Silver(I) and Copper(I) Complexes of Semi-Bulky Nitrogen-Confused C-Scorpionates

Article abstract of DOI:10.1002/ejic.202000173

Two new sterically demanding nitrogen-confused C-scorpionate ligands with a bis(3,5-diisopropylpyrazol-1-yl)methyl group bound to the 3- position of a normal pyrazole (^HL^iPr2) or an N-toluenesulfonyl pyrazole (^TsL^iPr2) have been prepared. Reactions between the ligands (^xL^iPr2) and silver trifluoromethanesulfonate, AgOTf, gave four new compounds of the types [Ag(^xL^iPr2)](OTf) (x = Ts, 1a; x = H, 2a) or [Ag(^xL^iPr2)₂](OTf) (x = Ts, 1b; x = H, 2b) depending on the initial metal:ligand ratio. Similarly, the reactions with [Cu(CH₃CN)₄](PF₆) produce four new compounds of the type [Cu(^xL^iPr2)(CH₃CN)](PF₆) (x = Ts, 3a; x = H, 4a) or [Cu(^xL^iPr2)₂](PF₆) (x = Ts, 3b; x = H, 4b). The solid-state structures of four derivatives (1a·acetone, 3a, 3b·CH₂Cl₂, and 4b·2THF) were determined by single-crystal X-ray diffraction while all complexes were characterized in CH₃CN solution by NMR spectroscopy and ESI(+) MS. The eight new complexes catalyze the aziridination of styrene. The copper complexes were generally (but not exclusively) more active catalysts than their silver counterparts.

Full text of DOI:10.1002/ejic.202000173

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Nitrene Transfer
Silver(I) and Copper(I) Complexes of Semi-Bulky Nitrogen-
Confused C-Scorpionates
Denan Wang,[a] Fathiya Jahan,[a] Kristen J. Meise,[a] Sergey V. Lindeman,[a] and
James R. Gardinier*[a]
Abstract: Two new sterically demanding nitrogen-confused four new compounds of the type [Cu(xLiPr2)(CH3CN)](PF6) (x =
C-scorpionate ligands with
a
bis(3,5-diisopropylpyrazol-1- Ts, 3a; x = H, 4a) or [Cu(xLiPr2)2](PF6) (x = Ts, 3b; x = H, 4b).
yl)methyl group bound to the 3- position of a normal pyrazole The solid-state structures of four derivatives (1a·acetone, 3a,
(HLiPr2) or an N-toluenesulfonyl pyrazole (TsLiPr2) have been pre- 3b·CH2Cl2, and 4b·2THF) were determined by single-crystal X-
pared. Reactions between the ligands (xLiPr2) and silver trifluoro- ray diffraction while all complexes were characterized in CH3CN
methanesulfonate, AgOTf, gave four new compounds of the solution by NMR spectroscopy and ESI(+) MS. The eight new
types [Ag(xLiPr2)](OTf) (x = Ts, 1a; x = H, 2a) or [Ag(xLiPr2)2](OTf) complexes catalyze the aziridination of styrene. The copper
(x = Ts, 1b; x = H, 2b) depending on the initial metal:ligand complexes were generally (but not exclusively) more active cat-
ratio. Similarly, the reactions with [Cu(CH3CN)4](PF6) produce alysts than their silver counterparts.
Introduction
Aziridines are important synthetic intermediates in organic syn-
thesis, medicinal chemistry, and polymer chemistry.[1,2] Catalytic
approaches to their syntheses have rapidly expanded in recent
years.[1,3–5] Transition metal-catalyzed reactions involving ni-
trene transfer to alkenes have gained prominence in this re-
gard.[3,5–9,10–19,20–27] Of these latter catalysts, copper(I) and
silver(I) scorpionate complexes [tris(pyrazolyl)borates[14–17,28,29]
(normal scorpionates, or TpR, Scheme 1, top left), tris-
(pyrazolyl)methanes (TpmR or C-scorpionates, Scheme 1, top
right),[30–32] certain C-heteroscorpionates (Scheme 1, bottom
left),[18] and certain nitrogen-confused C-scorpionates
(Scheme 1, bottom right)][27] have proven adept at catalytically
affording aziridines from alkenes and various nitrene sources
under mild conditions. Thus, Pérez and co-workers found that
Tp*Cu(C2H4)[14,17] or other CuTpR complexes[15] (prepared in-
situ) catalyze the aziridination of styrene, cyclooctene, or 1-hex-
ene over the course of a few hours at room temperature in
CH2Cl2 using PhI=NTs as a nitrene transfer agent. The CuTpR
could also catalyze aziridination using chloramine-T in CH3CN
to give more environmentally benign NaCl (vs. PhI) as a co-
product.[15] TpBr3Cu(CH3CN) was also found to catalyze nitrene
transfer from PhI=NTs to furans in room temperature CH2Cl2 to
give dihydropyridines.[33] AgTp* or Ag(Tp*,Br) were found to be
excellent catalysts for aziridination of styrenes, 2-alkenes, and
Scheme 1. Structures and abbreviations of representative ligands referred to
in this work. From left to right: scorpionate, C-scorpionate, C-heteroscorpion-
ate, and nitrogen-confused C-scorpionate ligands. Ts = p-toluenesulfonyl.
even E,E-hexadien-1-ol in room temperature CH2Cl2 using PhI=
[a] Dr. D. Wang, F. Jahan, K. J. Meise, Dr. S. V. Lindeman, Prof. J. R. Gardinier
Department of Chemistry, Marquette University,
NTs as a nitrene source.[16] For the latter substrate, the silver
complexes offered higher regio- and stereoselectivity than their
copper counterparts giving trans-aziridines vicinal to the alco-
hol group. Most recently, the Herres-Pawlis group[18] was able
1414 W. Clybourne St, Milwaukee, WI 53233, USA
E-mail: james.gardinier@marquette.edu
https://www.marquette.edu/chemistry/directory/james-gardinier.php
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000173.
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to trap and spectroscopically characterize the nitrene interme- Results and Discussion
diates {[(2-py)BpmR3R5]Cu(NTs)}+ in CH2Cl2 at –78 °C using (2-
The optimized synthetic route to the new bulky “confused”
tBuSO2)C6H4I=NTs[34] as a soluble nitrene source. The spectro-
scopic and magnetic data were consistent with diamagnetic,
singlet species at –78 °C. Calculations showed ground state trip-
let but all spin states [1/3CuI-nitrene or 1/3CuII-N·(iminyl)] and
nitrene binding modes (κ1N-, κ2N,O-) are thermally accessible.
These intermediates were capable of stoichiometrically transfer-
ring a nitrene unit to a variety of substrates. Then, different
{[(2-py)BpmR3R5]Cu(CH3CN)}(PF6) complexes were found to be
capable catalysts for nitrene transfer to unsubstituted or p-sub-
stituted styrenes giving modest yields (> 65 %) of aziridine un-
der mild ocnditions (PhI=NTs/CH2Cl2/295 K, 24 h).
scorpionate ligands is outlined in Scheme 2. The cobalt(II)-cata-
lyzed Peterson rearrangement reaction[35–38] between N-tosyl-
pyrazole-3-carboxaldehyde[39] and an excess of in-situ formed
bis(3,5-diisopropylpyrazolyl)sulfinyl, O=S(pziPr2)2, gave very high
yields (> 90 %) of the N-tosyl-protected ligand, TsLiPr2. The use
of excess O=S(pziPr2) ensured reproducibly high yields. The li-
gand TsLiPr2 could also be obtained in lower yields (ca. 65 %) in
a multi-pot reaction using O=C(pziPr2)2 in toluene instead of the
sulfinyl derivative in THF. As dipyrazolylcarbonyls are generally
more reactive than their sulfinyl counterparts,[36] the lower yield
by this latter route was initially surprising. It is noted, however,
In
a
previous study,[27] eight complexes of the type
that the syntheses of O=C(pziPr2
)
from triphosgene and
2
x
[Ag(xLR)n](OTf) (n = 1, 2 and LR
=
HL, TsL, HL*, and TsL*) were
H(pziPr2) was invariable complicated by a small amount (ca. 5 %)
of the starting heterocycle that is difficult to separate and likely
interferes with the subsequent rearrangement reaction. The N-
tosyl group of TsLiPr2 was quickly and quantitatively hydrolyzed
characterized and evaluated for their ability to catalyze aziridi-
nation of styrene. Both ESI(+) MS and NMR spectroscopic stud-
ies indicated that the analytically pure [Ag(xLR)n](OTf) com-
pounds did not retain their structure in CH3CN rather were in-
volved in multiple dynamic equilibria in solution. These dy-
namic processes remained in the fast exchange regime on the
NMR timescale down to the freezing point of the solvent,
thereby obfuscating structural information. These silver com-
plexes of nitrogen-confused C-scorpionates showed no or very
little capacity to participate in styrene aziridination using PhI=
NTs in room temperature CH2Cl2. However, [Ag(xLR)n](OTf)
showed modest catalytic activity at 80 °C in CH3CN when em-
ploying a nitrene generated in-situ from H2NTs and PhI(OAc).
Interestingly, the bulkiest derivative [Ag(TsL*)2](OTf) was re-
ported to have the highest activity giving 34 % yield of the
desired N-tosyl aziridine. This observation prompted the current
study to determine if further increasing steric bulk of the li-
gands would lead to an increase in catalytic activity of silver
complexes. Herein we report on the preparation of two new
semi-bulky nitrogen confused scorpionates, TsLiPr2 and HLiPr2 and
their silver(I) and copper(I) complexes. The copper(I) complexes
were prepared to compare catalytic activity with their silver
congeners, and possibly to demonstrate the generality of any
trends in ligand sterics on catalytic activity. It was also hoped
that the slower ligand exchange rates associated with the
smaller copper(I) compared to silver(I) would give more inform-
ative NMR spectra to shed light on possible solution structures
of the d10 metal complexes. During the course of study, we
found the reported activity of [Ag(TsL*)2](OTf) to be in error,
and outline a more reliable protocol and data for this series of
complexes.
under basic conditions to give HLiPr2
.
1
The H NMR spectrum of each ligand reveals a similar low
symmetry. Specifically, there is only one resonance near δH
=
6 ppm for the ring H4-pziPr2 hydrogen and two septet resonan-
ces near δH = 3.3 (J = 6.8 Hz) and 2.9 (J = 6.9 Hz) ppm for the
CHMe2 groups indicating equivalency of these two pyrazolyl
rings, top right of Figure 1. However, there are three doublet
resonances near δH = 1.1, 1.0, 0.9 ppm that integrate to 12, 6,
and 6 hydrogens for the isopropyl methyl groups. Moreover,
Figure 1. Left: Line drawing of a possible C1-symmetric ligand geometry (R =
Hpz or Tspz) with methine carbon and different isopropyl groups labeled.
Right: 1H NMR spectra of HLiPr2 in CDCl3 at 293 K (top) and 223 K (bottom).
The asterisk is for solvent resonance, the “cf” refers to the confused pyrazolyl
ring hydrogens.
Scheme 2. Optimized route to the new C-scorpionate ligands.
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the 13C NMR shows 6 resonances for isopropyl group carbons. phate anion. Each of the diligated complexes [Ag(xLiPr2)2](OTf)
The observed number of resonances is unusual, since a Cs-sym- (x = Ts, 1b, or x = H, 2b) or [Cu(xLiPr2)2](PF6) (x = Ts, 3b, or x =
metric ligand would be expected to give only two doublet iPr- H, 4b) is solvent free. The silver complexes were also prepared
CH3 1H resonances and four singlet 13C isopropyl carbon reso- using THF as a solvent with generally good yields (> 80 %) but
nances. Alternatively, as illustrated in Figure 1, a C1- symmetric care was needed in workup to achieve these reasonably high
species with free rotation of isopropyl groups and of pyrazolyl yields reproducibly. Many silver complexes of tris(pyraz-
rings is expected to give four doublet iPr-CH3 1H resonances olyl)methanes are insoluble in THF so this solvent is generally
and eight singlet 13C isopropyl carbon resonances (for groups used for their preparation. In this solvent, however, only 1b,
a–d, left of Figure 1). Thus, the unusual number of resonances gave a precipitate upon mixing reagents. The isopropyl groups
may occur if the local magnetic environment becomes progres- confer considerable solubility to the silver complexes in THF, or
sively equivalent (pseudo-C2 symmetric) with increasing dis- even Et2O (for 1a, 2a, 2b), so this method does not offer any
tance from the prochiral methine carbon, α, causing the reso- advantage over preparations using CH3CN.
nances for isopropyl (and H4/C4 pyrazolyl) groups (a and b, Fig-
ure 1) to have coincidental chemical shifts.
