Chiral heterobimetallic complexes of carbodiphosphoranes and phosphinidene-carbene adducts

Article abstract of DOI:10.1039/c3cc40382c

Heterobimetallic complexes derived from carbodiphosphoranes or phosphinidene-carbene adducts are reported, in which the central atom accepts two lone-pairs from two different donor ligands and-at the same time-donates two lone-pairs to two different metal

Full text of DOI:10.1039/c3cc40382c

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Chiral heterobimetallic complexes of
carbodiphosphoranes and phosphinidene–carbene
Cite this: DOI: 10.1039/c3cc40382c
adducts†
Received 16th January 2013,
Accepted 21st February 2013
Manuel Alcarazo,* Karin Radkowski, Gerlinde Mehler, Richard Goddard and
Alois Furstner*
¨
DOI: 10.1039/c3cc40382c
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Heterobimetallic complexes derived from carbodiphosphoranes or
phosphinidene–carbene adducts are reported, in which the central atom
accepts two lone-pairs from two different donor ligands and – at the
same time – donates two lone-pairs to two different metal centers.
Therefore these complexes are carbogenic yet captodative chiral entities.
Although the strong Lewis basicity and unusual coordination proper-
ties of carbodiphosphoranes were recognized early on,1 it was not
until the recent computational studies of Frenking and co-workers
that the chemical nature of such compounds was fully appreciated.2
These investigations suggested that carbodiphosphoranes cannot be
adequately described as heterocumulenes; rather, they are thought
Scheme 1 Basic coordination modes of carbodiphosphoranes.
to consist of two phosphine ligands coordinated to a central carbon in C (Scheme 1).10 All three situations have already been experi-
atom, which keeps its own valence electrons in the form of two mentally confirmed. What has not been realized so far, however,
orthogonal lone pairs and hence gains formally zero-valent char- is the possibility of employing A-type compounds as starting
acter. It was not least because of this provocative interpretation materials for a second metallation step using a different Lewis
of stable carbogenic compounds as captodative entities that the acid. Despite the plausible interest in the resulting hetero-
chemistry of carbodiphosphoranes and structurally related com- bimetallic species D from a structural point of view (they may
pounds has flourished. Moreover, it was suggested that the same offer a new platform for the study of metal–metal interactions)
bonding situation is found in carbodicarbenes,3 in ‘‘heteroleptic’’ and in the area of cooperative bimetallic catalysis, we are unaware
C0 moieties stabilized by one phosphane and one carbene ligand,4 of any precedent for such a molecular architecture. Outlined
as well as in related species in which the central carbon atom is below are the first examples of coordination compounds of this
formally replaced by isolobal BR,5 N+,6 or P+7 units.
type, which also demonstrate that zerovalent carbon centers can
Three possible coordination modes can be conceived for be rendered chiral when surrounded by two different internal
carbodiphosphoranes based on the rationale outlined above: (i) the phosphine donors and two different metal acceptor sites.
central carbon atom can donate only two electrons and hence
To this end, carbodiphosphorane 3 was chosen as a suitable
behave as a traditional s-donor ligand as shown in the generic starting material. While the strongly donating triphenylphos-
structure A;8 (ii) both electron pairs of the central carbon serve to phine and 2-(pyridyl)diphenylphosphine ligands impose C0
bind an electrophile each; if the latter are metal fragments, character onto its central atom, the pyridyl substituent
geminally dimetallated complexes B will ensue;4a,9 (iii) finally, it might assist in the coordination of a second Lewis acid by
is possible that all four available electrons are involved in simulta- chelation. Compound 3 was readily attained as a yellow solid upon
neous s- and p-donation to the same Lewis acid center as shown reaction of bromomethyl triphenylphosphonium bromide 1 with
(2-pyridyl)PPh2 and subsequent deprotonation of the resulting
¨
¨
Max-Planck-Institut fur Kohlenforschung, 45470 Mulheim an der Ruhr, Germany.
