Terminal titanium(IV) (trimethylsilyl)imides prepared by oxidatively induced trimethylsilyl abstraction

Article abstract of DOI:10.1021/om060168e

Titanium (trimethylsilyl)imide triflate complexes of the type (L)Ti=NSiMe₃(OTf) (L^- = [ArNC(CH₃)] ₂CH, Ar = 2,6-ⁱPr₂C₆H₃; L^- = N[2-P(CHMe₂)₂

Full text of DOI:10.1021/om060168e

Organometallics 2006, 25, 2725-2728
2725
Terminal Titanium(IV) (Trimethylsilyl)imides Prepared by
Oxidatively Induced Trimethylsilyl Abstraction
Brad C. Bailey, Falguni Basuli, John C. Huffman, and Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana UniVersity,
Bloomington, Indiana 47405
ReceiVed February 21, 2006
Summary: Titanium (trimethylsilyl)imide triflate complexes of
the type (L)TidNSiMe3(OTf) (L- ) [ArNC(CH3)]2CH, Ar )
2,6-iPr2C6H3; L- ) N[2-P(CHMe2)2-4-MeC6H3]2) can be
readily prepared by one-electron oxidation and subsequent
trimethylsilyl abstraction in the Ti(III) precursors (L)TiCl-
(N{SiMe3}2).
and tris-chelate ligands such as [ArNC(CH3)]2CH (Ar ) 2,6-
iPr2C6H3)10 and PNP- (N[2-P(CHMe2)2-4-MeC6H3]2).11
Previously, our group reported that one-electron oxidation
of titanium(III) bis-neopentyl- or bis-anilide (NHAr) promoted
R-hydrogen abstraction to afford terminal titanium alkylidene12
or arylimide13 complexes concurrent with alkane or aniline
formation. Realizing that alkane and aniline are both byproducts
generated in these types of reactions, we speculated whether
other groups could be also eliminated in one-electron-oxidation
processes. For this reason, we turned our attention to the hexa-
methyldisilazide group, since one would expect trimethylsilyl-X
(X- ) halide, pseudohalide) elimination to be a thermodynami-
cally favored process.
Early-transition-metal complexes containing the silylimide
functionality [NSiR3]2- are well-known.1 In the context of group
4 transition metals, the silylimide is typically prepared by
R-hydrogen abstraction,2 silyl chloride elimination,3,4 and, more
recently, transimination reactions.5 Other synthetic strategies to
generate the TidNSiR3 motif include oxidation reactions of low-
6,7
valent metal species with N3SiR3 or silyl-substituted benza-
midinates.8 In certain cases, polynuclear precursors such as
Accordingly, when a toluene solution of ([ArNC(CH3)]2CH)-
3
TiCl2(THF)12a or (PNP)TiCl2 is treated with an equimolar
12d
[Ti(NSiMe3)Cl2]8 can be converted to the corresponding
monomer via salt elimination reactions with sterically imposing
ligands.9 Despite the silylimide ligand being a common theme
in transition metals,1 few examples of the terminal (trimethyl-
silyl)imide group exist for group 4 metals.4,6,7,9 In this paper
we report a systematic approach to the terminal (trimethylsilyl)-
imide functionality on titanium(IV). The process involves a
synthetic strategy combining a one-electron oxidation coupled
with a trimethylsilyl abstraction step. Preparation of the silylim-
ide product is quantitative by NMR spectroscopy and may be
obtained in a highly pure form upon recrystallization. Most
notably, this technique is versatile and can encompass both bis-
amount of NaN{SiMe3}2, the complex ([ArNC(CH3)]2CH)TiCl-
(N{SiMe3}2) (1) or (PNP)TiCl(N{SiMe3}2) (2) is obtained in
good to moderate yields (1, 85%; 2, 64%) (Scheme 1).14
Solution magnetic moment measurements of 1 and 2 are
consistent with a d1 paramagnetic species (Evans method). The
molecular structure of 1 displays a four-coordinate Ti(III)
complex in a highly distorted tetrahedral environment.14 Salient
features for the molecular structure of 1 include a titanium-
amide distance of 1.961(2) Å with the silylamide N being
essentially planar. To avoid clashes with the â-diketiminate aryl
groups, one trimethylsilyl substituent is oriented away from the
isopropyl aryl groups and NCCCNTi framework. This feature
places one of the SiMe3 amide groups in close proximity to the
chloride ligand (∼3.76 Å, Figure 1). The orientation of such a
group implies that trimethylsilyl chloride elimination could be,
in principle, a viable reaction. On the other hand, the molecular
structure of 2 confirms a monomeric, five-coordinate bis-
(trimethylsilyl)amide titanium complex supported by the pincer
ligand PNP (Figure 1). Analogous to the molecular structure
of complex 1, one of the bis(trimethylsilyl)amide silyl groups
in 2 is oriented syn to the chloride ligand, but the distance is
significantly longer (∼4.12 Å, Figure 1).14
* To whom correspondence should be addressed. E-mail: mindiola@
indiana.edu.
