Carbene-stabilized diphosphorus

Article abstract of DOI:10.1021/ja807828t

The potassium graphite reduction of L:PCl₃ leads to the formation of carbene-stabilized diphosphorus molecules, L:P-P:L, 1 (L: = :C{N(2,6-Prⁱ₂C₆H₃)CH}₂) and 2 (L: = :C{N(2,4,6-Me₃

Full text of DOI:10.1021/ja807828t

Published on Web 10/21/2008
Carbene-Stabilized Diphosphorus
Yuzhong Wang, Yaoming Xie, Pingrong Wei, R. Bruce King, Henry F. Schaefer, III,
Paul v. R. Schleyer,* and Gregory H. Robinson*
Department of Chemistry and the Center for Computational Chemistry, The UniVersity of Georgia,
Athens, Georgia 30602-2556
Received October 3, 2008; E-mail: robinson@chem.uga.edu
Brand’s quest for the “philosopher’s stone,” by distilling residual
salts from evaporated urine, led instead to phosphorus, the only
element originally derived from the human body.1 The allotropy
of phosphorusswhite, red, and blacksis well documented. Py-
rolysis of white phosphorus, P4, yields the high temperature
diphosphorus allotrope, gaseous P2.2 In contrast to the legendary
inert nature of its ubiquitous lighter congener N2, P2 is highly
reactive and association-prone. Is it possible to stabilize the P2
molecule to explore its expected versatility? Indeed, diphosphorus
can function as four-, six-, and eight-electron donor ligands2 in
transition metal carbonyl complexes.3 Remarkably, Cummins and
co-workers reported the “mild thermal extrusion” of P2 from
niobium diphosphaazide complexes4 and that the Pt(0) species,
(C2H4)Pt(PPh3)2, may serve as a trap for W(CO)5-complexed P2
molecules.5 In all these examples, P2 behaves as a typical Lewis
base (i.e., an electron pair donor). We now demonstrate the
stabilization of P2 by its serving as an electron pair acceptor and,
thus, mimicking the behavior of a Lewis acid. Herein, we report
the syntheses,6 structures,6 and computations7 of N-heterocyclic
carbene (NHC, L:) stabilized P2 molecules, L:P-P:L, 1 (L′: ) :C-
{N(2,6-Pri2C6H3)CH}2) and 2 (L′′: ) :C{N(2,4,6-Me3C6H2)-
CH}2). While the free P2 molecule, :PtP:, possesses a phosphorus-
phosphorus triple bond, compounds 1 and 2 exhibit a unique bis-
phosphinidene structure: two phosphinidene units, each with two
lone pairs of electrons, bridged by a phosphorus-phosphorus single
bond.
Figure 1. Molecular structures of 1 and 2 (thermal ellipsoids represent
30% probability; hydrogen atoms omitted for clarity). Selected bond
distances (Å) and angles (deg): For 1, P(1)-P(1A) 2.2052(10), P(1)-C(1)
1.7504(17); C(1)-P(1)-P(1A) 103.19(6). For 2, P(1)-P(2) 2.1897(11),
P(1)-C(1) 1.754(3), P(2)-C(22) 1.754(3); C(1)-P(1)-P(2) 102.57(10),
C(22)-P(2)-P(1) 103.01(10).
Figure 2. Localized molecular orbitals (LMOs) of 1-H with C2h symmetry.
(a) P-P σ-bonding orbital; (b) P-C σ-bonding orbital; (c) lone pair orbital
(mainly p-character) with pπ back-donation to the empty p orbital of CNHC
;
(d) lone pair orbital (mainly s-character).
L:P(Ph), -53.5 ppm (L: ) tetramethylimidazol-2-ylidene) (3a) and
-23.0 ppm (L: ) :C{N(2,4,6-Me3C6H2)CH}2) (3b), but are quite
different from those (34 to 54 ppm) of diphosphabutadienes,
(R2N)2CdPsPdC(NR2)2 (R ) Me, Et).15 The lack of detectable
31P-1H coupling in 1 and 2 also supports the assigned structures.
Compound 1 has Ci symmetry and a trans-bent geometry with a
180.0° C(1)sP(1)sP(1A)sC(1A) torsion angle about the central
PsP bond. The PsP bond distance, 2.205 Å, compares well to
the 2.21 Å PsP single bond distance in Td P4.2 The C(1)sP(1)s
P(1A) bond angle (103.2°) corresponds to the 110° CsPsP angle
(av) in the L:[PsPdPsP]:L carbene adduct, 4,8 and the computed
CsPsP angles in both H2CdPsPdCH2 (5, C2h) (101.2°) and
H3CPdPCH3 (6, C2h) (99.9°).7 Hence, the CsPsP angle does not
help delineate the nature of the PsCNHC bonding. Notably, the two
imidazole rings and the PsP bond of 1 are almost coplanar (the
N(2)sC(1)sP(1)sP(1A) torsion angle is 2.3°; this value is 8.2°
(av) in 2). The PsC bond length (1.750 Å) in 1, similar to those
(1.75-1.79 Å) in 2-4, is between the 1.65-1.67 Å PdC double
bond lengths of the nonconjugated phosphaalkenes16 and the normal
Recently, Bertrand and co-workers reported P4- and P12-based
carbene complexes.8-10 Our laboratory recently reported carbene-
stabilized diborenes, L:(H)BdB(H):L,11,12 and diatomic silicon,
