Terminal alkylphosphanylidene organotantalum(V) complexes

Article abstract of DOI:10.1002/ejic.201300500

The first terminal organometallic alkylphosphanylidenetantalum(V) complexes [Cp*Ta{1,2-(NSiMe₃)₂C₆H ₄}(PR)] were obtained with cyclohexyl (2) and isopropyl groups (3) at phosphorus, whereas adamantyl and tert-butyl substituents resulted in the formation of the paramagnetic tantalum(IV) complex [Cp*Ta{1,2-(NSiMe ₃)₂C₆H₄}Cl] (4). DFT studies showed that the terminal cyclohexyl and isopropyl phosphanylidene complexes are stable towards dimerization and dissociation. The first terminal organometallic alkylphosphanylidenetantalum(V) complexes are obtained with cyclohexyl and isopropyl groups at phosphorus, whereas adamantyl and tert-butyl substituents result in the formation of a paramagnetic tantalum(IV) complex. DFT studies show that the terminal cyclohexyl and isopropyl phosphanylidene complexes are stable towards dimerization and dissociation. Copyright

Full text of DOI:10.1002/ejic.201300500

SHORT COMMUNICATION
DOI:10.1002/ejic.201300500
Terminal Alkylphosphanylidene Organotantalum(V)
Complexes
Anne Grundmann,[a] Menyhárt B. Sárosi,[a,b] Peter Lönnecke,[a][‡]
René Frank,[a][‡] and Evamarie Hey-Hawkins*[a]
Keywords: Tantalum / P ligands / Substituent effects
The first terminal organometallic alkylphosphanylidenetan-
talum(V) complexes [Cp*Ta{1,2-(NSiMe3)2C6H4}(PR)] were
obtained with cyclohexyl (2) and isopropyl groups (3) at
phosphorus, whereas adamantyl and tert-butyl substituents
resulted in the formation of the paramagnetic tantalum(IV)
complex [Cp*Ta{1,2-(NSiMe3)2C6H4}Cl] (4). DFT studies
showed that the terminal cyclohexyl and isopropyl phos-
phanylidene complexes are stable towards dimerization and
dissociation.
=
Ph, cyclohexyl (Cy), tBu][3e] and [Ta(PPh){OSi-
Introduction
(tBu)3}3],[6a] were reported in 1993 and 1994, respectively.
Subsequent reactions include phospha-Wittig reactions[3e]
and insertion of acetone.[6c]
Terminal transition-metal phosphanylidene complexes
[LnM=PR][1] are phosphorus analogues of the well-known
carbene complexes, which have many applications in me-
tathesis reactions and natural product synthesis.[2] As is the
case for carbene complexes, phosphanylidene complexes
can be regarded as electrophilic[1e,3a–3c] or nucleophil-
ic,[1e,3d–3f] depending on the reactivity of the phosphorus
atom.[3g] Reactions with alkynes, ketones, and protic rea-
gents[1e,3a–3f] have been reported. Remarkably, phospha-
alkenes can be obtained in a phospha-Wittig reaction of
nucleophilic phosphanylidene complexes.[3e,3f]
Since the first report of a stable organomolybdenum
complex with a terminal phosphanylidene ligand, namely,
[Cp2Mo(PMes)] (Cp = cyclopentadienyl, Mes = 2,4,6-
Me3C6H2),[4] organometallic terminal phosphanylidene
complexes have been reported for most d block elements.[5]
Organotantalum complexes with bridging phosphanylidene
ligands[6b,6d,6e] and one phosphanylphosphanylidene com-
plex[6f] are known, but there is only one report on a ter-
minal organometallic tantalum arylphosphanylidene com-
plex, in which the reactive terminal metal–phosphorus
bond is stabilized by sterically demanding ligands at tanta-
lum and phosphorus.[6c] The first terminal phosphanylidene
complexes of tantalum, [Ta(PR){N(CH2CH2NSiMe3)3}] [R
Results and Discussion
We herein report the synthesis of the first organometallic,
nucleophilic, terminal alkylphosphanylidenetantalum(V)
complexes. Upon treatment of [Cp*TaCl4][7] (Cp* = C5Me5)
with two equivalents of LiPHR[8] in toluene (to maintain a
low concentration of LiPHR in solution) at low tempera-
ture (to reduce side reactions), the deep-red phosphanylid-
ene-bridged tantalum(IV) complexes trans-[{Cp*TaCl(μ-
PR)}2] (R = Cy, tBu, Ph, Mes) were obtained in high
yields.[6d,6e] Reduction could be avoided by employing the
bisamido complex [Cp*Ta{1,2-(NSiMe3)2C6H4}Cl2][9] (1)
as the starting material. Thus, complexes [Cp*Ta{1,2-
(NSiMe3)2C6H4}(PR)] [R = Cy (2), iPr (3)] are readily avail-
able from 1 and two equivalents of lithium phosphanide
(LiPHCy, LiPHiPr)[8a] in toluene at –80 °C (Scheme 1).
[a] Faculty of Chemistry and Mineralogy, Institute of Inorganic
Chemistry, Universität Leipzig,
Johannisallee 29, 04103 Leipzig, Germany
Fax: +49-341-973-9319
Scheme 1. Synthesis of terminal alkylphosphanylidenetantalum(V)
complexes 2 and 3.
E-mail: hey@uni-leipzig.de
Homepage: http://www.uni-leipzig.de/chemie/hh
[b] Faculty of Chemistry and Chemical Engineering, Babes¸-Bolyai
University,
The 31P NMR spectra of compounds 2 and 3 exhibit a
broad singlet at δ = 443.0 and 454.7 ppm, respectively,
which is indicative of a bent MPR group, as was observed
in [Ta(PPh){OSi(tBu)3}3][6a] (δ = 334.6 ppm). Complexes
M. Kogalniceanu 1, 400084 Cluj-Napoca, Romania
˘
[‡] These authors performed the X-ray structure determinations.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300500.
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SHORT COMMUNICATION
with linear MPR groups are observed at higher field, for
Furthermore, the tantalum-bisamido group forms a five-
example, [Ta(PR){N(CH2CH2NSiMe3)3}][3e] [209.8 (R = membered ring with an envelope conformation (Figures 1
Cy) and 227.3 ppm (R = tBu)]. A low-field shift of the ipso- and 2) and fold angles TaN2/N2C2 of 137.0(1) (for 2) and
carbon atoms [13C NMR: C4, 46.0 ppm (for 2); C13, 135.6(2)° (for 3). The fold angle in the starting material 1
35.3 ppm (for 3); Figures 1 and 2] and their protons [1H [129.6(1)°, see Supporting Information] is about 7° smaller
NMR: 4.67 ppm (for 2); 4.78 ppm (for 3)] is caused by the than in 2 and 3 but 8° larger than that in the bis(triiso-
electron deficiency of these organometallic complexes.
propylsilyl) derivative [Cp*Ta{1,2-(NSiiPr3)2C6H4}Cl2]
(121.3°).[12] This difference is caused by the lower steric de-
mand of the phosphanylidene unit and the higher steric de-
mand of the isopropyl substituents. The nitrogen atoms are
in a trigonal-planar environment [sum of bond angles:
358.0(5)° (for 2), 359.5(3)° (for 3)]. The N–C bond lengths
of 141.7(2) pm (for 2) and 141.2(4) pm (for 3) are in the
range of single bonds; the corresponding bond lengths of
diimine complexes are about 128 pm.[13]
Density functional theory (DFT) calculations have
shown that only the highest occupied molecular orbitals
(HOMOs) of 2 and 3 are accessible, and their lowest unoc-
cupied molecular orbitals (LUMOs) are shielded by the
bulky substituents on the nitrogen atom (see the Supporting
Information for details). Thus, dimerization of these com-
plexes involving transfer of electron density from the
HOMO of one monomer to the LUMO of another cannot
occur. Furthermore, homolytic cleavage of the Ta–P double
bond with formation of a tantalum(III) species and cyclic
oligophosphanes can also be excluded (Supporting Infor-
mation). However, owing to the availability of the HOMOs,
2 and 3 should readily react with suitable small molecules.
Attempts to extend the synthetic approach shown in
Scheme 1 to the corresponding adamantyl and tert-butyl
phosphanylidene derivatives resulted in a redox reaction
and the formation of the corresponding tantalum(IV) com-
plex [Cp*Ta{1,2-(NSiMe3)2C6H4}Cl] (4, Scheme 2). The
EPR spectrum of 4 shows an eight-line pattern (g = 1.794,
A = 44 mT) caused by the spin I = 7/2 of 181Ta (N =
99.98%).[14] The formation of oligophosphanes, such as cy-
clotetraphosphanes and cyclotriphosphanes, as oxidation
products could be verified by 31P NMR spectroscopy. The
calculated thermodynamic properties alone do not explain
why reactions of 1 with Ad- and tBu-substituted lithium
phosphanides give tantalum(IV) species 4, whereas reac-
tions of 1 with the Cy- and iPr-substituted lithium phos-
phanides result in formation of 18 valence-electron terminal
phosphanylidene complexes 2 and 3 (PR as 4e– donor, bis-
amido ligand as 8e– donor; see the Supporting Information).
Figure 1. ORTEP diagram of 2 (ellipsoids are drawn at the 50%
probability level, hydrogen atoms are omitted for clarity). Selected
bond lengths [pm] and angles [°]: Ta–P 227.20(8), Ta–N1 202.9(2),
P–C4 186.6(3), N1–C14 141.7(2), Ta–P–C4 136.46(9), P–Ta–N1
105.87(4), N1–Ta–N1Ј 85.26(6).
Figure 2. ORTEP diagram of 3 (ellipsoids are drawn at the 50%
probability level, hydrogen atoms are omitted for clarity). Selected
bond lengths [pm] and angles [°]: Ta–P 226.5(1), Ta–N1 201.9(2),
P–C13 186.7(5), N1–C7 141.2(4), Ta–P–C13 138.2(2), P–Ta–N1
104.12(8), N1–Ta–N1Ј 85.5(1).
X-ray structure analysis of 2 and 3[10] confirmed the bent
geometry of the MPR unit [Ta–P–C4/C13 136.46(9)/
138.2(2)°] and short Ta–P bonds [Ta–P 227.20(8) pm in 2,
226.5(1) pm in 3]. The dependence of the Ta–P bond length
on the substituents and geometrical arrangement at phos-
phorus is also demonstrated in the complexes
[Ta(PCy){N(CH2CH2NSiMe3)3}][3e] [214.5(7) pm, TaPC is
linear] and [Ta(PPh){OSi(tBu)3}3][6a] [231.7(4) pm, Ta–P–C
Scheme 2. Redox reaction with formation of [Cp*Ta{1,2-(NSiMe3)2-
C6H4}Cl] (4).
Compound 4 crystallized from n-pentane and was struc-
110.2(4)°, bent TaPC]. These Ta–P bonds are shorter than turally characterized.[10] The molecule has a piano-stool
the sum of the covalent radii of Ta and P (277 pm).[11]
geometry with a four-coordinate tantalum atom (Figure 3).
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SHORT COMMUNICATION
1
46.0 [d, JCP = 39 Hz, CH, P-C(4)], 115.3 [s, C5(CH3)5], 123.0 (s,
m-CH), 124.5 (s, o-CH), 133.6 ppm (s, N–C). 31P NMR (160 MHz,
C6D6): δ = 443.0 ppm (s). ESI-MS (n-pentane/CH3CN, 1:2): m/z =
681.27 [M + H]+.
The five-membered TaN2C2 ring has only a slightly folded
envelope conformation [fold angle TaN2/N2C2 160.7(1)°
compared to 129.6(1)° in 1, see the Supporting Infor-
mation], as removal of one chloro ligand in the coordina-
tion sphere of tantalum leads to less steric interaction and
therefore to greater planarity of the chelate ring.
3: At –80 °C, nBuLi (1.57 m in n-hexane, 7 mL, 11 mmol) was
added dropwise to a solution of iPrPH2 (0.85 g, 11 mmol) in n-
pentane (20 mL). The reaction mixture was stirred and slowly
warmed to room temperature over 3 h and then filtered. A suspen-
sion of the slightly green precipitate in toluene (20 mL) was added
dropwise to a solution of 1 (2.20 g, 3 mmol) in toluene (30 mL) at
–80 °C. The reaction mixture was stirred for 15 h at room tempera-
ture. The volatile material was removed, and the remaining solid
was extracted with n-pentane. Red crystals of 3 were obtained at
1
7 °C, yield 0.81 g (36%). H NMR (400 MHz, C6D6): δ = 0.48 [s,
18 H, Si(CH3)3], 1.34 [m, 6 H, CH(CH3)2], 1.75 [s, 15 H,
C5(CH3)5], 4.78 [m, 1 H, CH(CH3)2], 7.05 (m, 2 H, o-CH),
7.21 ppm (m, 2 H, m-CH). 13C{1H} APT NMR (100 MHz, C6D6):
δ = 3.5 [s, Si(CH3)3], 10.7 [s, C5(CH3)5], 26.0 [s, CH(CH3)2], 35.3
1
[d, JCP = 41 Hz, PCH(CH3)2], 115.4 [s, C5(CH3)5], 123.0 (s, m-
CH), 124.5 (s, o-CH), 133.4 ppm (s, N–C). 31P NMR (160 MHz,
C6D6): δ = 454.7 ppm (s). ESI-MS (n-pentane): m/z = 641.2 [M +
H]+.
4: R = tBu: nBuLi (2.22 m in n-hexane, 8 mL, 19 mmol) was added
dropwise to a solution of tBuPH2 (1.69 g, 19 mmol) in n-hexane
(20 mL). The reaction mixture was stirred for 20 min and then fil-
tered. A suspension of the white precipitate in toluene (20 mL) was
added dropwise to a solution of 2 (4.73 g, 7 mmol) in toluene
(30 mL) at –80 °C. After stirring for 15 h, the solvent was removed,
and the residue was extracted with n-pentane. Red crystals of 4
were obtained at –60 °C, yield 1.61 g (36%).
Figure 3. ORTEP diagram of 4 (ellipsoids are drawn at the 50%
probability level, hydrogen atoms are omitted for clarity). Selected
bond lengths [pm] and angles [°]: Ta–Cl 234.22(8), Ta–N1 204.7(2),
Ta–N2 203.3(2), N1–C11 140.7(3), N2–C16 141.7(3), N1–Ta–N2
78.98(9), N1–Ta–Cl 109.41(8), N2–Ta–Cl 109.31(7).
4: R = Ad: nBuLi (2.22 m in n-hexane, 6 mL, 14 mmol) was added
dropwise to a solution of AdPH2 (2.46 g, 15 mmol) in n-hexane
(20 mL). The reaction mixture was stirred for 2 h and then filtered.
A suspension of the yellow precipitate in toluene (30 mL) was
added dropwise to a solution of 2 (4.40 g, 7 mmol) in toluene
(30 mL) at –80 °C. After stirring for 15 h, the solvent was removed,
and the residue was extracted with n-pentane. Red crystals of 4
were obtained at –60 °C, yield 1.28 g (16%). EPR: g = 1.794, A =
44 mT. ESI-MS (n-pentane/CH3CN, 1:2): m/z = 601.17 [M]+.
Conclusions
In summary, the formation of alkylphosphanylidenetan-
talum(V) complexes depends on the steric demand of the
substituent on the phosphorus atom. Thus, the first ter-
minal organometallic alkylphosphanylidenetantalum(V)
complexes were obtained with cyclohexyl and isopropyl
groups at phosphorus, whereas adamantyl and tert-butyl
substituents resulted in the formation of paramagnetic tan-
talum(IV) complex 4. The stability of 2 and 3 with respect
to dimerization was clarified by DFT calculations. Future
studies will focus on investigating the reactivity of these
compounds.
Supporting Information (see footnote on the first page of this arti-
cle): DFT calculations and further experimental details.
Acknowledgments
This work was supported by the Graduate School BuildMoNa
(doctoral grant to A. G.) and the EU-COST Action CM0802 Pho-
SciNet. The authors thank Chemetall GmbH and BASF SE for
generous donations of chemicals. Prof. R. Kirmse and his co-
workers (University of Leipzig) are thanked for EPR measurements
of 4.
Experimental Section
2: At 0 °C, nBuLi (1.44 m in n-hexane, 11 mL, 16 mmol) was added
dropwise to a solution of CyPH2 (1.90 g, 16 mmol) in n-hexane
(15 mL). After 2 h of stirring at room temperature, the reaction
mixture was filtered. At –80 °C, a suspension of the white precipi-
tate in toluene (20 mL) was added dropwise to a solution of 1
(4.79 g, 8 mmol) in toluene (40 mL). The reaction mixture was
stirred for 15 h at room temperature. After removal of the volatile
material, the remaining solid was extracted with n-pentane. Red
crystals of 2 were obtained at –20 °C, yield 2.24 g (41%). 1H NMR
(400 MHz, C6D6): δ = 0.49 [s, 18 H, Si(CH3)3], 1.10–1.58 (m, 6 H,
Cy), 1.70 [s with sh., 17 H, C5(CH3)5 and Cy], 2.10 (d, JHP
11 Hz, 2 H, Cy), 4.67 (m, 1 H, Cy), 7.04 (m, 2 H, o-CH), 7.22 ppm
(m, 2 H, m-CH). 13C{1H} APT NMR (100 MHz, C6D6): δ = 3.5
[s, Si(CH3)3], 10.7 [s, C5(CH3)5], 26.2 [s, CH2, Cy(6,6Ј)] (numbering
scheme in Figure 1), 27.5 [s, CH2, Cy(7)], 36.0 [s, CH2, Cy(5,5Ј)],
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Pbcm,
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P212121,
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μ = 4.492 mm–1, θmax = 30.51°, R = 0.0375, Rw = 0.0367. T
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Received: April 16, 2013
Published Online: May 15, 2013
Eur. J. Inorg. Chem. 2013, 3137–3140
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