Solid State
An alternate geometry with a Cs-symmetric ligand (with
eclipsed diisopropyl pyrazolyl rings) and with 5- (but not the 3-
) isopropyl groups locked into one position seems less likely
since such an arrangement with overlapping nitrogen lone
pairs on adjacent rings is energetically less favorable than a
geometry like that in Figure 1. Moreover, the unique proton of
the proposed 5-iPr CHMeMe′ group is expected to appear as a
quartet of quartets rather than the observed septet. The low
Two of the mono-ligated complexes, namely Ag(TsLiPr2)(OTf)·
acetone, 1a·acetone, and [(TsLiPr2)Cu(CH3CN)](PF6), 3a, gave
crystals suitable for single-crystal X-ray diffraction. Views of the
structures of these complexes are found in Figure 2, while
Table 1 collects selected bond lengths and angles. The structure
of 1a·acetone is comparable to the related complex
Ag(TsL*)(OTf) reported previously.[27] That is, silver center in
1a·acetone is tetracoordinate via bonding to a κ3N- ligand and
an oxygen (O3) of the triflate ion. The acetone solvate molecule
is not bound to silver; rather it occupies channels parallel with
the a-axis of the crystal. The Ag1–O3 distance of 2.275(1) Å in
1
temperature H NMR spectrum of the ligands in CDCl3 shows
that the free rotation of the confused pyrazolyl slows into the
intermediate exchange region near 223 K as the pyrazolyl dou-
blet resonances broaden and shift upfield (bottom right Fig-
ure 1). Concomitantly, the resonances for the 5-iPr group hydro-
gens (the septet near 3.3 ppm and the two upfield doublets)
broaden and shift upfield compared those in the high tempera-
ture spectrum. Unfortunately, the slow exchange limit is not
reached before the solvent freezing point. The slow exchange
limit was not even observed in the spectrum for CD2Cl2 solu-
tions on cooling to 183 K.
Four silver(I) trifluoromethanesulfonate and four copper(I)
hexafluorophosphate complexes were prepared in high yields
(> 85 %) by direct addition of either one or two equivalents of
ligands to the metal salts in CH3CN according to Scheme 3.
After drying under vacuum, the mono- ligated silver complexes
analyze as solvent-free [Ag(xLiPr2)(OTf)] where x = p-toluenesul-
fonyl = Ts, 1a, or x = H, 2a, whereas the monoligated copper(I)
complexes retain
a
molecule of acetonitrile to give
[(xLiPr2)Cu(CH3CN)](PF6) where x = p-toluenesulfonyl = Ts, 3a, or
x = H, 4a. The difference in composition likely reflects the
greater metal binding affinity of the triflate vs. hexafluorophos-
Figure 2. Structures of mono-ligated complexes (a) Ag(TsLiPr2)(OTf)·acetone,
1a·acetone and (b) [(TsLiPr2)Cu(CH3CN)](PF6), 3a, with atom labelling.
Hydrogen atoms and minor disorder components of C14 and C16 in 1a·ace-
tone are omitted for clarity.
Scheme 3. Bulk preparation of metal complexes of the new scorpionate li-
gands.
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1a·acetone is longer than the comparable distance of 2.224(2) of Cu–Npz distances. That is, in the distorted sawhorse approxi-
Å in Ag(TsL*)(OTf). In 1a·acetone, the two diisopropyl pyrazolyls mation, the two pseudoequatorial Cu–Npz bonds are longer
Ag–N bonds [Ag1–N12 2.366(2), Ag1–N22 2.335(2) Å; average than 2.0 Å while the pseudoaxial bonds are shorter than 2.0 Å,
2.35(2) Å] are shorter than that associated with the “confused” where the average distance of the four Cu–Npz bonds is 2.045(2)
pyrazolyl [Ag1–N2 2.415(2) Å]. This asymmetric binding mode Å for 3b·CH2Cl2 and 2.04(4) for 4b·2THF. The cation coordina-
is similar to that in Ag(TsL*)(OTf) where Ag–Npz* averaged tion geometry in each closely resembles those found in
2.34(1) Å and Ag–N2 was 2.427(2) Å. The bond angles about [Cu(H2Cpz2)2](ClO4) (avg. Cu–Npz 2.065 Å, τδ = 0.57),[50] or
silver in the AgN3O coordination environment give a τδ parame- {Cu[H2C(3,5-iPr2pz)2]2}(X) (X = CuICl2, avg. Cu–Npz 2.080 Å, τδ
=
ter[40,41] of 0.63 which puts the coordination polyhedron at the 0.67; X = ClO4, avg. Cu–Npz 2.068 Å, τδ = 0.64).[51] Finally, it is
–
borderline between distorted tetrahedral (τδ ≈ 0.63–0.9) and noted that in 3b·CH2Cl2 the PF6 anion has CH···F weak
distorted saw horse (τδ ≈ 0.45–0.63) geometries, slightly more hydrogen bonding interactions[45–49] with the acidic methine
tetrahedral than found for Ag(TsL*)(OTf) (τδ = 0.61).
hydrogen (C44H44···F4 2.582 Å, 149.4° ), the unique isopropyl
hydrogen of groups attached to the 5-pyrazolyl positions adja-
cent to the methine (C50H50···F4 2.579 Å, 140.1°; C50H50···F6
2.565 Å, 120.0°; C10H10···F2 2.557 Å, 148.1° C20H20···F2
2.627 Å, 146.2°) and a 3-tolyl ring hydrogen (C35H35···F6
2.489 Å, 147.0° ). In 4b·2THF, the PF6 anion bridges neighboring
cations via bifurcated C2-symmetric N-H···F···H-N weak
hydrogen bonding interactions (N1H1n···F2 2.343 Å, 138.0°)
that gives chains of complexes parallel with the crystallographic
a-axis (Figure S1).
Table 1. Bond lengths [Å] and angles [deg] for Ag(TsLiPr2)(OTf)·acetone,
1a·acetone and [(TsLiPr2)Cu(CH3CN)](PF6), 3a.
1a·acetone
3a
Bond lengths [Å]
Ag1–N2
Ag1–N12
Ag1–N22
Ag1–O3
2.4150(15)
2.3664(18)
2.3345(15)
2.2745(13)
Cu1–N2
2.1800(13)
2.0447(13)
2.0920(13)
1.8837(14)
Cu1–N12
Cu1–N22
Cu1–N1s
Bond Angles [°]
O3–Ag1–N2
O3–Ag1–N12
O3–Ag1–N22
N2–Ag1–N12
N2–Ag1–N22
N12–Ag1–N22
128.20(5)
133.89(5)
135.92(5)
80.99(5)
76.73(5)
79.16(6)
N1s–Cu1–N2
N1s–Cu1–N12
N1s–Cu1–N22
N2–Cu1–N12
N2–Cu1–N22
N12–Cu1–N22
127.48(5)
132.61(5)
121.39(5)
84.27(5)
92.25(5)
84.54(5)
The complex, [(TsLiPr2)Cu(CH3CN)](PF6), 3a, possesses a κ3-li-
gand with a copper-bound acetonitrile molecule (Figure 2b) to
give a distorted tetrahedral CuN4 kernel (τδ = 0.68). The average
Cu–N(pyrazolyl), Cu–Npz, distance of 2.11 Å is shorter than
2.12 Å found in the heteroscorpionate complex {[HC(3,5-
(CF3)2pz)2(Py)]Cu(CH3CN)}(PF6),[18] similar to 2.11 Å found in
{[HC(pz3tBu)3]Cu(CH3CN)}(PF6),[42] but longer than 2.09
Å
found in either {[HC(3,5-iPr2pz)3]Cu(CH3CN)}(ClO4)[43] or
[(HC(pz3–mesityl)3)Cu(CH3CN)]2(Cu2I4).[44] The Cu–N(acetonitrile)
distance of 1.884(1) Å in 3a is within the 1.86–1.89 Å range
found
in
other
complexes
of
the
type
{[tris(pyrazolyl)methane]Cu(CH3CN)}+.[42–44] Finally, it is noted
that the PF6 anion is docked close to the acidic methine and
the confused pyrazolyl's H4-ring hydrogen by two short CH···F
weak hydrogen bonding[45–49] interactions (C2H2···F1 2.434 Å,
160.7° and C4H4···F3 2.480 Å, 151.9°, respectively).
The
structures
of
two
di-ligated
complexes
[Cu(TsLiP2r)2](PF6)·CH2Cl2, 3b·CH2Cl2, and [Cu(HLiP2r)2](PF6)·2THF,
4b·2THF, are given in Figure 3a and b, respectively. Selected
interatomic distances and angles are collected in Table 2.
Despite the differences in substituents bound to the confused
pyrazolyl N1 ring atom, the local structures of the cations in
3b·CH2Cl2, and 4b·2THF are quite similar. In each, the ligands
are bound to copper(I) in a κ2- fashion via the 3,5-diisopropyl-
pyrazolyl donors; the “confused” pyrazolyl moieties are not
bound to the metal. The CuN4 kernel is best described as a
Figure 3. Views of the cation in (a) [Cu(TsLiPr2)2](PF6)·CH2Cl2, 3b·CH2Cl2, and
(b) [Cu(HLiPr2)2](PF6)·2THF, 4b·2THF, with partial atom labeling. Anions, solvate
molecules and most hydrogens are omitted for clarity.
Solution Behavior
distorted sawhorse by virtue of the combination of a borderline When the analytically pure powders or crystals of the new com-
τδ value of 0.63 and C2-symmetry (exact for 4b·2THF and ap- pounds are dissolved in acetonitrile a mixture of species is
proximate C2 for 3b·CH2Cl2) that arises from two disparate sets formed due to multiple equilibria, a property that is evident
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Table 2. Selected bond lengths [Å] and angles [deg] for
the copper derivatives were much more complex than ex-
pected. The major differences in the NMR spectra between
analogous complexes of each metal is due, in part, to the
greater exchange rate[58] of silver(I) (always in the fast exchange
regime on the NMR time scale) vs. the copper(I) counterparts
(which traverse the fast to slow exchange regime in the sol-
vent's liquid range).
First, NMR titrations were performed by adding substoichio-
metric quantities of metal salt in CD3CN to CD3CN solutions of
the various ligands at room temperature as well as by perform-
ing reverse titrations (adding substoichiometric quantities of li-
gand into an initial solution of ligand-free metal salt). The data
for the latter are more or less identical to the former, so the
former will be discussed. Overlays of the NMR spectra from ti-
tration experiments are given in Figures 4, 5, and S2 to S6.
The number and chemical shifts of resonances indicate that
complexes with 1:1 and 1:2 M/L stoichiometries are formed and
that there is fast exchange between free ligands and com-
plexes. As exemplified for the titration between HLiPr2 and
AgOTf, in Figure 4, solutions with excess ligand (Figure 4b) or
AgOTf (Figure 4f and g) with respect to complexes 2a/b gave
[Cu(TsLiPr2)2](PF6)·CH2Cl2, 3b·CH2Cl2, and [Cu(HLiPr2)2](PF6)·2THF, 4b·2THF.
3b·CH2Cl2
4b·2THF
Bond lengths [Å]
Cu1–N12
Cu1–N22
Cu1–N52
Cu1–N62
1.9995(15)
2.0760(15)
2.1276(15)
1.9763(15)
Cu1–N12
Cu1–N22
2.088(3)
1.988(3)
Bond Angles [°]
N62–Cu1–N12
N62–Cu1–N22
N62–Cu1–N52
N12–Cu1–N22
N12–Cu1–N52
N22–Cu1–N52
137.88(6)
113.69(6)
94.34(6)
93.23(6)
111.46(6)
102.46(6)
N12–Cu1–N22
N12–Cu1–N12′
N12–Cu1–N22′
N22–Cu1–N22′
94.07(12)
102.1(2)
111.88(12)
138.7(2)
from ESI(+) mass spectrometric and NMR spectroscopic data.
