Surprises in enolate binding at high valent molybdenum

Article abstract of DOI:10.1039/a800116b

Treatment of [Mo(NAr)₂Cl₂(DME)] (Ar = 2,6-diisopropylphenyl; DME = 1,2-dimethoxyethane) with LiOC(O-Bu^t)CMe₂ at low temperature affords the unexpected carbon-bound molybdenum(VI) enolate complex [Mo(NAr)₂

Full text of DOI:10.1039/a800116b

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Surprises in enolate binding at high valent molybdenum
Paul A. Cameron, George J. P. Britovsek, Vernon C. Gibson,*† David J. Williams and Andrew J. P. White
Department of Chemistry, Imperial College of Science, Technology and Medicine, Exhibition Road, London, UK SW7 2AY
Treatment of [Mo(NAr)2Cl2(DME)] (Ar = 2,6-diisopropyl-
orange crystalline product.‡ A second recrystallisation from
heptane gave X-ray quality crystals of 1 and the molecular
structure is shown in Fig. 1.§ Compound 1 decomposes slowly
in solution (t1/2 ca. 1 week) but can be stored for weeks in the
solid state at 220 °C.
phenyl; DME
= 1,2-dimethoxyethane) with LiOC(O-
But)CMe2 at low temperature affords the unexpected
carbon-bound molybdenum(VI) enolate complex [Mo(N-
2
Ar)2Cl{h -C(Me2)CO2But}] 1; the novel a-aminoenolate
2
derivative [Mo(NAr)2Cl{h -CH(NMe2)CO2Et}] 2 is obtained
The X-ray analysis of 1 reveals the unexpected bidentate
coordination of the enolate ligand via the carbonyl oxygen atom
O(2) and the a-carbon atom C(1). The geometry at Mo,
although five-coordinate, is probably best described as distorted
tetrahedral with the chelating enolate ligand occupying one of
the tetrahedral sites; the angles subtended at Mo by the two
imido N atoms and the Cl atom range between 105.4(4) and
111.3(3)°. The Mo–N distances are typical of ‘linear’ imido
ligands, the angles at the N atoms being 157.6(8) [N(10)] and
158.5(8)° [N(23)].17,18 The feature of most interest is the pattern
of bonding to, and within, the enolate ligand. The ‘stronger’
coordination is to the a-C atom [Mo–C(1) 2.208(11) Å]
whereas that to the carbonyl O atom is 2.370(6) Å. The
C(1)–C(2) linkage [1.50(2) Å] has lost all its double bond
character, though there is not a full pyramidalisation at C(1), the
sum of the C–C(1)–C angles being ca. 342°. There is evidence
for delocalisation between the two O atoms of the ester, the
C(2)–O(2) and C(2)–O(3) distances being 1.24(2) and 1.29(2)
Å, respectively.
under analogous conditions using LiNPri2/Me2NCH2CO2Et;
the structures of 1 and 2 confirm the carbophilic nature of
the [Mo(NAr)2] core.
Transition metal enolate chemistry nowadays features pro-
minently in organic synthetic methodology. The enolate ligand,
an intermediate between an allyl and an acetate unit, has two
different binding modes. Depending on the oxophilicity/
carbophilicity of the metal centre, the enolato functionality can
bind either via the ‘CH2’ unit or via its oxygen atom [Scheme
1(a)], giving rise to different reactivity in further transforma-
tions. In general, the oxygen binding mode I predominates for
early transition metals, whereas M–C bonding III is favoured
by the softer late transition metals. A number of oxygen bound
enolate complexes of early transition metals have been isolated
and structurally characterised.1–4 For molybdenum, only low
valent molybdenum enolate complexes as in [MoCp(CO)3R]
(R = enolate ligand) have been made and these are exclusively
carbon bound.5–9
With a view to exploiting the isolobal relationship between
the Cp2MIV and the (RN)2MVI cores,10,11 we became interested
in establishing whether or not the relatively ‘hard’ high valent
molybdenum centre would promote a switch from a carbon-
bound to an oxygen-bound enolate binding mode, since oxygen-
bound enolate complexes of early transition metal12,13 and rare
earth metallocenes14,15 are implicated as the active species in
the polymerisation of methyl methacrylate (MMA). Here, we
describe the first examples of high valent molybdenum enolate
complexes and their unusual and unanticipated structures.
