Nickel and Platinum PCP Pincer Complexes Incorporating an Acyclic Diaminoalkyl Central Moiety Connecting Imidazole or Pyrazole Rings

Article abstract of DOI:10.1021/acs.organomet.7b00396

This work explores avenues toward PCP pincer complexes with acyclic diaminocarbene central moieties that are suitable for ligand-assisted reactivity involving the carbene carbon. For this purpose, diphosphine chelating ligands 1 and 2, with bis(imidazolyl)methyl and bis(pyrazolyl)methyl backbones, respectively, were prepared in two high-yield synthetic steps. Nickel(II) and platinum(II) dihalide complexes 3 and 6, incorporating 1, were prepared and converted into PCP pincer ligands 5 and 7 upon deprotonation with KHMDS. Chelated nickel(I) complex 4 was obtained as a byproduct to 3. Although the solid-state structures of 5 and 7 presented geometric strain at the diaminomethyl carbon, this moiety could not be converted into a diaminocarbene by proton or hydride abstraction. 7 could be converted into platinum(IV) analog 8 by oxidation with PhICl₂. Ligand 2 proved more versatile than 1, or less dependable in its behavior, generating P,N-chelating cobalt(II) dibromide complex 9. Easily accessible ligands 1 and 2 provide a new and versatile PCP pincer platform.

Full text of DOI:10.1021/acs.organomet.7b00396

Article
pubs.acs.org/Organometallics
Nickel and Platinum PCP Pincer Complexes Incorporating an Acyclic
Diaminoalkyl Central Moiety Connecting Imidazole or Pyrazole Rings
Braulio M. Puerta Lombardi, Rudy M. Brown, Chris Gendy, Chia Yun Chang, Tristram Chivers,
and Roland Roesler*
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
S
* Supporting Information
ABSTRACT: This work explores avenues toward PCP pincer
complexes with acyclic diaminocarbene central moieties that
are suitable for ligand-assisted reactivity involving the carbene
carbon. For this purpose, diphosphine chelating ligands 1 and
2, with bis(imidazolyl)methyl and bis(pyrazolyl)methyl back-
bones, respectively, were prepared in two high-yield synthetic
steps. Nickel(II) and platinum(II) dihalide complexes 3 and 6,
incorporating 1, were prepared and converted into PCP pincer
ligands 5 and 7 upon deprotonation with KHMDS. Chelated
nickel(I) complex 4 was obtained as a byproduct to 3.
Although the solid-state structures of 5 and 7 presented
geometric strain at the diaminomethyl carbon, this moiety
could not be converted into a diaminocarbene by proton or
hydride abstraction. 7 could be converted into platinum(IV) analog 8 by oxidation with PhICl2. Ligand 2 proved more versatile
than 1, or less dependable in its behavior, generating P,N-chelating cobalt(II) dibromide complex 9. Easily accessible ligands 1
and 2 provide a new and versatile PCP pincer platform.
nonclassical NCN pincer complexes with bis(pyrazolyl)methyl
(Mo and W)6 and bis(azaindolyl)methyl (Pt)7 ligands.
INTRODUCTION
■
The term “metal−ligand cooperativity” was added to the
vocabulary of the discipline by Milstein in 2006,1 although the
concept has deeper roots in the chemistry literature. Over the
past decade, it has developed into a flourishing chapter of the
larger area of small-molecule activation and catalysis.2 We have
recently shown that, in stark contrast to the classical chemistry
of N-heterocyclic carbenes (NHCs), which are quintessential
ancillary ligands, the metal-coordinated carbene carbon in
selected ligands can act as proton acceptor.3 This enabled the
ligand-assisted, reversible activation of ammonia on a nickel(0)
NCN−pincer complex (Scheme 1). Derivatives featuring a
Aiming to identify other NHC-based architectures suitable to
metal−ligand cooperativity involving the carbene carbon for
small-molecule activation,8 we focused on complexes of types A
and B (Chart 1). The nitrogen lone pairs of electrons in these
ligands are integrated in the π systems of the aromatic azole and
pyrazole rings, respectively; consequently, the carbenic
structure is less stabilized. To date, only two pincer ligands
with central acyclic diaminocarbene moieties such as those
envisaged in ligands A and B have been reported, in metal
complexes C and D.9 They have been obtained in serendipitous
metal-templated self-assembly reactions, and their chemistry
has not been further investigated. In contrast, pincer ligands
featuring acyclic dialkyl and diarylcarbene moieties in the
central positions have been pioneered by Shaw four decades
ago (E, M = Ir),10 and their chemistry is rather extensive. Ru
and Os complexes E have been reported by Gusev.11 The
chemistry of derivatives F, first obtained via self-assembly in the
coordination sphere of the metal by Roper (R = Ph, M = Ru),12
has been examined extensively by Piers (R = iPr and tBu; R′ =
H; M= Ni, Rh, and Ir)13 and Iluc (R = iPr; R′ = H and tBu; M
= Pd)14 in recent years, and expanded by the former to analogs
G.15 Tantalum OCO−pincer complexes H, resulting from a
sequential proton and hydride abstraction at the methylene
Scheme 1. NHC−Ligand-Assisted Reversible Activation of
Ammonia
diaminoalkyl fragment coordinated to a transition metal,
N2C(H)−MLn, such as the ammonia activation product
shown in Scheme 1, remain rare for five-4 and six-membered5
cyclic frameworks, where deprotonation with formation of
NHC complexes (N2C → MLn) prevails. They are more
common for acyclic systems, most notably in strongly folded,
Received: May 27, 2017
© XXXX American Chemical Society
A
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Chart 1. Pincer Complexes with Acyclic Carbene Moieties in
the Central Position
Chart 2. P,P′- and N,N′-Complexes of Diphosphine Ligands
with Bis(pyrazolyl)methane and Bis(imidazolyl)methane
Backbones20
RESULTS AND DISCUSSION
■
Ligand Synthesis. Ligands 1 and 2 are easily accessible in
two simple steps in a manner very similar to that used for the
synthesis of their diphenylphosphino analogs (Scheme 2).20
The bis(imidazolyl)- and bis(pyrazolyl)methane precursors
were obtained in a neat reaction between the corresponding
nitrogen heterocycles and stoichiometric dichloromethane in
the presence of excess potassium hydroxide and a phase transfer
catalyst.22 We found that the mixture of nitrogen base,
potassium hydroxide, and phase transfer catalyst liquefied
easily and the viscous liquid solidified again within 60 min. The
best yields were obtained when dichloromethane was added to
the fluid mixture, although they were variable and in our hands
never approached the reported 100%. These highly exothermic
reactions were carried out in pressure glass reaction vessels.
