Oxygen Atom Transfer to Cationic PCPNi(II) Complexes Using Amine-N-Oxides

Article abstract of DOI:10.1021/acs.inorgchem.7b02766

Three PC_sp3P pincer ligands differing in the aryl group linking the phosphine arms with the anchoring carbon donor were used to support square planar Ni(II) bromide complexes 1-3_Br. Exchange of the coordinating bromide anion for the

Full text of DOI:10.1021/acs.inorgchem.7b02766

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Oxygen Atom Transfer to Cationic PCPNi(II) Complexes Using
Amine‑N‑Oxides
Etienne A. LaPierre,† Marissa L. Clapson,† Warren E. Piers,*,† Laurent Maron,*,‡ Denis M. Spasyuk,§
and Chris Gendy†
†Department of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4
‡LPCNO, Universite
́
de Toulouse, INSA, UPS, LPCNO, 135 avenue de Rangueil, F-31077 Toulouse, France
§Canadian Light Source, Inc., 44 Innovation Boulevard, Saskatoon, Saskatchewan, Canada S7N 2V3
S
* Supporting Information
ABSTRACT: Three PCsp3P pincer ligands differing in the aryl group linking the phosphine arms with the anchoring carbon
donor were used to support square planar Ni(II) bromide complexes 1−3Br. Exchange of the coordinating bromide anion for the
more weakly coordinating triflate (OTf) or hexafluoroantimonate (SbF6) anions was accomplished by treatment with AgX or
TlX salts to give compounds 1−3X; compounds 1OTf, 1SbF , 2Br, 2OTf, 3Br, and 3SbF were all characterized by X-ray
6
6
crystallography. The reactions of these Ni(II) compounds with the amine-N-oxide oxygen atom transfer agents ONMe3 and
ONMePh2 were explored. For ONMe3, reactions with 2 equiv gave products in which one arm of the pincer ligand was oxidized
to a PO unit, with the other amine-N-oxide ligated to the Ni(II) center, forming products 5−6X; compounds 4OTf, 5OTf, and
6SbF were characterized crystallographically. Transient amine-N-oxide adducts prior to ligand oxidation were observed in some
6
reactions. For the more effective O atom donor ONMePh2, reactions were very rapid and a second oxidation of the remaining
phosphine arm was observed, producing a Ni(II) species with an OCO pincer ligand (7SbF ). All compounds were fully
6
characterized. Experiments aimed at trapping transient Ni(IV) oxo intermediates (with cyclohexadiene, KH, and various Lewis
acids) indicated that such species were not involved in the reaction. This was supported by density functional theory (DFT)
computations at the B3PW91 level, which indicated that direct O atom insertion into the Ni−P bonds without the intermediacy
of a Ni oxo species was the low-energy pathway.
later-metal-element multiple bonds is through the use of
tridentate pincer ligands, which provide a rigid, chelating ligand
array that enforces a square planar geometry. While this
strategy has met with modest success, leading to the isolation
and structural characterization of iridium nitrido complexes,22,23
and the isolation of a compound proposed to be a platinum-oxo
complex,24 for the most part, intermediary or transiently
generated oxo and nitrido complexes have eluded explicit
structural elucidation.25−33 Spectroscopic observation has been
noted in some cases,25−27,30,34−36 but evidence for these species
often hinges on the characterization of the products of trapping
experiments with external reagents or isolation of “decom-
position” products28,29,31,32 resulting from the oxidation of one
or both of the arms (usually phosphines) of the pincer ligand
framework. Products arising from E-centered radical chemistry
INTRODUCTION
■
Late-transition-metal terminal oxo complexes represent a
challenging and intriguing target for synthetic inorganic and
organometallic chemists. These complexes are proposed as
intermediates in a variety of catalytic and biomimetic reactions,
such as oxidation of hydrocarbons1−7 and water splitting
schemes.8−16 However, late-transition-metal oxo complexes (as
well as related nitride complexes) are fundamentally unstable,
because of the population of M−E (E = N, O) π*-orbitals.17−20
Because of this destabilizing bonding situation, examples of
transition-metal multiple bonds in complexes with a tetragonal
ligand field past group 8 are sparse, since the high oxidation
states required for necessarily low enough d-electron counts are
less accessible.
The degree of π*-orbital population is dependent on
molecular symmetry and d-electron count, with square planar
complexes tolerating higher d-electron counts than octahedral
complexes.18,21,22 Thus, one strategy for the stabilization of
Received: October 29, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02766
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have also been observed, such as radical coupling of nitride
ligands22,27 or H atom abstraction.28,32 Perhaps the most
notable example of the former is the slow conversion of
Milstein’s aforementioned platinum-oxo complex (which, to
date, is the only claimed example of an isolated terminal group
10 metal multiple bond in a tetragonal ligand field) to the PCO
pincer complex in which the oxo has been inserted into the Pt−
P bond (Scheme 1a).24 In other systems, such as the
Vlugt chemistry of Scheme 1b. The PCP system is somewhat
more electron-donating,37,38 in comparison, and the anchoring
carbon donor can be sp3 or sp2 hybridized, creating the
possibility of ligand cooperativity.39 Furthermore, modifications
to the aryl group linking the ligand arms to the central donor
can provide another lever for electron-donating control.40
Nickel(II) complexes of these ligands can be readily
prepared,41−43 and herein we explore the reactivity of three
such complexes (Scheme 1d) with oxygen atom transfer agents.
The parent phenyl linked system L1 can be made more
electron-donating via incorporation of an NMe2 group para to
the central carbon donor, (L2) while the benzothiophene linked
ligand, L3,44 is a more planar, rigidified structure. With this
array of ligands, the potential intermediacy of a high-valency
nickel-oxo complex was explored; however, in each system,
oxygen atom insertion into the Ni−P bonds of the ligand was
observed, with no experimental or computational evidence for
Ni oxo species on the reaction path. The implications of these
observations for further ligand design are discussed.
Scheme 1
EXPERIMENTAL SECTION
■
For general experimental considerations and modified syntheses of
starting materials L1 and 1Br,41 see the Supporting Information.
Synthesis of Diphenylmethylamine-N-oxide (ONPh2Me). To
a 150-mL round-bottom flask was added 1.073 g of methyldiphenyl-
amine (6.22 mmol) dissolved in ca. 75 mL of chloroform. In a second
150 mL round-bottom flask, 2.015 g of 72% m-chloroperbenzoic acid
(11.0 mmol, 1.25 equiv) was dissolved in 50 mL of chloroform. The
solution of methyldiphenylamine was cooled in an ice bath and the
solution of m-chloroperbenzoic acid was added dropwise to the
solution of the amine over 10 min. The solution was warmed to room
temperature and stirred for 18 h. A color change from clear to light
orange was observed. Solvent was removed in vacuo and the crude
mixture purified by column chromatography on 60 g of basic alumina
using 3:1 CHCl3:CH3OH as an eluent. Fractions containing product
(Rf = 0.9) were combined and solvent removed in vacuo. The residue
was dissolved in minimal hot acetone, layered with hexanes and cooled
to −20 °C for 72 h to yield a beige powder (0.838 g, 4.21 mmol, 72%
yield). 1H NMR (500 MHz, Chloroform-d) δ 7.73 (dd, J = 7.9, 1.8 Hz,
4H, Ar-H), 7.40 (m, 4H. Ar-H), 7.37−7.31 (m, 2H, Ar-H), 3.93 (s,
3H, CH3). 13C NMR (126 MHz, methylene chloride-d2) δ 129.15,
128.64, 121.43, 62.53. HRMS (ESI) Calcd: 200.107, Found: 200.1076.
