Redox non-innocence permits catalytic nitrene carbonylation by (dadi)TiNAd (Ad = adamantyl)

Article abstract of DOI:10.1039/c6sc05610e

Application of the diamide, diimine {-CHN(1,2-C₆H₄)N(2,6-ⁱPr₂-C₆H₃)}₂^m ((dadi)^m) ligand to titanium provided adducts (dadi)TiL_x (1-L_x; L_x = THF, PMe₂Ph, (CNMe)₂), which possess the redox formulation [(dadi)^4-]Ti(iv)L_x, and 22 πe^- (4n + 2). Related complexes containing titanium-ligand multiple bonds, (dadi)TiX (2X; X = O, NAd), exhibit a different dadi redox state, [(dadi)^2-]Ti(iv)X, consistent with 20 πe^- (4n). The Redox Non-Innocence (RNI) displayed by dadi^m impedes binding by CO, and permits catalytic conversion of AdN₃ + CO to AdNCO + N₂. Kinetics measurements support carbonylation of 2NAd as the rate determining step. Structural and computational evidence for the observed RNI is provided.

Full text of DOI:10.1039/c6sc05610e

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Redox non-innocence permits catalytic nitrene carbonylation by
dadi)Ti=NAd (Ad = adamantyl)
(
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
Spencer P. Heins, Peter T. Wolczanski,* Thomas R. Cundari,* and Samantha N. MacMillan
i
m
m
DOI: 10.1039/x0xx00000x
6 4 2 6 3 2
Application of the diamide, diimine {-CH=N(1,2-C H )N(2,6- Pr -C H )} ((dadi) ) ligand to titanium provided adducts
4
-
-
(dadi)TiL
x
(1-L
x
; L
x
= THF, PMe
2
Ph, (CNMe)
2
), which possess the redox formulation [(dadi) ]Ti(IV)L
x
, and 22 πe (4n+2).
www.rsc.org/
Related complexes containing titanium-ligand multiple bonds, (dadi)Ti=X (2=X; X = O, NAd), exhibit a different dadi redox
2
-
-
m
state, [(dadi) ]Ti(IV)X, consistent with 20 πe (4n). The Redox Non-Innocence (RNI) displayed by dadi hampers binding by
CO, and permits catalytic conversion of AdN + CO to AdNCO + N . Kinetics measurements support carbonylation of 2=NAd
as the rate determining step. Structural and computational evidence for the observed RNI is provided.
3
2
Introduction
i
The diamide, diimine {-CH=N(1,2-C H )N(2,6- Pr -C H )} (m =
m
1-20
Redox non-innocent (RNI) ligands like dadi can support
m
6
4
2
6 3 2
m
to -4), i.e. (dadi) , was previously introduced as
11-17
0
a
1
catalytic activity
by expanding the redox capability of a
tetradentate chelate ligand capable of at least 5 redox states.
system, essentially electronically buffering the transition metal
In the initial study, (dadi)M (M = Cr(THF), Fe) was shown to center. As Fig. 1 illustrates, a ligand capable of RNI can keep
m+2
transfer azide-generated nitrenes into carbon-carbon bonds the metal in a stable configuration (M ) by being in a
p
2
-
-
p-2
via aziridination. In the process the dadi ligand (20 π
e , 4n) reduced state (L -> L ), and releasing the electrons to the
-
p-2 m+2
p
m
-> L M ->
was derivatized to a 22
i
)N(2,6- Pr
π
] M (R = 2,6- Pr
e (4n+2) ligand (i.e., [RN{CH=N(1,2- metal upon oxidative addition of XY (L
M
2- II
i
p
m+2
C
6
H
4
2
-C
6
H
3
)}
2
2
C
6
H
3
, adamantyl (Ad)) L M XY). Transfer of XY to a substrate, perhaps via a binding
step, causes reduction of the metal, but its formal oxidation
of considerable stability.
m
state is maintained (M -> M ) by the RNI of the ligand.
m+2
m
In its dianionic (n = -2) state, (dadi) possessed a 4n
π
system that is intrinsically susceptible to redox events, and
-
8
,9
redox non-innocence that can potentially support unusual
2
reactivity. Chelation of titanium(II) by (dadi) was a target,
-
4
-
with the expectation that RNI leading to (dadi) Ti could
IV
potentially stabilize the system, as the tetraanion has a 4n+2
π
-system. This report describes (dadi)Ti(X/L) species that
manifest RNI, including a rare instance in which carbon
monoxide does not affect redox activity at the metal, and does
not bind, thus permitting its use as a substrate in catalysis.
Figure 1. Conventional oxidative addition (blue), and corresponding
-
1
- and 2-e redox non-innocent (RNI) configurations (green) that can
p
Results
π-system can be
m
electronically buffer the metal center. The ligand, L ,
4n+2) -> 4n or 4n -> (4n-2) depending on its compatibility with M .
(
Synthesis of (dadi)TiL
CNMe)
n
2
(n = 1, L = THF, PMe Ph; n = 2, L =
(
dadi)Ti(THF). Deprotonation of {-CH=N(1,2-C
1
6
H
4
)NH(2,6-
i
Pr
2
-C
6
H
3
)}
2
2 2
, i.e., (dadi)H , with 2 equiv NaN(TMS) afforded
2
the disodium salt of (dadi) in 94% yield according to Scheme
-
1. The dianion is bright blue in THF solution, and exhibits a
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-
1
-1
= 22,000 M cm ) that
dominant absorption band at 770 nm (
ε
features a ~1500 cm progression plausible for a diimine.
