A C2 Fragment as Four-Electron σ Donor

Article abstract of DOI:10.1002/zaac.201700204

The formation and X-ray structure analysis of the Pt^IV complex [(CH₂C{PPh₂C₆H₄}₂)PtI₂] is reported. The molecule possesses the orthometalated ligand (PPh₃)₂C→CH₂, which serves via its C₂ fragment as an unprecedented four-electron σ donor.

Full text of DOI:10.1002/zaac.201700204

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201700204 SHORT COMMUNICATION
Zeitschrift für anorganische und allgemeine Chemie
A C2 Fragment as Four-Electron σ Donor
Wolfgang Petz,*[a] Diego M. Andrada,[a] Markus Hermann,[a] Gernot Frenking,*[a,b] and Bernhard Neumüller*[a]
Dedicated to Prof. Dieter Fenske on the Occasion of his 75th Birthday
Abstract. The formation and X-ray structure analysis of the PtIV com-
the orthometalated ligand (PPh3)2CǞCH2, which serves via its C2
plex [(CH2C{PPh2C6H4}2)PtI2] is reported. The molecule possesses fragment as an unprecedented four-electron σ donor.
Introduction
The chemistry of imidazolin-2-ylidenes, which are generally
termed N-heterocyclic carbenes (NHCs), has become an inten-
sively explored area in chemical research[1] since the first iso-
lation as a free species by Arduengo in 1991.[2] Prior to the
synthesis of the free carbene,[3] transition metal (TM) com-
plexes with NHCs as ligands were already known since 1968
when Öfele[4] and Wanzlick[4b] reported the isolation of the first
adducts with the general formula NHCǞ[TMLn].[5] Experi-
mental and theoretical studies suggest that NHC is a strong
two-electron σ donor and generally weak[6] π acceptor, which
is comparable to phosphines PR3.[5,7] But for main group com-
pounds bearing N-heterocyclic carbenes, significant π back-
donation was found theoretically.[8] The methylene adduct 2-
methylene imidazoline A[9] may either be sketched with a
Scheme 1. (a) Schematic representation of the bonding situation in 2-
double bond NHC=CH2 or with a dative bond[10] NHCǞCH2
methylene imidazoline (A) with electron-sharing double bonds and
dative bonds. (b) Schematic representation of the bonding situation in
the hypothetical methylene adduct of a carbone species L2CǞCH2.
and concomitant π backdonation NHCǟCH2, as shown in
Scheme 1a. Experimental studies suggest that A is better
described with the latter form, because it binds as η1
coordinated ligand through its terminal carbon atom
(NHCǞCH2)Ǟ[TMLn] rather than as side-on bonded ole-
fin.[11]
The stability of A can be explained with the π backdonation
of the p(π) lone-pair of the methylene group to the formally
vacant p(π) AO of the carbene NHCǟCH2. The occurrence of
such π backdonation is not possible when the methylene group
is attached to a carbone[12] L2C (Scheme 1b). Carbones are
divalent carbon(0) compounds where two σ donor ligands L
1
are bonded to a carbon atom in the D state, which retains its
four valence electrons as σ and π lone pairs LǞCǟL.[13] It
has experimentally been shown that carbenes CR2, which have
one electron lone pair and carbones CL2, which possess two
lone pairs, exhibit different chemical reactivities in related re-
actions that can be explained with the number of lone-pair
electrons.[14] Also, the NHC cation [NHC–CH3]+ can readily
be deprotonated to NHC=CH2, whereas deprotonation of
[(PPh3)2C–CH3]+ abstracts H+ from a phenyl group.[15]
The carbodiphosphorane[16] C(PPh3)2 (1) may be considered
as parent compound for the class of carbones.[17] The more
recent carbodicarbenes[18] C(NHC)2 are presently a topic of
intensive experimental studies.[19] In accordance with the
bonding situation of carbones, the species (PPh3)2CǞCH2 (B)
is not an energy minimum structure.[20] An equivalent descrip-
tion of B would be an olefin where the π and the π* orbitals
are occupied. We are not aware of a molecule where such
bonding situation has been observed.
