The 1 : 1 adduct of 1,3-diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2- ylidene and nitrous oxide

Article abstract of DOI:10.5560/ZNB.2013-2310

The nucleophilic carbene 1,3-diisopropyl-4,5-dimethyl-2,3-dihydroimidazol- 2-ylidene (7) captures nitrous oxide under formation of both syn- (8) and anti-1,3-diisopropyl-4,5-imidazolium diazotate (9). The syn-isomer is the thermodynamically more stable fo

Full text of DOI:10.5560/ZNB.2013-2310

The 1 : 1 Adduct of 1,3-Diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-
ylidene and Nitrous Oxide
M. Go¨hnera, P. Haissb, N. Kuhna, M. Stro¨belea, and K.-P. Zellerb
a
Institut fu¨r Anorganische Chemie, Universita¨t Tu¨bingen, Auf der Morgenstelle 18, D-72076
Tu¨bingen
b
Institut fu¨r Organische Chemie, Universita¨t Tu¨bingen, Auf der Morgenstelle 18, D-72076
Tu¨bingen
Reprint requests to Prof. Dr. N. Kuhn. E-mail: norbert.kuhn@uni-tuebingen.de or
Prof. Dr. K.-P. Zeller. kpz@uni-tuebingen.de
Z. Naturforsch. 2013, 68b, 539 – 545 / DOI: 10.5560/ZNB.2013-2310
Received November 26, 2012
Dedicated to Professor Heinrich No¨th on the occasion of his 85th birthday
The nucleophilic carbene 1,3-diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-ylidene (7) cap-
tures nitrous oxide under formation of both syn- (8) and anti-1,3-diisopropyl-4,5-imidazolium di-
azotate (9). The syn-isomer is the thermodynamically more stable form. As shown by X-ray crystal-
lography, the two isomers are statistically distributed in the crystalline state.
Key words: N-Heterocyclic Nucleophilic Carbenes, Nitrous Oxide, Stereochemistry, X-Ray
Crystallography
Introduction
The present interest in the chemistry of nitrous oxide
behavior. The qualitative VB treatment indicates elec-
trophilic character at both N atoms (Scheme 1).
The reactivity of N2O towards nucleophilic com-
is mainly focussed on its role as an atmospheric trace pounds has been known for a long time and is briefly
gas in global warming and destruction of the strato- summarized with the following examples. The reac-
spheric ozone layer [1, 2]. Furthermore, the forma- tion of alkali metal amide melts with N2O yields al-
tion and decomposition of nitrous oxide by enzymes kali metal azides (Wislicenus reaction) [7]. By means
of anaerobic bacteria is part of the global nitrogen of 15N-labelled nitrous oxide 2, Clusius et al. [8, 9]
cycle (denitrification process) [3]. The interaction of elucidated the mechanism of this process of industrial
N2O with the metal centers of the active sites of the importance as a combination of nucleophilic attack at
involved enzymes stimulated an increased interest in both, the terminal (a) and the central N atom (b). Like-
the chemistry of N2O-metal complexes [4]. The cat- wise the deprotonation products of anilines give aryl
alytic purification of N2O-polluted waste gases [4] and azides on reaction with N2O [10] (Scheme 2).
the utilization of N2O as oxidant in metal-catalyzed
Phosphanes react with nitrous oxide under oxygen
conversions [5] enhanced the research activities in the transfer to the phosphorus atom [11]. The phosphane-
field of N2O-metal complexes further. Compared to N2O adduct 3 which decomposes under N2 extrusion
these activities newer examples for the application of was suggested as an intermediate. The initial nucle-
nitrous oxide as reagent in metal-free reactions are ophilic attack of the phosphane at the terminal N atom
scarce.
of nitrous oxide is supported by the combined action
The bonding situation in nitrous oxide can be qual- of t-Bu3P and Ph3B or (C6F5)3B as so-called “frus-
itatively represented by mixing the zwitterionic VB trated Lewis pair” systems which yields the complex
structures 1a–c. The increased-valence structure 1d [6] 4 in 76% yield [12, 13]. In this reaction the dichoto-
with one electron πx(NO) and πy(NO) bonds and two mous character of N2O with electrophilic properties at
fractional πx(NN) πy(NN) bonds has also been sug- the terminal N atom and nucleophilic properties at the
gested to describe the N2O geometry and chemical O atom is elegantly exploited (Scheme 3).