Synthesis of 1OTf. To a solution of 1Br (300 mg, 0.557 mmol) in
THF (3 mL) was added 1.05 equiv of AgOTf (150 mg, 0.583 mmol),
resulting in the immediate precipitation of AgBr. The suspension was
filtered through a 0.1 μm PTFE syringe filter and the filtrate is
concentrated under reduced pressure to yield a red solid. The solid
was extracted into minimal toluene (∼2 mL) and resulting solution
was layered with ∼15 mL of n-pentane and refrigerated at −30 °C
overnight to yield red-orange block crystals of 1OTf. Yield: 277 mg
PNPNi(IV) nitrido complex proposed by van der Vlugt and
co-workers (Scheme 1b), the ligand oxidation is too rapid for
observation of the transient Ni nitride; the resulting product of
nitrido insertion rapidly activates the C−H bond of benzene.29
This contrasts with a variety of group 9 systems where the
primary decomposition pathway is ligand C−H bond
activation,28,32,34 or radical coupling of nitride ligands to yield
dinitrogen adducts.22,27 However, there are cobalt-based
systems in which ligand oxidation is the primary mode of
reactivity, such as Meyer’s tripodal Co(IV) nitride complex,30
and complexes arising from oxygen atom transfer to Caulton’s
PNPCo(I) complex (Scheme 1c).26
Attempted generation of a high-valency nickel-oxo species via
oxygen transfer reactions is of particular interest, given the
relative dearth of examples of tetragonal group 10 metal
compounds with multiple bonds, the reported evidence for
isolation of a Pt(IV) oxo complex,24 and the potential
importance of these species in Ni-catalyzed oxidation
reactions.36 We recently developed a rigid PCP pincer ligand
modeled after the PNP ligand system featured in the van der
1
(0.457 mmol, 82%). 31P NMR (243 MHz, benzene-d6) δ 44.1. H
NMR (600 MHz, benzene-d6) δ 7.07−7.02 (m, 2H, Ar-H), 6.97−6.90
(m, 4H, Ar-H), 6.87 (t, J = 7.2 Hz, 2H, Ar-H), 5.35 (s, 1H, Ni−CH),
2.67 (ddq, J = 14.3, 7.1, 4.4, 2.6 Hz, 2H, CH3−CH−CH3), 2.21 (ddq, J
= 10.5, 7.1, 3.2 Hz, 2H, CH3−CH−CH3), 1.45 (dd, J = 8.9, 7.1 Hz,
6H, CH−(CH3)2), 1.38 (dd, J = 7.7 Hz, 6H, CH−(CH3)2), 1.13 (dd, J
= 7.6 Hz, 6H CH−(CH3)2), 1.00 (dd, J = 6.6 Hz, 6H, CH−(CH3)2).
13C NMR (126 MHz, benzene-d6) δ 157.30 (t, J = 17.2 Hz, P-Caryl),
130.96, 130.71 (t, J = 19.5 Hz, Caryl), 129.50(Caryl), 125.52 (t, J = 6.7
Hz, Caryl), 124.65 (t, J = 3.2 Hz, Caryl), 34.69 (t, J = 7.8 Hz, Ni-C),
23.75 (t, J = 10.8 Hz, CH−(CH3)2), 23.15 (t, J = 9.2 Hz, CH−
(CH3)2), 19.02 (t, J = 3.2 Hz, CH−(CH3)2), 18.08 (CH−(CH3)2),
17.26 (CH−(CH3)2), 16.18 (CH−(CH3)2). 19F NMR (471 MHz,
benzene-d6) δ −78.2. Elemental Analysis: Calcd (%): C 51.42; H 6.14.
Found: C 51.50; H 6.11.
Synthesis of 1SbF . To a solution of 1Br (500 mg, 0.929 mmol) in
6
THF (3 mL) was added 1.00 equiv of AgSbF6 (320 mg, 0.929 mmol),
B
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resulting in the immediate precipitation of AgBr. The suspension was
filtered through a 0.1 μm polytetrafluoroethylene (PTFE) syringe filter
and the filtrate was concentrated under reduced pressure and the
residue was extracted into minimal DCM, layered with 15 mL of n-
pentane and refrigerated at −30 °C for 3 days to yield a red
microcrystalline solid. Crystals suitable for analysis by single-crystal X-
ray diffraction (XRD) studies were grown by layering 1 mL of a 50
= 3.5 Hz, NMe2-Caryl), 145.99 (t, J = 17.1 Hz, P-Caryl), 131.31 (t, J =
19.1 Hz, Caryl), 125.77 (t, J = 7.9 Hz, Caryl), 114.47 (Caryl), 113.61
(Caryl), 39.40 (N-(CH3)2), 33.51 (t, J = 6.5 Hz, Ni-CH), 23.83 (t, J =
10.4 Hz, CH(CH3)2), 23.30 (t, J = 8.9 Hz, CH(CH3)2), 19.15 (t, J =
3.3 Hz, CH(CH3)2), 18.28 (CH(CH3)2), 17.49 (d, J = 2.4 Hz,
CH(CH3)2), 16.57 (CH(CH3)2). 19F NMR (471 MHz, benzene-d6) δ
−78.2. Elemental analysis: Calcd (%): C 51.96; H 6.83; N 4.04.
Found: C 52.10; H 6.64; N 3.99.
mg/mL DCM solution of 1SbF with 3 mL of n-pentane and
6
Synthesis of 3Br. To a 25-mL thick-walled glass vessel equipped
with a Kontes needle valve was added L3 (315 mg, 0.614 mmol),
NiBr2(MeCN)2 (188 mg, 0.625 mmol), and NEt3 (0.24 mL). The
solids were dissolved in ca. 4 mL of bromobenzene and the solution
was heated for 18 h at 60 °C. The reaction solution was cooled and
filtered through a 0.1 μm PTFE syringe filter into a 50 mL round-
bottom flask. The flask was rinsed with ca. 2 mL of toluene and
filtered. Solvents were removed in vacuo, and the residue was
triturated with pentane. Solvents were again removed in vacuo to yield
a red-brown solid. Crystals suitable for analysis by single-crystal XRD
studies were grown by vapor diffusion using minimal toluene and
pentane and cooling the system at −30 °C for 24 h. Yield: 392 mg
refrigerating at −30 °C for 3 days, yielding orange plates. Yield: 627
mg (0.818 mmol, 88%). 31P NMR (243 MHz, methylene chloride-d2)
1
δ 43.1. H NMR (600 MHz, methylene chloride-d2) δ 7.47 (dq, J =
7.4, 3.6 Hz, 2H, Ar-H), 7.35−7.28 (m, 4H, Ar-H), 7.09 (dd, J = 6.2,
3.2 Hz, 2H, Ar-H), 5.55−5.44 (m, 1H, Ni−CH), 3.84 (s, 4H, THF
O−CH2), 2.58−2.49 (m, 2H, CH−(CH3)2), 2.42 (s, 2H, CH−
(CH3)2), 1.96 (s, 4H, THF O−CH2−CH2), 1.46 (dd, J = 8.0 Hz, 6H,
CH−(CH3)2), 1.42−1.36 (m, 6H, CH−(CH3)2), 1.39−1.26 (m, 12H,
CH−(CH3)2). 13C NMR (151 MHz, CD2Cl2) δ 156.4 (P-Caryl),
131.65 (Caryl), 131.08 (Caryl), 128.92 (Caryl), 126.35 (Caryl), 126.17
(Caryl), 25.15 (CH−(CH3)2), 24.66 (CH−(CH3)2), 19.15 (CH−
(CH3)2), 18.70 (CH−(CH3)2), 18.43 (CH−(CH3)2), 17.52 (CH−
(CH3)2). All peaks in the spectra are characteristically broad; the
carbon bound to nickel and the THF carbons are not observed.
Elemental analysis: Calcd (%): C 45.47; H 5.92. Found: C 42.94; H
6.09. (Carbon content low due to the presence of DCM in the crystal
lattice.)