Complex 1 was prepared according to Scheme 1(b), from
[Mo(NAr)2Cl2(DME)] and [LiOC(OBut)CMe2]16 at 278 °C.
Recrystallisation from pentane at 230 °C afforded 1 as an
Interestingly, complex 1 unveils a fundamental difference
between the isolobal Cp2MIV and the (ArN)2MVI cores.
Whereas the analogous Cp2MIV complex [Cp2Ti{O-
C(OMe)CMe2}Cl] was found to be an O-bound enolate
complex,4 complex 1 does not bind similarly via the O atom as
in complex A, but clearly establishes the carbophilic nature of
the (ArN)2MoVI core.
C(8)
C(9)
C(11)
(a)
O
O
O
N(10)
Mo
C(1)
C(2)
LnM
OR
LnM
OR
LnM
OR
I
II
III
O(3)
C(4)
(b)
N(23)
Me
O(2)
Cl
OBut
OBut
Cl
O
ArN
ArN
ArN
ArN
ArN
i
C(24)
Mo
Mo
O
O
Mo
Cl
ArN
O
Cl
Cl
1
A
Me
ii
EtO
ArN
O
Me2N
Me2N
Fig. 1 Molecular structure of 1. Selected bond lengths (Å) and angles (°);
Mo–C(1) 2.208(11), Mo–C(2) 2.579(12), Mo–N(10) 1.754(9), Mo–N(23)
1.730(9), Mo–Cl 2.362(3), C(1)–C(2) 1.50(2), C(2)–O(2) 1.24(2),
C(2)–O(3) 1.29(2); C(1)–Mo–O(2) 60.6(4), C(1)–Mo–Cl 128.1(3),
C(1)–Mo–N(10) 101.4(4), C(1)–Mo–N(23) 105.4(5), O(2)–Mo–Cl 82.4(2),
O(2)–Mo–N(10) 158.2(4), O(2)–Mo–N(23) 92.0(4), Cl–Mo–N(10)
102.6(3), Cl–Mo–N(23) 111.3(3), N(10)–Mo–N(23) 105.4(4), Mo–
C(1)–C(2) 85.9(7), C(1)–C(2)–O(2) 115.1(11), C(2)–O(2)–Mo 85.1(6),
C(11)–N(10)–Mo 157.6(8), C(24)–N(23)–Mo 158.5(8).
OEt
ArN
ArN
ArN
OEt
Mo
Cl
O
Mo
Cl
NMe2
Mo
Cl
O
ArN
ArN
C
B
2
Scheme 1 (a) Potential binding modes of the ester enolate ligand. (b)
Reagents and conditions: i, LiOC(OBut)CMe2 (1 equiv.), Et2O, 278 °C ?
room temp.; ii, LDA, Me2NCH2CO2Et, THF, 278 °C ? room temp.
