Synthesis and reactivity of 16-electron cycloheptatrienyl-molybdenum(0) complexes with bis(imidazolin-2-imine) ligands

Article abstract of DOI:10.1021/om700953u

The reaction of the ligands 1,2-bis(1,3-diisopropyl-4,5-dimethylimidazolin- 2-imino)ethane (BL^iPr) and 1,2-bis(1,3,4,5-tetramethylimidazolin-2- imino)ethane (BL^Me) with the cycloheptatrienyl-molybdenum complex [(η⁷-C₇H₇)Mo(CH₃CN) ₃]X (X = BF₄, PF₆) leads to acetonitrile substitution and formation of stable 16-electron half-sandwich complexes (η⁷-C₇H₇)Mo(BL^Me)]BF ₄, [1]BF₄, and [(η⁷-C₇H ₇)Mo(BL^Me)]X, [2]X (X = BF₄, PF₆), two of which could be crystallographically characterized to reveal undistorted two-legged piano stool geometries. Cyclovoltammetric studies exhibit very negative redox potentials of E^o = -1.095 V and E^o = -1.138 V versus the ferrocene/ferrocenium couple (0 V), indicative of very electron-rich metal complexes. Oxidation of [2]X with (η⁷-C ₇H₇)BF₄ or [Fe(η⁵-C ₅H₅)₂]PF₆ furnished the Mo(I) complexes of general formula [(η⁷-C₇H ₇)Mo(BL^Me)(CH₃CN)]X₂, [3]X ₂ (X = BF₄, PF₆), which display a three-legged piano stool geometry in the solid state by coordination of one additional acetonitrile molecule to molybdenum. The same geometry is observed upon reaction of [1]BF₄ and [2]PF₆ with 2,6-dimethylphenyl isocyanide (XyNC) to afford the isocyanide complexes [1(CNXy)]BF₄ and [2(CNXy)]PF₆. Their CN stretching vibrations are observed at 2041 and 2030 cm^-1, confirming the electron richness and strong π-electron-releasing capability of the molybdenum-imine moiety.

Full text of DOI:10.1021/om700953u

778
Organometallics 2008, 27, 778–783
Synthesis and Reactivity of 16-Electron
Cycloheptatrienyl-Molybdenum(0) Complexes with
Bis(imidazolin-2-imine) Ligands
Dejan Petrovic, Cristian G. Hrib, Sören Randoll, Peter G. Jones, and Matthias Tamm*
Institut für Anorganische and Analytische Chemie, Technische UniVersität Carolo-Wilhelmina,
Hagenring 30, 38106 Braunschweig, Germany
ReceiVed September 26, 2007
The reaction of the ligands 1,2-bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-imino)ethane (BLiPr) and
1,2-bis(1,3,4,5-tetramethylimidazolin-2-imino)ethane (BLMe) with the cycloheptatrienyl-molybdenum
complex [(η7-C7H7)Mo(CH3CN)3]X (X ) BF4, PF6) leads to acetonitrile substitution and formation of
stable 16-electron half-sandwich complexes (η7-C7H7)Mo(BLiPr)]BF4, [1]BF4, and [(η7-C7H7)Mo(BLMe)]X,
[2]X (X ) BF4, PF6), two of which could be crystallographically characterized to reveal undistorted
two-legged piano stool geometries. Cyclovoltammetric studies exhibit very negative redox potentials of
E° ) -1.095 V and E° ) -1.138 V versus the ferrocene/ferrocenium couple (0 V), indicative of very
electron-rich metal complexes. Oxidation of [2]X with (η7-C7H7)BF4 or [Fe(η5-C5H5)2]PF6 furnished the
Mo(I) complexes of general formula [(η7-C7H7)Mo(BLMe)(CH3CN)]X2, [3]X2 (X ) BF4, PF6), which
display a three-legged piano stool geometry in the solid state by coordination of one additional acetonitrile
molecule to molybdenum. The same geometry is observed upon reaction of [1]BF4 and [2]PF6 with 2,6-
dimethylphenyl isocyanide (XyNC) to afford the isocyanide complexes [1(CNXy)]BF4 and [2(CNXy)]PF6.
Their CN stretching vibrations are observed at 2041 and 2030 cm-1, confirming the electron richness and
strong π-electron-releasing capability of the molybdenum-imine moiety.
saturation.9 Accordingly, the stabilization for instance of 16-
Introduction
electron half-sandwich ruthenium complexes of the type [(η5-
Coordinatively unsaturated organometallic complexes play
a fundamental role as intermediates in homogeneous transition
metal-catalyzed reactions, and their occurrence is rationalized
by the 16- and 18-electron rule proposed by Tolman.1 Numerous
examples have been prepared to date, and their electronic and
magnetic properties have been studied.2 Among these systems,
16-electron ruthenium half-sandwich complexes containing
cyclopentadienyl-ruthenium moieties [(η5-C5R5)Ru] have re-
ceived considerable attention,3–6 since complexes of this type
can for instance be used for the stabilization of highly reactive
species such as vinylidenes or allenylidenes by metal coordina-
tion.7 In addition, these unsaturated complexes may serve as
models for the active catalysts generated in ruthenium-catalyzed
C-C bond forming reactions.8
C5Me5)Ru(PR3)X] (X ) OR, NHR, Cl, Br, I) can be achieved
by electron donation from the heteroatomic anionic ligand.10
Ligand π-coordination has also been suggested to exist in highly
reactive ruthenium-amidinate complexes such as [(η5-
C5Me5)Ru{iPrNC(Me)NiPr}].11 In contrast, the cationic 16-
electron tmeda complex [(η5-C5Me5)Ru(Me2NCH2CH2NMe2)]+
can be isolated despite the lack of nitrogen-to-metal π-donation.