Addition of small amounts of water to aid liquefying and mixing
led to a substantial drop in yields. The pure products were
isolated easily via sublimation from the crude reaction mixtures.
Ligand synthesis was accomplished via double deprotonation
of the bis(imidazolyl)- and bis(pyrazolyl)methane precursors in
THF with n-butyl lithium, followed by reaction with 2 equiv of
chlorodi(iso-propyl)phosphine, iPr2PCl. It proved crucial for
the success of this step that the starting materials were
completely dissolved prior to lithiation. Bis(pyrazolyl)methane
was well soluble, but its imidazole counterpart required a
solvent to solute ratio of ca. 200:1. The crude products were
reproducibly obtained in moderate (1) or high (2) yield as
viscous oils with good solubility in organic solvents, including
n-pentane. They had remarkably high purity and were used as
such for complex synthesis. The ligands could be handled in air
for brief periods of time but decomposed when exposed to air
over several hours, especially in solution. They were
characterized by 31P NMR resonances at −19.4 (1) and
−20.8 (2) ppm. The triplet resonances corresponding to the
methylene bridge were observed at 6.54 and 7.03 ppm,
respectively, in the 1H NMR spectra and at 55.3 and 61.0 ppm,
respectively, in the 13C NMR spectra.
bridge, have been described by Kawaguchi.16 Ruthenium
OCC−pincer complexes featuring an acyclic diaminocarbene
central moiety and NHC pendant arms have been reported by
Blechert and Grubbs as decomposition products of olefin
metathesis catalysts.17
Ruthenium and palladium complexes I (R = iPr), with a
structure very similar to that proposed for A and B, have been
described by Ozerov.18 Metal carbene (Ar2C → MLn) and
metal alkyl (Ar2C(H)−MLn) species were identified for these
systems, confirming that the constraining geometry of the
ligand does not prevent their formation. Later work with iron,
cobalt, and nickel complexes of less electron-rich PCP ligand
analogs (R = Ph) was limited to the metal alkyl species.19
We identified ligands 1 and 2 (Scheme 2) as promising,
easily accessible precursors for the synthesis of PCP−pincer
Scheme 2. Synthesis of Ligands 1 and 2
Nickel Complexes. Complex 3 was obtained as a dark
green microcrystalline product upon mixing 1 with Ni(dme)Br2
in dichloromethane (Scheme 3). Its effective magnetic moment
determined using Evans’ method23 had a value of 3.76, which is
in the expected range for tetrahedral Ni(II) complexes.24
Single-crystal X-ray analysis of the paramagnetic compound
revealed the expected pseudotetrahedral geometry (Figure 1).
Ligand 1 is strongly folded, with a dihedral angle of 100.5°
between the imidazole rings and an NCN angle at the
methylene bridge of 112.8(3)°. The methylene bridge is
conveniently positioned for pincer coordination, only 3.666(3)
Å away from the nickel center (cf. the sum of the van der Waals
radii for carbon and nickel of 3.3 Å). However, heating to 110
°C in toluene for 24 h failed to promote HBr elimination, and
complex 3 was recovered unchanged.
complexes via C−M bond formation upon deprotonation at the
methylene bridge. For the reasons discussed above, these metal
alkyl derivatives were postulated to have increased stability with
respect to diaminocarbene pincer analogs A and B and
potentially allow for carbene−carbon-assisted transformations.
The synthesis and P,P′, as well as N,N′, coordination chemistry
of diphenylphosphino analogs of ligands 1 and 2 have been
described nearly two decades ago (Chart 2).20 More recent
investigations targeted the P,P′, N,N′, and P,N coordination
chemistry of these ligands.21 Remarkably, the potential of these
ligands to generate pincer complexes via deprotonation or
C−H bond activation has not been explored; our investigations
of this chemistry will be described below.
