Bonding and structure of copper nitrenes.
ABSTRACT Copper nitrenes are of interest as intermediates in the catalytic aziridination of olefins and the amination of C-H bonds. However, despite advances in the isolation and study of late-transition-metal multiply bonded complexes, a bona fide structurally characterized example of a terminal copper nitrene has, to our knowledge, not been reported. In anticipation of such a report, terminal copper nitrenes are studied from a computational perspective. The nitrene complexes studied here are of the form (beta-diketiminate)Cu(NPh). Density functional theory (DFT), complete active space self-consistent-field (CASSCF) electronic structure techniques, and hybrid quantum mechanical/molecular mechanical (QM/MM) methods are employed to study such species. While DFT methods indicate that a triplet (S = 1) is the ground state, CASSCF calculations indicate that a singlet (S = 0) is the ground state, with only a small energy gap between the singlet and triplet. Moreover, the ground-state (open-shell) singlet copper nitrene is found to be highly multiconfigurational (i.e., biradical) and to possess a bent geometry about the nitrene nitrogen, contrasting with the linear nitrene geometry of the triplet copper nitrenes. CASSCF calculations also reveal the existence of a closed-shell singlet state with some degree of multiple bonding character for the copper-nitrene bond.
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ABSTRACT: Recent developments in catalytic C-H amination are discussed in this feature article. The careful design of reagents and catalysts now provides efficient conditions for exquisitely selective intramolecular as well as intermolecular nitrene C-H insertion. The parallel emergence of C-H activation/amination reactions opens new opportunities complementary to those offered by nitrenes.Chemical Communications 09/2009; · 6.38 Impact Factor
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ABSTRACT: We carried out a principle study on the reaction mechanism of rhodium-catalyzed intramolecular aziridination and aziridine ring opening at a sugar template. A sulfamate ester group was introduced at different positions of glycal to act as a nitrene source and, moreover, to allow the study of the relative reactivity of the nitrene transfer from different sites of the glycal molecule. The structural optimization of each intermediate along the reaction pathway was extensively done by using BPW91 functional. The crucial step in the reaction is the Rh-catalyzed nitrene transfer to the double bond of the glycal. We found that the reaction could proceed in a stepwise manner, whereby the N atom initially induced a single-bond formation with C1 on the triplet surface or in a single step through intersystem crossing (ISC) of the triplet excited state of the rhodium-nitrene transition state to the singlet ground state of the aziridine complexes. The relative reactivity for the conversion of the nitrene species to the aziridine obtained from the computed potential energy surface (PES) agrees well with the reaction time gained from experimental observation. The aziridine ring opening is a spontaneous process because the energy barrier for the formation of the transition state is very small and disappears in the solution calculations. The regio- and stereoselectivity of the reaction product is controlled by the electronic property of the anomeric carbon as well as the facial preference for the nitrene insertion, and the nucleophilic addition.Chemistry 11/2009; 16(2):588-94. · 5.93 Impact Factor
