Triatomic EP2 triangles (E = Ge, Sn, Pb) as μ2: η3,η3-bridging ligands

Article abstract of DOI:10.1002/anie.200500707

(Chemical Equation Presented) Divalent Group 14 salts react with the [Na(thf)_x]⁺ derivative of the terminal niobium phosphide anion [P≡Nb{N(Np)Ar}₃]^- (Np = neopentyl, Ar = 3,5-Me₂C₆H_3<

Full text of DOI:10.1002/anie.200500707

Communications
Bridging Ligands
Triatomic EP2 Triangles (E = Ge, Sn, Pb) as
m2Dh3,h3-Bridging Ligands**
Joshua S. Figueroa and Christopher C. Cummins*
Three-membered, 2p-electron rings comprised of main-group
elements are of interest as compact manifestations of 4n + 2
Hückel aromaticity in which n = 0.[1] The cyclopropenium ion
[cyclo-C3H3]+ serves as the prototypical organic representa-
tive of this class,[2] while the prospect of substituting one or
more CH units of the cyclopropenium ring by an isolobal
heteroatom has spurred investigations seeking to extend the
concept of p delocalization throughout the p block of the
[*] J. S. Figueroa, Prof. Dr. C. C. Cummins
Department of Chemistry
Massachusetts Institute of Technology, Room 2-227
77 Massachusetts Avenue, Cambridge, MA 02139-4307 (USA)
Fax: (+1)617-258-5700
E-mail: ccummins@mit.edu
[**] We gratefully acknowledge the US National Science Foundation
(CHE-0316823) for financial support, Dr. Peter Müller for assistance
with the X-ray crystallographic studies, and Prof. Daniel G. Nocera
for a generous gift of computer time.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200500707
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periodic table.[3,4] Reactive unsaturated rings, such as cyclo-
butadiene, may be stabilized through complexation with a
metal center. With this idea in mind, we present herein a study
on the synthesis of triatomic {cyclo-EP2} triangles (E = Ge,
Sn, and Pb) stabilized within the coordination sphere of a
sterically protected diniobium unit. This particular family of
{cyclo-EP2} triangles has not been considered previously in
either its free or complexed form. Furthermore, divalent
atoms from Group 14 have received relatively little attention
as components in three-membered, 2p-electron rings. In fact,
only the neutral cyclopropene carbene cyclo-DC(C2H2), dia-
zacyclopropene carbene cyclo-DCN2, and silacyclopropenyli-
dene cyclo-DSi(C2H2) molecules have been considered theo-
retically,[5,6] with three-membered rings containing the heavi-
er atoms from Group 14 having been completely neglected to
date.
product (1a: 65%, 1b: 40%, 1c: 30%) after removal of the
salt by-product and crystallization from Et2O (Scheme 1).
Diamagnetic complexes 1a–c are obtained as crystalline
solids, which are soluble in ethereal and aromatic solvents.
Complexes 1a and 1b retain their integrity in solution at
elevated temperatures ([D6]benzene, 80–1008C, 2 days),
whereas complex 1c decomposed in solution at room temper-
ature to Pb0 and the known bridging diphosphide ligand
complex [(m2Dh2,h2-P2){Nb[N(Np)Ar]3}2] (3),[11] over several
hours; furthermore, the absence of light did not retard the
decomposition of 1c. Therefore, considering the fragile
nature of 1c, the mild conditions of the present synthesis,[12]
condition which permitted it to be isolated in pure form, can
be further appreciated.
The solid-state structures for complexes 1a–c are depicted
in Figure 1. The m2Dh3,h3 disposition of the {cyclo-EP2} ring
between the two niobium centers is evident in each structure.
