P-hydrogen-substituted 1,3,2-diazaphospholenes: Molecular hydrides

Article abstract of DOI:10.1021/ja057827j

P-Hydrogen-substituted 1,3,2-diazaphospholenes 1 were prepared by an improved procedure from diazadienes and were characterized by spectroscopy and in one case by X-ray diffraction. A unique hydride-type reactivity of the P-H bonds was documented by exten

Full text of DOI:10.1021/ja057827j

Published on Web 03/03/2006
P-Hydrogen-Substituted 1,3,2-Diazaphospholenes: Molecular
Hydrides
Sebastian Burck,† Dietrich Gudat,*,† Martin Nieger,‡ and Wolf-Walther Du Mont¶
Contribution from the Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart,
Pfaffenwaldring 55, 70550 Stuttgart, Germany, Institut fu¨r Anorganische Chemie,
UniVersita¨t Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany, and Institut fu¨r
Anorganische Chemie, Technische UniVersita¨t Braunschweig, Hagenring 30,
38106 Braunschweig, Germany
Received November 30, 2005; E-mail: gudat@iac.uni-stuttgart.de
Abstract: P-Hydrogen-substituted 1,3,2-diazaphospholenes 1 were prepared by an improved procedure
from diazadienes and were characterized by spectroscopy and in one case by X-ray diffraction. A unique
hydride-type reactivity of the P-H bonds was documented by extensive reactivity studies. Aldehydes and
ketones were readily reduced to diazaphospholene derivatives of the corresponding alcohols, with alkyl-
substituted ketones being converted at much lower rates than aldehydes or diaryl ketones. Reactions with
the tetrachlorides of group 14 elements proceeded via hydride/chloride metathesis to give either partially
chlorinated derivatives EHnCl4-n (n ) 0-3 for E ) C, Si) or HCl and phosphenium salts 16c[ECl3] (for E
) Ge, Sn) which were characterized by spectroscopic and X-ray diffraction studies. Tin dichloride was
readily reduced to the element. Reactions of 1c with the P-chloro-diazaphospholene 3c and the salt 16c[OTf]
allowed the first experimental detection of intermolecular exchange of a hydride, rather than a proton,
between phosphine derivatives. Computational studies indicated that the hydride transfer between 1c and
-
the cation 16c involves a transient H-bridged species with bonding properties similar to those of B2H7
.
The preference for the formation of these bridged intermediates over P-P bonded phosphenium-phosphine
adducts is attributed to the low electrophilicity of the diazaphospholenium cations and characterizes a novel
reaction mode for phosphenium ions.
P-H bond (Scheme 1a),1,2 as ionic reactions that are catalyzed
Introduction
by acids or bases,1,2 or under catalysis by transition metal
Phosphine (PH3), primary organophosphines (RPH2), and
secondary organophosphines (R2PH) are distinguished by the
presence of reactive phosphorus-hydrogen bonds whose ca-
pability to undergo chemical transformations makes these
species widely used reagents and synthetic intermediates.1 Many
typical reactions involve formal conversion of the P-H into
other P-E bonds (where E may be nearly any other metal or
nonmetal) and occur via either metathetic replacement of a
hydrogen or addition of a P-H bond to a carbon-carbon or
carbon-heteroatom multiple bond (“hydrophosphination”).
Several of these processes, e.g., the conversion into tertiary
phosphines which are valuable ligands in many catalytic
processes, are of considerable importance for both academic
research and industrial chemistry.1 From a mechanistic stand-
point, substitution reactions are frequently carried out in the
presence of strong bases,1 whereas hydrophosphinations may
proceed as radical processes under homolytic cleavage of the
complexes.3 Acid-catalyzed addition reactions of phosphines are
presumed to proceed by nucleophilic attack of the phosphine
on the substrate which was previously activated by the acid and
subsequent deprotonation of the resulting phosphonium ion;2,4
in contrast, base-induced addition and substitution reactions are
initiated by deprotonation of the phosphine by a catalytic or
stoichiometric amount of base to give a phosphanide (R2P-M+),
which is then quenched by reaction with an electrophile or
addition to a multiple bond.2,5 Both reaction mechanisms are
in accord with the generally accepted concept of phosphines as
weak acids (Scheme 1b).6
Since, as a consequence of the similar electronegativities of
hydrogen (øAR 2.2) and phosphorus (øAR 2.06), the phosphorus-
hydrogen bond lacks a pronounced polarity, it is conceivable
that P-H-substituted phosphines can react not only as a source
of radicals or as acids but also as hydride donors (Scheme 1c).
An ambiguous behavior of this type is well established for
† Universita¨t Stuttgart.
‡ Universita¨t Bonn.
(2) Elsner, G. Methoden der Organischen Chemie (Houben-Weyl), 1952-, 4th
ed.; Georg Thieme Verlag: Stuttgart, 1980; Vol. 13/E1, p 122 ff.
(3) Baillie, C.; Xiao, J. Curr. Org. Chem. 2003, 7, 477.
(4) Rauhut, M. M.; Hechenbleikner, I.; Currier, H. A.; Schaefer, F. C.;
Wystrach, V. P. J. Am. Chem. Soc. 1959, 81, 1103-1107.
(5) Hoff, M. C.; Hill, P. J. Org. Chem. 1959, 24, 356.
¶ Technische Universita¨t Braunschweig.
(1) (a) Quin, L. D. A Guide to Organophosphorus Chemistry; Wiley-
Interscience: New York, 2000. (b) Corbridge, D. E. C. Phosphorus World
2005 (http://www.phosphorusworld.com). (c) The Chemistry of Organo-
phosphorus Compounds, Vol. 1; Hartley F. R., Ed.; John Wiley & Sons:
Chichester, 1990.
(6) Hudson, H. R. In ref 1c, p 473 ff.
9
3946
J. AM. CHEM. SOC. 2006, 128, 3946-3955
10.1021/ja057827j CCC: $33.50 © 2006 American Chemical Society

P-Hydrogen-Substituted 1,3,2-Diazaphospholenes
A R T I C L E S
Scheme 1. Possible Ways for Cleavage of P-H Bonds
Scheme 3
Scheme 2
hydrogermanes, where the electronegativity situation (øAR 2.02
for Ge) is similar:7 the polarity of the Ge-H bonds is generally
low, and substituent influences or even special reaction condi-
tions can have a determinative influence and may lead to a shift
from radical to either “proton” or “hydride” reactivity. Such
changes can have serious consequences on the outcome of
reactions and may, e.g., result in an inverted regioselectivity
(“Umpolung”), as exemplified in Scheme 2 for the addition of
Et3GeH and Cl3GeH to carbonyl compounds, which can yield
either a germylcarbinol (reflecting “proton” character of the
Ge-H bond) or a germyl ether (reflecting “hydride” character
of the Ge-H bond).7 A dualism between acidic and hydride
character even in the presence of a somewhat larger electrone-
gativity difference is likewise known for the reactions of Si-H
bonds in silanes (øAR 1.74 for Si).8
Scheme 4. Synthesis of Diazaphospholenesa
That substituent-induced inversion of the E-H bond polarity
is, in principle, also feasible in phosphorus compounds was first
established by the observation of hydride-type reactivities for
some hypervalent Lewis base adducts of phosphines.9 Some time
ago, we described 2-H-diazaphospholenes 1 as the first three-
coordinate phosphine derivatives showing “Umpolung” of the
P-H bond polarity. This behavior is due to a hyperconjugative
interaction between nonbonding π-electrons in the C2N2 unit
and the σ*(P-H) orbital (expressed by resonance between the
canonical structures 1-1′′, Scheme 3), which induces a weaken-
ing of the P-H bond and an increase of negative charge density
at the hydrogen atom.10 A preliminary survey of the chemical
reactivity of 1 revealed that the hydride character became evident
in the ability to react with acids under dehydrocoupling and to
convert benzaldehyde into a benzyloxy-diazaphospholene 2
(Scheme 3), with concomitant reduction of the carbonyl to an
alcohol function.10
a R ) MeOCH2CH2; ECln ) SiCl4 or GeCl2; R1 ) tBu (1c-6c), Mes
(1d-6d), 2,6-iPr2C6H3 (DIPP, 1e-6e).
element compounds have as yet not been studied systematically.
However, in consideration of the present knowledge on the
reactivity of 1, it is conceivable that further explorations in this
field may lead to new applications for phosphines in organic
and organometallic synthesis. This prospect stimulated us to
evaluate the reactivity of the diazaphospholenes 1 toward various
types of substrates in more detail. In this work, we will give a
full account of the synthesis of 2-hydro-diazaphospholenes and
a crystal structure study of one derivative, and we will report
on a comprehensive investigation of the use of these species as
hydride-transfer reagents toward organic carbonyl compounds,
di- and tetrahalides of group 14 elements, and chlorophosphines.
