Novel double substrate insertion versus isocyanate extrusion in reactions of imidotitanium complexes with CO2: Critical dependence on imido N-substituents

Article abstract of DOI:10.1039/b102704m

Reaction of the cyclopentadienyl-amidinate supported imidotitanium complexes [Ti(η-C₅ Me₅){MeC(NⁱPr)₂}(NR)] (R = ^tBu 1a or Ar 1b where Ar = 2,6-C₆H₃Me₂) with CO_2<

Full text of DOI:10.1039/b102704m

View Article Online / Journal Homepage / Table of Contents for this issue
Novel double substrate insertion versus isocyanate extrusion
in reactions of imidotitanium complexes with CO2: critical
dependence on imido N-substituents†
Aldo E. Guiducci, Andrew R. Cowley, Michael E. G. Skinner and Philip Mountford*
Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR.
E-mail: philip.mountford@chemistry.oxford.ac.uk;
Received 23rd March 2001, Accepted 2nd April 2001
First published as an Advance Article on the web 12th April 2001
Reaction of the cyclopentadienyl–amidinate supported
imidotitanium complexes [Ti(ꢀ-C5Me5){MeC(NiPr)2}(NR)]
(R = tBu 1a or Ar 1b where Ar = 2,6-C6H3Me2) with CO2
proceed via initial cycloaddition reactions, but depending
on the imido N-substituent go on to yield products of
either isocyanate extrusion or double CO2 insertion, the
latter forming [Ti(ꢀ-C5Me5){MeC(NiPr)2}{O(CO)N(Ar)-
(CO)O}] 4; the double CO2 insertion reaction leading to 4 is
the first example for any transition metal imide.
ligands.4 We have found that the C5Me5 and MeC(NiPr)2 sup-
porting ligands in combination provide a good balance of steric
protection and electron donation conducive with promoting
reactivity at the Ti᎐NR bond. The extremely high-yielding
᎐
syntheses of the new imidotitanium compounds [Ti(η-
C5Me5){MeC(NiPr)2}(NR)] (R = tBu 1a or Ar 1b, where Ar =
2,6-C6H3Me2)‡ are summarised in eqn. (1) starting from the
readily-available [Ti(η-C5Me5)(NBut)Cl(py)].5
Group 4 organoimido complexes were only first reported just
over ten years ago1 and continue to be the focus of considerable
attention.2 The main point of interest in these systems is
the reactivity of the M᎐NR imido linkage itself. A variety of
᎐
[2 ϩ 2] cycloaddition, insertion and NR group transfer reac-
tions of this moiety with a range of saturated and unsaturated
organic and organometallic substrates have been described. Of
the various classes of Group 4 imido complex reported to date,
the most intensively and systematically studied are the bis-
(cyclopentadienyl), diamido–amine and macrocyclic systems.
Specific derivatives are shown by way of example in I–III
The new compounds 1a,b are the first structurally character-
ised cyclopentadienyl–amidinate supported Group 4 imido
complexes. The solid state structure§ of [Ti(η-C5Me5){MeC-
(NiPr)2}(NAr)] 1b is shown in Fig. 1 and reveals an approx-
imately linear Ti᎐N–Ar linkage with a Ti-N
distance
᎐
imide
consistent with a formal metal–nitrogen triple bond (σ2π4).1
The monomeric pseudo-tetrahedral geometry at Ti(1) is
reminiscent of that in the bis(cyclopentadienyl) Group 4 imido
systems of the type I, except that the formal valence electron
count in 1a,b is only 16. Initial studies have shown that the
compounds 1a,b undergo productive reactions with a range
of unsaturated substrates including alkynes and the hetero-
allenes RCNO, CS2, RCNS, COS and CO2 thus demon-
strating the viability of this supporting ligand set in developing
metal imido chemistry. It is the reactions with CO2 that are
especially interesting and novel, and we therefore focus on these
here.
Cycloaddition reactions of metal imides with CO2 are very
uncommon1 and only two structurally authenticated metalla-
cyclic products of this type of reaction have been reported.2b,6
Reaction of 1a,b with CO2 (1 atm) for 10 or less min followed by
immediate isolation gave the cherry red N,O-bound carbamate
complexes [Ti(η-C5Me5){MeC(NiPr)2}{O(CO)NR}] (R =
tBu 2a or Ar 2b) in good isolated yield (Scheme 1). The
respectively.2a–c We were interested to continue our studies of
the influence of the supporting ligand set(s) on the reactivity
of the Ti᎐NR linkage. We report here preliminary results on
᎐
the synthesis of new cyclopentadienyl–amidinate supported
imidotitanium complexes that show novel and interesting
chemistry in their reactions with CO2, and the outcomes of
these depend critically on the imido N-substituent.
Titanium imido complexes with certain amidinate ligands
have been reported previously by us and others3 but chemistry
at the Ti᎐NR linkage has never been explored in these systems.
᎐
One attraction of the combined cyclopentadienyl–amidinate
supporting group is the potential for widely varying the elec-
tronic and steric properties of this system as demonstrated in
the context of olefin polymerisation catalysts containing these
† Electronic supplementary information (ESI) available: characterising
data for compounds 1–4. See http://www.rsc.org/suppdata/dt/b1/
b102704m/
1392
J. Chem. Soc., Dalton Trans., 2001, 1392–1394
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102704m