The {cyclo-EP2} units in 1b and 1c are near-perfect isosceles
triangles, in which the aP-E-P angle in 1c (48.85(5)8) is
slightly more acute than that in 1b (51.23(7)8) as a result of
the larger covalent radius of the lead center. Indeed, the
The
title
complexes
[(m2Dh3,h3-cyclo-EP2){Nb-
[N(Np)Ar]3}2] (1a–c; E = Ge, Sn, and Pb, respectively; Np =
neopentyl, Ar= 3,5-Me2C6H3) were formed as a consequence
ꢀ
of the propensity of the niobium phosphide anion [P Nb-
{N(Np)Ar}3]À (2)[7] to undergo electronic rearrangement
when functionalized at the phosphorus atom.[7,8] This ten-
dency has already afforded both a new synthesis of phos-
phaalkynes[8] and of diorganophosphanylphosphinidene com-
plexes.[7] The current manifestation of this phenomenon,
which is characterized by the exchange of a niobium/
phosphorus multiple-bond interaction for a main-group
element/phosphorus bonding interaction, is simply derived
from salt elimination reactions of divalent Group 14 halides
with the [Na(thf)x]+ derivative of anion 2.[7]
À
À
À
average E P bond lengths in 1b and 1c (Sn P: 2.571Š, Pb
P: 2.677 Š) follow the trend of the covalent radii going from
À
À
Sn to Pb and reflect values typical of Sn P and Pb P single
bonds.[13] Additionally, the P P distances of 2.223(2) and
À
2.213(3) Š for 1b and 1c, respectively, are in the range typical
of P P single bonds, thus indicating a saturated electronic
[14]
À
framework for these {cyclo-EP2} rings when sandwiched
between two reducing d2 niobium centers.[11] Complex 1a,
however, was found to crystallize in the cubic space group
P213 with its Nb–Nb vector coincident with a crystallographic
C3 axis. Unfortunately, this morphology resulted in a crystal-
lographically imposed threefold compositional disorder of the
ring atoms in the {cyclo-GeP2} unit. Consequently, chemically
suspect metrical parameters were obtained for the complexed
{GeP2} ring, in which the edge distance of 2.460 Š clearly
31P{1H} NMR spectroscopic analysis on a room-temper-
ature solution containing equimolar quantities of [Na(thf)x]·2
and SnCl2 after mixing in cold THF revealed a single new
resonance centered at d = 47.8 ppm, which is well upfield of
the range expected for a niobium phosphinidene complex.[7,9]
X-ray structural analysis of a crystal harvested from the
reaction mixture established the identity of complex 1b as
containing a central {SnP2} ring and as the product of a double
addition of [Na(thf)x]·2 to SnCl2 with elimination of NaCl. A
purposeful synthesis was then devised and extended to the
Ge- and Pb-containing derivatives. Accordingly, the slow
addition of 0.47 equivalents of the respective divalent
Group 14 source (GeCl2·dioxane, SnCl2, or Pb(OTf)2[10]) to
[Na(thf)x]·2 in cold THF provided dark-red 1a and forest-
green 1b and 1c, respectively, in moderate yields of isolated
À
exceeds the value for a P P single bond and reflects structural
dominance by the larger Ge component.[15]
To garner further insight into the geometrical structure of
1a, the model construct [(m2Dh3,h3-cyclo-GeP2){Nb(NH2)3}2]
(4a) was subjected to full geometric optimization at the
density functional level (ADF 2004.01, ZORA-TZ2P/BP86).
Using the experimental metrical parameters of 1a as the basis
for the initial computational model structure, 4a converged to
À
À
a geometry with P P (2.242 Š) and P Ge (2.4465 Š av)
Scheme 1. Formation of complexes [(m2Dh3,h3-cyclo-EP2){Nb[N(Np)Ar]3}2] (1a–c). E=Ge, Sn, or Pb, respectively; Np=neopentyl, Ar=3,5-Me2C6H3,
OTf=O3SCF3.
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Figure 2. Selected frontier molecular orbitals calculated for 4b.
calculated multipole-derived charges (MDC-q)[17] on the ring
atoms in 4a–c indicate that each complexed {cyclo-EP2} unit
bears a net charge of approximately À1.0 (ꢀMDC-q = À1.186,
À0.917, and À0.837 a.u. for 4a–c, respectively). Thus,
although formally neutral as free entities, complexation by
two electropositive d2 Nb centers renders these {cyclo-EP2}
rings moderately anionic. The latter interpretation is consis-
tent with the saturated nature of the {cyclo-EP2} framework,
as determined crystallographically for 1b and 1c.
It is noteworthy that the net negative charge calculated for
the {cyclo-EP2} rings in models 4a–c decreases in the order
Ge > Sn > Pb according to the decreasing electronegativity of
the Group 14 atom.[18] Indeed, the negative charge at the
phosphorus center is calculated to remain relatively constant
between models 4a–c (À0.384 Æ 0.016 a.u.), and the variation
in the net charge between each {cyclo-EP2} ring is dictated by
the charge at the E center (MDC-q = À0.390, À0.185, and
À0.057 a.u. for 4a–c, respectively). A similar dependence on
the identity of the Group 14 atom is observed experimentally
in the 31P{1H} NMR spectra of complexes 1a–c in solution
(d31P = À15.7, 47.8, and 115.2 ppm for 1a–c, respectively).