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M. Go¨hner et al. · 1 : 1 Adduct of 1,3-Diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-ylidene and Nitrous Oxide
Ph3P–CH2
N
N
O
N
N
O
N
N
O
Ph3P–O
H2C
N
N
1a
1b
1c
O
N
N
Scheme 5.
N
N
O
N-heterocyclic carbenes 5a, b. This article prompted
us to report on our study concerning the reaction
of 1,3-diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-
2-ylidene (7) with N2O.
1d
Scheme 1.
NH2
(a)
(b)
*
*
O
N
N
O
N
N
N
N
N
NH2
Results and Discussion
2, * = 15N
78%
The N,N0-dimesityl and N,N0-diisopropylphenyl-
substituted nucleophilic carbenes 5a, b react with N2O
to give the anti-imidazolium diazotates 6a, b [26]
(Scheme 6).
*
O
N
*
*
N
N
N
O
N
N
N
NH2
22%
NH2
Similarly, introduction of nitrous oxide into a so-
lution of 1,3-diisopropyl-4,5-dimethyl-2,3-dihydro-
imidazol-2-ylidene (7) in toluene furnished a 1 : 1
adduct as a yellow precipitate. However, the stereo-
chemistry of the product formed from 7 differs remark-
ably. Structure elucidation has indicated that both, syn-
(8) and anti-1,3-diisopropyl-4,5-dimethylimidazolium
diazotate (9) are simultaneously obtained with the syn-
adduct being the main isomer (Scheme 7).
Scheme 2.
N
N
O
R3P
3
O
BPh3
t-Bu3P
N
N
O
BPh3
N
N
t-Bu3P
4
Scheme 3.
R2
R2
N
N
O
M+
R–M
N
N
O
products
R
R = alkyl, aryl
M = Li, Na
R1
R1
R1
R1
R1
Scheme 4.
R1
N
N
+
N
N
N
O
N2O
–
–
Organolithium compounds [14 – 16] and triphenyl-
methylsodium [17] react with N2O to give alkane-
(arene) diazotates as initial products, which depending
on the substituent R are further transformed into a va-
riety of compounds (Scheme 4).
+
N
R1
R1
In a formally Wittig-like reaction triphenylphospho-
nium methylide and N2O produce diazomethane in low
yields [18] (Scheme 5).
Nucleophilic carbenes are powerful neutral C nucle-
ophiles, and their reactivity towards a variety of elec-
trophiles is well-documented in the literature [19 – 25].
In a recent communication Severin et al. [26] de-
R2
R2
5a,b
6a,b
b: R1 = CH(CH3)2, R2 = H
a: R1 = R2 = CH3
scribed the covalent capture of nitrous oxide by the Scheme 6.
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M. Go¨hner et al. · 1 : 1 Adduct of 1,3-Diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-ylidene and Nitrous Oxide
541
–
O
–
N
N
O
N
+
N
–
O
N
–
N
N
+
+
+
+
+
N
N
N
N
N
8
7
9
Scheme 7.
The FAB mass spectrum of the precipitate obtained D2O were recorded immediately after the solutions
in the reaction of 7 with N2O (3-nitrobenzylic alcohol had been prepared. At the beginning a syn/anti ratio
as matrix) shows the [M+H]+ ion (m/z = 225) as base of 51.5 : 48.5 was found. This ratio steadily increased
peak and the M+• ion (m/z = 224, rel. int. 40%). Both, over 24 h to 75 : 25. When the solution was kept at
M+• and [M+H]+ expel NO under formation of ions 80 ◦C for 10 min a ratio of 77.6 : 24.4 was obtained.
with m/z = 195 (rel. int. 95%) and m/z = 194 (rel. int.