1
(0.603 mmol, 98%). 31P NMR (162 MHz, benzene-d6) δ 40.7. H
3
NMR (400 MHz, benzene-d6) δ 7.60 (d, JHH = 8.1 Hz, 2H, Ar-H),
3
3
7.49 (d, JHH = 8.1 Hz, 2H, Ar-H), 7.01 (t, JHH = 7.5 Hz, 2H. Ar-H),
2
3
5.40 (s, 1H, Ni−CH), 1.57 (dq, JHP = 15.8, JHH = 7.8 Hz, 4H,
CH(CH3)2), 1.29 (q, 3JHP = 3JHH = 7.8 Hz, 12H, CH(CH3)2), 1.16 (q,
3JHP
=
3JHH = 7.2 Hz, 12H, CH(CH3)2). 13C NMR (126 MHz,
Synthesis of 2Br. To a solid mixture of 475 mg of L2 (0.976 mmol)
and 300 mg of NiBr2(MeCN)2 (0.976 mmol) in a 50-mL thick-walled
glass vessel with a Kontes needle valve was added 25 mL of
tetrahydrofuran (THF) and 1 mL of NEt3. The vessel was sealed and
heated to 65 °C for 18 h to yield a dark brown solution. The solvent
was removed in vacuo and the residue was extracted into toluene and
filtered to remove triethylammonium bromide. The filtrate was
concentrated, dissolved in 50 mL of Et2O, and filtered, leaving behind
a blue residue, which was discarded. The filtrate was concentrated and
the residue was recrystallized by layering a concentrated toluene
solution (∼2 mL) with 15 mL of n-pentane and cooling to −30 °C for
2 days, yielding orange-red plate crystals of 2Br. Yield: 301 mg (49%,
0.478 mmol). 31P NMR (243 MHz, benzene-d6) δ 45.4 1H NMR (600
MHz, benzene-d6) δ 7.38 (dd, J = 8.5, 1.3 Hz, 2H, Ar-H), 6.76 (q, J =
3.5 Hz, 2H, Ar-H), 6.59 (dd, J = 8.6, 2.6 Hz, 2H, Ar-H), 5.53 (s, 1H,
Ni-H), 2.71−2.64 (m, 2H, CH(CH3)2), 2.63−2.55 (m, 2H
CH(CH3)2), 2.54 (s, 12H, N-(CH3)), 1.63 (dd, J = 7.3 Hz, 6H,
CH(CH3)2), 1.55 (dd, J = 7.6 Hz, 6H, CH(CH3)2), 1.34 (dd, J = 7.2
Hz, 6H, CH(CH3)2), 1.28 (dd, J = 6.8 Hz, 6H, CH(CH3)2). 13C NMR
(151 MHz, benzene-d6) δ 148.19 (t, J = 3.3 Hz, NMe2-Haryl), 147.91
(t, J = 18.1 Hz, P-Haryl), 135.01 (t, J = 17.4 Hz, Haryl), 126.61 (t, J = 8.1
Hz, Haryl), 115.14 (Caryl), 114.37 (Caryl), 44.48 (t, J = 7.9 Hz, Ni−CH),
40.24 (N-CH3), 25.19 (t, J = 11.4 Hz, CH(CH3)2), 24.75 (t, J = 9.6
Hz, CH(CH3)2), 19.28 (t, J = 2.6 Hz, CH(CH3)2), 18.77
(CH(CH3)2), 18.47 (CH(CH3)2), 17.87 (CH(CH3)2). Elemental
analysis: Calcd (%): C 55.80; H 7.59; N 4.49. Found: C 56.18; H 7.66;
N 4.26.
Synthesis of 2OTf. To a solution of 2Br (100 mg, 0.160 mmol) in
DCM (3 mL) was added 1.05 equiv of TlOTf (60 mg, 0.168 mmol),
resulting in the immediate precipitation of TlBr. The suspension was
filtered through a 0.1 μm PTFE syringe filter and the filtrate was
concentrated under reduced pressure to yield a red solid. The solid
was extracted into minimal toluene (∼2 mL), and the resulting
solution was layered with ∼15 mL of n-pentane and refrigerated at
−30 °C overnight to yield an off-white residue. The mother liquor was
decanted and cooled to −30 °C overnight to yield orange block
crystals of the 2OTf. Yield: 68 mg (61%, 0.098). 31P NMR (203 MHz,
benzene-d6) δ 43.2. 1H NMR (500 MHz, benzene-d6) δ 7.12−7.07 (m,
2H, Ar-H), 6.62 (dd, J = 3.6 Hz, 2H, Ar-H), 6.44 (dd, J = 8.7, 2.6 Hz,
2H, Ar-H), 5.40 (s, 1H, Ni−CH), 2.74 (tt, J = 7.1, 2.6 Hz, 2H,
CH(CH3)2), 2.46 (s, 12H), 2.32 (td, J = 7.3, 3.5 Hz, 2H, CH(CH3)2),
1.54 (dd, J = 8.9, 7.2 Hz, 6H, CH(CH3)2), 1.46 (dd, J = 7.5 Hz, 6H,
CH(CH3)2), 1.27 (dd, J = 7.4 Hz, 6H, CH(CH3)2), 1.11 (dd, J = 6.5
Hz, 6H, CH(CH3)2). 13C NMR (126 MHz, benzene-d6) δ 147.77 (t, J
benzene-d6) δ 169.58 (vt, JPC = 26.3, 25.7 Hz, Ar-C-P), 144.71 (vt, JPC
= 3.4 Hz, Ar-C), 136.70 (s, Ar-C), 128.37 (s, Ar-C), 124.73 (s, Ar-C),
123.91 (s, Ar-C), 122.95 (s, Ar-C)), 122.22 (s, Ar-C), 34.18 (s, Ni-
CH), 24.74 (t, J = 10.8 Hz, C(CH3)2), 24.14 (t, J = 12.5 Hz,
C(CH3)2), 18.98 (s, CH(CH3)2), 18.72 (s, CH(CH3)2), 18.62 (s,
CH(CH3)2), 18.31 (s, CH(CH3)2). Elemental analysis: Calcd (%): C,
53.48; H, 5.88. Found: C, 54.03; H, 5.74.
Synthesis of 3SbF . In a 1 dram vial, 3Br (279 mg, 0.318 mmol) and
6
AgSbF6 (147 mg, 428 mmol) were dissolved in 1.5 mL of THF. The
reaction mixture was agitated for 1 min and allowed to sit at room
temperature for 1 h, resulting in the formation of a white precipitate.
The reaction mixture was filtered through a 0.1 μm PTFE syringe filter
into a 50 mL round-bottom flask. Solvent was removed in vacuo, and
the resulting crude solid triturated with ca. 2 mL of pentane. The final
product was dried in vacuo to yield a dark purple powder. Crystals
suitable for analysis by single-crystal XRD studies were grown by
dissolving a 15 mg portion of the final product in minimal
dichloromethane, layered with hexanes in a small vial, and placing in
the freezer at −30 °C. Red crystals were collected after 2 days. Yield:
194 mg (0.221 mmol, 69%). 31P{1H} NMR (203 MHz, methylene
1
chloride-d2) δ 37.2. H NMR (500 MHz, methylene chloride-d2) δ
7.82 (d, 3JHH = 7.9 Hz, 2H, Ar-H), 7.78 (d, 3JHH = 8.0 Hz, 2H, Ar-H),
7.44−7.33 (m, 4H, Ar-H), 5.59 (s, 1H, Ni−CH), 2.80 (s, 4H,
CH(CH3)2), 1.69−1.60 (m, 6H, CH(CH3)2), 1.60−1.50 (m, 6H,
3
3
CH(CH3)2), 1.43 (p, JHP = JHH = 7.0 Hz, 12H, CH(CH3)2). 13C
NMR (126 MHz, methylene chloride-d2) δ 168.24 (s, Ar-C), 144.68
(s, Ar-C), 137.47 (s, Ar-C), 125.69 (s, Ar-CH), 124.88 (s, Ar-CH),
123.76 (s, Ar-CH), 123.58 (s, Ar-CH), 25.55 (s, CH(CH3)2), 23.44 (s,
CH(CH3)2), 20.58 (vt, CH(CH3)2), 20.10 (s, CH(CH3)2), 19.58 (s,
CH(CH3)2), 18.45 (vt, CH(CH3)2). Elemental analysis: Calcd (%): C,
42.40; H, 4.92. Found: C, 42.09; H, 4.68.
