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compared to biphenylene (d,,,, = 278, 248 r ~ m ) ~and
' ~ dimethylbiphenylene (1,w1,8-mixture, d,,~,,=357, 254 nm).
This is an indication of perturbation of the n-electron system; analogous perturbations were observed in the transition from naphthalene to 1,8-methan0naphthalene.~'~~~
I t has been shown by MNDO calculations['o1that the
spatial structure of the molecule 3 is consistent with the
results of the X-ray crystal structure analysis. To ensure
reaching as near as possible the absolute energy minimum,
the structure was optimized in three ways: a) simultaneous
optimization of all bond lengths and angles, using approximate standard values for bond lengths and angles as starting values; b) simultaneous optimization of all bond
lengths and angles using the structural data determined by
an X-ray structure analysis as starting values; c) stepwise
optimization, involving successive optimization of the carbon skeleton of the benzene ring, then the sulfur bridge,
and then the H atoms, and repeating this procedure in the
same succession after each individual optimization. In addition, optimization was begun with a U-shaped biphenylene skeleton, to make certain that the optimization led to
a planar carbon skeleton. All three complete optimizations
afforded the values listed in Table 1 . Good agreement is
found between experimental and calculated values.
large distortion of the four-membered ring. From the existence of 3 and 4 it can be safely predicted that further
three-membered bridged biphenylenes such as 3 (X =0,
NR, CR,) with even higher ring strain and larger distortions could be synthesized.
Table I Calculated bond lengths [pml and angles ["I of 3 and 5 and, for
comparison. the values obtained by an X-ray structure analysis of 3 [a].
[ I ] J . K. Fawcett, J. Trotter, Acta Crys/al/ogr.20 (1966) 87. A. Yokozeki, C .
F. Wilcox, Jr., S. H. Bauer, J . Am. Cliem. Soc. 96 (1975) 1026.
[2] For the ring strain of biphenylene see. M. Mulin, 2. Nafurforsch.5 2 8
(1973) 478, where a value of 364 kJ/mol is quoted.
131 A longer bridge with four C atoms does not lead to any significant
changes in the bond lengths: cf. C . F. Wilcox, Jr., J. P. Uetrecht, G. D.
Grantham, K. G. Grohmann, J . Am. Chem. SOC.97 (1975) 1914: C . F.
Wilcox, Jr., D. A. Blain, J. Clardy, G. Van Duyne, R. Gleiter, M. EckertMakric, h i d . 108 (1986) 7693.
141 Cf. E. Hammerschmidt. W. Bieber. F. Vogtle, Clrenr Ber I 1 1 (1978)
2445. For further details of cesium-assistance cf. W. Kissener, F. Vogtle,
Angew. Chem. 97 (1985) 782: Anyen. Cliem. In/. Ed. Enyl. 24 (1985)
151 J. Sandstrom: Dynamic N M R Sprcrro.wip~.Academic Pres\, New York
[6] Further details of the crystal structure investigation of 3 and 4 are available on requesl from the Fachinformationszentrum Energte. Physik.
Mathematik GmbH, D-7514 Eggenstein-Leopoldshafen 2 (FRG) on
quoting the depository number CSD-52236, the names of the authors,
and the journal citation.
171 UV-Atlas of Organic Compounds, Vol. 2, Verlag Chemie, Weinheimi
Butterworths, London 1966.
[S] Cf. R. J. Bailey, H. Shechter, J Am C/rem. Soc. Y6 (1974) X I 16.
[9] The photoelectron spectrum of 3 can, as expected, be interpreted as a
superposition of the biphenylene spectrum and a sulfide band. We
thank Prof. Dr. E . Heilbronner. Basel (Switzerland), for collaboration.
[lo] M. J. S. Dewar, W. Thiel, J. Am. Chem. SOC.YY (1977) 4899. 4907.
3 (calc.)
5 (calc.)
3 (exp.)
