Redefining the coordination geometry and reactivity of guanidinate complexes by covalently linking the guanidinate ligands. Synthesis and reactivity of [RN{NH(R)}CN(CH2)2NC{NH(R)}NR]M(CH2 Ph)2 (R = iPr; M = Ti, Zr)

Article abstract of DOI:10.1021/om020830g

Reaction of the new biguanidine (ⁱPr)HN{NH(ⁱPr)}CN(CH₂)₂ NC{NH(ⁱPr)}N(ⁱPr)H (1) with M(CH₂ Ph)₄ affords the hydrocarbyl complexes [ⁱPrN{NH(ⁱPr)}CN(CH₂)₂ NC{NH(ⁱPr)}NⁱPr]M(CH₂Ph)₂ ([L₄^Pr]M(CH₂Ph)₄; M = Ti (2), Zr (3)). Structure determinations of 2 and 3 and the reactivity of 3 with 2,6-dimethylphenyl isocyanide indicate that the linked guanidinate ligand system provides a more open metal coordination sphere than do the unlinked analogues.

Full text of DOI:10.1021/om020830g

Organometallics 2003, 22, 387-389
387
Red efin in g th e Coor d in a tion Geom etr y a n d Rea ctivity of
Gu a n id in a te Com p lexes by Cova len tly Lin k in g th e
Gu a n id in a te Liga n d s. Syn th esis a n d Rea ctivity of
[RN{NH(R)}CN(CH2)2NC{NH(R)}NR]M(CH2P h )2
i
(R ) P r ; M ) Ti, Zr )
Tiow-Gan Ong, Glenn P. A. Yap, and Darrin S. Richeson*
Department of Chemistry and the Center for Catalysis Research and Innovation,
University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
Received October 3, 2002
Summary: Reaction of the new biguanidine (iPr)HN-
{NH(iPr)}CN(CH2)2NC{NH(iPr)}N(iPr)H (1) with M-
(CH2Ph)4 affords the hydrocarbyl complexes [iPrN{NH-
(iPr)}CN(CH2)2NC{NH(iPr)}NiPr]M(CH2Ph)2 ([L4Pr]M-
(CH2Ph)4; M ) Ti (2), Zr (3)). Structure determinations
of 2 and 3 and the reactivity of 3 with 2,6-dimethylphe-
nyl isocyanide indicate that the linked guanidinate
ligand system provides a more open metal coordination
sphere than do the unlinked analogues.
This contribution describes the first linked guanidi-
nate ligand system and its application in group 4
organometallic chemistry. We demonstrate that tether-
ing of the two guanidinate moieties results in changes
to ligand geometry and metal coordination behavior and
that the reactivity of these constrained complexes differ
from that of their unlinked analogues.
The direct reaction of 2 equiv of diisopropylcarbodi-
imide with ethylenediamine in toluene at 100 °C
produces, in 60% yield, biguanidine 1, in which the two
guanidine moieties are linked via an ethylene bridge
(eq 1).
Controlling metal coordination environment and re-
activity through modification of supporting ligation is
a common theme in organometallic chemistry. Consid-
erable recent interest has focused on the development
of amidinate and guanidinate anions, [N(R)C(R′)N(R)]-
and [N(R)C(NR′2)N(R)]-, as sterically and electronically
flexible ligands for the design of new transition-metal
and main-group-metal complexes.1,2 We have been
particularly interested in these species for this role due
to their amenability to modification through directed
variation of the substituents, R and R′.3 Despite this
potential, the reaction chemistry of complexes possess-
ing these ligands is still relatively undeveloped.
Synthetic and reactivity studies of transition-metal
guanidinate complexes have focused on the mono-
(guanidinate) and bis(guanidinate) frameworks.2 We are
interested in further revealing the versatility of the
guanidinate system by covalently linking two of these
ligands as a means of further defining and constraining
complex geometry. Such concepts have been elegantly
applied to ansa-metallocene complexes4 and other an-
ionic functionalities including amidinates5 and ami-
dates.6
The direct synthesis of 1 from commercial reagents and
the ready availability of other diamines with a variety
of tethers should allow considerable flexibility for
modifications to this potential ligand. Compound 1
possesses reactive NH groups that can be employed for
its introduction to an appropriate metal complex. For
example, 1 reacts smoothly with tetrakis(benzyl) com-
plexes of Zr and Ti to yield complexes 2 and 3,
respectively (Scheme 1). The NMR spectra of 2 and 3
indicate that they possess a dianionic linked biguanidi-
nate ligand and two benzyl groups. Furthermore, these
spectral data indicate a symmetric bonding arrange-
ment for the biguanidinate ligand, as exemplified by
only two signals for the iPr moieties and a single
resonance for the ethylene bridge.
