NCN trianionic pincer ligand precursors: Synthesis of bimetallic, chelating diamide, and pincer group IV complexes

Article abstract of DOI:10.1021/ic100232s

This report details the synthesis of new NCN trianionic pincer ligand precursors and metalation reactions to form group (IV) complexes. N,N-[1,3-phenylenebis(methylene)]bis-2,6-diisopropylaniline [2,6- ⁱPrNCN]H₃ (8) was converted to

Full text of DOI:10.1021/ic100232s

Inorg. Chem. 2010, 49, 5143–5156 5143
DOI: 10.1021/ic100232s
NCN Trianionic Pincer Ligand Precursors: Synthesis of Bimetallic,
Chelating Diamide, and Pincer Group IV Complexes
€
Soumya Sarkar, Kevin P. McGowan, Jeffrey A. Culver, Adam R. Carlson, Jurgen Koller, Andrew J. Peloquin,
Melanie K. Veige, Khalil A. Abboud, and Adam S. Veige*
Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611
Received February 9, 2010
This report details the synthesis of new NCN trianionic pincer ligand precursors and metalation reactions to form group
(IV) complexes. N,N0-[1,3-phenylenebis(methylene)]bis-2,6-diisopropylaniline [2,6-iPrNCN]H3 (8) was converted to
the N,N0-substituted Si(IV), Sn(IV), Mg(II), and Zn(II) derivatives. [2,6-iPrNCHN](SiMe3)2 (9-Si) and [2,6-iPrNCHN]-
(SnMe3)2 (9-Sn) form by first treating 8 with MeLi followed by Me3MCl, where M=Si or Sn. Single crystal X-ray
experiments indicate 8, 9-Si, and 9-Sn have similar structural features in the solid state. [2,6-iPrNCHN](μ-MgCl
3
THF)2 (12) forms by treating 8 with MeMgCl, and its solid state structure revealed a bis-μ-MgCl bridging unit. The 1H
NMR spectrum of 12 reveals a dynamic process occurs in solution. A variable temperature 1H NMR experiment failed
to quench the dynamic process. {[2,6-iPrNCHN]Zn}2 (13) forms upon treating {[2,6-iPrNCHN]Li2}2 (10) with
anhydrous ZnCl2 and is a dimer in the solid state. Again, dynamic 1H NMR behavior is observed, and a mechanism is
provided to explain the apparent low symmetry of 13 in solution. Extension of the aliphatic arm of the NCN ligand
provides the new NCCCN pincer ligand precursors N,N0-(2,20-(1,3-phenylene)bis(ethane-2,1-diyl))bis(3,5-bis-
(trifluoromethyl)aniline) [3,5-CF3NCCCN]H3 (16) and [3,5-CF3NCCHCN](SiMe3)2 (17). A more rigid ligand architec-
ture was accessed by synthesis of the anthracene derived pincer ligand anthracene-1,8-diylbis(N-3,5-bistrifluor-
methylaniline) [3,5-CF3NCCanthCN]H3 (18). Treating {Zr(NMe2)4}2 with 2 equiv of 16 provides the dimer {(μ-3,5-
CF3NCCHCN)Zr(NMe2)3NHMe2}2 (19). Treating Hf(NMe2)4 with 18 provides the bimetallic complex (μ-3,5-
CF3NCCHanthCN){Hf(NMe2)3NHMe2}2 (20) in which one ligand bridges two Hf(IV) ions. Salt metathesis between 10
and ZrCl2(NMe2)2(THF)2 provides the mononuclear complex [2,6-iPrNCHN]Zr(NMe2)2 (21) in which the NCN
ligand is bound as a chelating diamide. Thermoysis of 21 does not lead to formation of a trianionic pincer complex.
Instead, treating HfCl4 with {[2,6-iPrNCN]Li3}2 (11) followed by MeLi provides the trianionic pincerate complex
[2,6-iPrNCNHfMe2][Li(DME)2] (23). In the solid state the Hf ion has distorted trigonal bipyramidal geometry.
1. Introduction
[(3,5-MeNCN)2Hf][Li(DME)]2 (2), in which two ligands
bind one metal.1
Trianionic pincers are a new and potentially general class
of ligand capable of supporting reactive metal species. Figure
1 depicts seven trianionic pincer(type) ligand architectures.
They constrain three anionic metal-ligand bonds to the
meridional planeand, thoughtheyoccupythree coordination
sites, they can only contribute a maximum of 10-12 electrons
(formal oxidation state method). Consequently, a broad
scope of coordinatively and electronically unsaturated metal
species is plausible. However, choice of ligand precursor and
metal substrate determine whether effective complexes form.
For example, during metalation of HfCl4(THF)2 with 2,6-
iPrNCN3- (A), an additional Cl- ligand adds to form the
pincerate complex [(2,6-iPrNCN)HfCl2][Li(DME)3] (1), and
the yield is low.1 Altering the N-aryl substituent to 3,5-
MeC6H3 results in the coordinately saturated complex
A more promising metalation approach begins with neu-
tral pincer precursors and MRx and M(NR2)x as substrates
to eliminate alkane and amine, respectively. Remarkably,
when metalating 3,300,5,500-tetra-tert-butyl-1,10:30,100-terphe-
nyl-2,200-diol (B; R,R0 =tBu) with Me3TaCl2, C-H σ-bond
metathesis occurs prior to the second Ta-Me alcoholysis.
This is a unique instance of high-valent metal σ-bond meta-
thesis within the same coordination sphere as a protic group.2
Zirconium and titanium complexes bearing dianionic bis-
phenolate versions of B with furan, thiophenes, and pyridine
central rings in place of benzene are precatalysts for propyl-
ene polymerization and oligomerization.3 Generation of a
(2) (a) Agapie, T.; Bercaw, J. E. Organometallics 2007, 26, 2957–2959.
(b) Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2008, 27, 6123–
6142.
(3) (a) Agapie, T.; Henling, L. M.; DiPasquale, A. G.; Rheingold, A. L.;
Bercaw, J. E. Organometallics 2008, 27, 6245–6256. (b) Golisz, S. R.; Bercaw,
J. E. Macromolecules 2009, 42, 8751–8762.
*To whom correspondence should be addressed. E-mail: veige@chem.ufl.edu.
(1) Koller, J.; Sarkar, S.; Abboud, K. A.; Veige, A. S. Organometallics
2007, 26, 5438–5441.
r
2010 American Chemical Society
Published on Web 04/29/2010
pubs.acs.org/IC

5144 Inorganic Chemistry, Vol. 49, No. 11, 2010
Sarkar et al.
Figure 1. Seven examples of trianionic pincer(type) ligand architectures.
single component catalyst is possible when the central ring is
benzene,3b but the activity is modest.4
Wolczanski et al. exploit 1,3-di-2-pyridyl-2-azaallyl
(smif)11 ligands to modulate the field strength of coordina-
tion compounds of the first row. Among the possible deri-
vatives, and with the best chance to generate a very strong
field complex, is the trianionic diarylazaallyl (D).12 However,
isolation of stable complexes bearing this form of the ligand
remain elusive. Evidence supports the intermediacy of an
Fe(III) complex, but H-atom abstraction occurs and the
ligand converts to the dianionic imine form. Reminiscent of
this, metalation reactions between the trilithio salt of A and
MClx substrates often result in brightly colored solutions at
-35 ꢀC, but upon warming form intractable mixtures with
stoichiometric formation of the neutral free ligand, a result of
H-atom abstraction. Trianionic ONO and NNN pincer-type
ligands (E-G) may be advantageous because they do not
require formation of a metal-carbon bond, hence metalation
is easier.13 Recent examples indicate these ligands are non-
innocent and are redox active.14
Metalating a multianionic ligand is challenging. To gen-
eralize this ligand-type, multiple trianionic pincer ligand
precursors are necessary to accommodate the diversity of
properties across the transition metal series. No trianionic
pincer ligand complexes exist beyond group 6. Within this
report we describe a new set of pincer ligand precursors. It is
anticipated these precursors will be useful for creating new
reactive fragments bearing a trianionic pincer ligand. In
addition, using zirconium and hafnium as the metal sub-
strate, herein are examples of metalations that result in
bimetallic species, a mononuclear complex with an unacti-
vated ligand C-H aryl bond, and a trianionic pincer com-
plex. The bimetallic species are a consequence of metalating
without prior activation of the aryl C-H bond of the pincer.
NH bonds are also stable in the presence of pincer high-
valent M-C bonds. Smooth aminolysis of Mo(NMe2)4 with
the 3,300-di-tert-butyl-1,10:30,100-terphenyl-2,200-diol (B: R,R0 =
tBu,H) results in formation of the amido-amine complex
[tBuOCO]MoNMe2(NHMe2)2 (3).5 Subsequent treatment of
3 with NaN3 results in the formation of the anionic Mo-nitride
dimer {[tBuOCO]MotN(NMe2)Na(DMF)}2 (4). Exemplify-
ing the potential reactivity of these pincer complexes, 4 swiftly
transfers its N-atom to acid chlorides (RCOCl, R=Me, Ph,
tBu) to form nitriles.6,7 Another metalation approach involves
double salt metathesis and aryl CH σ-bond metathesis.
t
For example, the dipotassium salt of B (R,R0 = Bu,H),
[tBuOCHO]K2(THF) (5), reacts with MeCrCl2(THF)3 to from
[tBuOCO]Cr(THF)3 (6), KCl, and CH4. [tBuOCO]Cr(THF)3
(6) is an active precatalyst for the aerobic oxidation of PPh3.8
In search for low-valent uranium species, Gambarotta et
al. used the 1,3-[2,5-iPr2PhNC(=CH)2]2C6H42- (7) dianionic
ligand9 precursor to form UIII, and UIV complexes.10 For
both UIII and UIV, the aryl-CH bond of the central ring
remains intact, and the ligands are dienamides. Somewhat
unexpectedly, upon reduction, the aryl-CH bond activates
creating the trianionic pincer form of the ligand (C), con-
comitant with solvent degradation. NaH or Li/naphthalene
reduction leads to formation of formally UI and UII com-
plexes, respectively, though DFT studies indicate extensive
π-back-donation, indicating the oxidation states are more
likely UIII and UIV.10
(4) [tBuOCHO]Zr(CH2Ph)2 is a single component catalysts for ethylene
polymerization (80 psi ethylene), 30 mg (0.053 mmol) [tBuOCHO]Zr(CH2Ph)2,
toluene (25 mL), triisobutylaluminum (10 equiv water scavenger), 24 h; yield
120 mg polyethylene. Kuppuswamy, S.; Veige, A. S. unpublished results.
(5) Sarkar, S.; Carlson, A. R.; Veige, M. K.; Falkowski, J. M.; Abboud,
K. A.; Veige, A. S. J. Am. Chem. Soc. 2008, 130, 1116–1117.
2. Experimental Section
General Considerations. Unless specified otherwise, all ma-
nipulations were performed under an inert atmosphere using
standard Schlenk or glovebox techniques. Pentane, hexanes,
toluene, diethyl ether, tetrahydrofuran (THF), and 1,2-dime-
thoxyethane were dried using a GlassContour drying column.
(6) Sarkar, S.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2008, 130,
16128–16129.