Synthesis of 4OTf. To a solution of 1OTf (100 mg, 0.165 mmol) in
1 mL of THF was added 2.05 equiv of ONMe3 (26 mg, 0.340 mmol)
and allowed to stand until homogeneous. n-Pentane was introduced to
the solution via vapor diffusion overnight to yield analytically pure red
needles of 4OTf. Yield: 60 mg (51%, 0.083 mmol). 31P NMR (243
MHz, THF-d8) δ 72.76, 63.92. 1H NMR (600 MHz, THF-d8) δ 7.56−
7.46 (m, 2H, Aryl-H), 7.38−7.33 (m, 1H, Aryl-H), 7.30 (t, J = 7.7 Hz,
1H, Aryl-H), 7.23−7.19 (m, 1H, Aryl-H), 7.14 (t, J = 7.8 Hz, 1H, Aryl-
H), 7.06 (d, J = 7.8 Hz, 1H, Aryl-H), 6.32 (dd, J = 8.1, 3.7 Hz, 1H,
Aryl-H), 4.13 (s, 1H, Ni−CH), 3.30 (d, J = 2.6 Hz, 9H, ON(CH3)3),
2.94−2.84 (m, 1H, CH(CH3)2), 2.74−2.63 (m, 1H, CH(CH3)2),
2.54−2.44 (m, 1H, CH(CH3)2), 2.25−2.16 (m, 1H, CH(CH3)2), 1.54
C
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(dd, J = 16.6, 7.0 Hz, 3H, CH(CH3)2), 1.49−1.30 (m, 18H,
CH(CH3)2), 1.19 (dd, J = 16.1, 6.9 Hz, 3H, CH(CH3)2). 13C NMR
(151 MHz, THF-d8) δ 157.84 (d, J = 29.5 Hz, CAryl), 156.76 (d, J = 5.5
Hz, CAryl), 135.05 (d, J = 47.2 Hz,, P-CAryl), 132.08 (d, J = 2.6 Hz,
CAryl), 131.62 (d, J = 11.2 Hz, CAryl), 130.85, CAryl, 130.63 (d, J = 2.6
Hz, CAryl), 129.29 (d, J = 14.6 Hz, CAryl), 126.63 (d, J = 9.3 Hz, CAryl),
126.33 (d, J = 6.1 Hz, CAryl), 122.55 (d, J = 83.9 Hz, O = P-CAryl),
121.95 (d, J = 12.1 Hz, CAryl), 59.24 (O−N(CH3)3), 32.28 (dd, J =
20.4, 6.7 Hz, Ni−CH), 26.29 (d, J = 64.3 Hz, CH(CH3)2), 24.83 (d, J
= 22.6 Hz, CH(CH3)2), 24.55 (d, J = 2.5 Hz, CH(CH3)2), 22.50 (d, J
= 23.6 Hz, CH(CH3)2), 18.57 (dd, J = 9.2, 2.8 Hz, CH(CH3)2), 17.68
(d, J = 3.4 Hz, CH(CH3)2), 16.39 (d, J = 4.2 Hz, CH(CH3)2), 15.80
(CH(CH3)2), 15.43 (dd, J = 21.5, 3.2 Hz, CH(CH3)2), 14.50 (d, J =
3.4 Hz, CH(CH3)2). 19F NMR (471 MHz, THF-d8) δ −78.2.
Elemental analysis: Calcd (%): C 49.87; H 6.64; N 2.01. Found: C
49.81; H 6.72; N 2.10.
= 13.0 Hz, Ar-C), 137.24 (d, JCP = 2.2 Hz, Ar-C), 131.18 (d, JCP = 42.4
Hz, Ar-C), 129.39 (d, JCP = 90.4 Hz, Ar-C), 126.65 (s, Ar-C), 126.11
(s, Ar-C), 125.87 (d, JCP = 10.9 Hz, Ar-C), 124.82 (d, JCP = 12.4 Hz,
Ar-C), 124.26 (s, Ar-C), 123.52 (s, Ar-C), 122.03 (s, Ar-C), 111.74 (d,
JCP = 85.6 Hz, Ar-C), 60.41(ONCH3), 27.65 (d, JCP = 69.5 Hz,
1
CH(CH3)2), 26.20 (d, JCP = 11.5 Hz, CH(CH3)2), 25.72 (s,
1
2
CH(CH3)2), 25.19 (d, JCP = 23.5 Hz, CH(CH3)2), 24.00 (d, JCP
=
24.2 Hz, CH(CH3)2), 21.41 (s, CH(CH3)2), 20.35 (s, CH(CH3)2),
19.96−19.71 (m, CH(CH3)2), 18.90−18.62 (m, CH(CH3)2), 16.79 (s,
CH(CH3)2), 16.19 (d, 2JCP = 3.1 Hz, CH(CH3)2), 15.85 (d, 2JCP = 4.1
Hz, CH(CH3)2), 15.39 (d, 2JCP = 3.5 Hz, CH(CH3)2). HRMS (APCI)
Calcd: 585.1109, Found: 585.1211.
Synthesis of 7SbF . To a solution of 1SbF (100 mg, 0.130 mmol) in
6
6
4 mL of THF was added 2.05 equiv of ONPh2Me (55 mg, 0.340
mmol), resulting in an immediate color change to violet. n-Pentane
(15 mL) was layered onto the reaction mixture and left to stand at
room temperature overnight to yield a purple oil. The oil was dried
under high vacuum to yield a purple solid. The solution was taken up
in 4 mL THF and the precipitation of the product as an oil using n-
pentane was repeated twice, followed by drying under high vacuum to
Synthesis of 5OTf. To a solution of 2OTf (10 mg, 0.0165 mmol) in
0.7 mL of THF-d8 was added ∼2 equiv of ONMe3 (3 mg, 0.04 mmol)
and allowed to stand for 3 h until homogeneous. Analysis by 31P{1H}
NMR spectrometry shows complete conversion of the starting
material to one major product affording 1:1 peaks. n-Pentane was
introduced to the solution via vapor diffusion overnight to yield red
crystalline needles of 5OTf. However, because of contamination with
ONMe3, no yield was determined and the sample was unsuitable for
yield an analytically pure sample of 7SbF . Crystals suitable for analysis
6
by single-crystal XRD studies were grown by layering 1 mL of a 5 mg/
mL 1,4-dioxane solution of the title complex with cyclohexane,
yielding purple blocks. Yield: 67 mg (65%, 0.084 mmol). 31P NMR
1
elemental analysis. 31P NMR (243 MHz, THF-d8) δ 71.9, 63.7. H
1
(243 MHz, methylene chloride-d2) δ 81.0, 71.0. H NMR (600 MHz,
NMR (600 MHz, THF-d8) δ 6.86−6.79 (m, 2H, Aryl-H), 6.75 (dd, J =
13.1, 2.8 Hz, 1H, Aryl-H), 6.66 (dd, J = 8.0, 2.4 Hz, 1H, Aryl-H), 6.58
(dd, J = 9.0, 2.7 Hz, 1H, Aryl-H), 6.30 (dd, J = 8.9, 4.5 Hz, 1H, Aryl-
H), 3.92 (s, 1H, Ni−CH), 3.29 (s, 9H, ON(CH3)3), 2.96 (s, 6H,
N(CH3)2), 2.90 (s, 6H, N(CH3)2), 2.68−2.58 (m, 1H, CH(CH3)2),
2.44 (dp, J = 9.5, 6.9 Hz, 1H, CH(CH3)2), 2.18 (dq, J = 13.5, 6.6 Hz,
1H, CH(CH3)2), 1.55 (dd, J = 16.2, 7.1 Hz, 3H, CH(CH3)2), 1.46
(dd, J = 15.3, 7.1 Hz, 3H, CH(CH3)2), 1.42−1.31 (m, 15H,
CH(CH3)2), 1.20 (dd, J = 15.8, 7.0 Hz, 3H, CH(CH3)2). The fourth
multiplet for P−CH(CH3)2 is unresolved from the singlets for
N(CH3)2. 13C NMR (126 MHz, THF-d8) δ 148.78 (d, J = 7.2 Hz,
CAryl), 145.35 (d, J = 13.9 Hz, CAryl), 145.08 (d, J = 29.8 Hz, CAryl),
143.27 (d, J = 5.4 Hz, CAryl), 134.65 (d, J = 46.3 Hz, CAryl), 128.38 (d, J
= 16.9 Hz, CAryl), 127.40 (d, J = 14.5 Hz, CAryl), 127.07 (d, J = 11.1 Hz,
CAryl), 122.18 (d, J = 83.9 Hz, CAryl), 115.65 (dd, J = 68.9, 2.6 Hz,
CAryl), 114.29 (d, J = 13.4 Hz, CAryl), 111.66 (CAryl), 58.58 (ON(CH3)),
39.16 (N(CH2), 39.07 (N(CH2), 29.80 (dd, J = 19.5, 6.7 Hz, Ni−CH),
25.70 (d, J = 64.2 Hz, CH(CH3)2), 21.90 (d, J = 22.7 Hz, CH(CH3)2),
18.19 (d, J = 2.7 Hz, CH(CH3)2), 18.09 (d, J = 4.0 Hz, CH(CH3)2),
17.20 (d, J = 3.7 Hz, CH(CH3)2), 16.11 (d, J = 3.7 Hz, CH(CH3)2),
15.25 (CH(CH3)2), 15.11 (d, J = 3.5 Hz, CH(CH3)2), 14.88 (d, J = 2.3
Hz, CH(CH3)2), 14.12 (d, J = 3.4 Hz, CH(CH3)2). Two signals for P-
C(CH3)2 are unresolved from the residual solvent peak. 19F NMR
(471 MHz, THF-d8) δ −80.1.