(1) For reviews of benzamidinate complexes see: (a) Edelmann, F.
Coord. Chem. Rev. 1994, 94, 403. (b) Barker, J .; Kilner, M. Coord.
Chem. Rev. 1994, 94, 219.
(2) For a recent review of guanidinate complexes see: Bailey, P. J .;
Pace, S. Coord. Chem. Rev. 2001, 214, 91.
(3) (a) Lu, Z.; Yap, G. P. A.; Richeson, D. S. Organometallics 2001,
20, 706. (b) Foley, S. R.; Yap, G. P. A.; Richeson, D. S. J . Chem. Soc.,
Chem. Commun. 2000, 1515. (c) Thirupathi, N.; Yap, G. P. A.; Richeson,
D. S. Organometallics 2000, 19, 2573. (d) Foley, S. R.; Yap, G. P. A.;
Richeson, D. S. J . Chem. Soc., Dalton Trans. 2000, 1663. (e) Foley, S.
R.; Zhou, Y.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 2000, 39,
924. (f) Thirupathi, N.; Yap, G. P. A.; Richeson, D. S. J . Chem. Soc.,
Chem. Commun. 1999, 2483.
(4) For a recent examination of the effect of bridging groups in ansa-
metallocenes see: Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B.
M.; Parkin, G.; Brandow, C. G.; Bercaw, J . E.; J ardine, C. N.; Lyall,
M.; Green, J . C.; Keister, J . B. J . Am. Chem. Soc. 2002, 124, 9525 and
references therein.
(5) (a) Hagadorn, J . R. Chem. Commun. 2001, 2144. (b) Chen, C.-
T.; Rees, L. H.; Cowley, A. R.; Green, M. L. H. J . Chem. Soc., Dalton
Trans. 2001, 1761. (c) Bambirra, S.; Meetsma, A.; Hessen, B.; Teuben,
J . H. Organometallics 2001, 20, 782. (d) Bambirra, S.; Brandsma, M.
J . R.; Brussee, E. A. C.; Meetsma, A.; Hessen, B.; Teuben, J . H.
Organometallics 2000, 19, 3197. (e) Hagadorn, J . R.; Arnold, J . Angew.
Chem., Int. Ed. 1998, 37, 1729. (f) Brandsma, M. J . R.; Brussee, E. A.
C.; Meetsma, A.; Hessen, B.; Teuben, J . H. Eur. J . Inorg. Chem. 1998,
1867.
(6) Giesbrecht, G. R.; Shafir, A.; Arnold, J . Inorg. Chem. 2001, 40,
6069.
10.1021/om020830g CCC: $25.00 © 2003 American Chemical Society
Publication on Web 01/01/2003

388 Organometallics, Vol. 22, No. 3, 2003
Communications
Sch em e 1
Compounds 2 and 3 are related to the reported
unlinked bis(guanidinate) complexes {(Me2N)C(NiPr)2}2-
M(CH2Ph)2 (M ) Ti (I), Zr (II)) and [{(iPrNH)C(Ni-
Pr)2}2Zr(CH2Ph)2] (III).7,8 These recently reported spe-
cies exhibit structures with two chelating bidentate
guanidinate ligands and cis-oriented hydrocarbyl groups,
resulting in pseudo-C2-symmetric, distorted-octahedral-
based coordination geometries.
The single-crystal X-ray structures of 2 and 3 revealed
that these two species exhibit similar molecular struc-
tures and that they differ substantially from I-III.
Consistent with their spectroscopic data, both of these
new complexes possess two benzyl groups and a chelat-
ing tetradentate dianionic version of biguanidine 1.9,10
The biguanidinate ligand in both 2 and 3 coordinates
to the metal center in a symmetrical fashion to yield
an MN4 arrangement that approaches planarity. This
coordination geometry is in striking contrast to the
structurally characterized unlinked bis(guanidinate)
species I-III. Specifically, the angles between the two
planar four-membered guanidinate chelate rings in 2
and 3 are approximately 20°, while for a typical un-
linked example such as II this angle is 99.3°. This
considerable change in coordination behavior is at-
tributed to the restrictions of the ethylene linkage of
the biguanidinate ligand.
Linking the guanidinate moieties has the effect of
opening the metal coordination sphere, as indicated by
an increase in the angle between the two benzyl ligands
in comparison with the unlinked analogues. For ex-
ample, the angles between the two undistorted η1-benzyl
groups in complexes 2 and 3 (i.e. CH2MCH2) are 134.2-
(1) and 128.4(2)°, respectively, while the corresponding
angles in I (88.6(2)°), II (89.7(1)°), and III (93.02(7)°)
are much smaller and are in accordance with the
distorted-octahedral geometries of these species.
