Synthesis and characterization of a new thiophene-bridged diamidophosphine [NPN] Donor set and its coordination chemistry with zirconium(IV): Unexpected deprotonation-lithiation sequence with a mesitylaminothiophene precursor

Article abstract of DOI:10.1021/om900612n

The synthesis and characterization of the thiophene-bridged diamidophosphine proligand [NPN]^sH₂ (2) (where [NPN] ^sH₂ = {[N-(2,4,6-Me₃C₆H ₂)(3-NH-SC₄H₂-2-)]<

Full text of DOI:10.1021/om900612n

Organometallics 2009, 28, 5253–5260 5253
DOI: 10.1021/om900612n
Synthesis and Characterization of a New Thiophene-Bridged
Diamidophosphine [NPN] Donor Set and Its Coordination Chemistry with
Zirconium(IV): Unexpected Deprotonation-Lithiation Sequence with a
Mesitylaminothiophene Precursor
ꢀ
Gabriel Menard, Howard Jong, and Michael D. Fryzuk*
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC,
Canada V6T 1Z1
Received July 13, 2009
The synthesis and characterization of the thiophene-bridged diamidophosphine proligand
[NPN]SH2 (2) (where [NPN]SH2={[N-(2,4,6-Me3C6H2)(3-NH-SC4H2-2-)]2PPh}) along with several
[NPN]SZrX2 (X = NMe2, Cl, I) complexes are presented. The ligand precursor [NPN]SH2 was
prepared from N-(2,4,6-Me3C6H2)(3-NH-4-Br-SC4H2) (1), tBuLi, and PhPCl2 in Et2O in 50% yield;
the stereochemistry of the ligand precursor was unexpected and likely resulted from a competitive
bromine-lithium exchange and deprotonation sequence with the thiophene starting material 1 and
tBuLi. A mechanism to rationalize the observed stereochemistry of the product is proposed following
deuteration experiments. [NPN]SZr(NMe2)2 (8) can be prepared in 80% yield by the direct reaction of
[NPN]SH2 and Zr(NMe2)4 in toluene. Both [NPN]SZrCl2 (9) and [NPN]SZrI2 (10) are prepared in
high yield by the reaction of 8 with excess Me3SiCl or Me3SiI, respectively.
Introduction
below; in A we have CH2SiMe2 linkers6 that build off of our
earlier ligand design work, and we have also initiated more
robust systems that involve o-phenylene connectors,4 as shown
in B, which are common motifs in recent ligand designs.14,15
Our interest in tailoring the coordination environment
around metal centers to make them suitable for dinitrogen
activation has been focused on combinations of amido
(NR2) and phosphine (PR3) donors arranged in multidentate
arrays.1-11 In particular, we have achieved consider-
able success with a tridentate combination involving two
amido donors flanking a central phosphine unit, which
we term NPN.1,4,6,12,13 We have examined two modifica-
tions that differ in the linker groups as shown generically
*Corresponding author. E-mail: fryzuk@chem.ubc.ca.
(1) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.;
Mason, S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960–3973.
(2) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science
1997, 275, 1445–1447.
(3) MacLachlan, E. A.; Fryzuk, M. D. Organometallics 2006, 25,
1530–1543.
(4) MacLachlan, E. A.; Hess, F. M.; Patrick, B. O.; Fryzuk, M. D. J.
As both of these systems (A and B) have shown success
in their ability to support N2 activation, we have conti-
nued to examine changes in the linker to monitor what
effect this might have on the stability and reactivity of the
resultant metal complexes. It is well known that rather small
changes in the ancillary ligands can evoke rather drama-
tic changes in coordination mode and reactivity of group
4 dinitrogen complexes.16-18 We anticipated that this change
would result in slightly different bite angles in the two
chelate rings for C as compared to B and that the phosphine
donor in C would be more electron rich than in B. How these
Am. Chem. Soc. 2007, 129, 10895–10905.
(5) Morello, L.; Ferreira, M. J.; Patrick, B. O.; Fryzuk, M. D. Inorg.
Chem. 2008, 47, 1319–1323.
(6) Morello, L.; Yu, P.; Carmichael, C. D.; Patrick, B. O.; Fryzuk, M.
D. J. Am. Chem. Soc. 2005, 127, 12796–12797.
(7) Fryzuk, M. D. Chem. Rec. 2003, 3, 2–11.
(8) Fryzuk, M. D.; Shaver, M. P.; Patrick, B. O. Inorg. Chim. Acta
2003, 350, 293–298.
(9) Studt, F.; Morello, L.; Lehnert, N.; Fryzuk, M. D.; Tuczek, F.
Chem.;Eur. J. 2003, 9, 520–530.
(10) Morello, L.; Love, J. B.; Patrick, B. O.; Fryzuk, M. D. J. Am.
(14) Gerber, L. C. H.; Watson, L. A.; Parkin, S.; Weng, W.; Foxman,
B. M.; Ozerov, O. V. Organometallics 2007, 26, 4866–4868.
(15) Liang, L.-C. Coord. Chem. Rev. 2006, 250, 1152–1177.
(16) Chirik, P. J. Dalton Trans. 2007, 16–26.
(17) Hirotsu, M.; Fontaine, P. P.; Zavalij, P. Y.; Sita, L. R. J. Am.
Chem. Soc. 2007, 129, 12690–12692.
Chem. Soc. 2004, 126, 9480–9481.
(11) Fryzuk, M. D.; Haddad, T. S.; Mylvaganam, M.; McConville,
D. H.; Rettig, S. J. J. Am. Chem. Soc. 1993, 115, 2782–2792.
(12) MacLachlan, E. A.; Fryzuk, M. D. Organometallics 2005, 24,
1112–1118.
(18) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527–
530.
(13) Fryzuk, M. D. Acc. Chem. Res. 2009, 42, 127–133.
r
2009 American Chemical Society
Published on Web 08/11/2009
pubs.acs.org/Organometallics

ꢀ
Menard et al.