(1) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239-482.
(2) (a) Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119,
10696-10719. (b) Schaller, C. P.; Cummins, C. C.; Wolczanski, P. T. J.
Am. Chem. Soc. 1996, 118, 591-611. (c) Cummins, C. C.; Schaller, C. P.;
Van Duyne, G. D.; Wolczanski, P. T.; Chan, E. A.-W.; Hoffmann, R. J.
Am. Chem. Soc. 1991, 113, 2985-2994. (d) Bennett, J. L.; Wolczanski, P.
T. J. Am. Chem. Soc. 1994, 116, 2179-2180.
(3) Titanium complexes with bridging (trimethylsilyl)imides have been
reported: (a) Alcock, N. W.; Pierce-Butler, M.; Willey, G. R. J. Chem.
Soc., Dalton Trans. 1976, 707-713. (b) Wollert, R.; Wocadlo, S.; Dehnicke,
K.; Goesmann, H.; Fenske, D. Z. Naturforsch. 1992, 47b, 1386-1392. (c)
Schlichenmaier, R.; Stra¨hle, J. Z. Anorg. Allg. Chem. 1993, 619, 1526-
1529. (d) Alcock, N. W.; Pierce-Bulter, M.; Willey, G. R. J. Chem. Soc.,
Chem. Commun. 1974, 627. (e) Vasisht, S. K.; Sharma, R.; Goyal, V. Indian
J. Chem. 1985, 24A, 37-39.
(4) (a) Putzer, M. A.; Neumuller, B.; Dehnicke, K. Z. Anorg. Allg. Chem.
1998, 624, 57-64. (b) Bu¨rger, H.; Wannagat, U. Monatsh. Chem. 1963,
94, 761.
(5) Li, Y.; Banerjee, S.; Odom, A. L. Organometallics 2005, 24, 3272-
3278.
(6) Gray, S. D.; Thorman, J. L.; Berreau, L. M.; Woo, L. K. Inorg. Chem.
1997, 36, 278-283.
(7) Hanna, T. E.; Keresztes, I.; Lobkovsky, E.; Bernskoetter, W. H.;
Chirik, P. J. Organometallics 2004, 23, 3448-3458.
(8) (a) Hagadorn, J. R.; Arnold, J. J. Am. Chem. Soc. 1996, 118, 893-
894. (b) Hagadorn, J. R.; Arnold, J. Organometallics 1998, 17, 1355-
1368.
(10) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead, M.
M.; Roesky, H. W.; Power, P. P. Dalton Trans. 2001, 3465-3469 and
references therein.
(11) (a) Ozerov, O. V.; Guo, C.; Papkov, V. A.; Foxman, B. M. J. Am.
Chem. Soc. 2004, 126, 4778-4787. (b) Fan, L.; Foxman, B. M.; Ozerov,
O. V. Organometallics 2004, 23, 326-328.
(12) (a) Basuli, F.; Bailey, B. C.; Tomaszewski, J.; Huffman, J. C.;
Mindiola, D. J. J. Am. Chem. Soc. 2003, 125, 6052-6053. (b) Basuli, F.;
Bailey, B. C.; Watson, L. A.; Tomaszewski, J.; Huffman, J. C.; Mindiola,
D. J. Organometallics 2005, 24, 1886-1906. (c) Basuli, F.; Bailey, B. C.;
Huffman, J. C.; Mindiola, D. J. Organometallics 2005, 24, 3321-3334.
(d) Bailey, B. C.; Huffman, J. C.; Mindiola, D. J.; Weng, W.; Ozerov, O.