We anticipated that the observed changes in coordi-
nation geometry that accompany the tethering of the
two guanidinate ligands should have important reactiv-
ity consequences. To probe this concept, we chose to
investigate the fundamental reactivity of these new
biguanidinate species with 2,6-dimethylphenyl isocya-
nide (Ar′NC). Aryl isocyanides react with unlinked bis-
(guanidinate) species, including I-III, by initially un-
dergoing insertion reactions with the hydrocarbyl
moieties forming η2-iminoacyl ligands for which subse-
quent transformations have been observed.8 This reac-
tivity serves as a benchmark for comparison with the
reactivity of 3 and Ar′NC that is summarized in Scheme
1.11
(7) (a) Mullins, S. M.; Duncan, A. P.; Bergman, R. G.; Arnold, J .
Inorg. Chem. 2001, 40, 6952. (b) Duncan, A. P.; Mullins, S. M.; Arnold,
J .; Bergman, R. G. Organometallics 2001, 20, 1808.
(8) (a) Ong, T.-G.; Wood, D.; Yap, G. P. A.; Richeson, D. S.
Organometallics 2002, 21, 1. (b) Ong, T.-G.; Yap, G. P. A.; Richeson,
D. S. Organometallics 2002, 21, 2839-2841.
(9) Crystals were obtained from a saturated toluene solution cooled
to -35 °C. Crystal data for 2: empirical formula C30H48N6Ti, T ) 203-
(2) K, λ ) 0.710 73 Å, space group P212121, a ) 12.440(4) Å, b ) 14.820-
(3) Å, c ) 16.836(5) Å, V ) 3103.9(15) Å3, Z ) 4, R indices (I > 2σ(I))
R1 ) 0.0414 and wR2 ) 0.0992. The structure was refined as a racemic
twin with a refined absolute structure parameter of 0.62(3).
(10) Crystals were obtained from a saturated toluene solution cooled
to -35 °C. Crystal data for 3: empirical formula C30H48N6Zr, T ) 203-
(2) K, λ ) 0.710 73 Å, space group P212121, a ) 12.6201(19) Å, b )
14.985(2) Å, c ) 16.743(3) Å, V ) 3166.2(8) Å3, Z ) 4, R indices (I >
2σ(I)) R1 ) 0.0442 and wR2 ) 0. 0882.
Complex 3 reacts at room temperature with 2 equiv
of Ar′NC within 1 h to yield the new bis(η2-iminoacyl)
complex 4 (Scheme 1). A 13C NMR resonance for 4 at δ
257.7 is assigned to the two equivalent iminoacyl
ligands, and the relative integration intensities observed
1
in the H NMR spectra of this compound suggest the
proposed empirical formula. A single-crystal X-ray

Communications
Organometallics, Vol. 22, No. 3, 2003 389
structural analysis has confirmed the proposed struc-
ture.12 This reactivity is consistent with that observed
for the bis(guanidinate) dimethyl complex [{(Me2N)C(Ni-
Pr)2}2ZrMe2], which produced the bis(iminoacyl) species
[{(Me2N)C(NiPr)2}2Zr(η2-Ar′NdCMe)2] (IV) when re-
acted with 2 equiv of Ar′NC.8(a)
tion of Ar′NC with Cp2M(CH2-py-Me)2 (M ) Zr, Hf; CH2-
py-Me ) 2-(6-methylpyridyl)methyl). After initially
generating iminoacyl complexes, these species ulti-
mately yield the vinylamido complexes Cp2M(Ar′NCHd
CH-py-Me)(CH2-py-Me) and Cp2M(Ar′NCHdCH-py-
Me)2, depending on the reaction stoichiometry.15 On the
basis of mechanistic studies, it appears that the coor-
dination of the pyridine nitrogen of the hydrocarbyl
ligands plays an important role in facilitating this
isomerization. Interestingly, low-temperature addition
of pyridine to the η2-acyl complex Cp*TaCl3(η2-COCH2-
CMe3) initially yields a pyridine adduct that isomerizes
to the enolate Cp*TaCl3(py)(η2-OCHdCHCMe3) at -10
°C.16
The subsequent reactivity of 4 diverges from that
observed for IV, and we attribute this to the linking of
the guanidinate ligands. Mild thermolysis of IV trans-
forms the two η2-iminoacyl groups into a metal-bound
enediamido ligand via a coupling reaction.8a,13 In con-
trast, the product isolated when complex 4 is heated to
1
90 °C displays H and 13C NMR spectra, indicating that
both iminoacyl groups have transformed, via an overall
1,2-hydrogen shift, into vinylamido groups. A resonance
for the vinylic proton adjacent to the phenyl group
appears at δ 4.81, while the other vinylic proton that is
adjacent to the amido nitrogen exhibits a more down-
field shift at δ 7.87. The coupling between these two
protons of 14 Hz is consistent with a trans arrangement
for the olefin. The 13C NMR resonance for the iminoacyl
has disappeared and is replaced with two signals for
the vinylic carbon atoms at δ 98.1 (NAr′CHdCHPh) and
150.1 (NAr′CHdCHPh).