5254 Organometallics, Vol. 28, No. 17, 2009
two changes would manifest in dinitrogen activation was not
easily predictable, and so we initiated an exploration of the
synthesis of this modified ligand system. In this study, we
document our attempts to replace the o-phenylene linker of B
with a 3,4-thiophene unit as shown in C. One of the surpris-
ing aspects of this work was the unanticipated stereochem-
istry of the final product that likely results from a
competitive deprotonation-lithiation sequence. We also
include the coordination chemistry of this new ligand system
with zirconium(IV).
and X-ray crystallography. The product is somewhat photo-
and thermally sensitive, turning red gradually over time in
the solid state. However, it can be stored indefinitely without
any color change in the dark at -35 °C under N2.
To the best of our knowledge, the synthesis of 1 is the
first high-yielding Pd-catalyzed N-aryl amination coupling
of 3,4-dibromothiophene to selectively yield the monosub-
stituted product under mild reaction conditions. Although
the analogous reaction of diphenyl amine and 3,4-dibro-
mothiophene to yield the monosubstituted product has
been reported,22 this product was not isolated, and a
yield of 12% determined by GC-MS was reported. Similar
reactions to produce either C-C bonded species, such as 3-
benzyl-4-bromothiophene from 3,4-dibromothiophene,23
or C-N bonded species, using monobromothiophenes24-26
or polysubstituted monobromothiophenes,27 are well
documented.
Results and Discussion
Synthesis of [NPN]SH2. The proposed retrosynthetic ana-
lysis for the synthesis of a new [NPN]S-type system that has a
3,4-thiophene-type linker is shown in Scheme 1; the key
intermediate is 3-mesitylamino-4-bromothiophene (mesi-
tyl = 2,4,6-trimethylphenyl = Mes) (1), which could be
obtained by a selective Buchwald-Hartwig N-aryl amina-
tion19,20 of 3,4-dibromothiophene.
Figure 1 displays the typical numbering scheme for thio-
phenes as well as the characteristic H-H coupling constants
found for thiophene aromatic protons in appropria-
tely substituted systems.28 This numbering scheme is also
applied to 1, as it will become important in the following
sections.
Scheme 1
This amination reaction to yield 1 was optimized by
screening several different Pd catalysts and reagent ratios;
the results with the approximate gas chromatography-mass
spectrometry (GC-MS) results are shown in Table S1 in the
Supporting Information.
Figure 1. Typical numerical assignments and H-H coupling
constants for different unsymmetrically disubstituted thio-
phenes shown on the left (without substituents); on the right,
the assignments that will be used for 1.
The N-heterocyclic carbene Pd complex (SIPr)Pd(allyl)Cl
(SIPr = [N,N0-bis(2,6-diisopropylphenyl)4,5-dihydroimida-
zol)-2-ylidene])21 provided the most promising initial results.
Optimal conditions were found to be with a 5% catalyst
loading at room temperature; higher catalyst loadings
yielded little improvement in yield, and higher temperatures
led to the production of small amounts of the disubstituted
N3,N4-dimesitylthiophene-3,4-diamine byproduct. Com-
plete conversion to yield 1 could be achieved using 3 equiv
of 3,4-dibromothiophene (eq 1). Separation of the product 1
from the excess 3,4-dibromothiophene is easily accom-
plished by silica gel column chromatography (Rf values
of 0.19 and 0.71 in hexanes, respectively); the unreacted
3,4-dibromothiophene can then be recycled.
The assignment of the stereochemistry of bromo-amine 1
having the substituents in the 3,4-positions was made on the
basis of the NMR coupling constant, J(2-5)=3.6 Hz, which
agrees with the values summarized in Figure 1; as already
mentioned, this was confirmed by a solid-state X-ray
crystal structure of 1, which is reported in the Supporting
Information. Thus it was expected that upon lithiation
and quenching with 0.5 equiv of PhPCl2, the PhP frag-
ment would be installed in the 4-position of each ring, in
accordance with our retrosynthetic analysis shown above in
Scheme 1.
Not the Expected Stereochemistry. The details for
the synthesis of the [NPN]SH2 ligand precursor, 2, are
shown in eq 2. All steps are done in situ beginning with the
(22) Watanabe, M.; Yamamoto, T.; Nishiyama, M. Chem. Commun.
2000, 133–134.
(23) Minato, A.; Tamao, K.; Suzuki, K.; Kumada, M. Tetrahedron
Lett. 1980, 21, 4017–4020.
(24) Hooper, M. W.; Utsunomiya, M.; Hartwig, J. F. J. Org. Chem.
2003, 68, 2861–2873.
(25) Luker, T. J.; Beaton, H. G.; Whiting, M.; Mete, A.; Cheshire, D.
Compound 1 was obtained in 74% isolated yield and fully
characterized by NMR spectroscopy, elemental analyses,
R. Tetrahedron Lett. 2000, 41, 7731–7735.
(26) Ogawa, K.; Radke, K. R.; Rothstein, S. D.; Rasmussen, S. C. J.
Org. Chem. 2001, 66, 9067–9070.
(27) Begouin, A.; Hesse, S.; Queiroz, M.-J.; Kirsch, G. Synthesis
(19) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1348–1350.
2005, 2373–2378.
(20) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609–3612.
(21) Navarro, O.; Nolan, S. P. Synthesis 2006, 366–367.
(28) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of
Organic Compounds; Wiley: New York, 1998.

Article
Organometallics, Vol. 28, No. 17, 2009 5255
deprotonation-lithiation reaction using tert-butyllithium
(tBuLi), followed by the addition of PhPCl2, and finally
protonation using excess trimethylammonium hydro-
chloride (Me3NHCl); the isolated yield of 2 is 50%. Also
formed in this reaction is the debrominated 3-mesityla-
t
minothiophene 3 in 25% yield. BuLi was used rather than
nBuLi since the latter generated considerable quantities
of unreacted 1, which complicated the purification of the
product.
