Synthesis and characterization of η5-1,2,4-diazaphospholide complexes of ruthenium

Article abstract of DOI:10.1021/om060041p

Treatment of the 1,2,4-diazaphospholide anions [3,5-tBu₂dp] ^- (1) and [3,5-Ph₂dp]^- (2) with [Cp*RuCl]₄ affords the two complexes [(η⁵-3,5- tBu₂dp)RuCp*] (3) and [(η⁵-3

Full text of DOI:10.1021/om060041p

1548
Organometallics 2006, 25, 1548-1550
Synthesis and Characterization of η5-1,2,4-Diazaphospholide
Complexes of Ruthenium†
Wenjun Zheng,* Guozhen Zhang,‡ and Kangnian Fan‡
Department of Chemistry, Fudan UniVersity, Handan Road 220,
Shanghai 200433, People’s Republic of China
ReceiVed January 16, 2006
Chart 1. 1H-1,2,4-Diazaphospholes (a) and
Summary: Treatment of the 1,2,4-diazaphospholide anions [3,5-
tBu2dp]- (1) and [3,5-Ph2dp]- (2) with [Cp*RuCl]4 affords the
two complexes [(η5-3,5-tBu2dp)RuCp*] (3) and [(η5-3,5-Ph2dp)-
RuCp*] (4) (dp ) 1,2,4-diazaphospholide), which are the first
examples of sandwich complexes deriVed from 1,2,4-diazaphos-
pholide ligands. The X-ray crystal structure analysis of 3 reVeals
that the metal atom is π-bonded to the 1,2,4-diazaphospholide
ligand. The bonding was rationalized by DFT calculations.
1,2,4-Diazaphospholides (b)
of recent reports on complexes with phosphorus- and nitrogen-
containing aromatic heterocyclic ligands as well as the emerging
application to molecular materials and catalysis,8-11 the use of
1,2,4-diazaphospholide ions as ligands shows promise as an
attractive developing area. Therefore, we set out to study metal
complexes with 1,2,4-diazaphospholide ligand coordination.
Herein, we report the synthesis and characterization of two
ruthenium(II) complexes bearing these ligands in η5 coordina-
tion, which are, to the best of our knowledge, the first such
complexes with this coordination mode.
Treatment of 3,5-di-tert-butyl-1,2,4-diazaphophole (H[3,5-
tBu2dp])1 and 3,5-diphenyl-1,2,4-diazaphosphole (H[3,5-Ph2-
dp])1 with metallic potassium in THF (tetrahydrofuran) afforded
potassium 3,5-di-tert-butyl-1,2,4-diazaphospholide ([3,5-tBu2-
dp]K (1), 93%) and potassium 3,5-diphenyl-1,2,4-diazaphos-
pholide-THF ([3,5-Ph2dp]K‚0.67THF (2), 88%), respectively,
as white solids with the evolution of hydrogen. Complex 1 is
soluble only in THF, while compound 2 is somewhat soluble
in ether. Both complexes were characterized by spectral and
analytical methods.12
1H-1,2,4-Diazaphospholes have been known for more than
20 years (Chart 1).1-5 As anions with six π electrons, the
deprotonated 1H-1,2,4-diazaphospholes exhibit evidence of a
certain degree of aromatic delocalization and are the heterocyclic
analogues of the cyclopentadienyl ligand (Cp-).5 The electron
distribution within the heterocycle, however, is uneven and
presents a charge shift toward the more electronegative nitrogen
atoms, on the basis of DFT calculations.6 The unique electronic
structure as well as the presence of donor lone pairs on the
heteroatoms make the 1,2,4-diazaphospholides potentially in-
teresting ligands for the metals across the periodic table. By
virtue of the electronic requirements, the resulting complexes
of the 1,2,4-diazaphospholides may present coordinations of the
types η1(N), η1(N1):η1(N2), η2(N1,N2), η1(N1):η1(N2):η1(P), and
η2(N1,N2):η1(P) via the nitrogen or/and phosphorus atom(s), of
the η5 type via the π-electron system, or even a combination of
the two. In comparison to the cyclopentadienyl ligand, however,
the related isoelectronic heterocyclic 1,2,4-diazaphospholide
ligands have received much less attention. There is one report
by Gudat and co-workers of the lithium complex [(η1:η1-dp)-
(µ-Li)(DME)]2, bearing a η1:η1-1,2,4-diazaphospholide ligand
(DME ) 1,2-dimethoxyethane),7 but so far other coordination
modes have not been substantiated. As attested to by the number
(7) Szarvas, L.; Bajko, Z.; Fusz, S.; Burck, S.; Daniels, J.; Nieger, M.;
Gudat, D. Z. Anorg. Allg. Chem. 2002, 628, 2303.