E-mail: alcarazo@mpi-muelheim.mpg.de; Fax: +49 208 3062994;
Tel: +49 208 3062428
diphosphonium salt 2 with NaNH2 in liquid NH3. The carbo-
diphosphorane nature of 3 is evident from the high-field 13C
NMR signal at d = 11.8 ppm (dd, 1JC–P = 122, 119 Hz), which is a
characteristic trait of all known compounds comprising a
R3P - C ’ PR3 entity. The structure in the solid state is also
informative, as it shows that the putative P–C–P ‘‘hetero-cumulene’’
† Electronic supplementary information (ESI) available: Characterization data for
all new compounds. CCDC 914763 (3), 914764 (4), 914765 (5), 914766 (6), 914767 (9),
914768 (10). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3cc40382c
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(1.906(2)–1.948(5) Å).12 Compound 5 also displays an additional
example of weak metallophilic interaction between Cu and Au
(Cu(1)–Au(1) bond distance 2.8483(10) Å).13 Finally, the Cu atom
also coordinates with the nitrogen of the pyridyl substituent.
Although the Cu(1)–N(1) bond distance in 5 (2.267(6) Å) is longer
than those in regular Cu(I)–pyridine complexes, it is clearly within
the sum of the van der Waals radii of the partners.14
In complex 6, the Au(2) atom introduced in the second auration
step binds tighter to C(1) than Au(1) (Au(2)–C(1) and Au(1)–C(1) bond
lengths are 2.080(9) and 2.127(8) Å, respectively). This is somewhat
counterintuitive, given the fact that Au(2) belongs to a neutral AuCl
fragment, whereas the Au(1) centre is, a priori, positively charged.15,16
We tentatively ascribe the weaker interaction with the Au(1) center to
the trans-effect of the PPh3 ligand. The Au(I)–Au(2) distance in 6
is 3.1274(6) Å and in contrast to complex 5, featuring a distinctive
Cu–N bond, the pyridine nitrogen atom in 6 interacts with neither of
the neighbouring gold atoms.
Scheme 2 Synthesis and molecular structure of 3 in the solid state (hydrogen
atoms and solvent molecules have been removed for clarity; ellipsoids at 50%
probability).11 Reagents and conditions: (a) 1, 1 equiv.; PPh2(2-pyridyl), 1 equiv.,
(PhO)3PO, 120 1C, 12 h, 24%; (b) NaNH2/NH3, À35 1C, 7 h, 88%.
axis is in fact significantly bent away from linearity (Y =
133.25(10)1; Scheme 2).
To the best of our knowledge, complexes 5 and 6 are the first
examples of heterodimetallated C0 compounds. In both cases, the
In line with our expectations, compound 3 reacted smoothly central C(1) atom carries four chemically different substituents and
with [Ph3PAuCl] in the presence of NaSbF6 to give the mono- hence constitutes a chiral center. In assessing this situation, one
aurated species 4 (Scheme 3). Despite its cationic character, this should bear in mind that the nature of the four bonds engaging
complex was able to intercept either CuCl or [(Me2S)AuCl] with C(1) is captodative and, therefore, fundamentally different from the
formation of the corresponding dimetallated complexes 5 and bonding situation in ordinary (chiral) organic molecules.
6, respectively, in remarkably good yields.
Unfortunately, all our attempts to coordinate non-linear metal
The comparison of the structures of 5 and 6 is highly informa- fragments to 4 failed, presumably because of the congestion about
tive. In the solid state, the Cu atom of 5 is tightly bound to the the C0 atom caused by the phosphine ligands. Therefore, we next
central carbon of the ligand; actually, the Cu(1)–C(1) bond length of focused our attention on phosphinidenes, a series of compounds
1.981(5) Å is only slightly longer than the one found in closely in which the central carbon from carbodiphosphoranes has been
related mono-metallated carbodiphosphorane–Cu complexes formally replaced by an isolobal [PR]-fragment (Scheme 4). It is
expected that the longer metal phosphorus bonds should facilitate
the coordination of a wider variety of metal fragments.17
Due to the experience of our group with cyclopropenylidenes,
these excellent two electron donors were employed as ancillary
ligands to stabilize the central PR group.18 An appropriate phos-
phinidene–carbene adduct 8 was obtained by reaction of 1-chloro-
2,3-bis(di-isopropylamino) cyclopropenium tetrafluoroborate 7 with
2,4,6-tris(isopropyl)phenylphosphine followed by deprotonation with
KHMDS (for details, see the ESI†).19
When compound 8 was allowed to react with two equivalents of
[(Me2S)AuCl], the bis-aurated complex 9 was isolated in good yield.