View Article Online
Scheme 1 Reagents and conditions: (i) CO2 (1 atm), 10 min (for 2a) or 3 min (for 2b), 60% (for 2a) or 66% (for 2b); (ii) 12 h, >95%; (iii) CO2 (1 atm),
24 h, 82%.
Fig. 1 Displacement ellipsoid (30%) plot of [Ti(η-C5Me5){MeC-
(NiPr)2}(NAr)] 1b. Hydrogen atoms are omitted for clarity. Selected
bond lengths and angles: Ti(1)–Cp*cent 2.085, Ti(1)–N(1) 2.094(2),
Ti(1)–N(2) 2.099(2), Ti(1)–N(3) 1.738(2) Å; Ti(1)–N(3)–C(9) 168.9(2),
Cp*cent–Ti(1)–N(1) 119.1, Cp*cent–Ti(1)–N(2) 119.5, Cp*cent–Ti(1)–N(3)
121.4Њ, where Cp*cent is the C5Me5 ring carbon centroid.
Fig.
2 Displacement ellipsoid (40%) plot of [Ti2(η-C5Me5)2-
{MeC(NiPr)2}2(µ-O)2] 3. Hydrogen atoms are omitted for clarity;
atoms carrying the suffix ‘B’ are related to their counterparts by the
operator [Ϫx, Ϫy, 1 Ϫ z]. Selected bond lengths: Ti(1)–Cp*cent 2.130,
Ti(1)–N(1) 2.188(3), Ti(1)–N(2) 2.168(3), Ti(1)–O(1) 1.855(2), Ti(1)–
O(1B) 1.869(2) Å where Cp*cent is the C5Me5 ring carbon centroid.
N,O-coordination for the O(CO)NR ligand is implied by the
Ti–NAr bond of [Ti(η-C5Me5){MeC(NiPr)2}(NAr)] 1b. The
geometry at Ti(1) is typical of a four-legged piano stool
complex; distances within and between the ligands are as
i
presence of two inequivalent Pr groups for the MeC(NiPr)2
1
ligand in the H and 13C NMR spectra, and by strong ν(C᎐O)
᎐
bands between 1646 and 1669 cmϪ1 in the IR spectra.2b
expected. The two ν(C᎐O) bands in the IR spectrum of 4 are
᎐
The tert-butyl derivative 2a does not react further with CO2
in solution (1–2 atm) and under these conditions (or in the
absence of CO2) quantitatively undergoes a retrocyclisation
process over 12 hours to yield the isocyanate tBuNCO and the
dimeric µ-oxo complex [Ti2(η-C5Me5)2{MeC(NiPr)2}2{µ-O)2] 3,
the dimeric nature of which has been confirmed by X-ray
crystallography (Fig. 2). Under otherwise identical conditions
the aryl derivative [Ti(η-C5Me5){MeC(NiPr)2}{O(CO)NAr}]
2b reacts smoothly with further CO2 to give a product 4 that
attributed to symmetric and antisymmetric combinations. The
type of double substrate insertion with complete cleavage of
the metal–imide bond is highly unusual and has not been
authenticated before for any allenes or their hetero-
analogues. The only related example is for the reaction of the
late transition metal imide [Ir(η-C5Me5)(NtBu)] with certain
alkynes.6 The O(CO)N(Ar)(CO)O fragment in 4 (derived
formally from the dianionic conjugate base of azamalonic
acid) has not been structurally characterised previously and
represents a new type of ligand.7
i
1
has equivalent Pr groups in its H and 13C NMR spectra and
features two strong bands attributable to ν(C᎐O) at 1651 and
The reasons why compound 2a does not insert a second CO2
into the Ti–Nimide bond while 2b does are as yet unclear, but
may reflect the electron-withdrawing ability of the aryl group
in comparison to the electron-releasing nature of tert-butyl.
Insertion of the second CO2 presumably causes polarisation of
the Ti–NR (R = tBu or Ar) of 2a,b in the transition state with
concomitant build-up of partial positive and negative charge on
᎐
1694 cmϪ1 in its IR spectrum. The structure proposed for [Ti(η-
C5Me5){MeC(NiPr)2}{O(CO)N(Ar)(CO)O}] 4 in Scheme 1 is
supported by the solid state structure determined by X-ray
diffraction (Fig. 3).
Fig. 3 clearly confirms that 4 contains two CO2 molecules
that have been activated and inserted into the original triple
J. Chem. Soc., Dalton Trans., 2001, 1392–1394
1393

