Ruthenium(II) anilido complex containing a bisphosphine pincer ligand: Reversible formation of amidinate ligands via intramolecular C-N bond formation

Article abstract of DOI:10.1021/om0497543

The reaction of the octahedral anilido complex (PCP)Ru(CO)(NHPh)(PMe ₃) with acetonitrile produces the amidinate complex (PCP)Ru(CO){N(H)C(Me)N-(Ph)}. Mechanistic studies indicate that the reaction proceeds through coordination of the nitrile to the Ru(II) metal center, followed by intramolecular nucleophilic addition of the amido ligand.

Full text of DOI:10.1021/om0497543

3094
Organometallics 2004, 23, 3094-3097
Ru th en iu m (II) An ilid o Com p lex Con ta in in g a
Bisp h osp h in e P in cer Liga n d : Rever sible F or m a tion of
Am id in a te Liga n d s via In tr a m olecu la r C-N Bon d
F or m a tion
J ubo Zhang, T. Brent Gunnoe,* and Paul D. Boyle
Department of Chemistry, North Carolina State University,
Raleigh, North Carolina 27695-8204
Received April 5, 2004
Summary: The reaction of the octahedral anilido com-
plex (PCP)Ru(CO)(NHPh)(PMe3) with acetonitrile pro-
duces the amidinate complex (PCP)Ru(CO){N(H)C(Me)N-
(Ph)}. Mechanistic studies indicate that the reaction
proceeds through coordination of the nitrile to the
Ru(II) metal center, followed by intramolecular nucleo-
philic addition of the amido ligand.
suggest that such systems might be applied toward
C-N bond-forming reactions.12-14 In such a reaction,
the metal complex would support a nucleophilic amido
moiety as well as serving as a σ-Lewis acid to bind and
activate the substrate toward intramolecular nucleo-
philic addition. In fact, such transformations may be
central to polymerizations of carbodiimides and iso-
cyanates.15-17 The key step of many reactions mediated
by late-transition-metal complexes with nondative ligands
may involve intramolecular nucleophilic additions; how-
ever, only in a few cases are such transformations well
characterized.18,19
The reaction of (PCP)Ru(CO)(Cl) (PCP ) [2,6-(CH2P-
tBu2)2C6H3]) with TMSOTf yields (PCP)Ru(CO)(OTf) (1),
and the addition of [Li][NHPh] to complex 1 allows the
isolation of the anilido complex (PCP)Ru(CO)(NHPh)
(2).20 The direct reaction of (PCP)Ru(CO)(Cl) with [Li]-
[NHPh] does not cleanly produce complex 2. Complex
2 has been characterized by multinuclear NMR and IR
spectroscopy. The IR spectrum of 2 reveals absorptions
at 1901 (νCO) and 3420 cm-1 (νNH), and the 1H NMR
spectrum displays a broad resonance at 4.75 ppm due
to the amido NH resonance. The octahedral complex
(PCP)Ru(CO)(NHPh)(PMe3) (3) can be prepared by
reaction of 2 with trimethylphosphine (Scheme 1).20
Complex 3 can also be prepared by metathesis of (PCP)-
Ru(CO)(PMe3)(Cl) with [Li][NHPh]. On the basis of
phosphorus-carbon coupling constants, the geometry
of 3 is tentatively assigned as having CO and PMe3
ligands in a cis disposition. The synthesis and reactivity
of a closely related (PCP)IrIII anilido complex have been
reported.21
Metathesis reactions of carbon-carbon multiple bonds
using transition-metal catalysts have been applied to
the synthesis of polymers as well as small organic
molecules.1-3 The extension of such methods to trans-
formations involving carbon-nitrogen multiple bonds
could allow the development of controlled routes for
C-N bond formation, including the preparation of
polymers that contain heteroatomic functionality.4 Net
metathesis reactions of acyclic imines catalyzed by
transition-metal complexes have been observed; how-
ever, studies of such transformations reveal a high
degree of mechanistic complexity. For example, catalytic
metathesis reactions of acyclic imines using transition-
metal nitrene complexes proceed by multiple pathways,
including the formation of diazametallacycles, the pro-
duction of HCl due to amine impurities, amido-mediated
reactions, and reaction of amine impurities present in
imines with metal imido ligands.5-8 The reversible
addition of acetonitrile to a Ti(IV) imido complex has
been reported.9 In addition, we have reported reactions
of ruthenium benzylidene complexes with acyclic imines
that occur via imine to enamine tautomerization.10 This
mechanistic complexity suggests that controlled trans-
formations of unsaturated C-N bonds may require
alternatives to transition-metal-catalyzed Chauvin-type
mechanisms.11 Recent reports of highly nucleophilic
amido complexes that possess high d-electron counts
Variable-temperature NMR spectroscopy of (PCP)Ru-
(CO)(NHPh)(PMe3) (3) reveals three fluxional processes
(Chart 1). At low temperature, the 31P NMR spectrum
* To whom correspondence should be addressed. E-mail:
brent_gunnoe@ncsu.edu.
