Reaction of a diarylgermylene with a phosphaalkyne: Formation of a germadiphosphacyclobutene with an exocyclic C=Ge double bond

Article abstract of DOI:10.1039/b008472g

The reaction of bis(2-tert-butyl-4,5,6-trimethylphenyl)germylene (Ar₂Ge:) with tert-butylphosphaalkyne furnishes a germadiphosphacyclobutene derivative with an exocyclic -C(Bu^t)=GeAr₂ group at one phosphorus atom, which was characterised by an X-ray structure analysis.

Full text of DOI:10.1039/b008472g

Reaction of a diarylgermylene with a phosphaalkyne: formation of a
germadiphosphacyclobutene with an exocyclic CNGe double bond
Frank Meiners, Wolfgang Saak and Manfred Weidenbruch*
Fachbereich Chemie, Universität Oldenburg, D-26111 Oldenburg, Germany
Received (in Liverpool, UK) 16th October 2000, Accepted 13th December 2000
First published as an Advance Article on the web
The reaction of bis(2-tert-butyl-4,5,6-trimethylphenyl)ger-
mylene (Ar2Ge:) with tert-butylphosphaalkyne furnishes a
germadiphosphacyclobutene derivative with an exocyclic
–C(But)NGeAr2 group at one phosphorus atom, which was
characterised by an X-ray structure analysis.
The addition of dialkyl-silylenes and -germylenes to the P·C
triple bonds of thermally stable phosphaalkynes represents a
simple method for the synthesis of three-membered ring
systems containing a PNC double bond that are difficult to
prepare by other routes. For example, the silylene R2Si: (R =
CMe3) reacts smoothly with phosphaalkynes to afford the
corresponding phosphasilirenes.1 The dialkylgermylene 12,3
also undergoes a [2 + 1] cycloaddition reaction with tert-
butylphosphaalkyne from which the germaphosphirene 2 can be
isolated (Scheme 1).4 The diarylsilylene Mes2Si: behaves
differently and furnishes a phosphadisilacyclobutene via step-
wise addition of two silylene molecules to the phosphaalk-
yne.5
Fig. 1 Molecular structure of 5 (50% probability ellipsoids, hydrogen atoms
omitted). Selected bond lengths (Å) and angles (°): P(1)–P(2) 2.1703(14),
P(2)–C(6) 1.702(4), C(6)–Ge(2) 1.982(4), P(1)–Ge(2) 2.4351(9), P(1)–C(1)
1.822(3), C(1)–Ge(1) 1.833(4); P(2)–P(1)–Ge(2) 78.73(4), P(1)–Ge(2)–
C(6) 79.15(11), Ge(2)–C(6)–P(2) 105.0(2), C(6)–P(2)–P(1) 93.39(15).
bonds. The remaining bond lengths and angles also did not
reveal any unusual features.
Scheme 1
The unexpected formation of 5 is without precedence in the
chemistry of germylenes and phosphaalkynes. It is, at best,
comparable with the addition of the germylene 4 to 1,3-diynes,
a process proceeding through the C–C bridged bis(germacyclo-
propenes) 6 as the intermediate on the way to the rearranged,
acetylene-linked bis(germaethenes) 7 (Scheme 3).8 In analogy
to the formation of 7, it may be assumed that here also the
reaction sequence is initiated by the addition of 4 to the
phosphaalkyne to afford a three-membered ring system of type
8 with subsequent opening of the P–Ge single bond. Cyclo-
dimerisation of this intermediate would then yield compound
5.
A cyclodimerisation of this type should produce several
conformers with varying orientations of the substituents on the
exocyclic C atom. In fact, the 31P NMR spectrum of 5 contains
two doublets for two-coordinate phosphorus atoms at d 269.0
and 271.5 as well as two doublets in the high-field region at d
28.1 and 234.4. These signals remain unchanged even on
heating a sample to 80 °C, presumably on account of the steric
crowding at the exocyclic carbon atom. The existence of two
conformers in solution is further supported by the observation
We have now addressed the question if, similar to the
silylenes, the use of a diarylgermylene would furnish a different
result. Thus, from the reaction of the germylene 4 (which, in
analogy to 1,6 exists as the digermene 3 in the solid state)7 with
tert-butylphosphaalkyne gave orange-coloured crystals which
were isolated in 59% yield. The analytical data for these crystals
were indicative of a 1+1 adduct composed of 4 and the
phosphaalkyne. However, the 31P NMR spectrum revealed the
presence of both two- and three-coordinated phosphorus atoms,
thus excluding the formation of a three-membered ring system
analogous to 2 and suggesting the presence of a larger ring
system (Scheme 2).
In agreement with the analytical and spectral data, an X-ray
crystallographic analysis (Fig. 1)‡ revealed that two molecules
each of the phosphaalkyne and 4 had reacted to furnish
compound 5† comprised of a germadiphosphacyclobutene with
an additional, exocyclic GeNC double bond. The bond lengths
of both the exocyclic GeNC and the endocyclic PNC double
bonds were in accord with those of previously reported, similar
Scheme 2
Scheme 3
DOI: 10.1039/b008472g
Chem. Commun., 2001, 215–216
This journal is © The Royal Society of Chemistry 2001
215

