New aspects of 1,3-diphosphacyclobutane-2,4-diyls

Article abstract of DOI:10.1002/hlca.201200442

1,3-Di(tert-butyl)-2,4-bis[2,4,6-tri(tert-butyl)phenyl]-1, 3-diphosphacyclobutane-2,4-diyl was formed from [2,4,6-tri(tert-butyl)phenyl] phosphaacetylene and t-BuLi. In addition, the X-ray diffraction analysis was carried out, together with theoretical ca

Full text of DOI:10.1002/hlca.201200442

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New Aspects of 1,3-Diphosphacyclobutane-2,4-diyls
by Masaaki Yoshifuji*a)b)c), Yuichi Hiranoa)d), Gregor Schnakenburga), Rainer Streubela),
Edgar Nieckea), and Shigekazu Itoe)
a) University of Bonn, Institute for Inorganic Chemistry, Gerhard-Domagk-Str. 1, D-53121 Bonn
b) The University of Alabama, Tuscaloosa, AL 35487-0336, USA
c) Tohoku University, Graduate School of Science, Department of Chemistry, Aoba, Sendai 980-8578,
Japan (e-mail: masaaki@yoshifuji.org)
d) Hiroshima University, Graduate School of Science, Department of Chemistry, Kagamiyama,
Higashi-Hiroshima 739 – 8526, Japan
e) Tokyo Institute of Technology, Department of Applied Chemistry, Graduate School of Science and
Engineering, Ookayama, Meguro, Tokyo 152-8552, Japan
Dedicated to Prof. Dr. Dieter Seebach on the occasion of his 75th birthday
1,3-Di(tert-butyl)-2,4-bis[2,4,6-tri(tert-butyl)phenyl]-1,3-diphosphacyclobutane-2,4-diyl was formed
from [2,4,6-tri(tert-butyl)phenyl]phosphaacetylene and t-BuLi. In addition, the X-ray diffraction analysis
was carried out, together with theoretical calculations of the structure and NMR data.
Introduction. – In recent years, 1,3-diphosphacyclobutane-2,4-diyls 2a [1] and 2b [2]
have become available as stable species by using phospha-alkene 1 or phospha-alkyne 4
with the 2,4,6-tri(tert-butyl)phenyl (abbreviated as Mes*) as a sterically bulky
substituent. Compounds 2 are of interest in terms of bonding, physical properties,
and chemical reactivity. Anionic systems 3a [3] and 3b [4] – derived from 2a or being
precursor of 2b – (Scheme 1) have been established as well and shall be used as
building blocks.
Scheme 1
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

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To gain better understanding of the bonding and properties of biradicals, the first
attempt was made to prepare a multi-biradical system such as 2c (Scheme 2) via
reaction of 1,3-diphosphacyclobutenyl anion 3b with 2a. Multi-biradicals, for which
interesting chemical and physicochemical features might be expected, are different
from the individual 2a and/or 2b units that possess singlet ground states. Oligomeric (or
polymeric) biradicals might serve as chemicals for hydrogen storage [5][6], oxidation/
reduction [7], and UV/VIS and CV-featured substance [8]. Accumulation of these
characters in one molecule [8 – 10] may lead to interesting materials in view of
electrochemical, electromagnetic, and optophysical properties.
Scheme 2
Results and Discussion. – Biradical 2a was reacted with 3b to afford a blue
crystalline product. Based on preliminary spectroscopic analysis of this product,
including NMR, UV/VIS, and CV, 2d was assumed to be formed instead of the desired
mixed multi-biradical system 2c, mainly due to the 1H-NMR spectrum, in which the Me
signals appeared as a triplet that was reasonably assigned to an AX2 spin system for 2d.
However, X-ray crystallographic analysis of the compound unambiguously confirmed
that the product was 2e [4], revealing that the desired framework of compound 2a was
not incorporated. Furthermore, in an attempt to get further insight into the assumed
structure of 3b, a deep colored crystal from the concentrated solution of 3b in Et2O in
the presence of 12-crown-4 was submitted to X-ray analysis, but again the structure
turned out to be that of 2e. Observation of a small amount of 4 in the 31P-NMR
spectrum indicated an equilibrium between 3b and the starting material, although the
equilibrium might be far right-shifted and product-oriented. A solution of the anion
formed under the same conditions in the presence of the crown ether followed by
quenching the reaction with MeI, gave the expected product 2b. Based on the finding
that no evidence was available for the source of an electrophilic t-Bu group in the 2e
formation besides 3b itself (even if t-BuLi remained in a slight excess, it was supposed
to serve as a nucleophile), transformation might occur during the isolation process of
3b involving concentration. Scheme 3 displays a proposed reaction mechanism: 3b can
serve as a nucleophile and an electrophile to give 2e and a cyclic dianion 3c. It should be
noted that 2e was alternatively obtained from anion 3b with t-BuI in good yield [4]. In