The PhP fragment, as shown in the retrosynthetic anal-
ysis in Scheme 1, was expected to be in the 4-position of
both thiophene rings since bromine-lithium exchange is
known to lead to C-Li bonds in those positions. For
example, the reaction of 3,4-dibromothiophene with suc-
cessive BuLi additions and quenches with various electro-
philes29,30 such as PriSSPri leads regiospecifically to the
formation of 3,4-disubstituted thiophene derivatives
with no evidence of the formation of 2- or 5-isomers. Thus,
we were surprised to find that the anticipated 3,4-pro-
duct was not formed; instead, the PhP moiety was deter-
mined to be in the 2-position of each thiophene ring, as
shown in eq 2.
Figure 2. 400 MHz 1H NMR spectrum (C6D6) of 2. The vicinal
coupling constant of 5.2 Hz between H4 and H5 is consistent
with the PhP fragment in the 2-position.
The 31P{1H} NMR spectrum in C6D6 shows one singlet
at -55.5 ppm, which only indicates that it is a single
compound. At first glance, the 1H and 13C{1H} NMR
spectra of 2 were consistent with the proposed 3,4-disubsti-
tuted structure. However, upon more detailed analysis of the
1
coupling constants on the thiophene unit in the H NMR
spectrum (see Figure 2), evidence mounted that the 2-
substituted isomer (eq 2) was in fact the product of this
reaction.
Figure 3. ORTEP drawing of the solid-state molecular struc-
ture of 2 (ellipsoids drawn at the 50% probability level). All
hydrogen atoms, except the N-H bonds, have been omitted for
˚
Two thiophene signals were observed: a doublet at
6.88 ppm and a doublet of doublets at 6.19 ppm (due to
coupling to both H and P) with JH-H values of 5.2 Hz (the dd
also had JP-H=2.6 Hz). As shown in Figure 1, this H-H
coupling constant value is indicative of two protons side-by-
side in the 4,5-positions of the thiophene ring and not across
the sulfur of the ring as in the expected 2,5-positions.28
Unequivocal confirmation of the structure was obtained by
single-crystal X-ray diffraction studies. The solid-state struc-
ture of the compound is shown in Figure 3.
clarity. Selected bond lengths (A) and angles (deg): C1-P1
1.8172(17), C7-P1 1.7893(17), C11-P1 1.7889(17), C7-S2
1.7307(17), C11-S1 1.7316(16), C8-N2 1.375(2), C12-N1
1.382(2), N2-C15 1.426(2), N1-C24 1.414(2), C1-P1-C7
101.70(8), C1-P1-C11 105.03(8), C7-P1-C11 105.76(8),
C12-N1-C24 123.97(16), C8-N2-C15 121.92(15).
experiments were performed. The first involved addition of
2 equiv of tBuLi31-33 to 1 followed by quenching with 2 equiv
of CF3COOD; the reaction times, temperatures, and con-
centrations used were identical to those used to prepare 2,
except that instead of quenching with PhPCl2, deuterated
trifluoroacetic acid was added. The averaged results of two
runs, with H/D ratios integrated by 1H NMR spectroscopy
in C6D6, are accurate to (5% (eq 3); clearly evident is the fact
that the starting bromo-amine 1 was consumed and partially
deuterated 3-mesitylaminothiophene 3 was the only product
observed. Deuterium was incorporated at the amine and
both the 2- and 4-positions in the ratios indicated; the fact
that a total of approximately 2 equiv of deuterium was
detected in 3(H/D) is consistent with a mixture of dilithio
species being present.
Deuteration Studies Related to [NPN]SH2 Formation. In
an effort to try to understand how the PhP fragment could
have ended up in the 2-position rather than the 4-position,
a series of deprotonation-lithiation, deuterium-quench
(29) Reddinger, J. L.; Reynolds, J. R. J. Org. Chem. 1996, 61, 4833–
4834.
(30) Wakefield, B.; Halter, R. J.; Wipf, P. Org. Lett. 2007, 9, 3121–
3124.
(31) Clayden, J. Organic Chemistry; Oxford University Press: Oxford,
2001.
(32) Corey, E. J.; Beames, D. J. J. Am. Chem. Soc. 1972, 94, 7210–
7211.
(33) Bailey, W. F.; Gagnier, R. P.; Patricia, J. J. J. Org. Chem. 1984,
49, 2098–2107.

ꢀ
Menard et al.
5256 Organometallics, Vol. 28, No. 17, 2009
be expected to proceed further to generate the dilithio
species 4. However, while 4 is the expected dilithio species,
the deuteration experiments and the observation of the
stereochemistry of 2 require other processes to be occur-
ring. One likely reaction is the intramolecular transfer of the
N-H proton in 3-CLi to generate 3-NLi, which sub-
t
sequently could react further with BuLi in competition
with or concurrently with the formation of 4 to generate
dilithio 5; this is the key species that is the presumed
precursor to generate 2 by further reaction with PhPCl2.
We can rationalize the formation of 5 from 3-NLi as being
due to the influence of two cooperative effects: the first is
that the deprotonated amine acts as a strong ortho-direct-
ing group36,37 for deprotonation at both the 2- and 4-
positions; the preference for deprotonation at the 2-posi-
tion over the 4-position is a result of the second effect, which
is that the C-H bonds at the 2,5-positions of thiophene
are approximately 6 orders of magnitude more acidic
that the C-H bonds at the 3,4-positions (pKa values of 33
and 39, respectively).38,39 This helps explain the formation
Most interestingly, deuterium was detected at the 2-position,
which is consistent with the formation of phosphine sub-
stitution at this position.
An identical experiment using just 1 equiv of BuLi, and
quenching with 1 equiv of CF3COOD, generates two pro-
t
ducts, as shown in eq 4 (ratios accurate to (5%).
t
of 5 from 3-NLi by reaction with BuLi; furthermore, the
expected dilithio product 4 can rearrange via intermole-
cular deprotonation to generate 5. Because there is deuter-
ium incorporation at the 4-position (see eq 3), the fact that
no phosphine substitution is observed at this position may
be kinetic in origin, as the 2-Li species 5 may react more
quickly than the 4-Li isomer 4. If any unreacted 4 remains
at the end of the reaction with PhPCl2, it would be proto-
nated by the added Me3NHCl and generate 3 (Scheme 2),
the only other product observed (eq 2); it should also
be noted that 3 could arise via protonation of other lithio
species such as 3-NLi or 5 that do not react with PhPCl2.