Although the potential of phosphines as reducing agents has
long been recognized,11 the reaction of 1 is remarkable as it
does not proceedsas usualsunder deoxygenation of the sub-
strate and conversion of the phosphine into a phosphine oxide,
but rather via transfer of a hydride. Reactions involving reductive
hydride transfer from phosphines to organic and main-group
Results and Discussion
Synthesis and Characterization of 2-Hydrido-1,3,2-diaza-
phospholenes. The P-hydrogen-substituted diazaphophos-
pholenes 1 are readily prepared via H/Cl exchange from the
P-chloro-diazaphospholenes 3.10 The latter are in turn accessible
from readily available diazadienes 4 in a two- or three-step
procedure (Scheme 4) involving (i) reduction of the starting
material to a dilithium diazadienide salt 5 and (ii) either direct
(7) Rivie`re, P.; Rivie`re-Baudet, M.; Satge´, J. In ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon:
Oxford, 1982; Vol. 2, p 399 ff.
(8) Armitage, D. A. In ref 7, Vol. 2, p 1 ff.
(9) (a) Carre´, F.; Chuit, C.; Corriu, R. J. P.; Mehdi, A.; Reye´, C. J. Organomet.
Chem. 1997, 529, 59. (b) Bezombes, J.-P.; Carre´, F.; Chuit, C.; Corriu, R.
J. P.; Mehdi, A.; Reye´, C. J. Organomet. Chem. 1997, 535, 81. (c) Alder,
R. W.; Read, D. Angew. Chem., Int. Ed. 2000, 39, 2879.
(10) Gudat, D.; Haghverdi, A.; Nieger, M. Angew. Chem., Int. Ed. 2000, 39,
3084.
12
metathesis of this intermediate with PCl3 or, alternatively,
(11) (a) Buckler, S. A.; Doll, L.; Lind, F. K.; Epstein, M. J. Org. Chem. 1962,
27, 794. (b) Studer, A.; Amrein, S. Synthesis 2002, 835.
(12) Carmalt, C. J.; Lomeli, V.; McBurnett, B. G.; Cowley, A. H. Chem.
Commun. 1997, 2095.
9
J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 3947

A R T I C L E S
Burck et al.
13
conversion of the diazadienide into a diazasilole or cyclic
germylene12 and subsequent ring metathesis with PCl3. The
direct substitution pathway is clearly advantageous from a
practical point of view, as it involves fewer and less time-
consuming steps, but a drawback is that it provides only
moderate yields of diazaphospholene (55% of 3c by metathesis
from 5c12). In connection with our recent studies on the synthesis
of N-heterocyclic arsenium and stibenium ions,14 we found now
that the yields of chloro-diazaphospholenes 3 can be substan-
tially improved when the dianions 5 are first quenched by
protonation with Et3NHCl and the resulting R-aminoaldimines
6 are then reacted with PCl3 in the presence of NEt3 as acid
scavenger. Isolation of the intermediates 5, 6 is not required so
that the whole synthesis can be carried out in one pot. Using
this protocol allows to prepare multigram quantities of the
N-tert-butyl- and N-aryl-P-chloro-diazaphospholenes 3c-e in
one step from diazadienes with oVerall yields of 70-80%.
The completion of the synthesis of the target compounds
1c-e from 3c-e requires as the last step the replacement of
the P-chloro-substituent by hydrogen. Although LiAlH4 and
LiBEt3H have been initially employed for this purpose,10 their
use suffers from the occurrence of side reactions (mainly
cleavage of P-N bonds due to uncontrolled over-reduction)
which reduce the yields and require extra efforts for the removal
of the byproducts during workup. We have found that using a
stoichiometric amount of sodium bis(methoxyethoxy)aluminum
dihydride (“Red-Al”) for hydride transfer gives superior results
by avoiding unspecific P-N bond cleavage and facilitating the
separation of chloroaluminate salts by filtration. Following these
protocols, pure P-hydrido-diazaphospholenes 1c-e are easily
isolated in yields of 50-70% after distillation (1c) or recrys-
tallization (1d,e).
Figure 1. Molecular structure of 1e in the crystal, ORTEP view. Thermal
ellipsoids are at the 50% probability level. Selected bond lengths (in Å):
P1-N5 1.691(1), P1-N2 1.712(1), P1-H1 1.48(1), N2-C3 1.408(2), N2-
C6 1.437(2), C3-C4 1.317(2), C4-N5 1.416(2).
distances in 1e (see caption to Figure 1) and 5 (PsN 1.709(3),
1.722(3); CsN 1.407(5), 1.410(5); CdC 1.327(5) Å10) are
indistinguishable. The characteristic pattern of deviations from
standard single- and double-bond distances has been interpreted
as the result of hyperconjugation between the six π-electrons
in the C2N2 unit and the σ*(P-H) orbital and was stated as the
dominant cause for the bond weakening and increased negative
charge density at the hydrogen atom that account for the unique
hydride nature of the P-H bonds in diazaphospholenes.10
Reactions with Carbonyl Compounds. Although the reduc-
tive dechlorination of phosgene is a well-established route for
the preparation of phosphinous and phosphonous chlorides, R2-
PCl and RPCl2, from primary and secondary phosphines,2 the
latter react with aldehydes and ketones not by reduction of the
carbonyl group but via P-C bond formation to give R-phos-
phino-carbinols. Having discovered10 the reduction of benzal-
dehyde by 1b (Scheme 3) as a remarkable exception to this
behavior, we wanted to evaluate the scope and limitation of
this reaction as a new approach to the reduction of organic
carbonyls. For this purpose we set out to study the behavior of
the diazaphospholene 1c, which was chosen as the synthetically
most easily accessible derivative, toward a larger range of
substrates. Additional sampling experiments revealed that 1d,e
behave very much the same way as 1c, whereas the reactions
of 1a,b were in some cases complicated by side reactions
involving the 4-chloro substituents (see below).
The pure diazaphospholenes 1c-e are light yellow liquids
(1c) or solids that are highly air and moisture sensitive. They
are readily soluble in hydrocarbons, ethers, and acetonitrile but
decompose when dissolved in protic solvents and in chlorinated
hydrocarbons (see below). The composition and identity of all
compounds were verified by analytical and spectroscopic data
and in the case of 1e by a single-crystal X-ray diffraction study.
The spectroscopic data of 1c-e display no peculiarities and are
listed in detail in the Experimental Section. The structure of
crystalline 1e is composed of isolated molecules, without
significant intermolecular interactions (Figure 1), whose indi-
vidual features bear close similarity to those of the previously
described 4-chloro-substituted diazaphospholene 1b.10 As in that
case, the five-membered ring exhibits a flat “envelope” con-
formation in which the phosphorus atom is out of the plane
containing the other four ring atoms. The position of the
hydrogen atom at phosphorus was located and refined freely
using isotropic thermal displacement parameters, showing that
the P-H bond takes on a flagpole position. The P-H distance
of 1.48(1) Å is quite similar to that in 1b (1.51(4) Å10) and
likewise notably longer than known P-H bond lengths in
phosphines (1.288 ( 0.09 Å 15), and the endocyclic bond
The reactions of 1c with aldehydes (2-methoxybenzaldehyde,
butyraldehyde) and benzophenone were completed within a
couple of minutes at ambient temperature (as judged by the
fading of the orange color of the starting material) and
proceeded, like the reaction of 1b with benzaldehyde, with
reduction of the carbonyl function to yield the diamidophos-
phinites 8-10 (Scheme 5) along with varying amounts (<5-
25%) of the phosphorous acid diamide 7; the formation of the
latter is explained by reaction of 1c with traces of water present
in the reaction.16 Reaction of 1c with cinnamaldehyde afforded
the 1,4-addition products 11a,b as a 3:1 mixture of E/Z
stereoisomers, together with a small amount of 7; the allylic
alcohol derivative 12, resulting from formal 1,2-addition of the
phosphine to the carbonyl moiety, was not detectable. The
benzylic derivatives 8 and 10 were isolated as colorless,
crystalline solids after workup of the reaction mixtures and were
characterized by analytical and spectroscopic data, whereas 9
(13) Denk, M. K.; Gupta, S.; Ramachandran, R. Tetrahedron Lett. 1996, 37,
9025.
(14) (a) Gudat, D.; Gans-Eichler, T.; Nieger, M. Chem. Commun. 2004, 2434.
(b) Gudat, D.; Gans-Eichler, T.; Nieger, M. Heteroat. Chem. 2005, 16,
327.
(15) Average and standard deviation as the result of a query in the CSD database
for P-H distances in three-coordinate phosphorus compounds HnY3-n
P
(Y ) substituent bound via a p-block element).
(16) Burck, S.; Gudat, D.; Nieger, M. Angew. Chem., Int. Ed. 2004, 43, 4801.
9
3948 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

P-Hydrogen-Substituted 1,3,2-Diazaphospholenes
A R T I C L E S
Scheme 5
Scheme 6
unknown, the conversion of 1b into 1d can be rationalized as
P-H/C-Cl bond metathesis involving the transfer of a hydride
from a phosphorus to a carbon atom. To elucidate the suitability
of diazaphospholenes 1 as hydride-transfer reagents on a broader
scale, we initiated a detailed study of the reactions of 1c with
ECl4 (E ) C, Si, Ge, Sn) and ECl2 (E ) Ge, Sn), respectively.
and 11a,b were unambiguously identified from the analysis of
multinuclear and multidimensional NMR spectra of the reaction
mixture (in these cases, reactions were performed in NMR tubes
using C6D6 as solvent), but no attempts were made to isolate
them. Assignment of the position and configuration of the double
bond in 11a,b is based on the values of nJHH and nJPH coupling
constants involving the hydrogen atoms in the double bond and
the adjacent CH2 moiety; not surprisingly, the values obtained
match closely the analogous couplings in the corresponding free
enols.17
The analogous reactions of 1c with acetophenone (45%
conversion after 12 h at ambient temperature) and diethyl ketone
(approximately 8% conversion after 12 h at ambient tempera-
ture) proceeded at much lower rates, and NMR signals of
unreacted starting materials remained visible over a period of
several days at ambient temperature. Although no attempts to
isolate individual products were made, NMR studies on the
reaction mixtures allowed us to establish unambiguously the
formation of the expected diamidophosphinites 13, 14 and the
hydrolysis product 7, and to disprove the presence of further
spectroscopically detectable side products or intermediates.