This complex, however, can only be stabilized by the use of
the noncoordinating [B{3,5-C6H3(CF3)2}4]- (BAr′4) counter-
anion.12 The same holds true for the rare 16-electron phos-
phane complexes [(η5-C5Me5)Ru(PP)]BAr′4 [PP ) (PEt3)2,
iPr2PCH2CH2PiPr2].13
(9) Caulton, K. G. New J. Chem. 1994, 18, 25.
Various factors can be responsible for the stabilization of these
species, among which the introduction of π-donor ligands
such as alkoxides, alkylthiolates, amides, or halides can be
successfully employed for the creation of π-stabilized un-
(10) Johnson, T. J.; Folting, K.; Streib, W. E.; Martin, J. D.; Huffman,
J. C.; Jackson, S. A.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1995,
34, 488.
(11) (a) Yamaguchi, Y.; Nagashima, H. Organometallics 2000, 19, 725.
(b) Kondo, H.; Yamaguchi, Y.; Nagashima, H. Chem. Commun. 2000, 1075.
(c) Kondo, H.; Yamaguchi, Y.; Nagashima, H. J. Am. Chem. Soc. 2001,
123, 500. (d) Kondo, H.; Kageyama, A.; Yamaguchi, Y.; Haga, M.-a.;
Kirchner, K.; Nagashima, H. Bull. Chem. Soc. Jpn. 2001, 74, 1927. (e)
Terasawa, J.-i.; Kondo, H.; Matsumoto, T.; Kirchner, K.; Motoyama, Y.;
Nagashima, H. Organometallics 2005, 24, 2713.
* Corresponding author. Tel: +49-531-3915309. Fax: +49-531-3915387.
E-mail: m.tamm@tu-bs.de.
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M.; Schmid, R.; Kirchner, K. Inorg. Chim. Acta 1999, 286–114. (c) Gemel,
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10.1021/om700953u CCC: $40.75
2008 American Chemical Society
Publication on Web 02/05/2008

16-Electron Cycloheptatrienyl-Molybdenum(0) Complexes
Organometallics, Vol. 27, No. 4, 2008 779
Scheme 1. Mesomeric structures for the bis(imidazolin-2-imine)
Scheme 2. Syntheses of 16-Electron Molybdenum(0) Complexes
ligands BLR (R ) Me, iPr)
[1]X and [2]X (X ) BF4, PF6)
Recently, we have reported the isolation of very stable
cationic 16-electron ruthenium complexes of the type [(η5-
C5Me5)Ru(BLR)]+ (R ) iPr, Me) containing the neutral 1,2-
bis(imidazolin-2-imino)ethane ligands BLiPr and BLMe, in which
the ruthenium atom even resists coordination by the chloride
counterion.14 This unusual stability can be ascribed to the strong
π-basic nature of the novel bis(imidazoline) ligands, in which
the ability of the imidazolium moiety to stabilize a positive
charge leads to highly basic ligands with a strong electron-
donating capacity (Scheme 1).15–18 This behavior leads to a weak
propensity of the Ru atom to coordinate other π-donor ligands
such as chloride, whereas strong binding of σ-donor/π-acceptor
ligands such as CO or isocyanides can be observed.14
In view of the growing number of well-characterized 16-
electron cyclopentadienyl-ruthenium complexes, it is surprising
that no analogous half-sandwich cycloheptatrienyl-molybdenum
complexes have been reported to date, although the similarity
between the isoelectronic and isolobal fragments [(η5-C5R5)Ru]
and [(η7-C7H7)Mo] is well recognized.19–22 The steric require-
ments of the C7H7 (Cht) ligand (cone angle ≈ 154°) make
comparison with the C5Me5 (Cp*) system (cone angle ≈
142°)23 particularly appropriate, and it could clearly be
demonstrated that complexes of the 16-electron [(η7-
C7H7)MoL2] auxiliary exhibit reactivity patterns analogous
to those of [(η5-C5Me5)RuL2] systems.20,21 In this contribu-
tion, we present the synthesis and structural characterization
of stable cationic 16-electron molybdenum complexes of the
type [(η7-C7H7)Mo(BLR)]+ (R ) Me, iPr) containing the
neutral 1,2-bis(imidazolin-2-imino)ethane ligands BLMe and
BLiPr and compare their properties with those of their [(η5-
C5Me5)Ru(BLR)]+ congeners.14
Results and Discussion
Reactions of the Ligands BLR with [(η7-C7H7)Mo-
(CH3CN)3]X (X ) BF4, PF6). The synthesis of the ligands 1,2-
bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-imino)ethane (BLiPr)
and 1,2-bis(1,3,4,5-tetramethylimidazolin-2-imino)ethane (BLMe
)
has been described elsewhere.14 The reactions of these poten-
tially bidentate ligands with the purple molybdenum(0) com-
plexes [(η7-C7H7)Mo(CH3CN)3]X (X ) BF4, PF6),24 followed
by precipitation with diethyl ether, afforded grass-green,
extremely air-sensitive solids (Scheme 2). The ligands BLiPr
and BLMe coordinate to molybdenum by substitution of the three
acetonitrile molecules in the starting material to form stable 16-
electron half-sandwich complexes [(η7-C7H7)Mo(BLiPr)]BF4,
[1]BF4, and [(η7-C7H7)Mo(BLMe)]X, [2]X (X ) BF4, PF6),
respectively, which was confirmed by elemental analyses. The
cations in [1]BF4 and [2]X exhibit C2V symmetry in solution
(14) Petrovic, D.; Glöge, T.; Bannenberg, T.; Hrib, C. G.; Randoll, S.;
Jones, P. G.; Tamm, M. Eur. J. Inorg. Chem. 2007, 3472.