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addition to the formation of 2e, 5 [11] was also obtained in a small amount as a result of
protonation product of 3b based on the 31P-NMR analysis. Although attempts to
isolate 3c have failed so far, probably due to the thermal instability, concentration of a
THF solution of 2e generated a thermally unstable compound exhibiting a 31P
resonance at 175 ppm together with 4, 5, and phospha-alkenes (Mes*C(H)¼P(t-Bu),
d(P) 281 and 292 ppm). The peak at d(P) 175 ppm might be responsible to that of the
DFT-optimized 3c1).
Scheme 3
Attempts to achieve a ꢂsimpleꢃ halogen/metal exchange on the ring of 2a did not
seem to take place. In fact, 2a gave diphosphabutadiene 6 after reaction with t-BuLi,
followed by MeI quenching, suggesting that the process includes the C2P2 ring cleavage
during the reaction, and no incorporation with MeI was observed.
X-Ray Analysis. The structure of compound 2e was unambiguously confirmed by X-
ray crystallography (Fig. 1)2). It is thus confirmed that the two t-Bu groups are
attached to the P-atoms and not to the C-atoms of the four-membered ring.
The sample turned out to be non-merohedrally twinned. The twin fractions
(0.67:0.33) are connected via the refined twin law ( ꢀ10.19394 0.00064 ꢀ1.19452
0.002000 ꢀ1.00000 ꢀ0.00393 ꢀ0.80567 ꢀ0.00044 0.19394). Furthermore, a slight
disorder of the P-atoms could be found and solved. The main orientation (ꢂPꢃ, and ꢂP#ꢃ;
85%) is depicted in non-transparent purple color, whereas the side occupation (ꢂPsꢃ,
ꢂPs#ꢃ; 15%) is displayed in faded pink.
The twinning and disorder problems could be solved, thus the outcome is a good
and publishable structure. The Table lists selected bond lengths and angles. In spite of
the disorder of the compound, it is confirmed that C2P2 ring is almost coplanar, and that
1
)
)
A GIAO calculation for 3c, optimized by DFT method [B3PW91/6-31 þ G(d)], was performed at
the wB97XD/6-31 þþ G(2d,p) level to estimate 31P chemical shifts, d(P), of 152.00 and 152.07 ppm.
Crystallographic data of 2e: C46H76P2: Mr 691.91, orthorhombic, Pccn (#56), a ¼ 14.2075(9), b ¼
2
25.6989(16), c ¼ 11.6699(8) ꢄ, V ¼ 4260.9(5) ꢄ3, Z ¼ 4, T¼ 100(2) K, rcalc ¼ 1.077 g cmꢀ3
,
m(MoKa) ¼ 0.131 mmꢀ1, 2 qmax ¼ 50.58, F000 ¼ 1528, 4450 observed reflections, 3766 unique reflec-
tions (Rint ¼ 0.086), R1 ¼ 0.062 (I > 2s(I))/0.074 (all data), wR ¼ 0.157 (I > 2s(I))/0.172 (all data),
S ¼ 1.132. CCDC-878538 contains the supplementary crystallographic data for this work. These data
can be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.