First, electrospray mass spectrometry utilizes soft ionization
such that this technique is not only useful for sampling the
solution structure of inert complexes,[52,53] but can also be used
to probe the solution behavior of labile metal complexes or
even supramolecular species held together by non-covalent in-
teractions.[54–57] When either analytically pure 1a or 1b are dis-
solved CH3CN (with added formic acid), the mass spectrum con-
sist of peaks for [Ag(TsLiPr2)2]+, [Ag(TsLiPr2)]+, and [H(TsLiPr2)]+ as
well as a ligand fragmentation peak [TsLiPr2 – pziPr2]+ where the
relative abundance of each ion varied sample to sample. Similar
data were found for the HLiPr derivatives, 2a or 2b. When
CH3CN/formic acid solutions of either 3a, 3b, 4a, or 4b were
analyzed, the ESI(+) spectra showed peaks for [Cu(xLiPr2)2]+,
[Cu(xLiPr2)(CH3CN)n]+ (n = 0,1), [H(xLiPr2)]+, [xLiPr2 – pziPr2]+, and,
in the cases of 3b or 4a, [Cu(CH3CN)2]+. All of the above data,
but especially the presence of [M(xL2)]+ ions in solutions of ana-
lytically pure [M(xLiPr2)(CH3CN)n](OTf or PF6) (n = 0,1), indicate
that the complexes do not remain intact in solution.
Figure 5. Portion of the 295 K 1H NMR spectrum for a CD3CN solution of HLiPr2
(a) before and after aliquots of [Cu(CH3CN)4](PF6) are added such as to give
M/L ratios of: (b) 0.25, (c) 0.5, (d) 0.75, (e) 1.0, (f) 1.5, and (g) 2.0. The green
diamonds and green dashed lines represent the unique resonances for
[Cu(HLiPr2)2]+ while the red dashed lines between 8 to 6 ppm follow resonan-
ces for confused pyrazolyl hydrogens assigned to [(HLiPr2)Cu(CH3CN)]+. Reso-
nances between 2.2 to 1.8 ppm are from solvent.
1
The H and 13C NMR spectroscopic data are also indicative
of solution equilibria. The NMR spectra did not match expecta-
tions based on the respective solid-state structures (for 1a, 3a,
3b, and 4b) or molecular models (for 1b, 2a, 2b, or 4a); the
spectra for the silver complexes were simpler while those for
Figure 4. Left: Overlay of a portion of the 1H NMR spectra obtained by titration of a concentrated CD3CN solution of AgOTf into to a CD3CN solution of HLiPr2
.
Molar equivalents of AgOTf added to HLiPr2: (a) zero; (b) 0.3; (c) 0.5; (d) 0.8; (e) 1.0; (f) 1.5; (g) 2.0. The doublet resonances for the confused pyrazolyl ring
hydrogens and one multiplet resonance for a CHMe2 group are tracked with orange dashed lines as a visual guide. The “r” represents residual CD2HCN
resonance while the asterisk “*” represents residual H2O in CD3CN. Right: Overlay of iPr-CH3 region of the NMR spectrum of the free ligand (bottom) and after
incremental additions of 0.1 equivalents of AgOTf until a 1:1 L/Ag ratio (top).
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only one set of resonances that show ligand exchange is fast the dynamic equilibria, described by Equations 1–4. Qualita-
on the NMR timescale. Moreover, a rapid equilibrium between tively, the equilibrium constant for Equation 1 must be large
(> 103) and that for Equation 3 must be small (< 10–3) since the
titration of L with [Ag(CH3CN)4](OTf) (or the reverse titration)
was complete at the stoichiometric limits.
silver containing species exists (vide infra) since the chemical
shifts vary smoothly between limits found for 1:1 and 2:1 L/M
x
1
stoichiometric ratios. The H NMR spectrum of [Ag(HLiPr2)](OTf),
2a, (Figure 4e–g) shows that most resonances are shifted down-
field from those in the free ligand (Figure 4a), as expected. Ex-
ceptions occur for the methine Hα resonance (δH ≈ 7.5 Fig-
ure 4a) and those resonances for one of the two sets of iso-
propyl groups near (δH ≈ 3.0 ppm Figure 4a and δH ≈ 1.2 ppm
right of Figure 4). These exceptional resonances are shifted up-
field from those in the free ligand by |Δδ| = 0.10 (Hα), 0.03
(CHMe2), and 0.005 (iPr CH3) ppm, respectively. The significant
anomalous upfield shift of the former is thought to arise from
close ion-pair contact with the triflate ion oxygen atoms, since
similar behavior has been observed in solutions of other metal
complexes[59–66] and because the triflate ion is often found to
be in close contact with the methine hydrogen of the ligands
in the solid state (including that of 1a·acetone).[67] It is noted
that in solution the triflate ion in [Ag(xLiPr2)](OTf) is not bound
to the metal, rather it is displaced by CD3CN. The 19F NMR spec-
trum of each of the four silver compounds is identical and
shows only a single resonance at –79.3 ppm, a chemical shift
that is identical to that in the spectrum of either NBu4OTf, [(κ3N-
xL*)Mn(CO)3](OTf) (x = Ts, H),[27] or [Fe(κ3N-HL)2](OTf)2,[39,68] spe-
cies with “free” triflate ions. Ion-pairing of triflate with cations
has minimal effect on the chemical shift of the 19F resonance
because the ion pair contact occurs only with the oxygen at-
The NMR data for titration of xLiPr2 with [Cu(CH3CN)4](PF6) are
more complicated than those of the silver analogues because
the slower exchange rate associated with Cu(I) permits the ob-
servation of multiple species. An overlay of a portion of some
of the spectra from titration experiments involving HLiPr2 is
given in Figure 5 while full data are provided in Figure S5. Addi-
tion of substoichiometric portions of either [Cu(CH3CN)4](PF6)
to a CD3CN solution of HLiPr2 (Figure 5) or of HLiPr2 to a CD3CN
solution of [(HLiPr2)Cu(CH3CN)]+ (Figure S6) gives qualitatively
similar spectra that consist of two sets of resonances of unequal
intensity: the minor component corresponds to [Cu(HLiPr2)2]+
while the major component of the spectra is due to
[(HLiPr2)Cu(CH3CN)]+ in fast exchange with free HLiPr2. It is noted
that when analytically pure crystals of either 4a or 4b are dis-
solved in CD3CN, the 1H NMR spectra match those from titration
experiments showing two unequal sets of resonances. In the
former titration, both components are observed until an equi-
molar metal:ligand ratio is achieved. However, when the
metal:ligand ratio is greater than one, then only resonances for
[(HLiPr2)Cu(CH3CN)]+ (in exchange with free HLiPr2) are clearly visi-
ble in the 295 K spectrum. As can be elucidated by comparing
spectrum for the free ligand in Figure 5a with those in Figure 4e
through 4g, most resonances for the ligand hydrogens in
[(HLiPr2)Cu(CH3CN)]+ are shifted downfield compared to those of
the free ligand. Exceptions occur for the methine resonance at
δH = 7.48 and the septet at δH = 3.30 ppm for one of the CHMe2
hydrogens that are shifted upfield from those of the free ligand
by |Δδ| = 0.23 and 0.03 ppm, respectively. Similar to the silver
complexes described above, these upfield shifts may be related
to the proximity of the anion (hexafluorophosphate, in this
case) to hydrogens on the cation. Fast exchange between the
free and complexed ligand in [(HLiPr2)Cu(CH3CN)]+ is indicated
by two features of the NMR spectra. First, there is only one set
of major resonances (i.e., momentarily disregarding those minor
resonances for [Cu(HLiPr2)2]+, vide infra) when either less than 0.5
equivalents [Cu(CH3CN)4](PF6) are added to HLiPr2 (Figure 4b–
d) or when aliquots of free ligand are added to solutions of
[(HLiPr2)Cu(CH3CN)]+ (in the reverse titration, Figure S6). Second,
the chemical shifts of these major resonances are weighted av-
erages of those of [(HLiPr2)Cu(CH3CN)]+ and the free ligand. At
295 K the resonances for [Cu(HLiPr2)2]+ are broad and of weak
intensity, with the characteristic ones demarcated by green dia-
monds and green dashed lines in Figure 5. The broadness of
the minor resonances arises because of dynamic molecular mo-
tion that falls in the slow to intermediate exchange rate regime
at room temperature. Their weak intensity arises because of
1
oms. The H NMR spectrum of 1a is similar to that of 2a, but
the resonance for H4 of the confused ring (nearest to the me-
thine Hα) and the resonances for the tolyl ring hydrogens are
also shifted upfield with respect to the free ligand; the tolyl CH3
resonance is unchanged. Additionally, there is only one set of
resonances for diisopropylpyrazolyl hydrogens indicating that
the tosyl group must rapidly rotate to average the signals for
the heterocycle hydrogens. If the solid-state structure was
maintained with frozen C1 point group symmetry, then two sets
of resonances would be anticipated. The 1H NMR spectra of the
di-ligated complexes 1b and 2b are similar to the mono-ligated
cases, but as shown for 2b in Figure 4c and the middle spec-
trum in the right of Figure 4, the H4-pzcf resonance (|Δδ| =
0.19 ppm) and one set of isopropyl resonances exhibit quite
large upfield shifts with respect to the free ligand (CHMe2
|Δδ| = 0.45 ppm; iPr-CH3 |Δδ| = 0.19 ppm) while the upfield
shift for the methine Hα resonance is modest (|Δδ| = 0.08 ppm).
These features are qualitatively in agreement with those ob-
served for previous [Ag(xL*)2](OTf) complexes,[27] which have
κ2N- ligands with non-bonded confused pyrazolyl rings in the
solid state and possibly interconverting to κ3N- ligands in solu-
tion due to low energy barriers for such conversions.[69,70] Given
the greater steric profile of diisopropyl pyrazolyls vs. dimethyl-
pyrazolyls, it is likely that the current complexes 1b and 2b
have κ2N- rather than κ3N- ligands in solution. Unfortunately,
the exchange was still rapid even after lowering the tempera-
ture of the solution to 243 K, near the freezing point of the
solvent. Thus, it was not possible to quantitatively evaluate the
equilibrium constants or thermodynamic parameters for any of
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equilibria (vide infra) that favors [Cu(HLiPr2)2]+ over intra or intermolecular short contacts. There are four resonances
[(HLiPr2)Cu(CH3CN)]+ and HLiPr2 only at low temperature. The for isopropyl methyl hydrogens at δH = 1.2 (24 H), 0.9 (12 H),
243 K 1H NMR spectrum of a 0.03
M
solution of [Cu(HLiPr2)2](PF6) 0.6 (6 H), and 0.5 (6 H) ppm. The former resonance overlaps
in CD3CN is shown in Figure 6, while an overlay of 1H NMR with the broad multiplet resonance(s) for [(HLiPr2)Cu(CH3CN)]+
spectra acquired between 343 and 243 K are found in Figure and the free ligand while the latter three are well resolved;
S7. The major resonances for [Cu(HLiPr2)2]+ are consistent with the integration of the former (and percent composition of the
expectations based on the solid-state structure. That is, in the mixture) is determined by the integration of these latter three
C2-symmetric [Cu(HLiPr2)2]+, there are two sets of diisopropyl- resonances. Again the tentative assignment of the three upfield
pyrazolyl rings; psuedoequatorial (top rings, colored green in resonances is based on the metrics of intracationic C-H···π inter-
Figure 6) and pseudoaxial (bottom rings, colored blue in Fig- actions (or long contacts at the van der Waals, vdW, limit), with
ure 6) and four sets of isopropyl groups (types A–D in Figure 6). the shortest (with type A shorter than type C, each with C-
Inspection of the cation geometry in Figure 3b and right of CH3 bonds parallel with the two-fold rotation axis) having the
Figure 6 indicates that the 3-isopropyl groups nearest to the greatest upfield shift. Given the relatively long distances of in-
metal (types A and C) have close intramolecular contacts with tracationic vdW contacts for methyls of types C, A′, and C′, re-
pyrazolyl rings within the cation and are probably locked into spectively, an alternate assignment where A′ and C are switched
position whereas the 5-isopropyl substituents only exhibit inter- in the spectrum shown in the left Figure 6 is possible. Regard-
1
molecular contacts (see Table S3 for full details of noncovalent less, the low temperature H NMR spectrum is consistent with
interactions). Moreover, the eight 3-isopropylpyrazolyl methyls the solid-state structure of [Cu(HLiPr2)2]+. Upon warming from
are further subdivided into two sets, depending on whether 243 to 343 K, the resonances for [Cu(HLiPr2)2]+, broaden, de-
the C-CH3 bond is oriented either nearly parallel (A or C, Fig- crease in intensity and are no longer observed at about 323 K.