(15) Frison, G.; Sevin, A. J. Chem. Soc., Perkin Trans. 2 2002, 1692.
(16) Tamm, M.; Petrovic, D.; Randoll, S.; Beer, S.; Bannenberg, T.;
Jones, P. G.; Grunenberg, J. Org. Biomol. Chem. 2007, 5, 523.
(17) For the synthesis of a related bidentate ligand with 1,3-dimeth-
ylimidazolin-2-imino groups, see: Henkel., G.; Kuhn, N.; Grathwohl, M.;
Steinmann, M. Z. Naturforsch. 1998, 53b, 997.
(18) For the synthesis of a related tridentate pincer-type ligand with 1,3-
di-tert-butylimidazolin-2-imino groups, see: Petrovic, D.; Bannenberg, T.;
Randoll, S.; Jones, P. G.; Tamm, M. Dalton Trans. 2007, 2812.
(19) Green, M. L. H.; Ng, D. K. P. Chem. ReV. 1995, 95, 439.
(20) (a) Lancashire, H. N.; Ahmed, R.; Hague, T. L.; Helliwell, M.;
Hopgood, G. A.; Sharp, L.; Whiteley, M. W. J. Organomet. Chem. 2006,
691, 3617. (b) Fitzgerald, E. C.; Grime, R. W.; Knight, H. C.; Helliwell,
M.; Raftery, J.; Whiteley, M. W. J. Organomet. Chem. 2006, 691, 1879.
(c) Aston, G. M.; Badriya, S.; Farley, R. D.; Grime, R. W.; Ledger, S. J.;
Mabbs, F. E.; McInnes, E. J. L.; Morris, H. W.; Ricalton, A.; Rowlands,
C. C.; Wagner, K.; Whiteley, M. W. J. Chem. Soc., Dalton Trans. 1999,
24, 4379. (d) Disley, S. P. M.; Grime, R. W.; McInnes, E. J. L.; Spencer,
D. M.; Swainston, N.; Whiteley, M. W. J. Organomet. Chem. 1998, 566,
151. (e) Beddoes, R. L.; Grime, R. W.; Hussain, Z. I.; Whiteley, M. W.
J. Chem. Soc., Dalton Trans. 1996, 3893. (f) Bitcon, C.; Breeze, R.; Miller,
P. F.; Whiteley, M. W. J. Organomet. Chem. 1989, 364, 181. (g) Adams,
J. S.; Bitcon, C.; Brown, J. R.; Collison, D.; Cunningham, M.; Whiteley,
M. W. J. Chem. Soc., Dalton Trans. 1987, 3049.
1
according to their H and 13C NMR spectra in acetone-d6. For
instance, the 1H NMR spectra of [2]BF4 and [2]PF6 each show
one singlet for the NCH2CH2N unit together with two singlets
for the two different types of methyl groups. In contrast, two
1
doublets are observed in the H NMR spectrum of [1]BF4 for
the diastereotopic isopropyl methyl groups, indicating a hindered
rotation of the imidazoline moieties around the exocyclic C-N
bonds at room temperature on the NMR time scale. The
resonance for the η7-coordinated cycloheptatrienyl ring appears
as a broad signal at 4.90 ppm. Upon coordination of BLiPr, the
1H NMR resonance of the bridging ethylene hydrogen atoms
is significantly shifted to higher field (from 3.47 to 2.18 ppm),
(21) (a) Brunner, H.; Klankermayer, J.; Zabel, M. Eur. J. Inorg. Chem.
2002, 2494. (b) Brunner, H.; Klankermayer, J.; Zabel, M. Organometallics
2002, 21, 5746. (c) Brunner, H.; Klankermayer, J.; Zabel, M. Z. Anorg.
Allg. Chem. 2002, 628, 2264.
(22) (a) Tamm, M.; Dressel, B.; Bannenberg, T.; Grunenberg, J.;
Herdtweck, E. Z. Naturforsch. 2006, 61b, 896. (b) Tamm, M.; Dressel, B.;
Lügger, T.; Fröhlich, R.; Grimme, S. Eur. J. Inorg. Chem. 2003, 1088. (c)
Tamm, M.; Dressel, B.; Urban, V.; Lügger, T. Inorg. Chem. Commun. 2002,
5, 837.
(23) Davies, C. E.; Gardiner, I. M.; Green, J. C.; Green, M. L. H.; Hazel,
N. J.; Grebenik, P. D.; Mtetwa, V. S. B.; Prout, K. J. Chem. Soc., Dalton
Trans. 1985, 669.
(24) Ashworth, E. F.; Green, J. C.; Green, M. L. H.; Knight, J.; Pardy,
R. B. A.; Wainwright, N. J. J. Chem. Soc., Dalton Trans. 1977, 1693.

780 Organometallics, Vol. 27, No. 4, 2008
PetroVic et al.
Figure 2. ORTEP drawing of the cation in [2]BF4 · CH3CN with
thermal displacement parameters drawn at 50% probability. Selected
bond lengths [Å] and angles [deg]: Mo-N1 2.076(3), Mo-N2
2.063(3), Mo-C(Cht) 2.226(4)–2.307(4), N1-C3 1.357(4), N2-C10
1.363(4); N1-Mo-N2 76.5(1).
Figure 1. ORTEP drawing of the cation in [1]BF4 · 2CH3CN with
thermal displacement parameters drawn at 50% probability. The
ethylene bridge is disordered. Selected bond lengths [Å] and
angles [deg]: Mo-N1 2.069(2), Mo-N2 2.073(2), Mo-C(Cht)
2.209(4)–2.280(4),N1-C31.358(3),N2-C141.353(3);N1-Mo-N2
77.83(9).
whereas downfield shifts are observed for all imine resonances.