The fact that no deuterium is incorporated at the 5-position
(eqs 3 and 4) is consistent with the cooperative ortho-
directing effect of the deprotonated amine; if this were just
a result of the more acidic C-H bonds at the 2- and
5-positions as compared to the 3- and 4-positions, then
deuterium incorporation should be observed at both
of the 2- and the 5-positions R to the sulfur heteroatom of
the thiophene ring. Most importantly, it is the ortho-
directing effect of the lithium mesitylamide in the 3-posi-
tion that changes the course of the reaction to regio-
selectively produce 2 with the phosphine moiety in the
2-position.
The major product (68%) is the debrominated 3-mesityla-
minothiophene 3(H/D), which is the only observed product
in the reaction of 1 with 2 equiv of tBuLi (eq 3), and the minor
product (32%) is the starting 4-bromo-3-mesitylaminothio-
phene (1) with deuterium incorporation at the N-H position
and at the 2-position (indicated as 1(H/D) in eq 4). This
last experiment indicates that bromine-lithium exchange
is a competitive process with deprotonation of the N-H
moiety;34,35 interestingly, the presence of deuterium in the
2-position of 1(H/D) in eq 4 suggests that deprotonation at
this position may also be occurring to some extent. This all
suggests that these processes may be under kinetic control
since thermodynamic control would have only deuterium at
the most acidic site, the amine N-H. The actual percentages
of deuterium incorporation are not easily interpreted due to
various intermolecular and intramolecular deprotonation-
protonation processes; nevertheless, both products show
that there is deuterium incorporation at the 2-position.
Scheme 2 tries to rationalize the results of the deuteration
It should be noted that the two dilithio species 4 and 5
shown in Scheme 2 are not the only intermediates that are
possible; the 4-bromo-dilithio derivative 6 could also be
t
formed by reaction of 1-NLi with BuLi by deprotonation
of the 2-C-H instead of bromine for lithium exchange.
Indeed, bromine-lithium exchange of dilithio 6 could gen-
erate the trilithio species 7, which could also be formed to
some extent by the reaction of tBuLi with 4. How important
these other species are to the formation of 2 is not known.
Interestingly, the reaction of 6 with CF3COOD can account
for the observed deuterium distribution found in 1(H/D)
shown in eq 4.
t
studies by considering the reaction of 1 with BuLi in a
stepwise manner. The first equivalant of tBuLi could gene-
rate two monolithio species, 1-NLi and 3-CLi, which result
from competitive N-H deprotonation and bromine-
lithium exchange reactions. Both of these species would
(36) Figuly, G. D.; Loop, C. K.; Martin, J. C. J. Am. Chem. Soc. 1989,
111, 654–658.
(37) Posner, G. H.; Canella, K. A. J. Am. Chem. Soc. 1985, 107, 2571–
2573.
(34) Beak, P.; Musick, T. J.; Chen, C. W. J. Am. Chem. Soc. 1988, 110,
3538–3542.
(35) Narasimhan, N. S.; Sunder, N. M.; Ammanamanchi, A.; Bonde,
(38) Fraser, R. R.; Mansour, T. S.; Savard, S. Can. J. Chem. 1985, 63,
3505–3509.
(39) Shen, K.; Fu, Y.; Li, J. N.; Liu, L.; Guo, Q. X. Tetrahedron 2007,
63, 1568–1576.
B. D. J. Am. Chem. Soc. 1990, 112, 4431–4435.

Article
Organometallics, Vol. 28, No. 17, 2009 5257
Scheme 2
Synthesis of [NPN]SZrX2 (X=NMe2, Cl, or I) Compounds.
The coordination chemistry of this new thiophene-derived
tridentate ligand is still of interest even though the stereo-
chemistry is not what was initially targeted. The synthesis of
a series of zirconium complexes can be easily undertaken
starting from the ligand precursor 2 and using transamina-
tion procedures as shown in Scheme 3. The [NPN]SZr-
(NMe2)2 (8), [NPN]SZrCl2 (9), and [NPN]SZrI2 (10) com-
plexes are all isolated in high yields and have been fully
characterized.
The synthesis of 8 involves the transamination of a 1:1
mixture of 2 and tetrakis(dimethylamido)zirconium(IV), Zr-
(NMe2)4, in toluene to generate an 80% yield of a yellow
solid. The 31P{1H} NMR spectrum of 8 in C6D6 shows a new
peak downfield from the free ligand precursor 2 (-55.5 ppm)
at -41 ppm, and the 1H NMR spectrum shows five singlets
in the aliphatic region, three of which can be assigned to
two inequivalent sets of mesityl o-methyl groups due to
hindered rotation, and the corresponding p-methyls, while
the remaining two singlets correspond to the inequivalent
NMe2 groups. The thiophene protons in 8 display the
same pattern as in 2 of a doublet and a doublet of doublets
(due to H-H and P-H coupling). The solution NMR
data suggest a Cs symmetric trigonal-bipyramidal complex
with NMe2 groups in both the equatorial and apical posi-
tions. These results are analogous to the related [NPN]*Zr-
(NMe2)2 ([NPN]*={[N-(2,4,6-Me3C6H2)(o-N-C6H4)]2PPh})
complex previously reported12 and are also consistent with
the solid-state structure as shown in Figure 4.
Figure 4. ORTEP drawing of the solid-state molecular struc-
ture of 8 (ellipsoids drawn at the 50% probability level).
All hydrogen atoms have been omitted for clarity. Selected
˚
bond lengths (A) and angles (deg): Zr1-N1 2.1663(18),
Zr1-N2 2.1464(19), Zr1-N3 2.022(2), Zr1-N4 2.0430(19),
Zr1-P1 2.8410(6), N1-Zr1-N2 117.97(7), N3-Zr1-N4
99.19(8), N1-Zr1-P1 74.80(5), N2-Zr1-P1 72.24(5), N1-
Zr1-N3 114.64(7), P1-Zr1-N3 88.78(6), P1-Zr1-N4
169.62(5).