(8) Selected recent references for the complexes with phosphorus- and
nitrogen-containing aromatic heterocyclic ligands are as follows. (a)
Phospholyl complexes: Burney, C.; Carmichael, D.; Forissier, K.; Green,
J. C.; Mathey, F.; Ricard, L. Chem. Eur. J. 2005, 11, 5381. (b) 1,3,5-
Triphospholyl complexes: Clentsmith, G. K. B.; Cloke, F. G. N.; Green, J.
C.; Hanks, J.; Hitchcock, P. B.; Nixon, J. F. Angew. Chem., Int. Ed. 2003,
42, 1038. Clark, T.; Elvers, A.; Heinemann, F. W.; Hennemann, M.; Zeller,
M.; Zenneck, U. Angew. Chem., Int. Ed. 2000, 39, 2087. Hitchcock, P. B.;
Johnson, J. A.; Nixon, J. F. Organometallics 1995, 14, 4382. (c) Pyrrolyl
complexes: Ascenso, J. R.; Dias, A. R.; Ferreira, A. P.; Galva˜o, A. C.;
Salema, M. S.; Veiros, L. F. Inorg. Chim. Acta 2003, 356, 249. (d)
Pyrazolato complexes: Zheng, W. J.; Mo¨sch-Zanetti, N. C.; Blunk, T.;
Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G. Organometallics 2001,
20, 3299. (e) Triazolato and tetrazolato complexes: Zheng, W. J.; Heeg,
M. J.; Winter, C. H. Angew. Chem., Int. Ed. 2003, 42, 2761 and references
therein.
(9) Reviews for phospholyl complexes as catalysts and molecular material
building blocks: (a) Mathey, F. Angew. Chem., Int. Ed. 2003, 42, 1578.
(b) Mathey, F. J. Organomet. Chem. 2002, 646, 15.
(10) Selected recent references for pyrrolyl complexes in catalysis: (a)
Hansen, J. G.; Johannsen, M. J. Org. Chem. 2003, 68, 1266. (b) Lo, M. M.
C.; Fu, G. C. J. Am. Chem. Soc. 2002, 124, 4572.
(11) Selected recent references for pyrazolato complexes in catalysis:
(a) Baricelli, P. J.; Lo´pez-Linares, F.; Bruss, A.; Santos, R.; Lujano, E.;
Sa´nchez-Delgado, R. A. J. Mol. Catal. A: Chem. 2005, 239, 130. (b) Most,
K.; Hossbach, J.; Vidoviæ, D.; Magull, J.; Mo¨sch-Zanetti, N. C. AdV. Synth.
Catal. 2005, 347, 463. (c) Torres, F.; Sola, E.; Elduque, A.; Martinez, A.
P.; Lahoz, F. J.; Oro, L. A. Chem. Eur. J. 2000, 6, 2120.
† Dedicated to Professor Alfred Schmidpeter.
* To whom correspondence should be addressed. E-mail: wjzheng@
fudan.edu.cn.
‡ Physical Chemistry.
(1) Schmidpeter, A.; Willhalm, A. Angew. Chem., Int. Ed. Engl. 1984,
23, 903.
(2) In the solid state, 1,2,4-diazaphosphole and 3,5-di-tert-butyl-1,2,4-
diazaphosphole are associated via NH‚‚‚N hydrogen bonds and feature a
helix or dimer: Polborn, K.; Schmidpeter, A.; Ma¨rkl, G. Willhalm, A. Z.
Naturforsch. 1999, 54B, 187.
