Zero-field splitting parameters of triplet nitreno-s-triazines: A new insight into the geometry of the nitrene centres of triplet and singlet nitrenes

Article abstract of DOI:10.1070/MC2002v012n06ABEH001683

The zero-field splitting parameters of triplet nitreno-s-triazines indicate that triplet and singlet nitrenes have sp³ and sp² hybridization of the nitrene centres, respectively.

Full text of DOI:10.1070/MC2002v012n06ABEH001683

Mendeleev Commun., 2002, 12(6), 227–229
Zero-field splitting parameters of triplet nitreno-s-triazines: a new insight into
the geometry of the nitrene centres of triplet and singlet nitrenes
Sergei V. Chapyshev
Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region,
Russian Federation. Fax: +7 096 515 3588; e-mail: chap@icp.ac.ru
10.1070/MC2002v012n06ABEH001683
The zero-field splitting parameters of triplet nitreno-s-triazines indicate that triplet and singlet nitrenes have sp3 and sp2 hybridiza-
tion of the nitrene centres, respectively.
interest. Earlier, EPR studies2,6 have shown that the derivatives of
triplet phenylnitrenes have D-values in a range of 0.87–1.05 cm–1,
N
N
N
those of triplet pyridylnitrenes,7 in a range of 1.05–1.11 cm–1,
and triplet 2-nitrenopyrimidine8 has a D-value of 1.217 cm–1. On
comparison with all these species, triplet 2a–c have the highest
values of D. This is explained by the effect of heterocyclic
nitrogen atoms, which are poor spin-holding centres due to a
small size. The more nitrogen atoms in a heterocyclic ring, the
higher the D-values of triplet heteroarylnitrenes. An increase in
the D-values of triplet 2a–c is accompanied by an elongation
of the C–N bond (Table 1), which significantly exceeds in length
the C–N bond of the phenylnitrene 1A1 state (Figure 1). There-
fore, it is not surprising that triplet 2a–c display strongly pro-
nounced singlet-type reactivity to form compounds 4 and 5 on
photolysis in the presence of the traps of singlet nitrenes.9 The
extraordinarily high E-values of triplet 2a–c indicate the non-
degeneracy of two magnetic orbitals of these species, owing to
which two spins in 2a–c flip at different energies. Almost all
previous EPR studies dealt with triplet arylnitrenes, which have
very small E-values (< 0.002 cm–1).2,6,7 High values of E were
found only for some m-substituted phenylnitrenes (0.003 cm–1),2
triplet 2-nitrenopyrimidine (0.005 cm–1)8 and triplet 3 (0.011 cm–1).4
As usually, high values of E for triplet nitrenes were explained
by the low symmetry of molecules.2 The finding that so highly
symmetrical molecules as triplet 2a and 2c have very high
values of E for the first time demonstrates that the nondegen-
eracy of magnetic orbitals in triplet nitrenes not always results
from low molecular symmetry. The origin of high values of E
for triplet 2a–c can be understood from a qualitative analysis of
the wavefunctions of triplet nitrenes.
1.359
1.338
1.276
1.413
1.425
1.473
1.391
1.399
1.386
1.404
1.367
1.426
N
N
N
1A1
3A2
1A2
Figure 1 CASSCF(8,8)/6-31G* calculated3(c) bond lengths (Å) and mole-
cular models of closed-shell 1A1 singlet, 3A2 triplet and open-shell 1A2
singlet phenylnitrenes.