* Prof. Dr. W. Petz
E-Mail: petz@staff.uni-marburg.de
* Prof. Dr. G. Frenking
E-Mail: frenking@chemie.uni-marburg.de
* Prof. Dr. B. Neumüller
E-Mail: neumuell@chemie.uni-marburg.de
[a] Fachbereich Chemie
Philipps-Universität Marburg
Hans-Meerwein-Strasse 4
35032 Marburg, Germany
[b] Donostia International Physics Center (DIPC)
P. K. 1072
Herein we report the serendipitous formation of the PtIV
complex [(CH2C{PPh2C6H4}2)PtI2] (2), where the orthomet-
alated species B is found as a four-electron σ donor ligand.[21]
20080 Donostia, Euskadi, Spain
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201700204 or from the au-
thor.
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Journal of Inorganic and General Chemistry
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the molecule, which are in good agreement with experiment.
The complete result of the geometry optimization is given in
the Supporting Information. Figure 2a shows a plot of the La-
placian distribution ٌ2ρ(r) of 2 in the Pt–C1–C11 plane. There
are two large areas of charge concentration [ٌ2ρ(r) Ͻ 0,
dashed red lines] at C1 and C11 pointing toward the platinum
atom along the carbon–metal bond paths. They can be iden-
tified with the donation of the two lone-pair electrons at the
carbon atoms toward platinum. The bonding situation in 2 can
thus be described as shown in Figure 2b. With the donation
of four electrons from the C2 fragment of the orthometalated
fragment B, the metal atom acquires 18 valence electrons. This
makes 2 an 18 electron PtIV complex.
Results and Discussion
When the platinum complex [(cod)PtI2] reacts with 1 in a
1:1 ratio in toluene, a colorless to beige insoluble precipitate is
formed (Scheme 2, reaction 1). After several runs of attempted
recrystallization using various solvents, we finally isolated few
crystals from a CH2Cl2/n-pentane solution, which turned out
to be the Pt complex 2. The majority of the crystals consisted
of the salt [HC(PPh3)2]I (3). The X-ray analysis showed that
2 contains two Pt–C σ bonds originating from orthometalation
of two phenyl groups of 1 (Figure 1). The most surprising fea-
ture of 2 is the occurrence of a methylene group that is bonded
to the C1 carbon atom. We think that the CH2 moiety stems
from the solvent CH2Cl2. The C1–C11 distance of 1.47 Å in 2
is longer than the C–C bond length of the methylene group in
NHCǞCH2 (1.357 Å).[9] The carbon–metal distances Pt–C1
(2.12 Å) and Pt–C11 (2.09 Å) are comparably long. The struc-
tural data of 2 indicates that the C1–C11 moiety of the ortho-
metalated fragment B binds side-on (η2) to the metal.
We calculated the geometry and analyzed the bonding situa-
tion of 2 with quantum chemical methods.[22] Figure 1 gives
also the most important theoretical bond lengths and angles of
Figure 2. Contour line diagram of the Laplacian distribution ٌ2ρ(r)
of 2 in the Pt–C1–C11 plane. Dashed red indicate areas of charge
concentration [ٌ2ρ(r) Ͻ 0], while solid blue lines show areas of charge
depletion [ٌ2ρ(r) Ͼ 0]. The thick solid lines connecting the atomic
nuclei are the bond paths. The green points are the bond critical points,
while the red point shows the ring critical point.
The assignment of the fragment (PPh3)2CǞCH2 (B) as 4
electron σ donor and the bonding situation as sketched in Fig-
ure 2b are supported by the calculated charge distribution of 2.
Table 1 gives the NBO charges of the most important atoms
and relevant Wiberg bond orders. The carbon donor atoms still
carry large negative charges of –0.90 e (C1) and –0.48 e (C11),
because only part of the lone pair electrons is donated to Pt,
which is essentially neutral. The bond orders of the CǞPt
bonds are 0.40 (Pt–C1) and 0.56 (Pt–C11), which are slightly
smaller than for the electron-sharing bonds Pt–C21 and Pt–
C51 (0.59). The bond order P(C1–C11) = 1.15 suggests that
Figure 1. X-ray crystal structure of [(CH2C{PPh2C6H4}2)PtI2] (2). the carbon–carbon bond has a small double bond character,
The ellipsoids are drawn at a 40% probability level. The hydrogen
atoms at the phenyl rings and the solvent molecules are omitted for
which can be explained with the donation from the formal π*
orbital.
