. J. Chem. 1995, 73, 1126 and references therein. (b) Gambarotta,
S.; Strologo, S.; Floriani, C.; Chiesi-Villa, A.; Guastitin, C. J. Am.
Chem. Soc. 1985, 107, 6278 and references therein. (c) Harris, A. D.;
Robinson, S. D.; Sahajpal, A.; Hursthouse, M. B. J. Chem. Soc., Dalton
Trans. 1981, 1327 and references therein.
(7) (a) Gambarotta, S.; Strologo, S.; Floriani, C.; Chiesi-Villa, A.;
Guastitin, C. Inorg. Chem. 1985, 24, 654. (b) Wilkins, J. D. J.
Organomet. Chem. 1974, 80, 349. (c) Drew, M. G. B.; Wilkins, J. D.
J. Chem. Soc., Dalton Trans. 1974, 1579. (d) Drew, M. G. B.; Wilkins,
J. D. J. Chem. Soc., Dalton Trans. 1975, 2611.
(8) For examples of the insertion reaction into main group metal-carbon
bonds, see: (a) Srinivas, B.; Chang, C.-C.; Chen, C.-H.; Chiang, M.
Y.; Chen, I.-T.; Wang, Y.; Lee, G.-H. J. Chem. Soc., Dalton Trans.
1997, 957. (b) Li, M.-D.; Chang, C.-C.; Wang, Y.; Lee, G.-H.
Organometallics 1996, 15, 2571. (c) Lechler, R.; Hausen, H.-D.;
Weidlein, J. J. Organomet. Chem. 1989, 359, 1. (d) Kottmair-Maieron,
D.; Lechler, R.; Weidlein, J. Z. Anorg. Allg. Chem. 1991, 593, 111.
(9) (a) Kennepohl, D. K.; Santarsiero, B. D.; Cavell, R. G. Inorg. Chem.
1990, 29, 5081 and references therein. (b) Schwarz, W.; Rajca, G.;
Weidlein, J.; Dehnicke, K. Z. Anorg. Allg. Chem. 1985, 525, 143.
(10) For examples with M ) Mg, see: Srinivas, B.; Chang, C.-C.; Chen,
C.-H.; Chiang, M. Y.; Chen, I.-T.; Wang, Y.; Lee, G.-H. J. Chem.
Soc., Dalton Trans. 1997, 957.
(11) For examples with M ) Si, Ge, Sn, see: (a) Matsuda, I.; Itoh, K.;
Ishii, Y. J. Organomet. Chem. 1974, 69, 353. (b) Chandra, G.; Jenkins,
A. D.; Lappert, M. F.; Srivastava, R. C. J. Chem. Soc. A 1970, 2550.
(c) George, T. A.; Jones, K.; Lappert, M. F. J. Chem. Soc. 1965, 2157.
(12) (a) Lindenberg, F.; Sieler, J.; Hey-Hawkins, E. Polyhedron 1996, 15,
1459. (b) Hey-Hawkins, E.; Lindenberg, F. Z. Naturforsch. 1993, 48b,
951.
Ta(NMe2)4[(CH3)2CHNC(NMe2)NCH(CH3)2] (3). The procedure
was similar to the synthesis of 1 using 0.200 g of Ta(NMe2)5 (0.498
mmol) and 0.062 g of (CH3)2CHNCNCH(CH3)2 (0.49 mmol) in hexane
followed by recrystallization from toluene at -30 °C. Complex 3 was
isolated in 94% yield (0.25 g): 1H NMR (C6D6, ppm): 3.75 (br, 2H,
CHMe2), 3.52 (s, 12H, TaNMe2), 3.39 (s, 12H, TaNMe2), 2.47 (s, 6H,
NMe2), 1.19 (d, 12H, CH(CH3)2). 13C NMR (C6D6, ppm): 163.80
(NC(NMe2)N), 48.51 (TaN(CH3)2), 48.10 (TaN(CH3)2), 47.54 (NCHMe2),
40.65 (CN(CH3)2), 25.00 (CH(CH3)2). Anal. Calcd for C17H44N7Ta: C,
38.71; H, 8.41; N, 18.59. Found: C, 39.02; H, 8.81; N, 18.20.