B
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Scheme 3. Synthesis of Ni Complexes 3 and 4 and
Formation of 5
Figure 2. Solid-state structure of 4 with thermal ellipsoids drawn at
50% probability level. All hydrogen atoms except those on the methine
bridge were omitted for clarity. Selected bond lengths (Å) and angles
(deg): Ni1−Br1 = 2.3275(7); Ni1−P = 2.1710(11), 2.1802(12); Ni1−
C1 = 1.965(3); P1···P2 = 4.328(1); C1−N = 1.471(4); 1.483(4); P1−
Ni1−P2 = 168.25(4); Br1−Ni1−C1 = 172.81(10); N1−C1−N3 =
111.1(2).
Paramagnetic Ni(I) complex 5 was identified by single-crystal
X-ray crystallography (Figure 3), owing to its propensity to
Figure 1. Solid-state structure of 3 with thermal ellipsoids drawn at
50% probability level. All hydrogen atoms except those on the
methylene bridge were omitted for clarity. Selected bond lengths (Å)
and angles (deg): Ni1−Br = 2.3332(8), 2.3722(7); Ni1−P =
2.3274(12), 2.3538(11); Ni1···C1 = 3.666(3); P1···P2 = 3.750(1);
C1-N = 1.446(4); 1.452(4); P1−Ni1−P2 = 106.48(4); Br1−Ni1−Br2
= 117.38(3); N1−C1−N3 = 112.8(3).
Figure 3. Solid-state structure of 5 with thermal ellipsoids drawn at
50% probability level. All hydrogen atoms except those on the
methylene bridge were omitted for clarity. Selected bond lengths (Å)
and angles (deg): Ni1−Br1 = 2.2997(5); Ni1−P = 2.2088(10),
2.2242(9); Ni1···C1 = 3.448(1); P1···P2 = 3.694(1); C1−N =
1.448(4); 1.458(4); P1−Ni1−P2 = 112.87(4); Br1−Ni1−P =
115.55(3), 131.57(3); N1−C1−N3 = 112.3(3).
Addition of 1 equiv of potassium hexamethyldisilazide
(KHMDS) to a THF solution of 3 (Scheme 3) yielded a
diamagnetic, air stable, orange compound featuring one 31P
NMR resonance at 31.5 ppm. The triplet resonance
corresponding to the methine bridge proton was observed at
5.8 ppm, while the methyl protons of the isopropyl groups were
characterized by virtual triplet signals typical for pincer
coordination in ligands containing phosphine pendant arms.
The number of resonances observed for the isopropyl groups
corresponded to the expected Cs symmetry. Solid complex 3
does not show signs of decomposition when exposed to the air
for months, and its solutions can be handled in the air without
significant deterioration.
The solid-state structure of 4 confirmed the PCP−pincer
coordination with a diaminomethyl moiety in the central
position (Figure 2). Although the ligand backbone remained far
from planar, the folding was reduced substantially upon pincer
formation, with the dihedral angle between the imidazole rings
in 4 being 148.4°. C−N bond formation has little impact on the
geometry of the bridge carbon, with the NCN angle measuring
111.1(2)°. The tetrahedral bridge carbon atom, C1, sits 0.15
and 0.26 Å outside of the best plane of the two imidazole rings,
reflecting substantial strain in the rigid structure. Very similar,
strained geometries were identified in the closely related metal
alkyl (Ar2C(H)−MLn) precursors to derivatives I (R = iPr;
MLn = Ru(CO)2Cl, PdCl, and PdOTf) where the dihedral
angles between the pyrrole rings measured 154.9−159.6°, while
the deviation of the bridge carbon from the best plane of the
pyrrole rings was between 0.10 and 0.31 Å.18
crystallize preferentially from mixtures, as a minor byproduct in
the formation of 4 via deprotonation. Its presence led to line
broadening in the NMR spectra of impure samples of 4. It was
not isolated on a large scale, and targeted synthesis was not
pursued. A number of Ni(I) halide complexes with chelating
phosphine ligands have been described, and they feature very
similar geometry at the trigonal planar nickel center.25 The
reductive properties of alkali metal amides have been
documented,26 and it is assumed that the side reaction involves
the formation of the highly unstable bis(trimethylsilyl)aminyl
radical.27 The solid-state structure of 5 reveals a trigonal planar
nickel center and a strongly folded ligand conformation, very
similar to that observed in the structure of 3. The dihedral angle
formed by the imidazole rings measures 108.7°, and the NCN
angle corresponding to the methylene bridge remains relatively
unchanged at 113.2(3)°. The Ni−Br bond in Ni(I) complex 5
(2.30 Å) is only marginally shorter than those in Ni(II)
complexes 3 (2.35 Å) and 4 (2.33 Å). The decrease in the
length of the Ni−P bond upon pincer formation is much more
drastic, from 2.34 Å in 3 to 2.17 Å in 4, where it is even shorter
than in tricoordinate complex 5 (2.21 Å). These values fall
within the ranges that are typical for the respective compound
classes.