Monitoring the time-dependent changes of signal intensities
revealed that the fraction of 7 increased substantially during
later stages of the reaction, suggesting that its formation arises
most likely from slow diffusion of traces of water into the NMR
tubes. In contrast to the reactions with aldehydes and ketones,
1c was found to be unreactive toward esters such as ethyl acetate
and methyl benzoate.
The reaction of CCl4 with 1 equiv of 1c at room temperature
was instantaneous and afforded the P-chloro-diazaphospholene
3c and a mixture of chloromethanes CHmCl4-m (m ) 0-3),
which were identified in situ by 1D and 2D NMR spectroscopy
(Scheme 6). Addition of 1c in excess resulted in eventual
consumption of CCl4 and formation of CH2Cl2 and CH3Cl as
main products. In contrast to the unselective reaction with CCl4,
reduction of SiCl4 with 1 equiv of 1c gave 3c and SiHCl3 as
the only products identifiable by NMR spectroscopy; both
species were not isolated. Further reaction with an excess of 1c
(1-2 molar equiv) was unselective, and NMR studies of the
reaction mixture disclosed the presence of unreacted SiHCl3 in
addition to three new silicon-containing products. The two major
1
components showed characteristic H and 29Si signals which
led to their assignment as the anticipated reaction products, SiH2-
Cl2 and SiH3Cl, even though the observed chemical shifts and
couplings differ markedly from the reported data for these
compounds.19 The origin of this divergence is attributable to
the presence of rapid (on the NMR time scale) dynamic chloride
exchange between the halosilanes and 3c (see Scheme 6); the
observed chemical shifts and couplings under these conditions
represent a population-weighted average of the data of SiHnCl4-n
and the silicates [SiHnCl5-n]-, and it was feasible to demonstrate
that changing the composition of the solution by adding either
3c or a mixture of silanes SiHnCl4-n (prepared by partial
reduction of SiCl4 with LiAlH4) shifted the positions of all 1H,
31P, and 29Si NMR signals and the magnitude of 1JSiH couplings.
From the size of these shifts it can be concluded that the chloride
affinity increases in the order SiHCl3 < SiH2Cl2 < SiH3Cl.
Chloride abstraction from 3c by GeCl4 had previously been
demonstrated.12
Reactions with Chlorides of Group 14 Elements. The
ability of the diazaphospholenes 1 to effect reductive dechlo-
rination of organic halides was discovered serendipitously when
it was noted that attempts to dissolve 1b in CH2Cl2 resulted in
formation of the 2-chloro-diazaphospholene 3d10 and that
reactions of 1b with olefins or carbonyls were often ac-
companied by the formation of 3d as a side product. While the
fate of the hydrogen atom in the first reaction remained
The third product of the reaction of SiCl4 with 1c (formed in
yields <5%) was distinguished by the absence of Si-H bonds
(18) tom Dieck, H.; Zettlitzer, M. Chem. Ber. 1987, 120, 795.
(19) Lo¨wer, R.; Vongehr, H.; Marsmann, H. C. Chem. Z. 1975, 99, 33.
(17) Bergens, S. H.; Bosnich, B. J. Am. Chem. Soc. 1991, 113, 958.
9
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A R T I C L E S
Burck et al.
Treatment of the phosphenium salts 16c[ECl3] with an excess
of 1c resulted in exothermic reactions which proceeded under
gas evolution and immediate formation of orange (E ) Ge) or
black (E ) Sn) insoluble precipitates. The supernatant solutions
contained the P-chloro-diazaphospholene 3c as the only spec-
troscopically detectable phosphorus-containing species. Identical
products were obtained when 16c[ECl3] were replaced by GeCl2
x dioxane, or SnCl2, respectively. More detailed studies of the
reactions involving 16c[SnCl3] and SnCl2 disclosed that com-
plete conversion of the starting materials required 1 molar equiv
of 1c and that the black precipitate consisted of elemental tin
and led thus to the formulation of the overall reaction as 1c +
SnCl2 f 3c + Sn + HCl.
In light of the previous findings and the results of Schlecht,23
who obtained elemental germanium as an orange solid by
reduction of GeCl2 x dioxane with Li[BEt3H], it was expected
that the same product might be formed by reduction of GeCl2
or 16c[GeCl3] with 1c. Even though a powder diffraction study
confirmed that the insoluble orange material obtained indeed
contains germanium, the diffraction pattern showed additional
peaks, and elemental analysis revealed the presence of substan-
tial amounts (>30%) of organic material. Furthermore, samples
from different batches deviated perceptibly in their composition.
As solid-state NMR studies allowed us to identify the cation
16c as a constituent of the solid, we conclude that the orange
material is a mixture of germanium with an ionic phase which
is composed of the cation 16c and a subvalent, presumably
oligomeric or polymeric, germanate anion of yet unknown
structure. Attempts to characterize the material obtained in more
detail are still in progress.
In comparing the reactivity of 1c toward different tetrahalides,
it appears at first glance that the reactions with CCl4 and SiCl4
(which proceed via substitution at the group 14 element) take
a different course than those with GeCl4 and SnCl4 (which
proceed via reduction of the group 14 element). Nonetheless,
all processes can be rationalized by a common mechanism if
one considers that 1c behaves as a hydride donor and reacts
with all starting materials via E-Cl/P-H bond metathesis to
give 3c and EHCl3 as initial products. The latter may then decay
via two competing reaction channels, undergoing either reduc-
tive elimination of hydrogen chloride to yield the dichlorides
ECl2 or subsequent metathesis of further E-Cl bonds under
consumption of more equivalents of 1c. The second alternative
is obviously preferred for the carbon and silicon halides, which
undergo reductive elimination to transient dichlorocarbenes or
-silylenes only in the presence of strong bases24 and were found
to react with excess 1c under successive metathesis of up to
three of four E-Cl bonds. It remains presently undecided if
the last chlorine atom is not replaced at all, or if the tetrahydrides
EH4 (E ) C, Si) evade spectroscopic detection because of their
high volatility. In contrast to their lighter homologues, GeHCl3
and SnHCl3 are known to coexist in donating solvents in
equilibrium with ECl2 and HCl.25 The dihalides formed in this
reductive elimination are Lewis acids that may react with 3c
under chloride abstraction, as was autonomously verified by
the independent synthesis of 16c[ECl3] from 3c and ECl2, and
Figure 2. Molecular structure of 16c[SnCl3] in the crystal, ORTEP view.
Thermal ellipsoids are at the 50% probability level. Atoms P1′ and Cl1′ of
neighboring ions and intermolecular contacts are depicted as dashed lines.
Selected bond lengths (in Å) and angles (in deg; values for 16c[GeCl3] in
brackets): P1-N1 1.660(2) [1.664(1)], N1-C1 1.363(3) [1.367(2)], C1-
C1# 1.336(5) [1.339(4)], Sn1-Cl1 2.453(1) [Ge1-Cl1 2.282(1)], Sn1-
Cl2 2.470(1) [Ge1-Cl2 2.308(1)], Cl1-Sn1-Cl2 93.60(2) [Cl1-Ge1-
Cl2 95.91(2)], Cl2#-Sn1-Cl2 95.56(3) [Cl2#-Ge1-Cl2 97.32(3)].
and was identified as the known diazasilole 1518 by comparison
of its spectral data with those of an authentic sample. The
formation of 15 is explained by ring metathesis between 3c and
SiCl4 (or the reduction products SiHnCl3-n) under formal
exchange of the PCl for an SiCl2 moiety; this reaction is the
reversal of the previously reported metathesis between 15 and
SiCl4 which had been utilized in the first synthesis of 3c.13
The reactions of GeCl4 and SnCl4 with equimolar amounts
of 1c afforded quantitative yields of the appropriate phosphe-
nium trichlorogermanate or trichlorostannate, 16c[ECl3] (E )
Ge, Sn). The formation of HCl as byproduct was proven by its
trapping as hydrochloride after addition of diazabicyclo[2.2.2]-
octane (DABCO). The salts 16c[ECl3] were isolated as light
yellow crystalline solids and were characterized by analytical
and spectroscopic data and single-crystal X-ray diffraction
studies. The crystals of both compounds are isomorphous (space
group Pnma) and consist of pairs of anions and cations (see
Figure 2) that lie on a crystallographic mirror plane. The ions
in each pair are connected by weak hydrogen bonds between
two chlorine atoms and the hydrogen atoms on the double bond
(Cl2-H1 2.63 Å). The third chlorine atom displays an additional
contact to the phosphorus atom of an adjacent ion pair (P‚‚‚Cl
3.45 (16c[SnCl3]), 3.39 (16c[GeCl3]) Å) which is slightly shorter
than the sum of van der Waals radii (3.55-3.75 Å) and may
be interpreted in favor of a weak intermolecular Lewis acid-
base interaction. If one considers both types of intermolecular
interactions, the crystal packing can be described in terms of
one-dimensionally infinite zigzag chains of alternating and
weakly interacting anions and cations. The molecular structure
of individual cations 16c is characterized by planar five-
membered rings whose endocyclic bond distances (cf. Figure
2) and angles are, within experimental error, indistinguishable
from each other and from those in other known diazaphos-
pholenium salts.12,20,21 The anions show slightly distorted (with
respect to local C3V symmetry) trigonal pyramidal structures
whose bond lengths and angles (Figure 2) match the data of
other salts with the same anions.22
(20) Gudat, D.; Haghverdi, A.; Hupfer, H.; Nieger, M. J. Eur. Chem. 2000, 6,
3414.