The same trends are observed upon coordination of BLMe, where
the resonance for the bridging ethylene hydrogen atoms is shifted
from 3.63 to 2.23 ppm. Again, the remaining imine resonances
are shifted to lower field, and a broad η7-C7H7 signal is observed
at 4.83 ppm.
Single crystals of [1]BF4 · 2CH3CN suitable for an X-ray
diffraction analysis were obtained by crystallization from
CH3CN/Et2O solution. The cationic half-sandwich complex
exhibits a two-legged piano stool geometry with the η7-C7H7
and the η2-diimine ligand adopting a pseudo trigonal-planar
coordination sphere around the molybdenum atom (Figure 1).
Examination of the crystal packing reveals the absence of any
relevant intermolecular contact between the cation and the
tetrafluoroborate anion (shortest Mo · · · F 4.64 Å). The atoms
Mo, N1, and N2, together with Ct, the centroid of the Cht ring,
are coplanar (mean deviation 0.02 Å), and the sum of the
N1-Mo-N2 angle [77.83(9)°] and the two Ct-Mo-N angles
is 359.7°. Consequently, the cation 1 can be regarded as being
almost perfectly C2V symmetric. Very short Mo-N bonds25 in
1 [Mo-N1 2.069(2) Å, Mo-N2 2.073(2) Å] are in agreement
with a strong electron-releasing capability of the BLiPr ligand,
which is implied by the ylidic resonance structure shown in
Schemes 1 and 2. Charge separation in coordinated BLiPr can
also be clearly deduced from the observation of perpendicularly
oriented imidazoline moieties with respect to the Mo-N-N
coordination plane (interplanar angles 89.7° and 87.1°). This
orientation rules out the possibility of π-interaction between the
exocyclic nitrogen atoms and the imidazoline rings, leading to
elongated C-N distances [N1-C3 1.358(3) Å, N2-C14 )
1.353(3) Å], much longer than expected for an N-C(sp2) double
bond (1.28 Å) and almost in the range of an N-C(sp2) single
bond (1.38 Å).26
Figure 3. Cyclic voltammetric traces of [1]BF4 (above) and [2]PF6
(below), 1.0 mM solution in acetonitrile, (TBA)PF6 (0.1 M), scan
rate 0.1 V/s, T ) 25 °C, against a Ag/AgCl reference electrode (3
M KCl), Pt working electrode; the ferrocene/ferrocenium couple
displays a redox potential E° ) +476 mV, and the values are
referenced to E°(Fc/Fc+) ) 0 mV.
Mo · · · F separations are 4.80 and 5.39 Å, respectively (Figure
2). The structural parameters of the cation in [2]BF4 · CH3CN
are very similar to those in [1]BF4 · 2CH3CN; short Mo-N
bonds [Mo-N1 2.076(3) Å, Mo-N2 2.063(3) Å] together with
elongated exocyclic C-N bonds [N1-C3 1.357(4) Å, N2-C10
1.363(4) Å] are observed. Again, the imidazoline rings are
almost perpendicularly oriented with respect to the Mo-N-N
coordination plane (interplanar angles 80.3° and 88.3°). It should
be noted that even though the complexes [1]BF4 and [2]BF4
crystallize as 16-electron species, the interaction with acetonitrile
molecules in solution should be taken into account, in particular
upon oxidation (Vide infra).
On the basis of the structural results, it can be assumed that
the unusual stability of the cationic complexes 1 and 2 stems
from the strong π-electron releasing capability of the diimine
ligands BLiPr and BLMe. Indeed, cyclic voltammetric studies
on [1]BF4 and [2]PF6 in acetonitrile afford very negative redox
potentials of E° ) -1.095 V and E° ) -1.138 V versus the
ferrocene/ferrocenium couple, indicating a slightly stronger
electron-releasing capability of the BLMe ligand compared to
BLiPr (Figure 3). These potentials are much lower than usually
observed for a reversible one-electron Mo(0)/Mo(I) oxidation
Similarly to [1]BF4 · 2CH3CN, single crystals of [2]BF4 ·
CH3CN were obtained from CH3CN/Et2O solution. Interestingly,
even with decreased steric protection by substitution of the
isopropyl groups in BLiPr for the methyl groups in BLMe, the
molybdenum complex remains stable as a 16-electron species
in the solid state, showing no tendency to coordinate acetonitrile.
Any interaction between the cations 2 and the CH3CN molecules
or BF4 anions can be excluded, since the shortest Mo · · · N and
(25) The Mo-N bonds in the cations 1 and 2 are shorter than any
previously observed bond in (η7-C7H7)Mo-N systems; see refs 20a, 21b,
21c, 27, and 29.
(26) March, J. AdVanced Organic Chemistry, 4th.; John Wiley & Sons,
Inc.: New York, 1992; p 21.

16-Electron Cycloheptatrienyl-Molybdenum(0) Complexes
Organometallics, Vol. 27, No. 4, 2008 781
Figure 4. ORTEP drawing of the dication in [(η7-
C7H7)Mo(BLMe)(CH3CN)][PF6]2 · CH3CN, [3](PF6)2 · CH3CN, with
thermal displacement parameters drawn at 50% probability. Selected
bond lengths [Å] and angles [deg] in [3][BF4]2/[3][PF6]2 · CH3CN:
Mo-N1 2.164(2)/2.144(2), Mo-N2 2.127(2)/2.116(3), Mo-N7
2.227(2)/2.241(2), Mo-C(Cht) 2.246(3)–2.322(3)/2.243(3)–2.307(3),
N1-C3 1.346(3)/1.350(4), N2-C10 1.350(3)/1.349(4), N7-C24
1.143(4)/1.131(4); N1-Mo-N2 76.55(8)/75.7(1), Mo-N7-C24
166.4(2)/169.0(3), N7-C24-C25 177.7(3)/178.6(3).
