Functionalization of complexed N2O in bis(pentamethylcyclopentadienyl) systems of zirconium and titanium

Article abstract of DOI:10.1021/om5002556

Methyl triflate reacts with the metastable azoxymetallacyclopentene complex Cp₂Zr(N(O)NCPhCPh), generated in situ from nitrous oxide insertion into the Zr-C bond of Cp₂Zr(η²-PhCCPh) at -78 °C, to afford the salt [Cp₂Zr(N(O)N(Me)CPhCPh)][O₃SCF ₃] (1) in 48% isolated yield. A single-crystal X-ray structure of 1 features a planar azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen position of the former Zr(N(O)NCPhCPh) scaffold. In addition to 1, the methoxy-triflato complex Cp₂Zr(OMe)(O₃SCF ₃) (2) was also isolated from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of Cp₂Zr=O(NC₅H₅) with MeO₃SCF ₃. Complex 2 could also be observed, spectroscopically, from the thermolysis of 1 (80 °C, 2 days). In contrast to Cp₂Zr(N(O) NPhCCPh), the more stable titanium N₂O-inserted analogue, Cp ₂Ti(N(O)NCPhCPh), reacts with MeO₃SCF₃ to afford a 1:1 mixture of regioisomeric salts, [Cp₂Ti(N(O)N(Me)CPhCPh)] [O₃SCF₃] (3) and [Cp₂Ti(N(OMe)NCPhCPh)][O ₃SCF₃] (4), in a combined 65% isolated yield. Single-crystal X-ray diffraction studies of a cocrystal of 3 and 4 show a 1:1 mixture of azoxymetallacyle salts resulting from methyl alkylation at both the β-nitrogen and the β-oxygen of the former Ti(N(O)NCPhCPh ring. As opposed to alkylation reactions, the one-electron reduction of Cp ₂Ti(N(O)NCPhCPh) with KC₈, followed by encapsulation with the cryptand 2,2,2-Kryptofix, resulted in the isolation of the discrete radical anion [K(2,2,2-Kryptofix)][Cp₂Ti(N(O)NCPhCPh)] (5) in 68% yield. Complex 5 was studied by single-crystal X-ray diffraction, and its solution X-band EPR spectrum suggested a nonbonding σ-type wedge hybrid orbital on titanium, d(z²)/d(x²-y²), houses the unpaired electron, without perturbing the azoxymetallacycle core in Cp ₂Ti(N(O)NCPhCPh). Theoretical studies of Ti and the Zr analogue are also presented and discussed.

Full text of DOI:10.1021/om5002556

Article
pubs.acs.org/Organometallics
Functionalization of Complexed N2O in
Bis(pentamethylcyclopentadienyl) Systems of Zirconium and
Titanium
Daniel J. Mindiola,*,† Lori A. Watson,‡ Karsten Meyer,§ and Gregory L. Hillhouse⊥
Searle Chemistry Laboratory, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: Methyl triflate reacts with the metastable azoxymetallacyclo-
pentene complex Cp*2Zr(N(O)NCPhCPh), generated in situ from nitrous
oxide insertion into the Zr−C bond of Cp*2Zr(η2-PhCCPh) at −78 °C, to
afford the salt [Cp*2Zr(N(O)N(Me)CPhCPh)][O3SCF3] (1) in 48%
isolated yield. A single-crystal X-ray structure of 1 features a planar
azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen
position of the former Zr(N(O)NCPhCPh) scaffold. In addition to 1, the
methoxy-triflato complex Cp*2Zr(OMe)(O3SCF3) (2) was also isolated
from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of
Cp*2ZrO(NC5H5) with MeO3SCF3. Complex 2 could also be observed, spectroscopically, from the thermolysis of 1 (80 °C, 2
days). In contrast to Cp*2Zr(N(O)NPhCCPh), the more stable titanium N2O-inserted analogue, Cp*2Ti(N(O)NCPhCPh),
reacts with MeO3SCF3 to afford a 1:1 mixture of regioisomeric salts, [Cp*2Ti(N(O)N(Me)CPhCPh)][O3SCF3] (3) and
[Cp*2Ti(N(OMe)NCPhCPh)][O3SCF3] (4), in a combined 65% isolated yield. Single-crystal X-ray diffraction studies of a
cocrystal of 3 and 4 show a 1:1 mixture of azoxymetallacyle salts resulting from methyl alkylation at both the β-nitrogen and the
β-oxygen of the former Ti(N(O)NCPhCPh ring. As opposed to alkylation reactions, the one-electron reduction of
Cp*2Ti(N(O)NCPhCPh) with KC8, followed by encapsulation with the cryptand 2,2,2-Kryptofix, resulted in the isolation of the
discrete radical anion [K(2,2,2-Kryptofix)][Cp*2Ti(N(O)NCPhCPh)] (5) in 68% yield. Complex 5 was studied by single-crystal
X-ray diffraction, and its solution X-band EPR spectrum suggested a nonbonding σ-type wedge hybrid orbital on titanium, d(z2)/
d(x2−y2), houses the unpaired electron, without perturbing the azoxymetallacycle core in Cp*2Ti(N(O)NCPhCPh). Theoretical
studies of Ti and the Zr analogue are also presented and discussed.
Given the disposition of nitrous oxide as being a poor ligand
in the realm of coordination chemistry, our group and others
are pursuing the use of nitrous oxide in O atom transfer
systems for the purpose of oxidation catalysis. Previously, we
reported that group 4 transition metals (Ti and Zr) can insert
nitrous oxide into a strained M−C bond without oxidation of
the metal center to form bis(pentamethylcyclopentadienyl)
systems, having coordinated N2O as part of the ligand scaffold
(Scheme 1).11,12 As a result, N2O functionalizes the ligand (e.g,
tolane) without immediate extrusion of N2. Although N2
extrusion can in one case occur, affording an oxametallacycle,
this process depends heavily on the metal in question
(zirconium vs titanium, Scheme 1).12 The fact that the
azoxymetallacycle complex Cp*2Ti(N(O)NCPhCPh)11,12,33
was not prone to dinitrogen elimination under ambient
conditions (unlike its Zr derivative), to give a hypothetical
oxametallacyclobutene system Cp*2Ti(OCPhCPh), led us to
speculate that such a system could be ideal for O atom and/or
functionalization reactions stemming from N2O complexation
(Scheme 1).11 In addition, the azoxymetallacyclobutene moiety
in complexes of the type Cp*2M(N(O)NCPhCPh) (M = Ti or
INTRODUCTION
■
Due to the thermodynamic oxidizing power of nitrous oxide
(ΔGf0 = 25 kcal/mol) and its kinetic inertness in the absence of
a suitable activating metal center, the development of systems
capable of catalyzing the oxidation of organic or inorganic
substrates using this greenhouse feedstock represents a
paradigm in inorganic chemistry.1,2 Unlike most common
powerful oxidants, nitrous oxide is cheap and very soluble in
organic solvents such as toluene, alcohols, ethers, oils, water,
and acids, with the advantage of such a chemical being
resourceful, nonflammable, and essentially harmless. Therefore,
it is not surprising that there has been considerable interest in
employing N2O for oxygen-atom transfer reactions.3−21
Unfortunately, and as noted before, this molecule is kinetically
inert due to its poor properties as a ligand,22−24 consequently
resulting in only a handful of systems being capable of
complexing N2O without the entropic and enthalpic driving
force to N2 ejection and metal oxo formation.11,12,18,22−31 Thus,
the use of N2O as an oxidizing substrate in transition metal
chemistry is often hampered by a combination of kinetic
inertness and inevitable oxidation of the metal center,
concomitant with entropy-assisted loss of a thermodynamically
stable molecule such as dinitrogen.32
Received: March 10, 2014
© XXXX American Chemical Society
A
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Article
the radical anion titanium(III) species [Cp*2Ti(N(O)-
NCPhCPh)]−, without significant perturbation of the azox-
ymetallacyclobutene core.