(7) (a) Bindl, M.; Stade, R.; Heilmann, E. K.; Picot, A.; Goddard, R.;
Furstner, A. J. Am. Chem. Soc. 2009, 131, 9468–9470. (b) Mendiratta, A.;
Cummins, C. C.; Kryatova, O. P.; Rybak-Akimova, E. V.; McDonough, J. E.;
Hoff, C. D. J. Am. Chem. Soc. 2006, 128, 4881–4891. (c) Figueroa, J. S.; Piro,
N. A.; Clough, C. R.; Cummins, C. C. J. Am. Chem. Soc. 2006, 128, 940–950. (d)
Figueroa, J. S.; Cummins, C. C. Dalton Trans. 2006, 2161–2168. (e) Curley, J. J.;
Sceats, E. L.; Cummins, C. C. J. Am. Chem. Soc. 2006, 128, 14036–14037. (f)
Mendiratta, A.; Cummins, C. C.; Kryatova, O. P.; Rybak-Akimova, E. V.;
McDonough, J. E.; Hoff, C. D. Inorg. Chem. 2003, 42, 8621–8623.
(8) O’Reilly, M.; Falkowski, J. M.; Ramachandran, V.; Pati, M.; Abboud,
K. A.; Dalal, N. S.; Gray, T. G.; Veige, A. S. Inorg. Chem. 2009, 48, 10901–10903.
(9) Nuckel, S.; Burger, P. Organometallics 2000, 19, 3305–3311.
(10) Korobkov, I.; Gorelsky, S.; Gambarotta, S. J. Am. Chem. Soc. 2009,
131, 10406–10420.
(11) (a) Frazier, B. A.; Wolczanski, P. T.; Lobkovsky, E. B. Inorg. Chem.
2009, 48, 11576–11585. (b) Frazier, B. A.; Wolczanski, P. T.; Lobkovsky, E. B.;
Cundari, T. R. J. Am. Chem. Soc. 2009, 131, 3428–3429.
(12) Volpe, E. C.; Wolczanski, P. T.; Lobkovsky, E. B. Organometallics
2010, 29, 364–377.
(13) Schrock, R. R.; Lee, J.; Liang, L. C.; Davis, W. M. Inorg. Chim. Acta
1998, 270, 353–362.
(14) (a) Nguyen, A. I.; Blackmore, K. J.; Carter, S. M.; Zarkesh, R. A.;
Heyduk, A. F. J. Am. Chem. Soc. 2009, 131, 3307–3316. (b) Zarkesh, R. A.;
Ziller, J. W.; Heyduk, A. F. Angew. Chem., Int. Ed. 2008, 47, 4715–4718.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5145
C6D6 and toluene-d8 (Cambridge Isotopes) were dried over
sodium-benzophenone ketyl, distilled or vacuum transferred,
and stored over molecular sieves. CDCl3 (Cambridge Isotopes)
was dried over anhydrous CaCl2, vacuum transferred, and
stored over molecular sieves. Sublimed {Zr(NMe2)4}2, Hf-
(NMe2)4, HfCl4, and ZrCl4 were purchased from Strem Chemi-
cals Co. and used without further purification. MeLi, 1.6 M in
diethyl ether (Et2O), LiAlH4 (95%), m-xylenedicyanide (99%),
and 2-bromomesitylene (99%) were purchased from Acros
and used as received. Anhydrous ZnCl2, Pd2(dba)3, 3,5-bis-
(trifluoromethyl)bromobenzene, MeMgCl (3.0 M in THF),
3,5-bis(trifluormethyl)aniline, sodium-tert-butoxide, chlorotri-
methylsilane (97%), and trimethyltin chloride (97%) were pur-
chased from Aldrich and used as received. rac-BINAP was
purchased from Fluka and used as received. [2,6-iPrNCN]H3
(8),15 {[2,6-iPrNCHN]Li2}2 (10),15 {[2,6-iPrNCN]Li3}2 (11),1
(75.36 Hz, C6D6) δ (ppm): 149.1 (s, C aromatic), 148.5 (s, C
aromatic), 142.3 (s, C aromatic), 132.7 (s, C aromatic), 129.6
(s, C aromatic), 125.7 (s, C aromatic), 124.3 (s, C aromatic), 61.6
(s, ArCH2N), 28.3 (s, CH(CH3)2), 25.7 (s, CH(CH3)2), 25.2 (s,
CH(CH3)2), -4.7 (s, Sn(CH3)3). Anal. Calcd for C38H60N2Sn2:
C, 58.34; H, 7.73; N, 3.58. Found: C, 58.49; H, 7.86; N, 3.54.
Synthesis of [2,6-iPrNCHN](μ-MgCl THF)2 (12). MeMgCl
3
(5.52 mL, 3.0 M, 16.6 mmol) in THF was added dropwise to
[2,6-iPrNCN]H3 (8) (3.69 g, 8.08 mmol) in THF (20 mL) with
stirring at -35 ꢀC. The reaction was warmed to room tempera-
ture and stirred for 1 h, and then the solution was concentrated
under reduced pressure. The product was precipitated as a white
powder by dropping the concentrated yellow THF solution into
cold diethyl ether (15 mL). The product was isolated by filtering
the solution and washing the residue with cold pentane. Yield
3.33 g (57.4%). 1H NMR (300 MHz, C6D6) δ (ppm): 8.19 (s, 1H,
Ar-H), 7.64 (d, J=6 Hz, 2H, Ar-H), 7.38 (t, J=8.8 Hz, 1H, Ar-
H), 7.19 (d, J=3 Hz, 2H, Ar-H), 7.09 (t, J=3 Hz, 4H, Ar-H),
4.44 (s, 4H, Ar-CH2N, overlapping), 4.42 (sept, J=6 Hz, 4H,
CH(CH3)2, overlapping), 3.33 (bs, 16H, OCH2CH2), 1.48 (d,
J=6 Hz, 12H, CH(CH3)2), 1.30 (d, J=6 Hz, 12H, CH(CH3)2),
1.12 (bs, 16H, OCH2CH2). 13C{1H} NMR (75.36 Hz, C6D6) δ
(ppm): 155.4 (s, C aromatic), 148.9 (s, C aromatic), 146.1 (s, C
aromatic), 130.5 (s, C aromatic), 130.0 (s, C aromatic), 128.9
(s, C aromatic), 123.9 (s, C aromatic), 122.8 (s, C aromatic), 69.5
(s, OCH2CH2), 63.0 (s, ArCH2N), 27.8 (s, CH(CH3)2), 27.0
(s, OCH2CH2), 25.5 (s, CH(CH3)2), 25.4 (s, CH(CH3)2). Anal.
Calcd for C40H58Cl2Mg2N2O2: C, 66.87; H, 8.14; N, 3.90.
Found: C, 66.57; H, 8.03; N, 3.85.
17
bis(bromomethylene)anthracene,16 and ZrCl2(NMe2)4(THF)2
were prepared according to literature procedures. NMR spectra
were obtained on Gemini (300 MHz), VXR (300 MHz), or
Mercury (300 MHz) spectrometers. Chemical shifts are reported
in δ (ppm). For 1H and 13C NMR spectra, the residual solvent
peak was referenced as an internal reference. Variable tempera-
ture NMR experiments were performed in toluene-d8. GC/MS
spectra were recorded on an Agilent 6210 TOF-MS instrument.
Combustion analyses were performed at Complete Analysis
Laboratory Inc., Parsippany, New Jersey.
Synthesis of [2,6-iPrNCHN](SiMe3)2 (9-Si). Chlorotrimethyl-
silane (122 mg, 1.12 mmol) in toluene (2 mL) was added
dropwise to {[2,6-iPrNCHN]Li2}2 (10) (250 g, 0.53 mmol) in
toluene (6 mL) with stirring at -35 ꢀC. As the solution warmed
to ambient temperature over 30 min, the color turned from pale
yellow to colorless. The solution was filtered through Celite and
concentrated under reduced pressure. White needles were obtai-
ned from the concentrated solution after 48 h. The crystalline
material was isolated by filtration and washed with cold pen-
tane. Yield 180 mg (56%). 1H NMR (300 MHz, C6D6) δ (ppm):
7.08 (t, J=6 Hz, 1H, Ar-H), 7.05 (s, 1H, Ar-H), 7.00 (s, 2H, Ar-
H), 6.98 (d, J=3 Hz, 2H, Ar-H), 6.90 (d, J=3 Hz, 4H, Ar-H),
6.05(bs, 1H, Ar-H), 3.99 (s, 4H, Ar-CH2N), 3.25 (sept, J=6 Hz,
4H, CH(CH3)2), 1.15 (d, J=6 Hz, 12H, CH(CH3)2), 0.83 (d, J=6
Hz, 12H, CH(CH3)2), 0.15 (s, 18H, -Si(CH3)3). 13C{1H} NMR
(75.36 Hz, C6D6) δ (ppm): 149.2 (s, C aromatic), 142.3 (s, C
aromatic), 140.1 (s, C aromatic), 132.7 (s, C aromatic), 129.3
(s, C aromatic), 126.6 (s, C aromatic), 124.5 (s, C aromatic), 56.0
(s, ArCH2N), 28.4 (s, CH(CH3)2), 25.6 (s, CH(CH3)2), 25.1
(s, CH(CH3)2), 1.2 (s, -Si(CH3)3). Anal. Calcd for C38H60N2Si2:
C, 75.93; H, 10.06; N, 4.66. Found: C, 76.10; H, 10.13; N, 4.86.
Synthesis of [2,6-iPrNCHN](SnMe3)2 (9-Sn). Trimethyl-
tinchloride (1.06 g, 5.33 mmol) in toluene (2 mL) was added
dropwise to {[2,6-iPrNCHN]Li2}2 (10) (1.19 g, 2.54 mmol) in
toluene (6 mL) with stirring at -35 ꢀC. The reaction was warmed
to room temperature and stirred for 20 h. The solution was
filtered through Celite and concentrated under reduced pres-
sure. White needles were obtained from the concentrated solu-
tion after 48 h. The product was isolated by filtration and the
solid washed with cold pentane. Yield 1.34 g (67.2%). 1H NMR
(300 MHz, C6D6) δ (ppm): 7.48 (s, 1H, Ar-H), 7.30-7.24 (m,
2H, Ar-H), 7.22-7.18 (m, 2H, Ar-H), 7.18-7.12 (m, 5H, Ar-H),
4.41 (s, 4H, Ar-CH2N), 3.79 (sept, J=6 Hz, 4H, CH(CH3)2),
1.32 (d, J = 6 Hz, 12H, CH(CH3)2), 1.24 (d, J = 9 Hz, 12H,
CH(CH3)2), 0.04 (t, J=27 Hz, 18H, Sn(CH3)3). 13C {1H} NMR
Synthesis of {[2,6-iPrNCHN]Zn}2 (13). A solution of {[2,6-
iPrNCHN]Li2}2 (10) (3.015 g, 6.434 mmol) in diethyl ether
(50 mL) was added to a solution of anhydrous ZnCl2 (877 mg,
6.434 mmol) in diethyl ether (15 mL) at -35 ꢀC with stirring. The
reaction was warmed to room temperature, stirred for 1 h, and a
white precipitate formed. The suspension was filtered to collect
the product which was dried in vacuo to remove all volatiles. The
solid was redissolved in chloroform (15 mL), and the solution
was filtered, reduced under vacuum, and added to pentane
(50 mL) with stirring to precipitate 13 as a white powder. Yield
2.276 g (2.188 mmol, 68%). X-ray quality crystals were obtained
by slow evaporation from a hot benzene solution. 1H NMR (300
MHz, C6D6) δ (ppm): 7.62 (d, 2H, J=9 Hz, Ar-H), 7.46 (t, 1H,
J=7.5 Hz, Ar-H), 7.28 (s, 1H, Ar-H), 7.20 (d, 4H, J=1.5 Hz, Ar-
H), 7.15 (t, 2H, J=7.5 Hz, Ar-H), 4.23 (s, 4H, Ar-CH2N), 3.91
(sept, J = 6 Hz, 4H, CH(CH3)2), 1.25 (d, J = 6 Hz, 24H,
CH(CH3)2). 13C{1H} NMR (75.36 Hz, C6D6) δ (ppm): 148.0
(s, C aromatic), 147.9 (s, C aromatic), 146.4 (s, C aromatic),
131.2 (s, C aromatic), 128.9 (s, C aromatic), 125.9 (s, C aroma-
tic), 125.3 (s, C aromatic), 124.4 (s, C aromatic), 61.4 (s,
ArCH2N), 28.3 (s, CH(CH3)2), 25.2 (s, CH(CH3)2). Anal. Calcd
for C64H84N4Zn2: C, 73.90; H, 8.14; N, 5.39. Found: C, 73.65;
H, 8.25; N, 5.43.