ure 6, right) with or perpendicular (A′ and C′, Figure 6 right) to At temperatures of 323 K or greater the 1H NMR spectrum con-
the C2 rotation axis. The former reside above the pi clouds of sists of one set of resonances for [(HLiPr2)Cu(CH3CN)]+ in ex-
the pyrazolyl rings whereas the latter do not. Thus, the 1H NMR change with free HLiPr2. Finally, as depicted in Figure S8 the
spectrum of [Cu(HLiPr2)2]+, has three resonances for hydrogens relative amount of [Cu(HLiPr2)2]+ vs. [(HLiPr2)Cu(CH3CN)]+ and free
of the confused pyrazolyl, one for the central methine HLiPr increases with initial concentration of [Cu(HLiPr2)2](PF6) in
hydrogen. The doublet for the H4-ring hydrogen at δH
=
CD3CN, which is a consequence of the equilibria described be-
1
5.42 ppm is significantly shifted upfield from that for the free low. In most respects, the H NMR spectra for the TsLiPr deriva-
–
ligand, presumably by contact with PF6 ion. The two resonan- tives, 3a and 3b, are similar to those of 4a and 4b. However,
ces near δH = 6 ppm are for the H4-ring hydrogen of the pseu- the N-tosyl group not only gives more resonances but also
doequatorial and pseudoaxial diisopropylpyrazolyl rings. There slows the rate of dynamic motion such that the resonances for
are three resonances for the unique isopropylmethine hydro- [Cu(TsLiPr2)2]+ are well-resolved at room temperature.
gens at δH = 3.3 (4 H, types B and D), 3.0 (2 H, type A) and 2.2
The NMR experiments indicate at least three equilibria deter-
(2 H, type C) ppm. The tentative assignment of the most upfield mine the speciation of the copper(I) complexes in acetonitrile
resonance to the hydrogen of the C-type isopropyl group is (Eq's 5–7). The first of the stepwise formation expressions (Equa-
based on the X-ray structure that shows that this hydrogen has tion 5) is thought to have a very large equilibrium constant
a shorter C-H···π interaction[71–74] than the A-type hydrogen (K5 > 105) while that for the ligand redistribution reaction (Equa-
(C20H20···CtN1, 2.80 Å, 139° vs. C10H10···CtN22, 2.90 Å, 162°) tion 7) is miniscule (K ≤ 10–2) since the appropriate titrations
while the other two hydrogens (types B and D) do not have (i.e., forward reaction of Equation 5 and reverse reaction of
Figure 6. Left: The 243 K 1H NMR spectrum of a 0.03
M
solution of [Cu(HLiPr2)2](PF6) in CD3CN along with labeling diagram. The resonances marked with an
“m” are for [(HLiPr2)Cu(CH3CN)]+, that with an “s” is for residual solvent, and those with an “i” are from impurities in the deuterated solvent. Right: Partly labeled
capped stick diagram of cation from X-ray structure with closest intracationic C-H···π interactions shown as dashed green, red, and orange lines and pyrazolyl
ring centroids shown as spheres. Most of the hydrogens are removed except those involved in the noncovalent interactions. The atom coloring emphasizes
spatial relationships of species in the line drawing to the left.
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Equation 7) are complete at stoichiometric limits. The latter is
corroborated by variable temperature NMR studies of
[(xLiPr2)Cu(CH3CN)](PF6) dissolved in CD3CN for which the equi-
librium constant associated with Equation 7, K7, at 293 K for the
tosyl derivative (x = Ts) was 1.2 × 10–4 with ΔH = –21 kJ/mol
and ΔS = –146 J/K mol. The corresponding values for the HLiPr2
derivative were K3 (293) = 3.2 × 10–3 with ΔH = –30 kJ/mol and
ΔS = –149 J/K mol. As expected for Equation 7, the mono-liga-
ted species is strongly entropically favored over the di-ligated
species. The enthalpy change difference between the two li-
gand systems indicates stronger Cu–Npz bonding for
[Cu(HLiPr2)2]+ vs. [Cu(TsLiPr2)2]+ which might be expected on the
basis of steric demands of the confused pyrazolyl N-substituent.
The greater capacity for HLiPr to participate in weak N-H···F
hydrogen bonding interactions with PF6 anions may also con-
tribute to the differences in enthalpy change. Finally, the titra-
tions, variable temperature, and variable concentration NMR
studies of pure [Cu(xLiPr2)2](PF6) dissolved in CD3CN suggest that
second stepwise formation expression (Equation 6) has a mod-
est equilibrium constant, K6, in the range of 102 to 103.
Table 3. Results of styrene aziridination reactions using PhI=NTs.[a]
Entry
Catalyst
NMR % yield ( %)
TON[b]
1
2
3
4
5
6
7
8
None
Ag(OTf)
< 1
–
2(1), 5(1)[d]
2(1)
1(1)
1(1)
5(1)
5(1)
4(1)
4(1)
5(1)
4(1)
4(1)
5(1)
11(1)
3(1)
25(2)
27(2)
26(1)
29(2)
11(1)
4(2)
4(1)
AgBF4
[Ag(TsL*)](OTf)
10(1)[c]
[Ag(TsL*)2](OTf)
9(1), 9(1)[c]
7(1)[c]
[Ag(HL*)](OTf)
[Ag(HL*)2](OTf)
8(1)[c]
[Ag(TsLiPr2)](OTf), 1a
[Ag(TsLiPr2)2](OTf), 1b
[Ag(HLiPr2)](OTf), 2a
[Ag(HLiPr2)2](OTf), 2b
[Cu(CH3CN)4](PF6)
Cu(OTf)2
9(1),[c] 12(1)[d]
8(1), 9(1)[c]
8(1)[c]
9
10
11
12
13
14
15
16
17
18
19
20
9(2)[c]
21(2), 31(2)[d]
5(1)
[Cu(TsL*)(CH3CN)](PF6)
[Cu(TsL*)2](PF6)
49(2)[c]
53(2)[c]
[Cu(TsLiPr2)(CH3CN)](PF6), 3a
[Cu(TsLiPr2)2](PF6), 3b
[Cu(TsLiPr2)2](OTf)2
[Cu(HLiPr2)(CH3CN)](PF6), 4a
[Cu(HLiPr2)2](PF6), 4b
52(1), 53(1),[c] 69(1)[d]
58(2)[c]
22(1)[c]
8(2)[c]
7(2)[c]
[a] Conditions: 0.5 mmol of styrene, 0.575 mmol of PhI=NTs, 0.01 mmol of
catalyst, 0.5 g of 4 Å molecular sieves, 5 mL of CH2Cl2, 24 h, 23 °C. [b] mmol
aziridine/mmol catalyst reported for pre-formed catalysts (rather than in-situ
formed catalysts unless data for the former not available). [c] Catalyst formed
in-situ. [d] 6 mol-% catalyst.
That is, K6 (293 K) for the TsLiPr2 derivative was found to be
3.7 × 102 with ΔH = –19 kJ/mol and ΔS = –16.0 J/K mol while
that for the HLiPr2 analogue is 2.5(2) × 103 at 293 K with ΔH =
–21 kJ/mol and ΔS = –5.5 J/K mol. The enthalpic differences
between complexes of the two different ligands likely arises
because the larger steric demand of the former ligand destabil-
izes [Cu(TsLiPr2)2]+ over [(TsLiPr2)Cu(CH3CN)]+ by weakening the
Cu–Npz bond to a greater extent than the HLiPr2 counterpart.
The difference in steric demands of the ligands is likely the
origin of the entropic difference where the loss of rotational/
vibrational degrees of freedom of confused and/or isopropyl
pyrazolyl groups upon on complexation to copper (compared
Table 4. Results of styrene aziridination using PhI(OAc)2/H2NTs in CH3CN.[a]
TON[b]
to the free ligand) is greater for TsLiPr2 than HLiPr2
.
Entry
Catalyst
NMR % yield ( %)
1
None
3(2)
2(1)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Ag(OTf)
3(2)
2(1)
8(3)
9(2)
6(2)
7(1)
8(1)
4(1)
31(1)
25(3)
33(2)
30(2)
34(1)
25(2)
36(2)
20(1)
38(2)
30(1)
[Ag(TsL*)](OTf)
16(2), 15(2)[c]
18(2), 16(2)[c]
12(3), 15(2)[c]
13(2), 9(2)[c]
15(2), 16(1)[c]
8(2), 10(2)[c]
61(2), 59(2)[d]
50(5)
Catalysis
[Ag(TsL*)2](OTf)
[Ag(TsLiPr2)](OTf), 1a
[Ag(TsLiPr2)2](OTf), 1b
[Ag(HLiPr2)](OTf), 2a
[Ag(HLiPr2)2](OTf), 2b
[Cu(CH3CN)4](PF6)
Cu(OTf)2
The ability of 1a, 1b, 2a, 2b, and related complexes to catalyze
the aziridination of styrene was explored by using two meth-
ods, as summarized in Table 3 and Table 4. Each entry of the
tables is an average of at least two independent runs where
the uncertainty is given in parentheses. First, in Method A, [(p-
tolylsulfonyl)imino]phenyliodinane, PhI=NTs, was used as a ni-
trene source in CH2Cl2 at room temperature under heterogene-
ous conditions similar to that reported[18] for [Cu(2-py)BpmR3R5]-
catalyzed reactions (direct comparisons of 6 mol-% catalyst
loadings are found in Table 3, entries 2, 8, 12, 16; otherwise
2 mol-% loadings were used). The second method (Method B)
employed the nitrene formed in-situ from H2NTs and PhI(OAc)2
in CH3CN at 80 °C, under conditions outlined by our previous
study.[27] All of the complexes are capable of catalyzing the
aziridination of styrene. The aziridination reactions do not occur
to any significant extent in the absence of metal ion or com-
[Cu(TsL*)](PF6)
65(4)[c]
[Cu(TsL*)2](PF6)
59(3)[c]
[Cu(TsLiPr2)(CH3CN)](PF6), 3a
[Cu(TsLiPr2)](OTf)2
[Cu(TsLiPr2)2](PF6), 3b
[Cu(TsLiPr2)2](OTf)2
[Cu(HLiPr2)(CH3CN)](PF6), 4a
[Cu(HLiPr2)2](PF6), 4b
67(2), 50(3)[c]
49(3)[c]
71(3), 50(2)[c]
40(2)[c]
61(3), 57(3)[c]
64(2), 59(2)[c]
[a] Conditions: 5 mmol of styrene, 1 mmol of PhI(OAc)2, 1 mmol of H2NTs,
0.02 mmol of [Ag], 1 g of 4 Å molecular sieves, 4 mL of CH3CN, 16 h, 80 °C.
[b] mmol aziridine/mmol catalyst, reported for pre-formed (rather than in-
situ formed) catalysts unless data for the former not available. [c] Catalyst
formed in-situ. [d] 1 mmol HOAc added.