The structure of 5 was determined and confirmed the
presence of the trans vinylamido groups and the bigu-
anidinate dianion (Scheme 1).14 The Zr coordination
geometry in 5 is nearly identical with that observed for
3; the biguanidinate ligand exhibited nearly identical
metrical parameters for the two compounds, and the
vinylamido groups of 5 occupy coordination sites similar
to those of the benzyl ligands in 3 (N(7)-Zr-N(8) angle
of 137.63(13)°). Conjugation of the nitrogen lone pair
with the vinyl moieties is suggested by the short
N-C(vinyl) bond lengths (1.388(5) and 1.392(5) Å), the
planar N(7) and N(8), and the slightly elongated Zr-N
distances of 2.176(3) Å.
We attribute the difference in reactivity between 4
and IV to the more open coordination environment of 4
that is provided by the biguanidinate ligand. This may
facilitate the observed conversion by allowing coordina-
tion of an external base to complex 4. One source of such
external ligands is provided by the uncoordinated
nitrogen centers of the biguanidinate ligand. We are still
attempting to clarify the mechanistic details for the
formation of 5 but can report that addition to pyridine
to a solution NMR sample of 4 in C6D6 appears to only
slightly accelerate its transformation to 5.
We have demonstrated that linking guanidinate
ligands via a covalent tether can alter the relative
orientation of the ligand array, open the metal coordi-
nation sphere relative to the analogous unbridged
systems, and have significant effects on the reactivity
of metal complexes. Biguanidinate ligands can be pre-
pared from commercial sources and should be amenable
to straightforward modification of the linkage and
nitrogen substituents. We expect that biguanidinates
will form an interesting set of ancillary ligands for new
transition-metal and lanthanide chemistry.
The only reported precedent for this type of iminoacyl
rearrangement involves complexes derived for the reac-
Ack n ow led gm en t. This work was supported by the
NSERC of Canada.
(11) For a review of the insertion chemistry of isocyanides see:
Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059 and references
therein.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details for compounds 1-5 and thermal ellipsoid plots
and tables of crystal data and structural solution and refine-
ment details, atomic coordinates, bond lengths and angles, and
anisotropic thermal parameters for compounds 2, 3 and 5. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(12) The single-crystal X-ray structure for compound 4 has been
determined. Full structural details will be reported in a future paper.
(13) The intramolecular coupling between two η2-iminoacyl groups
to yield enediamido ligands has been observed for group 4 complexes
with a variety of supporting ligand environments: Chamberlain, L.
R.; Durfee, L. D.; Fanwick, P. E.; Kobriger, L. M.; Latesky, S. L.;
McMullen, A. K.; Steffey, B. D.; Rothwell, I. P.; Folting, K.; Huffman,
J . C. J . Am. Chem. Soc. 1987, 109, 6068. Durfee, L. D.; McMullen, A.
K.; Rothwell, I. P. J . Am. Chem. Soc. 1988, 110, 1463. Scott, M. J .;
Lippard, S. J . Organometallics 1997, 16, 5857. Giannini, L.; Caselli,
A.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.; Re, N.;
Sgamellotti, A. J . Am. Chem. Soc. 1997, 119, 9709.
(14) Crystals were obtained from a saturated toluene solution cooled
to -35 °C. Crystal data: empirical formula C48H64N8Zr, T ) 203(2) K,
λ ) 0.710 73 Å, space group P21/n, a ) 14.5031(14) Å, b ) 18.1977(18)
Å, c ) 17.9802(18) Å, â ) 104.504(2)°, V ) 4594.2(8) Å3, Z ) 4, R indices
(I > 2σ(I)) R1 ) 0.0608 and wR2 ) 0.1399.
OM020830G
(15) Beshouri, S. M.; Chebi, D. E.; Fanwick, P. E.; Rothwell, I. P.;
Huffman, J . C. Organometallics 1990, 9, 2375. Beshouri, S. M.;
Fanwick, P. E.; Rothwell, I. P.; Huffman, J . C. Organometallics 1987,
6, 891.
(16) Arnold, J .; Tilley, T. D.; Rheingold, A. L.; Geib, S. J .; Arif, A.
M. J . Am. Chem. Soc. 1989, 111, 149.