Quaternization of the bridgehead nitrogen in silatranylosmium(II) complexes and reestablishment of the transannular N-Si bond following migratory insertion of the silatranyl ligand

Article abstract of DOI:10.1021/om971100p

The Si-N distance in the five-coordinate silatranyl complex Os(Si{OCH₂CH₂}₃N)Cl(CQ)(PPh₃) ₂ (1a, Q = O) is very long, and the geometry about nitrogen is planar. Accordingly, the nitrogen atom readily

Full text of DOI:10.1021/om971100p

504
Organometallics 1998, 17, 504-506
Qu a ter n iza tion of th e Br id geh ea d Nitr ogen in
Sila tr a n ylosm iu m (II) Com p lexes a n d Reesta blish m en t of
th e Tr a n sa n n u la r N-Si Bon d follow in g Migr a tor y
In ser tion of th e Sila tr a n yl Liga n d
Moayad T. Attar-Bashi, Clifton E. F. Rickard, Warren R. Roper,*
L. J ames Wright,* and Scott D. Woodgate
Department of Chemistry, The University of Auckland, Private Bag 92019,
Auckland, New Zealand
Received December 16, 1997
Sch em e 1
Summary: The Si-N distance in the five-coordinate
silatranyl complex Os(Si{OCH2CH2}3N)Cl(CQ)(PPh3)2
(1a , Q ) O) is very long, and the geometry about nitrogen
is planar. Accordingly, the nitrogen atom readily un-
dergoes methylation or protonation, and this increases
the Si-N separation even further. On addition of CO
to 1b (Q ) S), the coordinated CS inserts into the Os-
Si bond and the normal silatrane N-Si interaction is
restored in the product, Os(η2-C{S}Si{OCH2CH2}3N)-
Cl(CO)(PPh3)2 (4).
Silatranes, cyclic organosilicon ethers of tris(2-oxy-
alkylamine), are among the most interesting examples
of hypervalent silicon compounds and they have been
intensively studied since their discovery in 1961.1 They
are part of a larger family of compounds which have
been called “atranes”.2 The silatranes have aroused
interest because of questions relating to the nature of
the N-Si bond, variation in the transannular N-Si
distance and the dependence of this on the axial silicon
substituent, biological activity, and their architectural
beauty.3 The nature of the N-Si bond in silatranes has
been the subject of several recent theoretical papers, and
the results suggest that it is best described in terms of
a dative NfSi bond with some small contribution from
a three-center, four-electron interaction.4-6 Further-
more, the calculated potential curve for deformation of
the N-Si distance is very shallow.6
In this paper, we report the synthesis of a silatrane
that bears an osmium substituent at silicon. The N-Si
distance is exceptionally long in this compound, and this
allows, for the first time, the isolation of derivatives
resulting from the addition of electrophiles to the
silatrane nitrogen atom. The transannular N-Si bond
within the silatranyl ligand is reestablished when this
group undergoes a migratory insertion reaction, which
has the effect of replacing the osmium substituent on
silicon by carbon.
We have reported previously that a convenient route
to coordinatively unsaturated silyl compounds of ruthe-
nium and osmium involves treatment of the five-
coordinate phenyl derivatives M(Ph)Cl(CO)(PPh3)2 (M
) Ru, Os) with silanes.7,8 We now report that heating
Os(Ph)Cl(CO)(PPh3)2 with silatrane (HSi{OCH2CH2}3N)
in toluene results in the formation of the yellow, air-
stable, five-coordinate silatranyl complex Os(Si{OCH2-
CH2}3N)Cl(CO)(PPh3)2 (1a )9 in good yield (see Scheme
1). The single-crystal X-ray structure of this product
has been determined, and the molecular structure is
depicted in Figure 1.10 The five ligands are arranged
about the osmium center in a distorted square-pyrami-
dal geometry with the silatranyl group in the apical
position. The Os-Si distance and the Os-Si-O angles
are very similar to the corresponding parameters found
for the closely related trihydroxysilyl complex Os(Si-
{OH}3)Cl(CO)(PPh3)2.7 The most striking feature of the
structure of 1a is the long N-Si distance of 3.000(7) Å,
which is the longest distance recorded for a silatrane.11
The separation between the silicon and nitrogen atoms
(1) Frye, C. L.; Vogel, G. E.; Hall, J . A. J . Am. Chem. Soc. 1961, 83,
996.
