Reactivity of ruthenium(II) and copper(I) complexes that possess anionic heteroatomic ligands: Synthetic exploitation of nucleophilicity and basicity of amido, hydroxo, alkoxo, and aryloxo ligands for the activation of substrates that possess polar bonds as well as nonpolar C-H and H-H bonds

Article abstract of DOI:10.1002/ejic.200601072

The preparation and reactivity of late transition-metal complexes in low oxidation states with nondative-heteroatomic ligands (e.g., amido, hydroxo, alkoxo, and aryloxo ligands) are described. For such complexes the disruption of ligand-to-metal π-donation because of a filled dπ manifold can enhance the nucleophilic and/or basic reactivity at the nondative-heteroatomic ligand relative to transition-metal complexes in high oxidation states. The chemistry of five- and six-coordinate Ru complexes with amido, hydroxo, methoxo, and aryloxo ligands is described including Bronsted acid/ base reactions, coordination, and activation of polar substrates toward N-C bond forming reactions and metal-mediated activation of dihydrogen and C-H bonds. The impact of ancillary ligands and Ru oxidation states (Ru^II versus Ru ^III) are discussed. In addition, the preparation and reactivity of well-defined monomeric two- and three-coordinate Cu complexes with amido, alkoxo, hydroxo, and aryloxo ligands are presented including examples of reactions to probe fundamental reactivity patterns as well as catalytic addition of N-H and O-H bonds across C=C bonds of olefins. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200601072

MICROREVIEW
DOI: 10.1002/ejic.200601072
Reactivity of Ruthenium(II) and Copper(I) Complexes that Possess Anionic
Heteroatomic Ligands: Synthetic Exploitation of Nucleophilicity and Basicity
of Amido, Hydroxo, Alkoxo, and Aryloxo Ligands for the Activation of
Substrates that Possess Polar Bonds as well as Nonpolar C–H and H–H Bonds
T. Brent Gunnoe*[a]
Keywords: Nondative heteroatomic ligands / Ruthenium / Copper / C–H Activation
The preparation and reactivity of late transition-metal com-
plexes in low oxidation states with nondative-heteroatomic
ligands (e.g., amido, hydroxo, alkoxo, and aryloxo ligands)
are described. For such complexes the disruption of ligand-
to-metal π-donation because of a filled dπ manifold can en-
hance the nucleophilic and/or basic reactivity at the non-
dative-heteroatomic ligand relative to transition-metal com-
plexes in high oxidation states. The chemistry of five- and
six-coordinate Ru complexes with amido, hydroxo, methoxo,
and aryloxo ligands is described including Brønsted acid/
base reactions, coordination, and activation of polar sub-
strates toward N–C bond forming reactions and metal-medi-
ated activation of dihydrogen and C–H bonds. The impact of
ancillary ligands and Ru oxidation states (RuII versus RuIII)
are discussed. In addition, the preparation and reactivity of
well-defined monomeric two- and three-coordinate Cu com-
plexes with amido, alkoxo, hydroxo, and aryloxo ligands are
presented including examples of reactions to probe funda-
mental reactivity patterns as well as catalytic addition of N–
H and O–H bonds across C=C bonds of olefins.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
of late transition-metal complexes in low oxidation states
with formally anionic heteroatomic ligands, our group has
been studying the synthesis and reactivity of RuII and CuI
systems. Herein, the scope of complexes accessed and their
reactivity are reviewed with a focus on the development of
new stoichiometric and catalytic reactions that are based on
the nucleophilic/basic character of the nondative ligands.
Bonding between a transition metal and an anionic het-
eroatomic ligand results from a metal–ligand σ bond and
one ligand–metal π-interaction. Thus, when the metal cen-
ter possesses one or more empty orbitals of π-symmetry
metal–ligand multiple bonds result.[10] On the basis of
metal–X (X = OR, NR2 or SR ligand) bond polarity it
is anticipated that nondative-heteroatomic ligands should
exhibit inherent nucleophilicity and basicity; however, li-
gand-to-metal π-donation can serve to delocalize electron
density from the heteroatomic ligand to the metal center.
As a result, the electron density centered on the heteroat-
Introduction
Nondative-heteroatomic fragments are a common class
of ligand that formally bear anionic, dianionic, or trianionic
charge. Such ligands based on nitrogen, oxygen, or sulfur,
including amido, alk-/aryl-/hydr-oxo, alkyl-/aryl-/hydro-sul-
fido, oxide, sulfide, imido, and nitrido, have been utilized as
nonreactive ancillary supports for reactive transition-metal
systems as well as for active participation in “group trans-
fer” and other catalytic processes. Prominent examples of
the latter include olefin epoxidation and aziridination, ole-
fin dihydroxylation, and polymerization sequences.[1–9] As
part of our efforts to understand and develop the reactivity
[a] Department of Chemistry,
North Carolina State University, Raleigh, NC 27695-8204,
USA
Fax: +1-919-515-8909
E-mail: brent_gunnoe@ncsu.edu
T. Brent Gunnoe was born in Charleston, West Virginia, USA in 1971. He received his B. A. in chemistry from West
Virginia University in 1993 where he worked on early transition-metal aryloxo chemistry with Dr. Jeffrey Petersen. Under
the supervision of Dr. Joseph L. Templeton, he received a PhD in 1997 from the University of North Carolina at Chapel
Hill with research efforts focused on chiral group VI organometallic complexes. After working with Dr. W. Dean Harman
as a postdoctoral researcher at the University of Virginia on the development of metal-mediated dearomatization sequences
based on chiral ReI systems, he began as Assistant Professor at North Carolina State University in 1999. His current
title is Associate Professor of Chemistry with research interests that include organometallic and inorganic chemistry
especially applied to homogeneous catalysis.
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T. B. Gunnoe
MICROREVIEW
omic ligand can be attenuated and, in some cases, the het- omic ligand adjacent to a Lewis acidic metal center pro-
eroatomic ligand is rendered relatively inert. Schrock-type vides opportunities for the coordination and activation of
carbene complexes supported by alkoxo and imido ligands external organic substrates toward controlled bond-break-
represent classic examples of unreactive anionic heteroat- ing and bond-forming reactions (Scheme 2). From a funda-
omic ligands. For these complexes, the formally anionic and mental perspective, we have been studying the impact of
dianionic heteroatomic ligands are reaction inert relative to metal identity, ancillary ligand identity, and the heteroat-
the carbene moiety.[11] High oxidation states for metal cen- omic ligand identity and substituents on reactivity. Herein,
ters tend to facilitate the ligand-to-metal π-bonding and, we summarize results over the past five years and comment
hence, serve to decrease the impact of M–X bond polarity on future prospects.
and ligand-centered electron density and, consequently, to
decrease nucleophilic and/or basic reactivity at the heteroat-
omic ligand. The lack of reaction of TpOsIV(Cl)2NHPh
with HCl (a thermodynamic effect) serves as a striking ex-
ample that potentially illustrates the effect on reactivity of
reduced charge density at the anionic heteroatomic ligand
(due to increased ligand-to-metal π-donation).[12]
The majority of anionic heteroatomic ligands of early or
middle transition elements are coordinated to relatively
Scheme 2. Potential use of Lewis acidic metal centers with the basic
high oxidation state systems. In contrast, late(r) transition
metals, which are more predisposed toward low oxidation
states, often have high d-electron counts that disrupt ligand-
to-metal π-bonding (Scheme 1). Until relatively recently, ex-
amples of anionic heteroatomic ligands coordinated to late
transition-metal systems in low oxidation states were scant
compared with high oxidation state complexes of the early
and middle transition elements.[13–15] Recent efforts have
substantially increased the number of late transition-metal
complexes and revealed a wealth of interesting reactiv-
ity.[15–22] Careful studies of thermodynamic trends and reac-
tivity indicate that the bond dissociation energies (BDEs)
between heteroatom-based ligands and late transition-metal
centers can be substantial, and recent bonding models sug-
gest that these M–L bonds are relatively strong; however,
the disruption of metal-to-ligand π-bonding can afford en-
hanced reactivity by decreasing electron delocalization from
the ligand to the metal.[17]
anionic ligand “X” (X = NR2, OR, SR, etc.) to activate substrates
with polar and nonpolar bonds.