1
of numerous, in part overlapping, signals in the H and 13C
was solved by direct methods (SHELXS-97) and refined by the full-matrix
least-squares techniques against F2 (SHELXL-97). The tert-butyl groups
C2–C5 and C7–C10 are disordered and were refined on two positions with
an occupancy factor of 0.5 each. Hydrogen atoms were placed in the
calculated positions, and all other atoms were refined anisotropically; R1 =
0.0455, wR2 (all data) = 0.1064. CCDC 186/1877. See http://www.rsc.org/
suppdata/cc/b0/b008472g/ for crystallographic files in .cif format
NMR spectra that cannot be assigned unambiguously. The 1JPP
coupling constants of 16 and 21 Hz for the two conformers are
inexplicably small; related ring systems9 have values of ca. 250
Hz.
Financial Support by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully acknowl-
edged. We thank Professor Marsmann, Paderborn, for the 31P
NMR spectra.
1 A. Schäfer, M. Weidenbruch, W. Saak and S. Pohl, Angew. Chem., 1987,
99, 806; Angew. Chem., Int. Ed. Engl., 1987, 26, 776.
2 D. E. Goldberg, D. H. Harris, M. F. Lappert and K. M. Thomas, J. Chem.
Soc., Chem. Commun., 1976, 261.
Notes and references
† Preparation of 5: to a suspension of 3 (0.35 g, 0.41 mmol) in n-hexane (20
mL) was added a solution of tert-butylphosphaalkyne (0.30 g, 3.0 mol) in n-
hexane (20 mL) and the mixture was stirred for 24 h at room temperature.
The mixture was then filtered, the filtrate concentrated to a volume of 20
mL, and cooled to 230 °C to furnish 0.255 g (59% yield) of orange crystals
of 5, mp 118–120 °C. 31P{1H} NMR: d 271.5, 234.4 (1JPP 16 Hz), 269.0,
28.1 (1JPP 21 Hz). UV–VIS (THF): lmax/nm (e/dm3 mol21 cm21) 366
(29000) nm. Anal. Calc. for C62H94Ge2P2: C, 71.16; H, 9.05. Found: C,
71.27, H, 9.22%.
3 P. J. Davidson, D. H. Harris and M. F. Lappert, J. Chem. Soc., Dalton
Trans., 1976, 2268.
4 A. H. Cowley, S. W. Hall, C. M. Nunn and J. M. Power, J. Chem. Soc.,
Chem. Commun., 1988, 753.
5 M. Weidenbruch, S. Olthoff, K. Peters and H. G. von Schnering, Chem.
Commun., 1997, 1433.
6 P. B. Hitchcock, M. F. Lappert, S. J. Miles and A. J. Thorne, J. Chem.
Soc., Chem. Commun., 1984, 480.
7 M. Weidenbruch, M. Stürmann, H. Kilian, S. Pohl and W. Saak, Chem.
Ber./Recueil, 1997, 130, 735.
8 F. Meiners, W. Saak and M. Weidenbruch, Organometallics, 2000, 19,
2835.
9 F. E. Hahn, L. Wittenberger, D. L. Van, R. Fröhlich and B. Wibbeling,
Angew. Chem., 2000, 112, 2393; F. E. Hahn, L. Wittenberger, D. L. Van,
R. Fröhlich and B. Wibbeling, Angew. Chem., Int. Ed., 2000, 39, 2307.
‡ Crystal data for 5: C62H94Ge2P2·n-C6H14, M
= 1132.66, crystal
¯
dimensions 0.32 3 0.21 3 0.15 mm, triclinic, space group P1, a =
12.1315(6), b = 15.3937(8), c = 17.6016(7) Å, a = 87.207(5), b =
80.276(5), g = 87.056(6)°, V = 3232.9(3) Å3, Z = 2, Dc = 1.164 g cm23
,
l(Mo-Ka) = 0.71073 Å, T = 193(2) K, 2qmax = 52°, 11735 unique
reflections, 6979 observed [I > 2s(I)] 6979, 624 parameters. The structure
216
Chem. Commun., 2001, 215–216