Intriguing tetrasodium dication cluster Na4/2+ stabilized between two silyl(fluorosilyl)phosphanide shells [8]

Full text of DOI:10.1021/ja9822963

10774
J. Am. Chem. Soc. 1998, 120, 10774-10775
2+
Scheme 1
Intriguing Tetrasodium Dication Cluster Na4
Stabilized between Two Silyl(fluorosilyl)phosphanide
Shells
Matthias Driess,*,† Hans Pritzkow,‡ Markus Skipinski,† and
Uwe Winkler†
Lehrstuhl fu¨r Anorganische Chemie I
Ruhr-UniVersita¨t Bochum
UniVersita¨tsstrasse 150 D-44801 Bochum, Germany
Anorganisch-chemisches Institut der UniVersita¨t
Im Neuenheimer Feld 247
D-69120 Heidelberg, Germany
ReceiVed July 1, 1998
Subsequent fractional crystallization of the reaction mixture
Clusters of the alkali metals are one of the fascinating ways to
obtain a better understanding of structure-metallic property
relationships when increasing the size of the aggregate. They
possess intrinsical shallow potential surfaces and tend to form
different isoenergetic topologies with increasing cluster size.1
Recently, especially diamagnetic and paramagnetic alkali metal
cluster cations of sodium, potassium, rubidium, and cesium are
of considerable interest since it has been shown that they appear
in zeolites which are differently loaded with alkali metals.2 Thus,
in the case of M4n+ cations (n ) 2-4), it has been demonstrated
from extensive experimental and theoretical work that the
topology of the M4 cluster is dependent on electron delocalization
(inorganic electrides) and the nature of the intrazeolite cavities.3
To our knowledge, a molecular ion pair compound of an alkali
metal cluster cation has not been obtained thus far. We report
here on the synthesis of the unusual complex 1 bearing the
tetrasodium dication which is coordinatively stabilized by two
sterically congested silyl(fluorosilyl)phosphanide counterions.
Compound 1 is furnished as an unexpected side-product by
metalation of the corresponding secondary silyl(fluorosilyl)-
phosphane 2 with NaN(SiMe3)2 in the molar ratio of 1:2 in hexane/
toluene (5:1) at 60 °C (2 d). Previous investigations revealed
that 2 can only be completely converted to the expected
monosodium phosphanide 3 at room temperature if 2 molar equiv
sodium amide is employed, due to the initial formation of
heteroaggregate intermediates (Scheme 1).4 Apparently, the latter
incorporate two molecules of the sodium amide. Whereas the 1
m excess of NaN(SiMe3)2 may be recovered after the reaction at
room temperature, we showed that the same reaction at 60 °C
within 2 d leads additionally to the novel complex 1 and only
0.74 M excess of the amide can be recovered.
in a small amount of hexane at 25 °C afforded bright yellow
crystals of the sodium-rich phosphanide 1 in 8% yield, and further
crystallization at 4 °C yielded colorless cubes of excess amide.
Finally, the major product of the reaction is the expected
monosodium phosphanide 3, which crystallizes at -30 °C in the
form of pale yellow plates in 59% yield. Compound 3, which
has been described previously, is a solvent-free dimeric sodium
phosphanide with intramolecular Na-F bonds.4 The mechanism
of the formation of 1 is unknown. Evidently, the process involves
a redox reaction in which the N(SiMe3)2 anion acts very likely
as a base and reducing agent at the same time. Reducing
properties of the N(SiMe3)2 anion, however, are already docu-
mented for its reactions with different cationic main groups and
transition metal centers.5 That 1 indeed represents a Na metal-
containing sodium phosphanide, which accommodates two Na
atoms and two Na+ ions, is unequivocally proven by its
independent synthesis through Na metal consumption of 3. Thus,
the reaction of powdered elemental Na with 3 in toluene in the
presence of styrene as a catalyst at 40 °C within 2 d furnishes
crystalline 1 in 24% yield. The integrity of the samples has been
established by correct elemental analyses (C, H, P, F) and X-ray
diffraction (see below). The characterization of 1 by means of
different mass spectrometric techniques failed due to decomposi-
tion. EPR spectroscopic and magnetic susceptibility measure-
ments (10-298 K) clearly showed that 1 is diamagnetic. In
contrast to 3, compound 1 cannot be dissolved in any common
organic solvents without decomposition (31P and 19F NMR
spectroscopy), and attempts to record a solid-state CP-MAS NMR
spectrum (23Na, 19F, 31P) failed due to the huge line-broadening
by the 23Na ions (I ) 3/2) and strong spin-spin coupling.