(2) Verkade, J . G. Acc. Chem. Res. 1993, 26, 483.
(3) See, for example: Voronkov, M. G.; Dyakov, V. M.; Kirpichenko,
S. V. J . Organomet. Chem. 1982, 233, 1. Tandura, S. N.; Voronkov, M.
G.; Alekseev, N. V. Top. Cur. Chem. 1986, 131, 99. Lukevits, E.;
Pudova, O.; Sturkovich, R. Molecular Structure of Organosilicon
Compounds; Ellis Horwood: Chichester, 1989. Chuit, C.; Corriu, R. J .
P.; Reye, C.; Young, J . C. Chem. Rev. (Washington, D.C.) 1993, 93,
1371.
(4) Haaland, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 992.
(5) Gordon, M. S.; Carrol, M. T.; J ensen, J . H.; Davis, L. P.; Burggraf,
L. W.; Guidry, R. M. Organometallics 1991, 10, 2657.
(6) Schmidt, M. W.; Windus, T. L.; Gordon, M. S. J . Am. Chem. Soc.
1995, 117, 7480.
(7) Rickard, C. E. F.; Roper, W. R.; Salter, D. M.; Wright, L. J . J .
Am. Chem. Soc. 1992, 114, 9682.
(8) Hu¨bler, K.; Hunt, P. A.; Maddock, S. M.; Rickard, C. E. F.; Roper,
W. R.; Salter, D. M.; Schwerdtfeger, P.; Wright, L. J . Organometallics
1997, 16, 5077.
S0276-7333(97)01100-X CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/30/1998

Communications
Organometallics, Vol. 17, No. 4, 1998 505
in the solid-state structures of silatranes generally falls
in the range 2.00-2.26 Å with the less electronegative
axial substituents on silicon favoring longer transan-
nular N-Si distances.12 In some cases, these weak,
easily deformed bonds have been shown to increase in
length somewhat in the gas phase.13 The longest N-Si
silatrane distance that has been described in the
literature is that found in the platinum silatranyl
complex trans-Pt(Si{OCH2CH2}3N)Cl(CO)(PMe2Ph)2
(2.89(1) Å).14 This appears to be the only metal-
substituted silatrane that has been reported.
One consequence of the very long N-Si distance
within the silatrane cage of 1a is that the bonds about
nitrogen are almost perfectly trigonal planar. The sum
of the angles at N is 360.0°, and the nitrogen atom sits
within 0.015(8) Å of the plane defined by C(2), C(4), and
C(6). The trigonal-planar geometry about nitrogen and
the very long N-Si distance suggest that the nitrogen
could be available for reaction with electrophiles. In-
F igu r e 1. ORTEP drawing of 1a with 50% thermal
ellipsoids. Selected bond lengths (Å): Os-Si, 2.326(2);
N-Si, 3.000(7).
(9) The experimental details and spectroscopic data for the repre-
sentative compounds 1a , 2a , and 4 follows, and the information for
compounds 1b and 3 is provided as Supporting Information. Os(Si-
{OCH2CH2}3N)Cl(CO)(PPh3)2 (1a ): Freshly degassed toluene (20 mL)
was added via a syringe to a Schlenk tube containing Os(Ph)Cl(CO)-
(PPh3)2 (200 mg, 0.234 mmol) and HSi(OCH2CH2)3N (82 mg, 0.468
mmol). The purple solution was refluxed for 3 h, during which time a
yellow color developed. The Schlenk tube was allowed to cool, and the
solvent volume was reduced in vacuo. Ethanol (30 mL) was added to
precipitate a bright yellow product. The product was filtered, washed
with hexane (20 mL), and recrystallized from dichloromethane:ethanol
to yield bright yellow needles of pure 1a (209 mg, 94%): mp 214-217
°C; m/z 917 (M+•); IR 1913 ν(CO) 1365, 1263, 1109, 998, 911, 878, 862,
702, 662, 608 cm-1; 1H NMR (400.1 MHz, CDCl3, TMS ) 0.00) 2.63 (t,
3
3
6H, CH2N, J HH ) 5.1 Hz) 3.30 (t, 6H, OCH2, J HH ) 5.1 Hz) 7.34-
7.66 (m, 30H, PPh3); 13C NMR (100.6 MHz, CDCl3, TMS ) 0.00) 52.89
2,4
(CH2N), 61.75 (OCH2), 127.86 (t′, PPh3 ortho-C,
129.71 (s, PPh3 para-C), 134.82 (t′, PPh3 meta-C,
J
J
) 10.1 Hz),
) 10.1 Hz),
CP
2,4
CP
ipso-C of PPh3 not observed; 29Si NMR (79.5 MHz, CDCl3) -65.00 (t,
OsSi, J CP ) 16.3 Hz); 31P NMR (162.0 MHZ, CDCl3) 25.83 (s, PPh3).