Scheme 1. Nitrous Oxide Insertion into a Strained Metal−
Carbon Bond of an η2-Tolane
Bis(pentamethylcyclopentadienyl) Complex and N2
Extrusion to Form an Oxymetallacyclobutene
EXPERIMENTAL SECTION
■
General Considerations. Unless stated otherwise, all operations
were performed in an M. Braun Lab Master drybox under an
atmosphere of purified nitrogen or using high-vacuum and standard
Schlenk techniques under an argon atmosphere.37 HPLC grade or
anhydrous solvents such as benzene, toluene, diethyl ether, n-hexane,
pentane, and CH2Cl2 were purchased from Acros Chemicals or EM
Science and were further dried by passage through activated alumina
and/or Q-5 columns. THF was distilled from purple benzophenone
ketyl solution under an inert atmosphere. C6D6, THF-d8, and CD2Cl2
were purchased from Cambridge Isotope Laboratory (CIL), degassed
and dried over activated 4 Å molecular sieves. Celite, alumina, and 4 Å
molecular sieves were activated in vacuo overnight at 180 °C.
Cp*2TiCl2 was prepared according to the literature38 or purchased
from Strem Chemicals and used as received. KC8,39 Cp*2ZrCl2,40
Cp*2Zr(PhCCPh),38 Cp*2Ti(N(O)NCPhCPh),11 Cp*2Zr(N(O)-
NCPhCPh),11,12 Cp*2Zr(OCPhCPh),12 and Cp*2Zr(O)(NC5H5)15c
were prepared according to literature methods. Cp*2Ti(PhCCPh) was
prepared in 68% yield by a modified procedure from that reported in
the literature, via reduction of Cp*2TiCl2 with Na/Hg, and in the
presence of PhCCPh, using a protocol identical to that reported for
the zirconium derivative Cp*2Zr(PhCCPh).38 2,2,2-Kryptofix was
purchased from Aldrich and was dried by dissolving in a minimum of
dry THF, then filtered through activated alumina, and the filtrate was
dried under vacuum to afford a white powder. All other chemicals were
used as received. Elemental analysis was performed by Desert
Zr) provides an ideal opportunity to examine the charge
distribution about this rare functionality since several canonical
forms can be envisioned (Scheme 2). While resonance
Scheme 2. Proposed Canonical Forms for the
Azoxymetallacyclobutene Framework in Complexes of the
Type Cp*2M(N(O)NCPhCPh) (M = Ti and Zr)
1
Analytics (Tucson, AZ, USA). H, 13C, and 19F NMR spectra were
1
recorded on Bruker 500 and 400 MHz NMR spectrometers. H and
13C NMR spectra are reported with reference to solvent resonances
(residual C6D5H in C6D6, 7.16 and 128.0 ppm; residual H in CD2Cl2,
5.32 and 53.8 ppm; residual H in THF-d8, 1.73 and 3.58 ppm, and 65.6
and 23.5 ppm). 19F NMR spectra are reported with respect to external
CCl3F (0.0 ppm). Room-temperature solution (THF) magnetic
structures A and B represent zwitterionic forms with negative
charge being localized on β-O and the β-N, respectively,
resonance form C can be best represented as an ylide, having
more nucleophilic character on O (Scheme 2). Another
probable form in Cp*2M(N(O)NCPhCPh) is the formation
of an N-nitroso ketimine tethered alkylidene moiety, resonance
D, where most of the nucleophilic charge is presumed to reside
at the hindered alkylidene carbon (Scheme 2). Hence,
examining the reactivity of Cp*2M(N(O)NCPhCPh) with an
electrophile could provide some clues as to which site is likely
most nucleophilic, but also allow us to further functionalize the
N(O)N motif resulting from N2O insertion into a strained M−
C bond. Surprisingly, examples of homogeneous catalytic
oxidation reactions using N2O as an O atom transfer reagent
are exceedingly rare, and examples include the oxidation with
N2O of PPh3 using a cobalt(I) complex17 and the oxidation of
styrene by a ruthenium porphyrin system.16 Unfortunately, the
latter reaction has yet to be developed into an efficient catalytic
cycle. This lackluster situation in homogeneous catalysis is
rather unanticipated given the popular usage of N2O in
heterogeneous catalytic reactions such as the oxidation of
benzene to phenol over supported metals34 or Fe and Ru/
ZSM-5 zeolites35 and the oxidation of methane on silica-
molybdate catalyst.36
1
susceptibility measurements were determined by H NMR spectros-
copy using the method of Evans.41,42 The room-temperature X-band
EPR spectra were recorded on a Bruker EMX spectrometer.
Acquisition, simulation, and data postprocessing of the solution
spectra were performed using an integrated WIN-EPR software
package (Bruker). Cyclic voltammetry measurements were collected
with the assistance of an Eco-Chemie Autolab potentiostat (pgstat20)
and the GPES 2.0 software from Bioanalytical Systems (BAS). X-ray
diffraction data were collected on a Siemens Platform goniometer with
a charged coupled device (Smart Apex). Structures were solved by
direct or Patterson methods using the SHELXTL (version 5.1)
program library (G. Sheldrick, Bruker Analytical X-ray Systems,
Madison, WI, USA).43
Preparation of [Cp*2Zr(N(O)N(Me)CPhCPh)][O3SCF3] (1).