V. Organometallics 2005, 24, 1390-1393.
(13) Basuli, F.; Bailey, B. C.; Huffman, J. C.; Mindiola, D. J. Chem.
Commun. 2003, 1554-1555.
(14) See the Supporting Information.
(9) Gardner, J. D.; Robson, D. A.; Rees, L. H.; Muntford, P. Inorg. Chem.
2001, 40, 820-824.
10.1021/om060168e CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/22/2006

2726 Organometallics, Vol. 25, No. 11, 2006
Scheme 1. Synthesis of Complexes 1 and 2 and Subsequent Oxidation To Form 3 and 4a
Communications
a Complexes 3 and 4 can be converted into the Cl- derivatives 5 and 6, and subsequent Cl- abstraction can regenerate the OTf- derivatives. The
compounds shown in brackets have not been observed spectroscopically. Legend: (i) NaN{SiMe3}2, -35 °C, toluene; (ii) [FeCp2][OTf], -35 °C,
THF; (iii) MgCl2, THF; (iv) AgOTf, C6D6.
4 occur via an outer-sphere one-electron oxidation to form FeCp2
and the putative [(L)TiCl(N{SiMe3}2)][OTf] complexes, which
smoothly undergo coordination of the triflate ligand to promote
Me3SiCl extrusion concurrent with TidNSiMe3 linkage forma-
tion (Scheme 1).
The molecular structure of complex 314 shows a rare example2
of a terminal and four-coordinate titanium (trimethylsilyl)imido
complex with a short Ti(1)-N(33) bond length of 1.689(2) Å
(Figure 2). Most notably, the Ti-Nimido-Si angle (164.8(6)°)
is bent, which implies that a pseudo TitN-SiMe3 bond may
not be heavily involved in the bonding scheme for this system.
Sterics, however, could also account for the deviation from
linearity. In the molecular structure of 3, the metal center
deviates ∼1.14 Å above the NCCCN plane (Figure 1), and the
low-coordinate environment at the titanium center promotes
weak interactions of the metal with both â-carbons of the
NCCCN â-diketiminate ring (2.677(3) and 2.685(3) Å). Com-
plex 3 is a close relative to four-coordinate titanium arylimide
complexes reported previously.13,15
The greater coordination environment in 4 prompted us to
collect single-crystal X-ray diffraction data (Figure 2).14 As
expected, the molecular structure of 4 reveals a five-coordinate
titanium(IV) center containing a terminal (trimethylsilyl)imide
ligand (Ti-N31 ) 1.693(6) Å, Ti-Nimido-Si ) 173.7(1)°). For
comparison, the pincer-amide distance (Ti-N10 ) 2.050(5)
Å) is significantly longer than the imide linkage. As observed
Figure 1. Molecular structures of complexes 1 (top) and 2
with 3, deviation from linearity is also evident from the Ti-
(bottom), with thermal ellipsoids at the 50% probability level. H
atoms and Pr methyls (2) have been omitted for clarity. Selected
bond distances (reported in Å) and angles (reported in deg) are as
N-Si angle, and the PNP pincer framework displays “trans-
i
like” phosphines (P-Ti-P ) 144.70(2)°, Figure 2), with the
PNP amide nitrogen being essentially planar. We propose that
follows. For 1: Ti1-N34, 1.960(2); Ti1-N3, 2.051(2); Ti1-N7,
sp hybridization at the imide nitrogen in complex 4 might be
2.075(2); Ti1-Cl2, 2.2950(5); N34-Si35, 1.748(3); N34-Si39,
enforced more by sterics12d than an electronic argument. The
1.755(3); N3-Ti1-N7, 90.86(5). For 2: Ti1-N31, 1.929(3); Ti1-
poor quality of the X-ray data for 3, however, forbids us from
making any accurate comparisons with the structural features
of 4.
N10, 2.057(2); Ti1-P3 2.615(4); Ti1-P18, 2.690(2); Ti1-Cl2,
2.351(3); P3-Ti1-P18, 140.39(4).