Octahedral TpRuII Systems
Preparation and Characterization
We have used the monoanionic hydridotris(pyrazolyl)bo-
rate (Tp) ligand[39] as a foundation to build octahedral Ru
complexes
with
anionic
heteroatom-based
li-
gands.[24,26,31–36] Our strategy has been to utilize the tem-
plate [TpRu(L)(LЈ)X]n+ (L, LЈ = neutral, two-electron do-
nor ligands, X = anionic heteroatomic ligand; n = 0 or 1)
to access systems with variable ancillary ligands (L/LЈ),
heteroatomic ligands (X), and oxidation states. Two strate-
gies have been used for syntheses: (1) coordination of amine
or alcohols followed by deprotonation and (2) metathesis
of the anionic heteroatomic moiety with Ru–halide or Ru–
triflate linkages (Scheme 3).
Scheme 1. M–X (M = transition metal, X = NR2, OR, SR, etc.)
multiple bonding occurs if dπ orbital(s) is vacant. Filled dπ mani-
fold results in no net π-bond.
Our group has been interested in understanding and ex-
ploiting the reactivity of late transition-metal systems with
highly nucleophilic/basic heteroatom-based ligands includ-
ing amido and alk-/aryl-/hydroxo ligands coordinated to
Scheme 3. Two general routes used for the preparation of
TpRu(L)(LЈ)NHR and TpRu(L)(LЈ)OR systems.
For the RuII oxidation state, the hydroxo and parent
RuII and CuI.[23–38] From the perspective of catalyst devel- amido complexes exhibit upfield chemical shifts between –1
opment, the positioning of a nucleophilic/basic heteroat- and –4 ppm for the N–H/O–H resonances (1H NMR spec-
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Reactivity of RuII and CuI Complexes
MICROREVIEW
troscopy), which is indicative of electron-rich nondative- of electron density from the amido ligand to the aryl sub-
heteroatomic ligands (a hallmark of the disruption of li- stituent.[31] This ρ value is similar to the value of 3.4 ob-
gand-to-metal π-donation for amido and hydroxo ligands). tained for Cp*Ni(PEt3)(NHPh)/NH2Ar reactions.[42] In
Upfield chemical shifts have been observed for related RuII contrast, a study of equilibria for four-coordinate ReIII
–
systems.[26,31,34] For the anilido complexes [TpRu(L)(LЈ)- aryloxo complexes revealed a correlation with Hammett σ
1
NHPh], variable-temperature H NMR spectroscopy pro- parameters with a much smaller ρ value of 0.7.[43]
vides a convenient means to determine the Gibbs free en-
ergy of activation for rotation around the Ru–Namido and
the Cipso–Namido bonds (Scheme 4).[33] Although the data
set is limited there is an inverse correlation between the
Acid–Base Reactions
magnitudes of these two rotational barriers, a result that is
consistent with anticipated Ru–N multiple-bond character
in response to metal-electron density. That is, the presence
of strongly electron-donating and poorly π-acidic ancillary
L/LЈ reduces anilido-to-Ru multiple bonding (via three-cen-
ter, four-electron interactions), which most likely increases
the extent of anilido-to-phenyl π* back-bonding and hence
increases the Namido–Cipso multiple-bond character. Using
this rationale, an inverse correlation between Ru–Namido
and Cipso–Namido bond lengths is anticipated. In fact, com-
paring structural data for the series of octahedral RuII–anil-
ido complexes [TpRu(CO)(PPh3)NHPh],[35] [TpRu{P-
(OMe)3}2NHPh],[33] [(η6-C6Me6)Ru(Ph)(PMe3)NHPh],[40]
and [(PMe3)4Ru(H)NHPh][41] reveals such a trend (Fig-
ure 1). In addition, the equilibrium constants for a series of
reactions of [TpRu(PMe3)2NHPh] with p-substituted ani-
lines (NH2Ar) to form [TpRu(PMe3)2(NHAr)] (Ar = p-sub-
stituted phenyl) and aniline afforded a Hammett plot (using
The coordination of anionic heteroatomic ligands to late
transition-metal complexes in low oxidation states can im-
part substantial basicity/nucleophilicity to the ligand, a fea-
ture that has been attributed to polar metal–ligand bonds
and the disruption of ligand-to-metal π-donation. For ex-
ample, Bergman et al. have demonstrated that trans-
[(DMPE)2Ru(H)NH2] {DMPE = 1,2-bis(dimethylphos-
phanyl)ethane} forms an acid–base equilibrium upon reac-
tion with triphenylmethane (pKa ≈ 31.5 in THF) to form
trans-[(DMPE)2Ru(H)NH3][Ph3C],[44] and related octahe-
dral RuII and FeII parent amido complexes exhibit analo-
gous reactivity.[45,46] Similarly, [TpRu(L)(LЈ)NHR] (R = Ph,
H or tBu) complexes react with weakly acidic C–H bonds.
For example, the anilido complexes [TpRu(L)(LЈ)NHPh]
form acid–base equilibria with malononitrile (pKa ≈ 24 in
THF),[47] which is the least acidic substrate that we have
found to react at room temperature with the anilido com-
plexes (Scheme 5).[33] For [TpRu(L)(LЈ)NHR] (R = H or
tBu), the combination with phenylacetylene (pKa ≈ 28.7 in
DMSO)[48] results in the formation of ion pairs
–
σp ) with ρ = 4.1, which is consistent with the delocalization
[TpRu(L)(LЈ)NH2R][PhC2]
at
room
temperature
(Scheme 5).[31] Thus, the formal substitution of H or tBu
for phenyl as the amido substituent likely increases basicity
by several orders of magnitude. For [TpRu(PMe3)2-
(NH2tBu)][PhC2], heating to 80 °C cleanly forms free
tBuNH2 and [TpRu(PMe3)2(CϵCPh)].
Although no reaction is observed at room temperature,
heating solutions of [TpRu(L)(LЈ)NHPh] and phenylacetyl-
ene to 80 °C forms free aniline and [TpRu(L)(LЈ)(CϵCPh)];
however, detailed mechanistic studies have revealed that
these transformations do not likely proceed through the for-
mation of ion pairs [TpRu(L)(LЈ)(NH2Ph)][PhC2] (as do
the parent amido and tBu-amido systems, see above).[31]
Rather, small and undetectable quantities of [TpRu(L)(LЈ)-
Figure 1. Plot of Ru–Namido and Namido–Cipso bond lengths (Å),
which illustrates an inverse relationship for the series of Ru–anilido
complexes
[TpRu(CO)(PPh3)NHPh],[35]
[TpRu{P(OMe)3}2-
NHPh],[33] [(η6-C6Me6)Ru(Ph)(PMe3)NHPh],[40] and [(PMe3)4Ru-
(H)NHPh].[41]
Scheme 4. Impact of ancillary ligands on Ru–N and Namido–Cipso bond rotational barriers.
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T. B. Gunnoe
MICROREVIEW
2.0ϫ10–5 s–1 at 80 °C). The addition of free PMe3 does not
influence the rate of isomerization of the 1,4-cyclohexa-
diene. We have proposed that these reactions occur by the
acid–base pathway shown in Scheme 7. Although hydrogen
atom abstraction pathways are also possible and have been
observed for late transition-metal complexes with heteroat-
omic ligands in which the metal centers are in relatively
high oxidation states,[49–57] the failure of [TpRu(PMe3)2-
NHPh], which is much less basic than the parent or tBu-
amido complexes, to react with 1,4-cyclohexadiene is con-
sistent with the proposed acid–base pathway. Furthermore,
initial hydrogen atom abstraction would form a cyclohexa-
dienyl radical and [TpRuI(PMe3)2NH2R], which is a 19-
electron RuI complex that is likely to be a high-energy spe-
cies. For [TpRu(PMe3)2NHR] systems, the Ru(II/I) poten-
tial is negative of –2.0 V (vs. NHE), which supports the
suggestion that access to [TpRuI(PMe3)2NH2R] is most
likely a high-energy process. In comparison, an octahedral
FeIII–methoxo complex that converts 1,4-cyclohexadiene to
benzene by a proposed hydrogen atom abstraction pathway
exhibits an Fe(III/II) potential of 0.73 V.[52,53]
Scheme 5. Acid/base equilibria observed for [TpRu(L)(LЈ)NHR] (R
= H, tBu or Ph) systems upon treatment with malononitrile or
phenylacetylene.