Figure 5. ORTEP drawing of the cation in [1(CNXy)]BF4 · C3D6O
with thermal displacement parameters drawn at 50% probability.
Selected bond lengths [Å] and angles [deg]: Mo-N1 2.207(2),
Mo-N2 2.143(2), Mo-C32 2.096(2), Mo-C(Cht) 2.251(2)–2.302(2),
N1-C3 1.339(3), N2-C14 1.344(2), C32-N7 1.168(3); N1-Mo-
N2 75.22(6), Mo-C32-N7 178.1(2), C32-N7-C33 175.7(2).
process in cycloheptatrienyl-molybdenum complexes of the
general type [(η7-C7H7)MoL2X].20,27 The E° values for [1]BF4
and [2]PF6 are also more negative than those of their ruthenium
analogues [(η5-C5Me5)Ru(BLR)]OTf (R ) iPr, Me; OTf )
triflate), E° ) -569 mV and E° ) -588 mV.14 This trend is
in agreement with other comparative studies on [(η7-C7H7)Mo]
and [(η5-C5R5)Ru] systems.20a Since the redox potentials
observed for [1]BF4 and [2]PF6 lie far below E° of the Fc/Fc+
couple, their oxidation to dicationic complexes should easily
be possible by chemical oxidation with ferrocenium salts, or
even with much weaker oxidation agents such as tropylium salts,
for which a redox potential of E° ) -650 mV in acetonitrile
versus the Fc/Fc+ couple has been reported.28 Consequently,
treatment of a green solution of [2]BF4 in THF/CH3CN with
tropylium tetrafluoroborate, (η7-C7H7)BF4, afforded the Mo(I)
complex [(η7-C7H7)Mo(BLMe)(CH3CN)][BF4]2, [3][BF4]2 as an
orange crystalline compound (eq 1), whereas the oxidation of
[2]PF6 with [Fe(η5-C5H5)2]PF6 furnished orange crystals of
[3][PF6]2 · CH3CN after diffusion of diethyl ether into an
acetonitrile solution (eq 2).
Isocyanide Adducts of [1]BF4 and [2]PF6. Reactions of
[1]BF4 and [2]PF6 with 2,6-dimethylphenyl isocyanide (XyNC)
in acetone afforded the isocyanide complexes [1(CNXy)]BF4
and [2(CNXy)]PF6 in good yields (eqs 3 and 4). The complexes
were isolated as red crystals upon diffusion of diethyl ether into
their acetone solutions. In the 1H and 13C NMR spectra,
coordination of the isocyanides is clearly indicated by the
observation of the expected isocyanide resonances together with
significantly shifted resonances for the bidentate ligands com-
pared to those observed for [1]BF4 and [2]PF6. The IR spectra
exhibit strong CN stretching vibrations at 2041 cm-1 for
[1(CNXy)]PF6 and at 2030 cm-1 for [2(CNXy)]PF6, which is
strongly shifted to lower wavenumbers with respect to the
uncoordinated isocyanide.30 These values are in agreement with
strong π-back-donation from molybdenum(0) to the isocyanide
ligands, which is much more pronounced than in related Mo(0)
complexes, e.g., νCN ) 2161 cm-1 (in CH2Cl2) for [(η7-
C7H7)Mo(bipy)(CNtBu)]PF6.20d As previously deduced from the
cyclic voltammetric studies, the IR results also indicate a
stronger electron-releasing capability of the BLMe ligand
CH3CN
[2]BF4 + (C7H7)BF4
8 [3][BF4]2
(1)
(2)
compared to BLiPr
.
-1 ⁄ 2(C7H7)2
[1]BF4 + XyNC f [1(CNXy)]BF4
[2]PF6 + XyNC f [2(CNXy)]PF6
(3)
(4)
[2]PF6 + (Fc)PF4 CH CN8 [3][PF6]2
3
-Fc
The isocyanide complexes could be crystallized from acetone/
Et2O solution, and the molecular structures of the cations in
[1(CNXy)]BF4 · d6-acetone and [2(CNXy)]PF6 are shown in
Figures 5 and 6. Both complexes exhibit a three-legged piano
stool geometry with significantly elongated Mo-N bonds
compared to the starting materials [Mo-N1 2.207(2) Å,
Mo-N2 2.143(2) Å and Mo-N1 2.252(3) Å, Mo-N2 2.189(3)
Å]. In both cases, the aryl moieties of the isocyanide ligands
adopt an approximately perpendicular orientation, and the six-
membered benzene rings subtend angles of 19.8° and 17.0°,
respectively, with the plane containing the Cht centroid, the Mo
To unequivocally establish the formation of Mo(I) complexes,
the molecular structures of [3][BF4]2 and [3][PF6]2 · CH3CN were
established by X-ray diffraction analyses, and the dication in
the latter complex is shown in Figure 4. In both cases, the
dication [(η7-C7H7)Mo(BLMe)(CH3CN)]2+ (3) contains a coor-
dinated acetonitrile ligand and exhibits a three-legged piano stool
geometry with elongated Mo-N(imine) bonds in comparison
to the structure of [2]BF4 · CH3CN. The Mo-N(CH3CN) bonds
are significantly longer [Mo-N7 2.227(2) (BF4 salt), 2.241(2)
Å (PF6 salt)]; they are also longer than the Mo-N distance of
2.166(3) Å observed for the Mo(I) acetonitrile complex [(η7-
C7H7)Mo(CH3CN)I2].29
(29) Green, M. L. H.; Harrison, A.; Mountford, P.; Ng, D. K. P. J. Chem.
Soc., Dalton Trans. 1993, 2215.
(27) Tamm, M.; Bannenberg, T.; Dressel, B.; Fröhlich, R.; Holst, C.
Inorg. Chem. 2002, 41, 47.