1 I6
134.2- 135.6
138.2- 138.4
135.5- 136.4
140.9- 141.1
C 1 -CZ
C9-c I I
C8b-C I-C2
I I6
120.3- I 2 1.2
123.6- 124.4
115.2-1 15.4
127. I - 128.0
91.3- 92.3
[a] Deviations between the values calculated: I pm and 1-3", resp.; mean
deviations of the experimental values: 1-3 pm and 1-2", resp.
The hypothetical hydrocarbon skeleton 5 was also calculated in this way (Table l). A further shortening of the
C8a-C8b bond and a further lengthening of the C4a-C4b
bond are accompanied by a n even greater distortion of the
benzene ring (angles between 107 and 139"). According to
calculations, the bond between the two "bracket" carbon
atoms C9 and C11 is unusually long (161 pm).
If the enthalpy of formation AH,, is used as a measure of
the stability (or the ring strain), it follows from the calculated values AH,,=395 kJ/mol for biphenylene 112' and
43 1 kJ/mol for 3 that 5 (with 586 kJ/mol) is a considerably
less stable system. On the basis of these data it is easy to
comprehend that all previous attempts to prepare 5 met
without success.
From the calculated atomic charges of the molecules 1,
3 and 5 it follows that the charge distribution does not
essentially alter upon bracketing 1 to give 3.
3 and 4 are the shortest bracketed biphenylene derivatives known so far. The comparatively slight additional
ring strain introduced thereby results in a surprisingly
Airyen,. Chenr. I t i f . Ed. Engl. 26 11987) No. 8
2-Thia[3]( 1,S)biphenylenophane 3 : A mixture of benzene (1.6 L), elhanol
(1.2 L) and cesium carbonate (3.00 g, 9.20 mmol) 141 contained in a 4 L threenecked flask was heated to boiling and treated dropwise and synchronously
under N? for 13 h with solutions of the 56:44 mixture (formed during the
preparation [3]) of 1.8- and 1,5-bis(bromomethyl)biphenylene (2.88 g,
8.59 mmol) [corresponding to ca. 1.60 g (4.80 mmol of pure (611 in benzene
(250 mL) and NalS.9HZ0 (1.91 g, 8.60mmol) in ethanol (250mL). After
cooling, the yellow solution was evaporated down and the insoluhle cesium
compounds removed by filtration through a frit The crude product was separated by column chromatography (SO2, cyclohexaneichloroform). Aside
from unchanged starting compound, a dimeric product (C2XH21,S?)
and the
disulfide compound 2 could be detected mass spectrometrically. The desired
monosulfide 3 (240mg, 24%) was obtained as yellow crystals. m.p. 142.1 4 4 T (from n-hexaneichloroform 2 : I).-90-MHz 'H-NMR (CDCI,, TMS
int.): 6=3.78 (s, 4 H , CHZ), 6.5-6.7 (m, 6 H , aromatic H).
4 . M.p. 233°C (from acetone/cyclohexane I : I ) : M , =242 0402, high resolution MS: 242.0396.
Received: December 22, 1986:
revised: February 18, 1987 [Z 2020 IE]
German version: Angew. Chem. 99 ( 1987) 459
Dihydrocyclobutafuran" *
By Norbert Miinzel and Armin Schweig*
The thermodynamics and kinetics of the equilibrium
gas-phase reaction (a) of o-quinodimethane 1 and dihy-
Prof. Dr. A. Schweig, DipLChem. N. Miinzel
Fachbereich Physikalische Chemie der Universitat
Hans-Meerwein-Strasse, D-3550 Marburg (FRG)
Theory and Application of Photoelectron Spectroscopy, Part I I I. This
work was supported by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen 1ndustrie.-Part 110: F. Diehl, A. Schweig, Angew. Chem. 99 (1987) 348: Angew Chem. In,. Ed. Engl. 26 (1987) 343.