OTf] likely catalyze the reaction as shown in Scheme 6. For
the PMe3 system, late in the reaction the putative vinylidene
intermediate is trapped by free aniline to form the Fischer
carbene complex [Tp(PMe3)2Ru=C(CH2Ph)(NHPh)], a re-
action that is confirmed by independent conversion of
[Tp(PMe3)2Ru=C=C(H)Ph] and aniline to [Tp(PMe3)2-
Ru=C(CH2Ph)(NHPh)].
Scheme 7. Proposed pathway for isomerization of 1,4-cyclohexa-
diene to 1,3-cyclohexadiene by [TpRu(PMe3)2NHR] and ultimate
conversion to benzene, [TpRu(PMe3)2H], and amine.
We have recently extended our studies of octahedral RuII
complexes to [TpRu(PMe3)2OH].[24,26,36] Similar to the
amido complexes, the RuII–hydroxo complex is basic. For
example, reactions with MeOH, EtOH, or tBuOH result in
the formation of species whose NMR spectra are consistent
Scheme 6. Proposed pathway for the conversion of [TpRu(PMe3)2-
NHPh] and phenylacetylene to [TpRu(PMe3)2CϵCPh], which is
catalyzed by [TpRu(PMe3)2OTf].
At 80 °C, [TpRu(PMe3)2NHR] (R = H or tBu) com- with the ion pairs [TpRu(PMe3)2OH2][OR] [R = Me, Et,
plexes react with 1,4-cyclohexadiene to ultimately form ben- or tBu; Equation (1)].[36] In contrast to the parent amido
zene, free amine, and [TpRu(PMe3)2H] with a 1:1 molar complexes, [TpRu(PMe3)2OH] does not react with phenyl-
ratio of benzene and Ru–hydride.[31] Preceding the forma- acetylene at room temperature; however, at an elevated tem-
tion of benzene and Ru–hydride, the isomerization of 1,4- perature (80 °C) [TpRu(PMe3)2OH] and phenylacetylene
cyclohexadiene to 1,3-hexadiene is observed (kobs
=
are converted to water and [TpRu(PMe3)2(CϵCPh)]. Sim-
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Reactivity of RuII and CuI Complexes
MICROREVIEW
ilar to the amido complexes, at 85 °C [TpRu(PMe3)2OH] substrates that possess C–H bonds with bond dissociation
and 1,4-cyclohexadiene are converted to benzene, water, energies up to ca. 335 kJ/mol. Using the approximate pKa
and [TpRu(PMe3)2H] with a 1:1 molar ratio of benzene to of [TpRu(PMe3)2(OH2)]+ and the Ru(III/II) potential of
Ru–H [Equation (2)]. Consistent with an acid–base path- [TpRu(PMe3)2OH] a thermodynamic cycle has been used
way (see Scheme 7) and the reduced basicity of the hydroxo to estimate the O–H BDE of [TpRu(PMe3)2OH2]+ to be
complex relative to the parent and tBu-amido complexes, between 343 and 356 kJ/mol, which is consistent with the
the reaction of [TpRu(PMe3)2OH] and 1,4-cyclohexadiene experimental reactivity of the [TpRuIII(PMe3)2OH]+ com-
is only 33% complete after 9 days at 85 °C. In comparison, plex. Furthermore, DFT calculations of the model [(Tab)
[TpRu(PMe3)2NH2] and 1,4-cyclohexadiene produce ben- Ru(PH3)2(OH2)]+ [Tab = tris(azo)borate] reveal that the O–
zene and [TpRu(PMe3)2H] in about 50% conversion after 3 H BDE is 351 kJ/mol.
days at 75 °C.
(1)
(2)
Scheme 8. The conversion of 1,4-cyclohexadiene to benzene by
Single-electron oxidation of [TpRu(PMe3)2OH] yields the
paramagnetic and NMR spectroscopic silent RuIII system
[TpRu(PMe3)2OH]+.[36] For the cationic RuIII–hydroxo
complex, Evans NMR spectroscopic experiments reveal µeff
[TpRu(PMe3)2OH]+ most likely proceeds by hydrogen-atom ab-
straction.
From a synthetic perspective, our interest in anionic het-
= 1.78 µB (room temperature), which is consistent with a eroatomic ligands coordinated to late transition metals is
single unpaired electron and close to the spin-only value of derived predominantly from the positioning of a basic/nu-
1.73 µB. Similar tothe RuII–hydroxo complex [TpRu(PMe3)2- cleophilic moiety adjacent to a Lewis acidic metal center
OH], the RuIII system [TpRu(PMe3)2OH]+ converts 1,4-cy- (Scheme 2). Hence, opportunities exist to coordinate sub-
clohexadiene to benzene; however, experimental evidence strates to the Lewis acidic metal and to activate them to-
points to a pathway that involves net hydrogen atom ab- ward reactions with the heteroatomic ligands. For example,
straction rather than even-electron acid–base chemistry metal-mediated C–H activation by σ-bond metathesis path-
(Scheme 8). In the absence of an external radical trap, ways involve the conversion of M–R and RЈ–H to M–RЈ
[TpRu(PMe3)2OH]+ most likely reacts with 1,4-cyclohexa- and R–H via a transition state in which net H+ is transfer-
diene to produce [TpRu(PMe3)2(OH2)]+ and a cyclohexadi- red to a basic alkyl or aryl ligand “R” (Scheme 9). We have
enyl radical, the latter is probably consumed rapidly by a considered that related reactions of late transition metals
second equivalent [TpRu(PMe3)2OH]+ to form benzene and with nondative-heteroatomic ligands might provide facile
a second equivalent of [TpRu(PMe3)2(OH2)]+. The ob- and controlled C–H activation/functionalization. In this
served stoichiometry of products (2:1 molar ratio of case, the net C–H activation might be best viewed as an
[TpRu(PMe3)2(OH2)]+ and benzene) is consistent with the intramolecular proton transfer to the basic heteroatomic li-
proposed mechanism. When the radical trap TEMPO gand. In related examples, the net 1,2-additions of dihydro-
(TEMPO = 2,2,6,6-tetramethylpiperidinyloxy) is added the gen across Ru amido ligands have been implicated in enan-
cyclohexadienyl radical is trapped by TEMPO to give ben- tioselective ketone hydrogenation as well as Rh and Ir cata-
zene and hence a 1:1 ratio of [TpRu(PMe3)2(OH2)]+ and lysts.[58–62] Morris et al. have demonstrated that the reaction
benzene in the products. [TpRu(PMe3)2OH]+ reacts with of a RuII amido complex with dihydrogen yields a complex
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T. B. Gunnoe
MICROREVIEW
in which the H–H bond is intermediate between protic-hyd- the product forming reductive elimination from electroposi-
ridic and dihydrogen,[63] dihydrogen adds across an IrIII
tive early transition metals is likely to be substantially unfa-
–
amido bond,[64] and the chelating amido ligand of vorable. In contrast, N–C and O–C reductive elimination
{Ru(Cl)(PPh3)[N(SiMe2CH2PPh2)2]} deprotonates dihydro- reactions have been successfully incorporated into catalytic
gen to yield {Ru(Cl)(H)(PPh3)[NH(SiMe2CH2PPh2)2]}.[65] cycles involving late transition-metal systems (e.g., catalytic
In addition, early transition-metal imido complexes have arylamine or aryl ether preparation).[78–82] Thus, catalytic
been demonstrated to activate C–H bonds for 1,2-addition cycles by Route A are potentially more viable with late tran-
across the M = NR moiety to yield amido and alkyl or aryl sition-metal systems; however, the 1,2-addition of C–H
ligands (Scheme 9).[66–74] Studies have demonstrated that bonds across late transition-metal–oxide or –imido bonds
key factors in the imido-based C–H activation include coor- remains, to the best of our knowledge, unknown.
dination of the C–H bond to the metal center and the polar
Route B (Scheme 10) illustrates an alternative catalytic
nature of the metal–imido bond.[75–77] We have been pursu- cycle with three key steps: (1) the 1,2-addition of a C–H
ing the utilization of RuII systems for the activation of non- bond occurs across a metal–amido or metal–alkoxo (or
polar bonds such as dihydrogen and carbon–hydrogen aryloxo) bond, (2) dissociation of the alcohol or amine, and
bonds.
(3) net oxygen atom or nitrene insertion into the newly
formed M–R or M–Ar bond.[83] To the best of our knowl-
edge, prior to our work and efforts by Periana et al., evi-
dence for the 1,2-addition of C–H bonds across the bonds
of late transition metals and amido, alkoxo, and related li-
gands was unknown.[24,26,84] Macgregor et al. have sug-
gested a related pathway for a C–H cyclometalation by Pd–
acetate and Ir complexes.[85,86] Addition of a C–H bond ac-
ross a Pt–Cl bond has been proposed as a possibility in the
Shilov reaction.[87] Establishing the viability of these poten-
tial catalytic cycles requires an understanding of the metal-
mediated C–H activation by late transition-metal amido,
alkoxo, and related systems.