The downfield progression in the resonances is attributed
qualitatively to the increase in the paramagnetic component
spara of the total 31P nuclei shielding tensor stotal as the
Group 14 atom becomes less electronegative.[19] NMR calcu-
lations performed on models 4a–c are consistent with this
suggestion, thus revealing that variation in spara dominates
stotal and increases in the order Pb > Sn > Ge.[20] Mapping the
principal components of spara onto the molecular frame of 4a–
c (Figure 3) reveals that R(s11) mediates an occupied–virtual
Figure 1. ORTEP diagrams of a) 1a, b) 1b, and c) 1c at the 35%
probability level. All the hydrogen atoms and the neopentyl residues of
1a have been omitted for clarity.
À
À
separations consistent with both P P and P Ge single
bonds.[13,14] All other overall structural features for 4a were
similarly consistent with those found experimentally for 1b
and 1c. Furthermore, the calculated structural parameters for
models 4b and 4c were in excellent agreement with their
experimental counterparts (1b and 1c, respectively). There-
fore, we contend that 4a represents the molecular geometry
for complex 1a in the absence of crystallographic disorder.
Of particular interest are the electronic-structure attrib-
utes attendant with {cyclo-EP2} complexation in 1a–c. The
highest occupied molecular orbitals (HOMOs) calculated for
4b are shown in Figure 2.[16] The HOMO-1is part of the
s framework of the {cyclo-SnP2} unit, whereas complexation
of the Nb centers consists of a pair of mutually orthogonal
two-electron p backbonds. The HOMO involves the out-of-
plane valence p orbital of the Sn center as the acceptor
component in one of these bonds, whereas the HOMO-2
utilizes a P–P p* orbital as the acceptor component in the
other bond. Accordingly, a pair of d2 niobium trisamide
fragments is seen to be electronically complementary to a
formally neutral {cyclo-EP2} ring. However, summation of the
À
coupling between the Nb2 P2 p* backbond (occupied) and
the s* framework (virtual) of the {EP2} ring in the presence of
an applied magnetic field. Thus, a molecular-orbital descrip-
tion of 31P nuclei deshielding as influenced[21] by the identity
of the Group 14 atom[22] within the {cyclo-EP2} ring is
afforded.
In conclusion, new triatomic molecules are of interest
both as synthetic targets and theoretical constructs.[3,4] We
have identified that complexed forms of {cyclo-EP2} triangles
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satisfactory 13C{1H} NMR spectroscopic and elemental analyses could
not be obtained because of the rapid decomposition of 1c.[23]
Received: February 25, 2005
Published online: June 30, 2005
Keywords: aromaticity · bridging ligands ·
.
main-group elements · niobium · phosphorus
[1] P. J. Garratt, Aromaticity, Wiley, New York, 1986, p. 137.
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Soc. 1970, 92, 984.
Figure 3. Principal components of spara for models 4a–c.
[3] For example, see: a) M. W. Wong, L. Radom, J. Am. Chem. Soc.
1989, 111, 6976; b) Y.-G. Byun, S. Seabo, C. U. Pittman, Jr., J.
Am. Chem. Soc. 1991, 113, 3689; c) J. R. Flores, A. Largo, J.
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THEOCHEM 1995, 343, 49; e) Y. Xie, P. R. Schreiner, H. F.
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10635; f) W. Eisfeld, M. Regitz, J. Org. Chem. 1998, 63, 2814;
g) Y. Xie, P. R. Schreiner, H. F. Schaefer, X.-W. Li, Robinson,
G. H. Organometallics 1998, 17, 114; h) R. Salcedo, C. Olvera,
THEOCHEM 1999, 460, 221.
containing germanium, tin, and lead can be synthesized as a
direct consequence of the remarkable chemistry of the
niobium–phosphorus triple bond in phosphide anion 2.[7,8]
Future prospects in this area include chemical liberation of
the {cyclo-EP2} triangles, with an aim of establishing their
reactivity patterns.
[4] Experimentally realized heteroatom-containing 2p-electron
three-membered rings: a) {cyclo-BC2}: J. J. Eisch, B. Shafii,
A. L. Rheingold, J. Am. Chem. Soc. 1987, 109, 2526; J. J. Eisch,
B. Shafii, J. D. Odom, A. L. Rheingold, J. Am. Chem. Soc. 1990,
112, 1 847; b) [cyclo-PC2]+: K. K. Laali, B. Geissler, O. Wagner, J.
Hoffmann, R. Armbrust, W. Eisfeld, M. Regitz, J. Am. Chem.
Soc. 1994, 116, 9407; c) [cyclo-Ga3]2À: X.-W. Li, W. T. Penning-
ton, G. H. Robinson, J. Am. Chem. Soc. 1995, 117, 7578;
d) [cyclo-CP2]+: D. Bourissou, G. Bertrand, Top. Curr. Chem.