As seen in Figure 4, the solid-state structure is indeed
trigonal bipyramidal but distorted. The [NPN]S ligand binds
facially to the metal center as expected since the phosphine
donor is trigonal pyramidal. The N1-Zr1-P1 and N2-
Zr1-P1 angles are noticeably smaller than 90°, which make
the amide donors hinged out of the equatorial plane.
Furthermore, the angles around N1 and N2 add up to
359.59° and 359.72°, respectively, consistent with sp2 hybri-
dization at the amide donors.
The dichloride [NPN]SZrCl2 (9) can be readily synthesized
from 8 using an excess of trimethylsilyl chloride (Me3SiCl)
in toluene and isolated in 80% yield. The 31P{1H} NMR

ꢀ
Menard et al.
5258 Organometallics, Vol. 28, No. 17, 2009
Scheme 3
Figure 5. ORTEP drawing of the solid-state molecular struc-
ture of 9 (ellipsoids drawn at the 50% probability level). All
hydrogen atoms have been omitted for clarity. Selected bond
˚
lengths (A) and angles (deg): Zr1-N1 2.072(3), Zr1-N2
2.070(3), Zr1-Cl1 2.3773(12), Zr1-Cl2 2.3909(16), Zr1-P1
2.8352(12), N1-Zr1-N2 121.28(11), Cl1-Zr1-Cl2 101.27(5),
N1-Zr1-P1 72.64(8), N2-Zr1-P1 72.90(8), N1-Zr1-Cl2
115.28(9), P1-Zr1-Cl2 91.88(5), P1-Zr1-Cl1 166.84(4).
spectrum in C6D6 shows a singlet at -36 ppm. Similar to
compound 8, three peaks in the aliphatic region of the H
1
NMR spectrum can be assigned to the three separate methyl
groups on the mesityl rings, thussuggesting hinderedrotation.
The solution NMR data are consistent with a Cs symmetric
trigonal-bipyramidal complex, which was also the structure
found in the solid state as shown in Figure 5.
The solid-state structure of dichloride 9 displayed in
Figure 5 is largely analogous to the one shown in Figure 4.
The structure is again distorted trigonal bipyramidal with the
amide donors hinged out of the equatorial plane. One of the
expected differences in changing from NMe2 ligands to less
bulky Cl ligands is the N1-Zr1-N2 angle, which increases
slightly from approximately 118° to 121°, respectively. The
Figure 6. ORTEP drawing of the solid-state molecular struc-
ture of 10 (ellipsoids drawn at the 50% probability level). All
hydrogen atoms have been omitted for clarity. Selected bond
˚
lengths (A) and angles (deg): Zr1-N1 2.054(4), Zr1-N2
˚
˚
Zr-N (average 2.09 A for 8 and 2.07 A for 9), Zr-Cl
2.081(5), Zr1-I1 2.7812(8), Zr1-I2 2.7757(15), Zr1-P1
2.8357(16), N1-Zr1-N2 116.80(17), I1-Zr1-I2 95.51(3),
N1-Zr1-P1 70.08(12), N2-Zr1-P1 73.56(12), N1-Zr1-I2
114.40(12), P1-Zr1-I2 85.27(4), P1-Zr1-I1 177.40(4).
˚
˚
(average 2.38 A), and Zr-P (2.84 A) bonds in both 8 and 9
are typical.40,41
The diiodide complex [NPN]SZrI2 (10) can also be synthe-
sized in high yield using an excess of trimethylsilyl iodide
(TMSI). As shown in Scheme 3, this compound can be
synthesized via two different routes. The cleanest route to this
synthesis is by using 8 as the starting material and treating it
with excess TMSI. After 3 h, a pure orange solid can be
obtained following workup in 82% yield. The 31P{1H} NMR
spectrum in C6D6 shows a singlet at -35 ppm, slightly
shifted from the -36 ppm peak for the dichloride 9. The
1H NMR data are analogous to those for 9 and also suggest a
Cs symmetric trigonal-bipyramidal complex. The solid-state
structure, seen in Figure 6, shows this type of structure.
As in both 8 and 9, the structure is again distorted trigonal
bipyramidal with the amide donors hinged out of the equa-
Diiodide 10 can also be readily synthesized from the
dichloride species [NPN]SZrCl2 (9). The reaction is per-
formed in an analogous way using TMSI, but a longer
reaction time of approximately 15 h is required. The yield
is 88%. Although this may seem like the highest yielding
route, the overall yield is lower since this is a two-step
procedure. Finally, this complex can also be synthesized
using a one-pot protocol. Mixing [NPN]SH2 (2) with Zr-
(NMe2)4 in toluene for 1 h, followed by the direct addition of
excess TMSI, yields the desired product. The overall yield is
73%. Although this route provides the highest overall yield,
this reaction is often complicated by the appearance of
intractable impurities, requiring an extra filtration step.
The NMR analysis also tends to show trace impurities. Thus,
although convenient, this method does not yield the purest
product: the optimal method for synthesizing 10 is by the
first method described.
˚
torial plane. The Zr-N (average 2.07 A), Zr-I (average
˚
˚
2.78 A), and Zr-P (2.84 A) bonds are also typical in this
complex.40-42
(40) Skinner, M. E. G.; Li, Y.; Mountford, P. Inorg. Chem. 2002, 41,
1110–1119.
(41) Chien, P. S.; Liang, L. C. Inorg. Chem. 2005, 44, 5147–5151.
(42) King, W. A.; Di Bella, S.; Gulino, A.; Lanza, G.; Fragala, I. L.;
Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1999, 121, 355–366.