2
Anal. Calcd for
C43H42ClNO4OsP2Si: C, 54.22; H, 4.44; N, 1.47.
Found: C, 54.99; H, 4.77; N, 1.42. [Os(Si{OCH2CH2}3NMe)Cl(CO)-
(PPh3)2]CF3SO3 (2a ): Os(Si{OCH2CH2}3N)Cl(CO)(PPh3)2 (200 mg,
0.210 mmol) was added to a Schlenk tube containing dry dichlo-
romethane (25 mL). Methyl triflate (48 µL, 0.42 mmol) was added to
the solution, whereupon the color changed from bright yellow to a
slightly lighter yellow. Ethanol (10 mL) was added slowly, and the
yellow product which formed was collected and recrystallized from
dichloromethane:ethanol (25:5 mL) to give pure 2a (179 mg, 76%): mp
209-212 °C; m/z 968.1853, C45H45ClNO4OsP2Si requires 968.1897; IR
1924 ν(CO), 1279, 1259, 1225, 1157, 1063, 982, 934, 911, 849, 832, 799,
708, 639 cm-1; 1H NMR (400.1 MHz, CDCl3, TMS ) 0.00) 3.30 (s, 3H,
CH3) 3.45 (bs, 6H, CH2N), 3.75 (bs, 6H, OCH2), 7.39-7.58 (m, 30H,
F igu r e 2. ORTEP drawing of 2a with 50% thermal
ellipsoids. Selected bond lengths (Å): Os-Si, 2.297(2);
N-Si, 3.564(7).
deed, this was found to be the case, and on treatment
of 1a with either methyl triflate or methyl iodide, the
nitrogen atom is smoothly quaternized and the corre-
sponding cationic, N-methyl complexes [Os(Si{OCH2-
CH2}3NMe)Cl(CO)(PPh3)2]X (2a , X ) CF3SO3; 2b, X )
I),9 are formed (see Scheme 1). In the 1H NMR
spectrum of 2a , the methyl group is assigned to the
singlet signal at 3.30 ppm. The resonance of the silicon
atom in the 29Si NMR spectrum of 2a appears at -49.44
ppm, 15.56 ppm further downfield than that in the
parent compound, 1a .
An X-ray crystal structure determination of 2a has
been completed, and the molecular structure is depicted
in Figure 2.15 The triflate anion is not coordinated to
the metal center, and the arrangement of the five
ligands about the osmium center is similar to that found
for 1a . As would be expected, the quaternization of the
silatrane nitrogen causes a large change in the silatrane
cage geometry. The N-Si distance is increased to 3.564-
(7) Å (cf. 3.000(7) Å in 1a ), and the angles subtended
at the quaternary nitrogen atom by the carbon atoms
of the silatrane cage are decreased (average values
113.9° vs 120.0° for 1a ). In addition, the O-Si-O
PPh3); 13C NMR (100.6 MHz, CDCl3, TMS ) 0.00) 58.12 (s, CH2N),
2,4
63.95 (s, CH3), 66.66 (s, OCH2), 128.22 (t′, PPh3 ortho-C,
J
CP
) 10.1
) 11.1
CP
3,5
Hz), 130.33 (s, PPh3 para-C), 134.35 (t′, PPh3 meta-C,
J
1,3
Hz), 131.41 (t′, PPh3 ipso-C,
J
) 52.6 Hz); 29Si NMR (79.5 MHz,
CP
CDCl3) -49.44 (t, OsSi, 2J CP ) 12.3 Hz); 31P NMR (162.0 MHz, CDCl3)
33.61 (s, PPh3). Anal. Calcd for C45H45ClF3NO7OsP2SSi: C, 47.15; H,
4.00; N, 1.21. Found: C, 47.12; H, 3.91; N, 1.24. Os(η2-C{S}Si{OCH2-
CH2}3N)Cl(CO)(PPh3)2 (4): Os(Si{OCH2CH2}3N)Cl(CS)(PPh3)2 (200 mg,
0.207 mmol) was dissolved in CHCl3 (25 mL), and the resulting solution
placed in a Fischer-Porter bottle. The bottle was then pressurized with
CO (4 atm) for 5 min, whereupon the tan-yellow solution became
colorless. At this point, the CO pressure was decreased to 1 atm and
the solution was heated to 40 °C for 20 min. The resulting orange
solution was then reduced in volume on a rotary evaporator, and