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ABSTRACT: Mid-to-late transition metal complexes that feature terminal, multiply bonded ligands such as oxos, imides, and nitrides have been invoked as intermediates in several catalytic transformations of synthetic and biological significance. Until about ten years ago, isolable examples of such species were virtually unknown. Over the past decade or so, numerous chemically well-defined examples of such species have been discovered. In this context, the presentreview summarizes the development of 4- and 5-coordinate Fe(E) and Co(E) species under local three-fold symmetry.Coordination Chemistry Reviews 04/2011; 255(7-8):920-937. · 11.02 Impact Factor
Bonding and Structure of Copper Nitrenes
Thomas R. Cundari,*,†Adriana Dinescu,†and Abul B. Kazi†,‡
Center for AdVanced Scientific Computing and Modeling (CASCaM), Department of Chemistry,
UniVersity of North Texas, Box 305070, Denton, Texas 76203-5070, and Department of Chemistry
and Physics, UniVersity of Arkansas at Pine Bluff, Pine Bluff, Arkansas 71601
Received July 17, 2008
Copper nitrenes are of interest as intermediates in the catalytic aziridination of olefins and the amination of C-H
bonds. However, despite advances in the isolation and study of late-transition-metal multiply bonded complexes,
Inanticipationof suchareport, terminal copper nitrenesarestudiedfromacomputational perspective. Thenitrene
complexesstudiedhereareof theform(?-diketiminate)Cu(NPh). Densityfunctional theory(DFT), completeactive
space self-consistent-field (CASSCF) electronic structure techniques, and hybrid quantum mechanical/molecular
mechanical (QM/MM) methods are employed to study such species. While DFT methods indicate that a triplet
(S) 1) is the ground state, CASSCFcalculations indicate that a singlet (S) 0) is the ground state, with only a
small energygapbetweenthesinglet andtriplet. Moreover, theground-state(open-shell) singlet copper nitreneis
found to be highly multiconfigurational (i.e., biradical) and to possess a bent geometry about the nitrene nitrogen,
contrasting with the linear nitrene geometry of the triplet copper nitrenes. CASSCF calculations also reveal the
existence of a closed-shell singlet state with some degree of multiple bonding character for the copper-nitrene
Copper catalysts are widely used for nitrene (NR, where
R is most often a hydrocarbyl or functionalized hydrocarbyl
group) transfer, most notably the aziridination of unsaturated
organic substrates such as olefins.1For example, Jacobsen
and co-workers have suggested that aziridination by copper
diimine catalysts involves a copper nitrene intermediate.2
Brandt et al. clarified the mechanism of the copper-catalyzed
aziridination of alkenes through a combination of density
functional theory (DFT) calculations and kinetics experi-
ments.3In related chemistry, Perez and co-workers have
studied the mechanism of alkene aziridination and C-H bond
amination by copper scorpionate catalysts.4While aziridi-
nation has been widely studied from an experimental point
of view, another attractive target is nitrene transfer to CO to
yield isocyanates, which are widely used as intermediates
for the production of polymers.5Several researchers have
demonstrated the feasibility of aryl isocyanate (ArNCO)
synthesis by the reaction of CO with structurally character-
ized arylimido/nitrene complexes of late transition metals.
For example, Mindiola and Hillhouse have reacted a bis-
(phosphine)nickel nitrene with CO to form ArNCO.6Bart
et al.7and Peters et al.8have reported nitrene group transfer
from structurally characterized late-transition-metal nitrene
complexes (iron and cobalt, respectively) to CO to yield
ArNCO. There is considerable interest in phosgene-free
processes for the production of isocyanates from nitroaro-
matics (nitrenes are proposed as intermediates) using late-
transition-metal catalysts.5,9For example, Moiseev et al.
* To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
†University of North Texas.
‡University of Arkansas at Pine Bluff.