methylene chloride-d2) δ 7.53−7.41 (m, 3H, Aryl-H), 7.35−7.15 (m,
4H, Aryl-H), 6.28 (dd, J = 8.0, 3.5 Hz, 1H, Aryl-H), 4.37 (s, 1H, Ni−
CH), 3.66 (s, 4H, THF O−CH2), 2.62−2.42 (m, 1H, CH(CH3)2),
2.25 (m, 3H, CH(CH3)2), 1.89 (s, 4H, THF O−CH2−CH2) 1.72 (dd,
J = 16.1, 7.1 Hz, 3H, CH(CH3)2), 1.67 (dd, J = 15.6, 7.0 Hz, 3H,
CH(CH3)2), 1.29 (dd, J = 16.3, 7.2 Hz, 3H, CH(CH3)2), 1.22 (dd, J =
15.5, 7.1 Hz, 3H, CH(CH3)2), 1.17−1.07 (m, 12H, CH(CH3)2). 13C
NMR (151 MHz, methylene chloride-d2) δ 157.82 (d, J = 5.6 Hz,
CAryl), 148.14 (d, J = 4.4 Hz, CAryl), 134.34 (d, J = 2.7 Hz, CAryl), 132.21
(d, J = 11.8 Hz, CAryl), 132.13 (d, J = 2.5 Hz, CAryl), 130.16 (d, J = 12.5
Hz, CAryl), 129.93 (d, J = 9.7 Hz, CAryl), 126.41 (d, J = 12.1 Hz, CAryl),
125.37 (d, J = 9.3 Hz, CAryl), 123.34 (d, J = 84.6 Hz, O−P CAryl),
122.30 (d, J = 12.1 Hz, CAryl), 121.06 (d, J = 83.0 Hz, O−P CAryl),
25.94 (d, J = 65.4 Hz, CH(CH3)2), 25.07 (d, J = 23.7 Hz, CH(CH3)2),
25.05 (d, J = 66.1 Hz, CH(CH3)2), 24.63 (d, J = 26.0 Hz, CH(CH3)2),
15.88 ((CH(CH3)2), 15.72 (d, J = 2.7 Hz, (CH(CH3)2), 15.42 (d, J =
2.6 Hz, (CH(CH3)2), 15.38 (d, J = 4.1 Hz, (CH(CH3)2), 15.24 (d, J =
3.7 Hz, (CH(CH3)2), 15.11 (d, J = 3.2 Hz, (CH(CH3)2), 14.47 (d, J =
2.9 Hz, (CH(CH3)2), 13.98 (d, J = 3.4 Hz, (CH(CH3)2), 13.37−13.14
(m, Ni-C). Peaks for bound THF are not observed. HRMS (APCI)
Calcd: 489.1622, Found: 489.1625. Elemental analysis: Calcd (%): C
43.70; H 5.56. Found: C 43.80; H 5.47.
RESULTS AND DISCUSSION
Synthesis of 6SbF . To a 25 mL thick-walled glass vessel equipped
■
6
The ligand attachment procedure used to make the bromo
derivative of L1, 1Br,41,43 (Scheme 2) was employed to prepare
the new compounds 2−3Br. In order to explore oxygen atom
transfer to Ni(II) complexes of ligands L1−3, starting materials
with more weakly coordinating anions were required. Thus, the
bromo complexes were converted to the triflate and
hexafluoroantimonate synthons 1OTf, 2OTf, 1SbF , and 3SbF , as
with a Kontes needle valve were added 3SbF (51 mg, 0.058 mmol) and
6
trimethylamine-N-oxide (9 mg, 0.120 mmol). The solids were
suspended in 20 mL of toluene and sonicated for 18 h. The mother
liquor was decanted from the resulting pink precipitate. The solid was
washed with 6 mL of pentane in a 2-mL sintered frit funnel and dried
in vacuo to yield a pink solid. Crystals suitable for analysis via single-
crystal XRD studies were grown from a concentrated solution in
dichloromethane layered with hexanes and placed in a freezer at −30
°C for 3 days. Yield: 31 mg (0.035 mmol, 60%). 31P NMR (203 MHz,
6
6
depicted in Scheme 2. Bromide abstraction from 1Br with silver
salts AgX (X = OTf, SbF6), which is a strategy previously
shown to be effective in related POCOPNi(II) complexes,45
cleanly effected ion exchange, yielding AgBr as a precipitate and
1
acetone-d6) δ 72.9, 50.9. H NMR (500 MHz, acetone-d6) δ 7.97−
7.90 (m, 3H, Ar-H), 7.88 (d, 3JHH = 8.1 Hz, 1H, Ar-H), 7.47−7.40 (m,
3
3H, Ar-H), 7.37 (t, JHH = 7.7 Hz, 1H, Ar-H), 4.41 (s, 1H, Ni−CH),
3
3.43 (s, 9H, ON(CH3)3), 3.29 (q, JHH = 7.3 Hz, 1H, CH(CH3)2),
1OTf or 1SbF , respectively. In both cases, a color change from
6
3
3
2.87 (dp, JHH = 14.1, 7.2 Hz, 1H, CH(CH3)2), 2.70 (dt, JHH = 14.5,
dark orange to red was noted and a small upfield shift in the
31P{1H} NMR spectral resonance (44.5 to 44.1 (X = OTf) or
43.1 (X = SbF6) ppm, respectively) was observed. Structural
elucidation by single-crystal XRD experiments revealed that, in
the case of 1OTf, the counterion is bound directly to the Ni(II)
3
7.2 Hz, 1H, CH(CH3)2), 2.57 (dp, JHH = 13.8, 6.9 Hz, 1H,
3
3
CH(CH3)2), 1.65 (dd, JHP = JHH = 17.2, 7.0 Hz, 3H, CH(CH3)2),
1.59−1.30 (m, 21H, CH(CH3)2). 13C NMR (126 MHz, acetone-d6) δ
172.94 (d, JCP = 8.0 Hz, Ar-C), 168.95 (d, JCP = 39.4 Hz, Ar-C), 147.01
(d, JCP = 8.2 Hz, Ar-C), 140.69 (d, JCP = 15.3 Hz, Ar-C), 138.89 (d, JCP
D
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Article
Scheme 2
Figure 1. Molecular structure of 1OTf (top) and 2OTf (bottom).