0 VCH Verlag.~gesellsc/rafrmbH. 0-6940 Weinheim. 1987
057/~-/1R33/87/0505-/J471$ 02.50/0
47 I
drocyclobutabenzene 2 have already been the subject of
detailed studies." 'I The measured data1" suggest that 1
can be observed together with 2 either thermally at high
temperatures in equilibrium [equilibrium constants
K 5 X I =20.8 (measured1' 'I) and KXOIIcC=4
much more efficiently at low temperatures as intermediate
after the decomposition of a suitable starting compound in
a thermal flow reactor [rate constant (1-2)
k - z25"c = 3.3 s - ' ; this corresponds to a half-life z , , ~of ca.
0.2 s (estimated); reaction time f R in the reactor, e.g. in
combination with a UV photoelectron spectrometer, ca.
0. I s ' ~ ~The
] . latter strategy" 71 made it possible for the first
time to measure the photoelectron spectrum of 1
I n contrast, nothing is known about the analogous equilibrium reaction (b) between 2,3-dimethylene-2,3-dihydrofuran 3 and dihydrocyclobutafuran 4, apart from the de-
tection of 3 by trapping reactions, and MS and NMR
4, in particular, has remained unknown.
According to M N D O calculations, the standard reaction
enthalpy A r H e of the reaction 1-2 is -82.8 kJ mol-',
about 24.2 kJ rnol - below the experimental valuei3]
(mainly a result of the overestimated stability of the closed
form 2 coqpared to the open form 1 by the M N D O method). For the reaction 3-4 the M N D O method gives
A , H * = 7 5 . 3 kJ mol-' and, corrected by the same deviation as for 1+ 2 ( - 24.2 kJ mol - I), ca. 99.5 kJ mol - I . According to these results the reaction 3 1 4 is about twice as
endothermic as the reaction 1 - 2 is exothermic. The
standard reaction entropy ArS* of the reaction 1 2 is negativeC3](ring closure unfavorable). Using the same value
for the similar reaction 3-4 leads to an estimated value of
at 200°C to lo-' at 800°C for the equilibrium
constant of this unknown gas-phase reaction. It thus follows from these data that it can hardly be possible to observe the thermodynamically "unfavorable" compound 4
in equilibrium with 3 at high temperatures in the same
way as the thermodynamically "unfavorable" compound 1
in the equilibrium 1 ~ 2 .
1 could be generated thermally and observed as an adequately long-lived intermediate for the following gas-phase
secondary reaction
suitable starting compound + 1
First of all, the reaction 5 - 3 was carried out in a thermal flow reactor,['31which was installed as close as possible to the ionizing region of a UV photoelectron spectrometer. As a result of a product optimization under the conditions dictated by the photoelectron spectrometer, only 3 as
well as HCI can be detected photoelectron spectroscopically, even at 550°C (Fig. 1). The structure of 3 is substantiated by comparison with the photoelectron spectrum of
l t 6 I and by comparison of the measured ionization energies
with those calculated by the M NDO['01-PERTCI,"41
and HAM/3[l7' methods. Especially characteristic is the
band system of reduced intensity in the region of 1011.5 eV as a result of an unusually strong interaction of
two Koopmans configurations via the HOMO-LUMO
configuration (cf. Ref. 161). The PE spectrum (Fig. la) remains unchanged, even at very high pyrolysis temperatures. Thus, in contrast to 1 , 3 proves to be a thermally
very stable compound showing no tendency to undergo a
thermally induced ring-closure reaction in the gas phase
[according to the photoelectron spectra1"I of 2,3-dimethylfuran and 4,5,6,7-tetrahydrobenzofuranas well as the calculated ionization energies (Fig. l ~ ) ,the photoelectron
spectrum of 4 should differ considerably from the photoelectron spectrum of 31.
after completion of the first step in a thermal flow reactor
at low temperature. An analogous procedure for the generation of 4 is less likely to succeed, since the reaction 4 - 3
will probably be much more rapid than the reaction 1 2
(pre-exponential factor A presumably of the same order of
magnitude as for the reaction 2 1J3] activation energy E,
presumably lower than for the reaction 1 -+2I3l),the ratio
of the rates of forward and reverse reaction for (b) is much
more unfavorable than for (a) [k4--3/k4--Sii: l o i 3for (b) at
200°C, k , - , / k , , , = lo4 for (a) at a comparable temperature], and, not least, since a suitable starting compound,
i.e. one which first reacts to give 4 and only then to give 3,
is not in the offing.