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Inorg. Chem. 2008, 47, 10067-10072
10.1021/ic801337f CCC: $40.75
Published on Web 10/04/2008
2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 21, 2008 10067
reported the production of phenyl isocyanate from nitroben-
zene through the use of a palladium cluster catalyst; nitrenes
were implicated as intermediates in this transformation.10Liu
et al. discuss the formation of ArNCO from alkyl-nitro
complexes by a copper zeolite catalyst.11Kober et al. report
the formation of halogenated ArNCO from halogenated
nitroaromatics using a catalyst that is 5% PdCl2 and 5%
CuCl2supported on SiC.12
Although an isolated copper nitrene complex has not been
reported, there are structurally characterized examples of
copper(I) amido complexes, which could conceivably be
viewed as precursors to a copper nitrene. Many copper amido
complexes contain chelating amide ligands13or adopt
dinuclear,14trinuclear,15or tetranuclear15,16structures with
bridging amide groups. Recently, a monomeric copper(I)
anilido system with a bulky bis(phosphine) ligand has been
successfully synthesized and characterized.17Warren and co-
workers have recently isolated a nitrene bound between two
copper ?-diketiminate moieties.18These researchers further
propose an equilibrium between the bridging dicopper nitrene
and a terminal copper nitrene complex. Given the long-
standing interest of our group in multiply bonded species,19
we undertook an analysis of the bonding, structure, and
reactivity of copper nitrenes because such species have
intrigued synthetic chemists in terms of their putative
intermediacy in nitrene transfer catalysis.1
B3LYP20,21geometry optimization utilized the Gaussian0322
suite of programs. Complete active space self-consistent-field
(CASSCF)23calculations were carried out with the GAMESS
package.24Hybrid quantum mechanical/molecular mechanical (QM/
MM) calculations employed the SIMOMM scheme,25integrated in
GAMESS with the TINKER module.26The QM part was treated
with the CASSCF approach, while the MM part involved the MM3
force field.27The active space employed for the CASSCF calcula-
tions is discussed below in section 2. Stevens’ effective core
potentials (ECPs) and valence basis sets were employed.28Main-
group basis sets were augmented with d polarization functions. This
basis set combination, termed SBKJC(d), has been used extensively
in our laboratory in conjunction with a wide variety of wave-
function- and DFT-based methodologies and applied to ground
states, excited states, and transition states for metals from across
the entire transition series. Vibrational frequencies were calculated
at all DFT-optimized stationary points to confirm them as minima.
Modeling of triplet species with DFT employed unrestricted
Results and Discussion
Copper nitrene complexes are proposed as key intermedi-
ates in copper-catalyzed nitrene transfer. Furthermore, iso-
lated nitrenes of late transition metals such as Fe,7,8Co,8
and Ni6,29are known to transfer NR to CO to yield
isocyanates. Given their importance and experimental scar-
city, we undertook a proactive investigation of the bonding
in copper nitrene complexes. Models with phenylnitrene
(NPh) as a ligand were studied. Borden and co-workers30
have published extensively on the bonding and reactivity of
NPh. The choice of ?-diketiminate ligands as a supporting
ligand system was motivated by recent experimental work
from the Warren,31Gunnoe,32and Holland33groups. More-
over, Ghosh and co-workers have published a very interesting
series of papers on 3d metal nitrenes using ?-diketiminate
The copper-containing portion (L′Cu, where L′ is the
parent ?-diketiminate anion, C3N2H5-) of the nitrene models
is formally a closed-shell d10CuIion, and hence a singlet
ground state is expected for L′Cu (a supposition supported
by DFT calculations). For PhN, a singlet (1A2)/triplet (3A2)
splitting of ∼18 kcal/mol is derived from a combination of
multiconfiguration calculations and experiment.30The ground
state of PhN is a triplet. Note that the1A2state is an open-
shell singlet. There is a closed-shell singlet (1A1) state of
PhN that is calculated to be ∼40 kcal/mol above the3A2
Given the chemical bonding of its L′Cu and NPh con-
stituents, it is imperative to (a) address whether copper
(10) Moiseev, I. I.; Stromnova, T. A.; Vargaftik, M. N.; Orlova, S. T.;
Chernysheva, T. V.; Stolarov, I. P. Catal. Today 1999, 51, 595.
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Cant, N. W.; Haynes, B. S.; Nelson, P. F. J. Catal. 2001, 203, 487.
(12) Kober, E. H.; Martin, R. H.; Raymond, M. A. U.S. Patents 3,884,952
and 19,750,520, 1975.
(13) Hamilton, C. W.; Laitar, D. S.; Sadighi, J. P. Chem. Commun. 2004,
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C. J. Chem. Soc., Dalton Trans. 1987, 883.
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Chem. Soc. 2006, 128, 15056.
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(22) Frisch, M. J.; Pople, J. A.; et al. Gaussian 03, revision C.02; Gaussian
Inc.: Wallingford, CT, 2004.
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1993, 14, 1347.
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(29) Kogut, E.; Wiencko, H. L.; Zhang, L.; Cordeau, D. E.; Warren, T. H.