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plex. For Method A using PhI=NTs, the ligand free salts AgOTf in the catalytic performance of the monoligated vs. diligated
or AgBF4 were ineffective catalysts only giving a TON of 1 (en- copper(I) complexes. So, ligand dissociation equilibria is likely
tries 2 and 3, Table 3) at 2 % loading and AgOTf had TON = 2 responsible for the catalytic activity of [Cu(xLR)2]+ and the rather
at 6 % loading. The silver C-scorpionate complexes outper- modest performance increase of [Cu(xLR)(CH3CN)]+ vs. ligand-
formed the corresponding ligand-free silver salts with consist- free salt. It is also noted that for the copper catalyzed reactions
ent but rather modest turnover numbers (TON) near 5 (entries performed using Method B but in the absence of molecular
4–11, Table 3) regardless of the number of ligands or type of sieves, the yields of aziridine product decrease because a sacrifi-
pyrazolyl substituents. With a couple of exceptions described cial [3+2] cycloaddition reaction occurs between styrene and
later, the copper complexes generally outperformed their silver 2-phenyl-N-tosylaziridine to produce variable amounts of 2,4-
counterparts. As found in entry 12 of Table 3, the cuprous start- diphenyl-N-tosylpyrrolidine (as a diastereomeric mixture)[75]
ing material [Cu(CH3CN)4]PF6 outperformed all of the silver where the yields of aziridine and pyrrolidine are generally
complexes with a TON of 11 at 2 mol-% loading. It is noted that 40 7 % and 30 4 %, respectively. This pyrrolidine by-product
when the loading was 6 mol-%, the TON only increased to 16; was previously observed, albeit in very minor amounts (1–2 %),
we were unable to achieve the reported TON of 26 despite in [Ag(xLR)n]+ catalyzed reactions.[27] It is unclear how molecular
numerous attempts. Also, given the substantial oxidizing power sieves suppress the cycloaddition reaction. Moreover, this side
of PhI=NTs, the cupric salt Cu(OTf)2 was investigated as a poten- reaction is not found using Method A (with or without molec-
tial catalyst, but this species was less effective (TON = 3) than ular sieves). Finally, although relatively modest, the catalytic ac-
the cuprous starting material. In contrast to the silver com- tivity of the silver complexes increases between two- to ten-
plexes above, the catalytic performance of the copper C-scorpi- fold over the ligand-free silver salt, AgOTf. Within experimental
onate complexes were dependent on ligand substitution pat- error, all of the silver catalysis performed equally regardless of
terns. Copper(I) complexes of the N-tosyl ligands (TsL*, TsLiPr2
,
number of C-scorpionate ligands or their substituents. It is note-
Table 3, entries 14–17) were the best catalysts of those tested worthy that the newly acquired results for [Ag(TsL*)2](OTf)
with TONs between 25–29 for 2 % loading. When the catalyst (Table 4, entry 4), differ from the erroneous results reported
loading was 6 mol-% for 3a (entry 17) the TON increased to previously by our group [NMR yield 34(4), 27(3) (in-situ formed),
35 which is comparable to those reported[18] for the copper TON 15]. By using several NMR standards [1,3,5-(MeO)3C6H3,
heteroscorpionates [Cu(L3 or L4)(CH3CN)]PF6 (see Scheme 1) CH3NO2 and SiMe4 spiked CDCl3 solutions] and careful observa-
with TONs 31–32 under similar conditions. It is noted that, for tion, it was found that the original standard p-(Me3Si)2C6H4 will
TsLR complexes the nominally diligated species had the same partially sublime (artificially raising yields) from the crude mix-
(Table 3 entries 14 vs. 15) or only slightly better (entries 16 ture if a sample is subjected to an oil pump vacuum (10–4 Torr)
vs. 17) activities than the monoligated counterparts, which is while being heated by an external (70 °C) oil bath over the
reflective of ligand dissociation equilibria. On the other hand, course of an hour (or more), in an effort to assist in solvent
the copper(I) complexes 4a and 4b with an N-H substituted removal. Sublimation does not occur when solvent is simply
ligand were equally ineffective with TON of 4 (entries 19 and removed at room temperature. Fortunately, the error only af-
20). Finallly, it was found that the combination of Cu(OTf)2 with fected the anomalous result for [Ag(TsL*)2](OTf); the results of
two equivalents of TsLiPr2 more than tripled the aziridination other complexes were reproducible (and low). Moreover, the
activity (giving a TON of 11) compared to the ligand-free cupric new procedure described in the current experimental of using
salt but was much less active than that involving its copper(I) aliquots from stock solutions, minimizes errors in weighing
counterpart 3b.
small masses of catalysts. Finally it is noted that the Method B
aziridination reactions were quite temperature sensitive. When
reactions were performed at room temperature, instead of
80 °C, the yields of N-tosylaziridine obtained using copper cata-
lysts dropped to approximately 25 % of the values reported in
Table 3 while those using the silver complexes as potential cata-
lysts failed provide any product.
In an effort to enhance product yield and to provide a com-
parison to an earlier study, the aziridination of styrene using
method B (in-situ formed nitrene, 80 °C in CH3CN) was pursued.
The copper(I) complexes outperformed the corresponding sil-
ver(I) complexes. As might be expected from the difference in
reaction temperatures, the yield of aziridine obtained from
Method B was greater than Method A, but the improvement
The reactivity screening of the current complexes suggests
was modest being most significant for [Cu(CH3CN)4]PF6, the cu- that in addition to ligand dissociation, electronic factors are im-
pric catalysts, or 4a or 4b, (compare Table 3 entires 12, 13, 18– portant in determining the efficacy of nitrene transfer reactions
20 with Table 4 entries 9. 10, 16–18). The reaction catalyzed by mediated by group 11 C-scorpionate complexes. For instance,
[Cu(CH3CN)4]PF6 was found to be insensitive to either added the catalytic activity trends with the electron richness of the
acetic acid or a second equivalent of tosylamine. The drop-off copper complexes supported by normal vs. C-scorpionates[43]
in performance between cuprous and cupric salts (ca. TON 30 {e.g., E1/2 (Cu2+/Cu+) = 0.07 V, 0.43 V, 0.63 and 1.0 V vs. Ag/AgCl
to 25, Table 3, entrees 9 and 10) was not as substantial as in in CH3CN for CuTpiPr2(CH3CN), [Cu(TpmiPr2)(CH3CN)]+,
Method A which might be related to the ability of CH3CN (vs. [Cu(TsLiPr2)(CH3CN)]+, and [Cu(CH3CN)4]+,[76] respectively}. This
CH2Cl2) to potentially induce disproportionation and stabilize trend appears to be further extended if one were to include
copper(I) over copper(II). Interestingly, the copper complexes the silver complexes, whose oxidation potentials are expected
only showed a marginal increase in catalytic activity vs. ligand- to be much higher [E1/2 (Ag2+/Ag+) ≥ 1.6 V vs. Ag/AgCl[77–79]
free copper salts. Moreover, there was little or no differentiation than the copper derivatives. In the silver cases, the highly oxid-
]
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ative nature of any hypervalent iodine reagent {E1/2 [PhI(OAc)2/ coarsely tuned by the charge of the scorpionate and more
PhI] 2.2 V vs. Ag/AgCl[79]}, putative silver nitrene, or other uni- finely tuned by pyrazolyl substitutions. A detailed experimental
dentified Ag(II) intermediate may lead to oxidative decomposi- and computational study of the relationship between electronic
tion of styrene [E1/2 (styrene+/styrene) 2.0 V vs. Ag/AgCl,[80]], or properties of group 11 scorpionates and aziridination activity is
possibly of ligand, at a rate that becomes competitive with ni- underway and will be reported on in due course.
trene transfer, thereby reducing yields. Thus, as evident from
relative M(xLR)2+/M(xLR)+ redox potentials (Table S4), the superi-
ority of the copper(I) complexes over the silver relatives to ef-
fect aziridination of styrene might be traced to the enhanced
Experimental Section
General Considerations: The compound TspzC(O)H (Ts = p-
stability of the metal nitrene intermediate brought about by
copper's superior ability to back donate electrons into the un-
or partly-filled orbitals on the unsaturated nitrogenous frag-
ment (giving a bond with some metal-iminyl character). Unfor-
tunately, it has not yet proven possible to identify any group
SO2C6H4CH3) was prepared by the literature method.[39] PhI(OAc)2,
3,5-diisopropylpyrazole or H(pziPr2), H2NTs and styrene were pur-
chased from commercial sources and used as received. Commercial
anhydrous CoCl2, AgOTf (OTf = trifluoromethanesulfonate), and
[Cu(CH3CN)4]PF6 were stored under argon in a drybox. Commercial
solvents ethyl acetate (EtOAc), dichloromethane (DCM), methanol
(MeOH) were used as received while diethyl ether (Et2O), aceto-
nitrile, toluene, and tetrahydrofuran (THF) were dried by conven-
tional means and distilled under a nitrogen atmosphere prior to
use. The silver(I) complexes were prepared under argon using
Schlenk-line techniques, however, after isolation, were stored and
manipulated under normal laboratory atmospheric conditions, un-
less otherwise specified.
x
11 nitrene intermediates supported by these LR ligands using
PhI=NTs. However, it can speculated on the basis of VT NMR
data of the starting metal complexes, the elevated tempera-
tures required for aziridination in CH3CN, and the structural
similarity
with
{[(2-py)BpmR3R5]Cu}+,
that
[(κ(3–n)N-
xLR)(CH3CN)nM(NTs)]+ (n = 0–2), are candidates for the catalyti-
cally active species. For the diligated complexes, it is not yet
possible to exclude either [(κ2-xLR)(κ1-xLR)M(NTs)]+ or [(κ1-
xLR)2M(NTs)]+, as the catalytically active species, although this
seems less likely based on dissociation equilibria.
Instrumentation: Melting point determinations were made on
samples contained in glass capillaries using an Electrothermal 9100
apparatus and are uncorrected. 1H (400 MHz), 13C (101 MHz), 19F
(376 MHz), 31P (162 MHz) NMR spectra were recorded on a Varian
400 MHz spectrometer. Chemical shifts were referenced to partly
deuterated solvent resonances at δH = 7.26 and δC = 77.23 for CDCl3
or δH = 1.94 and δC = 118.26 for CD3CN. Abbreviations for NMR:
br (broad), sh (shoulder), m (multiplet), ps (pseudo-), s (singlet), d
(doublet), t (triplet), q (quartet), p (pentet), sept (septet), “confused”
pyrazolyl = pzcf, diisopropylpyrazolyl = pziPr. Electrochemical meas-
urements were collected under a nitrogen atmosphere for samples
Conclusion
Two new semi-bulky nitrogen-confused C-scorpionate ligands,
TsLiPr2 and HLiPr2, and their 1:1 and 1:2 M/L complexes of silver(I)
and copper(I) have been prepared. In the solid state, the new
C-scorpionate ligands bind silver(I) or copper(I) in similar fash-
ion. The monoligated complexes 1a and 3a have κ3N- ligands
similar to that found in [Ag(TsL*)](OTf). The ligands in each
[Cu(xLiPr2)2](PF6) and [Ag(xL*)2](OTf) (x = Ts, H) coordinate the
respective metals in similar κ2N- modes by using the “normal”
pyrazolyls, leaving the “confused” pyrazolyl unbound. In CH3CN
solution, all eight of the new metal complexes are involved in
ligand dissociation and ligand redistribution equilibria that fa-
vor [M(xLiPr2)(CH3CN)n]+ at room temperature and above. These
multiple equilibria distinguish the group 11 complexes of C-
scorpionates from the complexes with boron-centered (normal)
scorpionates that generally remain as monomeric single species
in solution. Each of the new complexes was capable of catalyz-
ing the aziridination of styrene at room temperature in CH2Cl2
using PhI=NTs as a nitrene transfer agent or in CH3CN at 80 °C
using in-situ formed PhI=NTs with the copper complexes gener-
as 0.1 mM solutions in CH3CN with 0.1 M NBu4PF6 as the supporting
electrolyte. A three-electrode cell comprised of an Ag/AgCl elec-
trode (separated from the reaction medium with a semipermeable
polymer membrane filter), a platinum working electrode, and a
glassy carbon counter electrode was used for the voltammetric
measurements. With this set up, the ferrocene/ferrocenium couple
had an E1/2 value of +0.44 V in CH3CN at a scan rate of 200 mV/s,
consistent with the literature values.[81] ESI(+) mass spectrometric
measurements were obtained on a Micromass Q-TOF spectrometer
where formic acid (ca. 0.1 % v:v) was added to the mobile phase
(CH3CN).