In addition to the C–H activation step, oxo or nitrogen
functionalization of the M–R or M–Ar bond is a substan-
tial challenge to the development of catalytic cycles shown
in Route B. For example, insertions of metal–oxo ligands
into metal–alkyl (or aryl) bonds are extremely rare, espe-
cially under nonphotolytic conditions.[88,89] However, Per-
iana et al. have recently reported kinetically facile conver-
sions of a ReVII–methyl ligand and external oxidants to a
Re–OMe linkage, and they have proposed that the key to
the relatively low activation barriers is a mechanism that
does not involve a rhenium–oxo ligand but rather resembles
the classic organic Baeyer–Villiger reaction.[83] Thus, both
of the reaction steps for catalytic oxy functionalization of C–
H bonds by Route B in Scheme 10 have been observed to be
kinetically accessible under reasonably mild reaction condi-
tions using late transition-metal systems. With a better un-
derstanding of both reactions, it might be possible to merge
the two steps with a single catalyst.
Scheme 9. Net 1,2-addition of C–H bonds across early transition-
metal imido fragments has been observed.
Several strategies exist for oxygen or nitrogen functionali-
zation of C–H bonds via metal-mediated activation, and
the net 1,2-addition of C–H bonds across metal heteroatom
bonds is a potentially viable step in catalytic sequences. For
example, Scheme 10 depicts two possible catalytic cycles for
C–H functionalization involving the 1,2-addition of C–H
bonds across M–X bonds (X = amido, imido, oxide, alkoxo,
etc.). Route A illustrates C–H functionalization with three
key steps: (1) 1,2-addition of a C–H bond across a metal–
oxide or metal–imido bond, (2) O–C or N–C reductive eli-
mination, and (3) formal oxidation of the metal to reform
the oxide or imido complex. Although the 1,2-addition of
C–H bonds across M–X bonds has been demonstrated for
early transition-metal–imido complexes (see above),[66–77]
[TpRu(PMe3)2OH] and [TpRu(PMe3)2NHPh] provide ki-
netic access to aromatic C–H activation sequences that in-
volve the net 1,2-addition of C–H(D) bonds across the Ru–
OH and Ru–NHPh bonds.[24,26] For example, heating these
systems in C6D6 results in H/D exchange between the deu-
terated benzene and the hydroxo and anilido ligands, which
1
2
has been observed by both H and H NMR spectroscopy.
In addition to the incorporation of deuterium at the hetero-
atomic ligand, highly regioselective H/D exchange is ob-
served at the Tp 4-position (Scheme 11). On the basis of
several experimental results including detailed kinetic stud-
ies and complementary computational studies,[24] the
mechanism in Scheme 12 has been proposed. Initial dissoci-
Scheme 10. Two possible routes to incorporate the net 1,2-addition
of C–H bonds across transition-metal–heteroatom bonds (e.g.,
imido, oxide, amido, alkoxo, etc. ligands) into catalytic cycles.
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Reactivity of RuII and CuI Complexes
MICROREVIEW
ation of PMe3 provides a pathway to coordinate and acti- Ru(PH3)2Ph] and free H2O is calculated to be endergonic
vate the aromatic substrate (benzene in Scheme 12). Acti- by 38 kJ/mol relative to starting materials. Similar to calcu-
vation of the aromatic C–H(D) bond imparts acidic charac- lations for C–H activation by isoelectronic [TpRu(L)R] (R
ter, which results in the transfer of the “protic” hydrogen to = alkyl) systems,[90,91] the calculated pathway for benzene
the basic hydroxo or anilido site (intramolecular transfer) C–H activation by [(Tab)Ru(PH3)OH] does not involve oxi-
or transfer to the most basic pyrazolyl position (the 4-posi- dative addition but rather more closely resembles a σ-bond
tion, intermolecular transfer). The proposed mechanism metathesis reaction that is typically observed with d0 metal
suggests that the C–H(D) activations are thermally disfa- systems (i.e., the C–H and O–H bonds are simultaneously
vored, and consistent with this notion heating broken and formed). The intramolecular C–H activations
[TpRu(PMe3)2Ph] and aniline in C6D6 initially results in the with Ru–X (X = OH or NHPh) are perhaps best considered
conversion to benzene and [TpRu(PMe3)2NHPh]. In closely as intramolecular deprotonations (see below).
related work, Periana et al. have directly observed the con-
version of an octahedral IrIII–methoxo complex, which is
isoelectronic to the [TpRu(PMe3)2X] systems, and benzene
to the corresponding IrIII–phenyl and methanol.[84]
While examples of C–H activation by late transition-
metal complexes that involve the overall conversion of M–
R + RЈH to M–RЈ and RH or oxidative addition of C–H
bonds are relatively common,[90–110] related examples in-
volving M–X + R–H to M–R + X–H (X = amido, alkoxo,
etc.) are rare. Thus, an important fundamental aspect of the
latter transformations is the relative predilection toward C–
H activation for systems that possess nondative-heteroat-
omic ligands. That is, what features of the metal center and
ligand “X” decrease the activation barrier for the C–H acti-
vation? Part of our efforts with late transition-metal non-
dative-heteroatomic complexes is directed toward elucidat-
ing such features. For example, our preliminary results and
calculations suggest that the net 1,2-addition of C–H bonds
across M–X (X = anionic heteroatomic ligand) may be more
facile than related reactions involving M–R bonds, which may
be explained by considering the C–H activation as an intra-
molecular proton transfer that is facilitated by the lone pair
on the hydroxo and related ligands (Scheme 14). For exam-
ple, Scheme 15 depicts benzene C–H activation by
Scheme 11. Observed H/D exchange from heating [TpRu(PMe3)2X]
(X = OH or NHPh) in C6D6 (N* = isotopically labeled 15N).
DFT calculations performed by Cundari et al. using the [TpRu(PMe3)X] (X = OH, NHPh, or Me) fragments. The
Tp model “Tab” [Tab = tris(azo)borate] and PH3 reveal the activated benzene C–H bond can either produce a net trans-
energetics for the conversion of [(Tab)Ru(PH3)2OH] and fer of H+ to the pyrazolyl ring of another Ru system (kinter
benzene to [(Tab)Ru(PH3)2Ph] and H2O (Scheme 13). Con- in Scheme 15) or the intramolecular transfer of H+ to X
sistent with the experimental evidence suggesting that the (kintra) in Scheme 15. For X = OH and NHPh, kintra is more
1,2-additions of the benzene C–H(D) bonds are thermally rapid than kinter; however, for X = Me, kinter is more rapid
disfavored, the conversion of [(Tab)Ru(PH3)2OH] and ben- that kintra, results which are consistent with the presence of a
zene to [(Tab)Ru(PH3)(OH2)Ph] and free PH3 is calculated lone pair on ligand X facilitating the intramolecular transfer
to be endergonic by 77 kJ/mol, and conversion to [(Tab) of a proton from the activated benzene C–H(D) bond. The
Scheme 12. Proposed mechanism for H/D exchange with [TpRu(PMe3)2X] (X = OH or NHPh) and deuterated arene solvents.
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T. B. Gunnoe
MICROREVIEW
Scheme 13. Calculated energetics for benzene C–H activation using a model complex [(Tab)Ru(PH3)2OH].
kinter for X = Me is an estimate based on an approximate nondative-heteroatomic ligand, identity of substituents on
half-life acquired in an NMR tube reaction of the nondative ligand, and the donating ability of the ancil-
[TpRu(PMe3)2Me] in C6D6 to produce [TpRu(PMe3)2- lary ligands. We are currently studying such details both
(Ph-d5)] and CH3D at 130 °C.
experimentally and computationally. However, these pre-
liminary experimental and computational results suggest
that 1,2-additions of C–H bonds across M–X (X = anionic
heteroatomic ligand) bonds with late transition metals in
low oxidation states might be viable reactions for catalytic
C–H functionalization.