Toluene (ca. 20 mL) was vacuum transferred into a Schlenk flask
equipped with a stir bar and Cp*2Zr(η2-PhCCPh) [554 mg, 1.027
mmol]. The solution was placed under an N2O atmosphere at −78 °C,
causing a rapid color change from green-brown to orange. The orange
solution was stirred under an atmosphere of N2O at −78 °C for 1 h
and then degassed. To the orange solution was added via cannula a
similarly cold (−78 °C) toluene solution (ca. 8 mL) of MeO3SCF3
[171 mg, 1.042 mmol] under an argon atmosphere. The mixture was
allowed to stir for 2 h at −78 °C, and the mixture was then slowly
warmed to −42 °C and stirred for an additional 0.5 h, upon which an
orange microcrystalline solid was observed to precipitate. Slow
warming of the solution to room temperature afforded a yellow-
orange solution with an orange precipitate. The reaction mixture was
transferred into the glovebox and cooled to −35 °C. The cold solution
was then filtered to remove the orange crystalline solid, which was
washed with cold toluene followed by cold pentane. The solid was
dried under vacuum to afford pure 1 [373 mg, 0.499 mmol, 48% yield
in one crop]. 1H NMR (22 °C, 500.1 MHz, CD2Cl2): δ 1.57 (s,
Herein, we report comparative studies of the reaction of an
electrophile, such as MeO3SCF3, with the azoxymetallacyclo-
butene scaffold in the compounds Cp*2M(N(O)NCPhCPh)
(M = Ti or Zr). Thermostability of the salt and the fate of
coordination of the methyl cation were found to be highly
dependent on the nature of the metal. In addition, it was also
observed that the azoxymetallacyclobutene scaffold of complex
Cp*2Ti(N(O)NCPhCPh) can be resistant to reducing
conditions and house one unpaired electron, in the form of
B
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C5(CH3)5, 30 H), 3.05 (s, NCH3, 3 H), 5.32 (d, o-Ph, 2 H), 6.75 (m,
Ph, 3 H), 7.08 (m, Ph, 2 H), 7.24 (m, Ph, 3 H). 13C NMR (22 °C,
125.8 MHz, CD2Cl2): δ 11.91 (q, C5(CH3)5), 37.20 (q, NCH3), 127.3
(s, C5(CH3)5), 128.3, 128.8, 130.4, 130.5, 130.7, 133.4 (s), 137.5 (s),
190.8 (s). 19F NMR (22 °C, 470.6 MHz, CD2Cl2): δ −81.7 (s,
O3SCF3). Anal. Calcd for C36H43F3N2O4SZr: C, 57.81; H, 5.79; N,
3.74. Found: C, 56.87; H, 5.65; N, 3.70. Complex 1 is thermally
unstable in solution and must be stored as a solid at −35 °C.
37.2 (q, NCH3), 12.9 (q, C5(CH3)5). 19F NMR (22 °C, 470.6 MHz,
CD2Cl2): δ −78.4 (s, O3SCF3). Anal. Calcd for C36H43F3N2O4STi: C,
61.36; H, 6.15; N, 3.98. Found: C, 61.39; H, 6.21; N, 3.86.
Preparation of [K(2,2,2-Kryptofix)][Cp*2Ti(N(O)NCPhCPh)]
(5). In a 300 mL round-bottom flask under an N2 atmosphere was
dissolved Cp*2Ti(N(O)NCPhCPh) [278 mg, 0.515 mmol] in C6H6
(ca. 30 mL). To the orange-red solution was added in small portions
freshly prepared KC8 [77 mg, 0.570 mmol], causing an immediate
color change of the solution to olive-green concomitant with
generation of black graphite. After 20 min, the solution was filtered
through a frit to remove the graphite and excess KC8. To the green
filtrate was added, while stirring, a cold diethyl ether solution (ca. 5
mL, −35 °C) with dissolved 2,2,2-Kryptofix [193 mg, 0.513 mmol].
Upon complete addition of the cryptand, a small suspension was
observed to form and the solution was allowed to stir for an additional
10 min. The solution was then concentrated to 10−15 mL to afford a
green solid along with an oily residue. To this mixture was added a
copious amount of pentane to induce precipitation of the green solid
along with a green oil (cooling of the mixture also induces
precipitation of green solids). The suspension-oily residue was then
filtered through Celite, and the green solids were washed with a
copious amount of pentane. The solids collected in the Celite were
extracted with 8 mL of THF and filtered, and the filtrate was
concentrated to ca. 6 mL. To the olive-green solution was added a few
drops of pentane, causing formation of a mist at the solvent interface.
To the misty solution was then added a few drops of diethyl ether to
redissolve the mist. The solution was then cooled to −35 °C for 2
days. Dark green single crystals of 5 formed over this period, which
were collected via filtration, washed with cold diethyl ether, and dried
under vacuum [287 mg, 0.300 mmol, 58.3%]. Concentration of the
filtrate after separation of the crystals and addition of excess diethyl
ether, followed by cooling of the solution for 2 more days, afforded a
second crop of crystals [46 mg, 0.0481 mmol, total yield combined
68%]. Mag. suscep. (Evans’ method, THF, 298 K): μeff = 2.28 μB. EPR
(THF, 295 K): giso = 1.9846, Aiso (47Ti, I = 5/2, 7.4%; 47Ti, I = 7/2,
5.4%) = 8.55 G. Aiso (14N, I = 1, 99.64%) = 2.0 G, and Aiso (14N, I = 1,
Preparation of Cp*2Zr(OMe)(O3SCF3) (2). Method A. The
analogous procedure and scale reported for 1 was followed, and after
separation of the toluene insoluble salt, 1, the filtrate was dried under
vacuum and extracted with pentane, the solution filtered through
Celite, the filtrate concentrated to ∼2 mL, and then the solution
cooled to −35 °C for 3−4 days under N2. The yellow solid was
collected via filtration and dried under vacuum to afford pure 2 as
evidenced by 1H NMR spectroscopy [143 mg, 0.264 mmol, 26% yield
in two crops]. Examination of the pentane filtrate (after separation of
2) revealed a mixture of free tolane (PhCCPh) along with compound
2, as well as some other minor intractable products. Complex 2 is
highly soluble in hydrocarbon solvents, but analytically pure complex
can be obtained from recrystallization of the solid from a saturated
1
solution in diethyl ether at −35 °C over 5−6 days. H NMR (22 °C,
500.1 MHz, C6D6): δ 1.80 (s, C5(CH3)5, 30 H), 4.03 (s, OCH3, 3 H).
13C NMR (22 °C, 125.8 MHz, C6D6): δ 10.99 (q, C5(CH3)5), 59.31
(q, OCH3), 123.2 (s, C5(CH3)5), 128.3 (q, O3SCF3). 19F NMR (22
°C, 470.6 MHz, C6D6): δ −78.7 (s, O3SCF3). Anal. Calcd for
C22H33F3O4SZr: C, 48.77; H, 6.14; N, 0.00. Found: C, 48.90; H, 6.03;
N, <0.09.