Nowadays,1 all physical and chemical properties of nitrenes are
explained using models for triplet and singlet nitrene centres,
which were introduced by E. Wasserman2 in 1971. According
to these models, the nitrene nitrogen atoms in both closed-shell
1A1 singlet and open-shell A2 triplet states are sp-hybridized
3
and differ only in the way of filling two orthogonal p-orbitals
with electrons (Figure 1). In 1992, it has been found that singlet
phenylnitrene can also have an electronic configuration of the
open-shell 1A2 singlet state with the same orbital occupancy as
3
in the A2 state but with the antiparallel orientation of spins.3
The DZ+d CISD+Q calculations showed that the orbitals of the
1A2 state are very similar to those of the A1 state and much
1
differ from the orbitals of the A2 state.3(a) Despite very dif-
3
ferent geometries of the 1A1, 1A2 and 3A2 states, no attempt was
made to analyse the hybridization of their nitrogen atoms. Here,
the results of EPR studies of triplet nitreno-s-triazines, pro-
viding strong evidence for different hybridization of the nitrene
nitrogen atoms in the 1A1 and 3A2 states, are presented.
N
N
N3
N
N
N
N
The irradiation of azides 1a–c† in a degassed 2-methyltetra-
hydrofuran (MTHF) solution frozen at 7 K with light at l = 254 nm
led to the appearance of the EPR spectra of triplet nitrenes 2a–c
in the form of two strong and well-resolved X- and Y-transitions
at 7800–8200 G (Figure 2). A similar EPR spectrum has been
reported recently4 for triplet diazidonitrene 3. Standard calcula-
tions5 showed that triplet 2a–c have the zero-field splitting D-
and E-parameters that are close to those of triplet 3 (Table 1).
Both D- and E-values of triplet 2a–c are of considerable
N
N
254 nm
N
N
N
N
MTHF, 7 K
R
R
R
R
N
N
N
N
1a–c
2a–c
3
N
a R = Cl
b R = OMe
c R = NMe2
Table 1 X- and Y-transitions, D- and E-parameters, and UB3LYP/6-31G*
calculated spin populations (qN) on the nitrene centres and the C–N bond
lengths of nitrenes 2a–c, 3 and PhN.
HN
N
N
N
N
N
N
N
N
N
Nitrene X-line/G Y-line/G |D/hc|/cm–1 |E/hc|/cm–1 qN
r(C–N)/Å
MeO
N
OMe MeO
OMe
MeO
N
OMe
2a
2b
2c
3
PhN
7735a
7834a
7938a
—
7968b
8100b
8165b
—
1.377
1.427
1.465
1.402c
0.999d
0.011
0.013
0.011
0.011c
0.000d
1.7549 1.366
4, 48%
2b
5, 52%
1.7791 1.371
1.7930 1.378
1.7728 1.371
1.6001 1.328
All triplet nitrenes can arbitrarily be considered as systems of
two unpaired electrons and described by wavefunctions YT and
potential energy curves analogous to those used for describing
diatomic systems (Figure 3). Deviations to any side from the
optimum distance r between two spins should lead to a decrease
—
—
aMid-point. bTop-point. cFrom ref. 4. dFrom ref. 6.
†
The synthesis of azides 1a–c was described elsewhere.12
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Mendeleev Commun., 2002, 12(6), 227–229
YT
(a)
(b)
R
Y
X
YD
YS
r
r
2000
4000
6000
8000
Magnetic field/G
Figure 2 EPR spectrum from the photolysis of azide 1a (u0 = 9.520 GHz,
H0 at 3400 G) with light at l = 254 nm for 10 min in MTHF at 7 K. The
peaks X, Y and R correspond to X- and Y-transitions of triplet nitrene 2a
and a radical from MTHF, respectively.
in the size of YT and to an increase in the potential energy. At
very small r, YT approaches the point with YT = 0, beyond
which the wavefunction of closed-shell singlet nitrenes YS starts
to develop (r becomes the distance between two electrons in the
same orbital). On the other side of YT, there is another point
with YT = 0 and another wavefunction. This new wavefunction
YD describes diradicals with two unpaired electrons on two dif-
ferent radical centres (r becomes very large). Since the discovery
Figure 3 (a) Wavefunctions and (b) potential energy curves for closed-
shell singlet (YS), triplet (YT) and open-shell singlet (YD) arylnitrenes.