Five- and Six-Coordinate (PCP)RuII Systems
Preparation and Characterization of (PCP)RuII Systems
In order to access coordinatively unsaturated RuII amido
complexes we employed the sterically bulky PCP ligand 2,6-
(CH2P-tBu2)2C6H3, which is strongly donating and coordi-
nates in a meridonal fashion.[111] The addition of ammonia
to the previously reported complex [(PCP)Ru(CO)Cl] re-
sults in reversible coordination of the amine to form
[(PCP)Ru(CO)(NH3)Cl], and reactions of the RuII–amine
complex with strong bases produce the parent amido com-
plex [(PCP)Ru(CO)NH2] (Scheme 16).[29] In contrast to the
Scheme 14. For late transition-metal systems in low oxidation states
the net 1,2-addition of C–H bonds across M–X (X = anionic het-
eroatomic ligand such as hydroxo, amido, etc.) may possess in-
herently lower activation barriers than analogous reactions of M–
R (R = alkyl or aryl) bonds.
Clearly, more work is needed to substantiate (or dis- octahedral 18-electron [TpRu(L)(LЈ)NH2] parent amido
prove) this possibility as well as to understand the influence complexes (see above), the resonance arising from the
of the metal oxidation state, metal identity, identity of the amido protons appears downfield at δ = 4.36 ppm. The
Scheme 15. Comparison of rates of inter- versus intramolecular transfer of the proton (D+) from the activated benzene C–D bond for a
series of complexes [TpRu(PMe3)(C6D6)X] (X = OH, NHPh or Me) {[a] Data at 90 °C, [b] calculated at 90 °C from the value acquired
at 80 °C (assuming constant ∆G‡), [c] Data at 130 °C}.
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Reactivity of RuII and CuI Complexes
MICROREVIEW
downfield chemical shift most likely reflects amido-to-Ru facile dissociation of PMe3 and hindered rotation about the
π-donation as a result of the metal’s coordinative and elec- Ru–Nanilido (∆G‡ = 47 kJ/mol at 271 K) and Nanilido–Cipso
tronic unsaturation.
(∆G‡ = 64 kJ/mol at 319 K) bonds (Scheme 18).
Reactivity of (PCP)RuII Systems
The (PCP)RuII amido complexes undergo reactions sim-
ilar to the TpRuII amido complexes, but also afford access
to transformations not observed with the Tp systems. For
example, similar to the TpRu parent amido complexes,
[(PCP)Ru(CO)NH2] reacts with phenylacetylene to produce
free ammonia and [(PCP)Ru(CO)(CϵCPh)]; however, in
contrast to the Tp systems, the putative intermediate(s)
[(PCP)Ru(CO)NH3][PhC2]
or
[(PCP)Ru(CO)(NH3)-
(CϵCPh)] are not observed (even at –78 °C) as they appar-
ently undergo rapid conversion to the five-coordinate ace-
tylide complex [(PCP)Ru(CO)(CϵCPh)] (Scheme 19).
Scheme 16. Preparation of five-coordinate [(PCP)Ru(CO)NH2].
The anilido complex [(PCP)Ru(CO)NHPh] has been pre-
pared via transformation of [(PCP)Ru(CO)Cl] and
TMSOTf to [(PCP)Ru(CO)OTf] followed by reaction with
[Li][NHPh].[27,28] The addition of PMe3 to [(PCP)Ru(CO)-
NHPh] allows the isolation of the octahedral complex
[(PCP)Ru(CO)(PMe3)NHPh] (Scheme 17). Variable-tem-
Scheme 19. Reaction of [(PCP)Ru(CO)NH2] and phenylacetylene
to produce [(PCP)Ru(CO)CϵCPh] and free ammonia.
The failure of the TpRu parent amido complexes to react
with dihydrogen is potentially attributable to the lack of an
open coordination site to bind and activate the dihydrogen.
Consistent with this notion, the five-coordinate complex
[(PCP)Ru(CO)NH2] rapidly reacts with dihydrogen at room
temperature to produce [(PCP)Ru(CO)H], and mechanistic
studies suggest the pathway displayed in Scheme 20.[29] In
related chemistry, Morris et al. demonstrated that the reac-
tion of a RuII amido complex with dihydrogen yields a com-
plex in which the H–H bond is intermediate between protic-
hydridic and dihydrogen bonding.[63] Fryzuk et al. have re-
ported the 1,2-addition across a Ru–amido bond in which
the amido is part of a chelated PNP ligand.[65] Rauchfuss
has recently reported the addition of dihydrogen across an
IrIII–amido bond.[64] Also, the 1,2-addition of dihydrogen
across Ru–amido linkages has been implicated for asym-
metric hydrogenation reactions.[58,59] The conversion of
[(PCP)Ru(CO)NH2] and dihydrogen to [(PCP)Ru(CO)H]
and ammonia suggests that nonpolar bonds can be acti-
vated with late transition metals in low oxidation states that
possess anionic heteroatomic ligands.
1
perature H and 31P NMR spectroscopy of [(PCP)Ru(C-
O)(PMe3)NHPh] reveals three fluxional processes including
Scheme 17. Preparation of [(PCP)Ru(CO)NHPh] and [(PCP)Ru(C-
O)(PMe3)NHPh].
Despite the facile activation of dihydrogen by [(PCP)-
Ru(CO)NH2], this system does not mediate the 1,2-addition
of C–H bonds of benzene or methane across the Ru–NH2
bond. Rather, intramolecular C–H activation of a tBu
group of the PCP ligand kinetically competes with intermo-
lecular C–H activation (Scheme 21). Fryzuk et al. have re-
cently reported a related intramolecular cyclometalation of
a triphenylphosphane ligand that involves the conversion
of an amido moiety to amine coordinated to RuII.[112] The
intramolecular C–H activation occurs for [(PCP)Ru(CO)X]
(X = NH2 or Me) with the rate of reaction approximately
Scheme 18. Fluxional processes observed for [(PCP)Ru(CO)-
(PMe3)NHPh] using variable-temperature NMR spectroscopy.
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T. B. Gunnoe
MICROREVIEW
vation is thermally disfavored, and these results are consis-
tent with both calculations and experimental observations
for [TpRuL2X] (X = amido or hydroxo) systems (see above).
These results suggest that direct observation of the 1,2-ad-
dition of C–H bonds across the bonds of late transition-
metal–heteroatomic ligands requires a more favorable
change in M–X BDE. Interestingly, Periana et al. have re-
ported that benzene C–H activation by an IrIII–methoxo
complex, which is isoelectronic to our RuII systems, to form
methanol and an Ir–phenyl complex is thermally fa-
vored.[84]
Scheme 20. Proposed pathway for the conversion of [(PCP)Ru-
(CO)NH2] and dihydrogen to free ammonia and [(PCP)Ru(CO)H].
five times faster for the methyl complex at 50 °C. Eyring
plots for both complexes over a temperature range of 30 °C
revealed identical ∆H‡ values of 75(4) kJ/mol with a ∆S‡
value of –23(4) eu for [(PCP)Ru(CO)NH2] and –18(4) eu for
[(PCP)Ru(CO)Me]. Thus, the difference in the reaction rate
for intramolecular 1,2-addition of C–H bonds across RuII–
X (X = NH2 or Me) bonds for these systems is predomi-
nantly due to a more ordered transition state for X = NH2
than X = Me. Interestingly, the comparative rates of intra-
molecular C–H activation for the PCP systems with X =
NH2 or Me are in contrast to the more rapid 1,2-addition
of benzene C–D bonds by [TpRu(PMe3)2OH] versus
[TpRu(PMe3)2Me] (see above).
Scheme 22. Calculations for 1,2-addition of H–H and C–H (meth-
ane) across the Ru–NH2 bond of [(PCP)Ru(CO)NH2].
In addition to reactivity with nonpolar H–H and C–H
bonds the combination of a Lewis acidic metal center and
nucleophilic amido ligand of [(PCP)Ru(CO)NHPh] can be
utilized to coordinate and activate polar bonds.[27,28] These
transformations are of potential relevance to transition-
metal-catalyzed polymerization of heterofunctionalized
monomers such as carbodiimides, carbon dioxide, lactams,
lactones, lactides, isocyanates, epoxides, and azirid-
ines,[6,8,9,113–124] catalytic hydration of nitriles,[125] catalytic
transamidation and transesterification,[126–135] and related
transformations.