Preparation of Cp*2Zr(OMe)(O3SCF3) (2). Method B. In a vial
loaded with a stir bar Cp*2Zr(O)(NC5H5) [100 mg, 0.219 mmol] was
partially dissolved in toluene (ca. 5 mL), and the suspension cooled to
−35 °C. A cold toluene solution (−35 °C, ca. 5 mL) containing
MeO3SCF3 [38 mg, 0.236 mmol] was added dropwise, and the brown-
yellow mixture allowed to stir for 1 h. The volatiles were removed
under reduced pressure, the yellow-brown residue was extracted with
pentane and filtered through Celite, and the filtrate was dried under
vacuum to afford pure 2 as a light yellow powder [106 mg, 0.196
mmol, 89% yield]. 1H NMR spectroscopy of the crude reaction
mixture confirmed clean formation of 2 by comparison with authentic
samples reported by method A.
1
99.64%) = 0.5 G. ν = 9.468 GHz, MA = 0.5 G, MF = 100 kHz. H
NMR (22 °C, 500.1 MHz, THF-d8): δ 1.95 (m, crypt), 2.20 (br s),
2.73 (m, crypt), 3.78 (m, crypt), 7.50 (br s). Anal. Calcd for
C52H74O7KTi: C, 65.32; H, 8.01; N, 5.86. Found: C, 65.40; H, 8.24;
N, 5.57.
Preparation of [Cp*2Ti(N(OMe)NCPhCPh)][O3SCF3] (3) and
[Cp*2Ti(N(O)N(Me)CPhCPh)][O3SCF3] (4). In a Schlenk flask
equipped with a stir bar Cp*2Ti(N(O)NCPhCPh) [180 mg, 0.333
mmol] was dissolved in toluene (ca. 30 mL), and the orange solution
was cooled to −78 °C. To the cold solution was added via cannula a
solution of MeO3SCF3 [58 mg, 0.353 mmol] dissolved in cold toluene
(ca. 10 mL, −78 °C), causing a rapid color change to yellow-brown
followed by immediate formation of a brown oil. The reaction mixture
was slowly warmed to room temperature, and the solution transferred
into the glovebox. The yellow mother liquor was decanted with a
pipet, and the brown oil rinsed with toluene (1−2 mL) followed by
diethyl ether (3 mL) and then n-hexanes (2 × 5 mL). The remaining
oil was dried under reduced pressure, extracted with 2 mL of CH2Cl2,
and filtered through a small pad of Celite, and the filtrate was
concentrated to ∼1 mL. Subsequently, the solution was carefully
layered with diethyl ether until a fine mist formed at the interface.
Cooling the CH2Cl2/Et2O layered solution to −35 °C for 6 days
afforded dark red-brown blocks. Decanting of the solution and
washing of the crystals with 2−3 mL of Et2O/pentane (1:1) afforded
Cyclic Voltammetry Measurements of Cp*2Ti(N(O)-
NCPhCPh). Cyclic voltammetry studies were performed in a predried
solution of THF (0.3−0.8 M solution, containing predried and
recrystallized tetrabutylammonium hexafluorophosphate, TBAH,
Aldrich). A platinum disk (2.0 mm diameter, Bioanalytical Systems),
a platinum wire, and a silver wire were employed as the working
electrode, the auxiliary electrode, and the quasi-reference electrode,
respectively. A one-cell compartment was used in the CV experiments.
The electrochemical response was collected with the assistance of an
Eco-Chemie Autolab potentiostat (pgstat20) and the Bioanalytical
System (BAS) 100 W electrochemical workstation. All the potentials
were reported against the ferrocenium/ferrocene couple (0 V)
measured as an internal standard. All spectra were recorded at a
scan rate of 100−200 mV/s under an N2 atmosphere. In all instances,
reversible waves were subjected to scan-rate dependence tests ranging
from 20 to 1000 mV/s. In a typical experiment 8−14 mg of
Cp*2Ti(N(O)NCPhCPh) were dissolved in a TBAH/THF solution at
room temperature.
EPR Measurements of [K(2,2,2-Kryptofix)][Cp*2Ti(N(O)-
NCPhCPh)] (5). The X-band EPR spectrum of 5 was recorded in
THF at 298 K. Typical concentrations ranged from 0.05 to 10 mmol
solutions in THF. The coupling constants were found by simulation to
be Aiso (47Ti, I = 5/2, 7.4%; 47Ti, I = 7/2, 5.4%) = 8.55 G, Aiso (14N, I =
1, 99.64%) = 2.0 G, and Aiso (14N, I = 1, 99.64%) = 0.5 G.
Crystal Structure Determinations. Data Collection and
Structure Solution and Refinement. The crystal was mounted and
centered on a Bruker SMART APEX system. The radiation used was
Mo Kα (λ = 0.71073 A, m = 4.413 mm−1). Rotation and still images
showed diffractions to be sharp, while frames separated in reciprocal
1
pure 3 and 4 in a 1:1 cocrystal mixture, as evinced by H and 13C
NMR spectroscopy [160 mg, 0.227 mmol, 68% yield combined from
two crops]. Note: The solution obtained after separating the crystals
can be cooled further for several more days to afford a second crop of
3 and 4. 1H NMR (22 °C, 500.1 MHz, CD2Cl2): δ 7.56 (m, Ph), 7.47
(m, Ph), 7.37 (m, Ph), 7.04 (m, Ph), 6.91 (t, Ph), 6.02 (br s, Ph), 5.58
(d, Ph), 3.93 (s, OCH3, 3 H), 3.56 (s, NCH3, 3 H), 1.93 (s, C5(CH3)5,
30 H), 1.90 (s, C5(CH3)5, 30 H). 13C NMR (22 °C, 125.8 MHz,
CD2Cl2): δ 225.8 (s, Ti-C), 204.3 (s, Ti-C), 141.3, 140.2, 139.0, 138.2,
133.9, 131.8, 131.6, 131.3, 130.7 (br s, O3SCF3), 130.4 (m), 130.2,
129.7, 129.6, 129.4, 129.1, 128.9, 128.7, 128.0, 127.7, 60.0 (q, OCH3),
C
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Scheme 3. Alkylation of an Azoxymetallacyclobutene Complex with MeO3SCF3 to Afford 1 and 2
space were obtained and provided an orientation matrix and initial cell
parameters. Final cell parameters were obtained from the full data set.
A “hemisphere” data set was obtained, which samples approximately
1.2 hemispheres of reciprocal space to a resolution of 0.84 Å using 0.3
degree steps in ω and using 10 s interval times for each frame.
Absorption corrections were applied using SADABS (references of all
software and sources of scattering factors are contained in the
SHELXTL (version 5.1) program library, G. Sheldrick, Bruker
Analytical X-ray Systems, Madison, WI, USA). The space group was
determined based on systematic absences and intensity statistics.
Direct or Patterson methods were used to locate the heavy atoms, C
atoms, and all other heteroatoms from the E-map. Repeated difference
Fourier maps allowed recognition of all expected H atoms. Prior to
location of H, other atoms were converted to and refined
anisotropically. Hydrogen atoms were refined isotropically and were
placed in calculated (dCH = 0.96 Å) positions. Structures were
collected at 100 K. Crystal data and structure analysis results are
shown in the Supporting Information.