In the absence of moisture and oxygen complexes 1 and 2
are indefinitely stable as a solid or in solution. However, when
a THF solution of 1 or 2 is treated with 1 equiv of [FeCp2]-
[OTf], trimethylsilyl chloride, FeCp2, and the silylimide ([ArNC-
(CH3)]2CH)TidNSiMe3(OTf) (3) or (PNP)TidNSiMe3(OTf) (4)
Conveniently, the triflato ligand in compounds 3 and 4 can
be readily displaced with MgCl2 in THF to afford the (trim-
ethylsilyl)imide chlorides ([ArNC(CH3)]2CH)TidNSiMe3(Cl)
(5) and (PNP)TidNSiMe3(Cl) (6) quantitatively.14 Conversion
of 5 and 6 back to 3 and 4, respectively, can be achieved with
addition of 1 equiv of AgOTf in C6D6 (Scheme 1).14 The NMR
spectra of 5 and 6 were analogous to those of 3 and 4, albeit in
the absence of the 19F NMR resonance for the triflato group.14
1
are formed quantitatively, as evidenced by H NMR spectros-
copy (Scheme 1). Separation of 3 or 4 from the FeCp2 is easily
achieved via recrystallization of the reaction mixture from
pentane or Et2O at -35 °C. Vacuum transfer of the volatiles
from either reaction reveals only Me3SiCl to be generated from
the one-electron trimethylsilyl abstraction process, thus sug-
gesting that Me3SiCl formation is thermodynamically favored
over Me3SiOTf formation. We propose that compounds 3 and
(15) (a) Basuli, F.; Clark, R. L.; Bailey, B. C.; Brown, D.; Huffman, J.
C.; Mindiola, D. J. Chem. Commun. 2005, 2250-2252. (b) Prust, J.;
Hohmeister, H.; Stasch, A.; Roesky, H. W.; Magull, J.; Alexopoulos, E.;
Uso´n, I.; Schmidt, H.-G.; Noltemeyer, M. Eur. J. Inorg. Chem. 2002, 2156-
2162.

Communications
Organometallics, Vol. 25, No. 11, 2006 2727
in the reaction mixture (Scheme 2). In fact, examination of the
volatiles from the reaction confirmed the formation of Me3-
SiCl and Me3SiOTf in approximately a 3:1 ratio.16 For this
reason, it appears that one-electron oxidation of 1 or 2 with
AgOTf is partially discriminative for SiCl vs SiOTf bond
formation, which could be attributed to AgOTf being an inner-
sphere one-electron oxidant as opposed to [FeCp2][OTf]. We
believe that a transmetalation reaction between AgOTf and 1
does not precede the oxidation step, given the greater formation
of Me3SiCl vs Me3SiOTf and the absence of Ti(III) byproducts.
Independent experiments between 3 or 4 and Me3SiCl show
this process to be very slow, thus insinuating that triflate for
chloride exchange is not a likely pathway.14,17 Despite the mix-
ture of imide derivatives, our ability to convert 5 or 6 to the
triflato analogue (vide infra) suggested that a second equivalent
of AgOTf might promote anion exchange (Cl- for OTf-).
Gratifyingly, when complex 1 or 2 is treated instead with 2
equiv of AgOTf in toluene at -35 °C, trimethylsilyl abstraction
cleanly and rapidly (∼5 min) occurs to form 3 or 4, respectively
(76-80% isolated yield; Scheme 2).14 However, we were
surprised to observe Me3SiOTf as the only Volatile byproduct
from the reaction mixture (e.g., for the 1 f 3 conversion with
2 equiv of AgOTf). This result suggested that AgOTf might be
not only oxidizing 1 to an intermediate such as “([ArNC(CH3)]2-
CH)TiCl(OTf)(N{SiMe3}2)” but also promoting Cl- for OTf-
exchange to afford the putative “([ArNC(CH3)]2CH)Ti(OTf)2-
(N{SiMe3}2)”. The latter complex would subsequently generate
3 and Me3SiOTf, along with Ag0 and AgCl precipitates (pathway
A, Scheme 2). Despite this possibility, and given our ability to
generate a mixture of silylimides from the 1 f 3/5 conversion
with 1 equiv of AgOTf (3:1 ratio; vide supra), we propose that
a different pathway is involved. Hence, 1 equiv of AgOTf reacts
with 1 or 2 to form a hypothetical “(L)TiCl(N{SiMe3}2)-
(AgOTf)” adduct. This reactive intermediate rapidly expels Ag0
metal, which promotes trimethylsilyl abstraction to form the
two silylimide derivatives. Subsequent Cl- for OTf- exchange
with the second equivalent of AgOTf furnishes only the
silylimide compound 3 or 4 (pathway B, Scheme 2). The
absence of Me3SiCl in the reaction mixture suggests that another
exchange might be taking place. Evidence for Me3SiCl con-
sumption is provided by an independent reaction involving
AgOTf and Me3SiCl, which rapidly afforded Me3SiOTf quan-
titatively along with a white precipitate (Scheme 2). As a result,
these observations suggest that an intermediate such as “(L)-
TiCl(N{SiMe3}2)(AgOTf)” leads to a mixture of silylimides (3/5
or 4/6, in approximately a 3:1 ratio),16 1 equiv of Ag0, 0.25
equiv of Me3SiOTf, and 0.75 equiv of Me3SiCl. The second
Figure 2. Molecular structures of complex 3 (top) and 4 (bottom),
with thermal ellipsoids at the 50% probability level. H atoms and
iPr methyls (4) have been omitted for clarity. Selected bond
distances (reported in Å) and angles (reported in deg) are as follows.