Synthesis of 2,20-(1,3-phenylene)diethanamine (15). An alter-
native synthesis of 15 was performed.18 Under argon flow
diethyl ether (500 mL) was added to LiAlH4 (60 g, 12.7 equiv,
1.58 mmol) under stirring, in a 1000 mL three-neck flask fitted
with a reflux condenser and a 500 mL dropping funnel. To the
dropping funnel was added m-xylene dicyanide (19.4 g, 0.124
mol) in diethyl ether (300 mL). The m-xylene dicyanide solution
was added dropwise under static argon over a period of 2 h with
vigorous stirring and then refluxed under argon for 48 h. The
resulting green suspension was cooled to 0 ꢀC and quenched by
adding water (100 mL) through the dropping funnel, followed
by NaOH solution (15% by wt, 100 mL). Extra diethyl ether was
added periodically. An additional 30 mL of water was added
to produce a free-flowing white suspension. Compound 15
was extracted from the white suspension with diethyl ether (6 ꢀ
(15) (a) Daniele, S.; Hitchcock, P. B.; Lappert, M. F.; Nile, T. A.;
Zdanski, C. M. J. Chem. Soc., Dalton Trans. 2002, 3980–3984. (b) Estler,
F.; Eickerling, G.; Herdtweck, E.; Anwander, R. Organometallics 2003, 22,
1212–1222.
(16) Ren, J.; Zhao, X. L.; Wang, Q. C.; Ku, C. F.; Qu, D. H.; Chang, C. P.;
Tian, H. Dyes Pigm. 2005, 64, 179–186.
(17) Brenner, S.; Kempe, R.; Arndt, P. Z. Anorg. Allg. Chem. 1995, 621,
2021–2024.
(18) Ruggli, P.; Prijs, B. Helv. Chim. Acta 1945, 28, 674–690.

5146 Inorganic Chemistry, Vol. 49, No. 11, 2010
Sarkar et al.
150 mL). The organics were combined and dried over Na2SO4.
Yellow oil was obtained by removing all volatiles under reduced
pressure, and was further purified by distillation at 170 ꢀC @ 20
mTorr. Yield 6.0 g (0.036 mol, 39%). 1H NMR (300 MHz, C6D6)
δ(ppm): 7.15 (t, J=7.30 Hz, 1H, Ar-H), 6.99 (s, 1H, Ar-H), 6.97
(s, 2H, Ar-H), 2.88 (t, J=6.86 Hz, 4H, -H2CH2CH2), 2.65 (t,
J=6.86 Hz, 4H, NH2CH2CH2Ar), 1.01 (br s, 4H, NH2).
aromatic), 130.5 (s, aromatic), 129.6 (s, aromatic), 129.1 (s, aroma-
tic), 127.0 (s, aromatic), 125.8 (s, aromatic), 119.1 (s, aromatic),
112.1 (s, aromatic), 111.0 (s, aromatic), 46.8 (s, CH2). HRMScalcu-
lated (found) for C32H20F12N2 (Mþ): 660.14 (660.1465).
Synthesis of {(μ-3,5-CF3NCCHCN)Zr(NMe2)3NHMe2}2 (19).
A solution of {Zr(NMe2)4}2 (91 mg, 0.170 mmol) in toluene
(1 mL) was added to 16 (200 mg, 0.340 mmol) in toluene (1 mL)
at -35 ꢀC with stirring. As the solution warmed to room tempe-
rature and stirred for 3 h, the color changed from pale yellow to
brown. The solution was filtered and concentrated in vacuo.
Crystallization occurred from a concentrated toluene solution
of 19 over 1 week. The crystals were filtered and washed with
cold pentane. Yield (222 mg, 38%). 1H NMR (300 MHz, C6D6)
δ(ppm): 7.37 (bs, 6H, Ar-H), 7.26 (s, 1H, Ar-H), 7.18 (t, J=6 Hz,
1H, Ar-H), 7.11 (d, J=6 Hz, 2H, Ar-H), 3.57 (m, 4H, ArNC-
H2CH2Ar), 2.81(s, 12H, N(CH3)2), 2.67 (m, 4H, ArNCH2-
CH2Ar), 1.36 (d, J=3 Hz, 6H, HN(CH3)2), 0.90 (sept, J=3
Hz, 2H, HN(CH3)2). 13C{1H} NMR (75.28 Hz, C6D6) δ(ppm):
154.9 (s, C, aromatic), 140.9 (s, C, aromatic), 133.3 (q, J=32.3
Hz, CF3), 130.2 (s, C, aromatic), 127.4 (s, C, aromatic), 126.8 (s,
C, aromatic), 123.2 (s, C, aromatic), 115.6 (m, CF3C), 110.3
(sept, J=5 Hz, CF3CCCCF3), 50.5 (s, Zr-N(CH3)2), 42.7 (s,
CCH2CH2), 38.8 (s, CCH2CH2), 35.3 (s, Zr-NHCH3)2). Anal.
Calcd for C64H74F24N10Zr2: C, 47.40; H, 4.60; N, 8.64. Found:
C, 47.07; H, 4.66; N, 8.45.
Synthesis of (μ-3,5-CF3NCCHanthCN){Hf(NMe2)3NHMe2}2
(20). Hf(NMe2)4 (53.6 mg, 0.151 mmol) was added to a solution
of [3,5-CF3NCCanthCN]H3 (18) (50.0 mg, 0.076 mmol) in ben-
zene (1 mL), and the resulting mixture was stirred for 10 min.
Stirring was then stopped, and the reaction mixture was allowed
to stand at room temperature for 1 h, during which a precipitate
formed. The solvent was decanted, and the solid product dried in
vacuo to yield 20 as a pale yellow solid (91 mg, 87%). 1H NMR
(300 MHz, C6D6) δ (ppm): 9.18 (s, 1H, Ar-H), 8.29 (s, 1H, Ar-
H), 7.74 (d, J=6 Hz, 2H, Ar-H), 7.63 (d, J=6 Hz, 2H, Ar-H),
7.47 (s, 4H, CCHCCF3), 7.30 (s, 2H, Ar-H, overlapping), 7.27
(dd, 2H, J=6 Hz, J=6 Hz, CHCHCH), 5.58 (s, 4H, Ar-CH2N),
2.75 (s, 32H, Hf-N(CH3)2), 1.69 (d, J=6 Hz, 12H, Hf-NH-
(CH3)2), 0.84 (sept, J = 6 Hz, 2H, Hf-NH(CH3)2). 13C{1H}
NMR (75.36 Hz, C6D6) δ (ppm): 158.9 (s, C aromatic), 136.2 (s,
C aromatic), 132.8 (s, C aromatic), 132.2 (q, J=31.6 Hz, CF3),
132.1 (s, C aromatic), 130.9 (s, C aromatic), 129.8 (s, C aro-
matic), 128.2 (s, C aromatic), 126.0 (s, C aromatic), 124.5 (s, C
aromatic), 116.6 (s, C aromatic), 115.3 (s, C aromatic), 107.8 (s,
ArCF3), 52.4 (s, ArCH2N), 42.3 (s, Hf-N(CH3)2), 39.0 (s, Hf-
NH(CH3)2). Anal. Calcd for C48H68F12N10Hf2 C, 42.08; H,
5.00; N, 10.22. Found: C, 41.89; H, 4.96; N, 10.29.
Synthesis of [2,6-iPrNCHN]Zr(NMe2)2 (21). ZrCl2(NMe2)4-
(THF)2 (168 mg, 0.426 mmol) in Et2O (3 mL) was added to
{[2,6-iPrNCHN]Li2}2 (10) (200 mg, 0.426 mmol) in Et2O (3 mL)
at -35 ꢀC with stirring. As the solution was warmed to room
temperature and stirred for 1 h, the color changed to pale yellow
with the formation of a white precipitate. The solution was
filtered, and the resulting pale yellow filtrate was concentrated in
vacuo. Pentane (1 mL) was added to the solution, and the
product was obtained as yellow crystals over a period of 3 days
at -35 ꢀC. Yield (114 mg, 42%). 1H NMR (300 MHz, C6D6) δ
(ppm): 9.34 (s, 1H, Ar-H), 7.20 (m, 4H, Ar-H), 7.12 (m, 3H, Ar-
H), 6.93 (d, 2H, J = 6 Hz, Ar-H), 4.83 (d, 2H, J = 15 Hz,
Ar-CH2N), 4.01 (d, 2H, J=15 Hz, Ar-CH2N), 3.93 (sept, 2H,
J=9 Hz, CH(CH3)2), 3.70 (sept, 2H, J=9 Hz, CH(CH3)2), 2.50
(s, 6H, N(CH3)2), 2.06 (s, 6H, N(CH3)2), 1.42 (d, 6H, J=9 Hz,
CH(CH3)2), 1.41 (d, 6H, J=9 Hz, CH(CH3)2), 1.35 (d, 6H, J=9
Hz, CH(CH3)2), 1.25 (d, 6H, J = 9 Hz, CH(CH3)2). 13C{1H}
NMR (75.36 Hz, C6D6) δ(ppm): 150.3 (s, C aromatic), 146.7 (s,
C aromatic), 146.3 (s, C aromatic), 145.1 (s, C aromatic), 133.7
(s, C aromatic), 127.0 (s, C aromatic), 125.4 (s, C aromatic),
124.6 (s, C aromatic), 124.2 (s, C aromatic), 63.7 (s, ArCH2N),
44.0 (s, N(CH3)2), 41.4 (s, N(CH3)2), 29.3 (s, CH(CH3)2), 27.8 (s,
Synthesis of N,N0-(2,20-(1,3-phenylene)bis(ethane-2,1-diyl))-
bis(3,5-bis(trifluoromethyl)aniline) [3,5-CF3NCCCN]H3 (16). To
a 100 mL round-bottom flask charged with a stir bar and toluene
(50 mL) were added 15 (1.50 g, 9.15 mmol), 3,5-bis(trifluoro-
methyl)bromobenzene (5.37 g, 2 equiv, 18.3 mmol), Pd2(dba)3
(0.13 g, 0.5%, 0.142 mmol), rac-BINAP (0.23 g, 1.5%, 0.357
mmol), and NaOtBu (2.64 g, 27.5 mmol). After refluxing for 72 h
under argon the solution was filtered through Celite while hot,
and all volatiles were removed in vacuo. Nonvolatile products
were then taken up in hot pentane and filtered again through
Celite. The final product was recrystallized two times from
1
pentane at -20 ꢀC. Yield 2.1 g (3.57 mmol, 39%). H NMR
(300 MHz, C6D6) δ(ppm): 7.22 (s, 2H, Ar-H), 7.11 (t, J=7.64 Hz,
1H, Ar-H), 6.81 (dd, J=7.64, 1.70 Hz, 2H, Ar-H), 6.72 (s, 1H,
Ar-H), 6.48 (s, 4H, Ar-H), 3.13 (t, J=5.52 Hz, 2H, NH), 2.73 (dt,
J = 6.94 Hz, 4H, NHCH2CH2), 2.38 (t, J = 6.94 Hz, 4H,
NHCH2CH2Ar). 13C{1H} NMR (75.36 Hz, C6D6, δ): 149.2 (s,
ArCNH), 139.8 (s, ArCCH2-), 133.0 (q, JC-F=32.7 Hz, -CF3),
129.8 (s, aromatic), 127.6 (s, aromatic), 126.5 (s, aromatic), 122.9
(s, aromatic), 112.3 (s, aromatic), 110.4 (s, aromatic), 44.6 (s,
NHCH2CH2Ar), 35.5 (s, NHCH2CH2Ar). HRMS calculated
(found) for C26H20F12N2 (MþH)þ: 589.1508 (589.1537).