Crystal Structure of 1. An orange thin plate of 1 grown from slow
evaporation of a CH2Cl2 solution was selected under a stereo-
microscope while immersed in Paratone oil (Exxon) to avoid minimal
contact with air. The crystal was removed from the oil using a tapered
fiber, which also served to hold the crystal for data collection. The
structure was solved by direct methods in conjunction with standard
difference Fourier techniques. All non-hydrogen atoms were refined
anisotropically, and hydrogens were placed in calculated positions.
Two chemically equivalent but crystallographically independent
molecules were confined in the asymmetric unit. No absorption
correction was applied. The least-squares refinement converged
normally.
Crystal Structure of 3 and 4. A dark red-brown block of 3 and 4
grown from a CH2Cl2 solution layered with Et2O at −35 °C was
selected under a stereomicroscope while immersed in Paratone oil
(Exxon) to avoid minimal contact with air. The crystal was removed
from the oil using a tapered fiber, which also served to hold the crystal
for data collection. The structure was solved by Patterson methods.
Considerable electron density was confined next to O1. To account for
this, the peak was selected as C1B and refined at nearly 1/2 occupancy
with C1A. No anomalous bond lengths or thermal parameters were
noted except for the methyl carbons C1a and C1b. Both atoms suffer
from disorder and were fixed at approximately 0.52889 and 0.47111
occupancies using the second variable in the FVAR card, respectively.
The thermal parameter was fixed for both atoms such that it would not
change during refinement. Both C1a and C1b were refined
anisotropically, but no hydrogens were placed at calculated positions
for each of these atoms. All other non-hydrogen atoms were refined
anisotropically, and hydrogens were placed in calculated positions. An
absorption correction was applied in the refinement using semi-
empirical methods from psi-scans (max. and min. transmission 0.8953
and 0.8639). The least-squares refinement converged normally. No
extinction coefficient was applied.
oil (Exxon) to avoid minimal contact with air. The crystal was
removed from the oil using a tapered fiber, which also served to hold
the crystal for data collection. The structure was solved by direct
methods in conjunction with standard difference Fourier techniques.
All non-hydrogen atoms were refined anisotropically, and hydrogens
were placed in calculated positions. No absorption correction was
applied in the refinement. The least-squares refinement converged
normally, and no extinction coefficient was applied.
Computational Details. All calculations were performed with the
Gaussian 09 package44 at the B3PW9145 level of theory. Basis sets used
included LANL2DZ for Ti and Zr and 6-31G(d) for H, C, N, and O.46
The basis set LANL2DZ is the Los Alamos National Laboratory ECP
plus a double-ζ valence on Ti/Zr.47 All optimizations were performed
with C1 symmetry, and all minima were confirmed by analytical
calculation of frequencies, which were also used to compute zero point
energy corrections without scaling. The initial geometry of Cp2Ti(N-
(O)NCMeCMe) and Cp2Zr(N(O)NCMeCMe) was adapted from a
refined crystal structure of (Cp*)2Ti(N(O)NCPhCPh)11 with all
methyl groups on the Cp* replaced with H and phenyl groups on the
metallacycle replaced by methyls.
RESULTS AND DISCUSSION
■
Reaction of Zr(N(O)NCPhCPh) and Cp*2Zr(O)(NC5H4)
with MeOTf. As reported in earlier work,11,12 treatment of a
toluene solution of Cp*2Zr(η2-PhCCPh) with N2O at −78 °C
results in insertion of nitrous oxide into the Zr−C(alkyne)
bond to afford the orange and thermally unstable complex
Cp*2Zr(N(O)NCPhCPh), which was not characterized.11
Upon generation of metastable Cp*2Zr(N(O)NCPhCPh), we
found that addition of a cold toluene solution of MeO3SCF3
(−78 °C) rapidly results in formation of the orange salt
[Cp*2Zr(N(O)N(Me)CPhCPh)][O3SCF3] (1) in 48% iso-
lated yield (Scheme 3). The ionic nature of 1 allows for facile
separation from the reaction mixture, and, unlike its
predecessor Cp*2Zr(N(O)NCPhCPh), compound 1 is rela-
tively stable as a solid when stored at −35 °C. The most salient
spectroscopic feature associated with 1 is the observation of a
singlet in the 1H NMR spectrum at 3.05 ppm, which
corresponds to the methyl resonance derived from addition
of the electrophile to the N(O)N motif. Complex 1 also
exhibits a singlet at 1.57 ppm for the methyl protons of the
ancillary Cp* as well as multiplets ranging from 5.3 to 7.3 ppm,
arising from the two inequivalent phenyl groups (10 hydrogens
total). The insoluble nature of 1 in nonprotic and nonpolar
solvents such as pentane, ether, and arenes suggests this system
to be a discrete salt, while the 19F NMR spectrum also implies a
OTf− counterion to be present in this species (−81.7 ppm).
The canonical forms, which are possible in the azoxyme-
tallacyclobutene moiety in Cp*2Zr(N(O)NPhCCPh) (vide
supra, Scheme 2), suggest the β-O and β-N atoms to be the
most exposed nucleophilic sites. In order to unambiguously
Crystal Structure of Complex 5. A dark green block of 5 grown
from a THF solution layered with a few drops of pentane/Et2O at −35
°C was selected under a stereomicroscope while immersed in Paratone
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address the site of alkylation, a single-crystal X-ray diffraction
study was undertaken. Accordingly, a single crystal was grown,
and the molecular structure of 1 is shown in Figure 1,
undergoes thermal extrusion of N2 to generate the oxy-
metallacyclobutene complex Cp*2Zr(OCPhCPh).12
The moderate to low yield associated with the preparation of
1 suggested that other products could be formed from this
reaction. Accordingly, it was found that the filtrate resulting
from the separation of 1 contained another Zr(IV) material,
namely, the alkoxide complex Cp*2Zr(OMe)(O3SCF3) (2) in
26% yield, subsequent to recrystallization from pentane at −35
°C (Scheme 3). The low isolated yield of the substance can be
attributed to its high lipophilicity in most common organic
solvents, including pentane and hexane, in addition to complex
1
1 being the major product. Compound 2 displays H NMR
spectral resonances consistent with two equivalent Cp* rings
(1.80 ppm), and a downfield singlet shift, which is diagnostic of
a methoxide ligand (4.03 ppm). In addition, formation of a
triflato complex was evidenced by a singlet at −78.7 ppm in the
19F NMR spectrum. Intuitively, the high solubility of 2 in most
common organic solvents suggests the triflate anion to be
coordinating to the coordinatively unsaturated zirconium(IV)
center, especially when similar Cp*2Zr(OR)(X) (R = SiMe3, X
= Cl; R = Me, X = I) have been reported.15c,48 To substantiate
the connectivity of 2, an independent and more convenient
route to its formation was undertaken. It was found that
complex 2 could be prepared in quantitative yield by alkylation
of Parkin’s terminal oxo species Cp*2Zr(O)(NC5H5)15c with
MeO3SCF3 (Scheme 4).48
Figure 1. Perspective view of the molecular structure of only the
cation component in complex 1 (and one crystallographically
independent molecule), depicting the atom-labeling ellipsoid plot at
the 50% probability level. H atoms have been omitted for clarity
purposes.