(30) Stephany, R. W.; de Bie, M. J. A.; Drenth, W. Org. Magn. Reson.
1974, 6, 45. In our hands, 2,6-dimethylphenyl isocyanide shows νCN
)
(28) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
2123 cm-1 in KBr.

782 Organometallics, Vol. 27, No. 4, 2008
PetroVic et al.
and br (broad, for unresolved signals). Elemental analysis (C, H,
N) and IR spectroscopy analysis were performed on a Elementar
Vario EL III CHNS elemental analyzer and on a Bruker Vertex
70. Mass spectrometry was performed with a Finnigan MAT 90
device. UV–vis spectra were recorded on a Varian Cary 50 device.
Cyclic voltammetric traces have been measured on a Metrohm
µAutolab type III device; complex concentrations were 1.0 mM in
acetonitrile, (TBA)PF6 (0.1 M), scan rate 0.1 V/s, T ) 25 °C, against
a Ag/AgCl reference electrode (3 M KCl) and using a Pt working
electrode. The ferrocene/ferrocenium couple displays a redox
potential E° ) +476 mV; the values are referenced to E°(Fc/Fc+)
) 0 mV. FcPF6 was obtained from Aldrich and used as received.
BL , ,
iPr 14 BLMe 14 and [(η7-C7H7)Mo(CH3CN)3]X (X ) BF4, PF6)24
were prepared according to literature procedures.
[(η7-C7H7)Mo(BLiPr)]BF4, [1]BF4. A solution of BLiPr (112.6
mg, 0.27 mmol) in acetonitrile (10 mL) was added dropwise to a
deep red solution of [(η7-C7H7)Mo(CH3CN)3]BF4 (102 mg, 0.257
mmol) in CH3CN (5 mL) at room temperature. During the addition
of the ligand, the reaction mixture turned green, and the solution
was stirred for 16 h. The volume of CH3CN was reduced to 5 mL,
and the product was precipitated with a mixture of Et2O and
n-hexane (20 mL/20 mL). Filtration, washing with n-hexane (2 ×
10 mL), and drying in vacuo afforded the product as a light green
Figure 6. ORTEP drawing of the cation in [2(CNXy)]PF6 with
thermal displacement parameters drawn at 50% probability. Selected
bond lengths [Å] and angles [deg]: Mo-N1 2.252(3), Mo-N2
2.189(3), Mo-C24 2.077(3), Mo-C(Cht) 2.271(4)–2.324(4), N1-C3
1.326(4), N2-C10 1.327(4), C24-N7 1.172(5); N1-Mo-N2
76.7(1), Mo-C24-N7 173.5(3), C24-N7-C25 174.3(4).
1
solid. Yield: 158 mg (89%). H NMR (200 MHz, acetone-d6): δ
atom, and the isocyanide carbon atom. The molybdenum–
carbon(isonitrile) bonds are short [Mo-C32 2.096(2) and
Mo-C24 2.077(3) Å], confirming strong metal-to-ligand π-back-
donation. For instance, these distances are considerably shorter
than the corresponding distance of Mo-C 2.135(3) Å observed
for the only related structurally characterized isocyanide com-
plex [(η7-C7H7)Mo(N,N′)(CNtBu)]BF4 · CH2Cl2 (N,N′ ) chiral
iminopyridine ligand).21b As expected, cyclic voltammetric
studies on [2(CNXy)]PF6 in acetonitrile afford a considerably
more positive redox potential of E° ) -941 mV versus the
Fc/Fc+ couple in comparison with that of [2]PF6 (E° ) -1.138
V, Vide supra), which can be ascribed to the better π-acceptor
properties of the isocyanide ligand compared to CH3CN.
Nevertheless, the redox potential of this isocyanide complex is
still far more negative than the potentials reported for other 18-
electron cycloheptatrienyl-molybdenum isocyanide complexes,
e.g., [(η7-C7H7)Mo(bipy)(CNtBu)]PF6,20d giving further evi-
dence for the extraordinary electron richness and donor proper-
ties of the BLMe ligand.
5.03 (2 H, sept, NCH), 4.90 (7 H, br, C7H7), 2.50 (12 H, s, CCH3),
2.18 (4 H, s, CH2), 1.87 (12 H, d, CHCH3) 1.42 (12 H, d, CHCH3).
13C NMR (50.32 MHz, acetone-d6): δ 156.6 (NCN), 121.5
(NCCH3), 81.4 (C7H7), 55.4 (CH2), 49.4 (NCH), 22.8 (CHCH3),
22.5 (CHCH3), 10.4 (CCH3). Anal. Calc for C31H51N6MoBF4: C,
53.92; H, 7.44; N, 12.17. Found: C, 53.58; H, 7.56; N, 12.31.
UV–vis (CH3CN): λmax/nm 395 (ꢀ, dm3 mol-1 cm-1 4900), 450sh
(1130) and 748 (259).
[(η7-C7H7)Mo(BLMe)]BF4, [2]BF4. A solution of BLMe (150
mg, 0.493 mmol) in acetonitrile (10 mL) was added dropwise to a
deep red solution of [(η7-C7H7)Mo(CH3CN)3]BF4 (186 mg, 0.469
mmol) in CH3CN (5 mL) at room temperature. During the addition
of the ligand, the reaction mixture turned green, and the solution
was stirred for 4 h. The volume of CH3CN was reduced to 5 mL,
and the product was precipitated with a mixture of Et2O and
n-hexane (20 mL/20 mL). Filtration, washing with Et2O (2 × 10
mL), and drying in Vacuo afforded the product as a light green
1
solid. Yield: 221 mg (81%). H NMR (200 MHz, acetone-d6): δ
4.83 (7 H, br, C7H7), 3.69 (12 H, s, NCH3), 2.42 (12 H, s, CCH3),
2.23 (4 H, s, CH2). 13C NMR (50.32 MHz, acetone-d6): δ 158.4
(NCN), 121.5 (NCCH3), 82.8 (C7H7), 53.9 (CH2), 31.9 (NCH3),
9.7 (CCH3). Anal. Calc for C23H35N6MoBF4: C, 47.77; H, 6.10; N,
14.53%. Found: C, 47.10; H, 5.95; N, 14.53.