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Helvetica Chimica Acta – Vol. 95 (2012)
Fig. 1. Molecular structure of 2e by X-ray analysis (only the major structure is depicted)
the two P-atoms have a pyramidal configuration, which is a typical structure for
diphosphacyclobutanediyls.
Table. Selected Bond Lengths [ꢄ] and Angles [8] of 2e
Bond
DFT Calculation
X-Ray Analysis
Angle
DFT Calculation
X-Ray Analysis
P1ꢀC1
P1ꢀC2
P2ꢀC1
P2ꢀC2
P1ꢀC3
P2ꢀC4
C1ꢀC5
C2ꢀC6
1.7568
1.7633
1.7623
1.7565
1.9016
1.9022
1.4830
1.4831
1.751(3)
1.766(3)
1.766(3)
1.751(3)
1.918(4)
1.918(4)
1.490(4)
1.490(4)
C1ꢀP1ꢀC2
C1ꢀP2ꢀC2
P1ꢀC1ꢀP2
P1ꢀC2ꢀP2
Sangles(P1)
Sangles(P2)
Sangles(C1)
Sangles(C2)
91.63
91.67
88.36
91.4(1)
91.4(1)
88.6(1)
88.6(1)
334.6
334.6
357.8
357.8
88.34
332.63
332.70
357.38
357.35
Optimum Structure. A theoretical calculation was carried out for 2e to indicate that
the optimum structure for the compound is almost identical to that of the real
compound, as shown in Fig. 2 and in the Table [12][13]. TD-DFT Calculation [M06-
2X/6-31G(d)] characterized HOMO to LUMO (593.77 nm, f ¼ 0.0476), HOMO to
LUMO þ 1 (344.90 nm, f ¼ 0.2475), and HOMOꢀ1 to LUMO (324.21 nm, f ¼ 0.1896)
transitions, which is comparable to the experimentally observed UV/VIS spectrum.
NMR Calculation. NMR Parameters were calculated using the GIAO program
[14][15] at the B3LYP/6-31G(2d,p)//M06-2X/6-31G(d) level of theory. 31P-NMR
Chemical shifts of the compound were calculated as 38.06 and 38.08 ppm, and
13C-NMR shifts for the ring C-atoms were both as 120.7 ppm. Although the coupling
constants between H- and P-atoms are not calculated for a molecule with free rotation,
3
in some cases J values are smaller than those of 5J, which might be due to the
appearance of a pseudo-triplet signal of H-atoms of t-Bu moiety directly bound to the P-
atom for 2e.
Conclusions. – We have been successful in obtaining further important insights into
the biradical system 2, including the features of anion 3b and the X-ray structure

Helvetica Chimica Acta – Vol. 95 (2012)
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Fig. 2. Optimum structure of 2e by calculation [M06-2X] on the basis of 6-31G(d)
analysis of 2e as a biradical di(tert-butyl)bis[2,4,6-tri(tert-butyl)phenyl] derivative.
These findings will not only provide basic and useful information to the R&D
community in main-group element chemistry, but will also enable us to pursue some of
the proposed lines of research.
Financial support from the Alexander von Humboldt Foundation is gratefully acknowledged
providing M. Y. and Y. H. for their research stay in Bonn (August to October 2011). R. S. is also grateful
to the COST action cm0802 ꢂPhoSciNetꢃ.
Experimental Part
Preparation of Starting Materials. Diphosphacyclobutanediyl 2a was prepared from dichlorophos-
pha-alkene 1 according to a method described in [1]. To a soln. of 1 (300 mg, 0.835 mmol) in THF (10 ml)
at ꢀ 1008 was added dropwise BuLi (0.29 ml, 0.46 mmol). The soln. was stirred between ꢀ 1008 and
ꢀ 808 for 1 h, and then warmed to r.t. overnight, while the color of the soln. changed from yellow to dark
red. After removal of the solvent in vacuo, to the crude was added pentane (15 ml ꢁ 4), and the soln. was
collected through a filter syringe, then the solvent was removed in vacuo to give 2a (220 mg, 0.343 mmol,
81%). 1H-NMR (CDCl3, 300 MHz): 7.45 (s, 4 H); 1.73 (br. s, 9 H); 1.33 (br. s, 9 H); 1.26 (s, 9 H). 31P{1H}-
NMR (CDCl3, 121 MHz): d 27.8 (s).
Diphosphacyclobutenyl anion 3b was prepared as described in [2]. To a soln. of 4 (87 mg,
0.30 mmol) in THF (2 ml) at ꢀ 788 was added dropwise t-BuLi (0.10 ml, 0.16 mmol). The mixture was
stirred at ꢀ 788 for 15 min, then at r.t. for 1 h; in the meantime, the color of the soln. changed from yellow
to dark blue via dark green.
Attempted Reaction of 2a with 3b. To a soln. of 2a (195 mg, 0.301 mmol) in THF (10 ml) at ꢀ 788 was
added 3b (0.66 mmol; 3b was prepared from a soln. of 4 (381 mg, 1.32 mmol) in THF (9 ml) and t-BuLi
(0.44 ml, 0.70 mmol) as described above). The soln. was stirred at ꢀ 788 for 1 h, then warmed to r.t.
overnight (the color of the soln. changed from red brown to dark red). After recording 31P-NMR,
removal of the solvent in vacuo, and then NMR recordings, CH2Cl2 (2 ml) was added to the residue, and
the mixture was kept at 08. After filtration through a filter syringe at 08, the residue was purified by
recrystallization from CH2Cl2 (8 ml) at 48 to give blue-purple crystals of 2e. M.p. 157 – 1588 (dec.). UV
(CH2Cl2): lmax (e) 638.5 (932), 524.0 (621), 389.0 (7358), 340.5 (10258), 229.5 (29110). The soln. color of
2e changed to blue-purple to green (lmax 700; 638.5-nm band disappeared). HR-EI-MS: 690.5417