Synthesis of [3,5-CF3NCCHCN](SiMe3)2 (17). To a solution
of THF (2 mL) containing 16 (1.01 g, 1.72 mmol) and a stirbar,
MeMgCl (1.3 mL 3 M, 3.25 mmol) in THF (2 mL) was added
dropwise at -35 ꢀC. The solution was stirred at ambient
temperature for 3 h. Chlorotrimethylsilane (610 mg, 5.65 mmol)
was added at -35 ꢀC. The solution was kept at -35 ꢀC for 1 h
then stirred at ambient temperature for 15 h. 1,4-Dioxane
(2 mL) was then added causing precipitation of MgCl2. The
solution was filtered, and volatiles were removed in vacuo
causing crystallization of the product. Yield 931 mg (1.27 mmol,
74%). 1H NMR (300 MHz, C6D6) δ(ppm): 7.40 (s, 2H, Ar-H),
7.35 (s, 4H, Ar-H), 7.02 (t, J=7.6 Hz, 1H, Ar-H), 6.77 (dd, J=
2.0, 7.61 Hz, 2H, Ar-H), 6.68 (s, 1H, Ar H), 3.23 (t, J=7.5 Hz,
4H, ArNCH2), 2.48 (t, J=7.3 Hz, 4H, ArCH2CH2), 0.00 (s, 18H
(15H observed), Si(CH3)3). 13C{1H} NMR (75.36 Hz, C6D6)
δ(ppm): 150.8 (s, aromatic), 140.1 (s, aromatic), 132.8 (q,
J = 32.2 Hz, CF3), 129.9 (s, aromatic), 129.6 (s, aromatic),
129.6 (s, aromatic), 122.9 (s, aromatic), 120.8 (s, aromatic),
118.7 (s, aromatic), 112.3 (s, aromatic), 49.0 (s, CH2NAr), 35.6
(s, -CH2Ar), 0.7 (s, Si(CH3)3). Anal. Calcd for C32H36F12N2Si2:
C, 52.45; H, 4.95; N, 3.82. Found: C, 52.47; H, 4.82; N,3.69.
Synthesis of [3,5-CF3NCCanthCN]H3 (18). To an anhydrous
N,N-dimethylformamide (DMF, 15 mL) solution of bis-
(bromomethylene)anthracene (2.81 g, 7.72 mmol) and K2CO3
was added 3,5-bis(trifluormethyl)aniline (2.65 mL, 17.0 mmol).
The mixture was stirred for 15 h, and then water (50 mL) was
added to form a yellow oil layer. The yellow oil was extracted
with CHCl3 (2 ꢀ 30 mL), washed with water, brine, dried with
MgSO4, and then filtered. After removing all volatiles, an oil
formed with some yellow solid. The oil was dissolved in hexanes
(50 mL), and the yellow solid (bis(bromomethylene)anthracene
starting material) was removed by filtration. The oil contained
18, aniline, and DMF which was separated by flash column
chromatography (100 g SiO2, 1:1 CHCl3:pentane) to provide
pure 18 as a pale yellow solid. Yield 2.36 g (3.57 mmol, 46%). 1H
NMR (300 MHz, C6D6, δ): 8.31 (s, 1H, Ar-H), 8.23 (s, 1H, Ar-
H), 7.77 (d, J=6 Hz, 2H, Ar-H), 7.40-7.00 (m, 6H, Ar-H), 6.49 (s,
4H, o-CHCF3), 4.00 (d, J=3.0 Hz, 4H, ArNCH2), 3.23 (t, J=3.0
Hz, 2H, NH). 13C{1H} NMR (75.36 Hz, C6D6) δ(ppm): 149.1 (s,
i-C-NH), 133.6 (s, aromatic), 133.1 (q, JCF=33.1 Hz, CF3), 132.7 (s,

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5147
CH(CH3)2), 27.3 (s, CH(CH3)2), 26.9 (s, CH(CH3)2), 25.8 (s,
CH(CH3)2), 25.0 (s, CH(CH3)2). Anal. Calcd for C36H54N4Zr:
C, 68.19; H, 8.58; N, 8.84. Found: C, 68.08; H, 8.64; N, 8.76.
Synthesis of [2,6-iPrNCNHfMe2][Li(DME)2] (23). HfCl4 (202
mg, 0.630 mmol) in THF (3 mL) was added to {[2,6-iPrN-
CN]Li3}2 (11) (300 mg, 0.316 mmol) in THF (3 mL) at -35 ꢀC
with stirring. As the solution was warmed to room temperature
and stirred for 45 min, the color changed from colorless to pale
yellow. MeLi (0.78 mL, 1.6 M, 1.26 mmol) in Et2O was added to
the reaction mixture, and the stirring continued for an addi-
tional 30 min. The solution was filtered, and all volatiles were
removed in vacuo. The residue was triturated and washed with
pentane, and the remaining solid was extracted into DME (2 ꢀ
3 mL). The organic layers were combined, filtered, and the
product was obtained as a pale yellow powder after removing
the solvent in vacuo. Yield (189 mg, 35%). Single crystals for
X-ray diffraction were obtained by diffusing Et2O into a satu-
rated DME solution of 23 at -35 ꢀC for 4 days. 1H NMR (300
MHz, CDCl3) δ(ppm): 7.31-7.19 (m, 9 H, Ar-H), 5.21 (s, 4H,
CH2N), 4.26 (sept, 4H, J = 6 Hz, CH(CH3)2), 3.66 (s, 12H,
CH3OCH2CH2O CH3), 3.51 (s, 18H, CH3OCH2CH2OCH3),
1.41 (d, 12H, J = 3 Hz, CH(CH3)2), 1.39 (d, 12H, J = 3 Hz,
CH(CH3)2), -0.49 (s, 6H, Hf-(CH3)2). 13C{1H} NMR (75.36
Hz, CDCl3) δ (ppm): 205.6 (s, Hf-C aromatic), 155.6 (s, C
aromatic), 154.3 (s, C aromatic), 148.4 (s, C aromatic), 124.6 (s,
C aromatic), 123.0 (s, C aromatic), 122.6 (s, C aromatic), 117.4
(s, C aromatic), 72.5 (s, ArCH2N), 71.0 (s, CH3OCH2CH2O
CH3), 59.6 (s, CH3OCH2CH2OCH3), 50.6 (s, Hf-CH3), 28.8 (s,
CH(CH3)2), 27.0 (s, CH(CH3)2), 24.5 (s, CH(CH3)2). Anal.
Calcd for C42H67HfLiN2O4: C, 59.39; H, 7.95; N, 3.30. Found:
C, 59.17; H, 7.92; N, 3.08.
3. Results and Discussion
Ligand Precursor Synthesis and Characterization. [2,6-
iPrNCHN](SiMe3)2 (9-Si), [2,6-iPrNCHN](SnMe3)2 (9-
Sn). Two independent reports15 detail the synthesis of N,
N0-[1,3-phenylenebis(methylene)]bis(2,6-diisopropylani-
line); the parent isopropyl derivative [2,6-iPrNCN]H3 (8)
as either a tacky oil or a crystalline solid. Free from impuri-
ties, 8 is crystalline, and large colorless crystals (2-3 cm)
seed from solvent-free viscous oils of the pure substance.
Figure 2A depicts the molecular structure of 8, and Table 1
lists crystallographic data. Characterization data of 8 (1H
and 13C NMR spectra) matches the reported values.15
[2,6-iPrNCHN](SiMe3)2 (9-Si) and [2,6-iPrNCHN](Sn-
Me3)2 (9-Sn) form by treating {[2,6-iPrNCHN]Li2}2 (10)15
with Me3SiCl and Me3SnCl, respectively (eq 1). Compounds
9-Si and 9-Sn possess similar spectroscopic and solid-state
properties. The 1H NMR (C6D6) spectrum of both 9-Si and
9-Sn display distinct resonances for the M(CH3)3 groups, at
0.15 ppm and 0.04 ppm, respectively. The methylene protons
on each derivative appear as singlets at 3.99 (9-Si) and 4.41
ppm (9-Sn), and each display two distinct isopropyl methyl
resonances and a corresponding methine septet (9-Si, sept,
3.25 ppm, J=6.0 Hz; 9-Sn, sept, 3.79 ppm, J=6 Hz).
Figure 2. Ortep drawings of the molecular structure of (A) [2,6-iPrN-
CN]H3 (8), (B) [2,6-iPrNCHN](SiMe3)2 (9-Si), and (C) [2,6-iPrNCHN]-
(SnMe3)2 (9-Sn) with ellipsoids presented at the 50% probability level and
hydrogen atoms removed for clarity.
contains a complete list of bond lengths and angles. The
asymmetric unit of 9-Si consists of half a molecule,
whereas 9-Sn contains the entire molecule. The key
features that distinguish the two compounds are the
orientation of the MMe3 and the N-M bond distances.
In 9-Si, both SiMe3 groups orient toward one side of the
molecule and possess a N-Si bond length of 1.7333(12)
˚
A. For 9-Sn, the SnMe3 groups are anti, and the N-Sn
˚
bond length is appropriately longer at 2.0525(17) A. In
compounds 8, 9-Si, and 9-Sn, the spatial location of the
pendant arms is dependent on the N-substituent and the
N-M bond length. In [2,6-iPrNCN]H3 (8) the centroid
Figure 2B and 2C depict the solid-state molecular
structures of 9-Si and 9-Sn, Table 1 lists pertinent crystal-
lographic information, and the Supporting Information
i
˚
of the Pr-aryl rings separate by approximately 6.35 A,
in [2,6-iPrNCHN](SiMe3)2 (9-Si) by 9.58 A, and in
˚

5148 Inorganic Chemistry, Vol. 49, No. 11, 2010
Sarkar et al.