ethanol was added. The orange crystals that formed were collected and
recrystallized from dichloromethane:ethanol to yield pure 4 (130 mg,
63%): mp 253-254 °C; m/z 997.1406, C44H42ClNO4OsP2SSi requires
997.1383; IR 1877 ν(CO), 1588, 1402, 1384, 1267, 1119, 1019, 938, 912,
794, 747, 724, 638, 592 cm-1
;
1H NMR (400.1 MHz, CD2Cl2, TMS )
3
3
0.00) 2.71 (t, 6H, CH2N, J HH ) 5.8 Hz) 3.49 (t, 6H, OCH2, J HH ) 5.9
Hz) 7.18-7.68 (m, 30H, PPh3); 13C NMR (100.6 MHz, CD2Cl2, TMS )
2,4
0.00) 50.37 (s, CH2N), 56.63 (s, OCH2), 127.27 (t′, PPh3 ortho-C,
J
CP
1,3
) 10.1 Hz), 129.54 (s, PPh3 para-C), 132.14 (t′, PPh3 ipso-C,
J
)
CP
3,5
51.3 Hz), 134.77 (t′, PPh3 meta-C,
J
) 10.1 Hz); 29Si NMR (79.5
CP
MHz, CD2Cl2) -87.41 (s, CSi); 31P NMR (162.0 MHz, CD2Cl2) 4.25 (s,
PPh3). Anal. Calcd for C50H42N3OsO2P3: C, 53.03; H, 4.25; N, 1.41.
Found: C, 53.18; H, 4.28; N, 1.21.

506 Organometallics, Vol. 17, No. 4, 1998
Communications
angles are contracted, on average, by 4.6° compared
with the corresponding angles in 1a .
The Os-Si distance of 2.297(2) Å in 2a is significantly
shorter than that found in the neutral precursor com-
pound (2.326(2) Å). Among the factors that may be
responsible for the decrease in this bond length are the
loss of any residual N-Si bonding interaction upon
quaternization of the nitrogen atom, an increase in
s-character of the Os-Si bond brought about by the
decreased O-Si-O angles in 2a , and electrostatic
effects.
Electrophilic attack at the silatranyl nitrogen atom
of 1a is not restricted to methylation. On treatment of
1a with triflic acid, protonation occurs at nitrogen and
the cationic compound [Os(Si{OCH2CH2}3NH)Cl(CO)-
(PPh3)2]CF3SO3 (3)9 is formed. The N-H signal appears
in the 1H NMR spectrum at 10.41 ppm and rapidly
exchanges on addition of D2O. Compelling evidence that
protonation has occurred at the silatranyl nitrogen
rather than at one of the equatorial oxygens comes from
the 29Si NMR spectrum of 3. The chemical shift of the
silicon atom in 3 (-51.75 ppm) is almost the same as
that in the cationic compound 2a (-49.44 ppm), where
the silatranyl ligand is methylated at nitrogen, but very
different from the higher-field value recorded for 1a
(-65.00 ppm), where the silicon atom is pseudopenta-
coordinate through weak interaction with the nitrogen
atom.16
Alkylation or protonation of a silatrane nitrogen atom
has not been reported before. Normally, the preferred
sites for electrophilic attack are the equatorial oxygen
atoms17 or the axial oxygen in alkoxy-substituted sila-
tranes.18 There has been a single report of the alkyla-
tion of the axial nitrogen in a substituted azasilatrane.16
The five-coordinate thiocarbonyl, silatranyl compound
Os(Si{OCH2CH2}3N)Cl(CS)(PPh3)2 (1b)9 can be pre-
pared by a similar route to that which gives 1a . On
treatment of 1b with CO, the silatranyl and thiocarbo-
nyl ligands undergo a migratory insertion reaction and
F igu r e 3. ORTEP drawing of 4 with 50% thermal el-
lipsoids. Selected bond lengths (Å): N-Si, 2.086(6); Os-
C(2), 1.987(5); Os-S, 2.484(1); S-C(2), 1.664(5).