(3) Schmidpeter, A.; Karaghiosoff, K. Azaphospholes: In Rings, Clusters
and Polymers of Main Group and Transition Elements; Roesky, H. W.,
Ed.; Elsevier: Amsterdam, The Netherlands, 1989.
(4) (a) Nyula´szi, L.; Veszpre´mi, T.; Re´ffy, J. J. Phys. Chem. 1993, 97,
4011. (b) Nyula´szi, L.; Veszpre´mi, T.; Re´ffy, J.; Burkhardt, B.; Regitz, M.
J. Am. Chem. Soc. 1992, 114, 9080.
(5) Schmidpeter, A. In ComprehensiVe Heterocyclic Chemistry II;
Katritzky, R., Rees, C. W., Scriven, E. F. V., Eds.; Pergamon Press: Oxford,
U.K., 1996.
(6) Our DFT calculations followed by natural bond orbital (NBO)
population analysis on the charge distribution of the 3,5-di-tert-butyl-1,2,4-
diazaphospholide anion indicated that two nitrogen atoms gain more weight
(N (-0.37), P (0.17)); see the Supporting Information for the calculations
on [3,5-tBu2dp]- ligand.
10.1021/om060041p CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/01/2006

Communications
Scheme 1. Synthesis of Compounds 1-4
Organometallics, Vol. 25, No. 7, 2006 1549
The reaction of [Cp*RuCl]413 and 1 and 2 in THF smoothly
gave the complexes (η5-3,5-di-tert-butyl-1,2,4-diazaphospho-
lide)(η5-pentamethylcyclopentadienyl)ruthenium(II) ([(η5-3,5-
tBu2dp)RuCp*] (3), 89%) and (η5-3,5-diphenyl-1,2,4-diaza-
phospholide)(η5-pentamethylcyclopentadienyl)ruthenium(II) ([(η5-
3,5-Ph2dp)RuCp*] (4), 75%), respectively, as orange crystals
(Scheme 1).12 Both compounds are soluble in common organic
solvents and could be readily sublimed under high vacuum. They
may be handled briefly (2-5 min) in air in the solid state.
Attempts to prepare an analogous complex, (η5-1,2,4-diaza-
phospholide)(η5-pentamethylcyclopentadienyl)ruthenium(II) ([(η5-
1,2,4-dp)Ru(η5-CpMe5)]), using the 1,2,4-diazaphospholide
ligand in a similar manner were unsuccessful.
An X-ray crystal structure determination of complex 3 was
carried out. The molecule has a sandwich structure with two
π-bonded ligands (Figure 1).12 The ligands appear to be
essentially coplanar with a centroid-ruthenium-centroid angle
of 176.3(3)° as well as a dihedral angle of 5.73(14)° between
the two five-membered rings. The latter is larger than that
observed in the pseudo-ruthenocene complex [((CH3)2C3HN2)-
RuCp*] (2.6(4)°).14 Interestingly, the cross-orientation of the
two cyclic ligands is a close to eclipsed conformation, the
corresponding torsion angles for C(12)-Ru-C(1)-C(7) and
P(1)-Ru-C(11)-C(16) being -5.4(3) and 2.9(3)°, respectively.
This orientation is consistent with that found in the related
species [Cp*Ru(η5-tBu2C4H2P)]15 and decamethylruthenocene.16
The metal-to-ring distances ruthenium-pentamethylcyclopen-
tadienyl (centroid, 1.817(3) Å) and ruthenium-1,2,4-diaza-
phospholide (centroid, 1.823(3) Å) demonstrate the π-bonding
of ruthenium to the two ligands. Within the 1,2,4-diazaphos-
pholide ligand, the ruthenium-phosphorus bond length is
2.4309(7) Å, while the ruthenium-nitrogen distances are 2.165-
(2) and 2.166(2) Å, respectively, which fall in the expected range
of ruthenium-phosphorus distances associated with the sand-
wich complexes [Cp*Ru(η5-tBu2C4H2P)] (Ru-P ) 2.397(1)
Å),15 [Cp*Ru(η5-Me2Ph(Si-iPr3)C4P)] (Ru-P ) 2.405(1) Å),17a
[(η5-C5Me4Et)Ru(η5-P5)] (2.43 Å (av)),17b and [Cp*Ru(η5-
tBu2C2P2Sb)] (2.419(2), 2.472(2) Å)17c and are slightly shorter
than ruthenium-nitrogen bond lengths in the sandwich complex
[Ru(η5-NC4Me4)2] (Ru-N ) 2.1820(13) Å).18
Figure 1. Molecular structure of 3 with thermal ellipsoids at the
50% probability level, as determined by a single-crystal X-ray
diffraction study. Hydrogen atoms are not shown for clarity.
