N-versus O-silylation in cis-[(tBuHN)O{double bond, long}P(μ-NtBu)2P{double bond, long}O(NHtBu)] and...

Article abstract of DOI:10.1016/j.jorganchem.2008.05.023

Title full: N-versus O-silylation in cis-[(^tBuHN)O{double bond, long}P(μ-N^tBu)₂P{double bond, long}O(NH^tBu)] and [Me₂Si(μ-N^tBu)₂P{double bond, long}O(NHPh)]. Solid-state structures of their silylation products, of co-crystalline cis-[(^tBuHN)O{double bond, long}P(μ-N^tBu)₂P{double bond, long}O(NH^tBu)], and of {[Me₂Si(μ-N^tBu)₂P{double bond, long}O(N(SiMe₃)Ph)]VCl₃}. The solid-state structure of cis-[(^tBuHN)O{double bond, long}P(μ-N^tBu)₂P{double bond, long}O(NH^tBu)] (A), isolated as a 2-phenyl-2-propanol/2-phenyl-2-propanehydroperoxide co-crystal (1), is reported. Treatment of pure A with two equivalents of n-butyllithium, followed by addition of Me₃SiCl, afforded the bis-O,O′-silylated cis-[(Me₃SiO)^tBuN{double bond, long}P(μ-N^tBu)₂P{double bond, long}N^tBu(OSiMe₃)] (2), whose solid-state structure was determined. When the cyclic N,N,N-phosphoramidate [Me₂Si(μ-N^tBu)₂P{double bond, long}O(NHPh)] (B) was subjected to the same treatment as A, however, the N-silylated [(Me₂Si(μ-N^tBu)₂P{double bond, long}O(N(SiMe₃)Ph)] (3), was obtained. Its solid-state structure was determined. The reaction of 3 with VCl₃(THF)₃ did not produce the intended N,O-chelated V(II) complex, but furnished instead the adduct {[Me₂Si(μ-N^tBu)₂P{double bond, long}O(N(SiMe₃)Ph)] VCl₃(THF)}, in which 3 binds the trigonal-bipyramidally coordinated vanadium center through its oxygen atom only.

Full text of DOI:10.1016/j.jorganchem.2008.05.023

Journal of Organometallic Chemistry 693 (2008) 2748–2754
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
N-versus O-silylation in cis-[(tBuHN)O@P(
l
-NtBu)2P@O(NHtBu)] and
[Me2Si(
l
-NtBu)2P@O(NHPh)]. Solid-state structures of their silylation products,
of co-crystalline cis-[(tBuHN)O@P( -NtBu)2P@O(NHtBu)],
l
and of {[Me2Si(
l
-NtBu)2P@O(N(SiMe3)Ph)]VCl3}
b
Dana C. Haagenson a, Graham R. Lief a, Lothar Stahl a, , Richard J. Staples
*
a Department of Chemistry, University of North Dakota, Grand Forks, ND 58202-9024, United States
b Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 09208, United States
a r t i c l e i n f o
a b s t r a c t
The solid-state structure of cis-[(tBuHN)O@P(
l
-NtBu)2P@O(NHtBu)] (A), isolated as a 2-phenyl-2-propa-
Article history:
Received 15 April 2008
Received in revised form 14 May 2008
Accepted 15 May 2008
Available online 24 May 2008
nol/2-phenyl-2-propanehydroperoxide co-crystal (1), is reported. Treatment of pure
equivalents of n-butyllithium, followed by addition of Me3SiCl, afforded the bis-O,O0-silylated cis-
[(Me3SiO)tBuN@P( -NtBu)2P@NtBu(OSiMe3)] (2), whose solid-state structure was determined. When
the cyclic N,N,N-phosphoramidate [Me2Si(
-NtBu)2P@O(NHPh)] (B) was subjected to the same treatment
as A, however, the N-silylated [Me2Si(
-NtBu)2P@O(N(SiMe3)Ph)] (3), was obtained. Its solid-state struc-
ture was determined. The reaction of 3 with VCl3(THF)3 did not produce the intended N,O-chelated V(II)
complex, but furnished instead the adduct {[Me2Si(
-NtBu)2P@O(N(SiMe3)Ph)] VCl3(THF)}, in which 3
binds the trigonal-bipyramidally coordinated vanadium center through its oxygen atom only.