2
(dadi)Ti(CNMe) . In contrast, exposure of 1-THF to carbon
-1
monoxide (1 atm) resulted in no displacement of THF by CO
2
1
2
in THF (Scheme 2). A d titanium center would likely provide a
Treatment of (TMEDA)
2
TiCl
2
with (dadi)Na
2
at -20°C for 3h yielded a green solution. After filtration in favorable backbonding situation to a carbonyl ligand, whereas
4
-
cyclohexane, (dadi)Ti(THF) (
yields as an olive-green powder that contained some residual
12. The five-coordinate environment of diamagnetic -THF center to generate blood red (dadi)Ti(CNMe)
renders the isopropyl groups inequivalent in its H NMR in the mono-adducts, -(CNMe) also exhibits a normal diimine
spectrum due to hindered rotation of the aryl, and they are hydrogen chemical shift (
6.16) in its H NMR spectrum, and a
1
-THF) was isolated in ~40-50% a more electrophilic [(dadi) ]Ti(IV) configuration would favor
-donors. It is noteworthy that MeNC binds to the titanium
-(CNMe) ). As
σ
C
6
H
1
2
(1
2
1
1
2
1
δ
-
1
observed as doublets in a ~12:6:6 ratio along with THF broad
resonances.
above the stretching frequency of free MeNC (2164 cm ),
consistent with a lack of -backbonding, and indicative of a
ν(NC) at 2199 cm in its IR spectrum. The latter is well
-1
π
4
-
TiCl
2
(TMEDA)
2
1) THF
20°C, 3h
NaCl
N
N
(dadi) formulation.
ꢀ
ꢀ
+
Ti
2
N
N
N
blue
THF
oliveꢀgreen
2
6 12
) C H
1
ꢀTHF
4
0ꢀ50%
N
iPrꢀ6,2
2,6ꢀ Pr
i
NNa
1
) THF
NaN
ꢀ20°C, 2h
NaCl
6 6
C D
PMe Ph
2
ꢀ
THF
iPrꢀ6,2
ꢀ 2
i
2
PMe Ph
2
,6ꢀPr
2
) PMe Ph
2
(
dadi)Na
NaN(TMS)
dadi)H
2
6 6
C H
N
N
N
Ti
Scheme 2. Methylisocyanide binds, but CO does not.
2
2
ꢀ 2 HN(TMS)
2
57%
N
maroon
(
2
2
1ꢀPMe Ph
iPrꢀ6,2
i
2,6ꢀ Pr
Scheme 1. Synthesis of (dadi)Na
PMe Ph, -PMe Ph).
2
and (dadi)TiL (L = THF,
1
-THF;
2
1
2
1
Fe, Mn), the
For related complexes (dadi)M (M
=
aforementioned, attenuated intraligand (IL) band was split into
-1
-1
components at ~720 nm and ~940 nm (ε ~ 5-12,000 M cm ) in
the UV-vis spectrum. Blue shifted, low intensity features at 495
-
1
-1
-1
-1
(
ε
~ 900 M cm ) and 590 nm (
remnants of these IL bands according to TDDFT calculations,
and are accompanied by a major absorption at 345 nm (
ε ~ 700 M cm ) in 1-THF are
ε
=
-
1
-1
3
2,000 M cm ). The spectrum suggested a more covalent
electronic environment than in the high spin Fe and Mn
4
-
a [(dadi) ](THF)Ti(IV)
species, and calculations support
2 2
Fig. 2. Molecular view of (dadi)TiPMe Ph (1-PMe Ph). Interatomic
distances (Å) and angles (°): Ti-P, 2.6049(7); Ti-N1, 2.0029(18); Ti-N2,
2.0194(18); Ti-N3, 2.0332(18); Ti-N4, 2.0088(18); N1-C13, 1.411(3);
configuration in which the tetraanionic, tetradentate chelate
-
possesses a 22e (4n+2) π-system.
1
0
1
Unlike previous systems, the H NMR spectroscopic C13-C18, 1.413(3); N2-C18, 1.396(3); N2-C19, 1.379(3); C19-C20,
1
.338(3); N3-C20, 1.381(3); N3-C21, 1.396(3); C21-C26, 1.408(3); N4-
chemical shift of the diimine hydrogens, δ 6.64 in (dadi)Ti(THF)
-THF), does not distinguish between redox states of (dadi) .
C26, 1.406(3); P-Ti-N1, 114.64(6); P-Ti-N2, 85.21(5); P-Ti-N3, 85.23(5);
P-Ti-N4, 102.35(5); N1-Ti-N2, 78.10(7); N1-Ti-N3, 143.60(7); N1-Ti-N4,
n
(
1
Calculations of pseudo-square planar (dadi)Ti suggest that it 123.37(7); N2-Ti-N3, 73.38(7); N2-Ti-N4, 148.83(7); N3-Ti-N4, 77.14(7).
3
-
ꢀ
ꢁ
1
would be best considered [(dadi) ]Ti(III) , a d titanium anti-
3
ferromagnetically coupled to (dadi) , but adduct formation
-
Structural studies of (dadi)TiL
n
(L = PMe
Ph, L = (CNMe) )
2 2 2
4
-
causes an electronic reorganization to the [(dadi) ](L)Ti(IV)
configuration.
(
dadi)Ti(PMe
dadi)TiPMe Ph (1-PMe Ph) supports the [(dadi) ](PhMe Ph)
2 2
2
Ph). The X-ray structure determination of
4
-
(
2
(dadi)Ti(PMe
2
Ph). Crystals amenable to an X-ray structural
Ti(IV) electronic configuration. Fig. 2 illustrates a molecular
view of -PMe Ph, revealing a pseudo-square pyramidal
structure that is distorted from phosphine-isopropyl
interactions. The PMe Ph is tilted toward the diimine ( P-Ti-
Nim = 85.22(2)° (ave)) and the orientation of its phenyl group
determination of the THF-adduct were elusive, thus a PMe Ph
2
1
2
2 2
derivative, (dadi)TiPMe Ph (1-PMe Ph) was prepared in situ by
adding the phosphine subsequent to metalation, as shown in
2
/
Scheme 1. The maroon phosphine adduct, which also
1
manifests a normal diimine H NMR spectroscopic shift at
δ
.73, can also be made in a stepwise fashion via substitution of
causes the P-Ti-N1 angle (114.64(6)°) to be greater than the
other amide (/P-Ti-N4 = 123.37(5)°).