Conclusions
In this contribution, we have presented 16-electron cyclo-
heptatrienyl-molybdenum half-sandwich complexes of unusual
stability, which can be ascribed to the strong π-basic nature of
the bis(imidazolin-2-imine) ligands. Despite the presence of
π-stabilization, these complexes still exhibit a strong propensity
to bind σ-donor/π-acceptor ligands such as isocyanides. This
behavior makes these highly stabilized yet reactive 16-electron
complexes suitable systems for the activation of small molecules
such as N2 and H2 and promising candidates for catalytic
applications. These investigations are currently in progress in
our laboratory.
[(η7-C7H7)Mo(BLMe)]PF6, [2]PF6. A solution of BLMe (131 mg,
0.43 mmol) in acetonitrile (10 mL) was added dropwise to a deep
red solution of [(η7-C7H7)Mo(CH3CN)3]PF6 (186 mg, 0.409 mmol)
in CH3CN (5 mL) at room temperature. During the addition of the
ligand, the reaction mixture turned green and the solution was stirred
for 4 h. The volume of CH3CN was reduced to 5 mL, and the
product was precipitated with a mixture of Et2O and n-hexane (20
mL/20 mL). Filtration, washing with Et2O (2 × 10 mL), and drying
in Vacuo afforded the product as a light green solid. Yield: 232 mg
1
(89%). H NMR (200 MHz, acetone-d6): δ 4.83 (7 H, br, C7H7),
3.69 (12 H, s, NCH3), 2.42 (12 H, s, CCH3), 2.23 (4 H, s, CH2).
13C NMR (50.32 MHz, acetone-d6): δ 158.4 (NCN), 121.5
(NCCH3), 82.8 (C7H7), 53.9 (CH2), 31.9 (NCH3), 9.7 (CCH3). Anal.
Calc for C23H35N6MoPF6: C, 43.40; H, 5.54; N, 13.20. Found: C,
42.50; H, 5.47; N, 13.27. UV–vis (CH3CN): λmax/nm 392 (ꢀ, dm3
mol-1 cm-1 5160), 440sh (1400) and 740 (217).
Experimental Section
General Methods. All operations were performed in an atmo-
sphere of dry argon by using Schlenk and vacuum techniques. All
solvents were purified by standard methods and distilled prior to
use. 1H and 13C NMR spectra were recorded on Bruker DPX 200,
Bruker DPX 400, and Bruker AV 600 devices. The chemical shifts
are given in ppm relative to TMS. The spin coupling patterns are
indicated as s (singlet), d (doublet), m (multiplet), sept (septet),
[(η7-C7H7)Mo(BLMe)(CH3CN)][BF4]2, [3][BF4]2. A solution of
tropylium tetrafluoroborate, (C7H7)BF4 (15.4 mg, 0.0865 mmol),
in THF/acetonitrile (1 mL/2 mL) was added dropwise to a green
solution of [(η7-C7H7)Mo(BLMe)]BF4 (50 mg, 0.0865 mmol) in

16-Electron Cycloheptatrienyl-Molybdenum(0) Complexes
Organometallics, Vol. 27, No. 4, 2008 783
Table 1. Summary of Crystallographic Data for [1]BF4 · 2CH3CN, [2]BF4 · CH3CN, [3](PF6)2 · CH3CN, [3](BF4)2, [1(CNXy)]BF4 · C3D6O, and
[2(CNXy)]PF6
[1]BF4 · 2CH3CN
[2]BF4 · CH3CN
[3][PF6]2 · CH3CN
[3][BF4]2
[1(CNXy)]BF4 · C3D6O
[2(CNXy)]PF6
formula
M
cryst size (mm)
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
C35H57BF4MoN8
772.64
C25H38BF4MoN7
619.37
C27H41F12MoN8P2
863.56
C25H38F8MoN7B2
706.18
C43H60BD6F4Mo N7O C32H44F6MoN7P
885.82
767.65
0.25 × 0.17 × 0.08
monoclinic
P21/n
0.24 × 0.20 × 0.19 0.24 × 0.19 × 0.15 0.16 × 0.12 × 0.07 0.30 × 0.15 × 0.08 0.37 × 0.29 × 0.28
monoclinic
P21/c
10.8124(10)
31.337(3)
12.0187(12)
90.00
107.634(3)
90.00
3880.9(6)
4
monoclinic
P21/c
14.507(3)
8.2464(16)
23.934(4)
90.00
98.753(4)
90.00
2829.9(9)
4
triclinic
orthorhombic
Pbca
20.015(2)
14.9142(14)
20.075(2)
90.00
90.00
90.00
5992.5(10)
8
1.565
monoclinic
P21/n
j
P1
11.5973(14)
12.5314(14)
14.8528(18)
65.387(3)
70.003(3)
66.079(3)
1754.5(4)
2
1.635
0.562
878
133
15.5178(15)
13.3193(14)
21.697(2)
90.00
8.0090(6)
27.588(2)
15.9818(12)
90.00
95.982(6)
90.00
100.119(2)
90.00
4460.0(8)
4
3476.3(4)
4
Z
Dcalcd (g/cm3)
1.322
0.392
1624
133
1.454
0.517
1280
133
1.319
1.467
µ (mm-1
F(000)
T/K
)
0.517
2888
133
0.351
0.489
1856
1584
133
133
2θmax
56
56
50036
6981
1.011
0.0404
0.1140
56
29964
8671
1.037
0.0434
0.1027
56
98316
7433
1.059
0.0366
0.0990
56
75034
11 069
1.097
0.0346
0.0921
56
64559
8639
1.032
0.0528
0.1428
no. of reflns measd 70496
no. of indep reflns 9628
GOF
R1
wR2
1.091
0.0532
0.1567
THF/acetonitrile (2 mL/2 mL) at room temperature. During the
addition, the reaction mixture turned orange, and the solution was
stirred for 2 h. The product was precipitated with Et2O (40 mL),
filtered, washed with Et2O (2 × 10 mL), and dried in Vacuo. Yield:
56 mg (92%). Anal. Calc for C25H38N7MoB2F8: C, 42.52; H, 5.42;
N, 13.88. Found: C, 41.87; H, 5.56; N, 13.23.