(21) Denk, M. K.; Gupta, S.; Lough, A. J. Eur. J. Inorg. Chem. 1999, 41.
(22) Average and standard deviation as the result of a query in the CSD database
for trichlorostannates and trichlorogermanates: Sn-Cl, 2.51 ( 6 Å; Cl-
Sn-Cl, 92 ( 3°; Ge-Cl, 2.30 ( 3 Å; Cl-Ge-Cl, 95.6 ( 1.5°.
(23) Schlecht, S. Angew. Chem., Int. Ed. 2002, 41, 1178.
(24) Karsch, H. H.; Schlu¨ter, P. A.; Bienlein, F.; Herker, M.; Witt, E.; Sladek,
A.; Heckel, M. Z. Anorg. Allg. Chem. 1998, 624, 295.
(25) Inorganic Reactions & Methods; Zuckerman, J. J., Ed.; VCH Publishers:
New York, 1989; Vol. 3, p 399 ff and references cited therein.
9
3950 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

P-Hydrogen-Substituted 1,3,2-Diazaphospholenes
A R T I C L E S
the coupling of both equilibria can explain the quantitative
formation of these salts in the reaction of 1c with ECl4. Reaction
of excess of 1c under these circumstances must occur with the
divalent chlorides rather than with EHCl3, and it can be
envisaged that, for E ) Sn, the reaction is completed by another
sequence of hydride-transfer and reductive elimination steps to
yield Sn, HCl, and a second equivalent of 3c.
Reactions with Chlorophosphines. Whereas 1c did not react
with phosphorus trichloride in a controlled way, the reaction
with diphenylchlorophosphine afforded diphenylphosphine and
3c as the only NMR spectroscopically detectable products. This
process boils down to the interconversion between two phos-
phine derivatives via exchange of chloride and hydride substit-
uents and is practically irreversible, as the products must be
considered thermodynamically more stable than the starting
materials.26 We wondered if similar reactions might likewise
be feasible under reVersible hydride transfer. As it was
anticipated that a reversible process is most easily observable
in a degenerate reaction where the thermodynamic driving force
is zero and mechanistic information is often conveniently
available from dynamic NMR, we decided to investigate first
the reaction of 1c with 3c. 1H and 31P NMR spectra of mixtures
of both species in CD3CN at ambient temperature displayed a
distinct broadening of all signals as compared to the spectra of
solutions of the pure compounds. The interpretation of this effect
as dynamic line broadening arising from slow (on the NMR
time scale) chemical exchange was confirmed by a 2D 1H EXSY
experiment (Figure 3a). The presence of intermolecular cor-
relations connecting the signals of olefinic and aliphatic
hydrogen atoms of both reactants proved unambiguously that
the two species are in dynamic equilibrium and undergo
reversible H/Cl exchange on a second time scale.
Figure 3. (a) Expansion of the olefinic region of a 2D 1H EXSY spectrum
of a mixture of 3c and 1c in CD3CN. The traces on top and left of the 2D
spectrum depict the 1H NMR spectrum of the same solution. The
off-diagonal correlations connecting the peaks of both reactants in the 2D
map prove clearly the presence of dynamic exchange during the mixing
time (τ ) 500 ms). (b,c) Expansion of the 1H (b) and 31P (c) NMR spectra
of an equimolar mixture of 1c and 16c[OTf] in CD3CN. The signals of the
olefinic and the PH protons are denoted with CH and PH, respectively.
The signals marked with asterisks are attributable to an impurity arising
from hydrolytic decomposition of the reactants.
Evidence for reversible hydride transfer was observed as well
when 3c was replaced by an ionic diazaphospholenium triflate.
Samples prepared by dissolution of equimolar amounts of 1c
and 16c[OTf] in CD3CN at ambient temperature showed a single
broad 31P NMR signal (Figure 3c) which sharpened somewhat
when the spectrum was recorded at higher temperatures. The
1H NMR spectrum at ambient temperature displayed a single
set of sharp resonances for the olefinic and aliphatic hydrogen
atoms of both components and an additional singlet for the
P-bound hydrogen atom (Figure 3b). The spectral features are
consistently interpreted by assuming that intermolecular scram-
bling of a hydride between the two reactants occurs at a rate at
(or close to) the fast exchange limit; this leads to coalescence
of the separate signals of the phosphorus and the aliphatic and
olefinic hydrogen atoms of both components and a collapse of
the doublet splitting of the signal of the P-bound hydrogen atom
Chart 1
of the pure compounds, which suggests that noticeable amounts
of transient intermediates are not involved. Even though no
precise rate constants could be extracted, the NMR spectra
indicate that interconversion between 1c and the chlorophos-
phine 3c is slower than that between 1c and the cation 16c.27
The reversible intermolecular transfer of a hydride from a
phosphine to a phosphenium ion, although conceptually straight-
forward, must still be regarded as a remarkable process, as it is
in sharp contrast to the established reaction pattern, viz. the
formation of P-P bonded adducts of the type 17 (Chart 1) via
dative interaction of the lone-pair of the phosphine (a Lewis
base) with the vacant orbital of the phosphenium cation (a Lewis
acid).28,29 Even though the P-P bond formation is reversible
and labile phosphenium-phosphine adducts may easily dis-
1
in 1c due to the scalar coupling via JPH. Cooling to ap-
proximately -30 °C led to broadening of all lines, in accord
with the expectation of the exchange slowing down with
decreasing temperature. At still lower temperatures, the salt
16c[OTf] precipitated and the signals of 1c reappeared in the
spectrum of the supernatant solution, confirming that the original
components were still present and had not reacted to a new
product. All changes were completely reversible upon warming
the sample again to ambient temperature. The chemical shifts
in the NMR spectra of mixtures of 1c/16c[OTf] always comply
with the value of the population-weighted average of the shifts
(27) A dynamic exchange similar to that for 1c/16c[OTf] was also observed
between the P-chloro-diazaphospholenes 3c-e and 16c-e[OTf], suggesting
that these species equilibrate in a similar way via intermolecular chloride
scrambling. A detailed discussion of this reaction will be given elsewhere.
(28) (a) Schultz, C. W.; Parry, R. W. Inorg. Chem. 1976, 15, 3046. (b) Cowley,
A. H.; Lattman, M.; Wilburn, J. C. Inorg. Chem. 1981, 20, 2916. (c)
Abrams, M. B.; Scott, B. L.; Baker, R. T. Organometallics 2000, 19, 4944.
(26) Gudat, D. Eur. J. Inorg. Chem. 1998, 1087.
9
J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 3951

A R T I C L E S
Burck et al.
Table 1. Computed Relative Energies (in kcal mol-1 at the
B3LYP/6-31+g(d,p) Level of Theory) and Important Bond Lengths
of 1g, 16g, and the Molecular Adducts 18 and 19
1g
16g
18
19-C2
19-C1
symmetry Cs
C2V
C1
C2
C1
∆E a
-10.7 (-10.3) -11.9 (-11.2) -11.9 (-11.2)
-10.0
-1.0
∆EZPE
∆G 298
PsN
-13.5
-13.3
-3.1
-2.1
1.747 1.695 1.760b
1.763b
1.709, 1.711
1.711b
1.713b
1.707c
1.709c
1.391b
1.393b
1.388c
1.389c
1.358b
1.359c
1.716b
1.834c
Figure 4. Representation of the computed molecular structures of 18 (a),
19-C2 (b). Intermolecular contacts in 18 are drawn as thin lines. Relevant
distances are listed in Table 1.
1.703c
1.702c
NsC
1.413 1.367 1.412b
1.414b
1.389, 1.391
sociate into the free fragments28 or partake in exchange reactions
with other phosphines,29 they tend to retain unsymmetric
molecular structures and refrain from undergoing 1,2-shifts of
substituents at the phosphorus atoms.28,29 In view of these
considerations and the incompatibility of the NMR data of
known phosphenium-phosphine adducts with the spectral data
of the exchanging solutions of 1c/16c[OTf], it appears highly
likely that the hydride-transfer reactions involve adducts of the
type 17 neither as stable products nor as transient intermediates.