Nb(NMe2)4[(CH3)2CHNC(NMe2)NCH(CH3)2] (4). Following a
procedure similar to the synthesis of 2 using 0.40 g of Nb(NMe2)5 (1.28
mmol) and 0.162 g of (CH3)2CHNCNCH(CH3)2 (1.28 mmol) in hexane
produced a blood red solution. After stirring for 24 h at room
temperature followed by workup and recrystallization from toluene at
-30 °C, complex 4 was isolated in 71% yield (0.40 g) as light yellow
1
crystals. H NMR (C6D6, ppm): 3.75 (br, 2H, CHMe2), 3.40 (s, 12H,
NbNMe2), 3.24 (s, 12H, NbNMe2), 2.52 (s, 6H, NMe2), 1.19 (d, 12H,
CH(CH3)2). 13C NMR (C6D6, ppm): 167.10 (NC(NMe2)N), 49.90
(NbN(CH3)2), 49.41 (NbN(CH3)2), 47.80 (NCHMe2), 40.81 (CN(CH3)2),
25.31 (CH(CH3)2). Anal. Calcd for C23H52N7Nb: C, 46.46; H, 10.09;
N, 22.31. Found: C, 46.10; H, 10.16; N, 22.04.
(5) Bradley, D. C.; Thomas, I. M. Can. J. Chem. 1962, 40, 1355. Bradley,
D. C.; Thomas, I. M. Can. J. Chem. 1962, 40, 449.

1000 Inorganic Chemistry, Vol. 38, No. 5, 1999
Tin et al.
Table 1. Summary of Crystal Data and Structure Refinement for
Ta(NMe2)4[CyNC(NMe2)NCy] (1)
Table 3. Selected Bond Angles (deg) for
Ta(NMe2)4[CyNC(NMe2)NCy] (1)
empirical formula
fw
temp
wavelength
cryst syst
space group
unit cell dimens
C23H57N7Ta
607.67
153(2) K
0.710 73 (Å)
triclinic
P1h
N(5)-Ta(1)-N(4)
N(5)-Ta(1)-N(6)
N(4)-Ta(1)-N(6)
N(5)-Ta(1)-N(7)
N(4)-Ta(1)-N(7)
N(6)-Ta(1)-N(7)
N(5)-Ta(1)-N(1)
N(4)-Ta(1)-N(1)
N(6)-Ta(1)-N(1)
N(7)-Ta(1)-N(1)
N(5)-Ta(1)-N(2)
N(4)-Ta(1)-N(2)
N(6)-Ta(1)-N(2)
N(7)-Ta(1)-N(2)
N(1)-Ta(1)-N(2)
N(5)-Ta(1)-C(13)
N(4)-Ta(1)-C(13)
N(6)-Ta(1)-C(13)
N(7)-Ta(1)-C(13)
N(1)-Ta(1)-C(13)
N(2)-Ta(1)-C(13)
105.3(2)
91.6(2)
89.7(2)
91.0(2)
90.4(2)
177.3(2)
98.4(2)
156.2(2)
87.9(2)
90.9(2)
157.0(2)
97.7(2)
89.9(2)
87.5(2)
C(14)-N(3)-C(15) 116.7(4)
C(16)-N(4)-C(17) 108.0(5)
C(16)-N(4)-Ta(1)
C(17)-N(4)-Ta(1)
126.2(4)
125.7(4)
C(18)-N(5)-C(19) 108.2(4)
C(18)-N(5)-Ta(1)
C(19)-N(5)-Ta(1)
125.3(4)
126.3(3)
a ) 10.3664(6) Å
b ) 14.6400(9) Å
c ) 18.549(1) Å
R ) 81.304(1)°
â ) 88.894(1)°
γ ) 89.103(1)°
2782.0(3) Å3, 2
1.451 Mg/m3
C(21)-N(6)-C(20) 107.1(4)
C(21)-N(6)-Ta(1)
C(20)-N(6)-Ta(1)
129.2(4)
122.5(4)
C(23)-N(7)-C(22) 108.8(4)
C(23)-N(7)-Ta(1)
C(22)-N(7)-Ta(1)
130.6(3)
120.5(3)
volume, Z
density (calcd)
abs coeff
F(000)
cryst size
θ range for data collection
limiting indices
C(36)-N(8)-C(29) 119.4(4)
3.973 mm-1
1248
58.66(14) C(36)-N(8)-Ta(2)
94.3(3)
127.9(2)
126.7(2)
87.2(2)
90.6(2)
C(29)-N(8)-Ta(2)
146.2(3)
0.10 × 0.10 × 0.10 mm
C(36)-N(9)-C(35) 121.6(4)
1.11-28.68°
C(36)-N(9)-Ta(2)
C(35)-N(9)-Ta(2)
94.4(3)
144.1(3)
-13 e h e 7, -19 e k e 19,
-24 e l e 24
17 104
29.50(13) C(36)-N(10)-C(37) 116.6(5)