2002, 220, 1; D. Bourissou, Y. Canac, M. I. Collado, A. Barceirdo,
G. Bertrand, J. Am. Chem. Soc. 1997, 119, 9923; D. Bourissou, Y.
Canac, H. Gornitzka, A. Barceirdo, G. Bertrand, Eur. J. Inorg.
Chem. 1999, 1479.
[5] B. S. Jursic, THEOCHEM 1999, 491, 33.
[6] G. Frenking, R. B. Remington, H. F. Schaefer III, J. Am. Chem.
Soc. 1986, 108, 2169.
[7] J. S. Figueroa, C. C. Cummins, Angew. Chem. 2004, 116, 1002;
Angew. Chem. Int. Ed. 2004, 43, 984.
[8] J. S. Figueroa, C. C. Cummins, J. Am. Chem. Soc. 2004, 126,
13917.
Experimental Section
[(m2Dh3,h3-cyclo-EP2){Nb[N(Np)Ar]3}2] (1a–c):[10] Solutions of [Na-
(thf)x]·2 (0.300 g, 0.380 mmol) in THF (5 mL) and the corresponding
divalent Group 14 salt (0.47 equiv; GeCl2·dioxane, SnCl2, or Pb(OTf)2
for 1a–c, respectively) in THF (2 mL) were frozen separately in a
glove-box cold well. On removal of the solutions from the cold well,
the thawing solution containing the salt was added dropwise
(approximately 0.6 mL) over 1min to the thawing solution of
[Na(thf)x]·2. The reaction mixture was stirred for an additional
3 min, whereupon both solutions were placed back into the cold well.
This procedure was repeated twice more until complete addition of
the divalent Group 14 salt was achieved. The reaction mixture was
then allowed to warm to room temperature and stirred for an
additional 30 min before being evaporated to dryness in vacuo. The
residue was extracted with n-pentane (3 mL), the extract filtered
through celite, and the filtrate evaporated to dryness in vacuo.
Crystallization of each complex was effected by storing a saturated
solution of Et2O at À358C for 1–3 days.
1
1a: Red crystals, 70% yield; H NMR (500 MHz, [D6]benzene,
208C): d = 6.69 (s, 6H, o-Ar), 6.60 (s, 3H, p-Ar), 4.42 (s, 6H, N-CH2),
2.20 (s, 18H, Ar-CH3), 1.02 ppm (s, 27H, tBu); 13C{1H} NMR
(125.7 MHz, [D6]benzene, 208C): d = 155.9 (ipso-aryl), 137.7 (m-
Ar), 126.4 (p-Ar), 125.2 (o-Ar), 80.3 (N-CH2), 37.3 (C(CH3)3), 30.8
(C(CH3)3), 21.9 ppm (Ar-CH3); 31P{1H} NMR (202.5 MHz, [D6]ben-
zene, 208C): d = À15.7 ppm (s); elemental analysis (%) calcd for
C78H120N6P2GeNb: C 64.07, H 8.27, N 4.97; found: C 65.50, H 8.98, N
5.50.
[9] A. H. Cowley, Acc. Chem. Res. 1997, 30, 445.
[10] The corresponding dichloride, PbCl2, did not react with [Na-
(thf)x]·2 under the reaction conditions employed because of its
low solubility in THF; a full description of general synthetic
procedures can be found in the Supporting Information; see also
the Supporting Information of reference [7].
[11] J. S. Figueroa, C. C. Cummins, J. Am. Chem. Soc. 2003, 125, 4020.
[12] An alternate synthesis of 1b was attempted by treatment of 3
with an excess of Sn dust in THF; however, no reaction was
observed when intermittently assayed for 24 h. We tentatively
attribute this observation to the inability of elemental Sn to
reduce the {P2} unit in 3.
1b: Green crystals, 40% yield; 1H NMR (500 MHz, [D6]benzene,
208C): d = 6.76 (s, 6H, o-Ar), 6.60 (s, 3H, p-Ar), 4.35 (s, 6H, N-CH2),
2.22 (s, 18H, Ar-CH3), 1.02 ppm (s, 27H, tBu); 13C{1H} NMR
(125.7 MHz, [D6]benzene, 208C): d = 156.2 (ipso-aryl), 137.7 (m-
Ar), 126.3 (p-Ar), 125.0 (o-Ar), 80.0 (N-CH2), 37.4 (C(CH3)3), 30.8
(C(CH3)3), 21.9 ppm (Ar-CH3); 31P{1H} NMR (202.5 MHz, [D6]ben-
zene, 208C): d = 47.8 ppm (t, 1J(Sn-P) = 205.2 Hz); 119Sn NMR
[13] L. Pauling, The Nature of the Chemical Bond, 3rd ed., Cornell
University Press, Ithaca, NY, 1960, chap. 11, p. 405.