6
THF from 1-THF.
2
| J. Name., 2012, 00, 1-3
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Plea sC eh de om ni oc ta al dS j uc si et nm c ae rgins
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DOI: 10.1039/C6SC05610E
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ARTICLE
The crucial d(CN ) of 1.379(3) and 1.381(3) Å are packed, and well separated from the others, partly as a
im
2
significantly longer than the expected 1.30 Å for an neutral consequence of DFT calculations. Orbitals dxz and dyz possess
3
1
8,19,22
imine,
and the d(CimCim) is also anomalous at 1.338(3) Å, modest
π-character, but are empty, as expected for the
4-
a value substantially shorter than a C(sp )-C(sp ) distance of (dadi) formulation. Overall, the "3d" orbitals are essentially
2
2
2
2
1
.43 Å. Titanium-nitrogen distances to the "imines" are also non-bonding, due to the multicomponent mixing intrinsic to
short, at 2.0194(18) and 2.0332(18) Å, values that reflect low symmetry systems. All filled orbitals shown possess some
greater covalency in the interaction of the tetraamide chelate d-character, so even though dadi is expressed as tetranionic,
to the Ti(IV) center. For comparison, the "diimine" Ti-N the metal/dadi admixture reflects the covalence of the system.
distances are nearly the same as the titanium arylamide-
nitrogen bond lengths of 2.0029(18) and 2.0088(18) Å.
d
ꢀy
z
2
ꢀ1.20
ꢀ1.24
ꢀ1.37
ꢀ1.66
ꢀ1.84
1
ꢀPMe
z
2
Ph
d
x
2
2
d
xz
dxy
y
PMe
2
Ph
d
yz
x
N
N
N
Ti
N
i
Prꢀ6,2
2,6ꢀPr
i
NCCN πb
Ar πb
ꢀ4.80
ꢀ5.65
ꢀ5.92
Ar(Am) π
b
2 2
Fig. 3. Molecular view of (dadi)Ti(CNMe) (1-(CNMe) ). Interatomic
distances (Å) and angles (°): Ti-N1, 2.0677(16); Ti-N2, 2.0331(15); Ti-
N3, 2.0353(16); Ti-N4, 2.0636(15); Ti-C39, 2.2484(18); Ti-C41,
2 2
Fig. 4. Truncated MO diagram of (dadi)TiPMe Ph (1-PMe Ph),
0
showing the unfilled orbitals of the d species, and three filled dadi-
based orbitals (energies in eV). Assignments are tentative due to the
low symmetry: M06/6-311+G(d) single point calculations for orbitals at
the ONIOM(M06/6-311+G(d)) optimized minimum derived from the X-
ray structure as an initial guess.
2
.2364(18); N1-C13, 1.396(2); C13-C18, 1.418(3); N2-C18, 1.403(2); N2-
C19, 1.373(2); C19-C20, 1.350(3); N3-C20, 1.371(2); N3-C21, 1.401(2);
C21-C26, 1.423(2); N4-C26, 1.395(2); C39-N5, 1.141(2); C41-N6,
1
.145(2); N1-Ti-N2, 77.04(6); N1-Ti-N3, 149.95(6); N1-Ti-N4, 132.75(6);
N2-Ti-N3, 73.96(6); N2-Ti-N4, 150.05(6); N3-Ti-N4, 76.83(6); N1-Ti-C39,
1.41(6); N2-Ti-C39, 84.33(6); N3-Ti-C39, 93.42(6); N4-Ti-C39, 90.98(6);
9
N
N
N1-Ti-C41, 91.37(6); N2-Ti-C41, 92.30(6); N3-Ti-C41, 82.13(6); N4-Ti-
C41, 90.14(6); C39-Ti-C41, 175.04(7).
74%
maroonꢀred
Ti
ꢀ N2
N
N
N
Ti
THF
+
1
ꢀTHF
C
6
H
6
N
N
N
N
N
N
23°C, 15 min
(
dadi)Ti(CNMe)
CNMe) ) molecule, and select core distance and angles are
given in the caption. Although -(CNMe) is six-coordinate, all
dadi metric parameters are within 0.015 Å to those
corresponding to (dadi)TiPMe Ph ( -PMe Ph), with d(C19-C20)
1.350(3) Å, and d(CNim) = 1.372(2) (ave),
2
.
Fig. 3 illustrates the (dadi)Ti(CNMe)
2
(
1
-
iPrꢀ6,2
i
2,6ꢀ Pr
N
(
2
iPrꢀ6,2
2,6ꢀ Pr
i
1
2
2 h
3°C
Me PO
1
2
N
2
O, ꢀ N
2
3
2
5 min
3°C
(1 atm)
2=NAd
C
6
H
6
3
C
6
H
6
PMe
O
3
2
1
2
O
Ti
2=O
1ꢀOPMe3
1
8,19,22
=
consistent
Ti(IV) formulation, as predicted by the
its IR spectrum. The d(Ti-N) are within 0.035 Å at 2.050(18) Å
ave), and describe four amide linkages, although slightly
longer than in -PMe Ph (2.016(13) Å (ave)). The dadi bite
angles of NamTiNim and NimTiNim are 76.94(15)° (ave) and Scheme 3. Imide, (dadi)Ti=NAd (
formation from (dadi)Ti(THF) ( -THF), AdN
7
0%
N
N
N
PMe
3
N
Ti
N
4
-
green
81%
with a [dadi] (CNMe)
2
6 6
C D
N
N
N
limeꢀgreen
iPrꢀ6,2
23°C, 3 d
i
iPrꢀ6,2
i
(
2,6ꢀ Pr
2,6ꢀ Pr
1
2
2=NAd), and oxo, (dadi)Ti=O (
2=O),
1
3
and N O, respectively.