Chem. Commun., 1998
737

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Notes and References
† E-mail: V.gibson@ic.ac.uk
‡ Satisfactory microanalyses and MS data were obtained for complexes 1
and 2. Selected spectroscopic data: for 1: IR (CsI, Nujol, cm21): 1567
[n(CNO)]; 1H NMR (C6D6, 250 MHz, 298 K), d 6.99–6.91 (m, 6 H, m- and
O(2)
3
p-C6H3Pri2-2,6), 3.92 [spt, 4 H, JHH 6.8 Hz, CH(CH3)2], 1.73 [s, 6 H,
C(20)
N(3)
C(CH3)2], 1.34 [s, 9 H, C(CH3)3], 1.20 [d, 12 H, 3JHH 6.8 Hz, CH(CH3)2],
1.13 [d, 12 H, 3JHH 6.8 Hz, CH(CH3)2]; 13C NMR (C6D6, 62.5 MHz, 298
K), d 185.7 (s, O–CNO), 153.3 (s, ipso-C6H3Pri2-2,6), 143.5 (s, o-C6H3Pri2-
2,6), 127.0 (d, p-C6H3Pri2-2,6), 123.1 (d, m-C6H3Pri2-2,6), 86.4 [s, Mo–
C(CH3)2], 56.7 [s, C(CH3)3], 28.8, 28.0, 24.6, 24.0 (4s, 2Pri Me, But Me,
Mo–C(Me)2, 23.0 [d, CH(CH3)2]. For 2: IR: 1718m [n (CNO)]; 1H NMR
(C6D6, 250 MHz, 298 K), d 7.1–6.8 (m, 6 H, Ar-H), 4.0–3.8 [4 spt., 4 H,
3JHH 6.8 Hz, CH(CH3)2], 3.85–3.57 (q, 2 H, 3JHH 7.1 Hz, OCH2CH3), 3.73
[s, 1 H, (CH3)2NCH], 3.12, 2.33 [2s, 6 H, (CH3)2NCH], 1.37, 1.24, 1.20,
O(3)
C(1)
C(2)
C(7)
N(1)
Mo
N(2)
C(6)
C(8)
Cl
3
3
1.14 [4d, 24 H, JHH 6.8 Hz, CH(CH3)2], 0.75 (t, 3 H, JHH 7.1 Hz,
OCH2CH3). 13C NMR (C6D6, 62.5 MHz, 298 K), d 170.0 (s, CNO), 154.2,
153.0, 146.6, 140.9, 128.3, 127.6, 125.3, 122.5 (8s, ArC), 60.2 (s,
OCH2CH3), 58.8 (s, NCH), 51.0, 42.8 [2s, (CH3)2N], 28.9, 28.5 [2s,
CH(CH3)2], 24.4, 23.9, 23.4, 23.2 [4s, CH(CH3)2], 14.1 (s, OCH2CH3).
§ Crystal data: for 1: C32H49ClMoN2O2, M = 625.1, monoclinic, space
group P21 (no. 4), a = 9.376(2), b = 11.061(4), c = 16.496(3) Å,
b = 103.18(2)°, U = 1665.8(7) Å3, Z = 2, Dc = 1.25 g cm23, m(Mo-
Ka) = 5.0 cm21, F(000) = 660. An orange plate of dimensions 0.33 3 0.32
3 0.07 mm was used. For 2: C30H46ClMoN3O2, M = 612.1, monoclinic,
space group C2/c (no. 15), a = 22.823(1), b = 8.981(1), c = 30.877(4) Å,
b = 90.34(1)°, U = 6329(1) Å3, Z = 8, Dc = 1.29 g cm23, m(Cu-
Ka) = 43.9 cm21, F(000) = 2576. An orange–red prism of dimensions
0.18 3 0.10 3 0.07 mm was used. 2661 (4325) Independent reflections
were measured at 203 K on Siemens P4 diffractometers with graphite
monochromated Mo-Ka and Cu-Ka (rotating anode source) radiation for 1
and 2 respectively using w-scans. The structures were solved by direct
methods and all the non-hydrogen atoms were refined anisotropically (with
absorption corrected data for 2) using full-matrix least squares based on F2
Fig. 2 Molecular structure of 2. Selected bond lengths (Å) and angles (°);
Mo–C(1) 1.149(6), Mo–N(1) 2.198(4), Mo–N(2) 1.753(4), Mo–N(3)
1.746(4), Mo–Cl 2.336(1), C(1)–N(1) 1.452(7), C(1)–C(2) 1.477(7),
C(2)–O(2) 1.219(7), C(2)–O(3) 1.337(7), C(1)–Mo–N(1) 39.0(2),
C(1)–Mo–N(2) 103.8(2), C(1)–Mo–N(3) 101.4(2), C(1)–Mo–Cl 124.3(2),
N(1)–Mo–N(2) 125.8(2), N(1)–Mo–N(3) 112.4(2), N(1)–Mo–Cl 85.8(1),
N(2)–Mo–N(3) 113.2(2), N(2)–Mo–Cl 105.9(1), N(3)–Mo–Cl 108.3(2),
Mo–C(1)–N(1) 72.3(3), Mo–N(1)–C(1) 68.7(3), C(8)–N(2)–Mo 171.1(4).