into acetone solution over several days yielded red crystals, which
were isolated, washed with additional Et2O (2 × 5 mL), and dried
in Vacuo. Yield: 42 mg (90%). 1H NMR (200 MHz, acetone-d6): δ
7.28–7.15 (3 H, m, Xy-H), 4.66 (7 H, br, C7H7), 3.69 (12 H, s,
NCH3), 2.80 (4 H, br, CH2), 2.48 (12 H, s, CCH3), 2.24 (6 H, s,
Xy-CH3). 13C NMR (50.32 MHz, acetone-d6): δ 160.6 (NCN),
134.9 (o-C) 130.0 (m-C), 129.8 (p-C), 120.4 (NCCH3), 88.2 (C7H7),
57.2 (CH2), 33.0 (NCH3), 20.0 (o-CH3), 9.9 (CCH3). Anal. Calc
for C32H44N7MoPF6: C, 50.07; H, 5.78; N, 12.77. Found: C, 49.98;
H, 6.27; N, 12.65. IR (ATR/ cm-1): 2030 (νCt N).
Single-Crystal X-ray Structure Determinations. Numerical
details are presented in Table 1. All X-ray data were collected with
monochromated Mo KR radiation (λ ) 0.71073 Å) on a Bruker
SMART 1000 CCD area detector. Absorption corrections were
based on multiple scans (program SADABS). The structures were
solved by direct methods and refined anisotropically by full-matrix
least-squares on F2 (Program SHELXTL: Prof. G. M. Sheldrick,
Univ. of Göttingen). H atoms were included using a riding model
(except methyl groups: refined as rigid groups). Special features
of refinement: In [1]BF4 · 2CH3CN, the acetonitriles of solvation
could not be refined adequately and were therefore removed using
the program SQUEEZE (A. L. Spek, University of Utrecht,
Netherlands). For [1]BF4 · 2CH3CN and [2]BF4 · CH3CN, the eth-
ylene bridge C1-C2 is disordered over two positions, which were
refined isotropically. Hydrogen positions of solvent methyls (ac-
etonitrile, acetone) should be regarded as tentative.
[(η7-C7H7)Mo(BLMe)(CH3CN)][PF6]2 · CH3CN, [3][PF6]2 · CH3CN.
A deep blue solution of FcPF6 (42 mg, 0.126 mmol) in THF/
acetonitrile (5 mL/5 mL) was added dropwise to a green solution
of [(η7-C7H7)Mo(BLMe)]PF6 (80 mg, 0.126 mmol) in THF/
acetonitrile (5 mL/5 mL) at room temperature. During the addition,
the reaction mixture turned orange, and the solution was stirred
for 24 h. Diffusion of Et2O afforded an orange crystalline material
that was washed with Et2O (2 × 10 mL) and dried in Vacuo. Yield:
91 mg (84%). Anal. Calc for C27H41N8MoP2F12: C, 37.55; H, 4.79;
N, 12.98. Found: C, 37.17; H, 5.17; N, 12.83.
[1(CNXy)]BF4. 2,6-Dimethylphenyl isocyanide (9.5 mg, 72.4
µmol) and [(η7-C7H7)Mo(BLiPr)]BF4 (50 mg, 72.4 µmol) were
mixed together and dissolved in acetone (1 mL). Diffusion of Et2O
into acetone solution over several days yielded red crystals, which
were isolated, washed with additional Et2O (2 × 5 mL), and dried
in Vacuo. Yield: 51 mg (86%). 1H NMR (400 MHz, acetone-d6): δ
7.30–7.27 (3 H, mult, Xy-H), 4.97 (4 H, sept, NCH), 4.66 (7 H, s,
C7H7), 2.93 (4 H, br, CH2), 2.56 (6 H, s, Xy-CH3), 2.39 (12 H, s,
CCH3), 1.75 (12 H, br, CHCH3), 1.40 (12 H, br, CHCH3). 13C NMR
(100.62 MHz, acetone-d6): δ 159.0 (NCN), 134.8 (o-C) 129.5 (p-
C), 129.3 (m-C), 121.2 (NCCH3), 85.2 (C7H7), 57.8 (CH2), 48.9
(NCH), 22.9 (CHCH3), 22.2 (CHCH3), 19.4 (o-CH3), 10.4 (CCH3).
Anal. Calc for C40H60N7MoBF4: C, 58.47; H, 7.36; N, 11.93. Found:
C, 57.48; H, 7.63; N, 11.24. IR (ATR/ cm-1): 2041 (νCt N).
[2(CNXy)]PF6. 2,6-Dimethylphenyl isocyanide (8.0 mg, 61
µmol) and [(η7-C7H7)Mo(BLMe)]PF6 (38.8 mg, 61 µmol) were
mixed together and dissolved in acetone (1 mL). Diffusion of Et2O
Supporting Information Available: Crystal data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM700953U