J. Am. Chem. Soc. 2005, 127, 11248.
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C. R.; Platz, M. S. Acc. Chem. Res. 2000, 33, 765.
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Philadelphia, PA, Aug 22-26, 2004; INOR-220. (b) Dai, X.; Warren,
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Cundari et al.
10068 InorganicChemistry, Vol. 47, No. 21, 2008
nitrenes possess a singlet or triplet ground state and (b)
compare open- and closed-shell bonding descriptions of the
singlet states of copper nitrenes. The ground state is of
significance with respect to catalysis, particularly amination
of C-H bonds. Using known carbenoid chemistry as a
model, triplet/singlet copper nitrene complexes may be
expected to favor radical abstraction/concerted insertion
pathways. The copper nitrene ground spin state will, of
course, depend on the nature and strength of the copper-
nitrene bond. While a copper(III) imido (i.e., formally NR2-)
description seems inconsistent with the group-transfer chem-
istry of copper aziridination catalysts, any degree of covalent
bonding in the Cu(NAr) linkage may shift the energetic
balance away from a triplet and toward a singlet ground
state.35Conversely, a triplet copper nitrene complex is
expected to result from a weaker, less covalent copper-
nitrene bond. However, given the intricate multireference
character revealed by Borden and co-workers30for metal-
free NPh, quantitative calculations on copper nitrenes are
essential to support/refute such predictions. Given the steady
progression in research on 3d late-transition-metal nitrene
complexes from Fe f Co f Ni, the time seems most
propitious for a proactive computational chemistry study of
copper nitrene complexes.
1. DFT Calculations. A DFT and CASSCF analysis was
conducted of (?-diketiminate)Cu(NPh). The latter method,
although computationally much more expensive than DFT,
was deemed prudent in light of results by Borden et al. on
NPh.30We begin by discussing the results of the DFT
1.1. Triplet State. A B3LYP/CEP-31G(d) geometry
optimization of triplet L′Cu(NPh) (C1 symmetry) yields a
minimum with a linear nitrene coordination mode; the Ph
substituent is coplanar to the ?-diketiminate ligand
(Cu-Nnitrene ) 1.779 Å; Nnitrene-Cipso ) 1.331 Å;
Cu-Nnitrene-Cipso) 180.0°). Unrestricted DFT computations
on3L′CuNPh indicate delocalized spin density. Spin polar-
ization effects are substantial, as can be seen from large
negative spin densities on the ipso and meta carbons of the
phenyl substituent (Figure 1) in addition to positive spin
densities on the ortho and para carbons. Interactions of π
type between the nitrene nitrogen and aryl substituents are
also reflected in the short Nnitrene-Cipso distances.30This
delocalization suggests the potential for substantial control
of copper nitrene reactivity through the choice of functional
groups on the aryl substituent, which is inherently reasonable
given the synthetic control observed for copper-catalyzed
aziridinations with a variety of nitrene precursors, including
Chloramine-T, iodonium imides (e.g., PhIdNTs), aryl azides
(ArN3), and so forth.36
1.2. Singlet State. A singlet state for L′Cu(NPh) (C1
symmetry) was also geometry-optimized by DFT methods
and yielded a minimum with a bent nitrene geometry
(Cu-Nnitrene ) 1.726 Å; Nnitrene-Cipso ) 1.356 Å;
Cu-Nnitrene-Cipso) 147.8°). The singlet state is calculated
to be 13.9 kcal/mol higher in energy than the corresponding
triplet state discussed in section 1.1. For L′Cu(NPh), the
singlet is approximately Cssymmetry. Unlike the triplet, the
singlet copper nitrene shows some evidence of Cu-N
multiple bonding (Figure 2, bottom). Like the triplet, there
is delocalization of the nitrene pπ orbital onto the aryl
substituent (Figure 2, middle).