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2
ox
1
ox
(C46H70P2; calc. 690.5422). CV:
E
¼ 0.50 V (looks like irreversible),
E
¼ 0.15 V (reversible),
1=2
1=2
1Eopx ¼ ꢀ1.0 V (irreversible).
Attempted Isolation of 3b. To a soln. of 4 (75 mg, 0.26 mmol) and 12-crown-4 (0.030 ml, 0.18 mmol)
in Et2O (2 ml) at ꢀ 788 was added dropwise t-BuLi (0.090 ml, 0.16 mmol). The mixture was stirred at
ꢀ 788 for 15 min, then at r.t. for 1 h. The color of the soln. changed from yellow to dark blue via dark
green. After removal of the solvent in vacuo, to the crude were added pentane (2 ml) and then Et2O
(4 ml). Recrystallization at 48 gave 2e (blue crystals), the structure of which was confirmed by X-ray
structure analysis.
Time-Dependent Change of 3b (31P-NMR Study). To a soln. of 4 (93 mg, 0.32 mmol) in THF (2 ml)
at ꢀ 788 was added dropwise t-BuLi (0.11 ml, 0.18 mmol). The mixture was stirred at ꢀ 788 for 15 min
and then at r.t. for 1 h (the color of the soln. started to change from yellow to dark blue via dark green).
Reaction monitoring was carried out by recording 31P-NMR (0.6 ml of soln., which was taken from the
reaction mixture after 1 h into a Young NMR tube); after 1 h, 1 d, and 5 d. From the NMR spectra, the
ratio (3b/2e 99 :1 after 1 h) indicated an equilibrium between 3b and 2e (3b/2e 93 :7 after 1 d) in soln.
After 5 d, the ratio did not change.
Data of 3b. 31P{1H}-NMR (THF, 121 MHz): 267.5 (d, J ¼ 87); 86.8 (d, J ¼ 87).
Data of 2e. 31P{1H}-NMR (THF, 121 MHz): 39.0 (s).
Preparation of an Authentic Sample of 2e. To a soln. of 4 (87 mg, 0.30 mmol) in THF (2 ml) at ꢀ 788
was added dropwise t-BuLi (0.18 ml, 0.29 mmol). The mixture was stirred at ꢀ 788 for 15 min and then at
r.t. for 1 h. The color change of the soln. was observed as described above. To the soln. was added t-BuI
(0.040 ml, 0.34 mml) at r.t., and the mixture was stirred for 2 min. After removal of the solvent in vacuo,
to the crude was added pentane (10 ml), and the soln. was filtered through a filter syringe. The solvent
was removed in vacuo, and the residue was recrystallized from CH2Cl2 (4 ml) at 48 to give 2e (22 mg,
0.031 mmol, 12%). Blue crystals. 1H-NMR (CDCl3, 300 MHz): 7.24 (s, 4 H); 1.57 (s, 36 H); 1.22 (s, 18 H);
0.88 (t, J ¼ 7, 18 H). 31P{1H}-NMR (CDCl3, 121 MHz): 38.7 (s).
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Received August 10, 2012