(11) Bell, S. A.; Meyer, T. Y.; Geib, S. J . J . Am. Chem. Soc. 2002,
124, 10698-10705.
(12) Fulton, J . R.; Sklenak, S.; Bouwkamp, M. W.; Bergman, R. G.
J . Am. Chem. Soc. 2002, 124, 4722-4737.
(13) Holland, A. W.; Bergman, R. G. J . Am. Chem. Soc. 2002, 124,
14684-14695.
(14) Conner, D.; J ayaprakash, K. N.; Wells, M. B.; Manzer, S.;
Gunnoe, T. B.; Boyle, P. D. Inorg. Chem. 2003, 42, 4759-4772.
(15) Shibayama, K.; Seidel, S. W.; Novak, B. M. Macromolecules
1997, 30, 3159-3163.
(16) Goodwin, A.; Novak, B. M. Macromolecules 1994, 27, 5520-
5522.
(17) Patten, T. E.; Novak, B. M. J . Am. Chem. Soc. 1996, 118, 1906-
1916.
(18) Hartwig, J . Acc. Chem. Res. 1998, 31, 853-860.
(19) Wolfe, J . P.; Wagaw, S.; Marcoux, J .-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805-818.
(1) Schrock, R. R. Tetrahedron 1999, 55, 8141-8153.
(2) Ivin, K. J .; Mol, J . C. Olefin Metathesis and Metathesis Polym-
erization; Academic Press: San Diego, CA, 1997.
(3) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29.
(4) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000, 100, 1479-1493.
(5) Zuckerman, R. L.; Krska, S. W.; Bergman, R. G. J . Am. Chem.
Soc. 2000, 122, 751-761.
(6) Burland, M. C.; Pontz, T. W.; Meyer, T. Y. Organometallics 2002,
21, 1933-1941.
(7) Bu¨hl, M. J . Mol. Model. 2000, 6, 112-118.
(8) McInnes, J . M.; Blake, A. J .; Mountford, P. J . Chem. Soc., Dalton
Trans. 1998, 3623-3628.
(9) Pugh, S. M.; Tro¨sch, D. J . M.; Wilson, D. J .; Bashall, A.; Cloke,
F. G. N.; Gade, L. H.; Hitchcock, P. T.; McPartlin, M.; Nixon, J . F.;
Mountford, P. Organometallics 2000, 19, 3205-3210.
(10) Zhang, J .; Gunnoe, T. B. Organometallics 2003, 22, 2291-2297.
10.1021/om0497543 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/27/2004

Communications
Sch em e 1. P r ep a r a tion of
Organometallics, Vol. 23, No. 13, 2004 3095
Sch em e 2. Tw o Rou tes for th e F or m a tion of
t
t
(P CP )Ru (CO)(NHP h )(P Me3) (3; P ) Bu 2P )
(P CP )Ru (CO)(N(H)C(Me)NP h ) (4; P ) Bu 2P )
was used to calculate ∆Gq ) 15.3(2) kcal/mol (319 K)
for Namido-Cipso bond rotation. We have previously
reported that rotational barriers for Namido-Cipso of
TpRuL2(NHPh) (L ) P(OMe)3, PMe3) complexes are
between approximately 9.8 and 12.8 kcal/mol.22 Finally,
dissociation of the PMe3 ligand has been observed. The
addition of excess PMe3 to a solution of 3 results in a
single coalesced resonance for free and bound phos-
phine.