To obtain mechanistic insight into the hydride exchange and
possibly to understand why this reaction is preferred over the
formation of P-P bonded adducts, we performed computational
studies of the reactions of diazaphospholenium ions with
diazaphospholenes. As 16[OTf] exists in solution as a solvent-
separated ion pair,20 we focused on modeling the interaction of
the P-H-diazaphospholene 1g with the isolated cation 16g. In
both species, the bulky N-tert-butyl or N-aryl groups were
downsized to methyl substituents in order to reduce the cost of
the calculations. All computations, including the calculations
of harmonic vibrational frequencies and zero-point energies
(ZPE), were performed at the B3LYP/6-31+g** level; single-
point energy calculations were further carried out at the given
geometries using more extended 6-311+g(2d,p) basis sets. The
assignment of a structure as a local minimum or transition state
on the potential energy hypersurface was established by analysis
of the second derivatives (minima have no and transition states
have one negative eigenvalue of the Hessian matrix).
1.373c
1.373c
CdC
PsX
1.348 1.373 1.349b
1.367c
1.480
1.358
1.771
1.462
a Values in parentheses denote energies calculated with 6-311+g(2d,p)
basis sets at the energy-optimized molecular geometry obtained at the
B3LYP/6-31+g(d,p) level. b Diazaphospholene unit. c Diazaphospholenium
cation unit.
with inclusion of zero-point energy but uncorrected for basis
set superposition error31) upon association presumably reflects
the electrostatic (ion-dipole) interaction between the two
molecular fragments.
As the orientation of the fragments in the dimer 18 is
unsuitable for intermolecular hydride transfer, we looked for a
conformation with a closer contact of the P-H bond to the
phosphenium center. This search resulted in the identification
of a structure 19-C2 (Figure 4b), which can be described as a
C2-symmetric assembly of two diazaphospholenium moieties
that are connected by a symmetric P-H-P bridge. The
endocyclic bond lengths range between those reported for
cationic 16g and neutral 1g (Table 1). The phosphorus and the
bridging hydrogen atom form a quasilinear assembly (P-H-P
179.1°). The P-H distances (1.771 Å) are clearly longer than
in 1g (1.480 Å) and indicate significant bonding interaction
between all three atoms. Although 19-C2 is even lower in energy
than 18 (∆EZPE ) -13.5 kcal mol-1 vs 1g/16g31), the analysis
of the second derivatives disclosed a negative eigenvalue of
the Hessian matrix that is associated with a motion of the
bridging hydride along the P-P vector, suggesting that 19-C2
is a possible transition state of a hydride-transfer reaction.
Further energy optimization without symmetry constraints
allowed us finally to identify a molecular structure 19-C1 with
an unsymmetrical P-H-P bridge as a local minimum on the
energy hypersurface. The dislocation of the bridging hydrogen
atom lengthens one P-H bond to 1.834 Å; the second bond
shortens to 1.716 Å but stays clearly longer than the P-H
distance in 1g computed at the same level of theory (1.480 Å).
The bond lengths and angles in the diazaphospholene moieties
(Table 1) and the P‚‚‚P distance (3.542 Å in 19-C2 vs 3.550 Å
in 19-C1) remain essentially unaffected. Quite interestingly, the
energies of 19-C2 and 19-C1 are virtually identical (see Table
1) and differ still by less than 0.2 kcal mol-1 when zero-point
energies are included. NBO population analyses reveal a marked
increase of negative atomic charge on the P-bound hydrogen
Attempts to determine an energy-optimized geometry for a
P-P bonded adduct between 16g and 1g did not lead to the
localization of a stable bonded structure but ended inevitably
in fragmentation, giving rise to a supramolecular complex 18
(Figure 4a). The bond distances and angles in each fragment
and the charge distributions obtained from NBO analyses30 are
closely similar to those computed for free 1g and 16g at the
same level of theory (Table 1). The shortest intermolecular
contacts of 3.17 and 3.22 Å are found not between the two
phosphorus atoms, as would be expected for a phosphenium-
phosphine adduct, but between the phosphorus atom of the
cationic fragment and the nitrogen atoms of the neutral fragment.
As according to the NBO results these atoms carry the largest
positive and negative atomic charges, it appears that the
assembly of the supermolecule 18 is charge controlled, and the
sizable gain in energy of ∆EZPE ) -10.0 kcal mol-1 (∆EZPE
ZPE(18) - EZPE(1g) - EZPE(16g) at the B3LYP/6-31+g** level
)
E
(29) (a) Burford, N.; Cameron, T. S.; Ragogna, P. J. J. Am. Chem. Soc. 2001,
123, 7947. (b) Burford, N.; Ragogna, P. J.; McDonald, R.; Ferguson, M.
J. J. Am. Chem. Soc. 2003, 125, 14404.
(30) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical
Chemistry Institute: University of Wisconsin, Madison, 2001 (http://
www.chem.wisc.edu/∼nbo5).
(31) Attempts to estimate the basis set superposition error by using a counterpoise
correction led to corrected energies of ∆EZPE,corr ) -9.0 kcal mol-1 for 18
and -12.3 kcal mol-1 for 19-C2.
9
3952 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

P-Hydrogen-Substituted 1,3,2-Diazaphospholenes
A R T I C L E S
Chart 2
e.g., alkyl ketones to be reduced at significantly lower rates
than diaryl ketones or aldehydes and permits the selective partial
reduction of SiCl4 to SiHCl3 or of germanium and tin tetra-
chloride to the dichlorides. In this, the compounds 1 show a
remarkable substrate selectivity which makes them potentially
useful reagents in organic or organometallic synthesis for
controlled reduction of multifunctional molecules. Possible
applications in the deposition of metal films from hydrocarbon
solutions, as well as the selective synthesis of new Ge(II) species
are currently under study.
Apart from synthetic applications, the observed reversible
hydride exchange between 1 and neutral chloro-diazaphos-
pholenes or diazaphospholenium cations is of particular interest
from a mechanistic point of view. Although computed energies
of isodesmic hydride exchange reactions with phosphines have
been previously considered as a means to assess relative
stabilities of phosphenium cations,38 these reactions are now,
for the first time, established by experiment. Computational
studies suggest that the transfer involves a transient hydride-
bridged intermediate whose bonding is closely related to that
of species such as [B2H7]-. The preference for the formation
of H-bridged dimers over the formation of P-P bonded Lewis
acid-base adducts is further unprecedented for phosphenium
ions. A reasonable explanation for this behavior can be given
by considering the extraordinarily low electrophilicity of the
cations, which disfavors an orbital-controlled reaction (formation
of a dative P-P bond) over a charge-controlled reaction
(formation of X-bridged adducts with a high degree of ionic
bonding).
atom in both species (q(H) ) -0.21) and a decrease of the
Wiberg bond index (WBI) for the P-H bonds (WBI ) 0.48/
0.37 in 19-C1 and 0.43 in 19-C2) as compared to the values for
1g (q(H) ) -0.13; WBI ) 0.88). Inspection of the natural bond
orbitals and a second-order perturbation theory analysis of the
Fock matrix in the NBO basis suggest a description of the
P‚‚‚H‚‚‚P bonding as a three-center, two-electron bond that is
formed by interaction of the electron pair of the P-H bond
with the empty p-orbital of the phosphenium center and may
be represented as shown in Chart 2 by resonance between
canonical structures A-C. Although such electron-deficient
single-hydride bridges are not common, they have precedence
in the chemistry of main-group hydrides,32 and 19 may be
regarded as isolobal to species like [B2H7]- 33 and [Me3AlHAl-
Me3]-.34 It should be noted that asymmetric hydride bridges
had been encountered in an earlier computational study on
[B2H7]-,35 although it was later shown that this effect depended
highly on the quality of the basis set and the presumption that
the potential hypersurface was very flat.36 Considering these
results, it remains unclear whether the unsymmetrical hydride
bridge in 19-C1 is real or merely an artifact of the theoretical
model employed.
In assessing the significance of the computational results, it
must be conceded that a comprehensive account of the molecular
dynamics is unfeasible as long as the influences of counterions
and solvation are excluded. Questions as to precise activation
energies or the symmetrical or unsymmetrical nature of a
hydride-bridged intermediate must thus remain open. However,
in the light of the present results and the known capability of
diazaphospholenes to form stable transition metal complexes37
and borane adducts,10 we conclude that the preference for
substituent exchange over the formation of phosphine-phos-
phenium is attributable to the outstandingly low Lewis acidity
of the diazaphospholenium ions10,26 rather than to insufficient
nucleophilicity of the diazaphospholene component.
Experimental Section
General Remarks. All manipulations were carried out under dry
argon. Solvents were dried by standard procedures. NMR spectra were
obtained on a Bruker Avance 400 instrument (1H, 400.1 MHz; 13C,
100.5 MHz; 31P, 161.9 MHz) at 303 K. Chemical shifts are referenced
to external TMS (1H, 13C), 85% H3PO4 (¥ ) 40.480747 MHz, 31P),
Me4Si, (¥ ) 19.867187 MHz, 29Si), or Me4Sn (¥ ) 37.290632 MHz,
119Sn). Coupling constants are given as absolute values. The prefixes
i, o, m, and p denote atoms of N-aryl substituents. Mass spectrometric
data were collected on a Varian MAT 711 instrument in EI mode at
70 eV. Elemental analyses were conducted with a Perkin-Elmer
2400CHSN/O analyzer. Melting points were determined in sealed
capillaries.