Obviously, the adduct precipitating in the reaction of
65%). The elimination of NO from [M+H]+ is an ex- 7 with N2O originally consists of an approximate 1 : 1
ception from the “even electron rule” enabled by the mixture of the isomers 8 and 9, which in solution equi-
high stability of the NO radical. A further fragment librate to isomeric mixtures varying from 75 to 85%
at m/z = 181 (rel. int. 45%) corresponds to the imi- in the syn-adduct depending on the solvent. From the
dazolium ion formed by transfer of the added proton solvents tested, D2O is the only one in which the equi-
of [M+H]+ to C-2 and N2O elimination. In addition to libration is slow enough to be detected by simple NMR
the M+• ion (m/z = 224, rel. int. 66%) the FD mass measurements.
spectrum exhibits cluster ions of the composition M2+•
The indiscriminate formation of both isomers means
(m/z = 448, rel. int. 10%) and [M + imidazolium]+ that the energy of the transition state is practically not
(m/z = 405, rel. int. 100%). affected by the orientation of the O atom. However, the
The X-ray structural analysis of the adduct crys- dominance of the syn-isomer in the equilibrium mix-
tallized from toluene (Fig. 1) has indicated substantial tures indicates that this isomer is the thermodynami-
disorder in the crystalline state caused by the presence cally more stable form. This is remarkable because in
of two stereoisomers which are statistically distributed arene diazotates, the anionic counterparts of the zwit-
with the imidazolium units occupying identical posi- terionic imidazolium diazotates, the anti-isomers are
tions. Although the disordering prevents the determi- more stable [27]. Furthermore, the problem remains to
nation of meaningful bond lengths from the electron explain the contradictory stereochemical results, when
density distribution, Fig. 1 clearly shows that the two the capturing of N2O by the nucleophilic carbene 7 is
isomers are syn- and anti-imidazolium diazotates with compared with the analogous reactions of 5a and b.
the N2O units in a near orthogonal position in relation
to the five-membered ring. A syn : anti ratio of 70 : 30 actions of 5a, b at one hand in contrast to the formation
can be estimated from the electron densities. of syn/anti-mixtures from 7 at the other hand are prob-
The exclusive formation of the anti-adduct in the re-
The position of the oxygen atom also influences the ably the results of different steric interactions of the
orientation of the isopropyl substituents attached at N- substituents located at the N atoms. The orientation of
2. To avoid steric strain, the methyl groups (C-10, C- the bulky aryl groups in 6a, b nearly perpendicular to
11) of the syn-adduct 8 are directed away from the oxy- the five-membered ring allows the introduced diazo-
gen atom O-1.
tate unit to adopt dihedral angles of 25.4 and 39.5◦, re-
1
In accordance with the X-ray data, the H NMR spectively [26]. Thus, resonance stabilization between
spectra of the adduct in [D6]benzene, [D8]toluene, the N2O unit and the imidazolium ring should still be
[D2]dichloromethane, D2O, and [D6]acetone shows possible to some extent. By way of contrast, the iso-
two sets of signals in ratios of 77 : 23, 76 : 34, 85 : 15, propyl substituents of the syn- (8) and anti-adduct (9)
75 : 25 and 79 : 21. The doubling of the signals is force the diazotate moiety towards orthogonality with
also evident in the 13C NMR spectra. An important respect to the plane defined by the imidazolium ring.
observation could be made when the NMR spectra in In this geometry, resonance between the diazotate unit
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M. Go¨hner et al. · 1 : 1 Adduct of 1,3-Diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-ylidene and Nitrous Oxide
Fig. 1. Molecular structure of 1,3-diisopropyl-4,5-imidazolium diazotates 8 and 9 showing the two refined positions in the
disordered parts [upper left (a) and right (b)] and (c) the superposition of the two possibilities. Displacement ellipsoids are at
25% probability level, hydrogen atoms are omitted for clarity.
and the imidazolium ring is interrupted, however, the of the imidazolium unit and the diazotate N atom is ex-
syn-configuration may gain some extra-stabilization by pected to be weaker in 8/9 compared to 6a. Therefore,
Coulomb interaction between the negatively charged O a dissociation-recombination mechanism to explain the
atom and the positive charge of the imidazolium ring. observed equilibration between 8 and 9 in solution can
This may cause the reversal in the order of stability be excluded and either rotation about the formal N,N
and, thus, favor 8 over 9.
double bonds or inversion at the N atom remain to ra-
In cross-over experiments between the adducts 6a/b tionalize the geometrical isomerization.
