Angewandte
Chemie
an inert atmosphere. In contrast, the N-alkyl-substituted salts
2
e,f are thermally labile, albeit storable for months at À808C.
The molecular structures of 2a and 2c were established
unequivocally by X-ray crystal structure determinations
[
24]
(
Scheme 1).
nitrilium ion with a typical NꢀC bond [2a: 1.125(3), 2c:
.140(4) ꢁ] which compares well with the only other struc-
turally characterized N-aryl-substituted nitrilium salt, that is,
They showed a linear conformation of the
1
[
25,26]
[
2,6-Me C H -NꢀC-Me][BF ].
2
6
4
4
To explore the reactivity of the nitrilium triflates, we
examined the reaction of 2a with primary phosphanes both
experimentally and computationally. Treatment of 2a with
phenyl- and cyclohexylphosphane for 10 minutes at À788C
afforded, quantitatively, the respective iminium triflates 3a
Scheme 3. Formation of 4c and 5c (top) and the computed isomers of
E/Z-4c and E/Z-5c at wB97X-D/6-31+G(d,p) level of theory (bottom).
3
1
1
31
and 3b [3a: d P = À50.5 ppm, J(P,H) = 255.6 Hz; 3b: d P =
NEt3 afforded two isomers of the iminophosphane 4c in
1
31
1
À29.0 ppm, J(P,H) = 243.2 Hz] by P-nucleophilic attack on
a
3:2 ratio [d P: À55.3 ppm, J(P,H) = 243.2 Hz and
1
the nitrilium ion followed by a [1,3] H-shift from phosphorus
to the more basic nitrogen site (Scheme 2). Subsequent
À56.7 ppm, J(P,H) = 252.9 Hz; Scheme 3]. Both isomers
tautomerized under the reaction conditions to the amino-
phosphaalkene 5c, which was isolated as a yellow oil in 88%
yield [d P = 102.0, d H = 5.26 (NH) ppm].
at the wB97X-D/6-31 + G(d,p) level of theory showed (Z)-
3
1
1
[28,29]
Calculations
[27]
5
8
c to be the thermodynamic product of this reaction [DG =
.1 ((E)-4c), 4.6 ((Z)-4c), 9.0 ((E)-5c), and 0.0 ((Z)-5c)
À1
[30]
kcalmol ; Scheme 3, bottom], and compares well with the
reported (Z)-DippP=C(p-CH C H )N(H)Dipp (Dipp = 2,6-
3
6
4
[
20b]
iPr C H ) by Boerꢂ and co-workers.
These findings
2
6
3
illustrate that a diverse set of P,N ligands is accessible by
simply changing the substituents.
Having the phosphaamidines 4a,b and 5c in hand, we
explored the synthesis and coordination chemistry of the
corresponding phosphaamidinates. Treatment of either 4a or
5
c with n-butyl lithium at À788C in THF provided the
orange-colored 6 in good yield upon isolation (6a: 86%,
d P = 10.9 ppm; 6c: 66%, d P = 38.6 ppm; Scheme 4). The
molecular structure of the Mes*-derivative 6c (Scheme 4),
3
1
31
Scheme 2. Synthesis of the phosphaamidines 4a,b and molecular
structures of 4a,b (hydrogen atoms are omitted for clarity, except for
H1, one crystallographic independent molecule of 4b is shown, and
displacement parameters are drawn at 50% probability level). 4b
crystallizes with two crystallographic independent molecules in the
asymmetric unit of which the values of the second are within square
brackets. Selected bond lengths [ꢀ] and torsion angles [8] for 4a:
P1–C1 1.8748(13), P1–H1 1.325(15), N1–C1 1.2720(16), N1–C6
[24]
obtained by a single-crystal X-ray structure determination,
shows an N-coordinated lithium ion [Li1-N1 2.013(4) ꢁ]
bearing two molecules of thf and an interaction with the Mes*
ring [i.e., Li1-C12 2.599(4) ꢁ]. The elongated Z-configured
P=C bond [P1-C1 1.750(2) ꢁ] and shortened NÀC bonds [N1-
C1 1.327(3) ꢁ] are indicative of a delocalized 1-aza-3-
[20a]
1
.4159(16); P1-C1-N1-C6 À1.21(17). 4b: P1–C1 1.8631 (13) [1.8648
phosphaallyl system as was highlighted by Niecke et al.
(
15)], P1–H1 1.330(14) [1.324(14)], N1–C1 1.2760(16) [1.2736(18)],
Our next step was to establish the coordination modes for
the phosphaamidinate ligand with transition metals, since no
N1–C6 1.4166(17) [1.4115(18)]; P1-C1-N1-C6 À0.72(18) [-0.39(19)].
addition of triethylamine at À788C afforded, after work-up, in
[
31]
such complexes have been reported in the literature. A soft
metal should favor P coordination, which is indeed the case,
but with an unexpected result. Reaction of 6c with
[AuCl(tht)] at room temperature afforded 7 as yellow crystals
3
1
good yield the secondary iminophosphanes 4a [85%; d P =
1
31
À64.7 ppm, J(P,H) = 231.9 Hz] and 4b [79%; d P =
1
31
À42.7 ppm, J(P,H) = 220.8 Hz] as colorless solids. Single-
(66%, d P = À7.8 ppm; Scheme 4). Its molecular structure
crystal X-ray diffraction analysis revealed unequivocally a Z-
configurated imine (Scheme 2) with C=N bond character [4a:
displays a unique gold(I) phosphanyl trimer with a six-
membered {P Au } core [e.g., Au1-P1 2.354(2) ꢁ] and three
3
3
1
1
.2720(16), 4b: 1.2736(18) ꢁ] and a typical PÀC bond [4a:
noncoordinating E imine substituents [e.g., N1-C1 1.269(9);
Scheme 4]. The P-Au-P angles are distorted from linearity
[
24]
[24]
.8748(13), 4b: 1.8648(15) ꢁ]. The E conformers of 4a,b
3
1
were not detected by P NMR spectroscopy, and concurs with
[e.g., P1-Au1-P3 155.35(7)8] and the {Au } triangle features
3
[
27]
[32]
the DFT calculations at the wB97X-D/6-31 + G(d,p) level
of theory, and indicates these E isomers to be the least stable
Au···Au distances of 3.0168(6), 3.0945(6), and 3.1030(6) ꢁ,
which are similar to the ones found for [{Au(PIs )} ] (Is =
2
3
À1
[33]
ones [DGE-Z = 5.4 (4a), 2.2 (4b) kcalmol ].
2,4,6-iPr C H ) as reported by Glueck et al.
3 6 2
Treatment of the nitrilium triflate 2a with 2,4,6-tri-tert-
Next, we targeted the P,N-chelating motif for transition
metals, as this is the predominant bonding mode in the
[34]
butylphenylphosphane (Mes*PH ) and subsequently with
2
Angew. Chem. Int. Ed. 2014, 53, 9068 –9071
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim