Carbene-stabilized parent phosphinidene

Article abstract of DOI:10.1021/om100335j

The lithiated N-heterocyclic carbene-phosphinidene adduct L′:P-H (3; L′: =:C{[N(2,6-Prⁱ₂C₆H₃)] ₂CHCLi(THF)₃}) unexpectedly resulted from the reaction of lithium metal with the carbene-stabi

Full text of DOI:10.1021/om100335j

4778 Organometallics 2010, 29, 4778–4780
DOI: 10.1021/om100335j
Carbene-Stabilized Parent Phosphinidene†
Yuzhong Wang, Yaoming Xie, Mariham Y. Abraham, Robert J. Gilliard, Jr.,
Pingrong Wei, 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 April 22, 2010
16
˚
Summary: The lithiated N-heterocyclic carbene-phosphini-
dene adduct L0:P-H (3; L0:=:C{[N(2,6-Pri2C6H3)]2CHCLi-
(THF)3}) unexpectedly resulted from the reaction of lithium
metal with the carbene-stabilized diphosphorus species L:P-P:L
(2; L:=:C{N(2,6-Pri2C6H3)CH}2). Compound 2 was previously
prepared by the potassium graphite reduction of L:PCl3 (1).
elongated carbon-phosphorus double bond (1.740(1) A),
[(CH3)2N]2CdP-H may be regarded formally as an acyclic
diaminocarbene (I)-parent phosphinidene adduct.17 While
such N-heterocyclic carbene (II)-PH complexes have been
investigated theoretically,11,12 experimental data have yet to be
reported. We recently utilized N-heterocyclic carbenes18-21 to
stabilize a series of highly reactive low-oxidation-state main-
group molecules, including the parent diborene(2) (H-Bd
B-H),22,23 disilicon (Si2),24 diphosphorus (P2),25 diarsenic
(As2),26 and a neutral Ga6 octahedron.27 We now report
the syntheses,28 structures,28 and computations29 of carbene-
stabilized phosphorus trichloride, L:PCl3 (1; L: = :C{N(2,6-
Pri2C6H3)CH}2) and the first lithiated NHC (III) parent phos-
phinidene adduct, L0:P-H (3; L0:=:C{[N(2,6-Pri2C6H3)]2CH-
CLi(THF)3}). While both I and II are well-known and exten-
sively investigated carbene ligands, III, in contrast, may be
regarded as a new anionic N-heterocyclic dicarbene, an anionic
C3N2 ring with two carbene centers: one is an anionic version of
an “abnormal” carbene (aNHC) center,30 while the other is a
neutral carbene center.
Free phosphinidenes (R-P), highly reactive group 15 ana-
logues of carbenes,1-5 typically have triplet ground states and
are studied at low temperature.6 Transition-metal complexation
is an effective means to stabilize these short-lived species. The
transition-metal complexes of phosphinidenes prefer singlet
ground states,7 where phosphinidenes may act as two- or four-
electron donors.1 However, the phosphorus valence shell in free
phosphinidenes is unsaturated. This enables adduct formation
with electron pair donors, in particular Lewis base ligands2 such
as phosphines8 and N-heterocyclic carbenes (NHCs).9,10
In contrast to the diverse chemistry of substituted phosphini-
denes,2,3,5 studies of the parent H-P molecule have largely
been computational.7,11,12 With a triplet ground state and a 22
kcal/mol triplet-singlet energy gap, diatomic H-P is a highly
reactive molecule.13 The synthesis and characterization of
its carbene complex [(CH3)2N]2CdP-H (I; Figure 1) were
achieved more than two decades ago.14-16 Consistent with an
The reaction of the carbene ligand (L:) with PCl3 in hexane
affords the hypervalent L:PCl3 complex 1 in almost quanti-
tative yield. Potassium graphite reduction of 1 gave the
diphosphorus carbene complex L:P-P:L (2).25 Phosphini-
dene 3 was isolated as yellow crystals from the reaction of 2
with lithium metal (Scheme 1). Although the mechanism is
† Part of the Dietmar Seyferth Festschrift. This paper is dedicated to
Professor Dietmar Seyferth for his dedicated service to Organometallics
and to the field of organometallic chemistry.
*To whom correspondence should be addressed. Tel.: 706-542-1853.
Fax: (þ1) 706-542-9454. E-mail: robinson@chem.uga.edu.
(1) Mathey, F. Angew. Chem., Int. Ed. 1987, 26, 275–286.
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Smirnov, A. I. J. Am. Chem. Soc. 1994, 116, 7899–7900.
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(20) Scheschkewitz, D. Angew. Chem., Int. Ed. 2008, 47, 1995–1997.
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R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am.
Chem. Soc. 2007, 129, 12412–12413.
(23) Wang, Y.; Quillian, B.; Wei, P.; Xie, Y.; Wannere, C. S.; King,
R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am.
Chem. Soc. 2008, 130, 3298–3299.
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H. V. R.; Goerlich, J. R.; Marshall, W. J.; Riegel, B. Inorg. Chem.
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Schleyer, P. v. R.; Robinson, G. H. Science 2008, 321, 1069–1071.
(25) Wang, Y.; Xie, Y.; Wei, P.; King, R. B.; Schaefer, H. F., III;
Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2008, 130, 14970–
14971.
(10) Arduengo, A. J., III; Carmalt, C. J.; Clyburne, J. A. C.; Cowley,
A. H.; Pyati, R. Chem. Commun. 1997, 981–982.
(26) Abraham, M. Y.; Wang, Y.; Xie, Y.; Wei, P.; Schaefer, H. F., III;
Schleyer, P. v. R.; Robinson, G. H. Chem. Eur. J. 2010, 16, 432–435.
(27) Quillian, B.; Wei, P.; Wannere, C. S.; Schleyer, P. v. R.; Robinson,
G. H. J. Am. Chem. Soc. 2009, 131, 3168–3169.
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(28) See the Supporting Information for synthetic and crystallo-
graphic details.
(29) In DFT computations the 3-H model was optimized at the
B3LYP/DZP level with the Gaussian 94 and Gaussian 03 programs:
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PA, 1995; Gaussian 03, revision C.02; Gaussian, Inc., Wallingford, CT, 2004
(see the Supporting Information for details).
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r
pubs.acs.org/Organometallics
Published on Web 05/28/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 21, 2010 4779
Figure 1. Examples of diaminocarbenes stabilizing the H-Pmole-
cule: (I) acyclic diaminocarbene; (II) N-heterocyclic carbene
(NHC); (III) lithiated N-heterocyclic carbene (lithium omitted
for clarity).
Scheme 1. Syntheses of Compounds 1, 2, and 3
Figure 2. Molecular structure of 1(thermal ellipsoids represent 30%
probability; hydrogen atoms omitted for clarity). Selected bond dis-
˚
tances (A) and angles (deg): P(1)-C(1)=1.871(11), P(1)-Cl(1)=
2.018(8), P(1)-Cl(2) = 2.492(10), P(1)-Cl(3) = 2.235(10); C(1)-
P(1)-Cl(1) = 102.5(4), C(1)-P(1)-Cl(2) = 86.5(4), C(1)-P(1)-
Cl(3)=91.6(5), Cl(1)-P(1)-Cl(2) =90.0(5), Cl(1)-P(1)-Cl(3) =
76.9(4), Cl(2)-P(1)-Cl(3)=166.1(5).
δ 34.3 ppm, 1J(PH)=174 Hz) of P-hydrogeno-C-phosphino-
phosphaalkenes.33
The tetrahedrally solvated Liþ (THF)3 cation completes the
3
trigonal-planar geometry around atom C(3) in 3 (Figure 3). The
natural charge distribution at Liþ (þ0.84) and P-H (-0.21)
supports the in-plane anionic character of the NHC fragment in
3,29 but this does not appear to significantly influence the nature
of the P-C bond.