and the complementary free carbenes 5b/a, no trans-
The adduct 8/9 expels dinitrogen under formation
fer of N2O was observed [26]. Similarly, the syn/anti of the urea 10 at temperatures above 60 ◦C in non-
mixture 8/9 shows no N2O transfer when mixed with protic solvents. In [D8]THF the decomposition into
the nucleophilic carbene 5a in [D8]THF, although, be- 10 reach 81% after 80 h at 60 ◦C and in [D8]toluene
cause of the lack of resonance, the bond between C-2 68% after 50 min at 80 ◦C. This is in line with what
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M. Go¨hner et al. · 1 : 1 Adduct of 1,3-Diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-ylidene and Nitrous Oxide
543
1H and 13C NMR spectra were recorded using a Bruker
AvanceII+400 instrument at 400 MHz and 100 MHz, respec-
tively. Chemical shifts are reported in ppm, multiplicities are
labelled s (singulet), d (doublet) and sept (septet).
The FAB mass spectra were recorded on a Finnigan
TSQ 70 instrument with 3-nitrobenzylic alcohol as matrix
material. FD mass spectra were obtained on a MAT 95
instrument.
N
N
Δ
8 / 9
O
10
Scheme 8.
Adducts of nitrous oxide with 1,3-diisopropyl-4,5-dimethyl-
2,3-dihydroimidazol-2-ylidene (8, 9)
has been found for the analogous compound 6a [26].
(Scheme 8)
A constant flow of nitrous oxide is passed into a so-
lution of 2.328 g (12.91 mmol) of the carbene 7 in 80 mL
toluene for 8 h. The solution turns yellow followed by the
formation of a yellow precipitate. The solvent is removed in
vacuo, and unreacted carbene (7) is removed by sublimation
to afford 2.53 g (87%) of a yellow solid. – H NMR (D2O,
8): δ = 1.42 (d, J = 7.0 Hz, 12 H, N1,3-CH-(CH3)2), 2.32
The urea 10 is identical with a specimen inde-
pendently synthesized by oxidation of 1,3-dihydro-
1,3-diisopropyl-4,5-dimethyl-2,3-dihydroimidazole-
2-thione with alkaline hydrogenperoxide. The decom-
position of 8/9 into the urea 10 is strongly solvent-
1
dependent. In D2O and [D4]methanol no reaction (s, 6 H, C4,5-CH3), 4.56 (sept, J = 7.0 Hz, 2 H, N1,3-CH-
(CH3)2). – 13C NMR (D2O, 8): δ = 8.5 (C4,5-CH3), 20.7
(N1,3-CH-(CH3)2), 50.0 (N1,3-CH-(CH3)2), 123.8 (C4,5),
145.5 (C2). – 1H NMR (D2O, 9): δ = 1.39 (d, 12 H,
N1,3-CH-(CH3)2), 2.29 (s, 6 H, C4,5-CH3), 4.27 (sept,
J = 7.0 Hz, 2 H, N1,3-CH-(CH3)2). – 13C NMR (D2O, 9):
δ = 8.6 (C4,5-CH3), 19.9 (N1,3-CH-(CH3)2), 49.9 (N1,3-CH-
(CH3)2), 124.2 (C4,5), 143.7 (C2). – 1H NMR (C6D6, 8):
δ = 1.07 (d, J = 6.9 Hz, 12 H, N1,3-CH-(CH3)2), 1.53 (s, 6
H, C4,5-CH3), 4.58 (sept, J = 6.9 Hz, 2 H, N1,3-CH-(CH3)2).
could be detected on heating to 80 ◦C (24 h) and 60 ◦C
(82 h). In these solvents, strong solvation by hydro-
gen bonding should stabilizes the imidazolium diazo-
tates 8/9. In the crystalline state the adduct 8/9 endures
heating in vacuo to 60 ◦C without the formation of the
urea 10.
Conclusion
–
13C NMR (C6D6, 8): δ = 9.4 (C4,5-CH3), 21.2 (N1,3
-
1,3-Diisopropyl-4,5-dimethyl-1,3-dihydroimidazol-
2-ylidene (7) captures N2O under formation of both,
the syn- and the anti-imidazolium diazotate 8 and 9.