2
7
3.96(6)°, essentially the same as in the phosphine adduct.
Synthesis of (dadi)Ti=X (X = NAd, O)
Electronic structure of (dadi)Ti(PMe Ph)
2
(
dadi)Ti=NAd. Organoazides were explored as a means to
2
3-25
26-28
prepare imide derivatives,
with mixed results.
Use of
The metric parameters found in X-ray crystal structure of
Me
3 3
SiN led to a mixture of products, although NMR
(dadi)TiPMe Ph (1-PMe Ph) prompted an electronic structure
2 2
spectroscopic signals consistent with (dadi)Ti=NSiMe3 were
i
seen, and 2,6- Pr C H N afforded a complex product devoid of
2 6 3 3
investigation to assess the proposed RNI. Fig. 4 illustrates a
truncated molecular orbital diagram featuring three filled
symmetry. Treatment of (dadi)Ti(THF) with adamantyl-azide in
benzene effervesced immediately, and maroon-red
orbitals, with a diimine CC-π-bonding orbital as the HOMO.
The unfilled orbitals, predominantly 3d in character, are tightly
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dadi)Ti=NAd (
Journal Name
(
2
2 2 2
=NAd) was isolated in 74% yield, as indicated in ligand of (dadi)TiPMe Ph (1-PMe Ph) and (dadi)Ti(CNMe) (1-
1
2
Scheme 3. H NMR spectra revealed an integrated 6:12:6 ratio (CNMe) ). The d(TiO) of 1.6361(11) Å is typical for the
of CH(CH3
)2 hydrogens, suggestive of a five-coordinate imide numerous Ti(IV) oxo species that have been characterized
(1.63(1) Å ave for 15 5-coord examples), and the remaining
complex.
(
dadi)Ti=O. Generation of the corresponding oxo complex amide distances are typical (2.059(7) (ave) Å).
proved to be equally interesting. Initially, common oxygen
atom transfer (OAT) agents, such as PhIO, Me NO, and PyN-O
3
29-34
ꢀ0.35
failed, but unexpectedly, nitrous oxide
Exposure of (dadi)Ti(THF) ( -THF) to N O (1 atm) in benzene
for ~30 min afforded a lightening of the green solution, and
lime green (dadi)TiO ( -O) was eventually precipitated from
proved fruitful.
1
little TiO π*
ꢀ0.97
2
ꢀ
ꢀ
1.32
1.66
2
NCCN π*
benzene in 70% yield, as Scheme 3 illustrates. Four sets of
1
isopropyl-methyl doublets were observed in the H NMR
TiO π*
d
xy
ꢀ
3.41
x
spectrum in a 6:6:6:6 ratio, and a resonance Raman (
λ
nm) spectrum showed an absorption at 1015.4 cm tentatively
= 475
NCCN π*
-1
z
y
O
Ti
2=O
dadi π
2
9
N
N
assigned to the
with a variety of substrates (olefins, CO, etc.), but only transfer
to PMe , resulting in green (dadi)Ti-OPMe -OPMe ), was
observed. The latter compound was independently prepared
from (dadi)Ti(THF) ( -THF) and 1 equiv of Me PO.
ν(TiO). OAT from (dadi)TiO (2=O) was tested
N
Ar
N
Ar
(
1
3
ꢀ5.53
ꢀ6.01
3
3
dadi π
ꢀ
6.66
ꢀ6.68
6.75
6.78
1
3
ꢀ
ꢀ
dadi π
dadi π
ꢀ
7.11
ꢀ
7.73
TiO π
ꢀ8.23
ꢀ
ꢀ
8.50
8.59
Fig. 6. Truncated molecular orbital diagram of (dadi)TiO (2=O);
orbitals not plotted are either diffuse virtual orbitals or pertain to 2,6-
i
2 6 3
Pr C H . Energies are in eV.
Electronic structure of (dadi)TiO
The electronic structures of (dadi)TiO (2=O) and (dadi)Ti=NAd
(
2
=NAd) were investigated as examples of species in which the
chelate was dianionic. The calculation of =O is featured
Ti-N3, 2.1603(13); Ti-N4, 2.0640(12); N1-C1, 1.3770(19); C1-C6, herein, since the experimental structure is available for
Fig. 5. Molecular view of (dadi)TiO (
and angles (°): Ti-O, 1.6361(11); Ti-N1, 2.0538(13); Ti-N2, 2.1938(12);
2=O). Interatomic distances (Å)
2
1
1
.419(2); N2-C6, 1.395(2); N2-C7, 1.301(2); C7-C8, 1.433(2); N3-C8,
.297(2); N3-C9, 1.388(2); C9-C14, 1.418(2); N4-C14, 1.3750(19); O-Ti-
comparison. As Fig. 6 illustrates, the TiO
buried below a group of occupied dadi-
05.66(5); N1-Ti-N2, 75.40(5); N1-Ti-N3, 144.73(5); N1-Ti-N4, HOMO, which has significant Np
-character, but no metal
π
-bonding orbitals are
π
-orbitals, including the
N1, 105.18(5); O-Ti-N2, 104.88(5); O-Ti-N3, 94.71(5); O-Ti-N4,
1
1
π
24.02(5); N2-Ti-N3, 71.44(5); N2-Ti-N4, 136.56(5); N3-Ti-N4, 75.94(5).
constituent. These contain components of NCCN bonding, but
there are no discrete "diimide" orbitals, although a pair of
Structural study of (dadi)TiO
unoccupied orbitals, including the LUMO, are clearly NCCN π*
in character. The diagram is consistent with the claim of a
A molecular view of (dadi)TiO (2=O) and accompanying metric
2
highly delocalized (dadi) ligand attached to a titanyl
-
parameters are given in Fig. 5. The core O-Ti-N angles average
05.2(4)°, except for /O-Ti-N3 = 94.71(5)°, as the oxo tilts
fragment, whose
and -8.59 eV.