[14] The experimentally determined P–P distance in P4 is 2.21Š:
N. N. Greenwood,A. Earnshaw, Chemistry of the Elements, 2nd
ed., Butterworth-Heinemann, Oxford, 1997, chap. 12, p. 479.
[15] Separation of the Ge and P centers within the asymmetric unit is
possible by allowing each atom to refine freely with the
appropriate site occupancy factor (namely, Ge0.33P0.67); however,
we suggest that the true molecular geometry in 1a is marred by
the crystallographically imposed symmetry, as the inherent
(186.5 MHz, [D6]benzene, 208C): d = À696.4 ppm (brs, n1/2
=
1300.8 Hz); elemental analysis (%) calcd for C78H120N6P2SnNb: C
62.11, H 8.02, N 5.57; found: C 61.75, H 7.91, N 5.66.
1c: Green crystals, 30% yield; 1H NMR (500 MHz, [D6]benzene,
208C): d = 6.78 (s, 6H, o-Ar), 6.62 (s, 3H, p-Ar), 4.45 (s, 6H, N-CH2),
2.23 (s, 18H, Ar-CH3), 1.02 ppm (s, 27H, tBu); 31P{1H} NMR
(202.5 MHz, [D6]benzene, 208C): d = 115.2 ppm (s with shoulders);
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translations of the {Nb[N(Np)Ar]3} fragments, which are
required to reveal a chemically sensible molecular geometry,
are masked relative to the central ring (see the Supporting
Information for full details).
[16] The molecular-orbital splitting patterns calculated for models
4a–c are qualitatively identical.
[17] M. Swart, P. T. van Duijnen, J. G. Snijders, J. Comput. Chem.
2001, 22, 79.
[18] a) Y.-R. Luo, S. W. Benson, J. Phys. Chem. 1989, 93, 7333; b) J.
Kapp, M. Remko, P. von R. Schleyer, Inorg. Chem. 1997, 36,
4241; c) C. H. Suresh, N. Koga, J. Am. Chem. Soc. 2002, 124,
1790.
[19] a) G. Schreckenbach, T. Ziegler, J. Phys. Chem. 1995, 99, 606;
b) T. M. Gilbert, T. Ziegler, J. Phys. Chem. A 1999, 103, 7537.
[20] The calculated spara values are À558.6, À626.9, and À682.7 ppm
for models 4a–c, respectively; the total 31P nuclei chemical
shielding stotal is the sum of the individual paramagnetic spara
,
diamagnetic sdia, and spin orbit sso tensor components; note that
sdia was found to be essentially invariant between models 4a–c
(961.9 Æ 0.5 ppm) and sso made a negligible contribution (13–
21ppm).
[21] a) Y. Ruiz-Morales, G. Schreckenbach, T. Ziegler, Organo-
metallics 1996, 15, 3920; b) Y. Ruiz-Morales, G. Schreckenbach,
T. Ziegler, J. Phys. Chem. 1996, 100, 3359; c) Y. Ruiz-Morales, G.
Schreckenbach, T. Ziegler, J. Chem. Phys. 1996, 104, 8605.
[22] Complex 1b gives rise to a broad, upfield 119Sn NMR signal at
d = À696.4 ppm (n1/2 = 1300.8 Hz, [D6]benzene, 208C). Typical
119Sn NMR chemical shifts for divalent Sn species are signifi-
cantly deshielded relative to that of 1b. We attribute this
disparity to ground-state electronic population of the normally
empty p orbital of the Snii center. For a discussion of 119Sn NMR
chemical shifts in divalent Sn complexes, see: a) B. E. Eichler,
B. L. Phillips, P. P. Power, M. P. Augustine, Inorg. Chem. 2000, 39,
5450; b) B. E. Eichler, A. D. Phillips, S. T. Haubrich, B. V. Mork,
P. P. Power, Organometallics 2002, 21, 5622. Attempts to locate
the corresponding 203Pb NMR shift for 1c were unsuccessful.
[23] CCDC-262173–262175 contain the supplementary crystallo-
graphic data for [(m2Dh3,h3-cyclo-SnP2){Nb[N(Np)Ar]3}2}] (1b),
[(m2Dh3,h3-cyclo-GeP2){Nb[N(Np)Ar]3}2] (1a), and [(m2Dh3,h3-
cyclo-PbP2){Nb[N(Np)Ar]3}2] (1c), respectively. These data can
be obtained free of charge from the Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data request/cif.
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