Ligand Syntheses
TsLiPr2: Method A. An argon-purged solution of 3,5-diisopropyl-
pyrazole (4.57 g, 30.0 mmol) in 20 mL of THF was transferred slowly
via cannula over 10 minutes to a suspension of NaH (0.75 g,
31.0 mmol) in 20 mL of THF under argon atmosphere. To ensure
ally being superior to their silver cousins. However, under the quantitative transfer, the flask originally containing 3,5-diisopropyl-
pyrazole was rinsed with THF (2 × 5 mL) and the washings were
transferred to the reaction mixture. After 10 minutes of stirring,
thionyl chloride (1.1 mL, 15.0 mmol) was added by syringe slowly
over 5 minutes; a colorless precipitate formed during the addition.
The suspension was stirred at room temperature for 10 minutes,
then CoCl2 (0.065 g, 0.5 mmol) and TsPzC(O)H (2.5 g, 10.0 mmol)
were added sequentially as solids under an argon blanket. The blue
suspension was heated at reflux under argon for 12 h, and then
was cooled to room temperature. Solvent was removed by vacuum
distillation and the solid residue was dissolved in 200 mL of a 1:1
latter conditions, the extensive ligand dissociation in CH3CN at
80 °C renders the ligand-free cuprous starting material,
[Cu(CH3CN)4]PF6, the superior catalyst. Under the former condi-
tions, where dissociation is less extensive, the activity of the
cationic [Cu(xLiPr2)(CH3CN)n]+ complexes appears, at least quali-
tatively, to be lower than that reported for charge neutral CuTpx
counterparts since yields of aziridine (obtained at room temper-
ature) are lower than those reported for the latter. That is, it
appears the reactivity of the group 11 scorpionate complexes
increases inversely with their M2+/M+ redox couple, which is biphasic mixture of H2O:ethyl acetate. The organic layer was sepa-
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rated and the aqueous layer was extracted with dichloromethane
(d, J = 2.2 Hz, 1 H, H5pzcf), 6.11 (d, J = 2.2 Hz, 1 H, H4pzcf), 6.00 (s,
(2 × 50 mL). The organic fractions were combined, dried with 2 H, H4pziPr2), 3.33 (sept, J = 6.9 Hz, 2 H, CHMe2), 2.85 (sept, J =
MgSO4, and filtered. Solvents were removed by rotary evaporation 6.9 Hz, 2 H, CHMe2), 1.19 (d, J = 6.9 Hz, 12 H, iPr-CH3), 1.07 (d, J =
to leave 6.02 g of crude product. The crude product was dissolved
in 50 mL of boiling MeOH and the solution was stored at –10 °C
for 1 h. The colorless crystalline product (4.5 g) was collected by
vacuum filtration and was dried at room temperature under oil-
pump vacuum. The mother liquor was concentrated to 10 mL and
was stored at –10 °C for overnight to give another 0.3 g of pure
product. The yield is 4.8 g (91 %).
6.8 Hz, 6 H, iPr-CH3), 1.00 (d, J = 6.8 Hz, 6 H, iPr-CH3). 13C NMR
(CDCl3): δC = 158.72 (C5pzcf), 152.37, 142.88, 135.36, 105.88 (C4pzcf),
100.45 (C4pziPr), 69.61 (Cmeth), 28.06 (CHMe2), 25.34 (CHMe2), 23.86
(iPr-CH3), 23.05 (iPr-CH3), 23.02 (iPr-CH3), 22.85 (iPr-CH3).
Metal Complex Syntheses
General Procedure. A: Under argon, the desired ligand (1 or two
equivalents) and either AgOTf or [Cu(CH3CN)4]PF6 were dissolved in
10 mL of CH3CN and stirred 3 h at room temperature. Solvent was
removed under vacuum and the residue was washed with 2 mL of
Et2O (or if too soluble, like 1b or 3b, 2 mL of hexane), filtered, and
the precipitate was dried under vacuum at 60 °C 2 h.
Method B. Under an argon atmosphere, a solution of 0.975 g
(3.28 mmol) of triphosgene in 20 mL of THF was added dropwise
to a solution of 3.00 g (19.7 mmol) of 3,5-diisopropylpyrazole and
2.75 mL (mmol) NEt3 in 80 mL of THF. After stirring 16 h at 22 °C,
the insoluble HNEt3Cl was removed by filtration and was washed
with THF (2 × 10 mL). Solvent was removed from the combined THF
fractions by vacuum distillation to leave a 95:5 mixture of (PziPr2)2C=
General Procedure. B: A solution of a given ligand (1 or 2 equiv.)
in 10 mL of THF was added to a solution of AgOTf in 10 mL of THF
by cannula transfer. The flask originally containing the ligand was
washed twice with 2 mL of THF, and the washings were transferred
to the reaction medium to ensure quantitative transfer of the rea-
gent. After the mixture has been stirred for 2 h, the solvent was
removed by vacuum distillation. The colorless residue was washed
with two 2 mL portions of Et2O and was dried under vacuum for
an hour. The quantities of the reagents used and of the products
obtained and characterization data for each of the four new com-
pounds are given below. An alternative workup in the case where
a precipitate was observed is also described.
1
O:H(pziPr2) as a colorless oil that was used directly. H NMR (CDCl3):
(Pzipr)2C=O: 6.12 (s, 2 H, H4pziPr), 3.34 (sept, J = 6.8 Hz, 2 H, CHMe2),
2.97 (sept, J = 6.9 Hz, 2 H, CHMe2), 1.27 (d, J = 6.8 Hz, 12 H, iPr-
CH3), 1.24 (d, J = 6.9 Hz, 12 H, iPr-CH3), H(pziPr2): 5.95 (s, 1 H, H4pziPr),
3.03 (sept, J = 6.8 Hz, 2 H, CHMe2), 1.34 (d, J = 6.8 Hz, 12 H, iPr-
CH3). Next, 2.15 g (9.85 mmol) TsPzC(O)H, 0.0640 g (0.490 mmol) of
CoCl2 and 50 mL of toluene were added and the mixture was
heated at reflux under argon for 16 h. Then, the resulting blue
mixture was cooled to room temperature, and the solvent was re-
moved by vacuum distillation. The residue was partitioned between
100 mL of H2O and 100 mL of ethyl acetate. The layers were sepa-
rated, and the aqueous layer was extracted with two 50 mL portions
of CH2Cl2. The organic fractions were combined, dried with MgSO4,
and filtered. Solvents were removed by vacuum distillation to leave
4.04 g (76 %) of white solid. Recrystallization by cooling a boiling
MeOH solution (50 mL) to –20 °C for 1 h and filtering gave 3.45 g
(65 % yield) of pure TsLiPr2 as colorless crystals after filtration and
[Ag(TsLiPr2)](OTf), 1a: Using General Procedure A, a mixture of
0.300 g (0.559 mmol) of TsLiPr2 and 0.140 g (0.559 mmol) of AgOTf
gave 0.417 g (94 %) of 1a as a colorless solid. Using General proce-
dure B, a mixture of 0.502 g (0.935 mmol) of TsLiPr2 and 0.240 g
(0.935 mmol) of AgOTf gave 0.609 g (82 %) of 1a as a colorless
solid. Mp: 180–182 °CC. Anal. Calcd (found) for C30H40AgF3N6O5S2:
C, 45.40 (45.43), H, 5.08 (4.95), N, 10.59 (10.70). 1H NMR (CD3CN):
δH = 8.20 (d, J = 2.7 Hz, 1 H, H5pzcf), 7.75 (d, J = 8.4 Hz, 2 H, TsAr),
7.51 (s, 1 H, Hmethine), 7.39 (d, J = 8.4 Hz, 2 H, TsAr), 6.19 (s, 2 H,
H4pziPr), 6.02 (br s, 1 H, H4pzcf), 3.18 (sept, J = 6.7 Hz, 2 H, CHMe2),
2.73 (br m, 2 H, CHMe2), 2.40 (s, 3 H, Ts-CH3), 1.20 (d, J = 6.8 Hz, 6
H, iPr-CH3), 1.17 (d, J = 6.7 Hz, 6 H, iPr-CH3), 1.12 (br s, 12 H, iPr-
CH3). 13C NMR (CD3CN): δC = 162.46, 154.91, 154.69, 147.93, 134.32,
133.96, 131.22, 128.89, 122.16 (q, J = 321 Hz, CF3), 110.02, 100.76,
64.61, 29.36, 26.47, 23.54, 22.96, 22.27, 21.71. 19F NMR (CD3CN):
–79.33 ppm. ESI(+) MS, m/z (rel. abund. %) [assignment]: ESI(+) MS,
m/z (rel. abund. %) [assignment]: 1182 (38) [Ag(L)2]+, 645 (100)
[Ag(L)]+, 537 (78) [H(L)]+, 385 (10) [L – pziPr2]+. Crystal of 1a·acetone
suitable for single-crystal X-ray diffraction were obtained by layer-
ing hexanes over a solution of 50 mg of 1a in 2 mL of acetone and
allowing solvents to slowly diffuse over 16 h.
1
drying under vacuum. Mp: 140–143 °CC. H NMR (CDCl3): δH = 8.03
(d, J = 2.7 Hz, 1 H, H5pzcf), 7.81 (d, J = 8.3 Hz, 2 H, TsAr), 7.64 (s, 1
H, CHmethine), 7.26 (d, J = 8.3 Hz, 2 H, TsAr), 6.35 (d, J = 2.7 Hz, 1 H,
H4pzcf), 5.85 (s, 2 H, H4pziPr), 3.19 (sept, J = 6.9 Hz, 2 H, CHMe2),
2.85 (sept, J = 6.9 Hz, 2 H, CHMe2), 2.40 (s, 3 H, Ts-CH3), 1.17 (d, J =
7.0 Hz, 12 H, iPr-CH3), 0.95 (d, J = 6.8, 6 H, iPr-CH3), 0.91 (d, J = 6.8,
6 H, iPr-CH3). 1H NMR (CD3CN): δH = 8.19 (d, J = 2.7 Hz, 1 H, H5pzcf),
7.81 (d, J = 8.4 Hz, 2 H, TsAr), 7.62 (s, 1 H, Hmeth), 7.49 (d, J = 8.4 Hz,
2 H, TsAr), 6.33 (d, J = 2.7 Hz, 1 H, H4pzcf), 5.98 (s, 2 H, H4pziPr), 3.14
(sept, J = 6.8 Hz, 2 H, CHMe2), 2.80 (sept, J = 6.9 Hz, 2 H, CHMe2),
2.40 (s, 3 H, Ts-CH3), 1.15 (d, J = 6.9 Hz, 12 H, iPr-CH3), 0.98 (d, J =
6.8, 6 H, iPr-CH3), 0.87 (d, J = 6.8, 6 H, iPr-CH3). 13C NMR (CDCl3):
δC = 158.47 (C5pziPr), 145.99, 134.34, 131.89, 130.00, 128.51, 109.82
(C4pziPr), 100.20 (C4pzcf), 69.99 (Cmeth), 28.00, 25.52, 23.73, 23.02,
22.98, 22.83, 21.88 (Ts-CH3).
HLiPr2: A solution of 20.0 mL of 5.00
M
NaOH (aq) (100.0 mmol),
[Ag(TsLiPr2)2](OTf), 1b: Using General Procedure A, a mixture of
3.50 g (6.52 mmol) TsLiPr2, and 20 mL of THF was heated at reflux 0.500 g (0.935 mmol) of TsLiPr2 and 0.120 g (0.468 mmol) of AgOTf
1
for 20 min until completion (as monitored by H NMR and/or TLC).