29.24(13) C(36)-N(10)-C(38) 119.5(5)
reflns collected
indep reflns
abs correction
refinement meth
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole
N(12)-Ta(2)-N(11) 104.7(2)
N(12)-Ta(2)-N(14) 89.9(2)
N(11)-Ta(2)-N(14) 91.4(2)
N(12)-Ta(2)-N(13) 89.8(2)
N(11)-Ta(2)-N(13) 91.7(2)
N(14)-Ta(2)-N(13) 176.9(2)
N(12)-Ta(2)-N(9) 158.4(2)
C(37)-N(10)-C(38) 116.4(5)
C(40)-N(11)-C(39) 108.6(4)
C(40)-N(11)-Ta(2) 125.5(4)
C(39)-N(11)-Ta(2) 125.8(3)
C(42)-N(12)-C(14) 109.0(4)
C(42)-N(12)-Ta(2) 126.2(3)
C(41)-N(12)-Ta(2) 124.5(3)
C(46)-N(13)-C(45) 108.4(4)
C(46)-N(13)-Ta(2) 131.2(3)
C(45)-N(13)-Ta(2) 120.4(3)
C(44)-N(14)-C(43) 108.7(4)
C(44)-N(14)-Ta(2) 124.6(3)
C(43)-N(14)-Ta(2) 125.7(3)
12 070 [R(int) ) 0.0260]
none
full-matrix least-squares on F2
12066/0/559
1.050
R1 ) 0.0357, wR2 ) 0.0747
R1 ) 0.0502, wR2 ) 0.0865
1.288 and -1.124 e‚Å-3
N(11)-Ta(2)-N(9)
N(14)-Ta(2)-N(9)
N(13)-Ta(2)-N(9)
N(12)-Ta(2)-N(8)
96.9(2)
90.3(2)
88.9(2)
99.8(2)
Table 2. Selected Bond Lengths (Å) for
Ta(NMe2)4[CyNC(NMe2)NCy] (1)
N(11)-Ta(2)-N(8) 155.5(2)
Ta(1)-N(5)
Ta(1)-N(4)
Ta(1)-N(6)
Ta(1)-N(7)
Ta(1)-N(1)
Ta(1)-N(2)
Ta(1)-C(13)
Ta(2)-N(12)
Ta(2)-N(11)
Ta(2)-N(14)
Ta(2)-N(13)
Ta(2)-N(9)
Ta(2)-N(8)
N(1)-C(13)
N(1)-C(6)
N(2)-C(13)
N(2)-C(12)
N(3)-C(13)
N(3)-C(14)
N(3)-C(15)
N(4)-C(16)
N(4)-C(17)
2.011(4)
2.013(4)
2.025(4)
2.038(4)
2.246(4)
2.280(4)
2.695(5)
2.015(4)
2.019(4)
2.041(4)
2.045(4)
2.258(4)
2.263(4)
1.331(6)
1.469(6)
1.318(6)
1.466(5)
1.427(6)
1.442(6)
1.442(7)
1.460(7)
1.461 (7)
N(5)-C(18)
N(5)-C(19)
N(6)-C(21)
N(6)-C(20)
N(7)-C(23)
N(7)-C(22)
N(8)-C(36)
N(8)-C(29)
N(9)-C(36)
N(9)-C(35)
N(10)-C(36)
N(10)-C(37)
N(10)-C(38)
N(11)-C(40)
N(11)-C(39)
N(12)-C(42)
N(12)-C(41)
N(13)-C(46)
N(13)-C(45)
N(14)-C(44)
N(14)-C(43)
1.454(7)
1.464(6)
1.455(7)
1.465(7)
1.455(6)
1.465(6)
1.328(6)
1.479(6)
1.334(6)
1.464(6)
1.415(6)
1.426(8)
1.459(8)
1.460(7)
1.458(7)
1.454(6)
1.459(7)
1.449(6)
1.462(6)
1.455(7)
1.451(7)
N(14)-Ta(2)-N(8)
N(13)-Ta(2)-N(8)
N(9)-Ta(2)-N(8)
C(13)-N(1)-C(6)
C(13)-N(1)-Ta(1)
C(6)-N(1)-Ta(1)
87.6(2)
89.4(2)
N(2)-C(13)-N(1)
113.6(4)
125.9(4)
120.5(4)
57.6(2)
56.2(2)
174.1(3)
112.5(4)
58.61(14) N(2)-C(13)-N(3)
119.3(4)
94.3(3)
N(1)-C(13)-N(3)
N(2)-C(13)-Ta(1)
N(1)-C(13)-Ta(1)
N(3)-C(13)-Ta(1)
N(8)-C(36)-N(9)
N(8)-C(36)-N(10) 121.6(4)
N(9)-C(36)-N(10) 125.8(4)
145.4(3)
C(13)-N(2)-C(12) 121.6(4)
C(13)-N(2)-Ta(1)
C(12)-N(2)-Ta(1)
93.1(3)
144.7(3)
C(13)-N(3)-C(14) 120.2(4)
C(13)-N(3)-C(15) 114.1(4)
i
140.2 ppm for Cy and Pr, respectively). Despite attempts to
provoke the incorporation of a second equivalent of carbodi-
imide through prolonged reaction time and increased temper-