Selected bond lengths (Å) and angles (deg): Ru-N(1) ) 2.165-
(2), Ru-N(2) ) 2.166(2), Ru-C(1) ) 2.213(3), Ru-C(2) ) 2.225-
(2), Ru-C(11) ) 2.170(3), Ru-P(1) ) 2.4309(7), C(1)-P(1) )
1.777(3), C(2)-P(1) ) 1.780(3), C(1)-N(1) ) 1.372(3), C(2)-
N(2) ) 1.368(3), N(1)-N(2) ) 1.395(3); N(1)-Ru-N(2) ) 37.59-
(8), N(1)-Ru-C(1) ) 36.50(8), C(1)-Ru-P ) 44.68(7), C(1)-
P(1)-C(2) ) 85.77(12), C(1)-N(1)-N(2) ) 112.0(2), N(1)-
C(1)-P(1) ) 115.09(18); N(2)-N(1)-C(1)-P(1) ) -0.9(3),
C(2)-P(1)-C(1)-N(1) ) 1.6(2), C(12)-Ru-C(1)-C(7) ) -5.4-
(3), P(1)-Ru-C(11)-C(16) ) 2.9(3).
The elemental analysis results are in complete agreement with
the formulas of compounds 3 and 4.12 The mass spectra of both
complexes exhibit molecular ion peaks with the correct isotopic
distributions ([M+] at m/z 434 for 3 and m/z 474 for 4). The 1H
NMR (C6D6, 23 °C) spectrum of complex 3 displays two sharp
resonances at δ 1.34 for the tBu groups in addition to δ 1.73
for the C5Me5 group, attributable to a rotation of the two ligand
rings about their metal-ring centroid axes at room tempera-
ture.19 The 31P{1H} NMR resonance (C6D6, 23 °C) is observed
at -61.89 ppm, which is drastically shifted upfield relative to
the corresponding signals of the free heterocyclic ligand H[3,5-
tBu2dp] (31P δ +65.4 ppm) and the potassium salt 1 (31P δ
+50.65 ppm).1,12 The observed significant upfield shift reso-
nance upon ligation of the anion to transition metals in the 31P-
{1H} NMR spectrum is generally diagnostic for the π-bonding
of metals to a phosphorus atom.15,20 The 13C{1H} NMR
spectrum (C6D6, 23 °C) displays resonances at δ 31.75 (s),
34.823 (d, 2JCP ) 30.0 Hz, CCH3), and 148.280 (d, 1JCP ) 273.0
Hz, PCN) for the 1,2,4-diazaphospholide ligand of complex 3.
At δ 31.75, however, the coupling constant (3JCP(Me)) is too
small to be observed (the 3JCP(Me) coupling constant (26.8 Hz)
is observed for 1).12,21 Surprisingly, a coupled resonance at δ
89.06 (d, JCP ) 26.0 Hz) for the carbon atoms of the
pentamethylcyclopentadienyl ring is observed in complex 3,
(12) Synthetic procedures, analytical and spectroscopic data for 1-4,
crystallographic data for 3, and details of the calculations for both the [3,5-
tBu2dp]- ligand and 3 are contained in the Supporting Information.
(13) Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J. Am. Chem. Soc. 1989,
111, 1698.
(14) Perera, J. R.; Heeg, M. J.; Schlegel, H. B.; Winter, C. H. J. Am.
Chem. Soc. 1999, 121, 4536.
(15) Carmichael, D.; Ricard, L.; Mathey, F. J. Chem. Soc., Chem.
Commun. 1994, 1167.