The effect of changing the o-phenylene linker in the
original [NPN]* ligand system to a 2,3-thiophene moiety as

Article
Organometallics, Vol. 28, No. 17, 2009 5259
found in [NPN]S can be seen in some of the bond angles and
bond distances of analogous complexes.12 In [NPN]*ZrCl2
the N-Zr-N bond angle is 113.96(9)°, whereas in 9 N1-
Zr1N2 is 121.28(11)°; the Zr-P bond length in [NPN]*ZrCl2
literature procedure.43 All other compounds were purchased
from commercial suppliers and were used as received.
Synthesis of N-3-(NHMes)-4-Br-SC4H2 (1). A 500 mL round-
bottom Schlenk flask equipped with a magnetic stir bar
was charged with (SIPr)Pd(allyl)Cl (1.17 g, 2 mmol), NaOtBu
(4.33 g, 45 mmol), and toluene (300 mL). The reaction mixture
was stirred for 5 min; then 3,4-dibromothiophene (13.7 mL,
124 mmol) and mesitylamine (5.8 mL, 41 mmol) were added all
at once. The solution quickly turned dark green-brown and was
allowed to stir overnight (15 h). The mixture was then poured on
300 mL of water in open air. The organic phase was separated
and washed with another 2 ꢀ 300 mL of water. The combined
aqueous extracts were washed with 2 ꢀ 100 mL of toluene.
The organics were combined and dried with MgSO4. The
mixture was filtered and the toluene was removed by roto-
evaporation. Column chromatography using silica gel of the
resulting crude black liquid separated the excess 3,4-dibro-
mothiophene (Rf=0.71) from the desired product (Rf =0.19)
using hexanes as the solvent. Once the 3,4-dibromothiophene is
separated, the polarity of the eluent can be increased to 2% Et2O
in hexanes. The total weight of the pure white product was
9.0 g (74% yield). Slow evaporation of a hexanes solution of
the compound yielded single crystals suitable for X-ray crys-
tallography.
˚
is 2.7229(8) A, while for 9, this bond lengthens to 2.8352(12)
˚
˚
A, a full 0.1 A longer. While it is clear that these linkers do
generate differences in the coordination complexes, the
ramifications of these changes to the chemistry of these
Zr(IV) complexes are currently under investigation.
Conclusions
In this report, the synthesis of [NPN]SH2 (2), a new
tridentate mixed donor ligand precursor with 2,3-thiophene
moieties linking mesityl amido and phosphine units, and its
coordination chemistry with Zr(IV) are reported. The stereo-
chemistry of 2 has the PhP fragment in the 2-position, which
was unexpected, as the 3,4-dibromothiophene precursor
should have led to the PhP moiety in the 4-position. Pre-
liminary mechanistic investigations suggest a mechanism
involving competitive deprotonation and bromine-lithium
exchange reactions in which the ortho-directing effect of the
deprotonated aryl amine in the 3-position can override the
usually regiospecific bromine-lithium exchange process. A
series of Zr(IV) complexes of this new ligand were synthe-
sized. The [NPN]SZr(NMe2)2 (8) complex is synthesized via
transamination of Zr(NMe2)4 with 2. The related [NPN]SZr-
Cl2 and [NPN]SZrI2 complexes can be prepared by deamina-
tion of 8 with trimethylsilyl chloride or trimethylsilyl iodide,
respectively. We are currently investigating the reduction of
these halide complexes for the activation of N2.
1H NMR (400 MHz, C6D6): δ 6.77 (s, 2H, Mes-H), 6.75 (d, J=
3.6 Hz, H5), 5.29 (d, J=3.6 Hz, H2), 5.07 (bs, N-H), 2.16 (s, 3H,
p-CH3), 2.03 (s, 6H, o-CH3). 13C NMR (100 MHz, C6D6):
δ 143.3, 137.2, 135.4, 135.2, 129.6, 122.4, 102.8, 96.8, 21.0 (p-
CH3), 17.9 (o-CH3). Anal. Calcd for C13H14BrNS: C, 52.71; H,
4.76; N, 4.73. Found: C, 53.00; H, 4.95; N, 4.96. EI-MS (m/z):
297 [M]þ, 216 [M - Br]þ.
Synthesis of [NPN]SH2 (2). A 1 L, two-necked round-bottom
Schlenk flask was equipped with a magnetic stirbar and two
dropping funnels. One was charged with 2.04 equiv of BuLi
t
(1.74 M in pentane, 60 mL, 104 mmol) and the other with
a dilute solution of 0.5 equiv of PhPCl2 (3.38 mL, 25 mmol) in
250 mL of Et2O. The round-bottom flask was loaded with 15 g
(50 mmol) of 1 in 300 mL of Et2O. The solution was stirred and
was cooled to -78 °C (dry ice/acetone bath) on the Schlenk line.
The tBuLi solution was added dropwise to the solution. The pale
yellow solution was kept at -78 °C while stirring for 1 h
following the addition. The PhPCl2 solution was then added
dropwise at -78 °C over 5 h. The solution gradually went from
pale yellow to dark orange-brown. The solution was allowed to
warm to room temperature overnight. The following morning,
an excess of Me3NHCl (11.95 g, 125 mmol) was added all at once
to the stirring solution. The solution was stirred for 3 h, after
which time the Et2O was removed in vacuo. The brown residue
was dissolved in ca. 200 mL of toluene and was filtered on a glass
frit with Celite. The Celite was then washed with toluene. The
toluene was thoroughly removed in vacuo to obtain a viscous
dark blackish-brown residue. The residue was mixed with ca.
200 mL of hexanes. A pale beige solid precipitated and was
filtered on a glass frit. Repeated trituration using hexanes
yielded 6.8 g (50% yield) of pure product. Slow evaporation of
a hexanes solution of the compound yielded single crystals
suitable for X-ray crystallography.