Y
X
1
of such diradicals in 1992, they were called as open-shell A2
singlet nitrenes despite the fact that the nitrogen atom in these
species is divalent (Figure 1).‡ The strongly pronounced singlet-
type reactivity of triplet 2a–c and very high spin populations on
their nitrene centres (small r)2 suggest that the YT of these
species are close to the YS of closed-shell 1A1 singlet nitrenes.
According to the Hund rule, two spins occupy two orbitals and
have parallel orientation only if such orbitals are degenerate.
Deviations to any side from the optimum r between two spins
should lift this orbital degeneracy. Namely, this finally leads to
a situation when two spins either prefer to occupy the same
low-lying orbital (the 1A1 states) or adopt the opposite spin
E
3A2
2a–c
1A1
3
1
Figure 4 Orbitals and energy levels in the A2 and A1 states and triplet
nitrenes 2a–c.
electrons can interact with each other only if their wavefunc-
tions are not orthogonal. Thus, both a theory and EPR spectra
suggest that typical triplet nitrenes have two nearly degenerate
sp3-hybrid magnetic orbitals and small values of E owing to
S2x » S2y in the third term of the triplet state spin Hamiltonian.11
^
1
orientation (the A2 states). High values of E for triplet 2a–c
reflect changes in the spatial orientation of magnetic orbitals,
which take place on the way from 3A2 to 1A1 nitrenes. Another
example of this series is highly symmetrical triplet 2-nitreno-
pyrimidine (|D/hc| = 1.217 cm–1, |E/hc| = 0.005 cm–1).8 These
changes in the spatial orientation of magnetic orbitals of triplet
nitrenes can be associated only with different hybridization of
H = gbHS + DS2z + E(S2x – S2y).
(1)
On approaching YT of the 3A2 states to YS of the 1A1 states,
one magnetic orbital gains more s component, while another one,
more p component. As a result, two orbitals adopt different
spatial orientations (Figure 4) and different energies despite the
high symmetry of molecules. The EPR spectra of triplet 2a–c
are the best illustration of such changes.
3
1
nitrene nitrogen atoms in the A2 and A1 states. As is known,
five valent electrons of nitrogen with zero valence are described
by two wavefunctions, Y(2s) and Y(2p). Since the nitrene nitro-
gen atom is univalent, its six valent electrons are described by
YS and YT, which are not pure s or p. However, in any case, YS
and YT should have maximum s and p components, respecti-
vely. YS has a maximum s component (33.3%) in two cases. In
one case, two electron pairs occupy two sp-orbitals, and one
pair of electrons occupies one of two degenerate p-orbitals (the
Wasserman model).2 However, the Hund rule says that two
electrons cannot occupy only one of two degenerate orbitals.
According to this rule, the Wasserman model for the singlet
nitrene centre is wrong. In the other case, all three electron
pairs occupy three sp2-hybrid orbitals leaving the high-energy
p-orbital as vacant in full accord with the Hund rule. Only this
type of orbital occupancy in singlet nitrenes meets all require-
ments of quantum chemistry. Similar considerations show that
YT has a maximum p component (75%) when six valent elec-
trons occupy four sp3-orbitals. The fact that the magnetic orbitals
of triplet nitrenes are hybrid was experimentally proved by
observations of the hyperfine splitting on the nitrene nitrogen
atoms (aN = 17–20 G)4,6 in the EPR spectra of triplet nitrenes.
Moreover, this orbital occupancy does not contradict to the basic
principle of wave mechanics,10 according to which two unpaired
I am grateful to Professor M. S. Platz for provided laboratory
facilities and helpful discussions.
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‡
By definition, nitrenes are compounds of univalent nitrogen with six
valent electrons.13 Because the open-shell 1A2 singlet states have divalent
nitrogen with seven valent electrons, these species represent a class of
divalent nitrogen compounds. Such compounds can also have singlet and
triplet spin states,14 which, similarly to singlet and triplet nitrenes, are
described by two different wavefunctions.
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Mendeleev Commun., 2002, 12(6), 227–229
9
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Received: 6th November 2002; Com. 02/2010
– 229 –