(16) Zanin, I. E.; Antipin, M. Yu. Crystallogr. Rep. 2003, 48, 249.
(17) (a) Carmichael, D.; Mathey, F.; Richard, L.; Seeboth, N. Chem.
Commun. 2002, 2976. (b) Scherer, O. J.; Bru¨ck, T.; Wolmersha¨user, G.
Chem. Ber. 1988, 121, 935. (c) Francis, M. D.; Hibbs, D. E.; Hursthouse,
M. B.; Jones, C.; Malik, K. M. A. Chem. Commun. 1996, 1591.
(18) McComas, C. C.; Ziller, J. W.; Van Vranken, D. L. Organometallics
2000, 19, 2853.
(19) Janiak, C.; Shumann, H. AdV. Organomet. Chem. 1991, 33, 291.
(20) (a) Gudat, D.; Nieger, M.; Schmitz, K.; Szarvas, L. Chem. Commun.
2002, 1820. (b) Bartsch, R.; Hitchcock, P. B.; Nixon, J. F. J. Chem. Soc.,
Chem. Commun. 1987, 1146.
(21) The 13C{1H} NMR spectra for H[3,5-tBu2dp] and H[3,5-Ph2dp] have
3
been studied, and the JCP(Me) coupling was reported for the former:
Claramunt, R. M.; Lo´pez, C.; Schmidpeter, A.; Willhalm, A.; Elguero, J.;
Alkorta, I. Spectroscopy 2001, 15, 27.

1550 Organometallics, Vol. 25, No. 7, 2006
Communications
Table 1. Natural Charge Distributions of Several
Ruthenium Species (NPA)
presumably due to the closer distance between the interacting
nuclei. The resonance at δ -58.55 in the 31P{1H} NMR
spectrum (C6D6, 23 °C) strongly supports the π-bonding of
ruthenium to the 1,2,4-diazaphospholide in complex 4 (vs 31P
δ +67.19 ppm for 2).12
P
N1 N2
Ru
ref
[3,5-tBu2dp]- (1)
[(3,5-tBu2dp)RuCp*] (3) 0.544 -0.311 -0.310
Cp2Ru
0.167 -0.372 -0.372
a
a
0.24
The observation of an η5-1,2,4-diazaphospholide coordination
mode in the phenyl- and tert-butyl-substituted 1,2,4-diazaphos-
pholide complexes is significant, in view of the fact that no
such coordination mode has been previously authenticated.
Apparently, steric protection due to the tert-butyl and phenyl
groups plays a critical role in the successful preparation of
complexes 3 and 4, where the substituents of the heterocyclic
ligands likely block the heteroatom lone pairs (N, N, P) and
prevent ruthenium from σ-coordination. This is consistent with
the notion that bulky substituents in the 2- and 5-positions of
the pyrrolyl and phospholyl ligands generally favor η5 coordina-
tion of the transition metal to pyrroyl and phospholyl ligands
over σ-coordination.15,17a,22 In addition, the steric and electronic
effects of the fragment [Cp*RuII] is the likely reason for the
η5-1,2,4-diazaphospholide coordination, as it is known to have
an extraordinary ability to form η5 complexes with five-
membered aromatic heterocycles.23 It is interesting to note that
several transition-metal complexes with 1H-1,2,4-diazaphosp-
hole ligands are known, in which the metals are exclusively
σ-bonding to the neutral ligands via heteroatoms.7
To gain a better understanding of the bonding, we have
performed a series of DFT calculations followed by natural bond
orbital (NBO) population analyses on the 3,5-di-tert-butyl-1,2,4-
diazaphospholide ion as well as on complex 3.12,24a,b The
calculation on the former reveals that the HOMO and HOMO-
2, which differ in energy by 0.49 eV, are both π-bonding
orbitals. Such characteristics are thus in favor of participating
in π-bonding with orbitals of the proper symmetry from the
metal and the pentamethylcyclopentadienyl ligand to form the
HOMO-3 and HOMO-4 in 3, indicating the π-orbital charac-
teristic of bonding to ruthenium.12 The NBO analysis of 3
0.25 24c
-0.08 24c
-0.88 24c
CpRu(P5)
Ru(P5)2
a This work.