C
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Deprotonation of 4 at the methylene bridge to yield a PCP
pincer complex with a central, acyclic diaminocarbene moiety
was attempted using KHMDS, KH, MeLi, LDA, or tBuOK in
toluene and THF. KHMDS was also employed in the presence
of Lewis bases (PPh3, PCy3, and tBuNC) meant to cap the
nickel coordination site vacated by the bromide ion. All
attempts were unsuccessful, leading to multiple unwanted
decomposition products. Hydride abstraction with trityl
tetrakis(pentafluorophenyl)borate ([CPh3][B(C6F5)4]) in
toluene, as reported for Pd complex I (R = iPr, L = Cl),18b
did not meet with success either, leading to a complex mixture
of products that was not further investigated. Numerous
nickel(II) pincer complexes featuring dialkylmethyl central
platforms are known,28 but none of these was converted to a
carbene complex. Remarkably, a PCP−pincer system with a
diarylmethyl backbone was recently shown to stabilize both
diarylmethyl and diarylcarbene complexes of nickel.13b
Figure 4. Solid-state structure of 7 with thermal ellipsoids drawn at
50% probability level. All hydrogen atoms except those on the methine
bridge were omitted for clarity. Selected bond lengths (Å) and angles
(deg): Pt1−Cl1 = 2.3587(6); Pt1−P = 2.2724(6), 2.2748(6); Pt1−C1
= 2.065(2); P1···P2 = 4.514(1); C1-N = 1.473(3); 1.492(3); P1−Pt1−
P2 = 166.23(2); Cl1−Pt1−C1 = 177.16(7); N1−C1−N3 = 113.0(2).
tBuNC). Similarly, hydride abstraction with trityl tetrakis-
(pentafluorophenyl)borate in toluene failed to yield a cationic
PCP−pincer carbene complex.
Platinum Complexes. Platinum(II) chelate complex 6 was
obtained upon combination of ligand 1 with platinum
dichloride in THF (Scheme 4). Both the platinum dichloride
In anticipation of an increase in the acidity of the
diaminomethine hydrogen upon oxidation of the platinum
center, the synthesis of platinum(IV) complex 8 by oxidation of
7 with PhICl2 was easily achieved (Scheme 4). A few
diphosphine trihalide complexes of platinum(IV) with a similar
coordination geometry at the metal have been reported and
were shown to be susceptible to photoreduction,31 but none of
these were pincer derivatives. Several NCN pincer complexes of
platinum(IV) are however known,32 and CCP analogs have
been reported as well.33 The 31P NMR resonance at 35.6 ppm
in 7 shifted slightly to 24.5 ppm in 8 upon oxidation, and the
Scheme 4. Synthesis of Pt Complexes 6−8
1J
195Pt31
P coupling constant in this compound measured 2266 Hz.
This was significantly larger than in the reported Pt(IV) CCP
pincer complexes (1371 and 1427 Hz)33 and mirrored the
difference in the magnitude of the coupling constants observed
for the respective Pt(II) precursors. The time-averaged Cs
reagent and the product had low solubility in organic solvents
and purification was best accomplished via Soxhlet extraction.
1
According to the H NMR spectrum, the chelated platinum
symmetry of the compound was maintained upon oxidation,
2
195Pt13
and the J
C
Pt H
complex had time-averaged Cs symmetry, with the inequivalent
methylene bridge protons featured as doublet resonances.
Deprotonation of 6 with KHMDS in toluene led to clean
formation of pincer complex 7 (Scheme 3), as indicated by the
195
1
and J
coupling constants (652.5 and 86.8
Hz, respectively) observed for the resonances corresponding to
the methine bridge confirmed that the Pt−C bond remained
intact. Attempted deprotonation of 8 with KHMDS resulted in
quantitative reduction to 7, which was identified based on its
NMR signature.
Due to the presence of more Lewis basic sites, ligand 2
promises a more varied coordination chemistry in comparison
to 1. Its reaction with CoBr2 (Scheme 5) yielded teal-colored
1
emergence in the H NMR spectrum of a resonance featuring
platinum satellites at 6.43 ppm, corresponding to the
diaminomethyl group bonded to the metal. The four doublet
of doublet signals attributed to the methyl groups in 6
converted upon deprotonation to an equal number of doublets
of virtual triplets typical of PCP pincer complexes with di-
isopropylphosphine moieties. The 31P NMR resonance shifts
downfield from −19.4 ppm in 1 to 6.9 ppm in 6 and 35.6 ppm
Scheme 5. Synthesis of Co Complex 9
in 7, and the magnitude of the 1J
coupling constant
195Pt31
P
increases upon pincer formation from 2242 Hz in 6 to 2756 Hz
in 7. Very few platinum pincer complexes with methine central
moieties are known,29 and only one of these features a
diaminomethyl central group.30
The solid-state structure of 7 (Figure 4) confirms pincer
formation and is very similar to the structure of the analogous
nickel pincer 4, with the exception of the longer bonds
involving the square-planar metal. The best planes of the
imidazole rings form a dihedral angle of 147.1°, and the
methine carbon lies 0.13 and 0.25 Å outside of these planes,
implying structural strain. In line with the observations made
on 4, the acidity of the diaminomethine hydrogen in 7 is low,
and this compound does not react with KHMDS in toluene or
THF in the presence or absence of Lewis bases (PPh3 and
paramagnetic complex 9, with an effective magnetic moment of
4.77, as determined using Evans’ method;23 this is in good
agreement with a pseudotetrahedral geometry at the metal.24
A
structural determination based on single-crystal X-ray diffrac-
tion revealed P,N-chelation with the formation of a seven-
membered metallacycle that adopted a boat conformation in
the solid state (Figure 5). The metal was even closer to the
methylene carbon than in the nickel complex 3 (3.24 vs 3.67
Å). Given that the PP bite of the ligand is easily adaptable to
D
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1
d7 (2.09 ppm, H) and toluene-d8 (20.4 ppm, 13C). All 13C and 31P
NMR spectra were acquired broad-band proton decoupled.