Thermal ellipsoids shown to 50% probability. Most hydrogen atoms
are omitted for the sake of clarity. Select distances for 1OTf: Ni1−C1,
1.963(3) Å; P1−Ni1, 2.189(1) Å; Ni1−P2, 2.230(1) Å; Ni1−O1,
2.003(3) Å. Select angles for 1OTf: ∠O1−Ni1−P2, 97.43(8)°; ∠O1−
Ni1−C1, 170.6(1)°; ∠O1−Ni1−P1, 95.73(8)°; ∠P2−Ni1−P1,
158.44(4)°; Caryl−Caryl torsion angle, 55.67°. Select distances for
2OTf: Ni1−C9, 1.975(6) Å; P1−Ni1, 2.230(2) Å; Ni1−P2, 2.174(1) Å;
Ni1−O1, 2.003(4) Å. Select angles for 2OTf: ∠O1−Ni1−P2, 92.5(1)°;
∠O1−Ni1−C1, 172.5(2)°; ∠O1−Ni1−P1, 97.0(1)°; ∠P2−Ni1−P1,
166.3(6)°; Caryl−Caryl torsion angle, 43.68°.
center in the solid state, whereas in 1SbF , the counterion is
displaced from the metal center by a THF solvent molecule.
6
The synthesis of 3SbF was successfully carried out from 3Br,
6
using the same protocol as 1SbF . This reaction was similarly
6
accompanied by a color change from red-brown to red-purple
and an upfield shift of the resonance in the 31P{1H} NMR
signal from 40.7 ppm to 37.2 ppm. Both 1SbF and 3SbF feature
6
6
broad signals in their NMR spectra, likely due to exchange of
the bound and free THF molecules on the NMR time scale.
In contrast, attempted abstraction of bromide from the
NMe2 substituted 2Br with silver salts resulted in an immediate
color change from orange to dark blue, the formation of silver
metal, and the complete loss of a 31P{1H} NMR spectral
signature, suggesting a paramagnetic product. This is perhaps
unsurprising, given the known propensity of silver salts to
oxidize PNPNi(II) complexes,29,46 and likely involves oxidation
of the pendant NMe2 group on the ligand. Attempted bromide
abstractions using KOTf and NaOTf were unsuccessful but ion
exchange was accomplished cleanly by using TlOTf (Scheme
2), resulting in a color change from orange to red, similar to
linking aryl rings of the pincer ligand arms). The precise origin
of the increased distortion in 1OTf is unknown.
Oxygen atom transfer to compounds 1−2OTf was explored by
reaction of these contact ion pairs with an excess of
commercially available ONMe3 in THF (Scheme 3). The
reaction proceeded smoothly, resulting in a products
characterized by two singlets integrating to 1:1 in the
1
31P{1H} NMR spectra. Furthermore, the H NMR spectra
demonstrate a decrease in symmetry characteristic of a change
from a Cs to a C1 symmetric compound and generation of 1
equiv of NMe3 was confirmed by the presence of a
characteristic singlet at 2.13 ppm integrating to 9 protons.
The reactions only required 2 equiv of ONMe3 to reach
completion. The reaction of 1OTf with ONMe3 was explored in
a variety of solvents ((CD3)2CO, CD2Cl2, C6D6, and THF-d8),
and, in all cases, the same product was formed within 1 h, with
a qualitative solvent rate dependence of (CD3)2CO > CD2Cl2 >
THF-d8 > C6D6. The observed rate dependence may be due to
the relatively low solubility of ONMe3 in C6D6 and THF-d8,
and/or the increased rate of reaction in more polar solvents.
Vapor diffusion of n-pentane into the reaction mixture allowed
for isolation of a red crystalline solid. Subsequent XRD studies
revealed the product to be the result of oxygen-atom transfer to
1
OTf, and an upfield shift in the 31P{1H} NMR spectra (from
45.4 ppm to 43.1 ppm).
Structurally, 1OTf and 2OTf are closely related (see Figure 1),
with comparable Ni−P bond lengths (2.209 Å vs 2.202 Å avg.),
Ni−C bond lengths (1.963(3) Å and 1.975(6) Å), and Ni−O
bond lengths (2.003(3) Å vs 2.003(4) Å). The angles around
the nickel center are different, with more distortion from
idealized square planar geometry in 1OTf than 2OTf (τ4 = 0.22 vs
0.15)47 and more torsion between the ligand arene rings
(55.67° vs 43.68°; throughout this discussion, these torsion
values refer to the angle between the planes defined by the
E
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Inorganic Chemistry
Article
Scheme 3
the one of the phosphorus arms of the ligand, resulting in a new
PCO ligand system, and displacement of the triflate ion by a
second equivalent of ONMe3 to yield 4OTf (see Scheme 3, as
well as Figure 2). This asymmetric PCO ligand is unusual, with
few examples of similar PCO ligands in the literature.48,49
However, unlike literature examples, no hemilability of the
oxygen arm is observed in 4OTf. Performing the reaction with 1
equiv of ONMe3 in THF results in a 50:50 mix of 4OTf and
starting material, and refluxing the reaction mixture does not
force the oxidation to completion, indicating that ONMe3 does
not undergo exchange between the product and the starting
material. At early reaction times, an intermediate was
observable by 31P{1H} NMR spectroscopy (transient signal at
34.7 ppm) that we hypothesize is due to coordination of
ONMe3 to the nickel center, forming an ion separated adduct
of 1−2OTf, as shown in Scheme 3. When the reactions were
carried out at lower temperatures, this species persisted for
lengthier periods, but no other intermediates were ever
observed.
Figure 2. Molecular structure of 4OTf (top) and 5OTf (bottom).
Thermal ellipsoids shown to 50% probability. Most hydrogen atoms
and counteranions omitted for clarity. Select distances for 4OTf: Ni1−
O2, 1.944(2) Å; O2−N1, 1.405(3) Å; Ni1−O1, 1.958(2) Å; P1−Ni1,
2.1195(6) Å; Ni1−C1, 1.948(2) Å; O1−P2, 1.521(2) Å. Select angles
for 4OTf: ∠O2−Ni1−C1, 171.53(9)°; ∠O2−Ni1−P1, 87.12(6)°;
∠O2−Ni1−O1, 96.31(8)°; ∠O1−Ni1−P1, 176.48(6)°; ∠Ni1−O1−
P2, 130.8(1)°; Caryl−Caryl torsion angle, 66.07°. Select distances for
5OTf: Ni1−C1, 1.945(5) Å; Ni1−P2, 2.114(2) Å; Ni1−O1, 1.938(4)
Å; Ni1−O2, 1.950(4) Å. Select angles for 5OTf: ∠O2−Ni1−C1,
173.4(2)°; ∠O1−Ni1−P2, 175.2(1)°; ∠O2−Ni1−O1, 96.2(2)°;
∠O2−Ni1−P2, 88.3(1)°; ∠C1−Ni1−O1, 89.2(2)°; ∠C1−Ni1−P2,
86.4(2)°; Caryl-Caryl torsion angle, 65.63°.
That direct coordination of the N-oxide to the metal center is
required in order to generate product that is supported by the
observation that, when compound 1Br was reacted with excess
ONMe3 in THF-d8, the conversion to a new product with
31P{1H} resonances at 68.7 and 68.0 ppm was significantly
slower than the reaction with 1OTf, with <15% conversion
observed after 24 h. Presumably, the much more coordinating
Br− anion slows the rate of the reaction. In a second control
experiment, the ability of ONMe3 to oxidize the ligand in the
absence of the metal center was probed by the reaction of
ONMe3 with the free ligand L1 in THF-d8. Again, while the
reaction proceeded, its rate was very slow, with free L1 still
present in the sample after 4 days. These results, in addition to
the observation of the intermediate postulated in Scheme 3,
suggest that the metal must be intimately involved in mediating
the oxidation of the ligand by the ONMe3.
requiring 3 h in THF, as opposed to 1 h. Monitoring the
reaction via 31P{1H} NMR spectroscopy reveals a rapid buildup
of the ion-separated intermediate adduct (signal at 34.7 ppm),
followed by relatively slow O atom transfer to yield the
unsymmetrical product 5OTf characterized by two singlets at
71.9 and 63.7 ppm integrating to 1:1 (Figure S1 in the
Supporting Information). This decrease in rate is unexpected
for a metal-centered oxidation, because the relatively more
electron-rich metal center should be more prone to oxidation.