For 3: Ti1-N33, 1.689(2); Ti1-O38, 2.0235(18); Ti1-N2, 1.985-
(2); Ti1-N6, 1.980(2); N2-Ti1-N6, 95.50(9); Ti1-N33-Si34;
164.8(6). For 4: Ti1-N31, 1.693(6); Ti1-N10, 2.050(5); Ti1-
O36, 2.019(3); Ti1-P2, 2.6015(6); Ti1-P18, 2.5850(6); P2-Ti1-
P18, 144.70(2); Ti1-N31-Si32, 173.7(1).
We wondered if other common oxidants could also promote
trimethylsilyl abstraction. In view of that, when 1 equiv of
AgOTf was added to 1 or 2 in toluene, a mixture of compounds
3/5 or 4/6 were observed spectroscopically by 1H and 19F NMR
(and 31P NMR in the case of 4/6) in a 3:1 ratio. The observation
that the solid of 3 or 4 was marred by the content of 5 or 6
implied that Me3SiCl and Me3SiOTf should be also generated
Scheme 2. Synthesis of Complexes 3 and 4 from the Addition of 2 Equiv of AgOTf to 1 and 2, Respectively, and Proposed
Mechanisma
a The reaction involving 1 equiv of AgOTf and 1 or 2 is also displayed. The ratios of Me3SiCl and Me3SiOTf are approximate values and have
been standardized to 3:1, respectively.16

2728 Organometallics, Vol. 25, No. 11, 2006
Communications
equivalent of AgOTf would then rapidly convert, in the
appropriate ratios, Me3SiCl into Me3SiOTf and AgCl and 5 into
3 and AgCl (Scheme 2). Since these reactions are so rapid, anion
exchange of the triflate derivative 3 or 4 with Me3SiCl should
not be a competing pathway (vide supra).
In conclusion, we have shown that one-electron oxidation of
Ti(III) bis(trimethylsilyl)amide chloride complexes promotes
trimethylsilyl abstraction to afford Ti(IV) systems bearing the
terminal (trimethylsilyl)imide functionality. Discrepancies ob-
served between the reagents [FeCp2][OTf] and AgOTf manifest
clearly the distinctions between outer-sphere and inner-sphere
one-electron oxidants. We are currently exploring the chemistry
of these (trimethylsilyl)imide scaffolds.
Acknowledgment. This work is dedicated to the memory
of Prof. Henry Taube, whose work has and will continue to
inspire scientists studying electron transfer reactions. We thank
Indiana University, the Dreyfus Foundation, the Sloan Founda-
tion, and the NSF (Grant No. CHE-0348941) for financial
support of this research. B.C.B. acknowledges the Lubrizol
Foundation for a Graduate Fellowship.
Supporting Information Available: Text giving complete
experimental details and spectroscopic and analytical data for
complexes 1-6 and CIF files giving crystallographic data for 1-4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(16) Conversion of 1 to 3 generated a 3:1 ratio of Me3SiCl and Me3-
SiOTf, respectively, while the conversion of 2 to 4 generated the same
byproducts in an approximately 5.7:1 ratio. For the purposes of clarity we
generalize that Me3SiCl and Me3SiOTf are formed in approximately a 3:1
ratio.
(17) Addition of Me3SiCl (∼3 equiv) to 3 led only to 25% conversion
to 5 after ∼3 h.
OM060168E