π-bonds are clearly rendered at -8.23
1
slightly toward N3. The diimine bite angle of 71.44(5)°, imine-
amide bite angles averaging 75.7(4)°, and the opening N1-Ti-
N4 angle of 124.02(5)° describe a slightly distorted square
The distortion in (dadi)TiO
(2=O), reproduced by
calculation, appears to be steric in origin, as removal of the
i
2
,6- Pr
to a more symmetric species. Note the similarities in the
diagrams of (dadi)TiPMe Ph ( -PMe Ph) and =O, which both
-system orbitals, an
*-orbital, and ligand antibonding and metal orbitals.
While both diagrams depict Ti(IV), the middle NCCN *-orbital
2 6 3
-C H groups and re-optimization of the geometry leads
pyramid with basal nitrogens and the apical oxo. The
backbone contains d(CN) = 1.299(3) (ave) Å and d(CC) =
.433(3) Å, consistent with a neutral diimine fragment of a
dadi
1
2
2
2
1
2
-
18,19,22
show significant gaps between dadi
NCCN
π
(dadi) ligand.
The titanium-nitrogen bond lengths to the
π
diimine are 2.1603(13 and 2.1938(12) Å, substantially longer
-
than the electrostatically contracted d(TiN) of the (dadi)
4
π
4
| J. Name., 2012, 00, 1-3
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ARTICLE
2
=O, i.e., (dadi) , while it is filled for
-
is empty for
2
1-PMe Ph, of
2=NAd with an excess of CO (pseudo first-order, 3.1-3.6
2
4
i.e., (dadi) , thereby revealing disparate redox states for each.
-
atm) in C D (with THF present) at 25-75°C afforded AdNCO,
6
6
CO
ꢀ4 atm
AdNCO
2
k
CO
1
2=NAd
N
N
N
Ti
N
N
N
N
55°C
Ti
N
N
6 6
C D
iPrꢀ6,2
2,6ꢀPr
i
iPrꢀ6,2
2,6ꢀPr
i
k
Im
N
2
AdN
3
k
deg
CO (3.1ꢀ3.6 atm)
THF
6 6
C D
(
slight excess)
(25ꢀ75°C)
unknown degradation
products
(
dadi)Ti(THF) (1ꢀTHF) + AdNCO
Scheme 5. Kinetic model for AdN
(
3
carbonylation catalysis by
black) and stoichiometric study of imide carbonylation (blue).
2=NAd
Table 1 Kinetics of
2
catalytic, in C D .
=NAd carbonylation, stoichiometric and
6 6
a
3
-1 -1
‡
kCO x 10 (M s ) ∆G (kcal/mol)
2
=NAd (M) CO (M)
T(°C)
Scheme 4. Plausible mechanisms for the catalyzed conversion of AdN
CO -> AdNCO + N by the imide, (dadi)Ti=NAd ( =NAd).
3
+
2
2
b
0
0
0
0
0
.047
.048
.047
.047
.047
0.023
0.025
0.026
0.028
0.030
0.033
25.0(5)
35.0(5)
45.0(5)
55.0(5)
65.0(5)
75.0(5)
1.25(6)
1.83(8)
3.58(15)
5.44(42)
7.95(86)
14.0(11)
21.4(1)
21.9(1)
22.2(1)
22.7(1)
23.1(1)
23.4(1)
b
Catalytic Carbonylation of Adamantyl Azide
b
b,c
b
n
The RNI of (dadi) is implicit in the failure of CO to compete
b
1
1-13
0.047
with THF in binding to titanium,
rendering it capable of
catalyzing carbonylation reactions. Scheme 4 illustrates a
c
c
0.047
plausible mechanism for the carbonylation of the nitrene 0.046
0.0105
0.023
55.0(5)
55.0(5)
4.62(69)
4.20(45)
22.8(1)
22.8(1)
derived from adamantyl azide (>20 turnovers) as catalyzed by
d
0
.0063
0.022
55.0(5)
3.13(24)
23.0(1)
(dadi)Ti=NAd (2=NAd). Insertion of the carbonyl into the
titanium-imide bond, perhaps via an initial cis-bonding event,
a
Calculated from Henry's Law and corrected for temperature
dependence. Rate constants obtained from an average of three trials;
b
affects formation of an isocyanate adduct, and subsequent
‡
‡
substitution by AdN
generates an azide adduct that releases N
Imide (dadi)Ti=NAd ( =NAd) is the only metal-containing
species observed when the catalysis is monitored by H NMR
spectroscopy, and its concentration diminishes during the 0.028 M. Catalytic rate obtained from fitting disappearance of [AdN
and [ =NAd] according to Scheme 5 (3 trials): p(CO) = 2.58 atm, total
mol CO = 1.82 x 10 , [AdN
Supporting Information for details.
, dissociative, associative, or interchange, from a weighted Eyring plot: ∆H = 9.6(9) kcal/mol, ∆S = -39.9(28) eu,
3
‡ ‡
∆
9
G (55°C) = 22.6(9) kcal/mol. An unweighted Eyring plot gives: ∆H =
2
to restart the cycle.