Experimental Section
General Considerations. Unless otherwise stated, all mani-
pulations were performed under an atmosphere of dry, oxygen-
free N2 or Ar by means of standard Schlenk or glovebox
techniques (Innovative Technology glovebox equipped with a
-35 °C freezer). Hexanes, toluene, pentane, and diethyl ether
were purchased anhydrous from Aldrich, sparged with N2, and
passed through columns containing activated alumina and
˚
Ridox catalyst. C6D6 was dried on activated 5 A molecular
sieves and freeze-pump-thaw degassed three times. 1H,
31P{1H}, and 13C{1H} NMR spectra were recorded on a Bruker
AV-300, a Bruker AV-400, or a Bruker AV-400inv spectro-
meter, operating at 300.1, 400.0, and 400.0 MHz for 1H spectra,
respectively. All spectra were recorded at room temperature. 1H
NMR spectra were referenced to residual protons in the deute-
rated solvent, C6D6 (7.16 ppm). 31P{1H} NMR spectra were
referenced to external P(OMe)3 (141.0 ppm with respect to 85%
H3PO4 at 0.0 ppm). 13C{1H} NMR spectra are referenced to
residual solvent, C6D6 (128.0 ppm). Chemical shifts (δ) listed are
in ppm, and absolute values of the coupling constants are in Hz.
GC-MS spectra were recorded on an Agilent series 6890 GC
system with a 5973 mass selective detector. Mass spectrometry
(EI-MS), elemental analysis (C, H, N), and X-ray crystallogra-
phy were all performed at the Department of Chemistry at the
University of British Columbia.
1H NMR (400 MHz, C6D6): δ 7.75-7.71 (m, 2H), 7.14-7.11
(m, 2H), 7.06-7.02 (m, 1H), 6.88 (d, J=5.2 Hz, 2H), 6.76 (s, 4H),
6.19 (dd, JH-H =5.2 Hz, JP-H =2.6 Hz, 2H), 5.79 (N-H) (d,
J
P-H=3.2 Hz, 2H), 2.15 (s, 6H), 2.10 (s, 12H). 31P{1H} NMR
(161 MHz, C6D6): δ -55.5. 13C NMR (100 MHz, C6D6): δ 153.7
(d, J=74.4 Hz), 138.0, 137.8, 135.6, 135.2, 132.2 (d, J=70.8 Hz),
131.3 (d, J=11.2 Hz), 129.5, 128.8 (d, J=25.6), 128.4, 119.3 (d,
J=14.8 Hz), 103.8 (d, J=61.2 Hz), 20.9, 18.4. Anal. Calcd for
C32H33N2PS2: C, 71.08; H, 6.15; N, 5.18. Found: C, 71.31; H,
6.29; N, 5.51. EI-MS (m/z): 540 [M]þ, 525 [M - CH3]þ.
Mesitylamine and PhPCl2 (Aldrich) were both distilled prior
to use. 3,4-Dibromothiophene (Alfa) was degassed by freeze-
pump-thaw and mixed with activated molecular sieves (5 A).
CF3COOD (purchased in sealed glass ampules), Me3NHCl,
NaOtBu, TMSCl, and TMSI were purchased from Aldrich
and used without further purification. Zr(NMe2)4 was pur-
chased from Strem and used without further purification.
(SIPr)Pd(allyl)Cl was prepared by literature methods.21 tBuLi
(∼1.7 M in pentane) was doubly titrated using a known
˚
(43) Gilman, H.; Cartledge, F. K. J. Organomet. Chem. 1964, 2, 447–
454.

ꢀ
Menard et al.
5260 Organometallics, Vol. 28, No. 17, 2009
Isolation of 3. This air-sensitive compound was extracted from
the reaction mixture in the synthesis of 2 following the Celite
filtration and removal of toluene. The Schlenk flask containing
the residue was fitted with a distillation bridge attached to a
flask submerged in liquid N2. The apparatus was attached to the
high-vacuum line, and the pressure was reduced to 10 mTorr.
The oily compound was distilled off by heating the flask to
125-135 °C.
C, 54.84; H, 4.46; N, 4.00. Found: C, 54.69; H, 4.65; N, 3.88. EI-
MS (m/z): 700 [M]þ.
Synthesis of [NPN]SZrI2 (10) from [NPN]SZr(NMe2)2 (8). To
a stirred yellow toluene solution (50 mL) of 8 (1.60 g, 2.23 mmol)
was added trimethylsilyl iodide (3.18 mL, 22.3 mmol) dropwise.
The solution rapidly turned orange and was stirred for 3 h. The
reaction mixture was taken to dryness to obtain an orange
powder that was collected on a frit, washed with pentane (3 ꢀ
5 mL), and dried (1.61 g, 1.82 mmol, 82%). Slow evaporation of
an Et2O solution of the compound yielded single crystals
suitable for X-ray crystallography.
1H NMR (400 MHz, C6D6): δ 6.82 (dd, J=3.2 Hz, J=5.2 Hz,
1H), 6.80 (s, 2H), 6.37 (dd, J=1.6 Hz, J=5.2 Hz, 1H), 5.59 (dd,
J=1.6 Hz, J=3.2 Hz, 1H), 4.56 (bs, 1H), 2.17 (s, 3H), 2.07 (s,
6H). 13C NMR (100 MHz, C6D6): δ 146.9, 138.3, 134.7 (2
overlapping carbons), 129.6, 125.2, 120.0, 98.3, 20.9, 18.1. Anal.
Calcd for C13H15NS: C, 71.84; H, 6.96; N, 6.44. Found: C,
72.22; H, 7.02; N, 6.38. EI-MS (m/z): 217 [M]þ, 202 [M - CH3]þ.
Synthesis of [NPN]SZr(NMe2)2 (8). Zr(NMe2)4 (0.989 g, 3.70
mmol) and 2 (2.00 g, 3.70 mmol) were mixed together, and
toluene (40 mL) was added to obtain a lemon yellow solution,
which was stirred for 1 h. The reaction mixture was taken to
dryness to obtain a yellow residue. Upon addition of minimal
hexanes, a bright yellow precipitate formed that was collected on
a frit and dried (2.11 g). Slow cooling to -35 °C of a concen-
trated Et2O solution of the compound yielded single crystals
suitable for X-ray crystallography.