Computational Studies. DFT calculations were carried out with
the Gaussian 03 program suite39 using Becke’s three-parameter
exchange functional with a Lee-Yang-Parr correlation energy func-
tional (B3LYP) and split-valence basis sets of double-ê quality
augmented by one set of polarization functions on all atoms and diffuse
functions on all heavy atoms (6-31+g(d,p)). Harmonic vibrational
frequencies and zero-point energies were calculated at the same level.
All structures reported are minima (only positive eigenvalues of the
Hessian matrix) or transition states (one negative eigenvalue) on the
potential energy surface. Single-point energy calculations were also
carried out with 6-311+g(2d,p) basis sets at the given geometries.
Analyses of electron populations were carried out with the NBO 5.0
module.30 Listings of atomic coordinates and absolute energies are given
as Supporting Information.
Conclusions
An efficient protocol for the synthesis of P-hydrido-1,3,2-
diazaphospholenes 1 from diazadienes via the P-chloro deriva-
tives 3 has been developed. Extensive studies of the reaction
behavior confirmed that the compounds 1 behave as molecular
hydrides that reduce organic carbonyl compounds and undergo
halide/hydride metathesis with halides of various main-group
elements. These reactions resemble those of complex hydrides
such as NaBH4 or LiAlH4 but are set apart by the good solubility
of the molecular hydrides, which permits reductions in nonpolar
hydrocarbon solvents, and a retarded reactivity, which causes,
(32) Treboux, C.; Trinquier, G. Inorg. Chem. 1992, 31, 4201.
(33) Watkins, M. I.; Bau, R. J. Am. Chem. Soc. 1982, 104, 7670.
(34) Atwood, J. L.; Hrncir, D. C.; Rogers, R. D.; Howard, J. A. K. J. Am. Chem.
Soc. 1981, 103, 6787.
General Procedures for the Preparation of the 2-Chloro-1,3,2-
diazaphospholenes 3. (a) From r-Aldimino-aldimines. The appropri-
(35) Sapse, A.-M.; Osorio, L. Inorg. Chem. 1984, 23, 627.
(36) Raghavachari, K.; Schleyer, P. v. R.; Spitznagel, G. W. J. Am. Chem. Soc.
1983, 105, 5917.
(37) Gudat, D.; Haghverdi, A.; Nieger, M. J. Organomet. Chem. 2001, 617,
383.
(38) Schoeller, W. W.; Tubbesing, U. J. Mol. Struct. (THEOCHEM) 1995, 343,
49.
(39) Frisch, M. J.; et al. Gaussian 03, Revision B.04; Gaussian Inc.: Pittsburgh,
PA, 2003.
9
J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 3953

A R T I C L E S
Burck et al.
ate R-aldimino-aldimine 6 (25 mmol) and triethylamine (50 mmol) were
dissolved in toluene (100 mL). The solution was cooled to -78 °C
and PCl3 (25 mmol) slowly added. The mixture was stirred for 36 h at
room temperature. The precipitate was filtered off and washed twice
with toluene (20 mL). Volatiles were evaporated in a vacuum, and the
remaining residue was washed twice with n-hexane (30 mL). The
products were obtained as pale yellow to brown solids.
(b) From Diazadienes. Lithium turnings (0.3 mol) were added to a
solution of the appropriate diazadiene (0.15 mol) in THF (300 mL),
and the mixture was stirred for 1 d under strict exclusion of oxygen.
Excess lithium was then filtered off, the filtrate cooled to -78 °C, and
triethylamine hydrochloride (0.3 mol) added in several portions. When
the addition was complete, the mixture was allowed to warm to room
temperature and stirred for 1 h. After the mixture was cooled again to
-78 °C, PCl3 (0.15 mol) was slowly added. The mixture was stirred
for an additional 36 h at room temperature before all solvents were
evaporated in a vacuum. The residue was transferred into a Soxhlet
extraction apparatus and extracted with diethyl ether (350 mL) for 2 d.
After completion of the extraction, volatiles were evaporated in a
vacuum and the residue was washed twice with n-hexane (100 mL).
The products were obtained as pale brown solids.
was warmed to room temperature and stirred for 1 h. Solvents were
evaporated in a vacuum, and then the residue was dissolved in hexane
(50 mL) and filtered. The product was obtained after fractionated
distillation of the filtrate in a vacuum (1c) or concentration of the filtrate
in a vacuum and crystallization at -20 °C. Following this procedure,
1c10 was obtained as a yellow oil with a yield of 0.77 g (77%).
Reaction of 1c with 2-Methoxybenzaldehyde. 1c (200 mg, 1 mmol)
was added to a solution of the aldehyde (136 mg, 1 mmol) in THF (10
mL) and the reaction stirred for 1 h at room temperature. The solvent
was evaporated in a vacuum and the residue dissolved in acetonitrile
(10 mL). Crystallization at -28 °C afforded 8 as a white powder that
was collected by filtration and dried in a vacuum. Yield: 240 mg (65%).
1H NMR (CD3CN): δ ) 6.90-7.30 (m, 4 H, Hphenyl), 6.18 (d, 2 H,
3
3JPH ) 1.9 Hz, N-CH), 4.26 (d, 2 H, JPH ) 4.3 Hz, CH2), 3.79 (s, 3
4
H, OCH3), 1.42 (d, 2 H, JPH ) 1.1 Hz, CH3). 13C{1H} NMR (CD3-
CN): δ ) 156.5 (o-C), 135.7 (i-C), 128.1 (m-C), 127.8 (o-C), 127.4
2
(p-C), 119.8 (p-C), 112.0 (d, JPC ) 9.7 Hz, N-CH), 67.0 (OCH3),
58.0 (d, 2JPC ) 3.9 Hz, C), 52.4 (d, 2JPC ) 15.8 Hz, CH2), 30.0 (d, 3JPC
) 10.3 Hz, CH3). 31P{1H} NMR (CD3CN): δ ) 92.7. MS: m/z (relative
intensity) ) 336 ([M] +, 78), 199 ([M - C8H9O2]+, 38), 159 ([M -
C11H13O2]+, 85), 104 ([M - C11H23N2OP]+, 100). Elemental analysis
for C18H29N2O2P calcd: C 64.27, H 8.69, N 8.33; found: C 63.89, H
8.38, N 8.34.
1,3-Bis(tert-butyl)-2-chloro-1,3,2-diazaphospholene13 (3c). Yield:
3.95 g (67%).
Reaction of 1c with Benzophenone. 1c (200 mg, 1 mmol) was
added to a solution of benzophenone (182 mg, 1 mmol) in THF (10
mL) and the mixture stirred for 1 h at ambient temperature. The solvent
was then evaporated in a vacuum and the residue dissolved in
acetonitrile (5 mL). Diethyl ether (2 mL) was added and the solution
stored at -28 °C until 10 precipitated as a white powder that was
collected by filtration and dried in a vacuum. Yield: 280 mg (67%).
1H NMR (C6D6): δ ) 7.43 (d, 4 H, 3JHH ) 7.0 Hz, o-CH), 6.90-7.20
2-Chloro-1,3-bis(2,4,6-trimethylphenyl)-1,3,2-diazaphospho-
lene20 (3d). Yield: 5.70 g (70%). Elemental analysis: calcd C 66.94,
H 6.74, N 7.81; found C 65.83, H 6.82, N 7.51.
2-Chloro-1,3-bis(2,6-diisopropylphenyl)-1,3,2-diazaphospho-
1
lene (3e). Yield: 7.75 g (70%). Mp: 212 °C. H NMR (C6D6): δ )
7.18 (t, 2 H, 3JHH ) 8.5 Hz, p-CH), 7.12 (d, 4 H, 3JHH ) 8.5 Hz, m-CH),
3
6.21 (s, 2 H, N-CH), 3.74 (m, broad, 4 H, CH), 1.35 (d, 12 H, JHH
3
) 6.8 Hz, CH3), 1.14 (d, 12 H, JHH ) 6.8 Hz, CH3). 13C{1H} NMR
3
(m, 12 H, p/m-CH), 5.97 (d, 2 H, JPH ) 1.9 Hz, CH), 5.54 (d, 1 H,
(C6D6): δ ) 146.6 (i-C), 133.0 (o-C), 128.0 (p-CH), 123.4 (m-CH),
4
3JPH ) 9.8 Hz, O-CH), 1.13 (d, 2 H, JPH ) 0.9 Hz, CH3). 13C{1H}
2
4
119.7 (d, JPC ) 8.7 Hz, N-CH), 27.7 (d, JPC ) 2.3 Hz, CH), 24.0
(CH3), 23.2 (CH3). 31P{1H} NMR (C6D6): δ ) 141.0. MS: m/e (relative
intensity) ) 442.2 ([M]+, 6.4), 407.2 ([M - Cl]+, 37.6), 162.1 ([M -
C14H20N2PCl]+, 100.0). Elemental analysis for C26H36ClN2P: calcd C
70.49, H 8.19, N 6.32; found C 67.44, H 8.74, N 5.88.
NMR (C6D6): δ ) 144.2 (i-C), 128.8 (m-C), 128.2 (o-C), 125.5 (p-
C), 111.8 (d, 2JPC ) 9.8 Hz, N-CH), 75.8 (d, 2JPC ) 5.1 Hz, O-CH),
2
3
51.8 (d, JPC ) 3.9 Hz, C), 29.2 (d, JPC ) 9.7 Hz, CH3). 31P{1H}
NMR (C6D6): δ ) 96.4.