For these reasons, dihydrocyclobutafuran-other than
dihydrocyclobutabenzene-can very probably not be generated thermally. This is consistent with the hitherto unsuccessful attempts". ' 'I to obtain 4 thermally.
We have therefore explored a photochemical route [eq.
(c)]. The first (thermal) reaction step, unlike in other methods already described in the
proceeds without formation of interfering by-products and is quantitative at pressures in the region of ca. lo-' mbar.['31The subsequent ring closure reaction 3-4 was then accomplished
photochemically in the consensed phase (argon-matrix).
0 V C H Verlag~gesellschajimbH. 0.6940 Wernherrn. 1987
Fig. I . a) He1 photoelectron spectrum (count rate vs ionization energy I €
l e v ) of 3 ; for the first three bands: assignment, measured vertical ionization
energies lev]: 'A"(n), 8.00 (vibrational structure: AG= 14502 150 c m - ');
'A"(n), 10.09, 'A"(n), 10.93. b) LNDO/S-PERTCI ionization spectrum of 3
(relative intensity I,,, vs I E few): for the first s i x ionizations: assignment,
calculated vertical IE [eVJ ( I , c , ) : 'A"(n), 8.21 (0.89); 'A"(n), 10.23 (0.82);
'A''(J& 10.92 (0.61); 'A"(n), 11.89 (0.27); 'A'(o), 12.04 (0.87); 'A'(o), 13.35
(0.86). c) LNDO/S-PERTCI ionization spectrum of 4 ; for the first four ionizations: 'A(n), 8.07 (0.91); ' A ( n ) , 9.96 (0.88); 'A(o), 11.02 (0.89); 'A(@,
13.54 (0.89).
0570-0833/87/0505-0472 $ 02.5#/0
Angew. Chern. Int Ed. Engl 26 (1987) No 5
The reaction 5 -+ 3 was then carried out at the same temperature (optimized at the photoelectron spectrometer),
but at a lower pressure (by about a power of ten as a result
of the experimental set-up) in a reactor of the same construction,'"' which was installed as close as possible to the
matrix carrier of a low-temperature cryostat. The gaseous
reaction mixture was condensed together with a large excess of argon onto a NaCI-carrier at 16 K. Figure 2a shows
the low-temperature IR spectrum of the reaction mixture
in the solid solution (argon matrix) thus prepared, while
Figure 2c shows the UV/VIS absorption spectrum of the
same low-temperature matrix. From Figure 2a it can be
seen that 5 does not react completely to give 3 under the
reaction conditions. However, this is of no importance for
the investigation described here. The UV/VIS spectrum
90 -
I '
(Fig. 2c) with a well-resolved vibrational structure of the
long-wave band at il= 3 18 nm is unequivocally shown, by
its similarity with the UV/VIS absorption spectrum of l[I9]
and by the agreement with calculated CNDO/S-SECI,"''
LNDO/S-PERTCI and HAM/3 excitation energies (Fig.
2c) and oscillator strengths, to be the (previously unknown) spectrum of 3.
If the argon matrix containing 3 is irradiated with UVlight of wavelength il=
300 nm for 13.5 h,lr3]the absorption
bands in the IR spectrum of 3 completely disappear (Fig.
2b), and the band system located at il=
3 18 nm in the UV/
VIS spectrum of 3 is likewise no longer observed (Fig. 2d).