Synthesis of Methylenebis(imidazole).22 Imidazole (20.00 g,
134.98 mmol), n-butylammonium bromide (2.43 g, 7.53 mmol), and
finely ground KOH (32.97 g, 582.75 mmol) were mixed in a thick-
walled round-bottomed flask equipped with a magnetic stirring bar.
The reaction mixture liquefied upon mixing and when the viscosity
started to increase again (30−60 min), CH2Cl2 (9.4 mL, 146.90
mmol) was added, the flask sealed, and the highly exothermic reaction
allowed to proceed overnight. The pure product was isolated as a
colorless crystalline solid (13.397 g, 61.56%) via sublimation from the
crude reaction mixture at 170 °C and 10−3 bar. A similar procedure
was used for the synthesis of methylenebis(pyrazole).
Figure 5. Solid-state structure of 9 with thermal ellipsoids drawn at
50% probability level. All hydrogen atoms except those on the
methylene bridge were omitted for clarity. Selected bond lengths (Å)
and angles (deg): Co1−Br = 2.3547(7), 2.3775(10); Co1−P1 =
2.3916(11); Co1−N4 2.017(3); Co1···C1 = 3.238(3); C1−N =
1.446(4); 1.465(4); P1−Co1−N4 = 101.68(9); Br1−Co1−Br2 =
115.98(3); N1−C1−N3 = 113.2(3).
Synthesis of Ligand 1. Methylenebis(imidazole) (2.00 g, 13.50
mmol) was dissolved in THF (400 mL) in a swivel frit apparatus
attached to vacuum line. The solution was cooled to −55 °C using a
liquid N2/ethanol bath. Then, nBuLi (18.1 mL, 28.90 mmol) was
added dropwise, and the mixture was allowed to slowly warm up to
−20 °C under continuous stirring. After 1 h at this temperature, the
reaction mixture was cooled again to −55 °C, and iPr2PCl was added
(4.12 g, 27.81 mmol) and allowed to react for 12 h. Solid NH4Cl (1.40
g, 26 mmol) was added to quench excess unreacted nBuLi, and the
reaction mixture was stirred for another 1 h. The solvent was removed
in vacuo, and the remaining brown solid was extracted with pentane
(200 mL). The product was obtained as a pale yellow thick oil (3.16 g,
61.44%) of satisfactory purity upon removal of the pentane in vacuo.
1H NMR (CD2Cl2, 400 MHz, 298 K): δ 0.97 (12H, dd, 3JPH = 12.6
Hz, 3JHH = 6.9 Hz, CH3), 1.11 (12H, dd, 3JPH = 15.9 Hz, 3JHH = 7.1 Hz,
CH3), 2.35 (4H, dsep, 3JHH = 7.0 Hz, 2JPH = 2.1 Hz, CH), 6.55 (2H, t,
4JPH = 3.3 Hz, CH2), 7.16 (s, 2H, CH), 7.26 (s, 2H, CH). 13C NMR
the size and geometry of the metal, it is likely that the
coordination mode involving the pyrazole nitrogen reflects the
preference of Co(II) for mixed ligation34 and not steric strain.
CONCLUSIONS
■
Ligands 1 and 2, easily accessible via two high-yield synthetic
steps from commercially available starting materials, proved to
have a diverse coordination chemistry. Most notably, the
chelated complexes of Ni(II) (3) and Pt(II) (6) incorporating
ligand 1 were converted to PCP pincer analogs 4 and 7,
respectively, in the presence of KHMDS. Ni(I) chelate complex
5 was obtained as a side product in the formation of 4. The
methine bridge hydrogen in complexes 4 and 7 proved to be
remarkably robust and could not be removed as either a proton
or a hydride to yield carbene-based PCP pincer complexes.
This research started from the premise that the involvement
of the nitrogen lone pair in the π electron system of the
aromatic imidazole ring would lead to weak stabilization of the
diaminocarbene structure and increase the stability of the
diaminomethyl complexes. The validity of this premise was
confirmed, and diaminomethyl complexes 4 and 7 were shown
to be stable toward air and moisture and survive reflux in
toluene. It is nevertheless surprising that the diaminomethyl
proton in complexes 4 and 7 cannot be removed with strong
bases, especially given the related chemistry outlined in Scheme
1, as well as that described for analogs F, G, and especially
I.13−15,18 The lack of such an observable derivative does not
preclude the involvement of such an intermediate in a ligand-
assisted catalytic process, and investigations in this direction are
pending. Notable for ligand 1 is the easily variable P···P ligand
“bite”, which measures 3.69, 3.75, 4.33, and 4.52 Å in 5, 3, 4
and 7, respectively, as well as the stability of its Ni(II)
derivatives in the air. Further investigations of ligands based on
the bis(imidazolyl)- and bis(pyrazolyl)methane frameworks are
in progress.