Reactions of 1c with Acetophenone, Butyraldehyde, trans-
Cinnamaldehyde, and Pentan-3-one. In an NMR tube equipped with
a Teflon-sealed screw cap, the carbonyl compound (0.25 mmol) was
dissolved in benzene-d6 (0.6 mL), followed by addition of 1c (50 mg,
0.25 mmol). The tube was closed and the reaction monitored by NMR
spectroscopy. The products 9, 11a,b (relative ratio 3:1), 13, and 14,
respectively, and the hydrolysis product 716 were identified by 1D and
2D NMR studies.
Preparation of 2-Hydrido-1,3,2-diazaphospholenes 1c-e. (a) By
Using LiAlH4. A solution of 3e (2.09 g, 5 mmol) in THF (50 mL) was
cooled to -78 °C, and a 1 M solution of LiAlH4 in THF (1.25 mL,
1.25 mmol) was slowly added. The mixture was stirred for 30 min at
-78 °C and then allowed to warm to room temperature. All solvents
were evaporated in a vacuum, and then the residue was extracted with
n-hexane (50 mL) and filtered. Concentration of the filtrate to a volume
of 10 mL and storage at -20 °C produced yellow crystals which were
collected by filtration and dried in a vacuum to yield 1.65 g (86%) of
1e. Mp: 112 °C. 1H NMR (C6D6): δ ) 7.39 (d, 1 H, 1JPH ) 132.6 Hz,
PH), 7.16 (t, 2 H, 3JHH ) 6.9 Hz, p-CH), 7.14 (dd, 2 H, 3JHH ) 6.9 Hz,
1
3
Data for 9. H NMR: δ ) 5.94 (d, 2 H, JPH ) 1.8 Hz, N-CH),
3.26 (dt, 1 H, 2JPH ) 4.8 Hz, 3JPH ) 6.3 Hz, OCH), 1.46 (dt, 2 H, 3JPH
3
3
3
) 6.3 Hz, JPH ) 7.2 Hz, CH2), 1.32 (tq, 2 H, JPH ) 7.5 Hz, JPH
7.2 Hz, CH2), 1.31 (s, 18 H, CH3), 0.83 (t, 3 H, 3JHH ) 7.5 Hz, CH3).
13C{1H} NMR: δ ) 112.6 (d, 2JPC ) 9.6 Hz, N-CH), 61.5 (d, 2JPC
)
3
4
4JHH ) 3.2 Hz, m-CH), 7.05 (dd, 2 H, JHH ) 6.9 Hz, JHH ) 3.2 Hz,
m-CH), 5.91 (d, 2 H, 3JPH ) 1.8 Hz, N-CH), 3.69 (sept, 2 H, 3JHH
)
)
2
3
3
3
5.4 Hz, N-CH), 53.1 (d, JPC ) 17.0 Hz, o-CH), 34.2 (d, JPC ) 1.7
Hz, CH3), 31.3 (d, 3JPC ) 10.1 Hz, CH2), 30.1 (d, 4JPC ) 4.0 Hz, CH2),
20.1 (s, CH3). 31P{1H} NMR: δ ) 93.1.
6.9 Hz, CH), 3.52 (sept, 2 H, JHH ) 6.9 Hz, CH), 1.35 (d, 6 H, JHH
) 6.9 Hz, CH3), 1.26 (d, 6 H, 3JHH ) 6.9 Hz, CH3), 1.19 (d, 6 H, 3JHH
3
) 6.9 Hz, CH3), 1.13 (d, 6 H, JHH ) 6.9 Hz, CH3). 13C{1H} NMR
1
3
2
Data for 11a. H NMR: δ ) 7.24-6.99 (m, 5 H, Hphenyl), 6.15
(C6D6): δ ) 149.4 (d, JPC ) 2.4 Hz, o-C), 148.3 (d, JPC ) 2.9 Hz,
(ddt, 1 H, 3JPH ) 10.1 Hz, 3JHH ) 6.1 Hz, 6JHH ) 1.5 Hz, CH), 5.96 (d,
2 H, 3JPH ) 1.9 Hz, NCH), 4.69 (ddt, 1 H, 3JHH ) 7.4 Hz, 3JHH ) 6.1
Hz, 4JPH ) 1.9 Hz, CH), 3.54 (d, 2 H, 3JHH ) 7.5 Hz, CH2), 1.31 (s, 18
H, CH3). 31P{1H} NMR: δ ) 95.0.
3
i-C), 137.6 (s, o-C), 137.4 (s, p-C), 124.0 (s, m-C), 123.7 (d, JPC
)
1.6 Hz, m-C), 122.5 (d, 2JPC ) 6.3 Hz, N-CH), 29.0 (s, CH), 28.3 (d,
4JPC ) 1.3 Hz, CH), 24.8 (s, CH3), 24.5 (d, 5JPC ) 1.6 Hz, CH3), 24.0
(d, 5JPC ) 1.6 Hz, CH3), 23.4 (s, CH3). 31P NMR: δ ) 71.6 (d, 1JPH
)
132.6 Hz). MS: m/e (relative intensity) ) 408.3 ([M]+, 44.5), 407.3
([M - H]+, 100.0), 365.2 ([M - C3H7]+, 4.8). Elemental analysis for
C26H37N2P: calcd C 76.43, H 9.13, N 6.86; found C 75.87, H 9.29, N
6.72.
Data for 11b. 1H NMR: δ ) 7.24-6.99 (m, 5 H, Hphenyl), 6.13 (d,
1 H, JHH ) 12.1 Hz, CH), 5.98 (d, 2 H, JPH ) 1.8 Hz, NCH), 5.27
(dt, 1 H, 3JHH ) 12.1 Hz, 3JHH ) 7.4 Hz, CH), 3.13 (d, 2 H, 3JHH ) 7.4
Hz, CH2), 1.30 (s, 18 H, CH3). 31P{1H} NMR: δ ) 93.5.
3
3
1
3
4
(b) By Using “Red-Al”. A solution of the appropriate chloro-
diazaphospholene 3c-e (5 mmol) in THF (50 mL) was cooled to -78
°C and a 3.5 M solution of sodium dihydrido-bis(2-methoxyethoxy)-
aluminate in toluene (0.7 mL, 2.5 mmol) slowly added. The mixture
Data for 13. H NMR: δ ) 7.74 (dd, 2 H, JHH ) 8.5 Hz, JHH
)
1.4 Hz, o-CH), 7.12 (t, 1 H, 3JHH ) 7.5 Hz, p-CH), 7.05 (dd, 2 H, 3JHH
3
3
3
) 8.5 Hz, JHH ) 7.5 Hz, p-CH), 5.99 (dd, 1 H, JPH ) 1.9 Hz, JHH
3
3
) 2.9 Hz, N-CH), 5.84 (dd, 1 H, JPH ) 1.7 Hz, JHH ) 2.9 Hz,
9
3954 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

P-Hydrogen-Substituted 1,3,2-Diazaphospholenes
A R T I C L E S
3
3
1
N-CH), 4.61 (dq, 1 H, JPH ) 9.7 Hz, JHH ) 6.5 Hz, O-CH), 1.37
1H EXSY spectrum (see discussion in the text). H NMR: δ ) 6.69
(broad s, 2 H, N-CH, 3c), 6.03 (broad s, 2 H, N-CH, 1c), 5.40 (broad
s, 1 H, PH), 1.55 (broad s, 18 H, CH, 3c), 1.22 (broad s, 18 H, CH3,
1c). 31P{1H} NMR: δ ) 146.7 (broad s, 3c), 55.9 (broad s, 1c).
3
4
(d, 3 H, JHH ) 6.5 Hz, CH3), 1.31 (d, 9H, JPH ) 0.9 Hz, CH3), 1.13
(d, 9H, 4JPH ) 0.8 Hz, CH3). 13C{1H} NMR: δ ) 146.6 (s, i-C), 127.8
(s, o-CH), 126.5 (s, p-CH), 125.7 (s, m-CH), 112.6 (d, 2JPC ) 9.2 Hz,
2
2
N-CH), 112.5 (d, JPC ) 9.2 Hz, N-CH), 71.1 (d, JPC ) 4.0 Hz,
Reaction of 1c with 16c[OTf]. 16c[OTf] (174 mg, 0.5 mmol) and
1c (100 mg, 0.5 mmol) were dissolved in acetonitrile-d3 (1 mL), and
the resulting mixture was analyzed by H and 31P NMR spectroscopy
at temperatures between +50 and -40 °C. At the lowest temperature,
a white solid precipitated which dissolved again upon warming. The
variation of the spectral features and the conclusions drawn are
discussed in the text. 1H NMR (at 30 °C): δ ) 5.55 (s, N-CH), 4.87
(s, PH), 0.76 (s, CH3). 31P{1H} NMR: δ ) 99.4 (broad).
2
2
OCH), 52.8 (d, JPC ) 8.2 Hz, N-C), 52.7 (d, JPC ) 8.0 Hz, N-C),
30.6 (d, 3JPC ) 10.1 Hz, CH3), 30.1 (d, 3JPC ) 10.1 Hz, CH3), 29.6 (d,
3JPC ) 4.2 Hz, CH3). 31P{1H} NMR: δ ) 97.0.