Ó 2008 Elsevier B.V. All rights reserved.
A with two
l
l
l
Keywords:
Diazadiphosphetidines
Diazasilaphosphetidines
O-Silylation
l
N-Silylation
N,N,N-Phosphoramidates
Vanadium(III)
1. Introduction
neutral phosphoramidates are restricted to metal source materials
with reactive metal–halide or metal–alkyl bonds, such as TiCl4 and
AlMe3, respectively [14,15]. In their anionic forms, both A and B are
often strongly reducing, leading to a significant amount of metal
reduction, rather than metathesis reactions. Besides, the anions
are incompatible with some protic solvents.
To introduce A and B as ligands into transition-metal complexes
we sought more soluble and milder starting materials of these
compounds. We therefore set out to synthesize the mono- and
bis-trimethylsilyl-substituted derivatives of B and A, respectively.
Our investigations showed that while the dianion of A is silylated
at oxygen, the mono-anion of the close structural analog B is sily-
lated at nitrogen. Although these dissimilar outcomes, especially
the N-silylation over the O-silylation, may seem surprising in light
of silicon’s oxophilicity, they can be rationalized in terms of the
overall reaction thermodynamics.
The trimethylsilyl moiety (–SiMe3) is used extensively to pro-
tect protic hydrogens, like N–H and O–H, and to control the stereo-
chemistry in multistep organic syntheses [1–3]. Inorganic and
organometallic chemists employ this group both, as a ligand in
its own right [4] and as a mild and clean transfer reagent for li-
gands, because it reacts with certain element chlorides under the
elimination of volatile chlorotrimethylsilane [5]. In this latter role
the trimethylsilyl group has also proven invaluable for the synthe-
sis of inorganic polymers, such as in the generation of polyphosp-
hazenes
from
trimethylsilyl-substituted
amino-,
and
iminophosphines [6–10].
Ambidentate ligands, like the bis(tert-butylamino)-substituted
phosphorus–nitrogen heterocycle A [11,12] and the mono(anili-
no)-substituted phosphorus–silicon–nitrogen heterocycle B [13],
can coordinate metals in a number of ways. These phosphorus(V)
compounds, which may also be considered bis- and mono-
(N,N,N-phosphoramidates), respectively, can be introduced into
their complexes in either their neutral or their anionic forms. Both
approaches, however, suffer from disadvantages. Reactions of the
2. Results
The bis(tert-butylamino)cyclodiphosph(III)azane cis-[(tBuHN)P-
(l
-NtBu)2P(NHtBu)] (C) may be oxidized with oxygen (from organic
peroxides), sulfur, and selenium to furnish the homologous
phosphorus(V) compounds cis-[(tBuHN)E@P( -NtBu)2P@E(NHtBu)]
(E = O, S, Se), respectively [15–19]. Notably, tellurium yields only
l
* Corresponding author. Fax: +1 701 7772331.
E-mail address: lstahl@chem.und.edu (L. Stahl).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.05.023

D.C. Haagenson et al. / Journal of Organometallic Chemistry 693 (2008) 2748–2754
2749
the mono-oxidation product, namely cis-[(tBuHN)Te@P( -NtBu)2-
l
Table 2
P(NHtBu)] [20]. The solid-state structures of the disulfide, the disel-
enide and the monotelluride had been reported, but that of the
dioxide was previously unknown.
Selected bond lengths (Å) and angles (°) for 1 and 2
1
2
P–N(term) avg.
P–N( ) avg.
1.622(2)
1.678(2)
1.467(2)
1.520(2)
1.696(2)
1.592(2)
1.670(2)
1.849(4)
We discovered that while pure cis-[(tBuHN)O@P( -NtBu)2P@
l
l
O(NHtBu)] precipitated in microcrystalline form only, co-crystals
of this dioxide with the oxidant cumene hydroperoxide and the
by-product cumyl alcohol were suitable for single-crystal X-ray
analysis. The crystal and refinement data of 1, as obtained from
this study, are listed in Table 1, while Table 2 contains selected
bond parameters.
P–O, avg.
Si–O, avg.
Si–C, avg.
C(23)–O(3)
C(32)–O(4)
O(4)–O(5)
1.485(4)
1.317(12)
1.460(12)
N(l
N(l
N(l
)–P–N(
)–P–N(term) avg.
)–P–O, avg.
l
) avg.
84.14(11)
111.12(11)
118.46(11)
95.84(11)
83.86(11)
Shown in Fig. 1 is the asymmetric unit of 1, which consists of
one molecule of the cyclodiphosph(V)azane, and one molecule
each of the hydroperoxide and alcohol co-crystallants. Co-crystals
are common in chemistry, the most widespread examples being
solvates and inclusion compounds [21]. Cases such as 1, however,
where the unit cell contains, with the exception of C, all reactants
and products of a reaction are rare. The ButN(H) groups are endo–
endo configured, namely, both amino-hydrogen atoms are located
above the heterocyclic ring. This arrangement is in contrast to
the structures of the homologous heavier dichalcogenides, which
display endo–exo conformations—most likely due to the larger
sizes of sulfur and selenium.
The P2N2 ring of the bis(N,N,N)-phosphoramidate is almost pla-
nar, both NPN wedges enclosing a dihedral angle of merely
2.09(9)°. The phosphorus–nitrogen bonds of the four-membered
ring have normal lengths, being 1.678(2) Å long on average, while
the exocyclic P–N bonds are shorter at 1.622(2) Å, as is typically
the case for these molecules [19,22]. The phosphorus–oxygen dou-
ble bonds (1.467(2) Å) fall into the expected range and so do the
remaining bond parameters.
Cumene hydroperoxide, a common oxidizer and an intermedi-
ate in the synthesis of phenol and acetone from cumene, is a liquid
at room temperature. To our knowledge the crystal structure of cu-
mene hydroperoxide has not been reported; we are also unaware
of any X-ray structurally studied co-crystals containing this mole-
cule. The crystal structure of 1, thus, provides the first opportunity
to inspect the bond parameters of this peroxide in the solid state.
Unfortunately, the most interesting part of this molecule,
namely the hydroperoxy moiety, O–O–H, is disordered with two
conformations, of which only the predominant one is shown in
Fig. 1. The carbon–carbon bonds have normal lengths, as does
the peroxide bond, which, at 1.460(12) Å, is equidistant with that
in H2O2, 1.458(4) Å [23].
125.74(14)
105.92(11)
95.23(11)
106.65(13)
134.02(14)
137.4(2)
P–N(l)–P, avg.
N(term)–P–O, avg.
P–O–Si, avg.
P–N(term)–C, avg.
111.05(11)
130.15(19)
Like cumene hydroperoxide, cumyl alcohol (2-phenyl-2-propa-
nol) has never been subjected to a single-crystal X-ray analysis,
although the structure of a dimolybdenum hexaalkoxide complex
of this alcohol has been reported [24]. The alcohol was structurally
characterized as a co-crystallant in a tin–alkoxide cluster, but the
disorder in the nonaromatic portion of the molecule was so severe
that the OH and CH3 groups were indistinguishable [25]. In 1 the
alcohol is ordered, and this allowed us to obtain precise bond
lengths and angles of this molecule, some of which are listed in Ta-
ble 2. The aliphatic and aromatic carbon–carbon framework shows
no unusual features, and neither does the carbon–oxygen bond,
with its typical length of 1.485(4) Å. The cumene hydroperoxide
is hydrogen-bonded to one of the phosphoryl oxygen atoms via a
strong hydrogen bond (2.614(7) Å), while a weaker, and hence
longer, hydrogen bond (2.901(3) Å) connects the alcohol with the
second phosphoryl moiety [26] (Scheme 1).
Co-crystalline 1 can be freed of both, cumene hydroperoxide
and cumyl alcohol, by washing with hexanes to yield analytically
pure A. Treatment of A with two equivalents of n-butyllithium, fol-
lowed by addition of two equivalents of chlorotrimethylsilane,
Scheme 2, afforded disilylated 2 in good yields. Norman and co-
workers had previously reported that the dilithiations and subse-
quent silyl-, germyl-, and stannylations of trans-[C6H5NHPS-
(l
-NtBu)2PS(NHC6H5)] produced the N,N0-disubstituted products
[27]. Although the NMR data of 2 supported a symmetrical prod-
uct, we were unable to discern from these spectra alone whether
Table 1
Crystal, data collection, and refinement data for 1–4
1
2
3
4
Molecular formula
C
34H62N4O5P2
C22H54N4O2P2Si2
524.81
C2/c (no. 15)
213
C19H38N3OPSi2
411.67
P21/c (no. 14)
213
C23H46Cl3N3O2PSi2V
641.07
P21/n (no. 14)
213
Formula weight
Space group
T (K)
668.82
Pbca (no. 61)
213
a (Å)
b (Å)
c (Å)
15.6840(17)
21.9943(12)
22.7020(12)
10.0283(13)
16.9911(13)
19.7028(18)
103.270(3)
3267.6(6)
4
9.6531(9)
12.1954(11)
20.7594(19)
98.584(2)
2416.5(2)
4
9.8603(12)
19.273(3)
17.341(2)
90.198(3)
3295.4(7)
4
b (°)
V (Å3)
Z
7831.3(7)
8
k (Å)
0.71073
1.135
0.152
0.71073
1.067
0.229
0.71073
1.132
0.226
0.71073
1.292
0.690
qcalc (g cmꢀ3
)
l
(mmꢀ1
)
Reflections collected
Independent reflections (Rint
Parameters
47040
8547 (0.0553)
420
0.0637
0.1862
9680
3398 (0.0399)
145
0.0666
0.1802
14778
5145 (0.0220)
235
0.0432
0.1347
21887
7989 (0.0911)
317
0.0826
0.2210
)
R(F)a [I > 2
r
(I)]
wR2(F2)b [all data]
P
P
a
R = jFo ꢀ Fcj/ jFoj.
P
P
b
Rw = {[ w(F2o ꢀ Fc)2/[ w(F2o)2]}1/2; w = 1/[
r
2(Fo)2 + (xP)2 + yP] where P = (Fo2 + 2F2c)/3.

2750
D.C. Haagenson et al. / Journal of Organometallic Chemistry 693 (2008) 2748–2754
Fig. 2. Solid-state structure and partial labeling scheme of 2. Thermal ellipsoids are
drawn at the 50% probability level, and the hydrogen atoms have been omitted for
improved clarity.
Fig. 1. Asymmetric unit and partial labeling scheme of 1. The thermal ellipsoids are
drawn at the 35% probability level. With the exception of the amino- and hydroxyl
hydrogens, all hydrogen atoms have been omitted to improve clarity. Hydrogen
bonds are indicated by dashed lines.
As Fig. 2 shows, and in notable contrast to the results obtained
by Norman et al., the product obtained from this reaction is the
O,O0-disilylated isomer. The molecules are located on the twofold
rotation axes of the space group C2/c and are, thus, rigorously C2
symmetric. This bis(N,N,N-phosphoramidate) consists of an inor-
the trimethylsilyl groups were attached to the oxygen or the nitro-
gen atoms. We therefore conducted an X-ray analysis on single
crystals of 2, the results of which are shown in Fig. 2. The crystal
and data collection parameters obtained from this study are listed
in Table 1, while Table 2 contains selected bond parameters.
ganic O(N@P(l-N)2P@N)O core that is studded with four tert-butyl
groups and two trimethylsilyl groups. The substantial steric repul-
sions of these bulky attachments dictate their relative conforma-
tions. Thus, the tert-butyl substituents of the imino-nitrogen
atoms (N2) have endo-conformations, whereas the trimethylsilyl
Scheme 1.
Scheme 2.

D.C. Haagenson et al. / Journal of Organometallic Chemistry 693 (2008) 2748–2754
2751
groups, with their longer N–Si bonds, are disposed in an exo fash-
ion. Due to the intramolecular repulsion of their tert-butyl substit-
uents, the separation of the exocyclic nitrogen atoms (5.099 Å) is
much larger than that of the oxygen atoms (3.317 Å). The silyla-
tions have converted the P–O double bonds to single bonds, while
P1 and N1 are now connected by a double bond. These changes are
reflected in phosphorus–nitrogen bonds that have shortened from
1.622(2) Å in 1 to 1.520(2) Å in 2. Analogously, the P–O bonds have
lengthened from 1.467(2) to 1.592(2) Å, while the newly-formed
Si–O bonds are 1.670(2) Å long—a typical bond length for silicon–
oxygen single bonds.
The metric parameters of the heterocycle changed but little
during the chemical transformation from 1 to 2. Thus, the P–N
bonds are only slightly longer in 2 (1.696(2) vs. 1.678(2) Å in 1),
while all the endocyclic angles of this ring remained virtually
unchanged.
We then carried the silylation reaction sequence out on B, a
mono-N,N,N phosphoramidate in which the exocyclic nitrogen
atom bears a phenyl, rather than a tert-butyl, substituent. The
NMR spectral analysis revealed that here, too, only one product
(3) had formed, but the spectra were again inconclusive regarding
the position of the trimethylsilyl group. A single-crystal X-ray
study showed (Fig. 3) that in this reaction the N-silylated phos-
phoramidate, 3, had been produced. Its crystal and refinement data
are listed in Table 1, while Table 3 contains selected bond param-
eters (Scheme 3).
Fig. 3. Thermal-ellipsoid (50% probability) plot and partial labeling scheme of 3.
To test the utility of these silylated phosphoramidates as ligand
transfer reagents, we treated a toluene solution of VCl3(THF)3 with
3, as shown in Scheme 4. Despite extended reaction times (12 h,
50 °C), however, we were unable to observe any signs of chlorotri-
methylsilane elimination. Instead, we isolated the vanadium com-
plex 4, as an adduct of 3 with VCl3(THF).
Hydrogen atoms have been omitted to improve clarity.
Table 3
Selected bond lengths (Å) and angles (°) for 3 and 4
3
4
P–N(term)
P–N( ) avg.
Si–N( ) avg.
Si–N(term)
P–O
1.6706(15)
1.6700(15)
1.7432(16)
1.7708(17)
1.4753(14)
1.664(6)
1.648(5)
1.750(5)
1.774(5)
1.496(4)
2.2419(18)
1.990(4)
2.145(4)
Fig. 4 shows the solid-state structure of 4, while selected crys-
tal- and refinement data, and bond parameters are listed in Tables
1and 3, respectively. The vanadium(III) complex has a trigonal-
bipyramidal coordination geometry, with the chloride ligands
occupying the equatorial plane, while the two-oxygen donor li-
gands (3 and THF) are in the axial positions. The vanadium–chlo-
ride bonds are, on average, 2.2419(18) Å long, a value that agrees
well with the lengths of such bonds in similar complexes [28].
The bond angles are close to those expected for trigonal-bipyrami-
dal geometry, with an almost linear O–V–O arrangement of
175.73(17)° and Cl–V–Cl angles that range from 116.31(8)° to
124.09(8)°. The V–O bonds are decidedly different in length, the
donor bond with the sp2-hybridized oxygen atom (O1) being sub-
stantially shorter (1.990(4) Å) than that with the THF ligand, which
is rather long at 2.145(4) Å. Steric factors appear to control the con-
formations of both O-donor ligands with respect to the VCl3 moi-
l
l
V–Cl, avg.
V–O(P@O)
V–O(THF)
N(term)–P–N(
N(term)–P–O
N(l)–P–O
N–Si–N
l
) avg.
110.75(8)
107.57(8)
119.40(8)
83.30(7)
113.2(3)
104.0(2)
118.9(2)
82.5(2)
126.8(3)
175.73(17)
124.09(8)
118.08(8)
116.31(8)
94.13(13)
85.96(13)
Si–N(term)–P
122.60(9)
O(1)–V(1)–O(2)
Cl(1)–V(1)–Cl(2)
Cl(2)–V(1)–Cl(3)
Cl(1)–V(1)–Cl(3)
O(1)–V(1)–Cl, avg.
O(2)–V(1)–Cl, avg.
Scheme 3.

2752
D.C. Haagenson et al. / Journal of Organometallic Chemistry 693 (2008) 2748–2754
Scheme 4.
gand, the tert-butyl groups, in turn, dip between the chloride li-
gands, while the THF ligand bisects two of the Cl–V–Cl angles.
Although the attempted N,O ligation of vanadium by 3 failed
under the conditions employed by us, the structure of 4 is nonethe-
less interesting because it presumably represents an intermediate
in the reaction coordinate to the targeted product. More forcing
conditions are likely necessary to effect the desired chlorotrimeth-
ylsilane elimination and the ligation of the vanadium ion by 3 in a
bidentate-chelating manner.
3. Discussion
Because of their ambidenticity phosphoramidates may be sily-
lated at either nitrogen or oxygen, and examples of both isomers
have been reported in the literature [29,30]. The silylations of the
phosphoramidates A and B with Me3SiCl yielded the bis-O-silylat-
ed product (2) in the former and the mono-N-silylated product (3)
in the latter. While the dissimilar outcomes of these reactions
might be rationalized in terms of kinetic versus thermodynamic
control, both, the O,O0-disilyl derivative, 2, and the N-silyl deriva-
tive, 3, are thermodynamic products.
Silicon has an exceptionally high affinity for oxygen, the sili-
con–oxygen single bond (445 kJ molꢀ1) being among the strongest
single bonds known; it is ca. 90 kJ molꢀ1 stronger than the analo-
gous silicon–nitrogen bond [30]. This substantial energy difference
dictates that in ambidentate molecules having both –OH and –NH
groups, silylation usually occurs at oxygen, as long as steric and ki-
netic effects are negligible. We hasten to point out, however, that
silylations are considerably more complex than suggested here,
as numerous experimental and computational studies have shown
[31–35] (Chart 1).
In phosphoramidates, by contrast, where silylation may also oc-
cur at either nitrogen or oxygen, the overall thermodynamics are
quite different. Because the conversion of the phosphoryl bond
(620 kJ molꢀ1) to a phosphorus–oxygen single bond (368 kJ molꢀ1
)
exacts a significant thermodynamic penalty, phosphoramidates are
usually silylated at nitrogen. The bond-enthalpy changes accompa-
nying phosphoramidate metallations with element trihalides were
studied in detail by Glidewell and Duncan [36,37]. Because com-
pounds 2 and 3 are thermodynamic products they are amenable
to the same bond-enthalpy estimates used by these researchers,
whose estimated bond enthalpies we adopt here. In Chart 2 we
contrast the two conceivable structural isomers for silylated phos-
Fig. 4. Thermal ellipsoid plot and partial labeling scheme of 4. The thermal
ellipsoids are drawn at the 50% probability level and all hydrogen atoms have been
omitted for clarity.
ety. For example, ligand 3 is disposed in such a manner that its ste-
rically least-demanding feature, namely the four-membered ring,
straddles one of the V–Cl bonds. The bulkiest substituents of the li-

D.C. Haagenson et al. / Journal of Organometallic Chemistry 693 (2008) 2748–2754
2753
Chart 3.
4. Summary
Chart 1.
Contrary to conventional assumptions about the oxophilicity of
silicon, phosphoramidates are preferentially silylated at nitrogen,
because this structural isomer preserves the strong phosphorus–
oxygen double bond of these molecules. If the nitrogen atom of
these ambidentate molecules bears significant steric bulk, how-
ever, the O-silylated isomer may be the thermodynamically more
stable product.
Although the trimethylsilyl-derivatized phosphoramidates 2
and 3 proved unsuitable as ligand transfer reagents to transition
metal under mild conditions, they may find uses in transforma-
tions involving the more reactive metalloid- and nonmetal halides,
such as SiCl4 and PCl3.
Chart 2.
5. Experimental
Table 4
Relevant bond enthalpies for 2 and 3, as obtained from Ref. [15]
All experiments were performed under an atmosphere of puri-
fied nitrogen or argon, using standard Schlenk techniques. Solvents
were dried and freed of molecular oxygen by distillation under an
atmosphere of nitrogen from sodium or potassium benzophenone
ketyl immediately before use. NMR spectra were recorded on a
Bruker AVANCE-500 NMR spectrometer. The 1H, 13C, 29Si, and 31P
NMR spectra are referenced relative to C6D5H (7.15 ppm), C6D6
(128.0 ppm), TMS (0.0 ppm) and P(OEt)3 (137.0 ppm), respectively.
Melting points were obtained on a Mel-Temp apparatus; they are
uncorrected. Elemental analyses were performed by E and R Micro-
analytical Services, Parsipanny, NJ.
2
kJ molꢀ1
3
kJ molꢀ1
Si–N
P–N
P@O
R
335
347
620
Si–O
P@N
P–O
R
445
459
368
1302
1272
phoramidates, while we list the relevant estimated bond enthal-
pies and their sums for each isomer in Table 4.
Discounting steric factors, and focusing strictly on bond enthal-
pies, N-silylation is favored over O-silylation by approximately
30 kJ molꢀ1 per P@O(N) moiety. This thermodynamic estimate is
supported by synthetic studies where N-silylated products pre-
dominate [36]. If the nitrogen atom bears any steric bulk at
all, however, the energy differences between both isomers are
substantially smaller. Thus, for example, based on variable-
temperature NMR studies, it was estimated that an N,O,O-trip-
henylphosphoramidate was only 7.1 kJ molꢀ1 more stable than
the corresponding O-silylated derivative [30]. Because of the low
activation energies for 1,3-N,O-trimethylsilyl migrations such
small energy differences between isomeric N- and O-silylated iso-
mers usually manifest themselves in fluxional molecules [29,30].
Therefore, in contrast to most silylation reactions, phosphoram-
idates are typically silylated at nitrogen, making the N-silylated 3
the ‘‘conventional” product, while the O-silylated 2 is the ‘‘uncon-
ventional” product. The absence of any fluxionality in the bis
(O-silylated) 2 further confirms that it must be substantially more
stable than the alternative bis(N,N0-silylated) isomer. It is easy to
see why this might be the case, because a bis(N,N0-silylated) deriv-
ative ( Chart 3) would suffer from excessive steric strain due to no
less than three repulsive interactions. The existence of isomer 2 is a
reminder that non-bonding interactions can play as large, if not a
larger, role than covalent bonds, even in the selection of thermody-
namic products.
Chlorotrimethylsilane and cumene hydroperoxide were pur-
chased from Petrarch and Aldrich, respectively; they were used
as received. The heterocycles [Me2Si(
l
-NtBu)2P@O(NHPh)] [38]
and cis-[(tBuHN)O@P( -NtBu)2P@O(NHtBu)] [15] were synthesized
l
according to published procedures.
6. Syntheses
6.1. cis-[(Me3SiO)tBuN@P( -NtBu)2P@NtBu(OSiMe3)] (2)
l
A two-necked, 100 mL round-bottomed flask equipped with a
dropping funnel, inlet, and stirbar, was charged with 1 (2.46 g,
6.48 mmol). This white solid was suspended in 30 mL of hexanes.
Then n-butyllithium (1.88 mL, 7.70 mmol) in 5 mL THF was added
dropwise, and the resulting yellow solution was heated to reflux
for 1 h. The solution was then allowed to cool to room temperature
and Me3SiCl (0.60 mL, 4.7 mmol) in 5 mL of hexanes was added
dropwise with vigorous stirring. The resulting milky-white mix-
ture was warmed to 50 °C and kept at that temperature for 16 h.
All liquids were removed in vacuo and the colorless solid was
extracted with 30 mL of hexanes, filtered on a fine frit and concen-
trated to half its volume. After the clear, colorless solution had
been kept in a ꢀ21 °C freezer for 2 days, colorless needles
(0.891 g) formed. Yield: 72.1%.

2754
D.C. Haagenson et al. / Journal of Organometallic Chemistry 693 (2008) 2748–2754
M.p.: 122 °C dec. 1H NMR (500.13 MHz, benzene-d6, 26 °C) d =
reduced with SAINT, which corrects for Lp and decay. An empirical
absorption correction was applied with SADABS [41]. The structures
were solved by direct methods with the SHELXS-90 [42] program
and refined by full-matrix least squares methods on F2 with SHEL-
XL-97 [43], incorporated in SHELXTL-PC, Version 5.03 [44].
1.49 (s, 18H, NtBu), 1.46 (s, 18H, NtBu), 0.36 (s, 18H, SiMe3).
13C{1H} NMR (125.76 MHz, benzene-d6, 26 °C) d = 53.6 (s, NtBu),
51.0 (s, NtBu), 34.7 (t, JPC = 5.97 Hz, NtBu), 31.3 (t, JPC = 4.72 Hz,
NtBu), 1.76 (s, SiMe3). 31P{1H} NMR (202.46 MHz, benzene-d6,
26 °C) d = ꢀ42.15 (s). Anal. Calc. for C22H54N4P2O2Si2: C, 50.35; H,
10.37; N, 10.67. Found: C, 50.02; H, 10.10; N, 10.81%.
Appendix A. Supplementary material
6.2. [Me2Si(
l
-NtBu)2P@O{N(SiMe3Ph)}] (3)
CCDC 680343, 680344, 680345, and 680346 contain the supple-
mentary crystallographic data for 1, 2, 3, and 4. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2008.05.023.
A sample of [Me2Si(
l
-NtBu)2P@O(NHPh)] (2.24 g, 6.60 mmol),
dissolved in 20 mL of toluene, was treated with n-butyllithium
(4.4 mL, 7.0 mmol, 1.6 M) dropwise at 0 °C. The solution remained
clear and colorless while it was refluxed for 1 h. Chlorotrimethylsi-
lane (0.89 mL, 7.0 mmol), dissolved in 5 mL of toluene, was added
dropwise to this solution at room temperature. The mixture was
refluxed for 16 h, during which time a white precipitate formed.
The solution was then filtered through a medium-porosity frit
and concentrated to dryness in vacuo. The remaining solid was ex-
tracted with hexanes, filtered and stored at ꢀ21 °C. Several crops of
colorless crystals yielded 2.14 g (79.0%) of product.
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M.p.: 98–100 °C. 1H NMR (500.13 MHz, benzene-d6, 26 °C) d
7.30 (d, 2H, J = 8.0 Hz, o-Ph), 7.08 (t, 2H, J = 7.6 Hz, m-Ph), 6.98
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-NtBu)2P@O(N(SiMe3)Ph)]VCl3(THF)} (4)
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43.09; H, 7.23; N, 6.55. Found: C, 42.78; H, 7.01; N, 6.77%.
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7. X-ray crystallography
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7.1. Compounds 1, 2, 3, and 4
A sample of A was synthesized as previously reported [15]. Sin-
gle crystals of 1 were obtained when the reaction mixture was con-
centrated in vacuo and stored at ꢀ20 °C for 5 days. Suitable, single
crystals were coated with oil, attached to a glass capillary, and cen-
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intensities were collected with a Bruker SMART CCD diffractome-
ter, equipped with an LT-2 low-temperature apparatus, operating
at 213 K. Data were measured using
x scans of 0.3° per frame for
30 s until a complete hemisphere had been collected. The first 50
frames were recollected at the end of the data collection to monitor
for decay. Cell parameters were retrieved using SMART [39] software
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