Dynamic behavior of intramolecularly base-stabilized phosphatetrylenes. insights into the inversion processes of trigonal pyramidal geramanium(II) and tin(II) centers

Article abstract of DOI:10.1021/om8011757

The reaction between SnCl₂ and either 1 or 2 equiv of the lithium salt [{(Me₃Si)₂CH}(C₆H ₄-2-CH₂NMe₂)P]Li gives the heteroleptic compound [{(Me₃Si)₂CH}(C

Full text of DOI:10.1021/om8011757

Organometallics 2009, 28, 3327–3337
3327
Dynamic Behavior of Intramolecularly Base-Stabilized
Phosphatetrylenes. Insights into the Inversion Processes of Trigonal
Pyramidal Geramanium(II) and Tin(II) Centers
Keith Izod,* John Stewart, Ewan R. Clark, William McFarlane, Ben Allen, William Clegg,
and Ross W. Harrington
Main Group Chemistry Laboratories, School of Chemistry, Bedson Building, UniVersity of Newcastle,
Newcastle upon Tyne, NE1 7RU, U.K.
ReceiVed December 11, 2008
The reaction between SnCl2 and either 1 or 2 equiv of the lithium salt [{(Me3Si)2CH}(C6H4-2-
CH2NMe2)P]Li gives the heteroleptic compound [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]SnCl (7) and the
homoleptic, intramolecularly base-stabilized diphosphastannylene [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]2Sn
(8), respectively, in good yields. The solid state structure of 8 shows that the tin(II) center is three-
coordinate, bound by the N and P atoms of a chelating phosphide ligand and the P atom of a second
phosphide ligand. Both 7 and 8 are highly dynamic in solution. Variable-temperature NMR spectra suggest
that compound 7 and its germanium analogue 5 are subject to two distinct dynamic processes in polar
solvents, which are attributed to the formation of adducts between either 5 or 7 and the free phosphine
{(Me3Si)2CH}(C6H4-2-CH2NMe2)PH (9) and interconversion between diastereomers of these adducts.
Adduct formation is observed only in polar solvents and may be associated with the formation of weakly
bound [[{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]E(L)]+ · · · Cl- ion pairs in solution. The dynamic behavior
of 8 has been studied by multielement and variable-temperature NMR experiments; at high temperatures
there is rapid equilibrium between diastereomers, but at low temperatures a single diastereomer
predominates and exchange between the chelating and terminal phosphide ligands is frozen out. DFT
calculations on the model compound {(Me)(C6H4-2-CH2NMe2)P}SnCl (7a) suggest that epimerization
occurs either through a vertex-inversion process at phosphorus [Einv ) 65.3 kJ mol-1] or an edge-inversion
process at tin [Einv ) 141.0 kJ mol-1], of which the former is clearly favored. DFT calculations on the
model complex {(Me)(C6H4-2-CH2NMe2)P}2Sn (8a) indicate that the lowest energy dynamic process
involves exchange between the chelating and terminal phosphide ligands via a pseudotrigonal bipyramidal
intermediate [E ) -12.6 kJ mol-1]. Inversion at tin in 8a (via an unusual hybrid edge/vertex-inversion
process) is calculated to have a barrier of 206.3 kJ mol-1, whereas the barriers to vertex-inversion at
phosphorus are 59.4 and 51.0 kJ mol-1 for the chelating and terminal phosphorus atoms, respectively.
phosphagermylenes [CH{(CMe)(2,6-iPr2C6H3N)}2]Ge(PR2) [PR2
) PH2, PH(SiMe3), P(SiMe3)2, 1/2(PH-PH)].9 In addition, Power
and co-workers recently reported the synthesis and spectroscopic
characterization of the monomeric diphosphastannylene [{2,6-
Introduction
Stable diaminotetrylenes, (R2N)2E [E ) Si, Ge, Sn, Pb], were
first reported by Lappert and co-workers in the 1970s, and since
then, examples of these compounds have become relatively
numerous.1-4 In contrast, examples of the corresponding diphos-
phatetrylenes, (R2P)2E, remain somewhat scarce. Crystallographi-
cally characterized diphosphatetrylenes are limited to just a
very few examples: the dimeric compounds {(tBu2P)2Pb}2,5
{((Me3Si)2P)2Pb}2,6 and {(iPr2P)2Ge}27 and the monomeric com-
pounds [{(Tripp)2FSi}(iPr3Si)P]2E [E ) Ge, Sn (1), Pb; Tripp )
2,4,6-iPr3C6H2];8 Driess and co-workers have also recently reported
the crystal structures of the heteroleptic ꢀ-diketiminate-supported
(4) For leading references to diamidogermylenes, -stannylenes, and
-plumbylenes see:(a) Fjeldberg, T.; Hope, H.; Lappert, M. F.; Power, P. P.;
Thorne, A. J. J. Chem. Soc., Chem. Commun. 1983, 639. (b) Olmstead,
M. M.; Power, P. P. Inorg. Chem. 1984, 23, 413. (c) Tang, Y.; Felix, A. M.;
Zakharov, L. N.; Rheingold, A. L.; Kemp, R. A. Inorg. Chem. 2004, 43,
7239. (d) Avent, A. G.; Drost, C.; Gehrus, B.; Hitchcock, P. B.; Lappert,
M. F. Z. Anorg. Allg. Chem. 2004, 630, 2090. (e) Gans-Eichler, T.; Gudat,
D.; Nieger, M. Angew. Chem., Int. Ed. 2002, 41, 1888. (f) Chorley, R. W.;
Hitchcock, P. B.; Lappert, M. F.; Leung, W.-P.; Power, P. P.; Olmstead,
M. M. Inorg. Chim. Acta 1992, 198, 203. (g) Braunschweig, H.; Hitchcock,
P. B.; Lappert, M. F.; Pierssens, L. J.-M. Angew. Chem., Int. Ed. Engl.
1994, 33, 1156. (h) Lappert, M. F.; Slade, M. J.; Atwood, J. L.; Zaworotko,
M. J. J. Chem. Soc., Chem. Commun. 1980, 621. (i) Bazinet, P.; Yap,
G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2001, 123, 11162.
(5) Cowley, A. H.; Giolando, D. M.; Jones, R. A.; Nunn, C. M.; Power,
J. M. Polyhedron 1988, 7, 1909.
* Corresponding author. E-mail: k.j.izod@ncl.ac.uk.
(1) For reviews see: (a) Barrau, J.; Rima, G. Coord. Chem. ReV. 1998,
178-180, 593. (b) Tokitoh, N.; Okazaki, R. Coord. Chem. ReV. 2000, 210,
251. (c) Kira, M. J. Organomet. Chem. 2004, 689, 4475. (d) Weidenbruch,
M. Eur. J. Inorg. Chem. 1999, 373. (e) Klinkhammer, K. W. In Chemistry
of Organic Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.;
Wiley: New York, 2002; Vol. 2, pp 283-357. (f) Veith, M. Angew. Chem.,
Int. Ed. Engl. 1987, 26, 1.
(2) For reviews of N-heterocyclic silylenes see: (a) Hill, N. J.; West, R.
J. Organomet. Chem. 2004, 689, 4165. (b) Gehrhus, B.; Lappert, M. F. J.
Organomet. Chem. 2001, 617, 209.
(6) Goel, S. C.; Chiang, M. Y.; Rauscher, D. J.; Buhro, W. E. J. Am.
Chem. Soc. 1993, 115, 160.
(7) Druckenbrodt, C.; du Mont, W.-W.; Ruthe, F.; Jones, P. G. Z. Anorg.
Allg. Chem. 1998, 624, 590.
(8) Driess, M.; Janoschek, R.; Pritzkow, H.; Rell, S.; Winkler, U. Angew.
Chem., Int. Ed. 1995, 34, 1614.
(3) For a review of N-heterocyclic germylenes see: Ku¨hl, O. Coord.
Chem. ReV. 2004, 248, 411.
(9) Yao, S.; Brym, M.; Merz, K.; Driess, M. Organometallics 2008,
27, 3601.
10.1021/om8011757 CCC: $40.75
2009 American Chemical Society
Publication on Web 05/05/2009

3328 Organometallics, Vol. 28, No. 12, 2009
Izod et al.
Scheme 1
(2,4,6-Me3C6H2)2C6H3}(Ph)P]2Sn(2).10Theatecomplexes(tBu2P)E(µ-
tBu2P)2Li(THF) [E ) Sn (3), Pb]11 and several homometallic Sn(II)
phosphinidene clusters and heterometallic Ca/Sn or Ba/Sn phos-
phide and phosphinidene clusters have also been reported.12
(C6H4-2-CH2NMe2)P]2Ge (6).16 Multielement and variable-
temperature NMR experiments clearly showed that inversion
at phosphorus is a relatively low energy process in these
compounds and that, consequently, interconversion between
diastereomers is rapid on the NMR time scale at room
temperature. We now report the synthesis and characterization
of the tin(II) analogues of 5 and 6 and present NMR and
theoretical data concerning the dynamic behavior of these
species.
The scarcity of diphosphatetrylenes may, at least in part, be
attributed to the perception that pπ-pπ interactions between
the vacant tetrel p-orbital and the lone pairs at phosphorus would
be poor and, thus, that the electron deficiency of the tetrel center
would be alleviated to only a very limited extent in these
compounds. This situation contrasts with the excellent pπ-pπ
overlap found in diaminotetrylenes, which contributes signifi-
cantly to the stability of these species. However, it has been
calculated that the inherent π-donor capability of phosphorus
is at least as great as that of nitrogen.13 The poor P-E pπ-pπ
overlap in diphosphatetrylenes is largely a consequence of the
considerable barrier to inversion of phosphorus and the attendant
difficulty in achieving the planar configuration necessary for
optimal pπ-pπ interactions. However, such planarity may be
promoted by a range of strategies, including the use of sterically
demanding substituents at the pnictogen center and/or incor-
poration of the heteroatoms into a cyclic system; these strategies
were recently employed by Bertrand and co-workers in their
landmark synthesis of the first P-heterocyclic carbene, 4.14 With
particular relevance to the current report, it has also been
demonstrated previously that electropositive substituents, e.g.,
SiR3, significantly lower the barrier to inversion of tertiary
phosphine and arsine centers.15
Results and Discussion
Synthesis and Structural Characterization. The reaction
between SnCl2 and 1 equiv of the lithium phosphide [{(Me3Si)2-
CH}(C6H4-2-CH2NMe2)P]Li in THF gives the heteroleptic
phosphastannylene [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]SnCl
(7) in good yield as a pale yellow, light-, heat-, and air-sensitive,
microcrystalline solid after workup (Scheme 1). Although
initially crystallized from cold diethyl ether, once isolated,
compound 7 is essentially insoluble in most common organic
solvents, including THF, but is sufficiently soluble in dichlo-
romethane to permit spectroscopic characterization (see below).
Due to its very low solubility, in spite of repeated attempts, we
were unable to obtain single crystals of 7 suitable for X-ray
crystallography; however, the identity of 7 was unambiguously
confirmed by elemental analysis and multielement (1H, 13C{1H},
31P{1H}, and 119Sn{1H}) NMR spectroscopy. Attempts to
derivatize 7 in order to obtain a soluble compound more
amenable to characterization have so far failed: treatment of 7
with a variety of alkylating agents such as NpLi, NpMgBr, and
Np2Zn (Np ) neopentyl) led to deposition of elemental tin and
formation of either the free phosphine {(Me3Si)2CH}(C6H4-2-
CH2NMe2)PH or the known diphosphine [{(Me3Si)2CH}(C6H4-
2-CH2NMe2)P]2.
The reaction between SnCl2 and 2 equiv of in situ generated
[{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]Li in THF gives the diphos-
phastannylene [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]2Sn (8) in
good yield (Scheme 1). The ready synthesis of 8 contrasts
markedly with the corresponding reaction between GeI2 and 2
equiv of [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]Li, which, under
the same conditions, gives the ate complex [{(Me3Si)2CH}(C6H4-
2-CH2NMe2)P]2Ge · Li2I2(OEt2)3; the homoleptic diphosphager-
mylene 6 may be prepared by the reaction between GeI2 and 2
equiv of the potassium salt [{(Me3Si)2CH}(C6H4-2-CH2-
NMe2)P]K.16 In contrast to 7, compound 8 is soluble in both
ethereal and hydrocarbon solvents; deep orange single crystals of
8 were obtained from cold n-hexane. Like 7 compound 8 has
limited thermal and photolytic stability; whereas the corresponding
diphosphagermylene 6 is stable at elevated temperatures, compound
8 begins to decompose above 50 °C in solution or on exposure to
ambient light.
We recently reported the synthesis and dynamic behavior
of the intramolecularly base-stabilized phosphagermylenes
[{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]GeCl(5)and[{(Me3Si)2CH}-
(10) Rivard, E.; Sutton, A. D.; Fettinger, J. C.; Power, P. P. Inorg. Chim.
Acta 2007, 360, 1278.
(11) Arif, A. M.; Cowley, A. H.; Jones, R. A.; Power, J. M. J. Chem.
Soc., Chem. Commun. 1986, 1446.
(12) For examples see: (a) Westerhausen, M.; Hausen, H.-D.; Schwarz,
W. Z. Anorg. Allg. Chem. 1996, 622, 903. (b) Westerhausen, M.; Krofta,
M.; Wiberg, N.; No¨th, H.; Pfitzner, A. Z. Naturforsch. B 1998, 53, 1489.
(c) Westerhausen, M.; Hausen, H.-D.; Schwarz, W. Z. Anorg. Allg. Chem.
1995, 621, 877. (d) Westerhausen, M.; Krofta, M.; Schneiderbauer, S.;
Piotrowski, H. Z. Anorg. Allg. Chem. 2005, 631, 1391. (e) Allan, R. E.;
Beswick, M. A.; Cromhout, N. L.; Paver, M. A.; Raithby, P. R.; Trevithick,
M.; Wright, D. S. Chem. Commun. 1996, 1501. (f) Garcia, F.; Hahn, J. P.;
McPartlin, M.; Pask, C. M.; Rothenberger, A.; Stead, M. L.; Wright, D. S.
Organometallics 2006, 25, 3275.
(13) Kapp, J.; Schade, C.; El-Nahasa, A. M.; Schleyer, P. von R.; Angew.
Chem., Int. Ed. Engl. 1996, 35, 2236.
(14) Martin, D.; Baceiredo, A.; Gornitzka, H.; Schoeller, W. W.;
Bertrand, G. Angew. Chem., Int. Ed. 2005, 44, 1700.
(15) (a) Beachler, R. D.; Casey, J. P.; Cook, R. J.; Senkler, G. H., Jr.;
Mislow, K. J. Am. Chem. Soc. 1987, 109, 338. (b) Beachler, R. D.; Mislow,
K. J. Am. Chem. Soc. 1971, 93, 773. (c) Beachler, R. D.; Andose, J. D.;
Stackhouse, J.; Mislow, K. J. Am. Chem. Soc. 1972, 94, 8060. (d) Driess,
M.; Merz, K.; Monse, C. Z. Anorg. Allg. Chem. 2000, 626, 2264.
Compound 8 is chiral at both phosphorus centers and at the
germanium atom; it crystallizes as discrete monomers in the
(16) Izod, K.; McFarlane, W.; Allen, B.; Clegg, W.; Harrington, R. W.
Organometallics 2005, 24, 2157.

Intramolecularly Base-Stabilized Phosphatetrylenes
Organometallics, Vol. 28, No. 12, 2009 3329
Figure 1. Molecular structure of one of the two independent
molecules of 8 with 40% probability ellipsoids and with H atoms
omitted for clarity. Selected bond lengths (Å) and angles (deg)
[values for the second molecule in square brackets]: Sn(1)-P(1)
2.5906(9) [Sn(2)-P(3) 2.6061(9)], Sn(1)-P(2) 2.6407(8) [Sn(2)-P(4)
2.6439(9)], Sn(1)-N(1) 2.422(3) [Sn(2)-N(3) 2.379(3)], P(1)-C(1)
1.865(3) [P(3)-C(33) 1.872(3)], P(1)-C(8) 1.821(3) [P(3)-C(40)
1.833(3)], P(2)-C(17) 1.892(3) [P(4)-C(49) 1.895(3)], P(2)-C(24)
1.846(3) [P(4)-C(56) 1.848(3)], P(1)-Sn(1)-N(1) 83.66(7)
[P(3)-Sn(2)-N(3) 85.92(8)], P(1)-Sn(1)-P(2) 90.80(3) [P(3)-
Sn(2)-P(4) 94.55(3)], N(1)-Sn(1)-P(2) 92.76(7) [N(3)-Sn(2)-P(4)
93.44(8)],C(1)-P(1)-C(8)108.69(15)[C(33)-P(3)-C(40)107.33(15)],
C(1)-P(1)-Sn(1) 112.74(11) [C(33)-P(3)-Sn(2) 107.91(11)],
C(8)-P(1)-Sn(1) 97.02(11) [C(40)-P(3)-Sn(2) 98.02(11)],
C(17)-P(2)-C(24) 102.18(14) [C(49)-P(4)-C(56) 101.95(15)],
C(17)-P(2)-Sn(1) 112.64(10) [C(49)-P(4)-Sn(2) 110.33(10)],
C(24)-P(2)-Sn(1) 101.47(10) [C(56)-P(4)-Sn(2) 100.95(10)].
Figure 2. Molecular structure of 5 with 40% probability ellipsoids
and with H atoms omitted for clarity. Selected bond lengths (Å)
and angles (deg): Ge-P 2.4205(16), Ge-N 2.191(5), Ge-Cl
2.2965(16), P-C(1) 1.866(6), P-C(8) 1.842(6), P-Ge-Cl 99.42(6),
P-Ge-N 88.70(13), N-Ge-Cl 95.73(14), Ge-P-C(1) 112.74(19),
Ge-P-C(8) 87.14(17), C(1)-P-C(8) 103.6(3).
compound suitable for X-ray crystallography, we now find that
single crystals may be obtained by slow evaporation of a
saturated solution of 5 in diethyl ether at room temperature.
The structure of 5 is shown in Figure 2, along with selected
bond lengths and angles.
Compound 5 is chiral at both the germanium and phosphorus
centers and crystallizes as a monomeric species possessing a
GeRPS configuration. The germanium atom is coordinated by
the P and N centers of a chelating phosphide ligand [P-Ge-N
bite angle 88.70(13)°] and by a chloride ligand, affording the
germanium center a highly pyramidal geometry [sum of angles
at Ge ) 283.85°]. The Ge-P distance of 2.4205(16) Å is slightly
longer than the Ge-P distances in the homoleptic compound 6
[Ge-P 2.4023(4) and 2.4114(4) Å],16 but is similar to Ge-P
distances in related compounds containing divalent germanium;
for example, the Ge-P distances in {(iPr2P)2Ge}2 range from
2.3981(11) to 2.4261(11) Å,7 whereas the Ge-P distance in
[CH{(CMe)(2,6-iPr2C6H3N)}2]Ge{PH(SiMe3)} is 2.4261(7) Å.9
The Ge-Cl distance of 2.2965(16) Å is similar to other
Ge(II)-Cl distances; for example, the Ge-Cl distance in
GeCl2(1,4-dioxane) is 2.2813(5) Å,17 whereas the Ge-Cl
distance in [HC{CMeN(2,6-i-Pr2C6H3)}2]GeCl is 2.295(12) Å.18
Although the Ge-Cl vectors of adjacent molecules in the unit
cell are aligned in an antiparallel fashion to give an elongated
Ge2Cl2 parallelogram, the intermolecular Ge · · · Cl distances are
4.535(2) Å, and so it is unlikely that there are any significant
intermolecular Ge · · · Cl interactions. For comparison, the Ge · · · Cl
distance between the essentially unassociated GeCl2 units in
GeCl2(1,4-dioxane) is 3.463(1) Å.17
j
space group P1 with two crystallographically independent
molecules of identical SnRPchRPtR stereochemistry in the asym-
metric unit (where Pch and Pt refer to the phosphorus atoms of
the chelating and terminal phosphide ligands, respectively),
which differ only trivially in their bond lengths and angles. The
molecular structure of 8 is shown in Figure 1, along with
selected bond lengths and angles. One phosphide ligand binds
the tin(II) center through its P and N atoms, forming a puckered,
six-membered chelate ring, while the second phosphide ligand
binds the tin center solely through its phosphorus atom; the lone
pair of the amino group of this second ligand is directed away
from the tin atom. The tin center is thus three-coordinate and
adopts a trigonal-pyramidal geometry due to the presence of a
stereochemically active lone pair [sum of angles at Sn ) 267.22°
(273.91° in the second molecule)].
The Sn-P distances to the chelating and terminal phosphide
ligands, 2.5906(9) and 2.6407(8) Å, respectively [2.6061(9) and
2.6439(9) Å, respectively, in the second molecule in the
asymmetric unit], are similar to the Sn-P distances in 1
[2.567(1) Å]8 and 3 [2.684(4), 2.702(3), and 2.671(4) Å].11 The
P-Sn-N bite angle of the chelating ligand [83.66(7)°; 85.92(8)°
in the second molecule] is significantly smaller than the
corresponding angle in the germanium analogue 6 [97.49(3)°],16
consistent with the larger size of tin compared to germanium.
The phosphorus atoms in both the chelating and terminal ligands
are distinctly pyramidal [sum of angles at P(1) 315.45°, P(2)
316.29°, P(3) 313.26°, P(4) 313.23°], suggesting very little
P-Sn pπ-pπ overlap, consistent with intramolecular base
stabilization of the Sn(II) center.
Solution Behavior of the Heteroleptic Phosphatetrylenes 5
and 7. Compounds 5 and 7 exhibit highly unusual dynamic
behavior in solution, which we attribute to a combination of (i)
inversion processes that interconvert the diastereomers of these
compounds and (ii) the formation of adducts between the
tetrylenes and the free phosphine {(Me3Si)2CH}(C6H4-2-
(17) Denk, M. K.; Khan, M.; Lough, A. J.; Shuchi, K. Acta Crystallogr.
Sect. C 1998, C54, 1830.
(18) Ding, Y.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G.; Power,
P. P. Organometallics 2001, 20, 1190.
During the course of this work we had cause to prepare again
the heteroleptic phosphagermylene 5 (see below).16 Although
we had previously been unable to obtain crystals of this

3330 Organometallics, Vol. 28, No. 12, 2009
Izod et al.
Scheme 2a
a
R ) CH(SiMe3)2.
CH2NMe2)PH (9). The evidence to support this proposal is
presented below.
We have previously shown that the epimerization of intramo-
lecularly base-stabilized heteroleptic phosphatetrylenes such as
5 may potentially proceed through (i) inversion at the tetrel
center via a planar transition state, (ii) inversion at the tetrel
center via E-N dissociation, rotation about the E-P bond, and
subsequent E-N recoordination (Scheme 2; E ) Ge, Sn, Pb),
and/or (iii) inversion at the phosphorus center. Theoretical
calculations suggest that, for compound 5, the favored epimer-
ization pathway is inversion at phosphorus, rather than at
germanium. We were interested to see what effect the increased
atomic number of the tetrel center in 7 would have on such
processes and so have undertaken a variable-temperature NMR
study of this compound. To our knowledge, the inversion of
trigonal-pyramidal tin(II) species has not previously been
probed; the diastereomeric nature of 7 and 8 and the relative
ease with which dynamic processes may be studied by 31P NMR
spectroscopy make these potentially excellent compounds for
gaining insight into such processes.
Once isolated in the solid state, compound 7 is almost
completely insoluble in most common organic solvents, includ-
ing THF; however, 7 dissolves sufficiently, although still rather
sparingly, in CD2Cl2 to obtain reasonable NMR spectra.
Compound 7 is readily hydrolyzed and decomposes on extended
exposure to ambient light or elevated temperatures, yielding the
free phosphine 9 as the only spectroscopically identifiable
product (we assume that the tin-containing byproduct of
hydrolysis is an insoluble tin hydroxide or oxide species that
cannot be observed spectroscopically, whereas thermolysis/
photolysis of 7 yields elemental tin). The combination of the
extremely limited solubility of 7 and its ready decomposition
has the consequence that NMR signals due to sparingly soluble
7 are always accompanied by signals due to the free phosphine
9, which is produced in small quantities as a byproduct of
decomposition during sample preparation, but which has a much
greater solubility than 7; in spite of our extensive experience
in the handling of highly air-sensitive materials, we were unable
to prepare a sample of 7 completely free from contamination
by 9. However, this contamination revealed rather unexpected
dynamic behavior for 7 and its germanium analogue 5, which
is described below.
Figure 3. Variable-temperature 31P{1H} NMR spectra of 7 in
CD2Cl2.
in an approximately 3:1 ratio. At the same time peak C broadens
and shifts to higher field, such that, at -69 °C, this peak lies at
-71.0 ppm (∆ν1/2 ) 340.3 Hz) [cf. -70.9 ppm (∆ν1/2 ) 7.7
Hz) for pure 9 at room temperature in this solvent]. A low-
temperature 31P EXSY spectrum indicates that, at -41 °C,
whereas the two species responsible for signals A and B are
undergoing rapid dynamic exchange, neither of these species
is exchanging with the free phosphine 9. In this context it is
notable that the variable-temperature 31P{1H} NMR spectra of
pure 9 in CD2Cl2 are invariant over the temperature range 20
to -80 °C, confirming that the broadening of peak C in the
31P{1H} spectra of 7 is a direct consequence of the presence of
the stannylene.
1
The room-temperature H NMR spectrum of 7 in CD2Cl2
consists of a single set of rather broad ligand resonances,
consistent with the presence of a single ligand environment on
the NMR time scale, along with a set of sharp signals due to 9.
At this temperature the diastereotopic N-methyl groups in 7 give
rise to a pair of equal intensity, very broad singlets, indicating
that the nitrogen center is not undergoing inversion and must,
therefore, be coordinated to the tin atom. As the temperature is
reduced, the signals due to 7 broaden and decoalesce, until, at
-80 °C, the spectrum exhibits two distinct sets of ligand
resonances in the approximate ratio 1.7:1. This is most clearly
manifested in the SiMe3 region of the spectrum, which contains,
in addition to signals due to 9, four singlets at -0.02, 0.09,
0.15, and 0.25 ppm, in the ratio 1:1.7:1.7:1, due to the
diastereotopic CH(SiMe3)2 groups.
The room-temperature 119Sn{1H} spectrum of 7 in CD2Cl2
consists of a broad doublet at 257 ppm (JPSn ) 1070 Hz). At
-41 °C the spectrum consists of a sharp doublet at 249 ppm
(JPSn ) 1102 Hz); no other signals are observed at this
temperature.
Although compound 7 is only very sparingly soluble in THF
once isolated, it proved possible to obtain variable-temperature
31P{1H} NMR data on the crude reaction mixture when 7 was
prepared in this solvent. Variable-temperature 31P{1H} NMR
spectra of the crude THF solution (containing a small amount
of d8-toluene) exhibit similar characteristics to those obtained
from solutions in CD2Cl2. At -66 °C the spectrum exhibits two
The room-temperature 31P{1H} NMR spectrum of 7 in CD2Cl2
consists of two very broad, overlapping singlets centered at
-41.1 (A) and -34.3 ppm (B), on which 117Sn/119Sn satellites
are not resolved, along with a sharp singlet at -69.5 ppm (C)
due to 9 (Figure 3). As the temperature is reduced, peaks A
and B sharpen and move to higher field, until, at -30 °C, the
spectrum consists of a sharp singlet at -43.3 ppm (A) exhibiting
satellites due to coupling to 117/119Sn (JPSn ) 1056 Hz) along
with a lower intensity, broader singlet at -36.0 ppm (B) (JPSn
) 1249 Hz). As the temperature is reduced further, signal A
sharpens and moves upfield slightly, while signal B broadens
and continues to shift to significantly higher field, until, at -69
°C, peaks A and B lie at -44.6 and -40.1 ppm, respectively,

Intramolecularly Base-Stabilized Phosphatetrylenes
Organometallics, Vol. 28, No. 12, 2009 3331
Figure 4. Variable-temperature 31P{1H} NMR spectra of (a) a mixture of 5 and 9 in CD2Cl2 and (b) a mixture of 5 and 9 in the presence
of an excess of NEt3 in CD2Cl2.
signals due to 7 at -48.0 and -49.2 ppm, while the free
phosphine signal lies at -72.1 ppm (∆ν1/2 ) 12.5 Hz).
The variable-temperature 1H and 31P{1H} NMR spectra of 7
in both CD2Cl2 and THF are clearly consistent with the presence
of two distinct phosphastannylene species, which are undergoing
rapid dynamic exchange at room temperature. At low temper-
atures this exchange process is frozen out and signals for both
species are evident, although one species (A) clearly predominates.
However, the low-temperature line broadening associated with
the free phosphine 9 was rather unexpected and suggests that a
second dynamic process is operating. No such line broadening
was observed in the variable-temperature 31P{1H} spectra of
the germanium analogue 5 in d8-toluene (in which the stannylene
7 is insoluble). This prompted us to investigate the behavior of
5 in the presence of the free phosphine 9 in CD2Cl2; the
increased solubility of 5 also enabled us to study this system in
somewhat greater detail than the relatively insoluble stannylene
7 (see below).
to a dynamic equilibrium between two species; although this
equilibrium does involve the free phosphine, it does not involve
direct exchange between either 5 or 7 and 9. The NMR data
suggest that the dynamic behavior observed for 5 and 7 in
CD2Cl2 may be attributed to two distinct processes: formation
of an adduct between 5 or 7 and the free phosphine 9 and
dynamic exchange between the diastereomers of these adducts.
The upfield shift in the position of the phosphine reso-
nances in these spectra suggests that both of the possible
stereoisomers of 5 (or 7) form an adduct with 9. Since the
free phosphine is itself chiral, each adduct possesses three
stereocenters, and thus the adducts may exist in up to four
diastereomeric forms. In the 31P{1H} spectra of a mixture of
5 and 9 (Figure 4a), the broad signals E and G observed at
low temperature may be assigned to the phosphide and
phosphine ligands, respectively, of one adduct diastereomer
and the sharp peaks D and H may be assigned to the
phosphide and phosphine ligands of a second stereoisomer.
Whereas peaks D and H remain sharp at low temperature,
peaks E and G broaden and move to higher field. This is
consistent with further decoalescence of signals E and G and
suggests that adduct E:G is undergoing moderately fast
epimerization, which we were unable to freeze out due to
the low temperature that this would require. In contrast,
adduct D:H does not exhibit this behavior; either D:H is
present as a single diastereomer or else epimerization of this
diastereomer is rapid on the NMR time scale at -80 °C and
so only an averaged signal is observed.
Unfortunately, with the available data it is not possible to
identify unambiguously the nature of these adducts. However,
the lack of any resolved 31P-31P coupling (cf. the 31P{1H}
spectra of 6 and 8 (see below)) suggests that the adduct is formed
via a Ge · · · N, rather than a Ge · · · P, interaction between the
tetrylene and the free phosphine; it is likely that a Ge · · · P
interaction would also be disfavored on steric grounds.
It is notable that there is no evidence for the formation of
adduct species in the NMR spectra of 5 in d8-toluene (nor
in the spectra of the homoleptic diphosphatetrylenes 6 and 8
in this solvent (see below)).8 The peaks due to residual 9 in
these spectra lie at the same position as that of a pure sample
The room-temperature 31P{1H} NMR spectrum of a mixture
of 5 and 9 in CD2Cl2 exhibits broad signals at -31.8 (D) and
-36.9 ppm (E), due to the phosphagermylene, along with a
sharp singlet at -70.5 ppm (F), due to 9 (Figure 4a). As the
temperature is reduced, peak D sharpens and remains at
essentially the same chemical shift, while peak E broadens and
moves upfield. Simultaneously, peak F broadens, moves upfield,
and decoalesces into two broad singlets G and H. Thus, at -80
°C, the 31P{1H} spectrum consists of a sharp singlet at -32.6
(D), a broad singlet at -44.6 (E), a moderately broad singlet at
-73.2 (G), and a sharp singlet at -76.7 ppm (H); in the absence
of proton decoupling, these latter signals are resolved as doublets
[JPH ) 222.4 Hz (G), 206.0 Hz (H)], clearly identifying these
signals as arising from secondary phosphine species. A 31P
EXSY spectrum of a mixture of 5 and 9 in CD2Cl2 at -70 °C
indicates that exchange between species D and E and between
species G and H is rapid, but that there is no exchange between
the pairs D/E and G/H at this temperature.
Thus, in d8-toluene compound 5 appears to be subject to a
dynamic equilibrium between two species (most likely diaster-
eomers), which does not appear to involve the free phosphine
9.16 In polar solvents (CD2Cl2 or THF) both 5 and 7 are subject

3332 Organometallics, Vol. 28, No. 12, 2009
Izod et al.
Scheme 3a
a
R ) CH(SiMe3)2.
of 9 (i.e., in the absence of tetrylene species) and remain
invariant over a wide temperature range, suggesting that
adduct formation occurs only in the presence of a polar
solvent. In this regard, it has been suggested by Barrau and
co-workers that the closely related, three-coordinate, hetero-
leptic, ꢀ-diketiminate-supported halogermylenes {PhNC(Me)-
CHC(Me)NPh}GeX [X ) Cl, I] may be regarded as weakly
bonded {PhNC(Me)CHC(Me)NPh}Ge+ · · · X- ions.19 Con-
sistent with this, it has been shown by Power and co-workers
that treatment of the sterically hindered ꢀ-diketiminate-
supported halogermylene {ArNC(Me)CHC(Me)NAr}GeX
with B(C6F5)3 in the presence of water readily yields the ion
pair [{ArNC(Me)CHC(Me)NAr}Ge][(HO)B(C6F5)3] [Ar ) 2,6-
iPr2C6H3].20a With this in mind, we tentatively suggest that
adduct formation in the present case may be associated with
the formation of weakly bonded [[{(Me3Si)2CH}(C6H4-2-
CH2NMe2)P]E(L)]+ · · · Cl- ion pairs in solution [E ) Ge, Sn;
L ) 9]. Adduct formation may then be attributed to the enhanced
Lewis acidity of the pseudo-two-coordinate tetrel center in the
putative cationic tetrylene intermediate [[{(Me3Si)2CH}(C6H4-
2-CH2NMe2)P]E]+. It is likely that [[{(Me3Si)2CH}(C6H4-2-
CH2NMe2)P]E(L)]+ · · · Cl- ion pairs would be significantly
stabilized in polar solvents, whereas the formation of such
species would be inhibited in nonpolar solvents, consistent with
our observation that broadening of the signal due to 9 occurs
only in the presence of 5 or 7 in CD2Cl2 or THF.
In order to obtain further evidence to support this proposal, we
undertook an NMR-scale experiment in which the free phosphine
9 was allowed to compete for the germanium center in 5 with the
strong base NEt3. The room-temperature 31P{1H} NMR spectrum
of a mixture of 5 and 9 in CD2Cl2, in the presence of an excess of
NEt3, contains two broad singlets at -31.5 and -44.5 ppm due to
phosphagermylene species, along with a sharp singlet at -70.9
ppm due to 9 (Figure 4b). As the temperature is reduced, these
three signals merely sharpen, without any significant changes in
chemical shift. This strongly suggests that, under these conditions,
NEt3 competes successfully for the Lewis acidic germanium center
and thus that the signals observed at -31.5 and -44.5 ppm may
be attributed to the two diastereomers of the adduct [{(Me3Si)2-
CH}(C6H4-2-CH2NMe2)P]Ge(Cl)(NMe3) [or of the weakly bond-
ed adduct ion pair [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]Ge-
(NMe3)+ · · · Cl-], neither of which interacts with 9. Thus, the
presence of NEt3 appears to inhibit the formation of an adduct
between 5 and 9.
Solution Behavior of the Diphosphastannylene 8. In
addition to the inversion processes (i-iii) accessible to 7,
compound 8 may undergo two further exchange processes: (iv)
inversion at the terminal, rather than the chelating, phosphorus
center and (v) exchange between the chelating and terminal
phosphide ligands. This latter process may proceed either via a
two-coordinate transition state (i.e., via a dissociative mecha-
nism, I) or via a four-coordinate, pseudotrigonal-bipyramidal
transition state (i.e., via an associative mechanism, II) (Scheme
3). Exchange via either mechanism may have two potential
outcomes, depending on the sense of the chelate ring formed
during this process, resulting in either retention or inversion of
stereochemistry at the tin center. For the dissociative mechanism,
both outcomes are equally possible; however, for the associative
mechanism the two possible pseudotrigonal-bipyramidal transi-
tion states differ considerably. The approach of Nb on an axis
bisecting the two phosphorus atoms generates a transition state
in which the two phosphorus centers occupy equatorial sites
and would, upon cleavage of the Na-Sn bond, lead to exchange
of Pa and Pb with retention of configuration at tin. In contrast,
the approach of Nb on an axis bisecting Na and Pb results in a
transition state in which the phosphorus centers occupy an axial
and an equatorial site and would lead to inversion at tin on
cleavage of the Na-Sn bond. Clearly, this latter pathway is likely
to be significantly disfavored on steric grounds, and so we
propose that exchange of the amino groups in 8 via an
associative pathway would most likely occur with retention of
configuration at tin (i.e., without interconversion of the diaster-
eomers).
The 31P{1H} spectrum of 8 in d8-toluene at 50 °C consists of
a very broad singlet at -49.3 ppm, exhibiting poorly resolved
satellites due to coupling to 117/119Sn (JPSn ≈ 1090 Hz) [we were
unable to obtain a good-quality spectrum of 8 at this temperature
due to its limited thermal stability]. As the temperature is
reduced, this signal broadens and begins to decoalesce, until,
at -40 °C, the spectrum consists of two sharp doublets at -49.1
and -59.5 ppm, each exhibiting poorly resolved satellites due
to coupling to tin (JPP ) 47.4 Hz; JPSn ) 1124 and 898 Hz,
respectively), consistent with the presence of distinct chelating
and terminal ligand environments at this temperature (Figure
5). Below -40 °C the 31P{1H} spectra of 8 do not change
significantly.
(19) Saur, I.; Miqueu, K.; Rima, G.; Barrau, J.; Lemierre, V.; Chros-
towska, A.; Sotiropolous, J.-M.; Pfister-Guillouzo, G. Organometallics 2003,
22, 3143.
(20) (a) Stender, M.; Phillips, A. D.; Power, P. P. Inorg. Chem. 2001,
40, 5314. (b) Driess, M.; Yao, S.; Brym, M.; van Wullen, C. Angew. Chem.,
Int. Ed. 2006, 45, 4349.
The room-temperature 1H NMR spectrum of 8 in d8-toluene
exhibits a single set of ligand resonances, consistent with
rapid exchange between the chelating and terminal phosphide

Intramolecularly Base-Stabilized Phosphatetrylenes
Organometallics, Vol. 28, No. 12, 2009 3333
omer of 8, which is present in solution at all temperatures and
which has an essentially identical structure to that observed in
the solid state; the only dynamic process operating in this case
would be interconversion between the chelating and terminal
phosphide ligands. An alternative explanation for the behavior
of 8 is that, in addition to the chelating-terminal ligand
exchange, a second dynamic process is operating at high
temperatures that involves epimerization via inversion at one
or both of the phosphorus centers and/or at the tin atom. DFT
studies (see below) suggest that 8 lies on a fairly shallow
potential energy surface and that the barriers to epimerization
of this compound are low, and so rapid exchange between
diastereomers is likely. At low temperatures only the most stable
of these diastereomers is observed.
We note that the heteroleptic compound 7 is highly
dynamic in solution and that the low-temperature 31P{1H}
spectra of 6, the germanium analogue of 8, clearly show the
presence of multiple diastereomers.16 In addition, previous
NMR studies have shown that stannyl substituents favor more
rapid inversion at phosphorus than germyl substituents (see
below). The foregoing, in combination with the DFT studies
presented below, suggests that the presence of a single
diastereomer of 8 in solution at room temperature is
somewhat unlikely. Thus, we propose that the variable-
temperature 1H and 31P{1H} NMR spectra of 8 are consistent
with rapid interconversion on the NMR time scale both
between diastereomers and between the chelating and ter-
minal phosphide ligands at room temperature and suggest
that epimerization is a lower energy, faster process than
exchange between the chelating and terminal ligands. This
allows rapid equilibration of the diastereomers, but the
relative stability of one of these has the consequence that, at
low temperatures, a single diastereomer predominates in
which exchange between the chelating and terminal ligands
is frozen out.
Figure 5. Variable-temperature 31P{1H} NMR spectra of 8 in d8-
toluene.
ligands on the NMR time scale. The spectrum consists of
broad singlets at 0.00 and 0.17 ppm due to the diastereotopic
SiMe3 groups, a doublet (JPH ) 6.4 Hz) at 0.68 ppm due to
the methine protons, a singlet at 2.13 ppm due to the NMe2
groups, two very broad singlets at ∼3.50 and 3.94 ppm due
to the CH2N protons, and a series of multiplets in the range
6.72-7.52 ppm due to the aryl protons. As the temperature
is reduced, these signals broaden and decoalesce until, at -60
°C, the spectrum exhibits two separate sets of signals in a
1:1 ratio, due to the chelating and terminal phosphide ligands.
The diastereotopic SiMe3 groups give rise to four equal
intensity signals at -0.16, 0.00, 0.06, and 0.19 ppm, while
the CH(SiMe3)2 protons give rise to singlets at -1.71 and
0.93 ppm. The diastereotopic NMe2 groups of the chelating
phosphide ligand give rise to two equal intensity signals at
1.35 and 2.24 ppm, and the NMe2 group of the terminal ligand
exhibits a single singlet of twice the intensity at 2.15 ppm
due to rapid inversion of the free amino group. The benzylic
protons of the terminal ligand give rise to a typical AB pattern
(JHH ) 14.35 Hz) centered at 4.24 and 4.09 ppm, the latter
signal exhibiting further coupling to phosphorus (JPH ) 4.25
Hz), whereas the CH2N protons of the chelating ligand give
rise to complex multiplets at 1.99 and 3.58 ppm; the signals
due to the aromatic protons lie in the range 6.25-7.82 ppm.
The simplicity of the low-temperature 1H and 31P{1H} spectra
of 8 clearly implies that a single diastereomer predominates
at low temperatures.
The significant differences between the dynamic behavior of
8 and 6 may be attributed to the different electronegativities of
Sn(II) and Ge(II). It has been shown previously that electro-
positive substituents such as SiMe3 or GeMe3 significantly
reduce the barrier to inversion at trigonal-pyramidal phosphorus
centers.15 For example, it has been reported that line-shape
1
analysis of the variable-temperature H NMR spectra of iP-
rPhP(GeMe3) and iPrPhP(CMe3) yield ∆Gq ) 89.5 and 136.8
kJ mol-1, respectively, for the phosphorus-inversion pro-
cess.15b The barrier to inversion at phosphorus decreases
further with decreasing electronegativity of the substituents,
such that, for iPrPhP(SnMe3) ∆Gq ) 80.8 kJ mol-1. Thus,
inversion at phosphorus in 8 should be more energetically
favorable than in 6.
The large chemical shift difference of 2.64 ppm between
the CH(SiMe3)2 protons observed at low temperature and the
unusually high-field shift (-1.71 ppm) of one of these protons
may be attributed to the orientation of these groups with
respect to the aromatic rings of the ligands. In the diastere-
omer observed by X-ray crystallography the methine proton
[H(1)] of the chelating phosphide ligand lies directly over
the plane of the aromatic ring of the terminal ligand at a
distance of 2.68 Å, and so this proton should experience a
strong upfield shift due to ring current effects. In contrast,
the methine proton [H(17)] of the terminal ligand lies close
to the plane of the same aromatic ring and so should
experience an opposite ring current effect to H(1), i.e., a
moderate downfield shift. This strongly suggests that the
diastereomer observed in the low-temperature 1H NMR
spectrum of 8 has the same configuration as that observed
crystallographically.