illustrating one of the two chemically equivalent, but crystallo-
graphically independent molecules in the asymmetric unit. One
interesting feature associated with the molecular structure of 1
is the retention of an azoxymetallacyclobutene ring in
Cp*2Zr(N(O)NPhCCPh), a previously reported unstable
complex that eluded crystallographic analysis due to its
propensity to eject N2 and subsequently form the oxy-
metallacyclobutene compound (Scheme 1).12 In addition, the
solid-state structure confirms methylation of the β-N position
of the metallacycle (N(2)−C(1), 1.457(8) Å). For comparison,
the N(1)−N(2) distance of 1.318(7) Å in the azoxymetalla-
cycle fragment is relatively elongated from that of the titanium
and neutral analogue, Cp*2Ti(N(O)NCPhCPh).11 Likewise,
the N−O distance is shorter than in Cp*2Ti(N(O)NCPhCPh),
implying a canonical structure such as the one depicted in
Scheme 3. The metrical parameters of 1 are overall consistent
with an approximately planar five-membered metallacycle
(Table 2) with a long N−N bond when compared to the
The formation of 2 from the alkylation of metastable
Cp*2Zr(N(O)NCPhCPh) suggested that 1, or the byproduct
resulting from N2 extrusion in Cp*2Zr(N(O)NCPhCPh),
Cp*2Zr(OCPhCPh), could be reacting with MeO3SCF3. It
1
was observed by H NMR spectroscopy that complex 1 slowly
decays in solution at −35 °C (several days) to afford a
complicated mixture of products, one of which was indeed 2. In
fact, thermolysis of complex 1 in chlorobenzene affords 2 and
tolane among many other byproducts, but isolation of any
zirconium complexes from the reaction mixture was hampered
by their high solubility as well as their formation in low yield
(Scheme 4). Independently, it was found that Cp*2Zr-
(OCPhCPh) does not react with MeO3SCF3 under similar
conditions to form 1, therefore suggesting that 2 is most likely
formed from the decomposition of 1. A less likely pathway
might involve alkylation of Cp*2Zr(η2-PhCCPh) preceding
N2O insertion. We also argue against this pathway since
MeO3SCF3 does not react with the diphenylacetylene complex
under similar conditions. Instead, we propose that complex 2 is
likely formed from alkylation at the β-nitrogen to form 1,
followed by migration to the β-oxygen, which then undergoes a
series of steps including ring expansion as well as N2 and tolane
elimination by retrocycloadditions. Another process could be a
concerted addition of the methyl cation directly to the β-O
atom of Cp*2Zr(N(O)NCPhCPh) (without going through 1),
followed by analogous transformation steps such as ring
expansion as well as N2 and tolane elimination or
retrocycloaddion. Regardless, methyl alkylation at the β-N
intuitively implies complex 1 to be a kinetic product. The
hypothesis of whether alkylation takes place at the β-oxygen or
β-nitrogen was further investigated by using a more stable
analogue of 1, namely, the complex Cp*2Ti(N(O)NCPhCPh),
prepared analogously from the reaction of Cp*2Ti(η2-PhCPh)
with N2O.11
Table 2. Selected Bond Lengths (Å) and Dihedral Angles
(deg) for Complex 1
Zr(1)−N(1)
Zr(1)−C(31)
N(1)−O(1)
N(1)−N(2)
N(2)−C(1)
N(2)−C(41)
Zr(1)−N(1)−N(2)−C(41)
N(1)−N(2)−C(41)−C(31)
Zr(1)−C(31)−C(41)−N(2)
2.262(5)
2.309(6)
1.256(6)
1.318(7)
1.457(8)
1.458(8)
−2.2(6)
2.1(7)
−1.0(7)
similar functional group in p-azoxyanisole, p-MeOC6H4N-
(O)NC6H4-p-OMe (N−N = 1.218(5) Å).33 As a result,
complex 1 could also possess canonical forms A and C, as
shown in Scheme 2, thereby forming a salt by positive charge at
the β-nitrogen atom. The trapping and isolation of a discrete
salt, 1, undoubtedly implies that the thermally unstable
intermediate Cp*2Zr(N(O)NCPhCPh) must be the species
reacting with the electrophile, since such an intermediate
Alkylation of Cp*2Ti(N(O)NCPhCPh) with MeOTf.
When a toluene solution of Cp*2Ti(N(O)NCPhCPh) was
treated with one equivalent of MeO3SCF3 at −78 °C, an
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Scheme 4. Independent Syntheses of 2 by Reaction of the Terminal Oxo Cp*2Zr(O)(py) with MeO3SCF3 or via Thermolysis of
1
Scheme 5. Alkylation of the Azoxymetallacyclobutene Complex Cp*2Ti(N(O)NCPhCPh) with MeO3SCF3 to Form an Equal
Mixture of 3 and 4
immediate precipitation of a brown oil was observed. Decanting
the solution and extraction of the oil with CH2Cl2 followed by
layering with Et2O afforded dark maroon single crystals of a
mixture of two salts, [Cp*2Ti(N(OMe)NCPhCPh)][O3SCF3]
(3) and [Cp*2Ti(N(O)N(Me)CPhCPh)][O3SCF3] (4), in
68% combined isolated yield (Scheme 5). The 1H NMR
spectrum of the salts revealed a nearly 1:1 ratio of compounds
with characteristic resonances for the methyl protons on the
Cp* rings as well as multiplets for a total of four different
phenyl environments (two per complex). The most definitive
evidence for formation of two complexes was the observation of
two singlets residing at 3.93 (O-methyl moiety in 4) and 3.36
(N-methyl moiety in 3) ppm. We tentatively assign the latter
resonances on the basis of electron-withdrawing effects of O vs
N as well as comparison with the observed resonance for the N-
bound methyl analogue, 1 (vide supra, 4.03 ppm). In addition,
the 13C NMR spectrum corroborates formation of a mixture of
salts via the observation of two distinct Ti−C resonances at
225.8 and 204.3 ppm, as well as the methyl carbon resonances
(formally from Me+ in MeOTf) at 60.0 and 37.2 ppm,
respectively.