Table 1. X-ray Crystallographic Structure Parameters and Refinement Data
8
9-Si
9-Sn
12
13
16
empirical formula
formula weight
crystal system
space group
C32H44N2
456.69
triclinic
P1
C19H30NSi
300.53
C38H60N2Sn2
782.26
C40H58Cl2Mg2N2O2 C64H84N4Zn2
718.40
C26H20F12N2
588.44
1040.09
monoclinic
P2(1)/c
monoclinic
C2/c
monoclinic
P2(1)/c
monoclinic
P21/n
triclinic
P1
crystal dimensions (mm) 0.20 ꢀ 0.18 ꢀ 0.05 0.26 ꢀ 0.15 ꢀ 0.09 0.15 ꢀ 0.13 ꢀ 0.12 0.15 ꢀ 0.11 ꢀ 0.07
0.31 ꢀ 0.26 ꢀ 0.20 0.11 ꢀ 0.08 ꢀ 0.05
˚
a (A)
10.7730(15)
11.3453(16)
13.5349(19)
112.047(2)
90.730(2)
113.013(2)
1386.4(3)
2
17.3753(2)
8.7733(5)
24.6156(14)
90
13.0126(6)
20.1984(10)
14.9236(7)
90
14.1942(13)
19.6408(18)
16.2155(15)
90
43.169(7)
14.132(2)
19.314(3)
90
11.9159(14)
14.003(2)
16.6156(18)
75.343(2)
71.727(2)
74.212(2)
2490.3(5)
4
˚
b (A)
˚
c (A)
R (deg)
β(deg)
γ (deg)
volume (A )
˚
Z (A)
92.947(2)
90
101.7060(10)
90
113.689(2)
90
90.577(3)
90
3
˚
3747.0(4)
8
0.121
3840.8(3)
4
1.326
4139.7(7)
4
0.221
11782(3)
8
1.855
absorption
coeff (mm-1
F (000)
calcd (g/cm3)
˚
0.063
0.156
)
500
1.094
1320
1.065
1608
1.353
1544
1.153
4448
1.173
1192
1.570
D
γ (Mo KR) (A)
Temperature (K)
0.71073
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
100(2)
0.71073
173(2)
θ range (deg)
2.09 to 27.49
95.6%
9182
1.66 to 27.49
98.0%
11752
1.60 to 27.50
99.6%
25592
1.62 to 27.50
98.7
26976
0.94 to 27.50
99.4%
84071
1.31 to 22.50
99.8%
10886
completeness to θmax
reflections collected
indep reflections [Rint
]
6079 [0.0503]
6079/0/315
R1=0.0489,
wR2=0.1213
[4187]
4227 [0.0317]
4227/0/191
R1=0.0414,
wR2=0.1041
[3473]
8782 [0.0404]
8782/0/379
R1=0.0251,
wR2=0.0616
[7781]
9382 [0.1544]
9382/0/430
R1=0.0669,
wR2=0.1601
[4900]
26885 [0.0361]
26885/0/1293
R1=0.0378,
wR2=0.0912
[20900]
6488 [0.1286]
6488/0/890
R1=0.0633,
wR2=0.1387
[3149]
data/restraints/param
final R1 indices
[I > 2σ(I)]
R indices
(all data)
R1=0.0757,
wR2=0.01347
0.235/-0.209
R1=0.0530,
wR2=0.1117
0.300/-0.253
R1=0.0302,
wR2=0.0636
0.590/-0.398
R1=0.1429,
wR2=0.1943
0.391/-0.415
R1=0.0553,
wR2=0.1000
0.909/ -0.544
R1=0.1350,
wR2=0.1701
0.310/-0.263
largest diff peak/
-3
˚
hole e.A
goodness
of fit on F2
1.042
1.022
1.025
1.004
1.015
0.896
i
˚
[2,6- PrNCHN](SnMe3)2 (9-Sn) by 10.2 A. Though ori-
ented differently in the solid-state, 1H NMR spectroscopy
indicates the -CH2NMC6H3(iPr)2 groups freely rotate in
solution.
[2,6-iPrNCHN](μ-MgCl THF)2 (12) and {[2,6-iPrNC-
3
HN]Zn}2 (13). Unlike the smooth synthesis of {[2,6-iPrN-
CN]Li3}2 (11)1 with MeLi, refluxing [2,6-iPrNCN]H3 (8)
in toluene with 3 equiv of MeMgCl only provides intrac-
table mixtures. The dimer of compound 11 contains a
compact hexalithio core with no solvent molecules.1
However, addition of 2 equiv of MeMgCl to 8 in
THF results in formation of [2,6-iPrNCHN](μ-MgCl
THF)2 (12) in 89% yield as a microcrystalline powder
(eq 2).
3
Figure 3. Thermal ellipsoid (50% probability) drawing of the molecular
structure of [2,6-iPrNCHN](μ-MgCl THF)2 (12) with hydrogen atoms
removed for clarity. Coordinated THF molecules are disordered and their
3
˚
atoms (except O2) are refined isotropically. Selected bond lengths (A)
and angles (deg): Mg1-N1, 1.947(3); Mg1-O1, 2.004(5); Mg1-Cl1,
2.4162(14); Mg1-Cl2, 2.3911(14); Mg2-N2, 1.941(3); Mg2-O2,
2.015(2); Mg2-Cl1, 2.3939(14); Mg2-Cl2, 2.3851(13); Mg1-Cl1-Mg2,
94.58(5); Cl1-Mg2-Cl2, 92.78(5); Mg2-Cl2-Mg1, 85.32(5); Cl2-
Mg1-Cl1, 92.07(5).
Figure 3 presents the results of a single-crystal X-ray
diffraction experiment for 12, and Table 1 lists crystal-
lographic information. The salt contains a Mg2Cl2 core.
Each Mg(II) ion completes its coordination sphere with a
THF and one amido connection to the ligand, which
chelates the core. Within the series of (9-Si), (9-Sn), (10),
and (12), compound 12 is the only derivative to contain a
single NCN fragment that chelates the cations. The
bridging chlorides are the obvious cause for this observa-
tion, providing a stable diamond core.
The ambient temperature 1H NMR (C6D6) spectrum of
12 is misleading and is not representative of a chelating
NCN fragment. The methylene protons on the NCN
arms are equivalent and appear as a broad singlet at
4.44 ppm, which overlaps with the methine septet. The
isopropyl methyls appear as two doublets (J=6.0 Hz) at
1.48 and 1.30 ppm. The solid-state structure indicates the
methylene protons must be diastereotopic, and each

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5149
Figure 4. (A) Ortep drawings of the molecular structure of {[2,6-iPrNCHN]Zn}2 (13) with ellipsoids drawn at the 50% probability level and hydrogens
removed for clarity. (B) Truncated view of 13 highlighting the two-coordinate Zn ions and the 16-membered Z-shaped metallocyle. Selected bond lengths
˚
(A) and angles (deg): Zn1a-N1a, 1.8100(15); Zn1a-N3a, 1.8119(15); Zn2a-N2a, 1.8161(15); Zn2a-N4a, 1.8141(15); N1a-Zn1a-N3a, 164.65(7);
N4a-Zn2a-N2a, 166.64(7).
Scheme 1
halves reside on inversion centers. The Zn(II) ions are two
coordinate creating a nearly linear geometry between two
ligand molecules (N-Zn-Navg=164.86(7)ꢀ). The closest
˚
contacts are all greater than 3 A from the Zn ions.
Structural details of two coordinate Zn-amido complexes
are scarce19 owing to a preference to dimerize.20 Com-
pound 13 is unique because it combines two, two coordi-
nate Zn(II) ions within the same compound. The average
isopropyl methyl is unique, thus fluxional processes must
equilibrate them. Rotation around the N-aryl bond
explains the observation of two doublets instead of four
for the iPr methyls. However, to equilibrate the methylene
protons the arene ring of the ligand must flip between two
equivalent conformers (Scheme 1). Variable temperature
(25 to -60 ꢀC) 1H NMR (toluene-d8) experiments failed
to quench these dynamic processes. The unlikely alter-
native mechanism requires cleavage of two μ-Mg-Cl
bonds, followed by rotation around the N-CH2 bond,
and reattachment.
˚
Zn-N bond length is 1.8116(5) A, which is much shorter
than the sum of covalent radii (2.13 A) and consistent
˚
with other short Zn-N bonds which involve Npπ to Znpπ
19e
˚
bonding in Zn[N(SiMePh2)2]2 (1.824(14) A),
Zn[N-
(SiMe3)(SiPh2 Bu)]2 (1.853(2) and 1.858(2) A), Zn[N-
t
19a
˚
19a
˚
(SiMe3)(Ad)] (1.827(2) and 1.828(14) A),
Zn[N(Si-
t
19d
˚
˚
Me3)( Bu)]2 (1.82 A), and Zn[N(SiMe3)2]2 (1.82 A, by
gas phase electron diffraction).19f Clearly, the short bond
distance is due to Npπ to Znpπ bonding. To maximize
π-donation the local environment of each N-atom twists
along the N-Zn-N axis, creating dihedral angles of 57ꢀ
and 55ꢀ between planes created by the three N-atom
substituents.
Anhydrous ZnCl2 reacts with {[2,6-iPrNCHN]Li2}2
(10)15 to provide {[2,6-iPrNCHN]Zn}2 (13) in 68% yield
(eq 3). A single crystal X-ray diffraction experiment con-
firms the bimetallic composition of 13 and reveals each
Zn ion is two-coordinate. Figure 4A displays the mole-
cular structure of 13, and a truncated picture highlighting
the 16-member metallocycle and its Z-shape conforma-
tion (Figure 4B).
Figure 5 is a space-filling representation of 13 and
indicates the Zn ions are protected by the arene rings
and the isopropyl groups. However, 13 reacts instanta-
neously with H2O to form [2,6-iPrNCN]H3 (8)15 and
(19) (a) Tang, Y. J.; Felix, A. M.; Boro, B. J.; Zakharov, L. N.; Rheingold,
A. L.; Kemp, R. A. Polyhedron 2005, 24, 1093–1100. (b) Schumann, H.;
Gottfriedsen, J.; Dechert, S.; Girgsdies, F. Z. Anorg. Allg. Chem. 2000, 626, 747–
758. (c) Gaul, D. A.; Just, O.; Rees, W. S. Inorg. Chem. 2000, 39, 5648–5654. (d)
Rees, W. S.; Green, D. M.; Hesse, W. Polyhedron 1992, 11, 1697–1699. (e)
Power, P. P.; Ruhlandtsenge, K.; Shoner, S. C. Inorg. Chem. 1991, 30, 5013–
5015. (f) Haaland, A.; Hedberg, K.; Power, P. P. Inorg. Chem. 1984, 23, 1972–
1975. (g) Burger, H.; Sawodny, W.; Wannagat, U. J. Organomet. Chem. 1965, 3,
113–&.