Line-shape analysis of the variable-temperature 31P{1H}
NMR spectra of 8 yields values of ∆Hq ) 37 ( 1 kJ mol-1
and ∆Sq ) -50 ( 3 J K-1 mol-1 for the chelating-terminal
phosphide exchange process in this molecule. The substantial
negative entropy of activation is consistent with the formation
of the pseudotrigonal-bipyramidal transition state required
for chelating-terminal ligand exchange via an associative
mechanism (see Scheme 3), whereas the moderate enthalpy
of activation may be attributed to the significant reorganiza-
tion energy associated with the formation of this transition
state and the substantial steric congestion present therein.
DFT Calculations. We have carried out a series of DFT
calculations in order to gain insight into the barriers to inversion
at the phosphorus and tin centers in both 7 and 8. In all cases
final geometries were obtained using the B3LYP hybrid
The variable-temperature 1H and 31P{1H} NMR spectra of 8
may potentially be explained as arising from a single diastere-

3334 Organometallics, Vol. 28, No. 12, 2009
Izod et al.
Scheme 4
germanium compound (5a1) that has a GeRPR configuration; the
two other minima, GeSPR (5a2) and GeRPS (5a3), diastereomers
of 5a1, were located in our previous study [relative energies
(B3LYP/6-31G(d,p)): 5a1 11.7, 5a2 16.3, 5a3 0.0 kJ mol-1].16
Previous theoretical studies have indicated that the substitution
patterns of tertiary pnictines profoundly affect the mechanism
of inversion at the pnictine center. Two inversion mechanisms
have been identified: classical inversion via a trigonal-planar
transition state (vertex-inversion) and inversion via a T-shaped
transition state (edge-inversion) (Scheme 4).24 It is widely
regarded that for tertiary pnictines the HOMO in trigonal-planar
transition states consists largely of a lone pair located in a pure
p-orbital on the pnictogen atom of a2′′ symmetry, which lies
perpendicular to the plane of the molecule, whereas the HOMO
in T-shaped transition states consists of an essentially s-type
lone pair of a1′ symmetry located on the pnictogen atom; the
LUMO in T-shaped transition states consists of a vacant
pnictogen p-orbital that lies perpendicular to the plane of the
molecule. Previous studies have shown that in pnictine systems
electronegative, σ-withdrawing ligands, such as F, favor edge-
inversion, whereas electropositive ligands, such as H, favor
vertex-inversion. In addition, ligands that themselves possess
lone pairs may potentially stabilize T-shaped transition states
through π-donation into the vacant pnictogen p-orbital.
Calculations by Arduengo and Dixon indicate that, whereas
vertex-inversion of PH3 has a barrier of 146.0 kJ mol-1, edge-
inversion has a barrier of 670.0 kJ mol-1; in contrast, the barriers
to vertex- and edge-inversion processes for PF3 are calculated
to be 356.9 and 225.1 kJ mol-1, respectively.24c In our previous
report we suggested that a similar preference exists for the
valence isoelectronic germanium(II) center in the trigonal-
pyramidal phosphagermylenes 5 and 6: whereas 6 favors a
vertex-inversion process, 5, which contains an electronegative
chloride ligand, favors an edge-inversion process.16
Figure 6. Optimized geometries of the three calculated minima of
7a (7a1-7a3) and of the transition states for vertex-inversion at
phosphorus (7a4) and edge-inversion at tin (7a5), with relative
energies (kJ mol-1) in square brackets; H atoms omitted for clarity.
functional,21 with a Lanl2dz effective core potential basis set
on tin22 and an all-electron 6-31G(d,p) basis set on the remaining
atoms.23 The location of minima and transition states was
confirmed by the absence or presence of one, imaginary
vibrational frequency, respectively; for transition states the
principal displacement vectors of the imaginary vibrational mode
were consistent with the appropriate inversion processes in each
case.
In order to determine the barriers to epimerization of 7 and 8,
we have used the model compounds {Me(C6H4-2-CH2NMe2)P}-
SnCl (7a) and {Me(C6H4-2-CH2NMe2)P}2Sn (8a), to save on
computational resources. Replacement of the sterically demanding
(Me3Si)2CH groups with small Me substituents is likely to
significantly underestimate the impact of steric effects on the
inversion processes studied. However, the model compounds retain
the overall structural skeletons of 7 and 8, and so we believe that
it is possible to draw meaningful conclusions regarding the dynamic
processes of these compounds from their models. In order that we
might compare the results of the present study directly with those
of our previous investigations into the corresponding germanium
compounds, we have also calculated single-point energies at the
B3LYP/6-31G(d,p) level of theory for the previously obtained
geometries of the ground and transition states of the model
compounds {Me(C6H4-2-CH2NMe2)P}GeCl (5a) and {Me(C6H4-
2-CH2NMe2)P}2Ge (6a).
For 7a we have located three minima of very similar energies,
which represent the ground state (SnSPS, 7a1), its phosphorus-
inverted epimer (SnSPR, 7a2), and an enantiomer of 7a2 in which
the chlorine atom lies in close proximity to the aromatic ring
of the phosphide ligand (SnRPS, 7a3) (Figure 6); minima 7a2
and 7a3 lie just 14.2 and 2.9 kJ mol-1 higher in energy than the
ground state, respectively. As a consequence of the foregoing,
we also located a third minimum energy configuration for the
For the heteroleptic phosphastannylene 7a transition states
corresponding to epimerization of the ground state diastereomer
were located for both vertex-inversion at phosphorus (7a4) and
edge-inversion at the tin center (7a5) (see Figure 6). We were
unable to locate transition states corresponding to vertex-
inversion at tin or edge-inversion at phosphorus, optimizations
(21) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P. J.;
Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98,
11623. (c) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997, 268, 345.
(22) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R.
J. Chem. Phys. 1985, 82, 299.
(23) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(b) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.;
DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
(24) (a) Dixon, D. A.; Arduengo, A. J., III; Fukunaga, T. J. Am. Chem.
Soc. 1986, 108, 2461. (b) Dixon, D. A.; Arduengo, A. J., III J. Chem. Soc.,
Chem. Commun. 1987, 498. (c) Dixon, D. A.; Arduengo, A. J., III J. Am.
Chem. Soc. 1987, 109, 338. (d) Schwerdtfeger, P.; Laakkonen, L. J.; Pyykko¨,
P. J. Chem. Phys. 1992, 96, 6807. (e) Schwerdtfeger, P.; Hunt, P. AdV.
Mol. Struct. Res. 1999, 5, 223. (f) Schwerdtfeger, P.; Boyd, P. D. W.;
Fischer, T.; Hunt, P.; Liddell, M. J. Am. Chem. Soc. 1994, 116, 9620. (g)
Go¨ller, A.; Clark, T. Chem. Commun. 1997, 1033.

Intramolecularly Base-Stabilized Phosphatetrylenes
Organometallics, Vol. 28, No. 12, 2009 3335
always converging to 7a5 and 7a4, respectively. Similarly, we
were unable to locate a transition state containing a two-
coordinate tin center, corresponding to inversion at the Sn center
via Sn-N dissociation/Sn-P rotation/Sn-N recoordination (see
Scheme 2), optimizations consistently converging to transition
state 7a5; this is consistent with our NMR observations, which
indicate that dissociation of the NMe2 group is slow on the NMR
time scale. It is notable that in our previous study of 5a we
were similarly unable to locate the corresponding transition
states for these latter processes. The calculated inversion
processes for the tin and phosphorus centers in 7a are consistent
with both Arduengo and Dixon’s and our own previous
observations regarding the effect of electronegative substituents
on the favorability of edge- and vertex-inversion processes.
Transition state 7a4, corresponding to inversion at phosphorus
via a vertex-inversion process, is 65.3 kJ mol-1 higher in energy
than the ground state 7a1. This compares with a barrier to
inversion at phosphorus of 84.9 kJ mol-1 for the germanium
analogue 5a at the same level of theory. The lower barrier to
phosphorus inversion in the phosphastannylene 7a compared
to the phosphagermylene 5a is again consistent with previous
observations that electropositive substituents lower the barrier
to vertex-inversion of tertiary pnictines (see above).
Transition state 7a5, corresponding to inversion of the tin
center via an edge-inversion process, is 141.0 kJ mol-1 higher
in energy than the ground state 7a1, and this process is clearly
disfavored in comparison to inversion at phosphorus via
transition state 7a4. For comparison, the corresponding barrier
to inversion at germanium in 5a is 179.1 kJ mol-1. The lower
barrier to inversion at tin compared to germanium is consistent
with previous calculations, which suggest that, whereas the
barrier to vertex-inversion of tertiary pnictines increases with
increasing atomic number, edge-inversion follows the opposite
trend. For example, the barriers to vertex-inversion of PH3,
AsH3, and SbH3 are calculated to be 146.0, 172.8, and 179.1
kJ mol-1, respectively, whereas the barriers to edge-inversion
of PF3, AsF3, and SbF3 have been calculated as 225.1, 193.7,
and 161.9 kJ mol-1, respectively.24c By analogy, the edge-
inversion barrier for the tin center in phosphastannylene 7a,
which is valence isoelectronic with a tertiary stibine, should be
lower than the edge-inversion barrier for the germanium center
in the phosphagermylene 5a, which is valence isoelectronic with
a tertiary arsine.