Ch a r t 1. Dyn a m ic Beh a vior of
(P CP )Ru (CO)(NHP h )(P Me3) (3) Obser ved by
Va r ia ble-Tem p er a tu r e NMR Sp ectr oscop y (P )
tBu 2P )
The reaction of (PCP)Ru(CO)(NHPh)(PMe3) (3) with
acetonitrile produces the RuII amidinate complex (PCP)-
Ru(CO){N(H)C(Me)N(Ph)} (4) (Scheme 2).23 Although
isolated yields are low (∼35%), NMR tube reactions
reveal nearly quantitative transformation. Complex 4
can also be prepared by the reaction of (PCP)Ru(CO)-
(Cl) and [Li][NHPh] in acetonitrile. Although group 1
displays three unique resonances (two doublet of dou-
blets due to the PCP ligand and a triplet due to PMe3),
indicating that the anilido ligand has a preferred
amides are known to react with some nitriles,24
a
13C
t
NMR spectrum of a mixture of acetonitrile and [Li]-
[NHPh] does not yield resonances consistent with the
formation of amidinate. Complex 4 is characterized by
νCO 1898 cm-1 and νNH 3360 cm-1 in its IR spectrum.
orientation in which the Bu2P moieties are chemically
inequivalent. As the temperature is increased, the
resonances due to the PCP ligand broaden and coalesce
(271 K) into a single time-averaged resonance, and the
1
t
The H NMR spectrum of 4 displays resonances at 3.84
symmetry equivalence of the Bu2P fragments at el-
ppm (broad singlet) and 1.58 ppm due to the N-H and
amidinate methyl groups, respectively. A solid-state
X-ray diffraction study of 4 confirms its identity with
one of the tBu groups exhibiting an orientational
disorder (Figure 1). The reactions of amido complexes
of titanium, zirconium, and tantalum with acetonitrile
have been reported to yield amidinate products; how-
ever, the metal products were only characterized by IR
spectroscopy and elemental analysis.25 Although mecha-
nistic studies were not reported, reactions of titanium
and tungsten amido ligands with nitriles have been
disclosed.26,27
evated temperatures is likely due to rapid rotation about
the Ru-Namido bond and inversion at N (∆Gq ) 11.2(2)
kcal/mol at 271 K). In addition, hindered rotation of the
phenyl ring is indicated by five distinct resonances due
to the anilido phenyl at room temperature. As the
temperature is increased, the resonances due to ortho
and meta protons broaden and coalesce. The coalescence
point (319 K) of the resonances due to the ortho protons
(20) Complex 2: to a 20 mL THF solution of 1 (0.2400 g, 0.357 mmol)
was added 2 equiv of [Li][NHPh], and the mixture was stirred for
approximately 30. The volatiles were removed, and the residue was
extracted with 10 mL of benzene. After filtration, the volatiles were
removed from the filtrate. The resulting dark green solid was washed
with 10 mL of pentane and dried in vacuo (0.1100 g, 50%). IR (THF
(22) Conner, D.; J ayaprakash, K. N.; Gunnoe, T. B.; Boyle, P. D.
Inorg. Chem. 2002, 41, 3042-3049.
solution): νCO 1901 cm-1, νNH 3420 cm-1
.
1H NMR (CD2Cl2, δ): 7.09
3
(2H, phenyl, d, J HH ) 7 Hz), 6.93 (2H, phenyl, t, J HH ) 7 Hz), 6.86
(1H, phenyl, t, J HH ) 7 Hz), 6.70 (2H, phenyl, d, J HH ) 7 Hz), 6.36
(1H, phenyl, t, J HH ) 7 Hz), 4.75 (1H, NH, br s), 3.41 (4H, PCP CH2,
m), 1.42 (18H, PCP CH3, vt, N ) 12 Hz), 1.14 (18H, PCP CH3, vt, N )
12 Hz). Additional data are presented in the Supporting Information.
Complex 3: (PCP)Ru(CO)(Cl) (0.3540 g, 0.56 mmol) in 40 mL of
benzene was added to approximately 0.3 mL of PMe3. Two equivalents
of [Li][NHPh] (∼0.110 g) was added, and the mixture was stirred for
∼12 h. The solution was filtered, and the filtrate was dried under
reduced pressure. The resulting residue was washed with 3 × 10 mL
of pentane to yield a yellow solid that was dried under vacuum (0.1500
g, 40%). IR (benzene solution): νCO 1904 cm-1, νNH 3342 cm-1. 1H NMR
(toluene-d8, δ): 7.20 (4H, phenyl, overlapping multiplets), 7.01 (1H,
(23) Complex 4: a benzene solution (50 mL) of (PCP)Ru(CO)(Cl)
(0.5600 g, 1.0 mmol) was added to excess PMe3 (0.45 mL, 5.0 mmol).