{OCH2CH2}3N)Cl(CO)(PPh3)2 (4)9 is formed (see Scheme
1). The X-ray structure of 4 has been determined, and
the molecular structure is depicted in Figure 3.19 The
most notable feature is that the N-Si distance has
decreased in this molecule to 2.086(6) Å, a value that is
at the short end of reported silatrane N-Si dis-
tances.11,20 The remaining structural parameters are
similar to those found in the related ruthenium com-
pound Ru(η2-C{S}SiMe2OEt)Cl(CO)(PPh3)2.21 The ob-
servation that a short N-Si distance is reestablished
when the silatranyl group undergoes a migratory inser-
tion reaction in which the direct bond to the metal is
broken reinforces the notion that it is the special nature
of the bond to the electron-releasing osmium center8
that is responsible for the very long N-Si distance in
1a .
Ack n ow led gm en t. We are grateful for support from
the Marsden Fund for this work and to The University
of Auckland for grants-in-aid and the award of a
postgraduate scholarship to S.D.W.
the η2-silathioacyl-containing compound Os(η2-C{S}Si-
(10) Crystal data for 1a at 203 K with Mo KR radiation (Siemens
SMART CCD diffractometer): C43H42ClNO4OsP2Si, fw ) 952.46, a )
9.6775(1) Å, b ) 12.1180(1) Å, c ) 18.1361(1) Å, R ) 94.532(1)°, â )
100.817(1)°, γ ) 105.204°, triclinic, P1h, Z ) 2, V ) 1997.41(3) Å3, Dc )
1.584 g cm-3. R ) 0.0389 for 6545 observed (I > 2σ(I)) reflections;
weighted R2 ) 0.0960.
(11) A search of the Cambridge Structural Database gave the
following results for the N-Si distances in 72 silatrane structures:
Mean, 2.138; SD, 0.064 Å.
(12) Greenberg, A.; Wu, G. Struct. Chem. 1990, 1, 79. Hencsei, P.
Struct. Chem. 1991, 2, 21.
(13) Shen, Q.; Hilderbrandt, R. L. J . Mol. Struct. 1980, 64, 257.
(14) Eaborn, C.; Odell, K, J .; Pidcock, A.; Scollary, G. R. J . Chem.
Soc., Chem. Commun. 1976, 317.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures and characterization data for all new
compounds as well as tables of data collection parameters,
atomic coordinates, bond lengths and angles, anisotropic
displacement parameters, and hydrogen coordinates (25 pages).
Ordering information is given on any current masthead page.
OM971100P
(15) Crystal data for 2a at 203 K with Mo KR radiation (Siemens
SMART CCD diffractometer): C45H45ClF3NO7OsP2SSi, fw ) 1116.56,
a ) 10.4535(2) Å, b ) 17.5941(2) Å, c ) 25.3620(1) Å, â ) 96.250(2)°,
monoclinic, P21/c, Z ) 4, V ) 4636.84 Å3, Dc ) 1.599 g cm-3. R ) 0.0525
for 7338 observed (I > 2σ(I)) reflections; weighted R2 ) 0.1265.
(16) Gudat, D.; Daniels, L. M.; Verkade, J . G. J . Am. Chem. Soc.
1989, 111, 8520.
(19) Crystal data for 4 at 203 K with Mo KR radiation (Siemens
SMART CCD diffractometer): C44H42ClNO4OsP2SSi, a ) 12.4896(4)
Å, b ) 14.3772(4) Å, c ) 23.8280(7) Å, â ) 103.867(1)°, monoclinic,
P21/n, Z ) 4, V ) 4154.0(2) Å3, Dc ) 1.593 g cm-3. R ) 0.0392 for 7418
observed (I > 2σ(I)) reflections; weighted R2 ) 0.0795.
(20) Narula, S. P.; Shankar, R.; Kumar, M.; Chadha, R. K.; J anaik,
C. Inorg. Chem. 1997, 36, 1268.
(17) Timms, R. E. J . Chem. Soc. A 1971, 1969.
(18) Garant, R. J .; Daniels, L. M.; Das, S. K.; J anakiraman, M. N.;
J acobson, R. A.; Verkade, J . G. J . Am. Chem. Soc. 1991, 113, 5728.
(21) Rickard, C. E. F.; Roper, W. R.; Salter, D. M.; Wright, L. J .
Organometallics 1992, 11, 3931.