Its formation clearly parallels the chemistry of carbodiphosphoranes
and bears witness to the availability of two electron lone pairs at the
PI center. If 8 is first reacted with only one equivalent of [RhCl(cod)]2
followed by addition of half equivalent of [(Me2S)AuCl] to the
reaction mixture, the heterodimetallic species 10 could be obtained
(Scheme 5). The structure of 10 in the solid state was elucidated
using X-ray diffraction, which showed that the AuCl and the
RhCl(cod) moieties both coordinate to the phosphorus atom. It
seems, however, that there is no interaction between the two metals,
Scheme 3 Synthesis of 4–6 and molecular structures of 5 and 6 in the solid state
(hydrogen atoms, anions and solvent molecules have been removed for clarity; ellipsoids
at 50% probability).11 Reagents and conditions: (a) [Ph3PAuCl], NaSbF6, THF, 0 1C, 1 h,
97%; (b) CuCl, CH2Cl2, 0 1C, 12 h, 95%; (c) [(Me2S)AuCl], CH2Cl2, 0 1C, 2 h, 85%.
Scheme 4 Isolobal relationship between carbodiphosphoranes and phosphinidenes.
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11 CCDC 914763 (3), 914764 (4), 914765 (5), 914766 (6), 914767 (9), 914768
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Scheme 5 Synthesis and molecular structure in the solid state of 9 and 10
(hydrogen atoms and solvent molecules have been removed for clarity; ellipsoids
at 50% probability).11 Reagents and conditions: (a) [(Me2S)AuCl], 2 equiv., toluene,
À78 1C - rt, 16 h, 83%; (b) [RhCl(cod)]2, 0.5 equiv., toluene, À78 1C - rt, 1 h; then
[(Me2S)AuCl], 1 equiv., À78 1C - rt, 16 h, 54%; cod = 1,5-cyclooctadiene.
15 We do not imply that the positive charge resides on Au(1); rather it is
likely delocalized, at least over the [CAu2] core of 6. For the bonding
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since the Rh(I)–Au(I) distance is 3.891(5) Å. Similar to compounds 5
and 6, complex 10 is a chiral entity comprising a quaternary
phosphorous atom surrounded by two different donor ligands and
two different acceptor sites.20
In summary, we report herein the syntheses and structures
of various heterodimetallic complexes of carbodiphosphoranes
and phosphinidene–carbene adducts, in which the central
atom of these ligands donates electron density to two different
metal centers that are forced into close proximity; as a result,
these complexes are centrally chiral. Plausible applications of
these unusual chemical entities in polymetallic catalysis as well
as attempts to control their absolute configuration are currently
under investigation.
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Notes and references
19 For previous studies on the coordination chemistry of cyclopropenylidene
stabilized phosphinidenes, see: (a) N. H. T. Huy, B. Donnadieu,
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T. Huy, B. Donnadieu, G. Bertrand and F. Mathey, Organometallics, 2010,
29, 1302. For the synthesis of bis-(BH3) adducts of NHC-stabilized
phosphinidenes, see: (c) A. J. Arduengo, C. J. Carmalt, J. A. C. Clyburne,
A. H. Cowley and R. Pyatib, Chem. Commun., 1997, 981–982.
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(c) R. Tonner and G. Frenking, Angew. Chem., Int. Ed., 2007, 46, 20 For the only previous report on a heterodimetallated phosphinidene,
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8695–8698; (d) R. Tonner, F. Oxler, B. Neumu¨ller, W. Petz and
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.