Data for 14. H NMR: δ ) 5.88 (d, 2 H, JPH ) 1.9 Hz, N-CH),
3.56 (dquint, 1 H, JHH ) 10.2 Hz, JPH ) 5.4 Hz, O-CH), 1.49 (dq,
1
1
3
3
3
3
3
4 H, JHH ) 10.2 Hz, JHH ) 7.5 Hz, CH2), 1.29 (s, 18 H, CH3), 0.84
(t, 6 H, JHH ) 7.5 Hz, CH3). 31P{1H} NMR: δ ) 100.4.
3
Reactions of 1c with ECl4 (E ) C, Si, Ge, Sn). In an NMR tube
equipped with a Teflon-sealed screw cap, ECl4 (0.5 mmol) was
dissolved in acetonitrile-d3 (0.6 mL). 1c (100 mg, 0.5 mmol) was added,
the tube closed, and the reaction monitored by NMR spectroscopy.
The reaction products were identified in situ by 1D and 2D NMR
studies.
Crystal Structure Determination of 1e, 16c[GeCl3], and 16c-
[SnCl3]. Data were collected on a Nonius Kappa CCD diffractometer
at -150 °C using Mo KR radiation (λ ) 0.71073 Å). The structures
were solved by direct methods (SHELXS-9740). Non-hydrogen atoms
were refined anisotropically and H atoms with a riding model on F 2
(full-matrix least-squares, SHELXL-9741).
Reaction with CCl4. 1H NMR: δ ) 7.82 (s, CHCl3), 7.27 (s, 2 H,
1e: yellow crystals, C26H37N2P, M ) 408.6, crystal size 0.40 × 0.30
× 0.20 mm3, monoclinic, space group P21/n (No. 14), a ) 19.6744(3)
Å, b ) 6.3916(1) Å, c ) 20.4343(4) Å, â ) 107.570(1)°, V )
2449.75(7) Å3, Z ) 4, F(calcd) ) 1.11 Mg m-3, F(000) ) 888, µ )
0.13 mm-1, 23 982 reflections measured, 4330 unique reflections (Rint
) 0.041) used for structure solution and refinement with 265 parameters
and 1 restraint, no absorption correction, R1 (I > 2σ(I)) ) 0.038, wR2
4
NCH, 3c), 5.48 (s, CH2Cl2), 3.04 (CH3Cl), 1.65 (d, 18 H, JPH ) 1.8
Hz, CH3, 3c). 13C{1H} NMR: δ ) 122.3 (s, 3c), 78.2 (s, CHCl3), 57.4
(s, 3c), 54.0 (s, CH2Cl2), 29.1 (s, CH3, 3c), 26.8 (s, CH3Cl). 31P{1H}
NMR: δ ) 173.0 (3c).
Reaction with SiCl4. 1H NMR: δ ) 7.33 (s, 2H, CH, 3c), 5.71 (s,
1
SiHCl3), 1.65 (s, 18 H, CH3, 3c). 29Si NMR: δ ) -10.6 (d, JHSi
)
379.6 Hz, SiHCl3). 31P{1H} NMR: δ ) 176.3 (3c).
) 0.104, largest differential peak and hole 0.399 and -0.288 e Å-3
.
1
Reaction with GeCl4: Data for 16c[GeCl3]. H NMR: δ ) 8.24
(s, 2 H, CH), 1.84 (d, 18 H, 4JHP ) 1.8 Hz, CH3). 31P{1H} NMR: δ )
202.6.
16c[GeCl3]: orange crystals, C10H20Cl3GeN2P, M ) 378.2, crystal
size 0.55 × 0.50 × 0.40 mm3, orthorhombic, space group Pnma (No.
62), a ) 12.9640(4) Å, b ) 17.3985(6) Å, c ) 7.3944(2) Å, V )
1667.84(9) Å3, Z ) 4, F(calcd) ) 1.51 Mg m-3, F(000) ) 768, µ )
2.40 mm-1, 7370 reflections measured, 1527 unique reflections (Rint
) 0.035) used for structure solution and refinement with 82 parameters,
empirical absorption correction with multiple reflections (maximum
and minimun transmission 0.4125 and 0.3831), R1 (I > 2σ(I)) ) 0.021,
wR2 ) 0.051, largest differential peak and hole 0.254 and -0.271 e
1
Reaction with SnCl4: Data for 16c[SnCl3]. H NMR: δ ) 8.18
(s, 2 H, CH), 1.71 (s, 18 H, CH3). 31P{1H} NMR: δ ) 205.0. 119Sn
NMR (20 °C): δ ) -42.0.
Reaction of 3c with SnCl2 and GeCl2 x Dioxane. A solution of 3c
(1 mmol, 234 mg) and SnCl2 (1 mmol, 190 mg) or GeCl2 x dioxane (1
mmol, 232 mg), respectively, in CH2Cl2 (15 mL) was stirred for 1 h at
room temperature. Addition of hexane (25 mL) produced a white
precipitate which was collected by filtration and washed with hexane
(10 mL).
Data for 16c[GeCl3]. Yield: 348 mg (92%). 1H NMR (CD2Cl2): δ
) 8.24 (s, 2 H, CH), 1.84 (s, 18 H, CH3). 13C{1H} NMR (CD2Cl2): δ
) 133.6 (d, 2JPC ) 3.6 Hz, CH), 63.8 (d, 2JPC ) 7.4 Hz, NC), 32.1 (d,
3JPC ) 9.4 Hz, CH3). 31P{1H} NMR (CD2Cl2): δ ) 201.3. Elemental
analysis for C10H20Cl3N2PGe: calcd C 31.76, H 5.33, N 7.41; found C
30.56, H 5.11, N 6.96.
Å-3
.
16c[SnCl3]: orange crystals, C10H20Cl3N2PSn, M ) 424.3, crystal
size 0.40 × 0.35 × 0.30 mm3, orthorhombic, space group Pnma (No.
62), a ) 12.7132(3) Å, b ) 17.4583(5) Å, c ) 7.7723(2) Å, V )
1725.07(8) Å3, Z ) 4, F(calcd) ) 1.63 Mg m-3, F(000) ) 840, µ )
2.02 mm-1, 9453 reflections measured, 1574 unique reflections (Rint
) 0.036) used for structure solution and refinement with 82 parameters,
empirical absorption correction with multiple reflections (maximum
and minimum transmission 0.5682 and 0.5375), R1 (I > 2σ(I)) ) 0.020,
wR2 ) 0.046, largest differential peak and hole 0.441 and -0.490 e
Data for 16c[SnCl3]. Yield: 380 mg (90%). 1H NMR (CD2Cl2): δ
) 8.31 (s, 2 H, CH), 1.85 (s, 18 H, CH3). 13C{1H} NMR (CD2Cl2): δ
) 132.4 (d, 2JPC ) 3.6 Hz, CH), 62.2 (d, 2JPC ) 7.8 Hz, NC), 30.5 (d,
3JPC ) 9.4 Hz, CH3). 31P{1H} NMR (CD2Cl2): δ ) 199.4. 119Sn NMR
(CD2Cl2, -20 °C): δ ) -58.1. Elemental analysis for C10H20Cl3N2-
PSn: calcd C 28.31, H 4.75, N 6.60; found C 27.83, H 4.81, N 6.39.
Reaction of 1c with SnCl2. Addition of 1c (100 mg, 0.5 mmol) to
a solution of SnCl2 (95 mg, 0.5 mmol) in acetonitrile-d3 (0.5 mL) led
to immediate formation of a black precipitate which was filtered off
and characterized as elemental tin by powder diffraction. The solution
Å-3
.
Acknowledgment. We thank Dr. U. Kessler, University of
Bonn, for the powder diffraction measurements of tin samples.
Note Added after ASAP Publication. After this paper was
published ASAP on March 3, 2006, Figure 1 was modified to
properly display the thermal ellipsoids, as described in the
caption. The corrected version was published ASAP on March
6, 2006.
1
was transferred to a sure-sealed NMR tube and analyzed by H and
1
31P NMR spectroscopy. H NMR: δ ) 6.86 (s, 2H, N-CH), 1.49 (s,
18 H, CH3). 31P{1H} NMR: δ ) 159.8.
Supporting Information Available: IR and Raman data for
16c[GeCl3] and 16c[SnCl3]; complete ref 40; computational
details (extended geometric and energy data) (PDF); X-ray
crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
Reaction of 1c with Ph2PCl. Chloro-diphenylphosphine (110 mg,
0.5 mmol) and 1c (100 mg, 0.5 mmol) were dissolved in acetonitrile-
d3 (0.5 mL). The solution was analyzed by 31P NMR spectroscopy. 31
NMR: δ ) 152.0 (s, 3c), -40.8 (d, JPH ) 204.7 Hz, Ph2PH).
P
1
Reaction of 1c with 3c. 3c (117 mg, 0.5 mmol) and 1c (100 mg,
0.5 mmol) were dissolved in acetonitrile-d3 (0.5 mL). The resulting
JA057827J
1
solution was analyzed by H and 31P NMR spectroscopy and showed
broadened signals attributable to 1c and 3c, respectively. Dynamic
exchange between the two species was confirmed by a two-dimensional
(40) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(41) Sheldrick, G. M., University of Go¨ttingen, 1997.
9
J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 3955