clarity. Selected experimental bond lengths are given in Å and angles
We would like to point out that reaction (1) (Scheme 2)
in degree. Calculated values at BP86+D3(BJ)/def2-TZVPP are given
leads to different products if other solvents than in the pre-
sented study are used. We found earlier that the reaction of
[(cod)PtI2] with 1 in THF as solvent gives the PtII complex
[(η3-C8H11)Pt(C6H4PPh2CPPh3)] (4) in high yield under re-
lease of two equivalents of {HC(PPh3)2}I[23] (Scheme 3).
in brackets: C(1)–P(1) 1.80(1) [1.774], C(1)–P(2) 1.796(9) [1.774],
C(1)–C(11) 1.47(2) [1.481], C(1)–Pt(1) 2.12(1) [2.135], C(11)–Pt(1)
2.09(1) [2.073], C(21)–Pt(1) 2.09(1) [2.093], C(51)–Pt(1) 2.09(1)
[2.093], Pt(1)–I(1) 2.706(1) [2.752], Pt(1)–I(2) 2.662(1) [2.692],
C(21)–Pt(1)–C(51) 173.3(4) [174.4], P(1)–C(1)–P(2) 126.2(6) [126.2],
I(1)–Pt(1)–I(2) 96.18(4) [97.5], Pt(1)–C(1)–C(11) 68.8(7) [67.2],
Pt(1)–C(11)–C(1) 70.4(6) [71.6].
Compound
4 is unstable in halogenated hydrocarbons
Scheme 2. The reaction of [(cod)PtI2] with carbodiphosphorane (1) in toluene and subsequent recrystallizations gives protonated carbodiphospho-
rane (3) as major product and compound 2 as minor product.
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Table 1. NBO partial charges q and Wiberg bond orders P of [(CH2C{PPh2C6H4}2)PtI2] (2) at BP86+D3(BJ)/def2-TZVPP. See Figure 1 for the
numbering of the atoms.
q(Pt)
q(C1)
–0.90
q(C11)
–0.48
q(C21)
–0.11
q(C51)
–0.11
P(Pt–C1)
0.40
P(Pt–C11)
0.56
P(Pt–C21)
0.59
P(Pt–C51)
0.59
P(C1–C11)
1.15
–0.01
Scheme 3. The reaction of [(cod)PtI2] with carbodiphosphorane (1) in THF gives the orthometalated complex 4, which reacts with CH2Cl2 and
CHCl3 yielding the bis-orthometalated complex 5 in high yield. The arrows at platinum indicate the origin of the electron pairs.
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[6] There is evidence that NHCs may become sizeable π acceptor
and reactions in these solvents give the bis-orthometalated
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mention that the doubly charged cation [(PPh3)2CǞCH2]2+
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Conclusions
We report the isolation of the PtIV complex
[(CH2C{PPh2C6H4}2)PtI2] (2), which features a ligand with
a unique C2 fragment as four-electron σ donor moiety. The
phosphine and NHC complexes is given in: a) N. Holzmann, A.
Stasch, C. Jones, G. Frenking, Chem. Eur. J. 2013, 19, 6467; b)
G. Frenking, R. Tonner, S. Klein, N. Takagi, T. Shimizu, A.
serendipitous formation via reaction 1 may serve as a guide
for experiments where complexes with four-electron σ donor
ligands of the type L2CǞCH2 can be synthesized in high
yields.
Krapp, K. K. Pandey, P. Parameswaran, Chem. Soc. Rev. 2014,
43, 5106.
[8] a) G. Frison, A. Sevin, J. Phys. Chem. A 1999, 103, 10998; b) G.
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[10] For an enlightening controversy about the pro and contra of the
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Supporting Information (see footnote on the first page of this article):
Details about the experimental and theoretical methods and list of the
cartesian coordinates of the calculated structure.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft.
[11] N. Kuhn, M. Göhner, G. Frenking, Y. Chen, Unusual Structures
and Properties in Organometallic Chemistry (Eds.: M. Gielen, R.
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[12] R. Tonner, G. Frenking, Pure Appl. Chem. 2009, 81, 597.
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Keywords: Bonding analysis; Carbone; Four-electron donor;
Platinum; PtIV complex; Quantum chemical calculations
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Received: June 28, 2017
Published Online: ᭿
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W. Petz,* D. M. Andrada, M. Hermann, G. Frenking*,
B. Neumüller* ................................................................... 1–5
A C2 Fragment as Four-Electron σ Donor
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