Both stereoisomers are formed under kinetic control
in approximately equal amounts, however in solution
the syn-isomer 8 becomes the dominating species.
syn/anti-Ratios of ∼ 3 : 1 to ∼ 4 : 1 could be detected
CH-(CH3)2), 49.2 (N1,3-CH-(CH3)2), 120.7 (C4,5), 153.8
(C2). – 1H NMR (C6D6, 9): δ = 1.09 (d, J = 6.9 Hz, 12
H, N1,3-CH-(CH3)2), 1.48 (s, 6 H, C4,5-CH3), 4.16 (sept,
J = 7.0 Hz, 2 H, N1,3-CH-(CH3)2). – 13C NMR (C6D6, 9):
δ = 9.0 (C4,5-CH3), 20.6 (N1,3-CH-(CH3)2), 49.0 (N1,3-CH-
(CH3)2), 121.5 (C4,5), 153.4 (C2). – C11H20N4O: calcd. C
58.9, H 9.0, N 25.0; found C 58.5, H 9.5, N 25.5. FAB- and
FD-MS data see text.
1
by H NMR spectroscopy depending on the nature of
the solvents.
1,3-Diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-one
(10) by oxidation of 1,3-diisopropyl-4,5-dimethyl-
2,3-dihydroimidazol-2-thione
To the best of our knowledge, the parallel formation
of the syn- and anti-isomer in a single reaction step
is unique in the chemistry of diazotates. Furthermore,
the isomeric pair 8/9 represents the first documented
example with the syn-form as the thermodynamically
more stable diazotate.
Compound 10 was prepared by the method described in
ref [29]. An ice-cold solution of 1.0 g (4.71 mmol) thione and
six pellets of sodium hydroxide in 35 mL of ethanol and 2 mL
of water was mixed with 6 mL of hydrogen peroxide (30%),
and the mixture was stirred at 0 ◦C for 1 h and was then
stored in a refrigerator overnight. After dilution with 40 mL
Experimental Section
All reactions were carried out under an atmosphere of ar- of water and extraction with toluene, the toluene solution
gon. Solvents were dried using standard procedures and dis- was treated with sodium carbonate and evaporated to yield
tilled in an argon atmosphere prior to use. 1,3-Diisopropyl- 0.3 g (33%) of a colorless solid. – 1H NMR (D2O): δ = 1.36
4,5-dimethyl-2,3-dihydroimidazol-2-ylidene (7) was pre- (d, J = 7.0 Hz, 12 H, N1,3-CH-(CH3)2), 2.03 (s, 6 H, C4,5
pared according to literature methods [28].
-
CH3), 4.29 (sept, J = 7.0 Hz, 2 H, N1,3-CH-(CH3)2). – 13C
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544
M. Go¨hner et al. · 1 : 1 Adduct of 1,3-Diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-ylidene and Nitrous Oxide
NMR (D2O): δ = 8.4 (C4,5-CH3), 20.2 (N1,3-CH-(CH3)2), respectively. Selected bond lengths and angles are summa-
45.3 (N1,3-CH-(CH3)2), 114.5 (C4,5), 151.8 (C2). – EI-MS rized in Table 2.
(70 eV): m/z (%) = 196 (41) [M]+•, 154 (33) [M–C3H6]+•
,
CCDC 911586 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
112 (100) [M–2 C3H6]+•, 111 (39), 97 (18).
Thermolysis of adducts 8/9
The thermal decomposition of 8/9 was performed in
deuterated solvents in NMR tubes. The formation of 10 was
1
followed by H NMR spectroscopy. For reaction times and
˚
Table 2. Selected bond lengths (A) and angles (deg) for 1,3-
temperatures see text.
diisopropyl-4,5-imidazolium diazotate 8/9 with estimated
standard deviations in parentheses.