2. CASSCF Calculations. 2.1. C2W-Symmetric Copper
Nitrene Model Complexes. The CASSCF and DFT calcula-
tions yield different viewpoints of the copper nitrene
electronic structure, in particular singlet states. Several
CASSCF active space sizes were investigated, and the results
were found to be similar; i.e., the frontier b1and b2π orbitals
of the copper-nitrene linkage are the most strongly cor-
related orbitals. Hence, the discussion focuses on the results
from the largest active space studied. The 10-orbital, 10-
electron basis set was chosen to incorporate electron cor-
relation effects from the four highest energy doubly occupied
orbitals that subtend the a1+ a2+ b1+ b2representations,
and the four lowest energy unoccupied orbitals that span the
(35) (a) Cundari, T. R.; Gordon, M. S. J. Am. Chem. Soc. 1991, 113, 5231.
(b) Taylor, T. E.; Hall, M. B. J. Am. Chem. Soc. 1984, 106, 1576.
(36) (a) Zhdankin, V. V.; Stang, P. J. Chem. ReV. 2002, 102, 2523. (b)
Campbell, M. M.; Johnson, G. Chem. ReV. 1978, 78, 65. (c) Brase,
S.; Gil, C.; Knepper, K.; Zimmermann, V. Angew. Chem., Int. Ed.
2005, 44, 5188.
Figure 1. Spin density of (?-diketiminate)Cu(NPh). Color code: blue,
positive spin density; yellow, negative spin density.
Figure 2. Frontier orbitals of singlet (?-diketiminate)Cu(NPh): LUKSO
(top), HOKSO (middle), and HOKSO-3 (bottom).
Bonding and Structure of Copper Nitrenes
Inorganic Chemistry, Vol. 47, No. 21, 2008 10069
same representations, plus the singly occupied b1and b2of
the Hartree-Fock reference wave function. It is worth noting
that basis set requirements for wave-function-based correlated
methods are likely to be more stringent than those for DFT
but that disk space/memory issues constrained us to use of
the SBKJC(d) basis sets. While this will likely change the
specific energetics, the essential chemistry of the copper
nitrenes vis-a `-vis the singlet/triplet splitting and the multi-
reference nature of the singlet ground state is not expected
to be altered.
CASSCF/CEP-31G(d) geometry optimizations with a 10-
orbital, 10-electron active space were first carried out under
C2Vsymmetry for singlet and triplet (?-diketiminate)Cu(NPh)
for all possible state symmetries. The CASSCF(10,10)
calculations yield a3A2ground state within the constraint
of C2Vsymmetry. Low-energy1A1,1A2, and3A2states were
also obtained. Singlet and triplet states of B1 and B2
symmetry were much higher in energy (>73 kcal/mol). The
3A2 state obtained by CASSCF geometry optimization of
L′Cu(NPh) is reminiscent of the ground-state triplet found
by DFT calculations (vide supra). Furthermore, Borden et
al. have concluded from experimental and computational data
that the ground state of PhN is3A2, which is ∼18-20 kcal/
mol below the lowest-energy singlet state (1A2) of NPh.30
Hence, the3A2(?-diketiminate)Cu(NPh) state is conceptually
the product of the3A2NPh and1A1(?-diketiminate)Cu states
of its constituents. Pertinent structural data for C2Vnitrene
complexes are summarized in Table 1.
The singlet (1A2)/triplet (3A2) splitting at the CASSCF level
of theory, i.e., 2.0 kcal/mol [Figure 3 (left-hand side)], is
considerably less than that indicated by DFT calculations.
The lowest singlet1A1is now calculated to be only 4.8 kcal/
mol higher in energy than the3A2state of (?-diketiminate)-
Cu(NPh) for CASSCF/CEP-31G(d) geometries optimized
within the constraint of C2V symmetry. Hence, CASSCF
calculations greatly stabilize both the open-shell (1A2) and
10070 InorganicChemistry, Vol. 47, No. 21, 2008
closed-shell (1A1) singlet states of the copper nitrene relative
to the triplet state, suggesting that the former possess
significant multireference character.