molecular structure (and NMR spectroscopy) provides
concrete evidence for methylation taking place at both the β-
O and β-N, the metrical parameters about the Cp*2Ti(N(O)-
NCPhCPh) scaffold represent the average of the two systems,
therefore limiting our discussion of resonances from the
metrical parameters (Table 3 lists selected metrical parameters
Table 3. Selected Bond Lengths (Å) for Complexes
Cp*2Ti(N(O)NCPhCPh) and 5
Cp*2Ti(N(O)NCPhCPh)11
3/4
5
Ti−N(1)
Ti−C(31)
2.088(4)
2.210(5)
1.294(7)
1.281(6)
1.435(7)
1.344(7)
n/a
2.132(3)
2.187(3)
1.270(4)
1.321(3)
1.441(4)
1.357(4)
1.647(8)
1.449(6)
2.185(4)
2.208(5)
1.279(5)
1.295(5)
1.430(6)
1.349(7)
n/a
N(1)−N(2)
N(1)−O(1)
N(2)−C(41)
C(31)−C(41)
O(1)C(1B)
N(2)C(1A)
n/a
n/a
for the average of the core structures of 3 and 4 and
comparison to the neutral species Cp*2Ti(N(O)NCPhCPh).11
As anticipated, multiple attempts to separate complexes 3 from
4 were hindered by their similar solubilities.
Single-crystal X-ray diffraction studies revealed not only that
both 4 and 5 cocrystallize (one molecule per asymmetric unit)
but that the methyl group is both observed at the β-O and β-N
with nearly half-occupancies (Figure 2). Although our
Synthesis of the Ti(III) Radical Anion Salt [K(2,2,2-
Kryptofix)][Cp*2Ti(N(O)NCPhCPh)] (6) and Theoretical
Analysis of the Azoxytitanacyclobutene Fragment. Our
studies have demonstrated that the “N(O)N” moiety (derived
from N2O) in systems of the type Cp*2M(N(O)NCPhCPh)
(M = Zr and Ti) can be further functionalized with an
electrophile to form a metal-bound dialkylnitrosoamine, and in
the case of titanium, the O-methylated regioisomer can be
obtained as a mixture with the N-methylated product. Having
studied the reactivity of complexed nitrous oxide with an
electrophile, we inquired if the azoxymetallacyclobutane motif
would be stable to reducing conditions. Since N2O is by itself a
thermodynamically powerful oxidant, one would anticipate
systems such as Cp*2M(N(O)NCPhCPh) to also be good
oxidants. Accordingly, complex Cp*2Ti(N(O)NCPhCPh) was
examined by cyclic voltammetry to reveal an irreversible one-
Figure 2. Perspective view of the molecular structure of the cationic
component for a cocrystal for the average of complexes 3 and 4,
depicting the atom-labeling ellipsoid plot at the 50% probability level.
Both β-N and β-O methyl alkylated sites are shown.
+/0
electron anodic event at 0.30 V (vs FeCp2 referenced at 0.0
V). A cathodic scan showed a reversible one-electron process
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centered at −1.80 V, thus implying that complex Cp*2Ti(N-
(O)NCPhCPh) is not a good oxidant given such a highly
negative potential. Chemical reduction of Cp*2Ti(N(O)-
NCPhCPh) with one equivalent of KC8 in benzene afforded
an extremely air-sensitive green solution, from which the
Ti(III) radical anion [K(2,2,2-Kryptofix)][Cp*2Ti(N(O)-
NCPhCPh)] (5) has been isolated as an encrypted dark
green crystalline material in 68% yield (Scheme 6).
zirconium analogue of 5 were unsuccessful, even when
performing the reaction at −78 °C and in the presence of
cryptand as a trap.51
Single crystals of 5 reveal a discrete salt with an essentially
intact azoxymetallacyclobutene ligand, grossly similar to that of
its precursor Cp*2Ti(N(O)NCPhCPh).11 The only significant
perturbation is the Ti−N(1) bond distance of 2.185(4) Å,
which reflects slight elongation from that of its neutral
counterpart (Figure 4). Table 2 lists selected metrical
Scheme 6. Reduction of Cp*2Ti(N(O)NCPhCPh) with KC8
a
to Form the Ti(III) Complex Salt 5
a
H atoms have been omitted for clarity purposes.
Complex 5 is paramagnetic, therefore displaying only a
couple of extremely broad resonances in the 1H NMR spectrum
(2.20 and 7.50 ppm), while relatively sharp resonances for the
cryptand cationic portion (1.95, 2.73, and 3.70 ppm) are clearly
visible. At room temperature, a solution magnetic moment
measurement of complex 5 in THF solution was consistent
with this complex having one unpaired electron (μeff = 2.28 μB,
Evans’ method). In addition, the room-temperature X-band
EPR spectrum of 5 in THF revealed the unpaired electron to
reside primarily on titanium (giso = 1.9846) with Aiso = 8.55 G
(47Ti, I = 5/2, 7.4%; 47Ti, I = 7/2, 5.4%), in addition to some
superhyperfine coupling to the α-N with Aiso = 2.0 G (14N, I =
1, 99.64%), and coupling to the more distant β-N (Aiso = 0.5 G,
14N, I = 1, 99.64%) composing the azoxymetallacycle ring
(Figure 3). Although the coupling constant of the unpaired
electron with titanium is characteristic for Cp*2Ti(III)L or
Cp′2V(IV)L2 (Cp′− = C5H5 or C5H4Me) systems studied
previously,49,50 complex 5 represents the first example of a
stable transition metal radical species derived from N2O
complexation. Unfortunately, multiple attempts to prepare the
Figure 4. Perspective view of the molecular structure of complex 5,
showing the atom-labeling ellipsoid plot at the 50% probability level. H
atoms and a THF confined in the asymmetric unit have been omitted
for clarity.
parameters for Cp*2Ti(N(O)NCPhCPh)11 and the radical
anion core of 5. Intuitively, inclusion of an unpaired electron
should not deform the core structure in Cp*2Ti(N(O)-
NCPhCPh), but only permutate the Ti−N distance slightly,
since the orbital housing the unpaired electron should be a
nonbonding metal d orbital. Surprisingly though, the N−O and
N−N distances are relatively unperturbed when compared to
Cp*2Ti(N(O)NCPhCPh), even though the X-band EPR
spectrum implies some delocalization of the unpaired electron
onto these sites.
In order to understand the chemistry of the azoxymetalla-
cyclobutene moiety toward electrophiles or reductants, we
relied on theoretical studies using the Gaussian 09 package44 at
the B3PW91 level of theory to dissect the molecular picture of
the complexes Cp*2M(N(O)NCPhCPh) (M = Ti and Zr).
When inspecting the molecular representation of the simplified
models of Cp*2M(N(O)NCPhCPh), namely, Cp2M(N(O)-
NCMeCMe), it can be observed how the azoxymetallacyclo-
butene moiety dominates the frontier orbital picture. For
example, the HOMOs for each metal complex are nearly
identical and expose the NN π-bond, which is out of phase
with the β-O atom (Figure 5), and thus consistent with
resonance structures A or C depicted in Scheme 2 (vide supra).
