Journal of the American Chemical Society
Communication
Scheme 2. (a) Coordination of LNO2 and LNH2 with CuCl;
that DEAD acts as a single hydrogen atom abstractor (Figure
NHOH
38
(
(
b) Disproportionation of L
with [Cu(CH CN) ]PF ;
S10). Species 2 was characterized in solution by ESI-MS (m/
3
4
6
NHOH
NHOH
NO
+
1
c) Capture of L
by CuCl Leading to [L
z = 431.1 for L Cu , Figure S11), displays broadened H
NMR peaks in CDCl (Figure S12), and is EPR-silent in
3
frozen DCM solution at 77 K. Diffusion of pentane into a −30
°
C DCM solution of 2 produced by oxidation with DEAD
yielded light-red crystals. X-ray diffraction of these crystals
revealed that 2 dimerizes in the solid state as a (μ-Cl) Cu
2
2
NO
complex, [L CuCl] , 3, in which the ArNO moiety is not
2
coordinated (Figure 1) and remains a double-bonded
0
39
(
ArNO) (N1−O1 = 1.236 Å).
Given the redox noninnocence of ArNO species, two
NO 0
I
electromers are possible for 2: [(L ) Cu Cl], with an NO
NO•−
II
double bond, and [(L
)Cu Cl], an arylnitrosyl π-radical
1
with an N−O bond order of 1.5. Evans H NMR measure-
I
ments on solutions of 2 indicated paramagnetism with χ T ≈
with Cu Cl, however, led to a single well-defined species,
M
−1
NHOH
0.13 emu K mol (Figure S12), well below the expected value
[L
CuCl] (1), which precipitates as a yellow powder in
−
1
for two individual 1/2 spins (0.75 emu K mol ), which
7
5% yield (Scheme 2c). X-ray diffraction analysis of 1 shows a
I
strongly suggests that 2 is an antiferromagnetically coupled
Cu -radical species. The electronic structure of 2 was also
explored by Cu K-edge X-ray absorption spectroscopy (XAS,
Cu center in a T-shape geometry interacting weakly with the
ArNHOH (Cu···N1 = 2.657 Å) and the sulfonanide (Cu···N2
II
Figure 3, SI Section 6). The Cu K-edge XAS of 1 (15 mM in
I
frozen DCM or in the solid state) is consistent with Cu , with a
prominent edge feature at 8985.8 eV ascribed to the
I
40,41
characteristic Cu 4p ← 1s peak.
The Cu K-edge XAS of
2
(made with 3 equiv of DIAD, 15 mM in frozen DCM
solution), on the other hand, exhibits features more character-
istic of a Cu species, with an edge at 8985.0 eV, shifted 2.2 eV
II
with respect to that of 1, and a weak low-energy Cu 3d ← 1s
40
pre-edge feature at 8977.6 eV.
•
−
DFT and EXAFS data support that the (ArNO) moiety in
Figure 1. ORTEP view with 50% probability ellipsoids of 1 and 3.
Solvent molecules and H atoms were omitted for clarity, except those
on the NHOH function of 1.
II
2 is coordinated to Cu in a κN fashion. DFT calculations at
the CAM-B3LYP/Def2-TZVP level of theory (SI Section 8)
on 2 predict a distorted square-pyramidal coordination
geometry and a κN-(ArNO)• moiety having a NO bond
length of 1.28 Å (Figure 4). The antiferromagnetically coupled
singlet ( 2) and ferromagnetically coupled triplet ( 2) are
predicted to be nearly isoenergetic (Table S15). TD-DFT
calculations yield good agreements with the experimental XAS
−
complex with an intact primary hydroxylamine ligand (N1−O1
2
9−31
=
1.418 Å)
(some examples also exist with a ligated
1
3
−
18,20
37
ArNHO moiety
or secondary aminoxyl groups). Close
inspection reveals that the OH group is involved in a hydrogen
bond with the Cl atom (O···Cl = 3.094 Å), possibly explaining
the unusual stability of this ArNHOH moiety.
(
Figures S24−S26) and UV−vis (Figure S27) spectra. The 508
nm peak in the experimental UV−vis spectrum of 2 can be
The relative stability of complex 1 provides a unique
assigned as a ligand-to-metal charge transfer from the
NO
opportunity to access the L complex by 2e oxidation and
•−
II
(
ArNO) π system to the Cu d-hole (Figure S28). The
thus complete the nitrogen oxidation state series (Scheme 2).
Oxidation of 1 by diethyl or diisopropyl azadicarboxylate
II
same MO assigned as the Cu d-hole is the acceptor orbital in
the Cu 3d ← 1s pre-edge transition state of the TD-DFT-
calculated XAS of 2 (Figures S25, S26). Finally, the EXAFS
from the Cu K-edge XAS of 2 was fitted by taking the DFT-
optimized geometry as initial guess (Figure 4, SI Section 7).
The resulting fit (Figure 3, Table S10) is fully consistent with
(
(
(
(
DEAD or DIAD) led to the formation a deep-purple complex
λmax = 508 nm, ε508 = 2450 M− cm ), [L CuCl], 2
Scheme 3a). Titration of 1 with DEAD in dichloromethane
DCM) reached maximum absorbance at 2.0 equiv, suggesting
1
−1
NO
II
•−
the mononuclear Cu -κN-(ArNO) assignment for 2.
Scheme 3. (a) Oxidation of 1 by DEAD or DIAD to Form 2;
b) Dimerization to 3 upon Crystallization
Following these structural characterizations, we aimed to
study the reactivity of complexes 1 and 2, because they possess
the most reactive nitrogen oxidation states within the series of
ligands. Complex 1 is metastable and disproportionates upon
application of mechanical force (Figure 4a). Grinding a
microcrystalline sample of 1 at 30 Hz for 15 min in a
mechanochemical ball miller led to an amorphous purple
powder (λmax = 508 nm in DCM). ESI-MS analysis (Figure
(
NO
+
NH2
+
S14) revealed peaks for [L Cu] and [L Cu] , and XAS
I
II
supports the formation of a mixture of Cu and Cu species
Figure 3). Ion metathesis with NaBAr in DCM led to a dark
F
4
(
purple/magenta solution, from which dark purple crystals were
obtained (Figure 4b,c). X-ray diffraction revealed a dinuclear
B
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Zsombor-Pindera, Joseph