However, the molecular structure has been unequivocally elabo-
rated by single-crystal X-ray diffraction analysis (Figure 1).6
1 crystallizes in the monoclinic space group P21/n with two
molecules in the unit cell, whereas 3 is triclinic. The centrosym-
metric molecule consists of a rhomboidally distorted planar Na4
ring as the most interesting part of the aggregate. The distinctly
different Na-Na distances of 3.076(3) (Na1-Na2) and 3.202(3)
Å (Na1-Na1′) reflect attractive Na-Na interactions (3.82 Å in
elemental Na), whereas the Na1-Na2′ distance of 3.530(3) Å is
longer than that value in 3 (3.509(5) Å), indicating less attractive
interactions between Na1-Na2′. Since the compound is intensely
yellow and not red or blue as observed for Na-loaded zeolites
(e.g., Na43+ and related clusters), the residual metal electrons are
* Author for correspondence. E-mail: driess@ibm.anch.ruhr-uni-bochum-
.de.
† Ruhr-University Bochum.
‡ University of Heidelberg.
(1) (a) Maynau, D.; Malrieu, J. P.; J. Chem. Phys. 1988, 88, 3163. (b)
Bonacic-Koutecky, V.; Fantucci, P.; Koutecky, J. Chem. ReV. 1991, 91, 1035.
(c) Ro¨sch, N.; Pacchioni, G. In Cluster and Colloids; Schmid, G., Ed.; VCH:
Weinheim, Germany, 1994; pp 5-30.
(2) For a review see: Edwards, P. P.; Anderson, P. A.; Thomas, J. M.
Acc. Chem. Res. 1996, 29, 23.
(3) (a) Rabo, J. A.; Angell, C. L.; Kasai, P. H.; Schomaker, V. Discuss.
Faraday Soc. 1966, 41, 328. (b) Edwards, P. P.; Harrison, M. R.; Klinowski,
J.; Ramdas, S.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J. Chem. Soc.,
Chem. Commun. 1984, 982. (c) Harrosin, M. R.; Edwards, P. P.; Klinowski,
J.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J. Solid State Chem. 1984, 54,
330. (d) Anderson, P. A.; Barr, D.; Edwards, P. P. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1501. (e) Nakayama, H.; Klug, D. D.; Ratcliffe, C. I.;
Ripmeester, J. A. J. Am. Chem. Soc. 1994, 116, 9777. (f) Armstrong, A. R.;
Anderson, P. A.; Woodall, L. J.; Edwards, P. P. J. Phys. Chem. 1994, 98,
9279. (g) Armstrong, A. R.; Anderson, P. A.; Woodall, L. J.; Edwards, P. P.
J. Am. Chem. Soc. 1995, 117, 9087. (h) Anderson, P. A.; Armstrong, A. R.;
Porch, A.; Edwards, P. P.; Woodall, L. J. J. Phys. Chem. 1997, B 101, 9892.
(4) Driess, M.; Pritzkow, H.; Skipinski, M.; Winkler, U. Organometallics
1997, 16, 5108.
(5) (a) Andersen, R. A.; Faegri, K., Jr.; Green, J. C.; Haaland, A.; Lappert,
M. F.; Leung, W.-P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782. (b)
Bjo¨rgrinsson, M.; Roesky, H. W.; Pauer, F.; Stalke, D.; Sheldrick, G. M. Inorg.
Chem. 1990, 29, 5140.
(6) Crystal data of 1: C39H67FNa2PSi2, MW ) 688.06, a ) 10.919(6) Å,
b ) 26.140(14) Å, c ) 15.057(8) Å, â ) 102.78(4)°, V ) 4191(4) Å3,
monoclinic, space group P21/n, Z ) 4, final R1 ) 0.0625, wR2 ) 0.1849 (all
data) for 4131 [I > 2σ(I)] observed reflections, GOF (on F2) ) 1.029.
10.1021/ja9822963 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/03/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 41, 1998 10775
Scheme 2
is weakly coordinated by C-H bonds of a SiiPr3 group close by
(Na2-C15′ 3.012(6) Å). While calculations showed that the
2+
naked Na4 cluster, stabilized by four-center two-electron de-
2+
localization, prefers the tetrahedral arrangement (the only Na4
minimum!), which is 7.3 kcal mol-1 more stable than the
rhomboid form (D2h),9 apparently, the coordination of the Na
centers in 1 through the counterions and the steric congestion of
the substituents favor the D2h form of the cationic Na4-core. Since
1 contains 2 equiv Na metal, which could serve for reduction
processes, we investigated its reactivity toward 4 molar equiv
Me3SiCl in boiling diethyl ether (Scheme 2).