From the HOMO, the most obviously exposed nucleophilic
sites are the β-N and β-O. This pattern well explains why Me+
addition occurs at both the β-N and β-O for M = Ti. However,
it does not explain why alkylation occurs only (based on our
observations) at the β-N for M = Zr. In fact, close inspection of
the orbital populations for each individual atom do not reflect
any notable discrepancies in the atomic contributions from the
β-N and β-O atoms. For M = Ti, the LUMO and LUMO+1
illustrate a titanium d(x2−y2) orbital that interacts in a “slipped”
σ fashion with the α-N. In contrast to the HOMO orbitals, the
Figure 3. X-band EPR spectrum of complex 5, recorded in THF
solution at 298 K. The simulated spectrum (sim.) is shown above the
experimental spectrum (exp.).
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activated N2O is further functionalized at both the β-O and β-N
moieties. At this end, the apparent variation in reactivity of the
electrophile with each azoxymetallacycle remains unclear.
However, we suggest that the observed divergent reactivity is
due to the Zr−O versus Ti−O bond strengths and relates to
the thermodynamic stability of the titanium azoxymetallacycle
precursor complex relative to that of zirconium, which
spontaneously extrudes N2 to yield the oxymetallacyclobutene
complex. Using Cp*2Ti(N(O)NCPhCPh) however, we are
able to trap the O-methylated intermediate, resulting in N2 and
tolane extrusion in the case of Zr. The regioselectivity for Me+
addition suggests the HOMO in Cp*2M(N(O)NCPhCPh) to
be predominantly dominated by the lone pairs in the β-O and
β-N positions. In contrast, the LUMO for M = Ti represents a
slipped nonbonding metal-based σ orbital. This feature
contrasts M = Zr, where there is more azoxymetallacycle
character in the LUMO. Despite our shortcoming to use N2O
as a catalytic O atom transfer source, complexes of the type
Cp*2M(N(O)NCPhCPh) represent attractive synthetic targets
since N2O can be a reagent for the preparation of the
nitrosimine functionality, which can be further functionalized
with Me+ to a dialkylnitrosamine. Given the fact that N2O
binding to metal complexes has been predicted and shown to
occur preferentially at the terminal nitrogen atom,24b,52 other
systems can potentially harness the oxidizing power of this
resourceful, but underutilized reagent.
Figure 5. Most important frontier orbitals computed for the
complexes Cp*2M(N(O)NCPhCPh). Shown are the HOMO (top
left) and LUMO (bottom left) for M = Ti, and HOMO (top right)
and LUMO (bottom right) for M = Zr. The initial geometry of
Cp2Ti(N(O)NCMeCMe) and Cp2Zr(N(O)NCMeCMe) was adapted
from a refined crystal structure of Cp*2Ti(N(O)NCPhCPh),11 where
all methyl groups on the Cp* have been replaced with H and phenyl
groups on the metallacycle have been replaced with methyls.
LUMO of Cp2Zr(N(O)NCMeCMe) shows a π-like d orbital of
Zr having an in-phase combination with the β-N but where this
is significantly more delocalized about the azoxymetallacyclo-
butene moiety (Figure 5). This might explain why chemical
reduction of Cp*2Zr(N(O)NCPhCPh) is not clean since the
LUMO is not just an empty d orbital. In fact, the LUMO+1 for
Cp2Zr(N(O)NCMeCMe) has more similarities to the LUMO
observed in Cp2Ti(N(O)NCMeCMe), which shows a
hybridized nonbonding metal-based orbital with mostly
d(x2−y2) character (Figure 5). Petersen and Dahl have
described similar d1 bent metallocenes of vanadium, namely,
Cp′2VL2(4+), and have suggested the unpaired electron to
reside primarily on a vanadium al-type MO mainly composed of
d(z2), but augmented with some d(x2−y2).50 Due to the minor
structural differences observed between Cp*2Ti(N(O)-
NCPhCPh) and the Ti(III) derivative [Cp*2Ti(N(O)-
NCPhCPh)]− (vide supra), the above results suggest the
unpaired electron to virtually reside in a weakly bonding wedge
orbital (or nonbonding SOMO), which should slightly reduce
the bond order of the Ti and α-N, but also the NN π- and
N−O interaction. The nonbonding nature of the SOMO is not
surprising, since attempts to alkylate complex 5 with various
electrophiles resulted in clean oxidation to Cp*2Ti(N(O)-
NCPhCPh) rather than further functionalization. As a result,
the frontier orbitals predicted by theoretical methods explain
the mode of reactivity observed with Cp*2Ti(N(O)NCPhCPh)
well and corroborate our structural and spectroscopic findings
for its radical anion, [Cp*2Ti(N(O)NCPhCPh)]−. Unfortu-
nately, we are uncertain as to why Me+ does not alkylate the β-
O site (or why this species is not observed) of Cp*2Zr(N(O)-
NCPhCPh), but we do propose that the Me+ does migrate to
the β-O, resulting in degradation of the metallcycle.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files for the structures of compounds 1, 3/4, and 5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mindiola@sas.upenn.edu.
Present Addresses
†Department of Chemistry, University of Pennsylvania,
Philadelphia, PA 19104, USA.
‡Department of Chemistry, Earlham College, Richmond, IN
47374, USA.
§Department of Chemistry and Pharmacy, Inorganic Chem-
istry, Friedrich-Alexander University Erlangen-Nurnberg
(FAU), 91058 Erlangen, Germany.
̈
Notes
The authors declare no competing financial interest.
⊥Deceased March 6, 2014.
ACKNOWLEDGMENTS
■
This paper is dedicated to the memory of Prof. Gregory L.
Hillhouse, a terrific chemist, advisor, and beloved friend. In
order to pay our last respects to a great and unique scientist, we
voice both mentally and physically that a sad day has come, one
where we wish a fond farewell to a wonderful man, a man
whom we were all so very privileged to have known and worked
with. We thank the National Science Foundation for financial
support, Professor M. D. Hopkins and Dr. Daniel E. Haines for
access to a BAS potentiostat and software, and Dr. Ian Steele
for some assistance with X-ray crystallography. D.J.M. acknowl-
edges postdoctoral fellowship support from the Ford
Foundation and the National Institutes of Health.
CONCLUSIONS
■
In this work, we have shown that metal coordination of nitrous
oxide forms an azoxymetallacycle complex of the type
Cp*2M(N(O)NCPhCPh) (M = Ti and Zr), in which the
H
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M.; Tomaszewski, J.; Fan, H.; Caulton, K. G. J. Am. Chem. Soc. 2007,
129, 8706.
(30) Labahn, T.; Mandel, A.; Magull, J. Z. Anorg. Allg. Chem. 1999,
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