Synthesis of [NPN]SZrI2 (10) from [NPN]SZrCl2 (9). To a
stirred yellow toluene solution (50 mL) of 9 (0.9 g, 1.28 mmol)
was added trimethylsilyl iodide (1.83 mL, 12.8 mmol) dropwise.
The solution turned orange and was stirred overnight. The
reaction mixture was taken to dryness to obtain an orange
powder that was collected on a frit, washed with pentane (3 ꢀ
5 mL), and dried (1.00 g, 1.13 mmol, 88%).
Synthesis of [NPN]SZrI2 (10) from [NPN]SH2 (2) and Zr-
(NMe2)4: One-Pot Synthesis. Zr(NMe2)4 (0.495 g, 1.85 mmol)
and 2 (1.00 g, 1.85 mmol) were mixed together, and toluene
(40 mL) was added to obtain a lemon yellow solution, which was
stirred for 1 h. Trimethylsilyl iodide (2.63 mL, 18.5 mmol) was
added dropwise to the yellow solution. The solution rapidly
turned orange and was stirred for 3 h. The reaction mixture was
taken to dryness to obtain an orange powder that was collected
on a frit with Celite. The solids were washed on the Celite with
pentane (3 ꢀ 10 mL). The product was then extracted from the
Celite by washing it with excess toluene into an empty round-
bottom flask. Removal of the toluene in vacuo produced 1.20 g
(1.36 mmol, 73%) of product.
1H NMR (400 MHz, C6D6): δ 7.86-7.81 (m, 2H), 7.15-7.10
(m, 2H), 7.05-7.01 (m, 1H), 6.97 (d, J=5.2 Hz, 2H), 6.89 (bs,
2H), 6.87 (bs, 2H), 5.92 (dd, JH-H=5.2 Hz, JP-H=4.0 Hz, 2H),
2.99 (s, 6H), 2.36 (s, 6H), 2.26 (s, 6H), 2.25 (s, 6H), 2.17 (s, 6H).
31P{1H} NMR (161 MHz, C6D6): δ -41.4. 13C NMR (100 MHz,
C6D6): δ 168.6 (d, J=138.8 Hz), 146.4 (d, J=8.4 Hz), 135.31,
135.29, 134.9, 133.8, 133.7, 133.4, 131.9 (d, J=53.2 Hz), 129.7,
129.2, 128.7 (d, J=36.8 Hz), 120.1 (d, J=41.6 Hz), 99.4 (d, J=
84.0 Hz), 43.0, 42.8 (d, J=18.8 Hz), 20.9, 19.3, 19.2. Satisfactory
EA values for this compound could not be obtained due
to persistent residual solvents. EI-MS (m/z): 716 [M]þ, 672
[M - NMe2]þ.
1H NMR (400 MHz, C6D6): δ 7.81-7.76 (m, 2H), 7.10-7.05
(m, 2H), 7.02-6.98 (m, 1H), 6.85 (bs, overlapping singlet and
doublet, 4H), 6.78 (bs, 2H), 5.69 (dd, JH-H=5.2 Hz, JP-H=3.6
Hz, 2H), 2.60 (s, 6H), 2.25 (s, 6H), 2.09 (s, 6H). 31P{1H} NMR
(161 MHz, C6D6): δ -35.3. 13C NMR (100 MHz, C6D6): δ 167.0
(d, J=144.4 Hz), 138.9 (d, J=14.0 Hz), 138.6, 137.9, 136.9,
136.6, 132.0, 131.6 (d, J=48.0 Hz), 130.95, 130.92, 130.8 (d, J=
9.6 Hz), 129.1 (d, J=44.4 Hz), 118.3 (d, J=46.8 Hz), 106.6 (d, J=
118.4 Hz), 21.2, 20.8, 20.4. Anal. Calcd for C32H31I2N2PS2Zr: C,
43.49; H, 3.54; N, 3.17. Found: C, 43.39; H, 3.72; N, 3.08. EI-MS
(m/z): 882 [M]þ, 755 [M - I]þ.
Synthesis of [NPN]SZrCl2 (9). To a stirred yellow toluene
solution (85 mL) of 8 (2.55 g, 3.56 mmol) was added trimethyl-
silyl chloride (4.52 mL, 35.6 mmol) dropwise. The clear yellow
solution was stirred for 5 h. The reaction mixture was taken to
dryness to obtain a yellow powder that was collected on a frit,
washed with pentane (3 ꢀ 5 mL), and dried (2.0 g, 2.85 mmol,
80%). Slow evaporation of an Et2O solution of the compound
yielded single crystals suitable for X-ray crystallography.
1H NMR (400 MHz, C6D6): δ 7.89-7.84 (m, 2H), 7.07-7.04
(m, 2H), 7.00-6.96 (m, 1H), 6.88 (bs, overlapping singlet and
doublet, 4H), 6.74 (bs, 2H), 5.72 (dd, JH-H=5.2 Hz, JP-H=3.2
Hz, 2H), 2.46 (s, 6H), 2.36 (s, 6H), 2.08 (s, 6H). 31P{1H} NMR
(161 MHz, C6D6): δ -36.2. 13C NMR (100 MHz, C6D6): δ 166.6
(d, J=145.2 Hz), 140.7 (d, J=13.6 Hz), 137.6, 137.4, 136.4,
135.8, 131.6 (d, J=53.2 Hz), 131.2, 130.7 (d, J=8.8 Hz), 130.6,
130.5, 129.3 (d, J=42.4 Hz), 118.5 (d, J=46.0 Hz), 105.8 (d, J=
127.6 Hz), 21.0, 19.3, 19.1. Anal. Calcd for C32H31Cl2N2PS2Zr:
Acknowledgment. We thank NSERC of Canada for
generous financial support in the form of a Discovery
Grant to M.D.F. and a postgraduate scholarship for G.M.
Supporting Information Available: Optimization of condi-
tions for the preparation of bromo-amine 1; crystallographic
data for 1, 2, and 8-10 (CIFs); experimental details for X-ray
structure determinations. This material is available free of
charge via the Internet at http://pubs.acs.org.