(20) (a) Armstrong, D. R.; Forbes, G. C.; Mulvey, R. E.; Clegg, W.;
Tooke, D. M. J. Chem. Soc., Dalton Trans. 2002, 1656–1661. (b) Just, O.;
Gaul, D. A.; Rees, W. S. Polyhedron 2001, 20, 815–821. (c) Putzer, M. A.;
Dashti-Mommertz, A.; Neumuller, B.; Dehnicke, K. Z. Anorg. Allg. Chem.
1998, 624, 263–266. (d) Schumann, H.; Gottfriedsen, J.; Girgsdies, F. Z. Anorg.
Allg. Chem. 1997, 623, 1881–1884.
Crystallographic data is found in Table 1, and the
Supporting Information contains a complete list of bond
lengths and angles. The asymmetric unit consists of two
crystallographically independent bimetallic units in
which one dimer resides in a general position, and two

5150 Inorganic Chemistry, Vol. 49, No. 11, 2010
Sarkar et al.
Scheme 2
colorless microcrystalline powder after two recrystallizations
(eq 4).23 A 1H NMR (C6D6) spectrum of 16 reveals two sets
of resonances for the methylene protons as a doublet of tri-
plets at 2.73 ppm (J=6.0 Hz and J=6.0 Hz; -CH2CH2NH-)
and a triplet at 2.38 ppm (J=6.0 Hz; -CH2CH2NH-). The
NH proton resonates as a triplet (J=6.0 Hz) at 3.13 ppm.
Figure 5. Space-filling drawing of the molecular structure of {[2,6-iPrN-
CHN]Zn}2 (13).
Zn(OH)2, implying easy access to the Zn ions. Compound
13 exhibits an ambient temperature 1H NMR(C6D6) spec-
trum consistent with the formula [2,6-iPrNCHN]Zn, but
not the solid-state structure. Broad singlets appear at 1.24
and 4.23 ppm for the isopropyl methyl and methylene
protons, respectively. The methine protons appear as a
broadened septet at 3.91 ppm. The solid-state structure
indicates the methylene protons should be diastereotopic.
Single crystals grow from a saturated Et2O solution of 16
at -35 ꢀC, and Figure 6A depicts its solid-state molecular
structure determined from an X-ray diffraction experiment.
Crystallographic data is found in Table 1, and the Support-
ing Information contains a complete list of bond lengths and
angles. The asymmetric unit consists of two crystallogra-
phically unique molecules that connect through a triple
arene π-stacking interaction. Two of the trifluoroaryl
groups from one molecule wrap around a single arene group
of the other. Figure 6B depicts the π-stacking relationship
and highlights the approximately 3.84 A and 3.78 A separa-
tions between the centroids of the arene planes. To accom-
modate the trifuoro groups, the central ring staggers bet-
ween the two outer rings creating torsion angles between
CF3 groups of 37ꢀ and 44ꢀ; however, the two outer rings are
nearly eclipsed (—C24-C15-C21-C23 ≈ 7ꢀ). Presumably
the π-stacking and triflouro groups impart significant crys-
tallinity to 16 because the mesityl derivative [MesNCCCN]-
H3 is a viscous oil.24
Previous success at metalating the NCN trilithio salt
{[2,6-iPrNCN]Li3}2 (11)1 with group 4 ions prompted
attempts to synthesize {[3,5-CF3NCCCN]Li3}2. However,
deprotonation with RLi (R = tBu, Bu, Me) results in
lithiation of multiple sites, including the methylene
groups. As a consequence, other metalation strategies
were sought. Treating 16 with MeMgCl followed by
excess Me3SiCl provides [3,5-CF3NCCHCN](SiMe3)2
(17) in 74% yield as a crystalline white solid (eq 5).
1
A variable temperature H NMR (toluene-d8) experi-
ment indicates a fluxional process occurs, and at -60 ꢀC
the isopropyl methyl protons resolve into two doublets at
1.04 and 1.34 ppm. The coalescence temperature occurs at
-10 ꢀC corresponding to an activation energy of ΔGq=
12.6(3) kcal/mol. The resolution of the isopropyl groups
into two distinct signals is due to restricted rotation
around the N-aryl bond. However, even at -60 ꢀC a
rapid dynamic process still occurs; the methylene groups
still appear as a singlet. Low solubility in hydrocarbon
and aromatic solvent prevents analysis at temperatures
below -60 ꢀC. Scheme 2 depicts a plausible mechanism
for the methylene group coalescence. Compound 13 can
adopt either open or closed conformations. When closed,
the compound is C2h-symmetric, providing two distinct
environments for the methylene protons. When open, the
symmetry is D2h, and the protons become equivalent.
[3,5-CF3NCCCN]H3 (16) and [3,5-CF3NCCHCN](SiMe3)2
(17). We sought a two-carbon arm pincer ligand precursor.
Raney-Nickel catalyzed reduction of 2,20-benzene-1,3-diyl-
diacetonitrile (14) (90 atm H2, NH3 saturated MeOH, 90 ꢀC)
provides 2,20-benzene-1,3-diyldiethanamine (15) in 80%
yield.18 For convenience, the reduction may be accomplished
with LiAlH4 in Et2O,21 but provides 15 in only 39% yield
after purification by vacuum distillation. Cross-coupling22
between 15 and 3,5-trifluoromethylbromobenzene produces
N,N0-(1,3-phenylenebis(methylene))bis(3,5-bis(trifluoromet-
hyl)aniline) [3,5-CF3NCCCN]H3 (16) in 33% yield as a
˚
˚
(21) Cummins, C. C.; Beachy, M. D.; Schrock, R. R.; Vale, M. G.;
Sankaran, V.; Cohen, R. E. Chem. Mater. 1991, 3, 1153–1163.
A 1H NMR (C6D6) spectrum of 17 confirms the Me3Si-
substitution. Removal of the N-H proton reduces the
(22) (a) Guram, A. S.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 7901–
7902. (b) Paul, F.; Patt, J.; Hartwig, J. F. J. Am. Chem. Soc. 1994, 116, 5969–
5970. (c) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1348–1350. (d) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995,
36, 3609–3612. (e) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118,
7217–7218. (f) Wolfe, J. P.; Rennels, R. A.; Buchwald, S. L. Tetrahedron 1996,
52, 7525–7546. (g) Wolfe, J. P.; Wagaw, S.; Marcoux, J. F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805–818.
(23) (a) Wolfe, J. P.; Buchwald, S. L. J. Org. Chem. 2000, 65, 1144–1157.
(b) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118, 7215–
7216.
(24) Carlson, A. R. MSc. Thesis, University of Florida, Gainesville, FL,
2007.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5151
Figure 6. (A) Ortep drawing of the molecular structure of [3,5-CF3NCCCN]H3 (16) with ellipsoids presented at the 50% probability level. (B) Truncated
drawing of 16 highlighting the π-stacking between two molecules in the asymmetric unit.
doublet of triplets in 16 to a triplet for 17 at 3.23 ppm
(J=6.0 Hz) and the Me3Si protons resonate at 0.00 ppm.
The 13C{1H} NMR (C6D6) spectrum is complementary
and reveals a resonance at 0.7 ppm for the (CH3)3Si-
carbons and quartet at 132.8 ppm for the CF3 groups
(q, J = 32.2 Hz). Additional aliphatic (35.6 ppm (s, -
CH2-Ar), 49.0 ppm (s, -CH2N)) and aromatic reso-
nances (between 112.3 and 150.8 ppm) complete the
assignment.
[2,6-iPrNCN]H3 (8) and [3,5-CF3NCCCN]H3 (16) with
MCl4 or MCl4(THF)2 (M=Ti, Zr, and Hf) and pyridine
(as an HCl sponge) does not result in M-ligand bond
formation. McConville et al. metalated TiCl4 with the
disilyamide ligand precursor N1,N3-bis(2,6-diisopropyl-
phenyl)-N1,N3-bis(trimethylsiliyl)propane-1,3-diamine in
refluxing toluene.26 (9-Si and 17) and (9-Sn) ligand pre-
cursors offer a chance to first chelate through the amido
linkages via Me3SiCl and Me3SnCl elimination.27 Sub-
sequent arene C-H bond activation would provide a
trianionic ligand. However, refluxing 9-Si, 9-Sn, or 17
in the presence of MCl4 salts either provides intractable
mixtures or no reaction (eq 7).
[3,5-CF3NCCanthCN]H3 (18). A more rigid ligand is the
anthracene derivative [3,5-CF3NCCanthCN]H3 (18). Pale
yellow 18 forms by combining bis(bromomethylene)-
anthracene,16,25 3,5-triflouroaniline, and K2CO3 and
stirring for 12 h in DMF (eq 6). Purification by flash
column chromatography (1:1 CHCl3:pentane) provides
18 in 46% yield. 1H, 13C NMR (C6D6) spectroscopy and
1
mass spectrometry confirm the identity of 18. The H
NMR spectrum reveals a distinct doublet for the CH2
group at 4.01 ppm (J=5.0 Hz) which couples to the NH
proton, appearing at 3.24 ppm (J=5.0 Hz). Other note-
worthy resonances include a singlet for the four o-CH
protons at 6.47 ppm and two singlets at 8.32 and 8.24 ppm
for the opposing CH protons on the central arene ring
of the anthracene. Appropriate 13C NMR resonances
appear for the aromatic carbons of 18, but distinct signals
appear for the CH2 and CF3 carbons at 46.8 ppm
and 110.9 ppm, respectively, the latter being slightly
broadened.
{(μ-3,5-CF3NCCHCN)Zr(NMe2)2NHMe2}2 (19). Dir-
ect metalation with the parent diamine ligands is the most
convenient method to produce trianionic pincer ligand
complexes. Reasonable transition metal substrates are
homoleptic metal-amido and metal-alkyl complexes.
This method works with Mo(NMe2)4 and the terphenyl
diphenolate ligand, but the terphenyl framework seems to
be key, and this is not a general reaction. For instance,
Lappert et al. report the synthesis of the dinuclear
(25) (a) Akiyama, S.; Misumi, S.; Nakagawa, M. Bull. Chem. Soc. Jpn.
1960, 33, 1293–1298. (b) House, H. O.; Koepsell, D.; Jaeger, W. J. Org. Chem.
1973, 38, 1167–1173.
(26) Scollard, J. D.; McConville, D. H.; Vittal, J. J.; Payne, N. C. J. Mol.
Catal. A: Chem. 1998, 128, 201–214.
(27) Morrison, D. L.; Rodgers, P. M.; Chao, Y. W.; Bruck, M. A.;
Grittini, C.; Tajima, T. L.; Alexander, S. J.; Rheingold, A. L.; Wigley, D. E.
Organometallics 1995, 14, 2435–2446.
Synthesis and Characterization of Group 4 NCN Com-
plexes. Direct metalation of the parent NCN ligands

5152 Inorganic Chemistry, Vol. 49, No. 11, 2010
Sarkar et al.