The new spectra unambiguously identify 4 as photoproduct: The IR spectrum (Fig. 2b) is in full agreement with
the theoretical spectrum of 4 calculated according to the
AM1 method,'"' both as far as the number of observed 1R
bands as well as their frequency and intensity are concerned. The new UV/VIS absorption band at /z = 223 nm
(Fig. 2d) is best interpreted by the results of theoretical
studies as a HOMO-LUMO(n-n*) excitation of 4. Moreover, the UV/VIS spectrum of 4 excellently agrees with
the spectra of other 2,3-alkyl-substituted furans, whose
longest wave n-n*
transition is observed at /I =
220 nm.'"]
The results impressively show how a thermodynamically
unfavorable (because of restricted mobility, high ring
strain, and lack of resonance stability) and thus thermally
inaccessible compound can be generated photochemically
from its thermodynamically more favorable isomeric
Received: December 22, 1986:
revised: January 26, 1987 [Z 2023/2024 IE]
German version: Angew Chem. YY (1987) 471
CAS Registry numbers:
1, 32796-95-5; 2, 694-87-1: 3, 73567-98-3; 4, 6681-01-2; 5. 14497-33-7
[ I ] W. R. Roth, M. Biermann, H. Dekker, R. Jochems. C. Mosselman, H.
Hermann, Chem. Ber 1 1 1 (1978) 3892.
[2] W. R. Roth, B. P. Scholz, C/iem. Ber. 114 (1981) 3741.
[3] Standard enthalpies of formation A, HF ( ( 1)=254.4, A, H.E ( 2 ) =
199.6 kJ mol - '; standard reaction enthalpies A, HE ( 1 2 ) = - 54.8,
A, H?,,,.,,,,,,
( ( 1- 2 ) =
-58.6 kJ mol I : standard reaction entropy
A,S$,,b.,9,,,,c ( l + Z ) = -43.1 J K - ' m o l - ' : equilibrium constant
K , , , ( ( l - 2 ) = 2 0 . 8 : activation energy E,,(1-2)=
112.6, E,,(I - 2 ) = 166.9 kJ m o l - ' ; pre-exponential factor A ( I - Z ) =
2 . l ~
[4] The estimation is in agreement with the following experiment: After the
thermolysis of 2 in a thermal flow reactor [corundum tube 180 x 3 mm in
combination with a low-temperature cryostat, pressure < 10 ' mbar,
temperature (not optimized) 9 I O T ] and condensation of the reaction
mixture with excess argon on an NaCl carrier at 16 K, one can distinctly
recognize the most intense bands of I (C=869.2, absorbance A =0.05;
775.4 c m - ' , Ax0.03) together with the predominant bands of 2
(V=780.2, A=0.679. 715.4cm-', A=0.493) in the low-temperature IR
spectrum after subtraction of background: N. Miinzel. A. Schweig, u n published.
[ S ] R. Schulz, A. Schweig, J . Electron Sperirosc. Relat. Phenom. 28 (1982)
[6] J. Kreile, N. Miinzel, R. Schulz, A. Schweig, Chem. Phvp. Lerr. 108
(1984) 609.
171 For a further low-temperature method (using a plasma flow reactor) for
the generation of 1 cf. [19].
[8] J. Julien, J. M . Pechine, F. Perez, J. J. Piade, Tetrahedron Lett. 1979.
191 a) W. S. Trdhanovsky, T. J . Cassady, T. L. Woods, J . Am. Chem. Soc. 103
(1981)6691;b)C. H.Chou, W.S.Trahanovsky,IbId. 108(1986)4138;c)
J. Org. Chem. 51 (1986) 4208.
1101 M. J. S. Dewar, W. Thiel, J . Am. Chem. Soc 99 (1977) 4899.
[ I I 1 N. Miinzel, A. Schweig, unpublished results.
[I21 H. E Winberg, F. S. Fawcett, W. E. Mochel, C. W. Theobald, J . Am.
Chem. Soc. 82 (1960) 1428.