To the resulting yellow solution was added 1.5 equiv of [Li][NHPh]
(∼0.1500 g). After the mixture was stirred for 1 h, 10 equiv of CH3CN
(0.52 mL) was added, and the mixture was stirred for 24 h. The solution
was filtered, and the volatiles were removed from the filtrate. Extrac-
tion using 50 mL of hexanes was followed by filtration. The hexanes
filtrate was dried under reduced pressure, and the resulting solid was
washed with 10 mL of CH3CN to yield a light green powder (0.2300 g,
35%). IR (THF solution): νCO 1898 cm-1, νNH 3360 cm-1. 1H NMR (C6D6,
3
δ): 7.27 (2H, amidinate phenyl, t, J HH ) 8 Hz), 7.17 (2H, amidinate
3
phenyl, d, J HH ) 8 Hz), 7.01 (3H, PCP phenyl, br m), 6.84 (1H,
3
amidinate phenyl, t, J HH ) 8 Hz), 3.84 (1H, NH, br s), 3.20 (4H, PCP
amido phenyl, br t, 3J HH ) 7 Hz), 6.36 (1H, amido phenyl, br t, 3J HH
)
CH2, m), 1.58 (3H, CH3CN, s), 1.18 (18H, PCP CH3, vt, N ) 12 Hz),
1.04 (18H, PCP CH3, vt, N ) 12 Hz). Additional data are presented in
the Supporting Information.
3
7 Hz), 6.19 (1H, amido phenyl ortho, br d, J HH ) 7 Hz), 5.95 (1H,
3
amido phenyl ortho, br d, J HH ) 7 Hz), 3.53 (2H, PCP CH2, m), 3.30
(2H, PCP CH2, m), 1.71 (9H, PMe3, br s, at 283 K resonates as a doublet
(24) Edelmann, F. T. Coord. Chem. Rev. 1994, 137, 403-481.
(25) Bradley, D. C.; Ganorkar, M. C. Chem. Ind. 1968, 44, 1521-
1522.
(26) Broder, C. K.; Goeta, A. E.; Howard, J . A. K.; Hughes, A. K.;
J ohnson, A. L.; Malget, J . M.; Wade, K. J . Chem. Soc., Dalton Trans.
2000, 3526-3533.
2
with J PC ) 5 Hz), 1.21 (18H, PCP CH3, br vt), 1.10 (18H, PCP CH3,
br vt). Additional data are presented in the Supporting Information.
(21) Kanzelberger, M.; Zhang, X.; Emge, T. J .; Goldman, A. S.; Zhao,
J .; Incarvito, C.; Hartwig, J . F. J . Am. Chem. Soc. 2003, 125, 13644-
13645.

3096 Organometallics, Vol. 23, No. 13, 2004
Communications
F igu r e 2. Plot of (kobs
)
-1 versus equivalents of PMe3 (based
on complex 3) for the conversion of 3 to 4.
F igu r e 1. ORTEP (50% probability) of (PCP)Ru(CO)-
{N(H)C(Me)N(Ph)} (4). Bond lengths and angles can be
found in the Supporting Information.
F igu r e 3. Plot of kobs versus concentration of CH3CN for
the conversion of (PCP)Ru(CO)(PMe3)(NHPh) (3) to (PCP)-
Ru(CO){N(H)C(Me)N(Ph)} (4).
Sch em e 3. P r op osed Mech a n ism for th e
F or m a tion of (P CP )Ru (CO)(N(H)C(Me)NP h ) (4)
fr om th e Rea ction of (P CP )Ru (CO)(P Me3)(NHP h )
kinetic data provide evidence for the coordination of
acetonitrile and intramolecular C-N formation, since
a pathway involving intermolecular nucleophilic attack
of the amido ligand on uncoordinated acetonitrile is
anticipated to exhibit a reaction rate that is independent
of PMe3 concentration. At lower temperatures an inter-
molecular pathway may become viable, similar to a
previously reported intermolecular addition of a Ru(II)
anilido ligand to CO2.28 Although evidence has been
obtained that the N-C bond formation step is intra-
molecular, the nature of the proton-transfer step that
is required to complete the reaction has not been
discerned.