[(PCP)Ru(CO)(PMe3)NHPh] reacts with nitriles to form
the amidinate complexes. Detailed kinetic studies for the
reaction with NCMe are consistent with the rate law from
the mechanism shown in Scheme 23, which involves dissoci-
ation of PMe3, coordination of acetonitrile, and intramolec-
ular C–N bond formation. For reactions with 10 equiv. of
Scheme 21. Intramolecular C–H activation via 1,2-addition of the
C–H bond across the Ru–NH2 bond.
Consistent with experimental observations, DFT calcula-
tions {B3LYP/SBK(d)} by Cundari et al. suggest that the
conversion of [(PCPЈ)Ru(CO)NH2] (PCPЈ = model PCP li-
gand with tBu groups replaced with hydrogen) and dihydro-
gen to [(PCPЈ)Ru(CO)(NH3)H] is exothermic (∆H =
–71 kJ/mol) and exergonic (∆G = –39 kJ/mol; Scheme 22).
In contrast, the 1,2-addition of a C–H bond of methane to
[(PCPЈ)Ru(CO)NH2] to form [(PCPЈ)Ru(CO)(NH3)Me] is
calculated to be both endothermic (∆H = +17 kJ/mol) and
endergonic (∆G = +59 kJ/mol). While the calculated ender-
gonic nature of the methane conversion is not entirely unex-
pected (due to consumption of gaseous methane), the endo-
thermic nature is perhaps more difficult to rationalize. For
example, the enthalpic change from the cleavage of a C–H
bond of methane in combination with Ru–CMe and N–H
bond formation is calculated to be ca. 134 kJ/mol; however,
calculations also reveal that the conversion of the Ru–NH2
moiety to the dative RuǟNH3 bond comes at the expense
of approximately 167 kJ/mol (Scheme 22)! Thus, in addition
to intramolecular competition with the tBu groups, the cal-
Scheme 23. Proposed pathway for the conversion of [(PCP)Ru(C-
O)(PMe3)NHPh] and nitriles to amidinate complexes, and the cor-
culations reveal that the desired intermolecular C–H acti- responding rate law.
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Reactivity of RuII and CuI Complexes
MICROREVIEW
acetonitrile and 10 to 25 equiv. of PMe3, the rate of conver- and the slope from the plot in Figure 2, the ratio of k–1/k2
sion of complex 1 and acetonitrile to the amidinate complex was calculated to be 3; however, small deviations in the
2 is inverse first order in concentration of PMe3, which is slope can result in substantial deviations for the y intercept.
consistent with the term in the denominator “k–1[PMe3]”
dominating the term “k2[NCMe]” under the specific condi-
tions of these experiments with kobs under pseudo-first-order
conditions being equal to k1k2[NCMe]/k–1[PMe3]. A plot of
kobs versus the concentration of NCMe for reactions with
10 equiv. of PMe3 and between 10 and 50 equiv. of NCMe
reveals a first-order dependence on the concentration of
NCMe at low concentrations of NCMe while saturation ki-
netics are apparent at higher concentrations. Thus, in agree-
ment with the observation of an inverse dependence of reac-
tion rate on the concentration of PMe3 at lower concentra-
tions of NCMe, the term “k–1[PMe3]” is greater than
“k2[NCMe]” with the pseudo-first-order rate constant
equal to k1k2[NCMe]/k2[PMe3]. At elevated concentrations
of NCMe the term “k2[NCMe]” becomes greater than “k–
1[PMe3]” with kobs independent of the concentration of
NCMe. Monitoring the rate of disappearance of [(PCP)Ru-
(CO)(PMe3)NHPh] in the presence of 100 equiv. of acetoni-
trile with 10, 15, or 20 equiv. of PMe3 reveals that kobs is
independent of the PMe3 concentration and is equal to
(3)
Figure 2. Plot of 1/kobs versus [PMe3]/[NCMe] for the conversion
of [(PCP)Ru(CO)(PMe3)NHPh] and NCMe to the amidinate com-
plex (R2 = 0.99).
[(PCP)Ru(CO)(PMe3)NHPh] also reacts with carboxam-
1.29(7)ϫ10–5 s–1. For reactions that are pseudo-first order ides, isocyanates, carbodiimides, benzaldehyde, and water
in concentration of [(PCP)Ru(CO)(PMe3)NHPh], re- (Scheme 24). The reactions with carbodiimides, isocyanates,
arrangement of the rate law shown in Scheme 23 indicates and benzaldehyde follow similar pathways to the reaction
that a plot of 1/kobs versus [PMe3]/[NCMe] should be linear of [(PCP)Ru(CO)(PMe3)NHPh] with nitriles to form κ2-li-
with the slope equal to k–1/k1k2 and the y intercept equal gands, which most likely result from intramolecular C–N
to 1/k1 [Equation (3)]. From the plot shown in Figure 2, k1 bond formation. Thus, coordination of polar substrates
was estimated to be 1.8ϫ10–5 s–1, and using the value of k1 with Lewis basic sites by the RuII fragment activates the
Scheme 24. Reactions of [(PCP)Ru(CO)(PMe3)NHPh] including ORTEPs (30% probability) of three products (with hydrogen atoms and
tBu groups omitted).
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T. B. Gunnoe
MICROREVIEW
unsaturated molecules toward intramolecular nucleophilic
addition by the amido ligand. For benzaldehyde, the amid-
ate complex is isolated in only 40% yield because of side
reactions that produce [(PCP)Ru(CO)H] (5%), [(PCP)-
Ru(CO)(PMe3)H] (10%), and free PCPH (30%).
Two- and Three-Coordinate CuI Systems
Preparation and Characterization of CuI Systems
Copper has played an important role in the development
of metal-mediated organic synthesis. However, despite the
prevalence of anionic heteroatomic ligands with early and
middle transition metals, increasing evidence for the highly
reactive nature of late transition-metal complexes with an-
ionic heteroatomic ligands and the possibility that Cu–
imido complexes are intermediates in Cu-catalyzed olefin
aziridination,[3,136–140] the isolation and full characteriza-
tion of monomeric Cu amido complexes was, to the best of
our knowledge, unknown prior to our efforts. Multinuclear
systems with bridging-amido ligands have been re-
ported,[141–146] including an example of a nonbridging
amido that is part of a chelated ligand.[147] In addition, re-
ports of monomeric Cu–alkoxo and Cu–aryloxo complexes
are relatively scant.[148–154] Thus, we sought ligand plat-
forms and synthetic routes that would allow the isolation
of well-defined monomeric CuI systems with amido, alkoxo,
and related ligands. Sterically bulky ancillary ligands have
been incorporated in an effort to inhibit the formation of
dimeric or multinuclear Cu systems bridged by the heteroat-
omic ligands.
Scheme 25. Preparation of the monomeric CuI–anilido complex
[(dtbpe)Cu(NHPh)] (1) and its ORTEP diagram (30%).
Similar to the preparation of Cu–methyl complexes by
Sadighi et al,[156] we have employed NHC (NHC = N-
heterocyclic carbene) ligands to stabilize monomeric two-
coordinate CuI systems with anionic heteroatomic li-
gands.[23,25,38] The NHC ligand has been varied between
IMes {1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene},
IPr {1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene}, and
SIPr {1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene}.
Complexes with amido, alkoxo, and aryloxo ligands can be
prepared upon reaction of amines or alcohols with [(NHC)-
Cu(R)] (R = Me or Et) complexes or via metathesis using
[(NHC)Cu(Cl)] and lithium salts of the anionic heteroat-
omic ligand (Scheme 26). For the conversion of [(IPr)-
Cu(Me)] and aniline to methane and [(IPr)Cu(NHPh)] (2)
studies suggest that the reaction involves the initial coordi-
nation of aniline followed by protonation of the methyl li-
gand.[25]
Chloride/amido metathesis between [(dtbpe)Cu(µ-Cl)]2
{dtbpe
=
1,2-bis(di-tert-butylphosphanyl)ethane} and
[Li][NHPh] allows the isolation of monomeric [(dtbpe)-
Cu(NHPh)] (1) (Scheme 25).[30] The bulky dtbpe most likely
prevents the dimerization of 1 via bridging-anilido ligands,
and the strongly donating nature probably prevents re-
duction of CuI to Cu0 by the lithium amide salt (as is ob-
served for CuI systems with more weakly donating aryl–
phosphane ligands). The Cu–Namido bond length
{1.890(6) Å} of 1 is shorter than that of the corresponding
aniline complex [(dtbpe)Cu(NH2Ph)][OTf] by ca. 0.12 Å.