After the mixture had cooled to room temperature, the THF layer
was separated and the aqueous layer was extracted with dichloro-
methane (2 × 20 mL). The combined organic fractions were dried
gave 0.620 g (90 %) of 1b as a colorless solid. By adopting a modifi-
cation of General Procedure B, a mixture of 0.506 g (0.943 mmol)
of TsLiPr2 and 0.121 g (0.472 mmol) of AgOTf gave a colorless precipi-
tate immediately. Filtration and drying the precipitate under vac-
with MgSO4 and filtered. The organic solvents were removed by uum (no washing) gave 0.530 g (84 %) 1b as a colorless solid. On
vacuum distillation to leave 2.42 g (97 %) of pure HLiPr2 as a white
the other hand, if the general procedure is strictly followed (in a
separate experiment of the same scale), where the colorless precipi-
tate was collected by cannula filtration after 2 h, and the insoluble
portion is washed with two 2 mL portions of Et2O and dried under
vacuum for an hour, then 0.353 g (56 %) of 1b is obtained a color-
less solid. An additional 0.158 g (25 %) of 1b (81 % total yield) can
1
solid. Mp: 137–138 °CC. H NMR (CDCl3): δH = 7.83 (s, 1 H), 7.50 (d,
J = 2.0 Hz, 1 H, H4pzcf), 6.22 (d, J = 2.0 Hz, 1 H, H4pzcf), 5.89 (s, 2 H,
H4pziPr), 3.34 (sept, J = 6.8 Hz, 2 H, CHMe2), 2.92 (sept, J = 6.9 Hz,
2 H, CHMe2), 1.23 (d, J = 6.9 Hz, 12 H, iPr-CH3), 1.02 (d, J = 6.8 Hz,
6 H, iPr-CH3), 0.93 (d, J = 6.8 Hz, 6 H, iPr-CH3); N-H not observed. 1H
NMR (CD3CN): δH = 11.12 (br, s, 1 H, NH), 7.72 (s, 1 H, Hmeth), 7.56 be recovered from the THF soluble portion by removing solvent,
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European Journal of Inorganic Chemistry
washing the residue with 1 mL of Et2O, and drying under vacuum.
TsAr), 6.21 (s, 2 H, H4pziPr), 5.91 (br m, 1 H, H4pzcf), 3.18 (sept, J =
6.8 Hz, 2 H, CHMe2), 3.05 (br m, 2 H, CHMe2), 2.41 (s, 3 H, Ts-CH3),
1.23 (d, J = 6.8 Hz, 12 H, iPr-CH3), 1.20 (d, J = 6.8 Hz, 12 H, iPr-
CH3). 13C NMR (CD3CN): δC = 162.19, 154.80, 154.24, 148.00, 134.24,
133.71, 131.19, 128.94, 109.53 (C4pziPr), 100.53 (C4-pzcf), 64.22
(Cmethine), 28.96, 26.36, 23.63, 23.27, 22.85, 22.40, 21.70. 31P NMR
Mp: 128–130 °CC. Anal. Calcd (found) for C59H80AgF3N12O7S3: C,
1
53.27 (53.49), H, 6.06 (6.20), N 12.63 (12.39). H NMR (CD3CN): δH
=
8.18 (d, J = 2.8 Hz, 1 H, H5pzcf), 7.75 (d, J = 8.3 Hz, 2 H, TsAr), 7.53
(s, 1 H, CHmeth), 7.38 (d, J = 8.3 Hz, 2 H, TsAr), 6.11 (s, 2 H, H4pziPr),
6.01 (br s, 1 H, H4pzcf), 3.18 (sept, J = 6.7 Hz, 2 H, CHMe2), 2.45 (br
s, 2 H, CHMe2), 2.40 (s, 3 H, Ts-CH3), 1.12 (d, J = 6.7 Hz, 12 H, iPr-
CH3), 0.95 (br s, 12 H, iPr-CH3). 13C NMR (CD3CN): 162.26 (br s),
155.32, 154.95 (br s), 147.86, 134.43, 133.62, 131.21, 128.91, 122.17
(q, J = 321 Hz, CF3), 109.88 (C4pzcf), 100.78 (C4pziPr), 65.76 (Cmeth),
29.10 (br s), 26.47, 23.74, 23.36, 23.28, 21.89 (br s), 21.69. 19F NMR
(CD3CN): –79.34 ppm. ESI(+) MS, m/z (rel. abund. %) [assignment]:
1182 (33) [Ag(L)2]+, 645 (11) [Ag(L)]+, 537 (100) [H(L)]+, 385 (37)
[L – pziPr2]+.
(CD3CN): δp -144.64 (sept, JPF = 706 Hz). 19F NMR (CD3CN) δF
=
-72.93 (d, JFP = 706 Hz). ESI(+) MS, m/z (rel. abund. %) [assignment]:
1136 (15) [Cu(L)2]+, 640 (80) [Cu(L)(CH3CN)]+, 599 (100) [Cu(L)]+, 537
(97) [H(L)]+, 385 (73) [L pziPr2]+.
Crystals suitable for X-ray diffraction were grown by layering 4 mL
of hexane onto a solution of 50 mg of 3a in 1 mL of CH2Cl2 and
allowing solvents to diffuse 14 h.
[Cu(TsLiPr2)2](PF6), 3b: Using General Procedure A, a mixture of
0.100 g of (0.186 mmol) of TsLiPr2 and 0.0347 g (0.093 mmol) of
[Cu(CH3CN)4]PF6 gave 0.116 g (86 %) of 3b as a colorless powder
after drying under vacuum for an hour. Anal. Calcd (found) for
C58H80F6N12O4PS2Cu: C, 54.34 (54.46), H, 6.29 (6.27), N 13.11 (13.44).
Mp: 90–92 °CC. The NMR spectra have signals for an equilibrium
[Ag(HLiPr2)](OTf), 2a: Using General Procedure A, a mixture of
0.427 g (1.12 mmol) of HLiPr2 and 0.287 g (1.12 mmol) of AgOTf
gave 0.647 g (94 %) of 2a as a colorless solid. By using General
Procedure B, a mixture of 0.510 g (1.33 mmol) of HLiPr2 and 0.343 g
(1.33 mmol) of AgOTf gave 0.729 g (86 %) of 2a as a colorless solid.
Mp: 113–115 °CC. Anal. Calcd (found) for C23H34N6SO3F3Ag: C, 43.30
(42.99), H, 5.35 (5.36), N 13.14 (12.87). 1H NMR (CD3CN): δH = 7.66
(d, J = 2.5 Hz, 1 H, H5pzcf), 7.60 (s, 1 H, CHmethine), 6.25 (br s, 1 H,
H4pzcf), 6.16 (s, 2 H, H4pziPr), 3.30 (sept, J = 6.7 Hz, 2 H, CHMe2),
2.86 (sept, J = 6.9 Hz, 2 H, CHMe2), 2.39 (br s, 1 H, N-H), 1.24 (d, J =
6.9 Hz, 12 H, iPr-CH3), 1.18 (d, J = 6.7 Hz, 12 H, iPr-CH3). 13C NMR
(CD3CN): δC = 161.45, 153.81, 105.99, 100.54, 64.01, 29.34, 26.43,
23.81, 23.20, 22.90, 22.78, 22.67, CF3 not observed. 19F NMR
(CD3CN): –79.34 ppm. ESI(+) MS, m/z (rel. abund. %) [assignment]:
872 (100) [Ag(L)2]+, 530 (12) [Ag(L)(CH3CN)]+, 489 (67) [Ag(L)]+, 383
(8) [H(L)]+, 231 (6) [L – pziPr2]+, 190 (2) [Ag(CH3CN)2]+, 153 (7)
[H2pziPr2]+.
mixture of 3b, 3a, Ts iPr2, and [Cu(CH3CN)4]PF6 with relative compo-
L
sitions that are both temperature and concentration dependent
(vide infra), only those resonances for 3b are reported below. 1H
NMR (CD3CN, 295 K): δH = 8.13 (d, J = 1.3 Hz, 2 H, H5pzcf), 7.76 (d,
J = 7 Hz, 4 H, TsAr), 7.41 (s, 2 H, CHmeth), 7.35 (d, J = 7 Hz, 4 H, TsAr),
6.21 (s, 2 H, H4pziPr), 6.08 (s, 2 H, H4pziPr), 5.45 (d, 2 H, H4pzcf), 3.16
(sept, J = 6 Hz, 2 H, CHMe2), 3.04 (m, 1 H, CHMe2), 2.38 (d, 6 H, Ts-
Me), 2.26 (m, 1 H, CHMe2), 1.26–1.18 (m, 24 H, iPr-Me), 0.84 (d, J =
6.6 Hz, 6 H, iPr-Me), 0.80 (d, J = 6.6 Hz, 6 H, iPr-Me), 0.55 (dd, J =
6.5 Hz, 6 H, iPr-Me), 0.43 (dd, J = 6.5 Hz, 6 H). 31P NMR (CD3CN): δp
-144.64 (sept, JPF = 706 Hz). 19F NMR (CD3CN) δF = -72.93 (d, JFP
=
706 Hz). ESI(+) MS, m/z (rel. abund. %) [assignment]: 1136 (9)
[Cu(L)2]+, 640 (95) [Cu(L)(CH3CN)]+, 599 (100) [Cu(L)]+, 537 (35)
[H(L)]+, 385 (26) [L–pziPr2]+, 145 (2) [Cu(CH3CN)2]+. Crystals of
3b·CH2Cl2 suitable for single-crystal X-ray diffraction were obtained
by layering hexanes over a solution of 30 mg of 3b in 1 mL of
dichloromethane and allowing solvents to slowly diffuse over 16 h.
[Ag(HLiPr2)2](OTf), 2b: Using General Procedure A, a mixture of
0.300 g (0.784 mmol) of HLiPr2 and 0.101 g (0.392 mmol) of AgOTf
gave 0.374 g (90 %) of 2b as a colorless solid. By using General
Procedure B, a mixture of 0.524 g of (1.37 mmol) of HLiPr2 and
0.176 g (1.37 mmol) of AgOTf gave 0.559 g (80 %) of 2b as a color-
less solid after drying under vacuum for an hour. Mp: 125–127 °CC.
Anal. Calcd (found) for C45H68N12SO3F3Ag: C, 52.88 (52.89), H, 6.70
(6.67), N 16.44 (16.35). 1H NMR (CD3CN, 293 K): 11.23 (br s, 2 H, NH),
7.63 (s, 2 H, CHmeth), 7.60 (d, J = 2.1 Hz, 2 H, H5pzcf), 6.14 (d, J =
2.1 Hz, 2 H, H4pzcf), 6.09 (s, 4 H, H4pziPr), 3.30 (sept, J = 6.8 Hz, 4
H, CHMe2), 2.46 (br sept, J = 6.7 Hz, 4 H, CHMe2) 1.20 (d, J = 6.8 Hz,
12 H, iPr-CH3), 1.20 (d, J = 6.8 Hz, 12 H, iPr-CH3), 1.00 (d, J = 6.7 Hz,
12 H, iPr-CH3), 0.99 (d, J = 6.7 Hz, 12 H, iPr-CH3). 13C NMR (CD3CN):
δC = 161.80, 154.25, 148.41, 131.04, 122.11 (q, J = 324 Hz, CF3),
105.68 (C4pzcf), 100.45 (C4pziPr), 64.95 (Cmeth), 29.20, 26.52, 25.48,
23.66, 23.60, 23.06, 22.24. 19F NMR (CD3CN): –79.34 ppm. ESI(+) MS,
m/z (rel. abund. %) [assignment]: 872 (100) [Ag(L)2]+, 530 (3)
[Ag(L)(CH3CN)]+, 489 (28) [Ag(L)]+, 383 (92) [H(L)]+, 231 (79)
[L pziPr2]+, 153 (6) [H2pziPr2]+.