While being shorter than the P-C single bond distances (1.879
unclear, the formation of 3 involves both the cleavage of the
central P-P bond of 2 and the lithium-mediated C-H
activation of the imidazole ring. While the selective activa-
tion of aromatic C-H bonds by lithium has been achieved,31
the lithiation of NHC ligands has not been reported.
00
˚
˚
A (average) for 1; 1.828(2) and 1.856(2) A for L P(BH3)2Ph,
L00=:C{N(2,4,6-Me3C6H2)CH}2),10 the P-Cbondin3(1.763(2)
˚
A) compares well to that computed for [CH(CH3)N]2CP-H
(1.770 A)12 and is slightly longer than the experimental value for
˚
15
˚
The imidazole 1H NMR resonance of 1 resides at 6.51 ppm.
The 31P NMR resonance shifts from 219 ppm for PCl332 to 16.9
ppm for 1 due to the electron donation from the NHC ligand
(L:) to the phosphorus atom. In a manner similar to that for the
arsenic atom in L:AsCl3,26 the four-coordinate phosphorus
atom in 1 adopts a trigonal-bipyramidal geometry (Figure 2;
only one set of structural data of the disordered PCl3 core is
shown28). While two Cl atoms occupy the axial positions, the
ipso-C of the lithiated NHC ligand, one Cl atom, and one lone
[(CH3)2N]2CdP-H (1.740(1) A). Perhaps expectedly, the P-C
bond distance in 3 is much longer than the PdC double bonds in
P-hydrogeno-C-phosphinophosphaalkenes (1.713(2) A)33 and in
˚
34
˚
H2CdP-H (1.675 A, computed).
Much like the case for N-heterocyclic carbene stabilized phos-
phinidenes9,10 and bisphosphinidenes,25 two extreme P-C bond-
ing modes of lithiated 3 may be considered: a carbene-phos-
phinidene adduct (3A) and a phosphaalkene (3B). The pro-
nounced high-field 31P chemical shift of 3(-143.0 ppm), coupled
˚
with the elongated P-C bond (1.763(2) A; see above) also favor
˚
pair share the three equatorial sites. The 1.879 A (average) P-C
3A as the predominant formulation. The 102° (average) C-P-
H bond angle in 3 is comparable to the 103(1)° in [(CH3)2N]2Cd
P-H15 and to the computed 94.0° in [CH(CH3)N]2CP-H.12
bond length is comparable to other phosphorus-carbon single
bonds.25 The P-Cl(2) bond distance (2.471 A, average) is
˚
˚
significantly longer than that of P-Cl(3) (2.238 A, average)
˚
and of P-Cl(1) (2.032 A, average).
The 1HNMRof3shows the imidazole resonance at 6.24 ppm;
the P-H doublet (δ1.86 ppm, 1J(PH)=167 Hz) is shifted upfield
compared to that reported for [(CH3)2N]2CdP-H (δ 3.10 ppm,
1J(PH)=159 Hz).14 This is probably because the net electron-
donating ability of the ligand III in 3 is stronger than that of I in
[(CH3)2N]2CdP-H. The presence of the P-H fragment in 3
also is confirmed unambiguously by the 1H-coupled-31P NMR
spectrum. The 31P doublet at -143.0 ppm (1J=171 Hz) is upfield
Density functional theory (DFT) computations at the
B3LYP/DZP level on the simplified L00:P-H model (3-H;
1
when compared to that (δ -62.6 ppm, J(PH) = 159 Hz) of
[(CH3)2N]2CdP-H14 and those (δ 23.8 ppm, 1J(PH)=138 Hz;
(33) Bourissou, D.; Canac, Y.; Gornitzka, H.; Marsden, C. J.;
Baceiredo, A.; Bertrand, G. Eur. J. Inorg. Chem. 1999, 1479–1488.
(34) Weber, L. Eur. J. Inorg. Chem. 2000, 2425–2441.
(31) Tacke, M. Chem. Ber. 1995, 128, 91–92.