Figure 7. Optimized geometries of the four calculated minima of
8a (8a1-8a4), the calculated transition states for inversion at tin
(8a5), Pt (8a6), and Pch (8a7), and the pseudotrigonal-bipyramidal
intermediate (8a8), with relative energies (kJ mol-1) in square
brackets; H atoms omitted for clarity.
SnSPchRPtS configuration, although the SnSPchRPtR stereoisomer
(8a2), corresponding to the structure of 8 observed crystallo-
graphically, lies just 15.1 kJ mol-1 higher in energy [the
difference in chirality designations between 8 and 8a arises from
the replacement of the (Me3Si)2CH group in the former with a
Me group in the latter]. The SnSPchSPtS (8a3) and SnRPchRPtS (8a4)
diastereomers lie 0.4 and 6.7 kJ mol-1 above the ground state,
respectively.
The tin atom in 7a5 deviates only marginally from a strictly
T-shaped geometry. The P, Sn, N, and Cl atoms are coplanar
and the P-Sn-Cl angle is 180.0°; however, the P-Sn-N angle
is less than 90° due to constraints associated with chelate ring
formation (P-Sn-N 83.05°). The phosphorus center in 7a5
approaches planarity (sum of angles at phosphorus ) 349.84°),
although this atom is somewhat more pyramidal than that in
the corresponding transition state for 5a (sum of angles at
phosphorus ) 359.98°). The planarity of the phosphorus atoms
in 5a and 7a is entirely consistent with an edge-inversion
mechanism, in which the transition state is stabilized by pπ-pπ
interactions between the lone pairs at phosphorus and the vacant
p-orbital on the tetrel center (see Scheme 4). The more
pyramidal nature of the phosphorus atom in 7a5 compared to
5a5 may be attributed to less efficient overlap between the lone
pair on phosphorus and the large, diffuse 5p-orbital on Sn in
the former compared with overlap between the phosphorus lone
pair and the vacant 4p-orbital on Ge in the latter.
Unexpectedly, all attempts to obtain a transition state for
inversion at tin converged to transition state 8a5, in which the
tin atom adopts a planar geometry midway between trigonal
planar and T-shaped (P-Sn-P angle 157.73°, Figure 7). We
were unable to locate a transition state corresponding to either
pure edge- or pure vertex-inversion at the tin center for this
compound. For comparison, a trigonal-planar transition state
was located for inversion at the germanium center in 6a. The
barrier to inversion at tin via this hybrid edge/vertex-inversion
mechanism is 206.3 kJ mol-1; this compares with a barrier to
inversion at germanium in 6a via a vertex-inversion mechanism
of 191.2 kJ mol-1. The similarity of these two inversion barriers
appears to be a consequence of the differing inversion mech-
anisms in each case, since, by analogy with the valence
isoelectronic tertiary pnictines, the barrier to inversion via a
vertex-inversion process should increase, whereas the barrier
Minima were located for all four diastereomers of the model
compound 8a; these minima differ in energy by less than 16 kJ
mol-1 (Figure 7). The ground state diastereomer 8a1 adopts a

3336 Organometallics, Vol. 28, No. 12, 2009
Izod et al.
to inversion via an edge-inversion process should decrease with
increasing atomic number of the tetrel center.
low-energy process. Indeed, DFT calculations on the model
complex8asuggestthattheintermediateinthechelating-terminal
ligand exchange process, which contains a pseudotrigonal-
bipyramidal tin center, is 12.6 kJ mol-1 more stable than the
“ground state” diastereomer containing one chelating and one
terminal phosphide ligand.
As we found for 7a, we were unable to locate transition states
corresponding to edge-inversion of either the chelating or
terminal phosphorus centers in 8a, consistent with the presence
of the electropositive tin atom. The barriers to vertex-inversion
of the phosphorus centers in the terminal (8a6) and chelating
(8a7) ligands are calculated to be 51.0 and 59.4 kJ mol-1
,
Experimental Section
respectively; these values are substantially lower than those
calculated for the corresponding transition states in the germa-
nium analogue 6a (88.3 and 101.7 kJ mol-1, respectively). This
is consistent with our variable-temperature NMR results, which
indicate that interconversion between diastereomers is more
facile for 8 than for 6, and with previous NMR studies on the
inversion of tetrel-substituted tertiary pnictines (see above).
Attempts to locate a transition state containing a two-
coordinate tin center, corresponding to the transition state for
exchange between the chelating and terminal phosphide ligands
via a dissociative pathway, were unsuccessful; these calculations
consistently converged to give a four-coordinate pseudotrigonal-
bipyramidal tin center (8a8), corresponding to the transition state
for phosphide ligand exchange via an associative pathway. A
similar situation was found for the germanium analogue 6a, and
in keeping with our previous findings, at the B3LYP level of
theory, 8a8 is a minimum, rather than a transition state, with a
calculated energy 12.6 kJ mol-1 more stable than the “ground
state” 8a1. This implies that chelating-terminal ligand exchange
is favored over inversion at tin or phosphorus and runs counter
to our NMR observations, which suggest that, for 8, chelating-
terminal ligand exchange is less energetically favorable than
epimerization. Clearly, the steric bulk associated with the
(Me3Si)2CH substituents should significantly disfavor the forma-
tion of a pseudotrigonal-bipyramidal intermediate, and thus
exchange of the chelating and terminal ligands should be
disfavored with respect to the alternative epimerization pro-
cesses. Indeed, it is likely that an increase in steric bulk at the
tin and phosphorus centers in both 7 and 8 will result in
increased planarization of these atoms and a consequent
reduction in the barrier to inversion at these centers.
All manipulations were carried out using standard Schlenk
techniques under an atmosphere of dry nitrogen. Diethyl ether, THF,
n-hexane, and light petroleum (bp 40-60 °C) were dried prior to
use by distillation under nitrogen from sodium, potassium, or
sodium/potassium alloy. THF was stored over activated 4 Å
molecular sieves; diethyl ether, n-hexane, and light petroleum were
stored over a potassium film. Deuterated toluene was distilled from
potassium, and CD2Cl2 was distilled from CaH2; NMR solvents
were deoxygenated by three freeze-pump-thaw cycles and were
stored over activated 4 Å molecular sieves. Tin(II) chloride was
dried with chlorotrimethylsilane prior to use. The compounds
[{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]Li25 and [{(Me3Si)2CH}(C6H4-
2-CH2NMe2)P]GeCl (5)16 were prepared by a previously published
procedure; single crystals of 5 were isolated by slow evaporation
of a freshly prepared, saturated solution of this compound in diethyl
ether at room temperature. All other compounds were used as
supplied by the manufacturer.
1H and 13C{1H} NMR spectra were recorded on a JEOL
Lambda500 spectrometer operating at 500.16 and 125.65 MHz,
respectively, or a Bruker Avance300 spectrometer operating at
300.15 and 75.47 MHz, respectively; chemical shifts are quoted in
ppm relative to tetramethylsilane. 31P{1H} NMR spectra were
recorded on a JEOL Lambda500 or a JEOL Eclipse270 spectrometer
operating at 202.47 and 109.37 MHz, respectively, and 119Sn{1H}
NMR spectra were recorded on a JEOL Lambda500 spectrometer
operating at 186.50 MHz; 31P and 119Sn chemical shifts are quoted
in ppm relative to external 85% H3PO4 and external Me4Sn,
respectively. Phase-sensitive 31P EXSY spectra were obtained at a
measuring frequency of 202.47 MHz without proton decoupling
using a standard 90°-t1-90°-τm-90°Acq(t2) pulse sequence with
1024 data points in the t2 dimension and 128 in t1 and with a mixing
time of 40 ms. Elemental analyses were obtained by the Elemental
Analysis Service of London Metropolitan University.
Conclusions
[{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]SnCl (7). To a stirred
solution of SnCl2 (0.59 g, 3.11 mmol) in cold (-78 °C) THF (20
mL) was added a solution of [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]Li
(1.03 g, 3.11 mmol) in THF (20 mL), excluding light as much as
possible. The reaction mixture was allowed to attain room tem-
perature and was stirred for 16 h. Solvent was removed in Vacuo,
and the residue was extracted into Et2O (20 mL) and filtered. The
filtrate was cooled to -30 °C for 16 h, giving yellow crystals of 7.
Yield: 1.07 g, 72%. Anal. Calcd for C16H31NPSi2SnCl: C, 40.14;
Multielement and variable-temperature NMR spectroscopy
reveals that the intramolecularly base-stabilized phosphatetrylenes
5 and 7 are highly dynamic in solution. DFT calculations
indicate that, for these compounds, epimerization at the phos-
phorus centers via a vertex-inversion process is significantly
favored over epimerization via an edge-inversion process at the
tetrel centers. NMR spectroscopy also reveals that 5 and 7 are
somewhat Lewis acidic, forming adducts with the free phosphine
9 in polar solvents such as CD2Cl2 and THF. These adducts
appear to be formed through E · · · NMe2 contacts (E ) Ge, Sn)
between the tetrel center and the amino group of the free
phosphine; in the presence of triethylamine the formation of
adducts between the phosphatetrylene and 9 is inhibited. Adduct
formation is not observed in nonpolar solvents, and so we
tentatively propose that the adducts consist of weakly associated
[[{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]E(9)]+ · · · Cl- ion pairs,
which are stabilized in polar solvents.