[ 131 Thermal reactor: Stainless steel tube 200 x 4 mm: in combination with
the photoelectron spectrometer: pressure ca. lo-' mbar, optimum temperature 550°C; in combination with the low-temperature cryostat:
h [nrnlFig. 2 a ) IR spectrum (background subtracted) of the reaction mixture in
in a
argon matrix at 16 K after pyrolysis of 2-methyl-3-chloromethylfuran
thermal reactor, T=transmission; bands of 5 and 3 are indicated by 0 and
0, respectively: the five most intense absorptions ( V [cm-'I) of 5 . 1421.6
(m), 1137.2 (s). 723.0 (vs), 717.4 (vs), 709.8 (vs); of 3: 1639.6 (s), 1145.6 (vs),
1044.X (vs). 845.6(s), 816.8 ( s ) . b) IR spectrum (background subtracted) of the
matrix of a) irradiated with UV-light of wavelength A=300 nm with the five
most intense bands of 4 (indicated by 0 ) :1608.8 ( s ) , 1234.2 ( s ) , I1 15.4 (m),
1034.6 (s). 912.4 ( s ) . c) UV/VIS absorption spectrum (background subtracted)
of the reaction mixture isolated as described in a ) and H A M / 3 electron excitation spectrum of 3. A=absorbance: wavelength [nm] (oscillator strength):
I. 313.2 (0.661); 2: 237.4 (0.282): 3 : 216.0 (0.163). d) UV/VIS absorption
spectrum (background subtracted of the matrix of b) (1.e. after irradiation)
and HAM13 electron excitation spectrum of 4 : I : 225.3 (0.357).
C'liem. lnt. Ed. Enyl. 26 11987) No. 5
0 VCH Verlagrgesellschaft mhH. 0-6940 Weinherm, 1987
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pressure < l o - ' mbar, temperature not optimized. Radiation source for
the matrix irradiation: 450 W mercury high-pressure lamp, interference
filter with I0 nm in bandwidth.
1141 H. L. Hase, C . Lauer, K.-W. Schulte, A. Schweig, Theor. Chim. Acto 48
(1978) 47.
[IS] K.-W. Schulte, A. Schweig, Theor. Chim. Acta 33 (1974) 19, and references cited therein.
[I61 G. Lauer, K.-W. Schulte, A. Schweig, J . Am. Chem. SOC. 100 (1978)
1171 L. Asbrink, C. Fridh, E. Lindholm, Chem. Phys. Lett. 52 (1977) 63, 69,
[IS] N. Miinzei, A. Schweig, unpublished results; data for the Hel-photoelectron spectra in the sequence: assignment, LNDO/S-PERTCI ionization energies [ e q (calculated relative intensities), measured vertical ionization energies [ e q a) for 2,3-dimethylfuran: 'Afx), 8.36 (0.92), 8.22;
'A(rr), 9.87 (0.90), 9.70; 'A(o), 12.43 (0.89). 12.05; b) for 4,5,6,7-tetrahydrobenzofuran: 'A(x), 8.55 (0.93), 8.15; 'Afn), 10.16 (0.91), 9.65; 'A(cs),
11.84 (0.91). 11.09.
[I91 K. L. Tseng, J. Michl, J. Am. Chem. SOC.99 (1977) 4840.
1201 M. J. S. Dewar, E. G . Zoebisch, E. F. Healy, J. J. P. Stewart, J . Am.
Chem. Soc. 107 (1985) 3902.
121) H. Monti, M. Bertrand, Tetrahedron Lett. 1969, 123S:A,,,,,=218.0nm
n-hexane for 2.3-dimethylfuran; N. Miinzel, A. Schweig, unpublished
results: A , , , =220.9 nm in n-hexane for 4,5,6,7-tetrahydrobenzofuran.