L:SidSi:L.13 Extending this strategy, we prepared L:PCl3 by
combining L: ligands with PCl3. The potassium graphite reduction
of L:PCl3 (L:PCl3/KC8 ratio of 1:3.1) in THF affords the carbene-
stabilized diphosphorus L:PsP:L compounds, 1 (L: ) L′:) and 2
(L: ) L′′:), respectively (Figure 1). Both 1 and 2 were isolated as
moisture- and air-sensitive red crystals in moderate yields (1, 56.6%;
1
2, 20.7%). The H NMR imidazole resonances (CsH) of 1 and 2
are 5.98 and 5.71 ppm, respectively. The 1H-coupled 31P NMR
singlet resonances, -52.4 and -73.6 ppm for 1 and 2, respectively,
are comparable to those of other carbene-phosphinidene adducts,14
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10.1021/ja807828t CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
PsC single bond distance (i.e., the 1.839 Å PsC lengths in 3b
and the computed 1.87 Å in 6). Two interpretations of the bonding
in 1, 1A (bis-phosphinidene), and 1B (bis-phosphaalkene) (eq 1)
are akin to two resonance forms of carbene-phosphinidene
adducts.17 Donation of the two carbene electron lone pairs to P
decreases the phosphorus-phosphorus bond order from three in
:PtP: to one in 1A or 1B.
The PdC double bond character implied by 1B inhibits, however,
the imidazole π-delocalization and should be consistent with the
31P NMR chemical shifts (34 to 54 ppm) of the diphosphabuta-
dienes.15 Instead, the high-field 31P NMR chemical shift (-52.4
ppm) of 1 favors 1A as the predominate formulation.14,17,18
σ bond polarization is 64.8% toward carbon and 35.2% toward
phosphorus that has 20.7% s-, 78.6% p-, and 0.7% d-character.
The PsP bond is single (WBI ) 1.004) with 11.5% s-, 87.9% p-,
and 0.6% d-character. Thus, like the silicon atoms in L:SidSi:L13
and third-period elements generally, the phosphorus atoms in 1 and
2 do not hybridize extensively. Notably, the P2 unit in the L:PsP:L
molecules is demonstrated to serve as electron pair acceptors,
thereby mimicking the behavior of a Lewis acid.
Acknowledgment. We are grateful to the National Science
Foundation for support of this work: CHE-0608142 (G.H.R.), CHE-
0716718 (P.v.R.S. and R.B.K.), and CHE-0749868 (H.F.S.).
Supporting Information Available: Complete ref 7, full details of
the syntheses, computations, and X-ray crystal determination, including
cif files. This material is available free of charge via the Internet at
http://pubs.acs.org.
References
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Our DFT computations on the simplified L:P-P:L model, 1-H,
support this interpretation.7 Optimization of 1-H (C2h symmetry)
affords the same trans-bent conformation as that for 1, but with
one imaginary frequency corresponding to a rotational transition
state. Notably, the ca. 7 kcal/mol more stable gauche minimum of
1-H (C2 symmetry) (C-P-P-C torsion angle ) 98.6°) resembles
that of the isolobal H2S2 (H-S-S-H torsion angle ) 90.6°).19
The sensitivity of the conformation about the P-P bond to the steric
effects of the nearby carbene ligands (shown clearly by inspecting
space-filling models) was confirmed by preparing 2, which has a
smaller carbene ligand than 1. Although having bond distances
similar to those for 1, compound 2 adopts a gauche conformation.
The C(1)-P(1)-P(2)-C(22) torsion angle (134.1°) of 2 lies
between the 180° of 1 and the 98.6° of 1-H (C2 minimum).
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(with L: ) :C(NHCH)2) 1-H (optimized in both C2h (Figure 2) and
C2 symmetries) and 2-H (employing the X-ray coordinates of 2)
are quite similar.7 All LMOs have one PsP σ-bond (a), one PsC
σ-bond (b), and two lone-pair orbitals on each P atom (c and d).
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implied by 1B. This pπ back-donation, involving 64.8% P and
35.2% C components, results in modest PdC double bond character
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distance, and the 1.397 PsC Wiberg bond index (WBI)).7 The PsC
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HPdPH (C2h
) 2.038 (2.042 Å); H2PsPH2 (C2) 1.037 (2.258 Å);
H2CdPsPdCH2 (5, C2h) CsP 1.887 (1.673 Å), PsP 1.027 (2.235 Å);
CH3PsPCH3 (6, C2h) CsP 0.973 (1.871 Å), PsP 1.933 (2.039 Å).
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