Crystal structure determination
8/9
A suitable single crystal was selected and mounted on
a Stoe IPDS I diffractometer for data collection (graphite-
Bond lengths
N(1)–C(1)
N(1)–C(2)
N(1)–C(4)
N(2)–C(1)
N(2)–C(3)
N(2)–C(9A)
N(2)–C(9B)
C(1)–N(3B)
C(1)–N(3A)
C(2)–C(3)
C(9A)–C(11A)
C(9A)–C(10A)
C(9B)–C(11B)
C(9B)–C(10B)
O(1A)–N(4A)
1.332(2)
1.393(2)
1.486(2)
1.330(3)
1.388(3)
1.520(5)
1.523(8)
1.22(1)
1.523(6)
1.361(3)
1.541(9)
1.54(1)
˚
monochromatized MoKα radiation, λ = 0.71073 A, oscilla-
tion scan mode). The structure was solved by Direct Methods
and expanded using Fourier difference techniques with the
SHELXS/L-97 program package [30, 31] (Table 1). All non-
hydrogen atoms were refined anisotropically by full-matrix
least-squares refinements on F2. The hydrogen atom bound
to C(4) was found in the Fourier map, all other hydrogen
atoms were added in calculated positions, and all were re-
fined isotropically. The isopropyl group bound to N(2) and
the diazotate group bound to C(1) were refined with a disor-
der model in a ratio of 62(1) : 38(1), and 64(1) : 36(1) %,
1.48(2)
1.544(2)
1.248(7)
1.250(9)
1.27(2)
N(3A)–N(4A)
O(1B)–N(4B)
N(4B)–N(3B)
Table 1. Crystal structure data for 1,3-diisopropyl-4,5-
imidazolium diazotate 8/9.
1.380(19)
Bond angles
Formula
Mr
Crystal size, mm3
Crystal system
Space group
C11H20N4O
224.31
C(1)–N(1)–C(2)
C(1)–N(2)–C(3)
C(1)–N(2)–C(9A)
C(3)–N(2)–C(9A)
C(1)–N(2)–C(9B)
C(3)–N(2)–C(9B)
C(9A)–N(2)–C(9)B
N(3B)–C(1)–N(2)
N(3B)–C(1)–N(1)
N(2)–C(1)–N(1)
N(3B)–C(1)–N(3A)
N(2)–C(1)–N(3A)
N(1)–C(1)–N(3A)
C(3)–C(2)–N(1)
C(2)–C(3)–N(2)
N(2)–C(9A)–C(11A)
N(2)–C(9A)–C(10A)
C(11A)–C(9A)–C(10A)
C(11B)–C(9B)–N(2)
C(11B)–C(9B)–C(10B)
N(2)–C(9B)–C(10B)
N(4A)–N(3A)–C(1)
O(1A)–N(4A)–N(3A)
O(1B)–N(4B)–N(3B)
C(1)–N(3B)–N(4B)
109.12(16)
109.10(15)
114.9(3)
135.9(3)
139.7(6)
110.8(6)
25.2(4)
114.6(6)
136.9(6)
108.42(19)
16.9(6)
126.8(3)
123.5(3)
106.38(16)
106.97(16)
109.2(4)
105.7(4)
108.2(7)
105.9(8)
114.1(12)
108.9(7)
115.8(6)
116.0(7)
117.2(14)
108.0(10)
0.5 × 0.5 × 0.4
monoclinic
P21/c (no. 14)
10.048(1)
11.294(3)
11.321(2)
104.25(2)
1245.3(4)
4
˚
a, A
˚
b, A
˚
c, A
β, deg
3
˚
V, A
Z
Dcalcd, g cm−3
µ (MoKα ), cm−1
F(000), e
1.20
0.8
488
hkl range
θ range, deg
Refl. measured / unique / Rint
Param. refined
R1 / wR2a [I > 2σ(I)]
R1 / wR2b (all data)
GoF (F2)c
±11, ±13, ±13
2.59 to 25.03
12357 / 2126 / 0.0388
205
0.0503 / 0.1220
0.0772 / 0.1344
0.920
−3
˚
∆ρfin (max / min), e A
0.221 / −0.151
a
b
R1 = Σ||Fo| − |F ||/Σ|Fo|; wR2 = [Σw(Fo2 − F2)2/Σw(Fo2)2]1/2
,
c
c
w = [σ2(Fo2) + (AP)2 + BP]−1, where P = (Max(Fo2,0) + 2F2)/3;
c
c
GoF = [Σw(Fo2 −F2)2/(nobs −nparam)]1/2
.
c
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M. Go¨hner et al. · 1 : 1 Adduct of 1,3-Diisopropyl-4,5-dimethyl-2,3-dihydroimidazol-2-ylidene and Nitrous Oxide
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