t
(3) w ith Aceton itr ile (P ) Bu 2P )
The C-N bond formation between the anilido ligand
and NCMe that produces complex 4 is reversible. For
example, the reaction of a pentane solution of complex
4 with excess pentafluorobenzonitrile results in the
release of acetonitrile and the formation of (PCP)Ru-
(CO){N(H)C(C6F5)N(Ph)} (5) (eq 2). Complex 5 has been
A viable pathway for the formation of 4 from the
reaction of complex 3 and acetonitrile includes initial
formation of (PCP)Ru(CO)(NCMe)(NHPh) via PMe3/
NCMe ligand exchange followed by intramolecular
nucleophilic attack of the amido nitrogen on the bound
nitrile and proton transfer (Scheme 3). The anticipated
rate law for this mechanism is shown in eq 1. Kinetic
k1k2[3][NCMe]
rate )
(1)
k-1[PMe3] + k2[NCMe]
1
characterized by H, 13C, and 31P NMR spectroscopy as
well as IR spectroscopy. Salient features in the 1H NMR
spectrum of 5 include a downfield chemical shift for the
NH resonance (4.82 ppm) compared with 4. In analogy
to observations for the formation of 4, the production of
complex 5 likely proceeds through the formation of
(PCP)Ru(CO)(NHPh)(NCMe) followed by NCMe/NCC6F5
exchange and intramolecular C-N bond formation.
A series of recent reports has revealed that octahedral
RuII amido complexes exhibit reactivity consistent with
highly basic amido ligands.12-14 The reaction of (PCP)-
experiments for the conversion of 3 and acetonitrile to
the amidinate complex 4 under pseudo-first-order condi-
tions indicate an inverse first-order dependence on the
concentration of PMe3 (Figure 2). In addition, a plot of
kobs versus the concentration of NCMe exhibits a first-
order dependence on the concentration of NCMe at low
concentrations, while saturation kinetics are apparent
at higher concentrations (Figure 3). These results are
consistent with the rate law depicted in eq 1. Thus, the
(27) Batsanov, A. S.; Goeta, A. E.; Howard, J . A. K.; Hughes, A. K.;
J ohnson, A. L.; Wade, K. J . Chem. Soc., Dalton Trans. 2001, 1210-
1217.
(28) Hartwig, J . F.; Bergman, R. G.; Andersen, R. A. J . Am. Chem.
Soc. 1991, 113, 6499-6508.

Communications
Organometallics, Vol. 23, No. 13, 2004 3097
Ru(CO)(NHPh) with nitriles demonstrates that the
nucleophilic nature of such amido ligands can be
combined with the activating ability of a Lewis acidic
site on the metal center to achieve controlled C-N bond-
forming reactions. The “double activation” ability (i.e.,
Lewis acidic metal in tandem with nucleophilic nonda-
tive ligand) of other complexes has been suggested to
be central to related reactions. For example, the collec-
tive effect of Lewis acidic activation of cobalt-coordinated
nitrile and intramolecular nucleophilic attack by hy-
droxide ligand has been shown to increase the rate of
nitrile hydration by approximately 4 orders of magni-
tude relative to the metal center serving only as a Lewis
acid,29 and Tyler et al. have suggested a similar mech-
anism for a molybdenum catalyst.30 Furthermore, a
recent report of catalytic transamidation using metal
amido systems may implicate a dual role of the metal
amido intermediates.31 Work reported herein provides
direct evidence that C-N bond formation can be pro-
moted by “doubly activating” metal centers.
Ack n ow led gm en t is made to the National Science
Foundation (CAREER Award; CHE 0238167) and North
Carolina State University for support of this research.
We thank Dr. Peter White (UNC Chapel Hill) for X-ray
data collection.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails and complete tables of crystal data, collection and
refinement data, atomic coordinates, bond distances and
angles, and anisotropic displacement coefficients for (PCP)-
Ru(CO){N(H)C(Me)N(Ph)} (4). This material is available free
of charge via the Internet at http://pubs.acs.org.
OM0497543
(29) Kian, J . H.; Britten, J .; Chin, J . J . Am. Chem. Soc. 1993, 115,
3618-3622.
(30) Breno, K. L.; Pluth, M. D.; Tyler, D. R. Organometallics 2003,
22, 1203-1211.
(31) Eldred, S. E.; Stone, D. A.; Gellman, S. H.; Stahl, S. S. J . Am.
Chem. Soc. 2003, 125, 3422-3423.