displays the populations [Kr]5s0.164d7.585p0.04 on the ruthenium
center, a negative charge of -0.31 on the 1,2,4-diazaphospho-
lide, and a positive charge of 0.07 on the pentamethylcyclo-
pentadienyl ligand, where a significant difference is revealed
in the charge distribution between the two ligands. The more
negative charge on the 1,2,4-diazaphospholide in compound 3
may therefore represent an electron-withdrawing characteristic
relative to the pentamethylcyclopentadienyl ligand. With respect
to the charge distribution on the metal, however, the value
(+0.24) is very close to that (+0.25) in ruthenocene obtained
in previous calculations.24c This result may suggest that the
heterocyclic ligand in 3 is a poorer donor relative to the
cyclopentadienyl ligand (Table 1). Additionally, the NBO
analysis results suggest that the heteroatom lone pairs rarely
participate in bonding to ruthenium center.12
In conclusion, we present here an easy, high-yield synthetic
procedure and full characterization of the first sandwich
ruthenium complexes with η5 coordination of 1,2,4-diazaphos-
pholide ligands, whose π-coordination characteristics are dem-
onstrated by both experimental findings and DFT calculations.
The results of DFT calculations also indicated that the 3,5-di-
tert-butyl-1,2,4-diazaphospholide ligand is a poorer electron
donor relative to the pentamethylcyclopentadienyl ligand,
evidenced by the longer distances of ruthenium to the 1,2,4-
diazaphospholide compared to those of ruthenium to the
pentamethylcyclopentadienyl ligand in 3. Our work in this paper
suggests that the η5-1,2,4-diazaphospholide coordination is easily
accessible only with bulky groups on the heterocyclic ligands.
The results of the present study therefore argue that η5-1,2,4-
diazaphospholide ligand coordination with bulky substituents
may be possible in other low-valent transition-metal complexes.
We are currently exploring such 1,2,4-diazaphospholide chem-
istry with metals.
(22) (a) Kuhn, N.; Kockerling, M.; Stubenrauch, S.; Blaser, D.; Boese,
R. J. Chem. Soc., Chem. Commun. 1991, 1368. (b) Kuhn, N.; Henkel, G.;
Stubenrauch, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 778. (c) Tanski, J.
M.; Parkin, G. Organometallics 2002, 21, 587.
(23) Chaudret, B.; Jalon, F. A. J. Chem. Soc., Chem. Commun. 1988,
711.
(24) (a) DFT calculations were carried out at the B3LYP level with
6-311G*/CEP-121G basis sets, computed with Gaussian 03 (Revision B.03)
(Frisch, M. J. et al. Gaussian Inc., Pittsburgh, PA, 2003). The B3LYP hybrid
density functional method (Becke, A. D. Phys. ReV. A 1988, 38, 3098. Lee,
C.; Yang, W.; Parr, R. D. Phys. ReV. B 1988, 37, 785. Becke, A. D. J.
Chem. Phys. 1993, 98, 5648. Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H.
Chem. Phys. Lett. 1989, 157, 200) was employed in all the calculations,
combining a Stevens-Basch-Krauss triple-split valence basis set (Stevens,
W. J.; Basch, H.; Krauss, M. J. Chem. Phys, 1984, 81, 6026. Stevens, W.
J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem. 1992, 70, 612.
Cundari, T. R.; Stevens, W. J. J. Chem. Phys, 1993, 98, 5555) on ruthenium
and a 6-311G* basis set on other elements. The geometry of the isolated
3,5-di-tert-butyl-1,2,4-diazaphospholide was fully optimized: (b) Foster,
J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211 (Glendening, E. D.;
Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1). (c) Malar,
E. J. P. Eur. J. Inorg. Chem. 2004, 2723.
Acknowledgment. We are grateful to the National Natural
Science Foundation of China (NSFC) (Grant No. 20571017)
for the support of this research.
Supporting Information Available: Text, tables, and figures
giving synthetic procedures, analytical and spectroscopic data for
1-4, and details of the molecular orbital calculations for the 1,2,4-
diazaphospholide and 3 and a CIF file giving X-ray crystallographic
data for 3. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM060041P