2.2. Cs-Symmetric Copper Nitrene Model Complexes.
The singlet/triplet energy splitting obtained for C2V model
geometries of L′Cu(NPh) is likely to be less than the
foregoing estimates because the DFT evidence (see section
1.2) suggests that singlet copper nitrenes have an appreciably
more bent nitrene coordination mode than the corresponding
triplets. For this reason, utilizing the 10-orbital, 10-electron
CASSCF methodology, (?-diketiminate)Cu(NPh) was opti-
mized under Cs symmetry for singlet and triplet states of
both A′ and A′′ symmetry. When bending of the nitrene
ligand is permitted, the closed-shell1A′ state is now predicted
to be the ground state for L′Cu(NPh). Relative energies (kcal/
mol) of the other symmetry states are 6 (3A′′), 7 (1A′′), and
12 (3A′) (Figure 3). While the A′ states are bent (Table 2),
the A′′ states return to linear nitrene coordination (150° was
used as a starting guess for the geometry optimizations as a
compromise between sp and sp2hybridization of the nitrene
nitrogen) and thus3A′′ and1A′′ correspond to the3A2and
1A2states of C2VL′Cu(Ph), respectively (see Figure 3).
As with multireference calculations on NPh,30variations
are seen in the CASSCF-optimized bond lengths between
the ipso carbon and the nitrene nitrogen (Table 2) for the
different electronic states. Furthermore, interesting variations
are also seen in copper-nitrene nitrogen bond distances and
copper-nitrene-ipso carbon bond angles for the different
symmetry states of Cs(?-diketiminate)Cu(NPh). The lowest-
energy singlet (1A′) and triplet (3A′′) states of CASSCF(10,10)/
CEP-31G(d)-optimized L′Cu(NPh) are bent and linear,
respectively, a result consistent with the DFT geometry
optimizations of these nitrenes. Apart from the high-energy
3A′ state, Cu-N bonds to the nitrene are little changed from
1.84 to 1.85 Å (Table 2) for the different electronic states.
The nitrene nitrogen-ipso carbon distance for the1A′ ground
state is ∼0.06 Å longer than the analogous bond in the3A′′
state. Indeed, both A′ states have longer N-Cipsobond lengths
than the A′′ states, regardless of the spin state.
One important result that emerges from the present
calculations is that the1A′ ground state cannot be adequately
described by a single electronic configuration, as evidenced
by the computed natural orbital occupation numbers (NOONs)
of (πCuN)1.5(πCuN*)0.5(Figure 4). These values are far removed
from the single determinant occupation numbers of 2 and 0,
respectively, and suggest significant diradical character to
the copper-nitrene π bond. Analysis of the natural orbitals
from the CASSCF calculation of
Table 1. CASSCF(10,10)/CEP-31G(d)-Optimized C2VGeometries for
aCalculations were carried out with the phenyl substituent perpendicular
to the plane of the ?-diketiminate ligand because this was indicated to be
the most stable conformer from DFT calculations.
Figure 3. Qualitative correlation diagram for low-energy singlet and triplet
states of L′Cu(NPh) in C2V(left) and Cs(right) symmetry.
Table 2. CASSCF(10,10)/CEP-31G(d)-Optimized CsGeometries for
aCASSCF calculations were carried out with the phenyl substituent
perpendicular to the Csplane of the molecule because this conformation
was found to be most stable in DFT geometry optimization.
Cundari et al.
Cu(NPh) supports some degree of multiple bonding character
for the copper-nitrene bond (Figure 4).
The A′′ (∼A2) states (both singlet and triplet) of (?-
diketiminate)Cu(NPh) are quite different in their character
from the corresponding A′ states of L′Cu(NPh), the former
displaying perpendicular π symmetry orbitals that are highly
polarized on either the copper or nitrene nitrogen end (Figure
5). In this regard, the description of the copper-nitrene bond
of3A′′ (?-diketiminate)Cu(NPh) within the CASCF frame-
work is reminiscent of similar calculations on dioxygen.37
The nitrene nitrogen-polarized orbital remains partially
delocalized to the phenyl substituent [Figure 5 (bottom)].