‡
‡
c
.3(4) kcal/mol, ∆S = -40.7(13) eu, ∆G (55°C) = 22.6(6) kcal/mol. To
2
-5 -1
determine order in CO; kobs = 4.85(55) x 10
=
s at [CO] = 0.0105 M; kobs
-4 -1
1
-5 -1
9.67(95) x 10 s at [CO] = 0.023 M; kobs = 1.52(8) x 10 s at [CO] =
d
3
]
2
course of catalysis. The catalysis was modeled by a two-step
process in which the imide is clipped from =NAd by CO, and
renewed by the reaction of (dadi)Ti ( ) with AdN , as shown in
Scheme 5. A degradation path was added to account for loss of
catalyst over time. Initial kinetics experiments confirmed an and (dadi)Ti(THF) (
order in [CO], and while the concentration of CO in solution of (dadi)Ti (
). A first-order dependence on [CO] was found
was only ~3.5 times that of =NAd, the reaction was slow (-d[
=NAd]/dt = kCO[ =NAd][CO]), and an Eyring analysis of the
second order rate constants listed in Table 1 yielded a ∆H of
-
4
-5
-1
3
] = 0.123 M, kdeg = 3.2(3) x 10 Ms . See
2
1
3
1-THF), presumably resulting from trapping
1
2
2
2
‡
enough such that CO (~60 fold excess) was replenished from
the gas phase. As a consequence CO was a pseudo first-order
‡
‡
9
.6(9) kcal/mol, and a ∆S of -39.9(28) eu. The calculated ∆G
reagent, and the phenomenological rate (-d[AdN
=NAd][CO]) was zero-order since the catalyst
concentration also does not change, aside from the
degradation. Factoring out [CO] and [ =NAd] afforded a
second-order rate constant for carbonylation as 3.13(24) x 10
3
]/dt = kobs;
at 55°C is 22.6(9) kcal/mol, a value consistent with that
obtained during catalysis, supporting the claim of
carbonylation as the rate-determining step. The modest
enthalpy of activation thus reflects the balance between N=C
bond-formation and Ti=N bond-breaking in the transition
state, while the large negative entropy of activation value is
consistent with a 2nd-order process.
k
obs = kCO[2
2
-3
-1
-1
‡
M s (∆G (55°C) = 23.0(1) kcal/mol), as listed in Table 1.
A stoichiometric study supported carbonylation as the
rate-determining elementary step in the catalysis. Treatment
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ARTICLE
Journal Name
Calculated Free Energy Profile of AdN
3
Carbonylation
unlikely that steps subsequent to carbonylation, and preceding
loss of N from -N Ad are particularly consequential.
2
1
3
Fig. 7 illustrates the calculated free energy profile for the
catalytic carbonylation of AdN to AdNCO + N , a reaction that
3
2
is exergonic by -72.8 kcal/mol. Formation of the C=N bond in Discussion
AdNCO is -72.2 kcal/mol enthalpically more favorable than the
Transfers of X (NAd, O) from (dadi)Ti=X
2
2
2
2.1
0.8
.9
7
6
3
.2
.1
.6
ONIOM(M06/6ꢀ33+G(d):UFF)
1
4.9
4.7
UFF = DIPP & Ad (excluding Cα)
In the successful transfer of the imido functionality of (dadi)Ti=NAd
1
-
0.7
G (kcal/mol)
H (kcal/mol)
S (eu)
(
2-Ad) to CO, affording AdNCO, the redox non-innocence of the dadi
0.0
(
3
dadi)TiꢀN Ad
AdN
CO
THF
ligand was critical. Stabilization of presumed transient (dadi)Ti (1)
+
+
+
3
@STP
(dadi)Ti(THF)
-
70.3
-58.0
41.3
rateꢀdetermining
barrier for catalysis
3-
ꢀ
ꢁ
4-
+
2 AdN
3
as [(dadi) ]Ti(III) , (dadi)TiL (1-L) via (dadi) , and 2-Ad through
+
CO
AdN3 + CO ꢀ> AdNCO + N2
2-
ꢀ
27.3
29.2
(
dadi) permits the titanium to exist in favorable, higher oxidation
ꢀ
∆
G° = ꢀ72.8 kcal/mol
0.9
1'ꢀTHF
1'ꢀN
3
Ad
∆H° = ꢀ72.3 kcal/mol
S° = 1.7 eu
states while catalyzing the carbonylation, providing a textbook
example of the RNI in Fig. 1.
∆
2
0.9
.0
-43.3
-
-
30.0
29.7
1.0
8
ꢀ
ꢀ
4
48.2
37.2
4.2
ꢀ52.0
ꢀ34.2
67.0
ꢀ50.7
ꢀ51.5
4.6
The dadi ligand is more oxidizing to the titanium center than
ꢀ
57.3
-
ꢀ
58.9
1.9
one or two bound CO ligands, possessing the 22 πe (4n+2)
(
dadi)Ti=NAd
ꢀ
65.6
+
+
+
+
CO
AdN
THF
(dadi)Ti
+ OCNAd
+ AdN3
ꢀ66.1 14.9
configuration in (dadi)TiL (1-L), but less oxidizing than a nitrene (i.e.,
(
dadi)TiꢀOCNAd
14.6 etc.
5.6
3
-
1.0
-
+ AdN
3
NAd), where it resides in its 20 πe (4n) form. The MO diagrams for
N
2
+ THF
+ THF
+ N2
(dadi)TiꢀN
+ OCNAd
THF
3
Ad
+
N
2
2 2
(dadi)TiPMe Ph (1-PMe Ph) and (dadi)Ti=O (2=O), if taken as
etc.
1"ꢀN Ad
2
'=NAd
+
1
'ꢀOCNAd
1"
analogues for a potential carbonyl species, and (dadi)Ti=NAd (2-Ad),
3
+
N
2
respectively, support this key electronic rationale. Concerning Fig. 4
Fig. 7. Calculations of Scheme 4, with transition states (primes (1-PMe Ph), any backbonding interaction with one of the dπ-
2
indicate calculated states; activation energies italicized) pertaining to
orbitals would have to lower it below -4.80 eV in order to
N
(
2
loss from bound azide, (dadi)Ti(N
dadi)Ti=NAd ( =NAd). Association/dissociation transition states are
not included. Small inconsistencies are due to round-off errors. Apparently CO, a potent π-acid toward titanium, is incapable of
3 3
Ad) (1-N Ad) and carbonylation of
depopulate the NCCN π-orbital and render the dadi dianionic.