1
H, 6.53; N, 2.93. Found: C, 40.07; H, 6.62; N, 2.86. H NMR
(CD2Cl2, 20 °C): δ 0.19 (s, 18H, SiMe3), 1.02 (s, 1H, CHP), 2.25
(s, 3H, NMe2), 2.77 (s, 3H, NMe2), (s, 2H, CH2N) 7.07 - 7.29 (m,
4H, aryl). 13C{1H} NMR (CD2Cl2, 22 °C): δ 2.63 (SiMe3), 4.09 [d,
JPC ) 54.7 Hz, CHP], 47.88 (NMe2), 63.43 [d, JPC ) 17.3 Hz,
CH2N], 124.62, 130.44, 134.87, 140.27 (aryl). 31P{1H} NMR
(CD2Cl2, 20 °C): δ -41.1. 119Sn{1H} NMR (CD2Cl2, 20 °C): δ
257 [d, JSnP ) 1070 Hz].
[{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]2Sn (8). To a stirred solu-
tion of SnCl2 (0.29 g, 1.57 mmol) in cold (-78 °C) THF (20 mL)
was added a solution of [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]Li
(1.04 g, 3.14 mmol) in THF (20 mL), excluding light as much as
possible. The reaction mixture was allowed to attain room tem-
The diphosphastannylene 8 is also highly dynamic in solution.
Multielement and variable-temperature NMR studies, combined
with DFT calculations, suggest that the lowest energy dynamic
process is epimerization at the terminal phosphorus center via
a vertex-inversion process, although exchange between the
chelating and terminal phosphide ligands is also a relatively
(25) Clegg, W.; Doherty, S.; Izod, K.; Kagerer, H.; O’Shaughnessy, P.;
Sheffield, J. M. J. Chem. Soc., Dalton Trans. 1999, 1825.

Intramolecularly Base-Stabilized Phosphatetrylenes
Organometallics, Vol. 28, No. 12, 2009 3337
Table 1. Crystallographic Data for 5 and 8
5
before refinement because of reflection overlap. Programs were
Bruker AXS SMART, SAINT, SHELXTL, Oxford Diffraction
CrysAlisPro, and local programs.26
DFT Calculations. Geometry optimizations and single-point
energy calculations were performed on the gas phase molecules
with the Gaussian03 suite of programs (revision C.02)27 on a 224-
core Silicon Graphics Altix 4700, with 1.6 GHz Montecito Itanium2
processors, via the EPSRC National Service for Computational
Chemistry Software (http://www.nsccs.ac.uk).
8
formula
fw
C16H31ClGeNPSi2
432.6
C32H62N2P2Si4Sn
767.8
cryst size, mm
cryst syst
space group
a, Å
0.40 × 0.22 × 0.20 0.38 × 0.36 × 0.20
monoclinic
P21/c
triclinic
j
P1
19.1002(6)
10.1387(3)
11.5197(4)
12.7543(6)
12.8444(16)
25.4612(12)
90.789(2)
90.187(2)
101.872(2)
4081.4(3)
4
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
Ground state optimizations of the model compounds 7a and 8a were
carried out using the B3LYP hybrid functional21 with an Lanl2dz
effective core potential basis set22 for Sn and a 6-31G(d,p) all-electron
basis set on all other atoms23 [B3LYP/6-31G(d,p),Lanl2dz; default
parameters were used throughout]. Transition states were initially
located using the QST3 method at the HF/3-21G* level of theory prior
to a full transition state geometry optimization at the B3LYP/6-
31G(d,p),Lanl2dz level of theory. For several test cases final energies
for 7a and 8a were further calculated at the MP2/6-31G(d,p),Lanl2dz
level of theory;28 energy differences obtained from these calculations
differed only marginally from those calculated at the B3LYP/6-
31G(d,p),Lanl2dz level of theory, and so only the latter energies were
used. Minima were confirmed by the absence of imaginary vibrational
frequencies and transition states by the presence of a single imaginary
vibrational frequency; the accuracy of transition states was judged by
consideration of the principal displacement vectors of the imaginary
vibrational mode in each case. For direct comparison, final energies
for 5a were calculated at the B3LYP/6-31G(d,p) level of theory using
previously reported optimized geometries.16
106.099(4)
2143.32(12)
4
Z
F
calcd, g cm-3
1.341
1.738
3740
1.250
0.844
34 437
18 159, 0.034
14 131
0.740-0.850
0.045, 0.104
0.064, 0.111
1.049
µ, mm-1
no. reflns measd
no. unique reflns, Rint
no. reflns with F2 > 2σ(F2) 3270
transmn coeff range
0.543-0.723
a
R, Rw (F2 > 2σ)
0.047, 0.154
0.053, 0.156
1.149
a
R, Rw (all data)
Sa
refined params
209
740
1.88, -1.00
max., min. diff map, e Å-3 1.42, -0.54
a Conventional R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(Fo - Fc2)2/
2
2
2
∑w(Fo )2]1/2; S ) [∑w(Fo - Fc2)2/(no. data - no. params)]1/2 for all
data.
perature and was stirred for 16 h. Solvent was removed in Vacuo,
and the sticky brown solid was extracted into light petroleum (20
mL) and filtered. Solvent was removed in Vacuo from the filtrate,
and the sticky solid was recrystallized from cold (-30 °C) n-hexane
as orange blocks of 8. Yield: 0.94 g, 78%. Anal. Calcd for
C32H62N2P2Si4Sn: C, 49.40; H, 7.97; N, 3.72. Found: C, 49.29; H,
8.12; N, 3.66. 1H NMR (d8-toluene, 24 °C): δ 0.00 (18H, s, SiMe3),
0.17 (18H, s, SiMe3), 0.68 [2H, d, JPH ) 6.4 Hz, CHP], 2.13 (s,
12H, NMe2), ∼3.50 (br s, 2H CH2N), 3.94 (s, 2H, CH2N),
6.72-7.52 (m, 8H, aryl). 13C{1H} NMR (d8-toluene, 23 °C): δ 4.48
(SiMe3), 7.67 (CHP), 48.79 (NMe2), 64.98 [d, JPC ) 25.1 Hz,
CH2N], 125.34, 127.29, 130.58, 133.33, 139.40, 146.60 (aryl).
31P{1H} NMR (d8-toluene, 21 °C): δ -55.3 (s, br). 119Sn{1H} NMR
(d8-toluene, 23 °C): δ 558 [t, JSnP ) 1045 Hz].
Crystal Structure Determinations of 5 and 8. Measurements
were made at 150 K on Bruker SMART and Oxford Diffraction
Gemini A Ultra diffractometers using graphite-monochromated Mo
KR radiation (λ ) 0.71073 Å). Cell parameters were refined from
the observed positions of all strong reflections. Intensities were
corrected semiempirically for absorption, based on symmetry-
equivalent and repeated reflections. The structures were solved by
direct methods and refined on F2 values for all unique data. Table
1 gives further details. All non-hydrogen atoms were refined
anisotropically, and H atoms were constrained with a riding model;
U(H) was set at 1.2 (1.5 for methyl groups) times Ueq for the parent
atom. The crystal of 5 was nonmerohedrally twinned (9.5(2)%
minor component), preventing merging of equivalent reflections
Acknowledgment. The authors are grateful to the Royal
Society and the EPSRC for support. Dr. J. L. Bookham
(Northumbria University) is thanked for access to the JEOL
Eclipse270 NMR spectrometer.
Supporting Information Available: For 5 and 8 details of
structure determination, atomic coordinates, bond lengths and
angles, and displacement parameters in CIF format. For 7a and 8a
details of DFT calculations, final atomic coordinates, and energies.
Full details of ref 27. This material is available free of charge via
the Internet at http://pubs.acs.org. Observed and calculated structure
factor details are available from the authors upon request.
OM8011757
(26) (a) SMART and SAINT software for CCD diffractometers; Bruker
AXS Inc.: Madison, WI, 1997. (b) CrysAlisPro; Oxford Diffraction Ltd.:
Oxford, UK. (c) Sheldrick, G. Acta Crystallogr., Sect. A 2008, 64, 112.
(27) Frisch, M. J.; et al. Gaussian 03, ReVision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(28) (a) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett.
1988, 153, 503. (b) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. Chem.
Phys. Lett. 1990, 166, 275. (c) Frisch, M. J.; Head-Gordon, M.; Pople, J. A.
Chem. Phys. Lett. 1990, 166, 281. (d) Head-Gordon, M.; Head-Gordon, T.
Chem. Phys. Lett. 1994, 220, 122. (e) Saebo, S.; Almlof, J. Chem. Phys.
Lett. 1989, 154, 83.