- PPh3
Scheme I
molecular structure is shown in Figure 1. As expected, the
complex contains the linear seven-atom CCPMoPCC
First Examples of Complexes Containing
q'-Bonded PhosphaalkynesSynthesis and Crystal and Molecular Structure of
tratiS-lMO(AdCE P)2(EtzPCHZCH2PEt&I
(Ad = adamantyl)**
By Peter B. Hitchcock, Mohd Jamil Maah, John F. Nixon,*
Jalal A . Zora, G. Jeffery Leigh, and Mohamad Abu Bakar
In a series of papers"-21 we have described the wide variety of ligating behavior exhibited by phosphaalkynes
RC-P (R=rBu or adamantyl (Ad)). UV photoelectron
spectroscopic studies on RC-P ( R = H, F, Me, Ph, or rBu)
establish that the HOMO is of the n-type with the orbital
of the lone pair on the P atom significantly lower in energy. It is therefore not surprising that only q*-type coordination has hitherto been found for the coordination of
phosphaalkynes to transition metals.['] We had previously[*"] attempted to facilitate q '-coordination of a phosphaalkyne to a metal by treating [Pt(triphos)(PPh,)] (triphos=(Ph2PCH2)3CMe) with r B u C e P but obtained instead the q2-phosphaalkyne complex 1 (Scheme 1).
We now report the synthesis of 2-7, the first q'-phosphaalkyne complexes (Scheme 2). They are readily obtained by displacement of dinitrogen from appropriate
W ~ ~ ~ - [ M ( N ~ ) ~ ( R ; P C H ~ C H ~(M
P R=; Mo
) , I or W) comp l e x e ~ ' ~(Scheme
The formulation of complexes 2-7 is supported by their
solution "P( 'HJ-NMR spectra which give typical patterns
expected for [AA'BB'X] or [A4XZ]spin systems,[61and the
is typical for this type of pseudooctamagnitude of 'JpMp.
hedral MP,-complex ( P = phosphorus ligand). Much
smaller values of 'JpMp.
would be expected for an q2bonded phosphaalkyne complex.
Confirmation of the proposed q'-bonding mode comes
from a single crystal X-ray diffraction study on 6;"l the
[*I Prof. Dr. J. F. Nixon, Dr. P. B. Hitchcock, M. J. Maah, Dr. J . A. Zora
School of Chemistry and Molecular Sciences, University of Sussex
Brighton BN I 9QJ, Sussex (UK)
Prof. Dr. C . J. Leigh, Dr. M. A. Bakar
AFRC Unit of Nitrogen Fixation, University of Sussex
Brighton BNI 9RQ, Sussex ( U K )
This work was supported by the British Science and Engineering Research Council (SERC), the Malaysian Government (grants for M. J. M.
and M. A . B . ) , and Basrah University (grant for J. A. 2.).
0 VCH Verlagsgesellschafl mbH. 0-6940 Weinheim. 1987
f B U p-CICeH4
tBu Et
t B u Ph
tBu p-Tolyl
M o Ad Et
W fBu P h
Scheme 2
framework involving the two trans-q'-coordinated phosphaalkynes. Other features of interest in 6 are:
1) The short M o - P ~ , ~ ~distance
(2.305(3) A) compared
with the Mo-Pphorph.,nelength (ca. 2.433(4) A), reflecting
the smaller s p radius of phosphorus (0.94 A) compared
with the sp3 radius (1.07 A).
2) The short P-C bond distance ( 16520(12) A) compared
with an average value of 1.540(4) A observed in the free
RC=P ligands ( R = H, F, Me, or rBu)IX1and 1.672(17) A
in the $complex [Pt(PPh3)2(tBuC=P)].12h1
3) The metal-phosphorus network exploited here is such
that only ligands which are long and thin can approach
the metal to bind in the axial position. ExampIes include N2, CO, and MeNC. The network is also particularly robust, and consequently side-on coordination of
AdC=P is excluded o n steric grounds.
Studies to investigate whether the P=C bonds in 6 will undergo further q2-interaction with other metals are underway.
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