View Article Online
Notes and references
‡ Satisfactory characterising data have been obtained for all the new
compounds (see ESI).
§ Crystal data for 1b: C26H41N3Ti, M = 443.53, tetragonal, I41cd,
a = b = 20.5223(4), c = 24.3873(6) Å, U = 10271.1 Å3, Z = 16, T = 150
K, µ = 0.35 mmϪ1, 5910 independent reflections (Rmerge = 0.060), 4139
I > 3σ(I) used in refinement, final R indices: R = 0.0396, Rw = 0.0418.
For 3: C36H64N4O2Ti2, M = 680.71, monoclinic, P21/n, a = 9.7810(4),
b = 15.4630(4), c = 12.0930(5) Å, β = 92.625(2)Њ, U = 1827.1 Å3, Z = 2,
T = 150 K, µ = 0.47 mmϪ1, 3890 independent reflections (Rmerge = 0.07),
2403 I > 3σ(I) used in refinement, final
R indices: R = 0.0523,
Rw = 0.0395. For 4: C28H41N3O4Tiؒ0.5(C6H6), M = 570.61, orthorhom-
bic, Pbca, a = 14.4250(2), b = 17.2619(3), c = 24.3318(4) Å, U = 6058.7
Å3, Z = 8, T = 150 K, µ = 0.32 mmϪ1, 6916 independent reflections
(Rmerge = 0.055), 4551 I > 3σ(I) used in refinement, final R indices:
R = 0.0463, Rw = 0.0534. CCDC reference numbers 160966–160968. See
http://www.rsc.org/suppdata/dt/b1/b102704m/ for crystallographic data
in CIF or other electronic format.
1 For reviews see: D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239;
P. Mountford, Chem. Commun., 1997, 2127 (Feature Article).
2 For recent work and leading references: (a) L. H. Gade and P. Mount-
ford, Coord. Chem. Rev., 2001, in press; (b) A. J. Blake, J. M.
McInnes, P. Mountford, G. I. Nikonov, D. Swallow and D. J. Watkin,
J. Chem. Soc., Dalton Trans., 1999, 379; (c) Z. K. Sweeney, J. L.
Salsman, R. A. Andersen and R. G. Bergman, Angew. Chem., Int.
Ed., 2000, 2339; (d) J. L. Bennett and P. T. Wolczanski, J. Am. Chem.
Soc., 1997, 119, 10696; (e) C. J. Harlan, J. A. Tunge, B. M. Bridge-
water and J. R. Norton, Organometallics, 2000, 19, 2365; ( f ) J. L.
Thorman, I. A. Guzei, V. G. Young and L. K. Woo, Inorg. Chem.,
2000, 39, 2344.
3 P. J. Stewart, A. J. Blake and P. Mountford, Organometallics, 1998, 17,
3271; J. R. Hagadorn and J. Arnold, Organometallics, 1998, 17, 1355
and references therein.
4 K. C. Jayaratne and L. R. Sita, J. Am. Chem. Soc., 2000, 122, 958 and
references therein.
5 S. C. Dunn, P. Mountford and D. A. Robson, J. Chem. Soc., Dalton
Trans., 1997, 293.
6 D. S. Glueck, J. Wu, F. J. Hollander and R. G. Bergman, J. Am. Chem.
Soc., 1991, 113, 2041.
7 The X-ray structure of the homologous N,O-bound carbamate com-
plex [Ti(η-C5Me5){κ2-Me3SiNC(Ph)NCH2CH2NMe2}{O(CO)NtBu}]
has been determined: C. L. Boyd, A. E. Guiducci and P. Mountford,
unpublished results.
Fig. 3 Displacement ellipsoid (30%) plot of [Ti(η-C5Me5){MeC-
(NiPr)2}{O(CO)N(2,6-C6H3Me2)(CO)O}] 4. Hydrogen atoms are omit-
ted for clarity. Selected bond lengths: Ti(1)–Cp*cent 2.046. Ti(1)–N(2)
2.064(3), Ti(1)–N(3) 2.071(3), Ti(1)–O(1) 1.932(2), Ti(1)–O(3), N(1)–
C(1) 1.410(3), N(1)–C(2) 1.417(3), C(1)–O(2) 1.213(3), C(2)–O(4)
1.210(3) Å where Cp*cent is the C5Me5 ring carbon centroid.
Ti and N, respectively. Therefore an aryl-substituted carbamate
nitrogen would be better stabilised during this process. In this
regard we have recently found in preliminary studies that
fluoraryl analogues of 2b undergo the second CO2 insertion
reaction at faster relative rates compared to those of the
non-fluorinated homologues, consistent with the view that
electron-withdrawing groups can accelerate the novel double
insertion reaction. Further studies of the role of the imide
N-substituent on the pathway of these and related reactions are
underway, along with a survey of the chemistry of 1a,b and
their homologues in general. Moreover, it is clear that a better
understanding of the role and influence of the imido N-
substituent on the reactions of imido complexes will progress
the rational design of useful imido group transfer reagents.
We thank the EPSRC for support of this work.
1394
J. Chem. Soc., Dalton Trans., 2001, 1392–1394