Explanations for the decrease in rate include higher lability of
the coordinated ONMe3 in the intermediate for the more
electron-rich system, or an increase in Ni−P bond strength in
2OTf with the more electron-rich pincer ligand.
The structures of 4OTf and 5OTf are given in Figure 2 and
exhibit similar features. In both cases, the nickel center adopts
an almost idealized square planar geometry (τ4 = 0.08 and
0.09),47 with an increase in torsion between the arenes in the
ligand backbone, when compared to the starting complexes
(65.63° vs 56.57° and 66.07° vs 43.68°). The bound molecule
of ONMe3 in both cases shows little signs of activation in the
solid state; the oxygen atom does not appear to be inserting
into the remaining Ni−P bond, with O−Ni−P angles of
To investigate the importance of ligand donor properties on
the reaction, the oxidation of 2OTf incorporating the more
40
electron-rich L2 with ONMe3 was effected (Scheme 3). This
reaction proceeded in a similar fashion as the oxidation of 1OTf
,
yielding 5OTf as the major product by 31P{1H} NMR
spectroscopy, although the rate was slower, with full conversion
F
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Article
Scheme 4
88.3(1)° and 87.12(6)°, nor does there appear to be any
lengthening of the O−N bond, with lengths of 1.395(5) and
1.405(3) Å, which are comparable to the literature values of
1.388(5) and 1.404 Å, respectively, for free ONMe3.50,51
In order to discourage O atom insertion into the Ni−P bond
of the complex, the more-rigid L3 ligand was enlisted. The use
of a five-membered benzothiophene linking group in this
system relieves the steric interaction found between the phenyl
linkers of L1 and L2, creating an overall more planar ligand that
would increase this steric interaction upon insertion of an
oxygen atom into the N−P bond. Thus, when compared to
1
SbF , 3SbF features significantly less torsion between the arene
6 6
rings of the ligand framework (11.24° vs 51.25°), although the
Figure 3. Molecular structure of 6SbF . Thermal ellipsoids shown to
6
ligand framework still adopts a distorted square planar
geometry (τ4 = 0.19).47 In the reaction of 3SbF with ONMe3
50% probability. Hydrogen atoms and counteranions omitted for the
sake of clarity. Select distances: O1−Ni1, 1.953(4) Å; O2−Ni1,
1.933(3) Å; O2−N1, 1.425(5) Å; P2−Ni1, 2.147(2) Å; Ni1−C9,
1.956(4) Å. Select angles: ∠O2−Ni1−C9, 176.3(2)°; ∠P2−Ni1−O1,
176.1(1)°; ∠O1−Ni1−O2, 94.8(2)°; ∠P2−Ni1−O2, 89.1(1)°; ∠P2−
Ni1−C9, 88.0(1)°; ∠O1−Ni1−C9, 88.0(2)°; Caryl−Caryl torsion angle,
71.19°.
6
in THF (Scheme 4), similar to both 1OTf and 2OTf, formation of
an intermediate was signified by the emergence of a signal in
the 31P{1H} NMR spectra at 30.1 ppm, followed by slow
conversion to an asymmetric product characterized by two
downfield singlets at 72.5 and 51.1 ppm. However, for 3SbF , the
6
reaction proceeds far slower than the oxidation of 1OTf, with
starting material still present after 18 h. Attempts to force the
reaction to completion by extending the reaction time or by
heating the reaction mixture (up to reflux) leads to the
increased formation of unidentified side products. Isolation of
to other O atom transfer agents. The reagents pyridine N-oxide,
2,4,6-trimethylbenzonitrile-N-oxide and N2O were ineffective
and did not react with 1OTf in solution, even after 24 h.
Motivated by computational reports suggesting that the
substitution of methyl groups with phenyl groups in amine-
N-oxides leads to an increase in thermodynamic oxygen atom
donor strength,50 as well as experimental evidence for an
increase in the rate of oxygen transfer of aryl amine N-oxides
relative to alkyl amine oxides in decarbonylation reactions,
despite reduced basicity,52 the compound ONMePh2 was
synthesized via oxidation of the amine with m-CPBA.53 Indeed,
the primary product 6SbF was accomplished by performing the
6
reaction in toluene and recrystallization of the resultant
insoluble residue from methylene chloride/hexanes mixtures.
Single-crystal XRD studies revealed the product 6SbF to again
6
be the result of oxidation of a ligand phosphine arm (Figure 3).
While the starting material 3SbF features one of the smallest
6
torsion angles between ligand aromatic linkers, the PCO ligand
reaction of 2 equiv of ONMePh2 with 1SbF in THF resulted in
6
in 6SbF features a much larger torsion angle of 71.19°,
an immediate color change from red-orange to bright purple
and a new unsymmetrical product was evidenced in both the
31P{1H} and 1H NMR spectra. Signals at 81.0 and 71.0 ppm in
the 31P{1H} NMR spectrum and a complex 1H NMR spectrum
both suggested C1 symmetry for the product and furthermore,
the 1H NMR spectrum showed the consumption of both
equivalents of ONMePh2 and the formation of 2 equiv of free
amine. The compound was isolated as a microcrystalline purple
solid. Analysis of the reaction mixture via high-resolution mass
spectrometry revealed a mass (M = 489.1625 amu) that was
consistent with two oxygen transfers to the nickel-ligand core.
On the face of it, these data initially supported the formation of
a nickel oxo species. Repeated attempts to garner crystals
suitable for X-ray crystallography were plagued by poorly
diffracting samples, but crystals obtained from a solution
containing a small amount of dioxane did allow for
6
suggesting that O atom insertion does indeed engender strong
steric interaction between the linker groups of the PCO ligand.
The relatively slow rate of this oxidation may be related to this
large change in torsion between the ground state and the
transition state and/or the lower overall donor properties of
L3.40
Since the reaction of 1OTf with the trimethylamine oxide was
the most facile, further experiments focused on this parent
ligand system (L1). It was hypothesized that the adduct 4OTf
might be induced to transfer a further oxygen atom, but all
attempts at further oxidation by release of an oxygen atom from
the bound ONMe3 moiety met with failure. Refluxing the
sample in either THF or toluene led to slow decomposition
over several days and irradiation of the sample with 254 nm
light in THF also led to decomposition. Therefore, we turned
G
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unambiguous determination of the connectivity within the
molecule, showing it to contain the unusual OCO ligand, in
which both arms of the pincer ligand L1 have been oxidized
(see Figure S12 in the Supporting Information). Thus, despite
the C1 symmetry indicated by the NMR spectra of the
compound, the isolated product is assigned as the THF adduct
Scheme 6
7SbF depicted in Scheme 5, rather than a high-valency nickel
6
oxo species.
Scheme 5
While OCO pincer ligands have been employed in the main
group and early-transition-metal complexes,54−57 group 10
examples are rather rare.58,59 Compound 7SbF is thermally
6
stable, with no decomposition observed in refluxing THF after
4 h, although it has limited chemical stability, decomposing
with the addition of a variety of strong and weak donors, such
as PMe3, ONMe3, pyridine, 4,4′-bipyridine, water, or acetone,
although it does not decompose in the presence of PPh3, even
at elevated temperatures. As mentioned above, based on
nuclearmagnetic resonance (NMR) spectroscopy, the com-
pound has C1 symmetry, which is counterintuitive for a product
in which both arms of the phosphine are converted to
phosphine oxides. However, the expansion of both these
chelate rings greatly increases the barrier to exchange of the
products. In path B, insertion of the O atom proceeds directly
via the transition state depicted without the involvement of a
nickel oxo species. This question was probed both
experimentally via trapping experiments and using DFT
computational methods.