Recently, Johnson et al. have reported a multinuclear Cu9
system that possesses Cu–amido linkages.[147] The three-co-
ordinate d10 CuI complex 1 has a vacant p orbital of π-
symmetry with respect to the Cu–Namido bond, and the
amido ligand is oriented for overlap with the Cu-based p
orbital. For example, the P–Cu–N–Cipso torsion angle is
about 13°, while a torsion angle of 0° would provide an
orientation optimized for π-overlap (see Figure 4 below).
This amido orientation is approximately perpendicular to
that observed by Hillhouse and Mindiola for the closely
related d9 NiI complex (dtbpe)Ni{NH(2,6-CHMe2)
C6H3}.[155] The phenyl substituent of 1 is also oriented for
π-overlap with the amido lone pair (see Figure 4 below).
Scheme 26. Two routes for the preparation of [(NHC)Cu(X)] com-
plexes (X = NHPh, OEt or OPh).
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Reactivity of RuII and CuI Complexes
MICROREVIEW
Several of these complexes have been characterized by ent is Cu 3d (pamido–dCu π*) rather than 4p (Figure 5).[23]
single-crystal X-ray diffraction. In all cases the structures However, the KS HOMOs possess a substantial contri-
reveal approximately linear two-coordinate monomeric sys- bution from the phenyl π-system.
tems (Figure 3). We have structurally characterized three
Cu–NHPh
complexes:
[(dtbpe)Cu(NHPh)],
[(IPr)-
Cu(NHPh)], and [(SIPr)Cu(NHPh)]. In all three systems,
the plane of the anilido–phenyl group is approximately co-
planar with the H–N–Cphenyl plane, which optimizes delo-
calization of the amido lone pair into the phenyl π* system
(Figure 4). Consistent with the amido-to-phenyl electron
delocalization, the Namido–Cphenyl bond lengths {1.351(4) Å
to 1.359(3) Å} are shorter than a typical N–C single bond
(1.47 Å) and shorter than the corresponding N–C bond of
the aniline complex [(dtbpe)Cu(NH2Ph)][PF6], which is
1.444(4) Å.[23,30]
Figure 5. Highest occupied (left) and lowest unoccupied (right)
Kohn–Sham orbitals for [(dtbpe)Cu(NHPh)] (1) (top pair) and
[(IMes)Cu(NHPh)] (4) (bottom pair). Orbitals for other NHC CuI–
anilido complexes are similar.
Reactivity of CuI Systems
As a straight-forward probe of nucleophilic reactivity, the
rates of net SN2 reactions of RuII– and CuI–anilido com-
plexes with bromoethane to produce the corresponding
metal–bromide and ethyl aniline were compared.[23] The
complexes [TpRu(PMe3)2NHPh], [(dtbpe)Cu(NHPh)] (1),
Figure 3. ORTEPs (30%) of representative examples of [(NHC)-
Cu(X)] (X = nondative heteroatomic ligand) complexes including
[(IPr)Cu(NHPh)] (2), [(SIPr)Cu(NHPh)] (3), [(IPr)Cu(OEt)] (5), [(IPr)Cu(NHPh)] (2), [(SIPr)Cu(NHPh)] (3), and [(IMes)-
and [(IPr)Cu(OPh)] (8) (presented from left to right and top to
bottom).
Cu(NHPh)] (4) all react with bromoethane with the rate of
reaction for Ru being considerably slower than Cu
(Scheme 27 and Table 1). Using ∆G‡ values calculated at
Consistent with the solid-state structures that suggest a variable temperatures and calculating rate constants at
negligible Cu–Namido π-interaction for the CuI anilido com- 298 K, which assumes a constant ∆G‡, reveals that the reac-
plexes 1–4, the calculated Kohn–Sham (KS) HOMO of tions with the Cu complexes are 55000 to 250000 times
these systems has little Cu character, and that which is pres more rapid than for the TpRu system. These results suggest
Figure 4. Depiction of the orientation of phenyl substituents of anilido ligands of [(SIPr)Cu(NHPh)] (3) (left), [(IPr)Cu(NHPh)] (2)
(middle), and [(dtbpe)Cu(NHPh)] (1) (right) with most atoms removed.
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T. B. Gunnoe
MICROREVIEW
that the LnCu–X (X = anionic heteroatomic ligand) are
highly reactive and indicate that they may be useful for the
development of Cu-mediated catalytic organic syntheses.
Scheme 28. Equilibrium trends between metal–amido/dihydrogen
and metal–hydride/amine based on the identity and oxidation state
of the transition metal.
Scheme 27. Nucleophilic substitution reaction of amido complexes
with bromoethane.
Table 1. Kinetic data for the reaction of bromoethane with [(IPr)-
Cu(NHPh)] (2), [(SIPr)Cu(NHPh)] (3), [(IMes)Cu(NHPh)] (4),
[(dtbpe)Cu(NHPh)] (1), and [TpRu(PMe3)2(NHPh)] (see
Scheme 27). All reactions were carried out at room temperature
except [TpRu(PMe3)2NHPh], which was performed at 80 °C.
[a]
Complex
kobs [s–1]
t1/2
∆G‡[b]
[(dtbpe)Cu(NHPh)]
[(IMes)Cu(NHPh)]
[(IPr)Cu(NHPh)]
[(SIPr)Cu(NHPh)]
5.5(2)ϫ10–4
3.3(3)ϫ10–4
2.4(2)ϫ10–4
1.1(1)ϫ10–4
21.0(6)
35(3)
49(4)
109(8)
22[c]
91
92
93
94
Scheme 29. Observed equilibrium for Cu–hydride/aniline and Cu–
anilido/dihydrogen.
[TpRu(PMe3)2(NHPh)] 8.6(3)ϫ10–6
121
[a] Minutes. [b] kJ/mol. [c] Hours.
In general, the thermodynamics of reactions between
transition-metal amido (and related) complexes and dihy-
drogen to form metal–hydride complexes and free amines
appear to be dependent upon the identity of the metal cen-
ter. Early and middle transition-metal systems in relatively
high oxidation states tend to favor metal–amido/dihydrogen
while late transition-metal systems in low oxidation states
exhibit a trend toward the metal–hydride/free amine
(Scheme 28 and see below for examples). This trend can be
rationalized by increased metal–amido bond energies due
to amido-to-metal π-donation for metal centers in relatively
high oxidation states. For late transition-metal centers in
Scheme 30. Observed equilibrium for Cu–phenyl/aniline and Cu–
anilido/benzene.
low oxidation states the π-bonding is disrupted and the M– upon combination of trans-[(PEt3)2Pt(H)NHPh] and dihy-
Namido BDE is presumably decreased relative to systems in drogen,[158] and reaction of Cp*2M(H)2 (M = Zr or Hf)
which π-bonding is present. The [(NHC)Cu(NHPh)] sys- with ammonia to produce dihydrogen and the correspond-
tems exhibit reactivity that contrasts these general trends. ing parent amido complexes.[159] In contrast to the late tran-
For example, [(IPr)Cu(NHPh)] (2) does not react with dihy- sition metal and low oxidation state RuII and PtII systems,
drogen, while [[(IPr)Cu(µ-H)]2] rapidly reacts with aniline complex 2 favors the Cu–amido and dihydrogen. The con-
to form dihydrogen and complex 2 (Scheme 29), reactivity version of [(IPr)Cu(Ph)] and aniline to [(IPr)Cu(NHPh)] (2)
which is consistent with an early transition-metal system. and benzene also contrasts reactivity observed with another
Furthermore, [(IPr)Cu(Ph)] reacts with aniline to produce late transition-metal anilido complex. [(PCP)Ir(H)(Ph)] and
benzene and [(IPr)Cu(NHPh)] with an equilibrium constant free NH2Ph are thermally favored over [(PCP)-
Ͼ 1000 (Scheme 30).