2
(CD2Cl2, 101 MHz, 298 K): δ 19.5 (d, JPC = 8.8 Hz, CH3), 20.1 (d,
2JPC = 18.3 Hz, CH3), 25.0 (d, 1JPC = 5.8 Hz, CH), 55.3 (t, 3JPC = 16.7
Hz, CH2), 121.0 (vt, JPC = 3.0 Hz, CH), 131.4 (s, CH), 147.1 (d, 1JPC
=
16.8 Hz, PC). 31P{1H} NMR (toluene-d8, 162 MHz, 298 K): δ −19.44
(s).
Synthesis of Ligand 2. In a 100 mL two-necked round-bottomed
flask, 1,1′-methylenebis(pyrazole) (500 mg, 3.37 mmol) was dissolved
in THF (70 mL). The solution was subsequently cooled to −78 °C,
treated dropwise with 1.6 M nBuLi in hexanes (4.22 mL, 6.74 mmol),
and stirred for 2 h, while being allowed to warm up to −25 °C. After
cooling back to −78 °C, the mixture was treated dropwise with
iPr2PCl (1.03 g, 6.95 mmol) and allowed to warm up to room
temperature overnight. Solid ammonium chloride (370 mg, 6.87
mmol) was added, the mixture stirred for another hour, and the
solvent removed in vacuo. The solid residue was extracted with
pentane (80 mL), yielding a light yellow solution. After filtration, the
solvent was removed in vacuo, leaving behind the product as a yellow
1
transparent oil (1.20 g, 93%). H NMR (400 MHz, C6D6, 298 K): δ
3
3
0.97 (12H, dd, JPH = 12.0 Hz, JHH = 7.2 Hz, CH3), 1.02 (12H, dd,
3
3
3JPH = 12.0 Hz, JHH = 7.2 Hz, CH3), 1.94 (4H, dsep, JHH = 6.8 Hz,
2JPH = 1.6 Hz, CH), 6.16 (d, 2H, 3JHH = 1.8 Hz, CH), 7.03 (2H, t, 4JPH
= 3.2 Hz, CH2), 7.56 (2H, d, JHH = 1.8 Hz, CH). 13C NMR (C6D6,
3
101 MHz, 298 K): δ 19.6 (d, 2JPC = 10.1 Hz, CH3), 20.0 (d, 2JPC = 17.2
Hz, CH3), 24.6 (d, 1JPC = 9.1 Hz, CH), 61.0 (t, 3JPC = 15.2 Hz, CH2),
1
5
111.6 (s, CH), 139.8 (dd, JPC = 24.2, JPC = 2.4 Hz, PC), 140.2 (s,
CH). 31P{1H} NMR (400 MHz, C6D6, 298 K): δ −20.8 ppm (s).
Synthesis of Ni(II) Complex 3. Ni(dme)Br2 (520 mg, 1.68
mmol) was added to a solution of 1 (650 mg, 1.69 mmol) in toluene
(50 mL), and the reaction mixture was stirred for 12 h at room
temperature. The solvent was subsequently removed in vacuo, and the
solid was washed with pentane, leaving the product behind as a dark
green crystalline solid (919 mg, 91.2%). The magnetic susceptibility
μeff was determined to be 3.76 by Evans’ method using the chemical
shift of SiMe4 in toluene-d8 solution and a concentration of 3 of 9.13
mg/mL, 298 K. Elemental analysis (%): Calcd for C19H34N4Br2NiP2:
C 38.10, H 5.72, N 9.25. Found: C 37.86, H 5.76, N 8.44. Single
crystals suitable for X-ray crystallography were obtained via slow
evaporation of a THF solution.
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise specified, all oper-
ations were carried out with careful exclusion of air and moisture using
standard Schlenk and glovebox techniques. The solvents were dried
and deoxygenated prior to use. Starting materials were purchased from
commercial suppliers and used as received. All NMR spectra were run
on either a Bruker Avance RDQ-400 instrument and chemical shifts
are reported in δ units (ppm) using the solvent as an internal
reference: CH2Cl2-d1 (5.32 ppm, 1H) and CH2Cl2-d2 (54.00 ppm,
13C); THF-d7 (3.58 ppm, 1H) and THF-d8 (67.57 ppm, 13C); toluene-
E
DOI: 10.1021/acs.organomet.7b00396
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis of Ni(II) Complex 4. Solid KHMDS (336 mg, 1.68
mmol) was added at room temperature to a solution of 3 (873 mg,
1.46 mmol) in THF in a swivel frit setup. The reaction mixture was
allowed to stir for 2 h at room temperature, and the solvent was
subsequently removed in vacuo. The product was extracted with
toluene, the solvent removed in vacuo, and the remaining solid washed
with pentane. The product was isolated by filtration as a light brown
powder (656 mg, 86.9%). Ni(I) byproduct 5 was removed by exposing
a suspension of crude 4 in hexane to the air and subsequently
subjecting the resulting mixture to Soxhlet extraction using hexane for
48 h. Pure 5 separated in the collection flask as an orange powder.