[Cu(HLiPr2)(CH3CN)](PF6), 4a: Using General Procedure A, a mixture
of 0.300 g of (0.784 mmol) of HLiPr2 and 0.292 g (0.784 mmol) of
[Cu(CH3CN)4]PF6 gave 0.480 g (97 %) of 4a·CH3CN as an off-white
powder after drying at 60 °C under vacuum for an hour. Anal. Calcd
(found) for C24H37N7CuF6P: C, 45.61 (45.99), H, 5.90 (5.83), N 15.51
(15.14). Mp: 157–159 °CC. The NMR spectra have signals for an equi-
librium mixture of 4a, 4b, HLiPr2, and [Cu(CH3CN)4]PF6 with relative
compositions that are both temperature and concentration de-
pendent (vide infra), only those resonances for 4a are reported be-
low. 1H NMR (CD3CN, 293 K): 11.64 (br s, 1 H, NH), 7.66 (br d, J =
1 Hz, 1 H, H5-pzcf), 7.48 (s, 1 H, Hmeth), 6.46 (br d, J = 1 Hz, 1 H, H4-
pzcf), 6.12 (s, 2 H, H4-pziPr), 3.31 (sept, J = 6.7 Hz, 2 H, CHMe2), 3.06
(br sept, J = 6.6 Hz, 2 H, CHMe2), 1.28 (d, J = 6.7 Hz, 6 H, CH3),
1.25–1.21 (br m, 18 H, CH3). 13C NMR (CD3CN, 293 K): δC = 161.18
(C3/5-pziPr), 152.92 (C3/5-pziPr), 147.20 (C3/5-pzcf), 130.91 (C3/5-pzcf),
105.43 (C4-pzcf), 100.40 (C4-pziPr), 62.53 (Cmeth), 28.92 (CHMe2),
[Cu(TsLiPr2)(CH3CN)](PF6), 3a: Using General Procedure A, a mixture
of 0.500 g of (0.935 mmol) of TsLiPr2 and 0.347 g (0.935 mmol) of
[Cu(CH3CN)4]PF6 gave 0.655 g (89 %) of 3a as a pale yellow powder
after drying at 60 °C under vacuum for an hour. Anal. Calcd (found)
for C31H43N7CuF6O2PS: C, 47.18 (46.86), H, 5.49 (5.36), N 12.42
(12.77). Mp: 95–97 °CC. The NMR spectra have signals for an equilib-
rium mixture of 3a, 3b, TsLiPr2, and [Cu(CH3CN)4]PF6 with relative
compositions that are both temperature and concentration de-
26.29 (CHMe2), 24.08 (CH3), 22.93 (CH3), 22.76 (CH3), 22.49 (CH3). 31
P
NMR (CD3CN): δp -144.63 (sept, JPF = 707 Hz). 19F NMR (CD3CN) δF =
-72.93 (d, JFP = 707 Hz). ESI(+) MS, m/z (rel. abund. %) [assignment]:
827 (1) [Cu(L)2]+, 676 (1) [Cu(L)(L
–
pziPr2)]+, 486 (100)
[Cu(L)(CH3CN)]+, 445 (12) [Cu(L)]+, 383 (5) [H(L)]+, 231 (5) [L – pziPr2],
145 (5) [Cu(CH3CN)2]+.
[Cu(HLiPr2)2](PF6), 4b: Using General Procedure A, a mixture of
pendent (vide infra), only those resonances for 3a are reported be-
0.300 g of (0.784 mmol) of HLiPr2 and 0.146 g (0.392 mmol) of
1
low. H NMR (CD3CN): δH = 8.15 (d, J = 2.7 Hz, 1 H, H5pzcf), 7.76 (d, [Cu(CH3CN)4]PF6 gave 0.420 g (94 %) of 4b as colorless powder after
J = 8.3 Hz, 2 H, TsAr), 7.41 (s, 1 H, CHmeth) 7.38 (d, J = 8.3 Hz, 2 H, drying under vacuum for an hour. Anal. Calcd (found) for
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EurJIC
European Journal of Inorganic Chemistry
C44H68N12CuF6P: C, 54.28 (54.33), H, 7.04 (6.72), N 17.26 (17.07). Mp:
132–134 °CC. The NMR spectra have signals for an equilibrium mix- Next, either 0.4 mL of a 0.05
ture of 4a, 4b, HLiPr2, and [Cu(CH3CN)4]PF6 with relative composi- catalyst (0.02 mmol, 0.02 equiv.) or separate CH3CN solutions of
then was backfilled with argon, and cooled to room temperature.
M
stock solution of pre-formed metal
tions that are both temperature and concentration dependent (vide
infra), only those resonances for 4b are reported below. 1H NMR
(CD3CN, 293 K): 11.18 (br s, 2 H, NH), 7.59 (br s, 2 H, H5pzcf), 7.53
(s, 2 H, CHmeth), 6.35 (br s, H4pzcf), 6.15 (s, H4pziPr), 6.09 (s, H4pziPr),
5.29 (br s, H4pzcf), 3.29 (sept, J = 6.8 Hz, 4 H, CHMe2), 2.97 (br m, 2
H, CHMe2), 2.18 (br m, 2 H, CHMe2), 1.34 –1.09 (br m, 24 H, iPr-CH3),
0.89 (br s, 12 H, iPr-CH3), 0.59 (br s, 6 H, iPr-CH3), 0.45 (br s, 6 H, iPr-
metal salt (0.4 mL, 0.05
for monoligated complexes, or 0.8 mL of 0.05
M
, 0.02 equiv.) and ligand (0.4 mL of 0.05
M
M
for diligated com-
plexes), then enough CH3CN was added to give a total volume of
4 mL. After the mixture had been stirred five min, 0.171 g
(1.00 mmol) of tosylamine, and 0.322 g (1.00 mmol) of PhI(OAc)2
were added sequentially under an argon blanket giving either a
colorless (silver catalysts) or blue (copper catalysts) solution. The
CH3), see text for explanation. 31P NMR (CD3CN): δp -144.65 (sept, reaction flask was placed in an oil bath maintained at 80 °C and
JPF = 707 Hz). 19F NMR (CD3CN) δF = -72.92 (d, JFP = 707 Hz). ESI(+)
MS, m/z (rel. abund. %) [assignment]: 827 (50) [Cu(L)2]+, 486 (100)
allowed to equilibrate for 15 min. Then, 0.57 mL (0.52 g, 5.0 mmol)
of styrene was added by syringe, at which time the solution
[Cu(L)(CH3CN)]+, 445 (29) [Cu(L)]+, 383 (30) [H(L)]+, 231 (60) [L – changed color to either orange, or in some instances, orange-brown
pziPr2]. Crystals of 4b·2THF were grown under an Ar atmosphere in
the drybox by layering 5 mL of hexanes onto a solution of 30 mg
of 4b in 1 mL of THF and allowing solvents to diffuse over 12 h.
(for silver catalysts) or remained blue (for copper catalysts). After
the reaction mixture had been stirred at 80 °C for 16 h, it was
filtered through a sintered glass frit. The solid residue was washed
with two 2 mL portions of CH3CN. Next, between 20 and 30 mg
(89.9 to 135 μmol) of 1,4-bis(trimethylsilyl)benzene was added to
the filtrate as a nonvolatile NMR standard, and the solvent was re-
moved by rotary evaporation (1 Torr, 40 °C) to leave a brown-orange
oily residue that was subject to NMR analysis as above. A summary
of results (average values of at least three independent runs) is
collected in Table 4.
Catalytic Aziridination
For most consistent results that minimize uncertainties in weighing
small masses of solid catalysts, 0.05
M stock solutions of complexes,
or separate 0.05 solutions of ligands and metals were prepared
M
and used in catalysis reactions.
X-ray Crystallography
General Procedure. Method A: In an argon-filled drybox, a 20 mL
vial was charged with 0.5 g of activated 4 Å molecular sieves and
X-ray intensity data from a colorless plate of 1a·acetone, a colorless
block of 3a, a colorless prism of 3b·CH2Cl2, and a colorless plate of
4b·2THF were collected at 100.0(1) K with an Oxford Diffraction Ltd.
Supernova equipped with a 135 mm Atlas CCD detector using Cu-
Kα radiation, λ = 1.54184 Å. Raw data frame integration and Lp
corrections were performed with CrysAlis Pro (Oxford Diffraction,
Ltd.).[82] Final unit cell parameters were determined by least-squares
refinement of 22190, 17870, 29178, and 7422 reflections from the
data sets of 1a·acetone, 3a, 3b·CH2Cl2, and 4b·2THF, respectively,
each with I > 2σ(I). Analysis of the data showed negligible crystal
decay during collection in each case. Direct methods structure solu-
tions were performed with Olex2.solve[83] while difference Fourier
calculations and full-matrix least-squares refinements against F2
were performed with SHELXTL.[84] Empirical (Gaussian) absorption
corrections were applied using spherical harmonics implemented
in the SCALE3 ABSPACK scaling algorithm. Hydrogen atoms were
placed in idealized positions and included as riding atoms. For
1a·acetone, one of the isopropyl groups is unevenly (63:37 %) disor-
dered over two nearby positions, affecting C14 (C14a) and C16
(C16a). For 3a·CH3CN, the structure includes cavities apparently
partially populated by disordered water molecules (ca. 30 %). Their
contribution was accounted for by using a solvent-mask procedure
SQUEEZE. Similarly, the structure of 4b·2THF had large cavities
(1097.1 Å3) populated with severely disordered THF solvent mol-
ecules, that were accounted for by using the SQEEZE procedure. A
summary of crystal data and structure refinement is given in Tables
S1 and S2.
0.20 mL of a 0.05
(0.010 mmol, 0.02 equiv.). Alternatively, for in-situ formed catalysts
0.2 mL of a 0.05 solution of either AgOTf or [Cu(CH3CN)]PF6 in
CH2Cl2 and either a 0.2 mL or a 0.4 mL of CH2Cl2 solution that is
0.05
in XLR (for 1:1 and 1:2 M/XLR complexes, 0.01 mmol,
M stock solution of pre-formed metal catalyst
M
M
0.02 equiv.) was added. Then, CH2Cl2 was added to give a total
volume of 5 mL. The mixture was stirred for 5 min to generate the
metal catalyst, then 0.215 g (0.575 mmol, 1.15 equiv.) of PhINTs and
57 μL (0.5 mmol, 1.0 equiv.) of styrene were added. After the mix-
ture had been stirred for 24 h at room temperature, it was filtered
through a 2 cm silica pad and 3 mL of CH2Cl2 was used to rinse
the silica pad. Next, 11.1 mg (0.05 mmol) of 1,4-bis(trimethyl-
silyl)benzene was added to the filtrate as an NMR standard, and the
solvent was removed under vacuum at room temperature to leave
a brown-orange oily residue. This NMR standard is relatively nonvol-
atile under most mild conditions (rotary evaporator at house vac-
uum of 1 Torr); however, this compound will partly sublime if
heated at 70 °C under oil pump vacuum (10–4 Torr) for several
hours, so care should be taken to avoid extended heating under
modest to high vacuum. NMR yields of N-tosyl-2-phenylaziridine
(conversion % with respect to N-tosylamine) in the brown-orange
oily residue dissolved in CDCl3 were obtained by relative integra-
tions as follows. First the singlet resonance at δH = 0.26 ppm for
SiCH3 hydrogens was set to 18 H. Next the average integration
value for the resonances at δH = 3.78 (dd, J = 7.2, 4.6 Hz, 1 H) and
δH = 2.98 (d, J = 7.2 Hz, 1 H) for aziridinyl ring hydrogens was
taken (the third doublet resonance is obscured by the tosyl methyl
resonance near 2.4 ppm). The average integration value is then mul-
tiplied by the known μmol of C6H4(SiMe3)2 and 100 % to give the %
conversion to aziridine based on N-tosylamine. The average values
of three independent runs are collected in Table 3. The cited turn-
over numbers (TON) in the main text are calculated as the ratio of
mmol aziridine to mmol catalyst.
CCDC 1985192 (for 1a·acetone), 1985193 (for 3a), 1985194 (for
3b·CH2Cl2), and 1985195 (for 4b·2THF) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
Acknowledgments
Method B: A 1.00 g sample of activated 4 Å molecular sieves and
J. R. G. thanks the donors of the Petroleum Research Fund
a Teflon-coated magnetic stir bar were added to a Schlenk flask
under an argon blanket. The flask was flame-dried under vacuum, (#58705-ND3) and Marquette University for support.
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EurJIC
European Journal of Inorganic Chemistry
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Received: February 20, 2020
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Nitrene Transfer
D. Wang, F. Jahan,
K. J. Meise, S. V. Lindeman,
J. R. Gardinier* ................................. 1–16
Silver(I) and Copper(I) Complexes of
Semi-Bulky Nitrogen-Confused C-
Scorpionates
The copper and silver complexes of solution properties. The ability for the
bulky nitrogen-confused C-scorpion- complexes to catalyze styrene aziridi-
ates were prepared to compare effects nation with different nitrene sources
of ligand substitution, ligand number, was also evaluated.
and metal on their solid state and
doi.org/10.1002/ejic.202000173
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