2
4
-
Calculated
1'-L and 1"-L compounds have (dadi) configurations,
2- 3-
inducing the necessary change. Note that in Fig. 6 (2=O), the related
NCCN π-orbital at -3.41 eV is the LUMO in the system, having been
depopulated in favor of the imide π-bonding orbitals of low energy.
The catalytic carbonylation in Scheme 4 produces AdNCO, but
fortunately OAT to afford (dadi)TiO (2=O) and CNAd is calculated to
2
'
=NAd has a (dadi) structure, and 1" is computed as [(dadi) ꢀ]Ti(III)ꢁ
.
corresponding azide N=N bond in terms of free energy, and
constitutes the driving force for catalysis.
‡
The calculated ∆G (298.15 K) for carbonylation of the
3
6,37
be only slightly favorable (∆G° = -1.1 kcal/mol).
CNAd is also favored to scavenge any (dadi)Ti (1) and provide
dadi)Ti(CNAd) with ∆G°calcd = -22.1 kcal/mol. It is likely that the
barrier for OAT from AdNCO is sluggish compared to dinitrogen loss
from (dadi)Ti(N Ad) (1-N Ad) to form (dadi)Ti=NAd (2-Ad), because
The released
imide, (dadi)Ti=NAd (2=NAd) is 20.9 kcal/mol, in decent
agreement with experiment (21.5 kcal/mol at 25 °C). The slight
‡
discrepancy is spread between ∆H (calc'd 8.0 kcal/mol)
(
‡
kcal/mol) and ∆S (calc'd -43.3 eu). The latter is similar to the
‡
calculated ∆S = -42.3 eu for the microscopic reverse of N
3
3
2
the latter is exergonic by ∆G°calcd = -69.0 kcal/mol. Since all adduct
formations are expected to be rapid and reversible, OAT is likely not
competitive. Note that no IR spectral evidence of isocyanide
formation is observed in catalytic runs, even at the end of catalysis.
extrusion from (dadi)Ti(N
) + AdNCO from =NAd and CO is only favorable by ∆G°
-3.8 kcal/mol, and enthalpically unfavorable by ∆H° = 3.0
3 3
Ad) (1-N Ad). Formation of (dadi)Ti
(
1
2
=
kcal/mol, hence carbonylation has modest thermodynamic
impetus. With the Bond Dissociation Enthalpy (BDE(calc)) of
the AdN=CO bond at 81 kcal/mol, the titanium-imide bond
Attack by PMe
OPMe ) is calculated to be favorable by ∆G° = -22.5 kcal/mol, but is
relatively slow (>24 h, 23°C), presumably due to known orbital
3 3
on (dadi)TiO (2=O) to form (dadi)TiOPMe (1-
3
3
5
BDE(calc) is ~84 kcal/mol. Both adducts,
OCNAd, are calculated to be most stable as
species.
1-N Ad and 1-
3
3
8
1
symmetry constraints on OAT. The phosphine oxide adduct is
more favorable than (dadi)Ti(THF) (1-THF) by ∆G°calcd = -17.2
kcal/mol.
κ
-NAd-bound
Dinitrogen loss from
1
-N
transition state, at ∆G = 14.9 kcal/mol. The binding of AdN
dadi)Ti is favorable by only -13.6 kcal/mol, but the subsequent
loss of N to form =NAd is substantial, at -55.4 kcal/mol,
3
Ad possesses the next highest
‡
While the ligand redox states are consequential to nitrene
transfer, certain atom and group transfers are not so readily
explained. For example, the deoxygenation of (dadi)TiO (2=O) by CO
3
to
(
2
2
with 1 equiv THF present to form (dadi)Ti(THF) (1-THF) and CO
2
is
hence there is substantial thermodynamic influence on the
dinitrogen loss step. Associative or interchange transition
calculated to be exergonic by ∆G° = -19.2 kcal/mol (∆H° = -34.6
6,37
3
kcal/mol, ∆S° = -51.6 eu).
Decomposition is observed, perhaps
states for AdN
3
displacement of isocyanate from 1-OCNAd
indicative of additional reactivity from liberated CO ; further study
could not be found. Since dissociation of product OCNAd from
2
is warranted. In addition, attempts to generate other (dadi)Ti=NR
This journal is © The Royal Society of Chemistry 20xx
1
-OCNAd is calculated to be only ∆G° = 5.3 kcal/mol, it is
6
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Plea sC eh de om ni oc ta al dS j uc si et nm c ae rgins
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ARTICLE
2
3-25
1
(
2=NR) species
adamantyl group protects the imide in (dadi)Ti=NAd (2-Ad) such of carbonylation, but there is considerable lengthening of the d(CO)
that group transfer can be realized.
to 1.17 Å. The calculated metrics pertaining to the TS (dadi)Ti core
Attempts to catalyze carbodiimide formation akin to Heyduk's still conform to (dadi) , as the 2.12 and 2.14 Å Ti-N distances are
led to unanticipated reactivity, hence the 175° in the calculated κ -N-OCNAd adduct that is the initial product
2
-
am
12
zirconium system failed, as treatment of (dadi)Ti=NAd (2-Ad) with significantly shorter than the Ti-Nim bond lengths of 2.23 and 2.24 Å.
CNMe afforded (dadi)Ti(CNMe)2 (1-(CNMe) ) and only a trace In a late TS, the core geometry would be expected to more closely
2
4
-
amount of MeNCNAd. Apparently the strong sigma-donation of correspond to (dadi) , which is calculated to have similar amide and
CNMe, in contrast to the weak donor CO, imparts too much stability "imine" d(TiN).
to the diisocyanide, as exposure of 1-(CNMe)2 to AdN was also
3
ineffective.