1H NMR (toluene-d8, 400 MHz, 298 K): δ 1.07 (6H, dvt, 3JHH = 7.5
temperature to a solution of 7 (202 mg, 0.33 mmol) in the same
solvent (25 mL). The mixture was stirred for 6 h, and the volatiles
were removed in vacuo. The residue was washed twice with pentane
(25 mL) and collected, yielding 8 (104 mg, 46%) as a microcrystalline
solid.
3
1H NMR (CD2Cl2, 400 MHz, 298 K): δ 0.90 (6H, dd, JHH = 7.0
Hz, 3JPH = 17.2 Hz, CH3), 1.66 (6H, dd, 3JHH = 7.2 Hz, 3JPH = 20.6 Hz,
3
3
CH3), 1.76 (6H, dd, JHH = 7.4 Hz, JPH = 18.1 Hz, CH3), 1.77 (6H,
dd, 3JHH = 6.9 Hz, JPH = 17.2 Hz, CH3), 2.79 (2H, m, CH), 4.01 (2H,
2
m, CH), 6.91 (1H, s, JPtH = 86.8 Hz, PtCH), 7.51 (2H, s, CH), 7.52
(2H, d, CH). 13C NMR (CD2Cl2, 101 MHz, 298 K): δ 19.7 (s, CH3),
19.8 (s, CH3), 20.5 (s, CH3), 18.3 (s, CH3), 21.3 (s, CH3), 31.6 (d, 1JPC
= 28.3 Hz, JPtC = 15.2 Hz, CH), 32.9 (d, 1JPC = 33.3 Hz, JPtC = 16.7
2
2
3
Hz, JPH = 7.5 Hz, CH3), 1.26 (6H, dvt, JHH = 8.4 Hz, JPH = 8.4 Hz,
Hz, CH), 57.9 (s, 1JPtC = 652.5 Hz, PtCH), 123.3 (s, 3JPtC = 28.2 Hz),
CH3), 1.40 (6H, dvt, 3JHH = 8.4 Hz, JPH = 8.4 Hz, CH3), 1.42 (6H, dvt,
3JHH = 8.4 Hz, JPH = 8.4 Hz, CH3), 2.40 (2H, m, CH), 2.68 (2H, m,
CH), 5.84 (1H, s, NiCH), 6.85 (2H, s, CH), 7.39 (2H, s, CH). 13C
NMR (toluene-d8, 101 MHz, 298 K): δ 17.7 (s, CH3), 18.2 (s, CH3),
18.4 (s, CH3), 18.5 (s, CH3), 24.4 (vt, JPC = 14.9 Hz, CH), 25.7 (vt, JPC
= 12.8 Hz, CH), 65.6 (t, 3JPC = 17.3 Hz, NiCH), 121.4 (s, CH), 137.6
(vt, JPC = 3.5 Hz, CH), 148.1 (vt, JPC = 31.3 Hz, N2CP). 31P{1H} NMR
(toluene-d8, 162 MHz, 298 K): δ 31.5 (s). Elemental analysis (%):
Calcd for C19H33N4BrNiP2: C 44.05, H 6.42, N 10.82. Found: C 44.78,
H 6.35, N 11.02. Single crystals suitable for X-ray crystallography were
obtained via vapor diffusion of pentane into a toluene solution.
Synthesis of Pt(II) Complex 6. THF (120 mL) was condensed
onto a mixture of ligand 1 (808 mg, 2.12 mmol) and PtCl2 (565 mg,
2.12 mmol). The reaction vessel was immersed in an ultrasonic bath
for 1 h and then stirred at room temperature for 16 h. The solvent was
removed in vacuo, and the residue was washed with pentane (2 × 50
mL) and subsequently extracted with dichloromethane over 24 h in a
Soxhlet apparatus. Removal of the solvent yielded 6 (840 mg, 61%) as
a tan powder.
3
138.1 (d, JPC = 11.1 Hz, CH), 147.3 (d, JPC = 94.9 Hz, N2CP).
31P{1H} NMR (CD2Cl2, 162 MHz, 298 K): δ 24.5 (s, JPtP = 2266.4
1
Hz).
Synthesis of Co Complex 9. Solid cobalt(II) bromide (286 mg,
1.31 mmol) was added at −78 °C to a THF (60 mL) solution of 2
(500 mg, 1.31 mmol). The mixture was allowed to warm up to room
temperature overnight with stirring, during which time the color
turned from light green to dark teal. The mixture was filtered, and the
solvent was removed in vacuo. The solid teal colored residue was
washed with pentane (50 mL) and collected by filtration (715 mg,
91%). Its effective magnetic moment was determined by Evans’
method (μeff = 4.77) using a 16.7 mM solution in CD2Cl2. Elemental
analysis (%): Calcd for C19H34N4Br2P2Co: C 38.09, H 5.72, N 9.35.