3. QM/MM Calculations. With regards to possible steric
effects, bulky substituents were added to the ?-diketiminate
ligand: two mesityl and two methyl substituents (LMe,Mes),
while the nitrene substituent was set as 3,5-dimethylphenyl.38
These modifications model the putative terminal copper
nitrene intermediate recently reported by Warren and co-
workers.18Figure 6 depicts the optimized structures of singlet
(1A) and triplet (3A) states using the hybrid CASSCF(10,10)/
CEP-31G(d):MM3 method with no symmetry constraint to
the geometry optimization.
Optimized geometries of (LMe,Mes)Cu(N-3,5-C6H3Me2) us-
ing hybrid QM/MM methods show the same structural
patterns as those for smaller models: the triplet is closer to
linear (Cu-Nnitrene-Cipso ) 160.7°), while the singlet is
markedly bent (Cu-Nnitrene-Cipso) 130.4°). Still, the singlet/
triplet energy splitting of 21.5 kcal/mol is larger than those
in previous small-model calculations (Table 2), suggesting
a less favorable steric interaction between the nitrene and
Cu(?-diketiminate) substituents in the triplet state versus the
singlet ground state. The NOONs for the singlet (LMe,Mes)-
Cu(N-3,5-C6H3Me2) are very similar to those obtained for
the1A′ ground state of L′Cu(NPh), i.e., 1.5 e-and 0.5 e-
for natural orbitals akin to those shown in Figure 4 for the
L′Cu(NPh) model. However, in the case of the triplet
(LMe,Mes)Cu(N-3,5-C6H3Me2), the natural orbitals show that
both pπ orbitals of the nitrene nitrogen are occupied with
∼1 e-each. Thus, the QM/MM calculations suggest that
the increase of steric effects on the ?-diketiminate ligand
will tend to destabilize the triplet relative to the ground-
state singlet of copper nitrenes with these specific supporting
ligands and substituents.
Summary and Conclusions
Copper nitrene complexes are of considerable interest as
intermediates in nitrene transfer although they have thus far,
to our knowledge, eluded structural characterization. Thus,
DFT, CASSCF, and QM/MM methods were employed in
conjunction with ECP basis sets to study such species. The
copper nitrene complexes studied are of the form (?-
diketiminate)Cu(NPh),31-33,38and two models were con-
sidered. The “small” model [L′Cu(NPh)], where all substit-
uents on the ?-diketiminate and phenyl groups are replaced
with H atoms, was investigated under both C2V and Cs
symmetries. The larger model [(LMe,Mes)Cu(N-3,5-C6H3Me2)]
possesses methyl and mesityl substituents on ?-diketiminate
(37) Carter, E. A.; Goddard, W. A., III J. Phys. Chem. 1988, 92, 2109.
(38) (a) Badiei, Y. M.; Warren, T. H. J. Organomet. Chem. 2005, 690,
5989. (b) Dai, X.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085.
Figure 4. πCuN(bottom) and πCuN* (top) natural orbitals for the1A′ state
of (?-diketiminate)Cu(NPh). The NOONs are 1.5 e-for πCuNand 0.5 e-
Figure 5. πN(bottom) and πCu(top) natural orbitals for the3A′′ state of
(?-diketiminate)Cu(NPh). The NOONs are ∼1.0 e-for both.
Figure 6. Top (top) and side (bottom) views of (LMe,Mes)Cu(N-3,5-
C6H3Me2) optimized geometries using a QM/MM approach. The QM region
[CASSCF(10,10)/CEP-31G(d)] is illustrated in a ball-and-stick representa-
tion, while the MM region (MM3 force field) is illustrated in a stick
representation: (a) singlet state; (b) triplet state.
Bonding and Structure of Copper Nitrenes
Inorganic Chemistry, Vol. 47, No. 21, 2008 10071