Figure 7. (A) Ortep drawing of the molecular structure of {(μ-3,5-CF3NCCHCN)Zr(NMe2)2NHMe2}2 (19) with ellipsoids presented at the 50%
probability level and hydrogen and fluorine atoms removed for clarity. (B) Truncated drawing of 19 highlighting the trigonal bipyramidal Zr core. Selected
˚
bond lengths (A) and angles (deg): Zr-N1, 2.137(2); Zr-N2_i, 2.212(2); Zr-N3, 2.461(3); Zr-N4, 2.036(3); Zr-N5, 2.037(3); C28-N3-C27, 109.4(3);
N3-Zr-N2_i = 174.00(10); N4-Zr-N5, 114.44(11); N4-ZR-N1, 122.10(10); N5-Zr-N1, 121.45(10).
complex [μ-2,6-iPrNCHN]{Zr(NMe2)3}2 from [2,6-iPrN-
CN]H3 (8) and {Zr(NMe2)4}2.15a
The obvious difference between the dinuclear species
produced from [2,6-iPrNCN]H3 (8) and [3,5-CF3NC-
CCN]H3 (16) and {Zr(NMe2)4}2 is one contains a single
bridging ligand and one contains two. The more flexible,
extended-arm ligand comfortably spans two metals cen-
ters, though neither produces a trianionic ligand bound
complex. An attempt to activate the aryl C-H bond in the
central ring by refluxing 19 in toluene results in no
detectable reaction.
(μ-3,5-CF3NCCHanthCN){Hf(NMe2)3NHMe2}2 (20). If
a too-flexible ligand provides dinuclear species, then the
more rigid anthracene derivative [3,5-CF3NCCanthCN]H3
(18) may promote chelation. Another possibility is the
dinuclear composition of metal-precursor {Zr(NMe2)4}2
is the cause of dimer formation, not the ligand. However,
even the presumably more rigid 18 produces a dinuclear
complex, albeit with only one bridging ligand, from the
mononuclear metal source Hf(NMe2)4. Treatment of 18
with 2 equiv of Hf(NMe2)4 in benzene results in the
formation of pale yellow (μ-3,5-CF3 NCCHanthCN){Hf-
(NMe2)3NHMe2}2 (20) (eq 9). The reaction is complete
within 10 min at room temperature, but 20 precipitates
from the reaction medium after 1 h. Using 1 equiv of
Hf(NMe2)4 only results in reduced conversion. A combi-
Using the extended-arm ligand precursor 16 and
{Zr(NMe2)4}2 provides a similar result. The dimer {(μ-
3,5-CF3NCCHCN)Zr(NMe2)2NHMe2}2 (19) forms by
treating {Zr(NMe2)4}2 with 16 in toluene first at -35 ꢀC
followed by stirring at ambient temperature for 3 h
(eq 8). Figure 7 displays the results of an X-ray diffraction
experiment performed on single-crystals of 19 grown
from a saturated benzene solution. Table 2 lists refine-
ment data. The complex consists of two trigonal bipyr-
amidal Zr(IV) ions bridged by two NCCCN ligands. Each
Zr contains four amido connections; two from the brid-
ging NCN ligand and two from NMe2 groups. A fifth site,
(axial) contains one HNMe2 ligand produced in the
aminolysis reaction. The asymmetric unit contains half
the dimer and a benzene molecule. Because of disorder
within the CF3 groups, each F-atom requires refinement
over three positions to total site occupancy of one. The
1
nation of H and 13C NMR (C6D6) spectroscopy, com-
bustion analysis, and an X-ray diffraction study confirms
the identity of 20.
˚
Zr-N bond lengths span a difference of 0.4 A. The
˚
longest Zr-N bond length (d(Zr-N3) = 2.461(3) A) is
appropriate for the Zr-amine connection, and the two
˚
shortest are from the Zr-NMe2’s (d(Zr-N4)=2.036(3) A
˚
and d(Zr-N5)=2.037(3) A). Intermediate (d(Zr-N1)=
˚
˚
2.137(2) A and d(Zr-N2_i)=2.212(2) A) bond lengths
form with the NCN amido connection and may be due to
the slightly less π-basic character of the aryl-amide versus
alkyl-amide.
Single crystals amenable to an X-ray diffraction experi-
ment grow by pentane diffusion into a diethyl ether

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5153
Table 2. X-ray Crystallographic Structure Parameters and Refinement Data
19
20
21
23
empirical formula
formula weight
crystal system
C76H86F24N10Zr2
1777.99
triclinic
P1
0.19 ꢀ 0.19 ꢀ 0.06
9.1226(5)
12.3882(7)
18.5955(10)
77.615(1)
79.183(1)
88.269(1)
2016.04(19)
1
C48H68F12N10Hf2
1370.10
monoclinic
P21/c
0.17 ꢀ 0.04 ꢀ 0.04
17.3997(11)
20.3795(12)
15.9524(10)
90
93.829(1)
90
6544.0(6)
4
3.757
2712
1.612
0.71073
173(2)
C36H54N4Zr
634.05
triclinic
C46H77Hf LiN2O6
939.53
monoclinic
P2(1)/n
0.30 ꢀ 0.17 ꢀ 0.13
11.2841(13)
20.515(2)
20.8(2)
space group
crystal dimensions (mm)
P1
0.26 ꢀ 0.25 ꢀ 0.09
8.2682(14)
12.2632(16)
18.800(4)
107.72(2)
91.20(2)
108.229(19)
1710.4(5)
2
0.231
676
1.231
0.71073
173(2)
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
90
90.893(2)
90
4833(1)
4
2.203
1960
1.291
0.71073
100(2)
3
˚
volume (A )
˚
Z (A)
absorption coeff (mm-1
F (000)
)
0.362
908
1.464
0.71073
173(2)
Dcalcd (g/cm3)
˚
γ (Mo KR) (A)
temperature (K)
θ range (deg)
1.68 to 27.50
94.9%
13079
1.17 to 27.50
99.8%
36722
1.15 to 27.50
94.9%
12504
1.95 to 27.50
100.0%
68129
completeness to θmax
reflections collected
indep reflections [Rint
data/restraints/param
final R1 indices
[I > 2σ(I)]
]
8784 [0.0386]
8784/4/543
R1 = 0.0537,
wR2 = 0.1333 [7414]
R1 = 0.0638,
wR2 = 0.1398
0.902/-0.778
1.048
12966 [0.0962]
12966/4/697
R1 = 0.0469,
wR2 = 0.583 [5882]
R1 = 0.1412,
wR2 = 0.0736
1.643/-1.058
0.821
7445 [0.0148]
7445/0/374
R1 = 0.0337,
wR2 = 0.0950 [7081]
R1 = 0.0353,
wR2 = 0.0966
0.768/-0.732
1.043
11108[0.0223]
11108/0/521
R1 = 0.0182,
wR2 = 0.0443[10208]
R1 = 0.0211,
wR2 = 0.0464
1.116/-0.7631.131
1.027
R indices (all data)
-3
˚
largest diff peak/hole e A
goodness of fit on F2
˚
the dimethylamides and ligand amides are 2.017(14) A
˚
and 2.187(7) A, respectively, and the Hf-NHMe2
˚
bond length is longer at 2.428(8) A. Bond angles
around the hafnium atom deviate slightly from the
ideal trigonal bipyramidal values, with the average
N-Hf-N angle for the equatorial dimethylamides
being 119.2(5)ꢀ. The average N-Hf-N angle along
the axis between the NCN amide and the dimethyla-
mine is 178.1(3)ꢀ.
1
The H NMR spectrum of 20 agrees with the solid-
state analysis. One singlet at 2.59 ppm26 appears for all
the equatorial amido NMe’s. The proton on the axial
NHMe2 ligand does not exchange, at least on the NMR
time scale, with the other amido ligands. A distinct septet
appears at 0.68 ppm (J=6.3 Hz) for the N-H and the
corresponding doublet (N-Me) appears at 1.52 ppm.
Singlets for the central arene C-H’s resonate at 9.02
and 8.13 ppm. The proximity of hafnium to the C14
proton (see Figure 8) must deshield it downfield to
9.02 ppm. The 13C{1H} NMR spectrum of 20 further
supports its assignment and matches the solid and
solution state analysis. Thirteen aromatic resonances
appear between 107.8 and 159.5 ppm, but the key sig-
nals resonate at 52.4, 42.3, and 39.3 ppm for the
ArCH2N, Hf-N(CH3)2 and Hf-NH(CH3)2 carbons,
respectively.
Figure 8. Ortep drawing of the molecular structure of (μ-3,5-
CF3NCCHanthCN){Hf(NMe2)3NHMe2}2 (20) with ellipsoids presen-
ted at the 50% probability level and hydrogen atoms removed for
˚
clarity. Selected bond lengths (A) and angles (deg): Hf1-N2, 2.188(5);
Hf1-N3, 1.990(5); Hf1-N4, 2.032(6); Hf1-N5, 2.019(5); Hf2-N1,
2.185(5); Hf2-N6, 2.419(5); Hf2-N7, 1.992(5); Hf2-N8, 2.037(6);
Hf2-N9, 2.032(6); Hf2-N10, 2.436(6); C2-C1-N1, 118.0(5);
C12-C16-N2, 118.6(5); N2-Hf1-N6, 178.83(19); N1-Hf2-N10,
177.4(2).
[2,6-iPrNCHN]Zr(NMe2)2 (21). The metalation exam-
ples above all result in dinuclear complexes, and they all
rely on aminolysis to attach the M-N bonds. Cleary,
once one NCN M-N bond forms, the second arm
reaches for a second metal. Reversible aminolysis will
exacerbate this problem, especially with the more flexible
ligands. An irreversible salt metathesis to first form the
M-N bond prevents dimer formation. Treating Zr-
solution of 20. Figure 8 depicts the molecular structure of
20, and Table 2 lists crystal and refinement data. The
geometry of each hafnium ion is trigonal bipyramidal,
and the overall symmetry of the compound is C2-
symmetric. One dimethylamine occupies the site trans
to the NCN amide, and three dimethylamides occupy
three equatorial sites. Average Hf-N bond lengths for
(NMe2)2Cl2(THF)2 with {[2,6-iPrNCHN]Li2}2 (10)15a
17

5154 Inorganic Chemistry, Vol. 49, No. 11, 2010
Sarkar et al.
interaction is evident by the short distance between the two
˚
1
atoms (d(Zr1-C29)=2.6561(19) A). The H NMR (C6D6)
spectrum of 21 provides additional information about the
Zr-Cipso interaction and matches the Cs symmetry in the
solid state. The CHipso proton appears well downfield at
9.34 ppm. More importantly, unlike in 12 above, the arene
interaction is not fluxional. Four distinct doublets (J=6.0
Hz) resonate at 1.42, 1.41, 1.35, and 1.24 ppm for each
unique isopropoyl methyl. The corresponding septets for
the methine CH’s appear at 3.92 and 3.70 ppm. The NMe2
ligands must freely rotate around the Zr-N bond because
two singlets appear at 2.49 and 2.06 ppm, instead of four.
Despite the seemingly activated and position of the aryl-
CH bond on the backbone, prolonged thermolysis of 21 in
1
C6D6 at 90 ꢀC and periodic examination by H NMR
spectroscopy indicates no reaction occurs (eq 11).
Figure 9. Ortep drawing of the molecular structure of [2,6-iPrNCHN]-
Zr(NMe)2 (21) with ellipsoids presented at the 50% probability level and
˚
hydrogenatoms removed for clarity. Selectedbond lengths (A) and angles
(deg): Zr1-N1, 2.1440(17); Zr1-N2, 2.1797(16); Zr1-N3, 2.0182(17);
Zr1-N4, 2.0465(17); Zr1-C29, 2.6561(18); N1-Zr1-N2, 128.94(6);
N1-Zr1-N4, 105.97(7); N2-Zr1-N4, 110.31(7); N3-Zr1-C29,
149.90(6); N1-Zr1-N3, 102.88(7); N2-Zr1-N3, 99.10; N3-Zr1-N4,
107.54(7); N4-Zr1-C29, 102.53(7).