ature, only the monoinsertion products were observed. Multiple
insertion products have been observed in the analogous reactions
of MexMCl5-x (M ) Ta, Nb; x ) 1, 2, 3) with carbodiimides
RNCNR (R ) isopropyl, cyclohexyl, p-tolyl) to give acet-
amidinate complexes MCl4[RNC(Me)dNR], MeMCl3[RN-
C(Me)dNR], and MCl3[RN-C(Me)dNR]2.7b-d Furthermore,
a double insertion product is reported for the reaction of
M(NMe2)5 (M ) Ti, Zr, Hf) with carbodiimides to yield bis-
(guanidinate) complexes [(p-tolylN)2CNMe2]2M(NMe2)2 and
[(CyN)2CNMe2]2M(NMe2)2.3a,b
1
Both the H and 13C NMR spectra were consistent with the
single insertion product. The most obvious changes from the
starting materials were a shift in signals for the alkyl groups
originating on the carbodiimides and the division of the dimeth-
ylamino protons signal (singlets at 3.25 and 3.12 ppm for the
Ta and Nb staring materials, respectively) into three singlets.
The integrated ratios of the signals assigned to the NMe2
groups (1:2:2) as well as their chemical shifts provided the first
direct evidence for the number of guanidinate and amido ligands
within the compounds. The observation of a single resonance
for the cyclohexyl and isopropyl protons on the R carbon
indicated a structure with either a symmetric arrangement of
the ligands or one that was fluxional. 13C NMR was consistent
with the proton spectrum in all cases and revealed that the cen-
tral, sp2 C of the guanidine ligands had undergone a substan-
tial downfield shift from the starting carbodiimides (142.1 and
These observations might be attributed to two possible factors.
The first is the increased steric congestion of complexes 1-4
compared to the monoinsertion products MCl4[RN-C(Me)d
NR] and [(RN)2CNMe2]M(NMe2)3 compounds. A second possi-
bility for the decreased reactivity of 1-4 toward further insertion
may arise from the increased π-donating ability of amido versus
chloro and the consequent reduction in the acidity of the Ta or
Nb center. Further support for the idea that steric constraints
may play a role in these reactions is provided by the fact that
attempts to get a similar insertion reaction with bis(trimethyl-
silyl)carbodiimide have been completely unsuccessful.

Guanidinate Complexes of Ta(V) and Nb(V)
Inorganic Chemistry, Vol. 38, No. 5, 1999 1001
In order to confirm the structural details of complexes 1-4
a single-crystal X-ray analysis of one of these compounds was
undertaken. Complex 1 crystallized in the triclinic space group
P1h with two independent molecules in the asymmetric unit.