The reaction is complete after 4 h, affording a pale yellow
solution and solid NaCl. Interestingly, while the phosphanide
anions of 1 are silylated to give the corresponding disilyl-
(fluorosilyl)phosphane 4 (δ (31P) ) -243, δ (19F) ) -122.3)10
in quantitative yield, the only other molecular product, detectable
by means of GC/MS analysis and 29Si NMR spectroscopy, is the
disilane Me3Si-SiMe3 5, which has been isolated by fractional
condensation in 68% yield. Furthermore, hydrolysis of 1 in
diethyl ether leads, under evolution of H2, to the starting silyl-
(fluorosilyl)phosphane 2 (31P, 19F NMR) in quantitative yield.
In summary, we have succeeded in the synthesis of the first
Na-cluster cation. Its existence suggests a terra incognita of other
novel and exciting types of molecular alkali metal cluster
compounds.
Figure 1. Molecular structure of 1. Selected bond lengths (Å) and angles
(deg): Na1-Na2 3.076(3), Na1-Na1′ 3.202(3), Na1-Na2′ 3.530(3),
Na1-F1 2.284(3), Na1-F1′ 2.395(3), Na2-F1′ 2.209(3), Na1-P3 2.911-
(2), Na2-P3 2.905(3), P3-Si2 2.233(2), P3-Si1 2.254(2), Si1-F1 1.618-
(3), Na1-C1′ 2.712(4), Na2-Na1-Na1′ 68.39(7), Na1-Na2-Na1′
57.50(6), Na2-P3-Na1 63.87(6), Si2-P3-Si1 117.07(7), Si1-P3-Na2
104.36(7), Si1-P3-Na1 71.97(6), Na1-F1-Na1′ 86.35(10), Na2′-F1-
Na1′ 83.75(11).
probably much less delocalized. The Na42+ cluster is embedded
between the two silyl(fluorosilyl)phosphanide counterions. The
latter impose that each P atom is µ2-bonded to a Na2 edge and
each F atom coordinates to three Na centers. The Na-P distances
of 2.911(2) and 2.905(3) Å are in the typical range of values of
other sodium phosphanides.7 The three different Na-F distances
(2.284(3), 2.395(3), 2.210(3) Å) reflect steric restrictions but
resemble those values observed for other sodium fluorosilylphos-
phanides (2.26-2.38 Å).8 Interestingly, the Si-F distance of
1.618(3) Å is shorter than that in 3 (1.673(3) Å), although the F
atoms in the latter are only two-coordinate, but this can be
explained by a higher polarity of the Si-F bond in 1. The
different Si-P distances (2.253(2), 2.232(2) Å) are unremarkable
and resemble those of other metalated silylphosphanes.7 The Na1
center benefits additionally from an attractive interaction with
the ipso-C atom of a aryl ring (Na1-C1′ 2.712(4) Å), while Na2
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft, the University of Bochum, and the Ministry of
Science and Research Nordrhein-Westfalen, Germany.
Supporting Information Available: Crystallographic data with
complete tables of bond lengths, bond angles, and thermal and positional
parameters for 1 (7 pages, print/PDF). The crystallographic file, in CIF
format, is available through the Internet only. See any current masthead
page for ordering information and Web access instructions.
JA9822963
(9) Glukhovtsev, M. N., Schleyer, P. v. R.; Stein, A. J. Phys. Chem. 1993,
97, 5541.
(7) (a) Adrianarison, M.; Stalke, D.; Klingebiel, U. Chem. Ber. 1990, 123,
71. (b) Driess, M.; Huttner, G.; Knopf, N.; Pritzkow, H.; Zsolnai, L. Angew.
Chem., Int. Ed. Eng. 1995, 34, 316. (c) Koutsantonis, G. A.; Andrews, P. C.;
Raston, C. L. J. Chem. Soc., Chem. Commun. 1995, 47. See also ref 4.
(8) See ref 4 and 7a.
(10) 4: colorless oil; 31P NMR (C6D6) δ -243.0 (s); 19F NMR (C6D6, CFCl3
1
ext. standard) δ -122.3; 29Si NMR (C6D6) δ -2.81 (dd, SiF, J (Si, F) 303
1
1
3
Hz, J (Si, P) 48 Hz), 18.30 (dd, SiMe3, J (Si, P) 37 Hz, J (Si, F) 8.8 Hz),
28.81 (d, SiiPr3, 1J (Si, P) ) 39 Hz). HRMS Calcd for C42H76FPSi3 M+
716.296. Found M+ 716.291.