Conclusions
m
Structural and computational investigations show that dadi is
4
-
capable of binding as a tetraanion in [dadi ]Ti(IV)L (1-L ; L = THF,
x
x
x
2
-
2 2
PMe Ph, (CNMe) ) and as a dianion in [dadi ]Ti(IV)X (2=X; X = O,
m
NAd). The redox non-innocence of dadi is crucial in making CO
binding to (dadi)Ti essentially devoid of π-backbonding, and
uncompetitive with THF adduct formation by ∆G°(calc'd) = 7.7
kcal/mol. In contrast, formation of oxo or adamantyl-imido multiple
bonds is competitive with dadi reduction, ultimately permitting the
successful catalytic carbonylation of AdN
changes occur at dadi rather than titanium.
3
in which the redox
Carbonylation catalysis with titanium is quite unusual, with
a.
40
Buchwald's enone synthesis as the primary example, and catalytic
nitrene transfer from titanium is limited to Tonks' recent pyrrole
4
1
synthesis. While the former is unlikely to involve formal RNI, the
4
2
capability of Cp to covalently distribute charge is remarkable;
a
bound pyrrole product in the latter system could act in a redox non-
innocent fashion.
Experimental Section
b.
Full experimental details are given in the Supplemental Information,
including kinetics and computational procedures in addition to
syntheses. Brief descriptions of the syntheses, kinetic
measurements, and computations are given in the Schemes, and
Figure and Table captions. Elemental analyses on all (dadi)Ti-based
compounds failed normal standards, despite multiple attempts on
crystalline samples, and utilization of several companies. As a
consequence, full spectral characterizations, including all pertinent
NMR spectra, are provided in the Supplemental Information.
Fig. 8. Views of the computed transition state for carbonyl addition to the
imide of (dadi)Ti=NAd (2=NAd): a. metrics (distances (Å), angles (°)) of the
imide-carbonyl fragment; b. distances of the core and dadi ligand.
Transition State of Imide Carbonylation
The transition state (TS) for imide carbonylation of (dadi)Ti=NAd
2 55 4
Crystal data for 1-PMe Ph: C46H N PTi, M = 742.81, monoclinic,
(
2=NAd) was found via DFT calculations, and views of the TS and its
P2
1
/c, a = 18.3497(6), b = 10.3436(3), c = 25.5974(8) Å, β =
acompanying metrics are illustrated in Fig. 8. The CO attacks the
titanium at an angle consistent with its lone pair interacting with
3
1
=
0
08.022(2)°, V = 4620.1(3) Å , T = 223(2) K, λ = 0.71073 Å, Z = 4, Rint
0.0524, 40189 reflections, 8152 independent, R (all data) =
.0672, wR = 0.1340, GOF = 1.048, CCDC-1522529.
Crystal data for 1-(CNMe) : C H N Ti, M = 758.92, triclinic, P-1,
1
d
xz/yz, while the imide connection is made via an Npπ->COπ*
2
interaction. Related investigations of early metal nitride
2
47 62 6
3
9
carbonylations show initial CO binding to be important, and the a = 10.2323(6), b = 13.2844(7), c = 16.9919(9) Å, α = 100.164(3)° β
3
=
0
104.521(2)° γ = 95.151(2)°, V = 2179.0(2) Å , T = 223(2) K, λ =
.71073 Å, Z = 2, Rint = 0.0344, 29680 reflections, 8887 independent,
d(Ti-C) of 1.79 Å shows an interaction with the titanium.
The TS appears early in the reaction coordinate, as the d(TiN) of
R
1
(all data) = 0.0639, wR
Crystal data for 2=O: C49
3.1179(8), b = 13.2082(8), c = 13.7748(8) Å, α = 105.738(3)° β =
2
= 0.1178, GOF = 1.026, CCDC-1252531.
1
.79 Å is only 0.11 Å longer than the calculated d(TiN) in 2=NAd, the
62 4
H N
OTi, M = 770.92, triclinic, P-1, a =
TiNC(Ad) angle is still large (162.5°), and the d(NC) is long at 1.69 Å.
The accompanying NCO angle is 120.3°, significantly bent from the
1
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3
9
9.727(3)° γ = 98.022(3)°, V = 2220.5(2) Å , T = 223(2) K, λ
Å, Z = 2, Rint = 0.0313, 40966 reflections, 9425 independent, R
data) = 0.0532, wR = 0.1215, GOF = 1.041, CCDC-1522530.
=
0.71073 15 S. S. Karpiniec, D. S. McGuiness, G. J. P. Britovsek, J. Patel, J.
Organometallics 2012, 31, 3439-3442.
1
(all
1
1
6 R. Sikari, S. Sinha, U. Jash, S. Das, P. Brandäo, B. de Bruin, N.
D. Paul, Inorg. Chem. 2016, 55, 6114-6123.
7 (a) A. L. Smith, K. I. Hardcastle, J. D. Soper, J. Am. Chem. Soc.
2
Acknowledgements
2
010, 132, 14358-14360; (b) C. A. Lippert, S. A. Arnstein, C. D.
Sherrill, J. D. Soper, J. Am. Chem. Soc. 2010, 132, 3879-3892;
(c) C. J. Rolle III, K. I. Hardcastle, J. D. Soper, Inorg. Chem.
Support from the National Science Foundation (CHE-1402149
PTW); CHE-1531468 (UNT computer facility); CHE-1531632 (NMR
facility)), the Department of Energy Office of Basic Energy Sciences
DE-FG02-03ER15387 (TRC)), and Cornell University is gratefully
2
008, 47, 1892-1894.
(
1
1
2
8 C. C. Lu, E. Bill, T. Weyhermüller, E. Bothe, K. Wieghardt, J.
Am.Chem. Soc. 2008, 130, 3181-3197.
9 V. A. Williams, E. B. Hulley, P. T. Wolczanski, K. M. Lancaster,
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