Selected bond distances and angles are provided in Tables 1-3
for both molecules. Figure 1 provides an ORTEP of one of these
molecules and reveals the Ta(V) center in a distorted octahedral-
based coordination geometry. The coordination environment of
Ta contains a tetrasubstituted, bidentate guanidinate ligand de-
rived from the insertion of a dicyclohexylcarbodiimide into
one of the Ta-NMe2 ligands of the starting material along with
four remaining NMe2 ligands. Completing the equatorial plane
defined by the coordinated N-C-N function are two dimeth-
ylamido ligands (N(4) and N(5) or N(12) and N(13)). Two axial
dimethylamido functions (N(6) and N(7) or N(14) and N(15))
complete the Ta coordination sphere (average N-Ta-N angle
of 177.1°). The guanidinate ligand binds to Ta through two
nitrogen atoms to yield a planar four-membered ring of sp2-
hybridized N and C centers with an average bite angle of 58.6°.
The delocalized π interaction within the NCN moieties (N(1)-
C(13)-N(2) and N(8)-C(36)-N(9)) leads to partial double
bonded C-N distances which average 1.33 Å. The distances
and angles within this cyclic arrangement are reminiscent of
those in the structurally characterized seven-coordinate bis-
(diisopropylacetamidinato) and bis(dicyclohexylacetamidinato)
Figure 1. ORTEP diagram and atom-numbering scheme for Ta-
(NMe2)4[CyNC(NMe2)NCy] (1) (Cy ) cyclohexyl) showing one of
the two independent molecules in the asymmetric unit. Thermal
ellipsoids are drawn at 30% probability. Hydrogen atoms have been
omitted for clarity.
average 2.03 Å, which is compatible with a π interaction,13 and
the orientation of the amido groups is consistent with d-p
interactions between the nitrogen p orbitals and metal dxy, dxz,
dyz orbitals of the distorted octahedral geometry described above.
The angles between the mean planes of the axial amides are
nearly orthogonal (84.4° and 88.4°), an alignment appropriate
for these two amides to interact with two orthogonal tantalum
d orbitals (i.e. dxz and dyz). The two equatorial amido ligands
also exhibit a preferred orientation with the mean Me-N-Me
planes at an average of 64° with respect to the plane defined
by the Ta and NCN of the guanidine anion. It would appear
that the maximum π overlap with the remaining tantalum d orbi-
tal (dxy) would be achieved for these ligands if this angle were
90°. Opposing this orientation are the steric interactions between
the axial and equatorial amido ligands and the fact that the two
amido ligands are competing for donation to a single orbital.
In conclusion, we have shown that guanidinate ligands can
be generated by the insertion reaction of carbodiimides into Ta
and Nb amido functions. The structure of complex 1 indicates
some of the steric and electronic features of these tetrasubstituted
guanidinate ligands by providing initial details on the role of
the bulk of the organic substituents and π delocalization in the
ligand. Our current efforts and future reports will be directed
at clarifying these issues through changes to the guanidinate
framework, variation of the metal centers, and manipulation of
ancillary ligands.
i
complexes Cl3Ta(RNC(CH3)NR)2 (R ) Pr, Cy) (B).7c,d
The third guanidinate nitrogen (N(3) and N(10)) was derived
from a dimethylamido function and lies just slightly out of the
ligand plane. For example, the N(3)-C(13)-Ta(1) angle is
174.1(3)°. Although this exocyclic nitrogen appears to be
distorted toward planar, sp2 hybridization (Table 3), the angles
of the mean planes defined by the C-NMe2 and the bidentate
NCN fragments average 80.7°. This feature militates against
conjugation of the N-C-N π system and the lone pair on these
NMe2 groups. In accord with this are the N(3)-C(13) and
N(10)-C(36) distances (average ) 1.42 Å) which are consistent
with an N-C single bond. Rather than being conjugated with
the ligand π system, it appears that, in fact, this orientation of
the dimethylamino group effectively adds a third dimension to
the guanidinate ligand.
Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council of Canada.
Supporting Information Available: Listing providing a description
of the structural solution and ORTEP drawings for compound 1. This
material is available free of charge via the Internet at http://pubs.acs.org.
IC980709V
The four amido groups in each molecule are planar, a feature
consistent with π donation of the nitrogen lone pair into a metal
d orbital. Furthermore the values for the Ta-NMe2 distances
(13) Chisholm, M. H.; Tan, L.-S.; Huffman, J. C. J. Am. Chem. Soc. 1982,
104, 4879.