Found: C 38.17, H 5.83, N 8.73.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00396.
3
1H NMR (CD2Cl2, 400 MHz, 298 K): δ 0.87 (6H, dd, JHH = 6.6
Hz, 3JPH = 15.7 Hz, CH3), 1.26 (6H, dd, 3JHH = 7.0 Hz, 3JPH = 15.8 Hz,
3
3
CH3), 1.35 (6H, dd, JHH = 6.8 Hz, JPH = 19.2 Hz, CH3), 1.52 (6H,
3
3
dd, JHH = 6.9 Hz, JPH = 17.8 Hz, CH3), 2.92 (2H, m, br, CH), 3.68
Complete NMR spectra for all new diamagnetic
compounds, as well as the relevant crystallography tables
(PDF)
(2H, m, br, CH), 6.09 (1H, d, 2JHH = 13.7 Hz, CH2), 7.16 (2H, s, CH),
7.38 (2H, s, CH), 7.89 (1H, d, JHH = 14.1 Hz, CH2). 13C NMR
2
(CD2Cl2, 101 MHz, 298 K): δ 18.6 (s, CH3), 19.5 (s, CH3), 20.8 (s,
CH3), 21.6 (s, CH3), 27.1 (d, 1JPC = 36.6 Hz, CH), 29.0 (d, 1JPC = 39.0
Hz, CH), 57.1 (s, CH2), 125.1 (s, CH), 131.4 (d, 3JPC = 10.0 Hz, CH),
Cartesian coordinates (XYZ)
Accession Codes
1
136.4 (d, JPC = 70.6 Hz, N2CP). 31P{1H} NMR (CD2Cl2, 162 MHz,
CCDC 1552638−1552642 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, UK; fax: +44 1223 336033.
298 K): δ 6.9 (s, 1JPtP = 2242.1 Hz). Elemental analysis (%): Calcd for
C19H34N4Cl2P2Pt: C 35.30, H 5.30, N 8.67. Found: C 35.74, H 5.68, N
8.20.
Synthesis of Pt(II) Complex 7. Toluene (70 mL) was condensed
onto a mixture of 6 (683 mg, 1.06 mmol) and KHMDS (211 mg, 1.06
mmol), and the suspension was stirred at room temperature for 24 h.
The solids were filtered off, and the toluene was removed in vacuo.
The residue was triturated with pentane (25 mL), which was then
removed in vacuo. This operation was repeated three times to fully
remove excess HMDS. The solid was washed with pentane (50 mL),
and 7 was isolated as an off-white powder (505 mg 78%).
1H NMR (toluene-d8, 400 MHz, 298 K): δ 0.91 (6H, dvt, 3JHH = 7.2
AUTHOR INFORMATION
Corresponding Author
*E-mail: roesler@ucalgary.ca.
■
ORCID
3
Hz, JPH = 8.8 Hz, CH3), 1.17 (6H, dvt, JHH = 7.2 Hz, JPH = 8.8 Hz,
Roland Roesler: 0000-0002-1235-3976
Notes
The authors declare no competing financial interest.
CH3), 1.35 (6H, dvt, 3JHH = 7.2 Hz, JPH = 9.2 Hz, CH3), 1.38 (6H, dvt,
3JHH = 7.2 Hz, JPH = 7.2 Hz, CH3), 2.52 (2H, m, CH), 2.90 (2H, m,
CH), 6.43 (1H, s, 2JPtH = 84.8 Hz, PtCH), 6.97 (2H, d, 3JHH = 0.8 Hz,
CH), 7.54 (2H, d, JHH = 0.8 Hz, JPtH = 12.0 Hz, CH). 13C NMR
(CD2Cl2, 101 MHz, 298 K): δ 17.7 (s, 3JPtC = 24.2 Hz, CH3), 18.1 (vt,
JPC = 2.5 Hz, CH3), 18.3 (s, 3JPtC = 10.5 Hz, CH3), 18.7 (s, 3JPtC = 17.9
3
5
ACKNOWLEDGMENTS
■
Financial support for this work was provided by the Natural
Sciences and Engineering Research Council of Canada in the
form of Discovery Grants to T.C. and R.R.
Hz, CH3), 23.6 (vt, JPC = 17.3 Hz, 2JPtC = 21.9 Hz, CH), 26.9 (vt, JPC
=
15.6 Hz, 2JPtC = 22.4 Hz, CH), 54.2 (t, 3JPC = 3.9 Hz, 1JPtC = 899.6 Hz,
PtCH), 121.8 (vt, JPC = 2.5 Hz, 3JPtC = 41.2 Hz, CH), 137.8 (vt, JPC
=
4.7 Hz, CH), 147.3 (vt, JPC = 37.5 Hz, N21CP). 31P{1H} NMR
(toluene-d8, 162 MHz, 298 K): δ 35.6 (s, JPtP = 2755.6 Hz).
Elemental analysis (%): Calcd for C19H33N4ClP2Pt: C 37.41, H 4.45,
N 9.19. Found: C 37.12, H 5.22, N 8.80.
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DOI: 10.1021/acs.organomet.7b00396
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