The reaction of 1OTf with ONMe3 in the presence of 50
equiv of 1,4-cyclohexadiene in THF proceeded cleanly to 4OTf
with no observable formation of either water or benzene.36
Furthermore, attempted trapping with phosphines24,35 in THF
proceeded with no oxidation, in the case of PPh3, whereas
PMe3 formed a strong complex with 1OTf, preventing formation
of the necessary adduct with ONMe3. Attempted interception
of NiO with KH24 in THF yielded only known compound
L1NiH41 and 4OTf as products, with no detection of known
compound L1NiOH.41 The addition of excess Sc(OTf)3 or
B(C6F5), potential Lewis acid trapping agents for a Ni oxo
intermediate,34,60 to the reaction mixture in THF led to no
consumption of the 1OTf starting material, presumably due to
sequestering of ONR3. Finally, freeze-quenched reactions
between 1OTf and ONR3 in 2-MeTHF showed no observable
EPR signals, suggesting a lack of paramagnetic intermediates
that are not observable via 31P{1H} NMR spectroscopy. Taken
together, along with the observation that the more electron-rich
2OTf system reacts more slowly with the amine-N-oxides, the
results of these trapping experiments strongly suggest the lack
diastereotopic P environments in 7SbF , because of the steric
6
interaction between the aryl linker rings as the linker arms “flip”
conformation. The two phosphines are thus not in exchange on
the NMR time scale, and high-temperature NMR spectroscopy
reveals that coalescence does not occur up to 65°C in THF.
While the ultimate products of single and double oxygen
atom transfer to the Ni(II) cations used here are the result of
ligand oxidation, the question remains as to whether or not
nickel oxo compounds are involved as intermediates on the
reaction path of cations 1−3X with amine-N-oxide reagents.
Two possible mechanisms for the oxidation of complex 1OTf by
ONR3 are shown in Scheme 6. The experimental evidence
described above suggests that the first step involves displace-
ment of either a weakly coordinating anion or a coordinated
THF molecule by the amine-N-oxide to give the transient
adducts observed spectroscopically. From this ONR3 ligated
species, transfer of an O atom to the metal with loss of NR3
(path A, Scheme 6) would yield a transient nickel(IV) oxo
complex, which would proceed rapidly to the observed
H
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Article
Figure 4. DFT (B3PW91) computed enthalpy reaction profile at 298 K for the oxygen transfer from ONMePh2 to [L1NiTHF]+.
of a high-valency nickel−oxygen intermediate and favor the
direct insertion pathway B in Scheme 6.
The reaction of ONMePh2 with the cationic [L1NiTHF]+
fragment was further studied using density functional theory
(DFT) methods and the low-energy reaction coordinate for
two sequential oxygen atom transfer events is shown in Figure
4. The first step of the reaction was found to be the barrier-less
displacement of THF and coordination of N-oxide to metal
center; these species likely are in equilibrium, and this is
completely consistent with experimental observations. This is
followed by a low barrier insertion (13.0 kcal/mol) of the
oxygen into the Ni−P bond. The geometry of the transition
state for the first oxygen transfer (TS1, Figure 5) exhibits
considerable N−O bond lengthening in the N-oxide fragment
when compared to the optimized adduct geometry (1.852 Å vs
1.389 Å), concurrent with a shortening of the P−O bond
distance (2.334 Å vs 2.838 Å) and a decrease in P−Ni−O angle
(68.37° vs 82.00°). Notably, there is very little lengthening of
the Ni−P bond distance, suggesting that phosphine dissociation
is not a prerequisite for insertion and consistent with the low
calculated barrier for this insertion. Oxidation of the phosphine
is exothermic, relative to the starting material, by −66.5 kcal/
mol and is further favored by an additional 23.5 kcal/mol
through the binding of a second molecule of ONMePh2 to give
a product analogous to the isolated compounds 4−5OTf and
6
SbF . Further oxygen transfer from this species is calculated to
6
Figure 5. DFT (B3PW91) optimized geometries of the transition
states for the first (TS1, top) and second (TS2, bottom) oxygen
transfer reactions.
have a notably higher barrier of 22.5 kcal/mol, when compared
to the first oxidation, but is of a magnitude consistent with the
rapid reaction observed under ambient conditions for this
second oxygen atom transfer when using ONMePh2. Similar to
TS1, TS2 (Figure 5) features an elongated N−O (1.978 Å vs
1.389 Å) distance, a decrease in O−P (2.386 Å vs 2.910 Å)
distance and a decrease in P−Ni−O (73.57° vs 88.08°) angle,
relative to the adduct, providing no evidence of a divergent
mechanism for the second oxidation. Oxidation of the second
phosphine arm is similarly exothermic, although the magnitude
of thermodynamic stabilization is 10.2 kcal/mol less. This
difference in enthalpy could be explained by the increased
I
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Inorganic Chemistry
Article
ligand strain involved in undergoing a second ring expansion
within the pincer ligand framework.
X-ray crystallographic data (Cartesian coordinates)
(XYZ)
These computations suggest that a Ni(IV) oxo species, such
as that shown in Scheme 6, analogous to the Pt(IV) species
reported previously,24 does not lie along the pathway of this
reaction. Indeed, while DFT minimization of this species does
converge on a local minimum, the ground-state structure is a
triplet that is best described as a Ni(III) oxyl complex with
significant spin density on the oxo ligand (see Figure S13 in the
Supporting Information). Furthermore, it is 54.2 kcal/mol
higher in energy than the first PCO insertion product, on an
energy surface that is not connected to the reaction coordinate
in Figure 4. Thus, the formation of such an intermediate can be
ruled out for the first insertion reaction. A similar geometry
optimization of a putative Ni oxo species formed from the
second oxygen atom transfer en route to 7X was also carried out
and again, this is a triplet ground state Ni oxyl that is much
higher in energy (56.8 kcal/mol) than the second PCO
insertion product (amine adduct in Figure 4). Therefore,
forming such Ni oxo/oxyl species, even transiently in these
reactions, seems quite unlikely, based on the totality of the
experimental and computational results presented.
Accession Codes
CCDC 1582263−1582271 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: wpiers@ucalgary.ca (W. E. Piers).
*E-mail: laurent.maron@irsamc.ups-tlse.fr (L. Maron).
ORCID
Warren E. Piers: 0000-0003-2278-1269
Laurent Maron: 0000-0003-2653-8557
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
CONCLUSIONS
■
Funding for this work was provided by NSERC of Canada
(Discovery Grant) the Canada Research Chair secretariat (Tier
I CRC 2013−2020) to W.E.P. The Alexander von Humboldt
Foundation (W.E.P. and L.M.) and the Chinese Academy of
Science (L.M.) are acknowledged for financial support. L.M. is
member of the Institut Universitaire de France. L.M. and W.P.
also acknowledge CNRS for a bilateral grant through the PICS
program. CalMip is acknowledged for a generous grant of
computing time.
The implication and harnessing of highly reactive late-
transition-metal oxo or nitrido complexes is of current interest.
For the former, oxygen atom transfer agents are often used in
an effort to generate such species. Here, we have studied the
reactions of cationic Ni(II) complexes supported by three
pincer ligands with differing steric and electronic properties.
Facile reactions with amine-N-oxide reagents were discovered,
and the products characterized indicated oxygen atom insertion
into one or two of the Ni−P bonds of the pincer ligand
framework, depending on the stoichiometry of the reaction and
the oxygen atom transferring ability of the ONR3 reagent. The
possibility that nickel oxo complexes are involved as
intermediates in these reactions was explored both exper-
imentally and computationally, and these investigations indicate
that they are not involved on the reaction path. Direct insertion
of the oxygen atom into the Ni−P bond(s) offers a much lower
energy path to the products than one that goes through a nickel
oxo derived from oxygen atom transfer to the metal. This
probably is in part due to the flexibility of the pincer ligand and
its ability to undergo ring expansion, not only once but twice.
Computations do indicate that the barrier to a second O atom
insertion is higher than the first, suggesting that increased
rigidification through linking the aryl rings40,61 of the ligand
may be a strategy to employ in raising the barrier to direct O
atom insertion into the Ni−P bonds. This may make Ni−O
formation competitive with direct insertion and allow for
generation and implication of a NiO, which computations
suggest will be a Ni(III) oxyl species.
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02766.
Experimental and characterization details for all new
compounds, including spectroscopic data and computa-
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