Ir(H)(NHPh)] and free C6H6 with Keq = 105.[160] In con-
Examples of systems that follow the relative thermody- trast, the [(IPr)CuI] fragment demonstrates a thermo-
namics for reactions of amido complexes and dihydrogen dynamic propensity for the formation of a Cu–NHPh bond
(see paragraph above) include the conversion of and free benzene, which seems to suggest a shift in the rela-
[Cp*Ru(PMe3)2(NPh2)] (Cp*
tadienyl) and dihydrogen to free amine and Ru–hydride M–Nanilido BDEs from established norms.
complexes,[157] reaction of [(PCP)Ru(CO)NH2] and dihy-
Analysis of the Cu–Namido BDE using B3LYP/CEP-
=
pentamethylcyclopen- tive magnitude of M–H/M–Nanilido and M–Cphenyl/
drogen to produce free ammonia and Ru–hydride com- 31G(d) calculations provides a rationale for the observed
plexes,[29] release of aniline and formation of a Pt–dihydride equilibria. For [(IPr)Cu(NHPh)], the Cu–Namido BDE is
1198
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Reactivity of RuII and CuI Complexes
MICROREVIEW
Scheme 31. DFT calculated changes in enthalpy (kJ/mol) for conversion of transition-metal–hydride complexes and aniline to transition-
metal–anilido complexes and dihydrogen.
calculated to be 364 kJ/mol. In comparison, the calculated cently reported to catalyze the hydroamination of ole-
M–Namido BDEs of [Cp*2Zr(NHPh)(H)] (284 kJ/mol; Cp* fins.[169] The [(NHC)Cu(X)] and [(dtbpe)Cu(X)] (X = an-
=
pentamethylcyclopentadienyl),
trans-[(Et3P)2Pt(H)- ionic heteroatomic ligand) complexes catalyze the conjugate
(NHPh)] (180 kJ/mol), and [Cp*Ru(PMe3)2NHPh] (146 kJ/
mol) are substantially weaker (Scheme 31).[23] The differ-
ence in M–Namido BDE for the Zr and Pt/Ru systems is
easily rationalized by, at least in part, the π-bonding com-
ponent of the Zr–Namido bond, which is likely to be negligi-
ble for PtII and octahedral RuII. The large BDE for the Cu–
Namido is more difficult to rationalize, and future studies
will address whether this is an anomaly or a central feature
of CuI systems with anionic heteroatomic ligands. At this
point it can be stated that in terms of ground-state energetics
the Cu–NHPh systems appear similar to early/middle transi-
tion-metal systems in high oxidation states. In contrast, the
reactivity of Cu–NHPh related systems is more reminiscent
of highly nucleophilic complexes in low oxidation states.
The hydroamination or hydroalkoxylation of electron-de-
ficient olefins under acidic or basic conditions is known;
however, such reactions typically require relatively harsh
conditions, including high temperatures that are often in-
compatible with functional groups and do not allow the
control of regio-, chemo- or stereoselectivity.[161] In con-
trast, transition-metal-catalyzed reactions offer the promise
of more ambient conditions and greater control over prod-
uct selectivity. For example, both early and late transition-
metal complexes as well as lanthanide systems have been
developed for olefin hydroamination, with many of these
systems limited to specific substrates or more facile intra-
molecular reactions.[162–167]
Table 2. Hydroamination of electron-deficient olefins catalyzed by
[(IPr)Cu(NHPh)] (2) and uncatalyzed control experiments.
Copper catalysts for the enantioselective conjugate ad-
dition of carbon-based nucleophiles have been reported,[168]
and we have become interested in related reactions for het-
[a] All reactions performed with 5 mol-% catalyst at room tempera-
ture in C6D6. [b] % Conversions determined by 1H NMR spec-
troscopy. [c] Control experiments reported after similar reaction
ero-conjugate additions. Cu(OTf)2 systems have been re- times and given in % conversion. [d] Reaction conducted at 80 °C.
Eur. J. Inorg. Chem. 2007, 1185–1203
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1199

T. B. Gunnoe
MICROREVIEW
addition of N–H and O–H bonds across the C=C bonds
We have recently proposed a reaction mechanism for the
of electron-deficient olefins (Table 2 and Table 3).[37] In all copper-catalyzed hydroamination (and related reactions) of
examples studied the reactions are regioselective for the electron-deficient olefins in which initial intermolecular nu-
“anti-Markovnikov” product (Scheme 32). Under condi- cleophilic addition of the amido ligand to the olefin pro-
tions identical to catalysis some of the substrates undergo duces an unobserved zwitterionic intermediate.[38] Subse-
transformation in the absence of a Cu catalyst; however, quent proton transfer yields a new copper amido complex
comparison of reaction rates clearly indicates that the Cu and reaction with aniline (presumably via coordination to
complexes are serving as catalysts. For comparison, Table 2 Cu) yields a free organic product and regenerates the origi-
and Table 3 depict results from control experiments under nal Cu system (Scheme 33). The hydroalkoxylation reac-
identical reaction conditions and times for the Cu-catalyzed tions are proposed to proceed through a similar mechanism
transformations.
with the exception that product formation likely originates
from the zwitterionic intermediate or a linkage isomer Cu–
alkyl complex. The relative rate of catalysis is consistent
with a nucleophilic addition pathway. For example, reactiv-
ity decreases in the order N(H)nPr Ͼ N(H)Ph Ͼ OEt Ͼ
OPh for reactions with acrylonitrile. It is also possible that
olefin coordination precedes N–C or O–C bond formation;
however, this pathway seems unlikely as it does not neces-
sarily account for the observed anti-Markovnikov regiose-
lectivity. Furthermore, the addition of coordinating tBuNC
slightly accelerates the rate of catalysis for the addition of
aniline to acrylonitrile. If olefin coordination precedes N–
C bond formation, competition for coordination between
acrylonitrile and the isocyanide would be anticipated to re-
Table 3. Hydroalkoxylation and hydroaryloxylation of olefins and
uncatalyzed control reactions.
[a] All reactions performed with 5 mol-% catalyst in C6D6. [b] %
1
Conversions determined by H NMR spectroscopy. [c] Control ex-
periments are reported after similar reaction times to catalyzed
variants and are given in % conversion. [d] Transesterification ob-
served. [e] Addition of tBuNC (1 equiv. based on EtOH) allows
observation of hydroalkoxylation. [f] Approximately 10% transes-
terification also observed.
Scheme 33. Proposed catalytic cycle for Cu–amido catalytic hy-
droamination reactions.
Scheme 32. Cu-catalyzed additions of N–H and O–H across C=C
bonds of electron-deficient olefins are selective for the “anti-Mar-
kovnikov” products.
In general, reactions that incorporate stronger nucleo-
philes react more rapidly. For example, among amines and
alcohols, catalysis is generally slower and less broad in
scope for alcohols than for amines. Amongst a series of
amines, alkylamines react more rapidly than arylamines.
Table 2 shows comparative reactions for aniline and alk-
ylamines with acrylonitrile. The exception to this generali-
zation is the sterically bulky tBuNH2, which reacts more
slowly with acrylonitrile than aniline.
Scheme 34. Proposed mechanism and derived rate law for addition
of aniline to acrylonitrile catalyzed by [(IPr)Cu(NHPh)] {[Cu] =
(IPr)Cu in scheme, A = aniline, O = acrylonitrile, P = product}.
1200
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Reactivity of RuII and CuI Complexes
MICROREVIEW
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Summary and Future Outlook
Work presented herein and reported by others have dem-
onstrated that anionic heteroatomic ligands coordinated to
metal centers in low oxidation states results in highly reac-
tive heteroatomic ligands. Such substrates are potentially
useful for catalytic polymerizations as well as transforma-
tions of small molecules. Using well-defined RuII and CuI
systems, we have demonstrated controlled stoichiometric
and catalytic N–C bond formation using polar substrates,
N–C and O–C bond formation using olefins as well as
metal-mediated activation of dihydrogen and carbon–hy-
drogen bonds. Mechanistic studies indicate that, in some
cases, the combination of a Lewis acidic metal center and
nucleophilic/basic heteroatomic ligand form the foundation
of these transformations.
Future efforts will be directed toward continued develop-
ment of catalysts using these, and related, complexes. For
example, we are presently pursuing strategies to extend the
scope and selectivity of Cu-catalyzed hydroamination and
hydroalkoxylation reactions as well as to develop other
methods of N–C and O–C bond formation using these reac-
tive Cu systems. Furthermore, efforts toward using the 1,2-
addition of C–H bonds across metal–heteroatom bonds as
a key step for C–H functionalization sequences are continu-
ing. In terms of the reactivity of anionic heteroatomic li-
gands, future work will be directed toward an increased un-
derstanding of the impact of ancillary ligands, metal ident-
ity, and metal oxidation state on accessible reactions.
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The National Science Foundation, the Alfred P. Sloan Foundation,
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Energy, and the Petroleum Research Fund administered by the
American Chemical Society are gratefully acknowledged for sup-
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Received: November 15, 2006
Published Online: February 22, 2007
Eur. J. Inorg. Chem. 2007, 1185–1203
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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