(32) Morrow, B. A.; Lang, S. J.; Gay, I. D. Langmuir 1994, 10, 756–760.

4780 Organometallics, Vol. 29, No. 21, 2010
Wang et al.
Figure 3. Molecular structure of 3(thermal ellipsoids represent 30%
probability; hydrogen atoms on carbons omitted for clarity). Selected
bond distances (A) and angles (deg): P(1)-C(1)=1.763(2), P(1)-
H(1)=1.232(19), Li(1)-C(3)=2.116(5); C(1)-P(1)-H(1)=102(2),
N(1)-C(1)-P(1) = 127.19(16), N(2)-C(1)-P(1) = 129.14(17),
C(2)-C(3)-N(2) = 101.27(18), C(2)-C(3)-Li(1) = 120.7(2),
N(2)-C(3)-Li(1) = 135.68(19).
Figure 4. Localized molecular orbitals (LMOs) of 3-H: (a)
C-Li σ-bonding orbital; (b) P-C σ-bonding orbital; (c)
lone pair orbital (mainly p character) with some pπ back-
donation to the Cipso p orbital of the lithiated NHC ligand;
(d) P-H σ-bonding orbital; (e) lone pair orbital (mainly P s
character).
˚
L00:=:C[(NH)2CHCLi(THF)3]) support the bonding analysis
similar to that in 2 (64.8% C and 35.2% P). Notably, the
Wiberg bond index (WBI) of the P-C bond of 3-H (1.332) is
even less than that of 2 (1.397). These computational results
reveal that the pπ-bonding interaction between the ipso-
carbon of the lithiated NHC ligand and phosphorus is not
developed effectively, further supporting 3A as the predomi-
nant depiction of 3.
of 3.29 The computed bond distances (P-C=1.775 A, C-Li=
˚
˚
2.095 A) and 180° H-P-C-N torsion angle match the experi-
˚
˚
mental values of 3 (P-C=1.763(2) A, C-Li=2.116(5) A) and
180° H(1)-P(1)-C(1)-N(2) torsion angle. The steric repulsion
between one of the bulky 2,6-diisopropylphenyl groups in 3 and
the P-H hydrogen atom is responsible for the C-P-H bond
angle in 3 (102°, average) being larger than that computed for
3-H (93.2°).
Acknowledgment. We are grateful to the National
Science Foundation for support of this work: Grant
Nos. CHE-0608142 (G.H.R.), CHE-0953484 (G.H.R.,
Y.W.), CHE-0749868 (H.F.S.), and CHE-0716718 (P.v.
R.S.). Either as a Harvard graduate student contempor-
ary (P.v.R.S.), freshman chemistry professor at MIT (H.
F.S.), or valued mentor (G.H.R.), Professor Seyferth has
touched us all in various capacities. We, much like the
entire organometallics community, will forever remain
indebted to our dear friend Dietmar.
Localized molecular orbitals (LMOs) (Figure 4) and natural
bond orbital (NBO) analysis support the bonding description of
3A. The C-Li σ bond is strongly polarized to carbon, which has
40.5% s and 59.5% p character (LMO a). The localized phos-
phorus MOs include a P-C σ bond (LMO b), a P-H σ bond
(LMO d), and two lone pairs (LMOs c and e). LMO e has
mainly s character (68.1% s, 31.9% p), whereas LMO c is essen-
tially pure p character (0.0% s, 99.8% p, 0.2% d). Notably, the
latter involves modest pπ back-donation to the p orbital of ipso-
C of the lithiated NHC ligand (71.2% P and 28.8% C compo-
nents). This back-donation interaction is weaker than that
reported for 2 (64.8% P and 35.2% C components).25
In addition, the P-C σ-bond polarization of 65.7% to-
ward carbon and 34.3% toward phosphorus in 3-H is very
Supporting Information Available: Text, tables, and CIF files
giving full details of the syntheses, computations, and X-ray
crystal determinations. This material is available free of charge
via the Internet at http://pubs.acs.org.