Angewandte
Chemie
concurrent increase of only three new signals in a 1:1:2 ratio
at, respectively, À125.7 (P1), À153.3 (P4) and À322.4 ppm
(P2,3) (AMX2 spin system). From the reaction mixture, 46%
of white solid 1a could be isolated that is storable under inert
conditions.
The molecular structure of 1a obtained by an X-ray
crystal structure determination (Figure 1)[15] shows an RP4
butterfly anion (fold angle = 93.61(3)8) capped with a tolu-
ene-complexed lithium cation that is coordinated to the P1
Figure 2. Molecular structure of 1b in the crystal[15] (ellipsoids are set
at 50% probability; hydrogen atoms, non-coordinated benzene solvent
molecules, mesityl rings, and two C6F5 rings are omitted for clarity).
Selected bond lengths [ꢀ], angle, and torsion angle [8]: P1–P2/P3
2.2180(4)/2.2131(4), P4–P2/P3 2.2121(4)/2.2184(4), P2–P3 2.1793(4),
P1–P4 2.8251(5) C1–P1 1.8413(12), P4–B1 2.0692(14), Li1–P1/P4
2.623(2)/2.661(2), Li1–F3i 2.149(3), P1-Li-P4 64.64(6), P1-P2-P3-P4
94.16(2). Symmetry code (i): 1Àx, 1Ày, 1Àz.
À
observed P4 B1 bond length (2.0692(14) ꢁ) and its com-
puted strength (23.8 kcalmolÀ1) are similar to those of 1a.
Bicyclo[1.1.0]tetraphosphabutanes 1a and 1b are to the
best of our knowledge the first isolated monofunctionalized
P4 anions. B3LYP/6-311 + G(d)//6-31 + G(d) calculations
reveal 1b to result from nucleophilic attack of DmpLi onto
Figure 1. Molecular structure of 1a in the crystal[15] (ellipsoids are set
at 50% probability; hydrogen atoms are omitted for clarity). Selected
bond lengths [ꢀ], angle, and torsion angle [8]: P1–P2/P3 2.2166(6)/
2.2286(6), P4–P2/P3 2.2031(6)/2.2177(7), P2–P3 2.1694(7), P1–P4
2.8183(7), C1–P1 1.8628(17), P4–B1 2.064(2), Li1–P1/P4 2.677(4)/
2.504(4); P1-Li1-P4 65.81(8); P1-P2-P3-P4 93.61(3).
P4, causing a 20.6 kcalmolÀ1 exothermic scission of one P P
À
bond that is further enhanced (by 23.8 kcalmolÀ1) by addition
of the Lewis acid (Figure 3). No minimum could be found for
end-on or side-on coordination of B(C6F5)3 to P4 and neither
À
À
and P4 atoms. The P1 P2 (2.2166(6) ꢁ), P1 P3 (2.2286(6) ꢁ)
À
À
and P4 P2 (2.2031(6) ꢁ), P4 P3 (2.2177(7) ꢁ) bonds are
À
elongated compared to the bridgehead P2 P3 bond
(2.1694(7) ꢁ), which resembles those of neutral bicyclic
analogues.[16] The “Lewis” P4 B1 bond (2.064(2) ꢁ) is of
À
similar length to that of H3P·B(C6F5)3 (2.046(5) ꢁ[17]), but
B3LYP/6-311 + G(d)//6-31 + G(d) calculations[18] suggest it to
be 17 kcalmolÀ1 stronger (1a 23.0; H3P·B(C6F5)3 6.1 kcal
molÀ1), which is attributable to the higher basicity of the
anionic P4 fragment.
The slow conversion of P4 into 1a, caused by the low
solubility of Mes*Li in toluene, could not be improved by
longer reaction times and variation in temperature.[19] We
then decided to use DmpLi (Dmp = 2,6-dimesitylphenyl),
which readily dissolves in toluene. Reaction of a 1:1:1 mixture
of DmpLi, B(C6F5)3, and P4 at room temperature gave only 1b
within one hour and in 70% isolated yield (Scheme 2; d
31P{1H}: À154.6 (P1), À172.0 (P4), and À342.9 ppm (P2,3));
31P{1H} NMR spectroscopy showed that full conversion of P4
(instead of 84%) could be achieved on using 1.5 equivalents
of DmpLi and B(C6F5)3.
Figure 3. Calculated reaction pathway (E, kcalmolÀ1) of the reaction of
P4 with DmpLi and B(C6F5)3, and the HOMO of the calculated
geometry of monomer 1b.[18]
could such complexes be detected by 31P NMR spectroscopy.
This suggests a reaction sequence in which DmpÀ attacks P4
upon which the incipient anion is trapped by the Lewis acid.
In fact, no Dmp-substituted P4 anion could be detected
without the Lewis acid, not even at À788C, thereby further
exemplifying its trapping role; only a mixture of polyphos-
phides resulted, in analogy to the studies by Rauhut et al.[8]
To further functionalize P4, we targeted the alkylation of
1b, which would be expected to occur at the phosphorus atom
bonded to the B(C6F5)3 group, because it contributes most to
the HOMO (Figure 3). Indeed, adding MeI to a solution of 1b
in toluene caused a downfield shift of the three 31P{1H} NMR
resonances to À57.5 (P4), À154.2 (P1), and À296.4 ppm
Compound 1b crystallizes as a centrosymmetric dimer
(Figure 2) in which the Li+ cations are in tetrahedral environ-
ments[20] with intramolecular Li···F interactions (2.149(3) ꢁ)
that are much shorter than the sum of the van der Waals radii
(ꢀr(vdW)[Li, F] = 3.28 ꢁ[21]), but also much larger than the
bond length computed for lithium fluoride (1.595 ꢁ[22,23]); the
Li···F interactions were not observed in solution, as no Li–F
7
coupling was detected in the Li and 19F NMR spectra. The
Angew. Chem. Int. Ed. 2014, 53, 12836 –12839
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