Subsequently, an attempt was made to force the enolate
ligand into an O-bound coordination mode, by using an
a-aminoester enolate ligand [Scheme 1(b)]. It was anticipated
that this ligand should produce a five-membered chelate, bound
via the N and the carbonyl O donor to the metal centre as in
complex B [Scheme 1(b)]. However, the strong carbophilic
nature of the (ArN)2MoVI fragment again became apparent,
resulting in the C-bound enolate complex 2. Moreover, a three-
membered chelate with an intramolecular coordination of the
amine donor is formed. Thus, chelation of the carbonyl O to
give a four-membered ring as in complex C is not observed
[Scheme 1(b)]. Similar preferred arrangements have been
reported previously for low valent Mo complexes.19,20
The preparation of complex 2 involved the in situ formation
of the lithium enolate from LDA and Me2NCH2CO2Et in THF
at 278 °C. Addition to [Mo(NAr)2Cl2(DME)] at this tem-
perature and extraction of the product with pentane at room
temp. gave complex [Mo(NAr)2{CH(NMe2)CO2Et}Cl] 2 as a
dark red crystalline solid.‡ Suitable crystals for X-ray analysis
were obtained from pentane, and the molecular structure is
given in Fig. 2.§
The X-ray structure of 2 again reveals an unexpected
coordination of the a-aminoenolate ligand, with chelation via
the amino atom N(1) and the a-C atom C(1) [with consequent
loss of double bond character for C(1)–C(2)], rather than the
carbonyl O atom O(2). As in 1, the coordination at Mo can be
considered as distorted tetrahedral, the enolate occupying a
single coordination site [the N–Mo–N and N–Mo–Cl angles are
in the range 106.9(1)–113.2(2)°]. The Mo–N(imido) distances
are again typical of ‘linear’ species, with angles at N of 161.0(4)
and 171.1(4)° for N(2) and N(3) respectively. As in 1, the
‘strong’ bond to the a-aminoenolate ligand is to the a-C atom
[2.149(6) Å] whereas that to N(1) is 2.198(4) Å. This contrasts
with the pattern observed for related chelation to a MoII centre
where the Mo–C bond is noticeably longer than that to N.20 In
the absence of coordination to the ester carbonyl O atom there
is no delocalisation between the two O atoms.
to give R1
= 0.057 (0.046), wR2 = 0.097 (0.098) for 2088 (3430)
independent observed reflections [ıFoı > 4s)ıFoı), 2q ! 47° (115°)] and
343 (339) parameters for 1 and 2 respectively. The absolute chirality of 1
could not be unambiguously determined. CCDC 182/776.
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In conclusion, we have shown that enolate complexes of the
bis(imido)MoVI core are readily accessible and that the metal–
carbon bonded form is favoured over the oxygen-bonded
arrangement, confirming the surprisingly carbophilic nature of
the MoVI centres in these complexes.
ICI Acrylics (CASE Award to P. A. C.) and EPSRC are
gratefully acknowledged for financial support.
Received in Cambridge, UK, 5th January 1998; 8/00116B
738
Chem. Commun., 1998