The equivalent activation is facile at ambient tempera-
ture for Mo(NMe)2 and the OCO ligand derivative to
form the bisamine complex [tBuOCO]MoNMe2(NHMe2)2
(3).5,6 The anticipated NCN trianionic complex 22 may
form, but the reaction is a reversible one in which the equi-
librium heavily favors 21. Addition of an equivalent of
pyridine to potentially stabilize 22 did not lead to a reaction
at 90 ꢀC.
provides the mononuclear complex [2,6-iPrNCHN]Zr-
(NMe2)2 (21) in 42% yield as a yellow microcrystalline
powder (eq 10).
[2,6-iPrNCNHfMe2][Li(DME)2] (23). Poor yields re-
sult when {[2,6-iPrNCN]Li3}2 (11) and HfCl4(THF)2
combine to form [(2,6-iPrNCN)HfCl2][Li(DME)3] (1).1
Obvious from the studies above, leaving the central aryl-
CH proton in place prior to metalation results in bimetal-
lic species with a bridging ligand. Part of the problem with
the synthesis of 1 is salt separation during purification. A
remedy is to alkylate and create a more soluble complex.
Thus, generating 1 in situ and adding 2 equiv of MeLi
provides colorless [(2,6-iPrNCN)HfMe2][Li(DME)2] (23)
in 35% yield (eq 12).
Single crystals grow at -35 ꢀC from saturated pentane/
ether solutions of 21. Figure 9 depicts the results from an
X-ray diffraction experiments performed on 21, and
Table 2 lists crystal and refinement data. [2,6-iPrNCH-
N]Zr(NMe2)2 (21) is pseudo Cs-symmetric with a highly
distorted trigonal bipyramidal Zr center and a chelating
diamido ligand.26,28 The equatorial positions comprise
the NCN amidos and a NMe2 ligand, whereas the axis
comprises a second NMe2 and an agostic Zr-Cispo bond.
The axis is significantly bent from linearity (—N3-Zr1-
C29=149.90(6)ꢀ) and the equatorial angle between the
NCN amidos is splayed (—N1-Zr1-N2=128.94(6)ꢀ).
Consequently, two smaller angles form between the remain-
ing equatorial positions (—N1-Zr1-N4=105.97(7)ꢀ and
N2-Zr1-N4=110.31(7)ꢀ). Clear indication of a Zr-Cipso
Exhibiting C2v-symmetry in solution, the 1H NMR
(CDCl3) spectrum of 23 reveals a singlet for the methyl
protons at -0.49 ppm, and two doublets and one methine
appear at 1.41, 1.39, and 4.26 ppm, respectively, for the
iPr groups. The furthest downfield signal in the 13C{1H}
NMR (CDCl3) spectrum of 23 is at 205.7 ppm and attri-
butable to the Cipso-Hf carbon. The corresponding Cipso in
the dichloride complex 1 resonates slightly upfield at
201.5 ppm, and for [(3,5-MeNCN)2Hf][Li2(DME)2] (2) it
appears at 204.3 ppm.1 Symmetric Hf-CH3 carbons ap-
pear at 50.6 ppm, and the CH2N carbon resonates at 72.5
ppm. Signals attributable to two equivalent DME mole-
cules bound to Liþ appear at 71.0 (CH3OCH2CH2OCH3)
(28) (a) Burger, H.; Wiegel, K.; Thewalt, U.; Schomburg, D. J. Organo-
met. Chem. 1975, 87, 301–309. (b) Brauer, D. J.; Burger, H.; Wiegel, K.
J. Organomet. Chem. 1978, 150, 215–231. (c) Burger, H.; Beiersdorf, D. Z.
Anorg. Allg. Chem. 1979, 459, 111–118. (d) Burger, H.; Geschwandtner, W.;
Liewald, G. R. J. Organomet. Chem. 1983, 259, 145–156. (e) Herrmann, W. A.;
Denk, M.; Albach, R. W.; Behm, J.; Herdtweck, E. Chem. Ber. 1991, 124, 683–
689. (f) Guerin, F.; Mcconville, D. H.; Vittal, J. J. Organometallics 1995, 14,
3154–3156. (g) Scollard, J. D.; Mcconville, D. H.; Vittal, J. J. Organometallics
1995, 14, 5478–5480. (h) Guerin, F.; McConville, D. H.; Payne, N. C.
Organometallics 1996, 15, 5085–5089. (i) Guerin, F.; McConville, D. H.; Vittal,
J. J. Organometallics 1996, 15, 5586–5590. (j) Scollard, J. D.; McConville, D. H.;
Vittal, J. J. Organometallics 1997, 16, 4415–4420.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 5155
Figure 10. (A) Ortep drawing of the molecular structure of [2,6-iPrNCNHfCl2][Li(DME)3] (23) with ellipsoids presented at the 50% probability level and
˚
hydrogen atoms removed for clarity. (B) Alternative perspective of 23 without the cation. Selected bond lengths (A) and angles (deg): Hf1-N1, 2.1318(14);
Hf1-N2, 2.1251(13); Hf1-C1, 2.2191(16); Hf1-C33, 2.2555(18); Hf1-C34, 2.2310(19); N1-Hf1-N2, 143.89(5); C1-Hf1-C33, 131.72(7);
C1-Hf1-C34, 120.518(7); C33-Hf1-C34, 108.09(8).
and 59.6 (CH3OCH2CH2OCH3), which are shifted from
free DME in CDCl3 (71.8 and 59.08).
Complementary to the solution-state characterization,
a single crystal X-ray diffraction experiment confirms the
trianionic form of the ligand on 23 and its C2v solid-state
symmetry.29 Figure 10 depicts two different perspectives
of the molecular structure of 23, Table 2 lists crystal-
lographic refinement data, and a full list of bond lengths
and angles appears in the Supporting Information. The
Figure 11. Structural motifs observed for diamido NCN transmetala-
tion reagents.
The different arrangements are as follows: (I) one ligand
binds two independent metal ions, (II) one ligand binds two
metal ions bridged by mutual counterions, and (III) two
ligands bind two independent metals ions. None of the pre-
cursors 9-Si, 9-Sn, 12, 13, or 17 undergo transmetalation to
form a trianionic pincer complex. However, it is conceivable
these reagents will find use as M-diamido precursors. For
instance, {[2,6-iPrNCHN]Li2}2 (10)15a reacts smoothly with
ZrCl2(NMe2)2(THF)2 to form [2,6-iPrNCHN]Zr(NMe2)2
21, but the ligand functions as a diamide with an intact
aromatic C-H bond. Two new NCN derivatives were pre-
pared with the intention of probing metalation reactions
with both a more rigid and more flexible framework than 8.
As such, the flexible extended arm [3,5-CF3NCCCN]H3 (16)
and the rigid anthracene [3,5-CF3NCCanthCN]H3 (18) pre-
cursors were prepared and characterized. Metalation with
M(NMe2)4 (M = Hf, and Zr) results in the bimetallic
complexes {(μ-3,5-CF3NCCHCN)Zr(NMe2)3NHMe2}2 (19)
and (μ-3,5-CF3NCCHanthCN){Hf(NMe2)3NHMe2}2 (20).
The more flexible ligand adopts the structural motif III
(Figure 11), and the “rigid” anthracene version only allows
one ligand to bind two metal ions, akin to type I (Figure 11).
From this study it appears that metalation without prior
activation of the central aromatic C-H bond predisposes the
formation of bimetallics. In addition, ligand precursors
possessing a flexible CH2 group in the pincer arm are more
prone to bimetallic species, considering the OCO derivatives
(B; Figure 1) readily chelate to form trianionic pincer
complexes.2,3b,5,6,8 Removing the backbone C-H bond prior
to metalation can provide trianionic pincer complexes, but
geometry of the hafnium ion deviates from trigonal
bipyramidal along the N-Hf-N axis. Instead of 180ꢀ,
the five-atom metallocycle creates a N-Hf-N angle of
143.89(5)ꢀ.
In 23, the Hf ion rests within the trigonal plane,1,30
and the methyl groups create a 108.09(8)ꢀ angle between
them, but are not symmetrically displaced. Hf1-C33
˚
˚
(2.2555(4) A) is 0.0245(19) A longer than Hf1-C34
˚
(2.2310(19) A), and an 11.54(9)ꢀ difference results bet-
ween C1-Hf1-C33 (131.72(7)ꢀ) and C1-Hf1-C34
(120.18(7)ꢀ). No obvious electronic rationale explains
this observation, and it must be due to the closer
proximity of the Li(DME)3 counterion complex to
C34 in the lattice. The closest distances between
˚
C34-Li and C33-Li are approximately 4.9 and 6.0 A,
respectively.
Conclusions
Preparation of NCN ligand transmetalation reagents is
straightforward. N,N0-[1,3-phenylenebis(methylene)]bis-2,6-
diisopropylaniline [2,6-iPrNCN]H3 (8) was converted to the
N,N0-substituted Si(IV), Sn(IV), Mg(II), and Zn(II) deriva-
tives. Figure 11 depicts three different structural motifs that
occur depending on the metal ion chosen for transmetalation.
(29) As a single crystal the Li ion is coordinated by three DME solvent
molecules, but routine isolation of 23 provides a composition with only two
DME.
(30) [(2,6-iPrNCN)HfCl2][Li(DME)3] (1) is distorted square pyramidal,
˚
and the Hf ion sits 0.4 A above the basal plane.

5156 Inorganic Chemistry, Vol. 49, No. 11, 2010
Sarkar et al.
access to trilithio or other trialkali salts of a variety of NCN
derivatives is not likely. An attempt to form a trilithio salt of
16 results in lithiation of the CH2 group and additional
aromatic sites. Ultimately the trianionic pincer complex
[2,6-iPrNCNHfMe2][Li(DME)2] (23) forms when {[2,6-iPrN-
CN]Li3}2 (11) reacts with HfCl4(THF)2 in THF followed by
alkylation with MeLi. By alkylating, the yield and scale
improves over the previous synthesis of the dichloride com-
plex [(2,6-iPrNCN)HfCl2][Li(DME)3] (1) because 23 is in-
soluble in Et2O.1 Complex 23 precipitates cleanly from DME
upon slow addition of Et2O, whereas 1 remains dissolved,
complicating isolation. We continue to search for conditions
that yield trianionic pincer complexes and are concentrating
our efforts by starting with the parent neutral ligands and
M-alkyl substrates to eliminate alkanes as the thermody-
namic impetus.
Acknowledgment. A.S.V. thanks UF, the ACS-PRF-
(G) (#44063-G3), NSF CAREER (CHE-0748408), and
the Camille and Henry Dreyfus Foundation for financial
support. K.P.M. and A.R.C. are UF Alumni Fellows. K.
A.A. thanks the NSF (CHE-0821346) and UF for fund-
ing the purchase of X-ray equipment.
Supporting Information Available: Text describing experimen-
tal procedures, analytical, spectroscopic, crystallographic data,
tables of bond lengths and angles, and cif files. This material is
available free of charge via the Internet at http://pubs.acs.org.