Carbamoyl complexes of divalent tungsten, molybdenum, and iron and the unexpected formation of an aminomethylidyne complex

Article abstract of DOI:10.1021/om970738y

Sequential treatment of [M(CO)₆] (M = W, Mo, but not Cr) with 1 equiv of LiNⁱPr₂, iodine, and PPh₃ provides [M(η²-OCNⁱPr₂)I(CO)₃(PPh ₃)], which serve as precursors for a wide range of bidentate carbamoyl complexes; however, if for M = W, an excess of LiNⁱPr₂ is employed, the aminomethylidyne complex [W(≡CN-ⁱPr₂)I(CO)₃(PPh₃)] is also obtained.

Full text of DOI:10.1021/om970738y

Organometallics 1997, 16, 5595-5597
5595
Ca r ba m oyl Com p lexes of Diva len t Tu n gsten ,
Molybd en u m , a n d Ir on a n d th e Un exp ected F or m a tion of
a n Am in om eth ylid yn e Com p lex
Stephen Anderson, Darren J . Cook, and Anthony F. Hill*
Department of Chemistry, Imperial College of Science Technology and Medicine,
South Kensington, London SW7 2AY, U.K.
Received August 18, 1997X
Sch em e 1a
Summary: Sequential treatment of [M(CO)6] (M ) W,
Mo, but not Cr) with 1 equiv of LiNiPr2, iodine, and PPh3
provides [M(η2-OCNiPr2)I(CO)3(PPh3)], which serve as
precursors for a wide range of bidentate carbamoyl
complexes; however, if for M ) W, an excess of LiNiPr2
is employed, the aminomethylidyne complex [W(tCN-
iPr2)I(CO)3(PPh3)] is also obtained.
Carbamoyl (carboxamide) complexes of the group 6
metals have been prepared previously by a variety of
routes.1 Generally, these routes involve nucleophilic
attack by amines on electrophilic carbonyl ligands,
although notable examples of the C-H activation of
formamides have also been observed.2 As a class of
ligands, carbamoyls have, however, been somewhat
neglected, in favor of related acyl and aroyl ligands, for
which a more direct industrial relevance to the catalytic
activation of carbon monoxide is appreciated. Our
studies on the synthesis of aminomethylidyne com-
plexes3 and in particular those of iron4 have focused on
the O-acetylation or phosphorylation of anionic carbam-
oylate complexes. Thus, e.g., the reaction of [Fe{dC-
(OLi)NiPr2}(CO)4] with (CF3CO)2O and triphenylphos-
phine has been shown to be solvent dependent: In
diethyl ether, the carbamoyl complex [Fe(η2-OCNiPr2)-
(CF3)(CO)2(PPh3)] is obtained,5 while in dichloromethane
the aminomethylidyne salt [Fe(≡CNiPr2)(CO)3(PPh3)]-
(CF3CO2) is formed.4 This aminomethylidyne complex
has also been unexpectedly obtained from the reaction
of [Fe(η2-OCNiPr2)(CF3)(CO)2(PPh3)] with iodine,4 and
this unusual result has prompted us to investigate, in
more detail, the oxidation of carbamoylate complexes.
During studies on the iron system, a convenient entry
point to carbamoyl complexes in general was found,
based on the sequential treatment of [Fe(CO)5] with
LiNiPr2, I2 and PPh3 to provide [Fe(η2-OCNiPr2)(CO)2-
(PPh3)] (1) (Scheme 1). Both this complex and its
derivatives all displayed the less common bidentate η2-
O,C coordination mode for the carbamoyl ligand. We
wished to establish whether this was a peculiarity for
divalent iron or a more general feature of these ligands.
Reagents and Conditions: Et2O; (i) LiNiPr2, 25 °C; (ii) I2,
a
-78 °C; (iii) PPh3, -78 to 25 °C.
Herein, we wish to report (i) the reactions of the car-
bamoylates [M{C(OLi)NiPr2}(CO)5] (M ) Mo, W) with
iodine, which provide convenient access to a wide range
of carbamoyl complexes of these metals, the majority
of which feature bidentate carbamoyl coordination. (ii)
The reaction of [W{C(OLi)NiPr2}2(CO)4] with iodine or
bromine and triphenylphosphine, which unexpectedly
provides the aminomethylidyne complexes [W(tCNi-
Pr2)X(CO)3(PPh3)] (X ) Br, I), via the presumed and
unprecedented oxidative coupling of two carbamoyl
ligands.
Treating [M(CO)6] (M ) Mo, W) with LiNiPr2 provides
the carbamoyl metalates [M{dC(OLi)NiPr2}(CO)5], which
have previously served as entry points to the chemistry
of aminomethyene and aminomethylidyne complexes of
these metals.6 Treating these compounds (generated in
situ) with iodine results in the formation of thermolabile
complexes presumed to be [M(OCNiPr2)I(CO)x] (x ) 4,
5, ?); however, if triphenylphosphine is added subse-
quently, then stable derivatives [M(η2-OCNiPr2)I(CO)3-
(PPh3)] (M ) Mo (2), W (3)) are obtained in reasonable
yields (40% (2), 45% (3)). The formulations follow from
spectroscopic data7 which confirm the gross composition
but reveal an interesting difference in metal stereo-
chemistry. In the case of 2, a fac-W(CO)3 geometry is
adopted; however, for 3, a mer-Mo(CO)3 arrangement
X Abstract published in Advance ACS Abstracts, November 15, 1997.
(1) (a) For a review of carbamoyl complexes; see: Angelici, R. J . Acct.
Chem. Res. 1972, 18, 335. (b) For a more general review of η2-acyl and
related ligands, see: Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988,
88, 1059.
(2) Mu¨ller, A.; Seyer, U.; Eltzner, W. Inorg. Chim. Acta, 1979, 32,
L65. Mu¨ller, A.; Sarkar, S. Z. Naturforsch., B. 1978, 33, 1053. Ishida,
T.; Mizobe, Y.; Tanase, T.; Hidai, M. Chem. Letts. 1988, 441.
(3) Anderson, S.; Hill, A. F. J . Organomet. Chem. 1990, 394, C24.
Anderson, S.; Cook, D. J .; Hill, A. F. J . Organomet. Chem. 1993, 463,
C3.
(6) (a) Fischer, E. O.; Reitmeier, R.; Ackermann, K. Z. Naturforsch.,
B. 1984, 39, 668 and references therein. (b) For a review covering the
chemistry of aminomethylidyne complexes of these metals, see: Mayr,
A.; Hoffmeister, H. Adv. Organomet. Chem. 1991, 32, 227.
(4) Anderson, A.; Hill, A. F. Organometallics 1995, 14, 1562.
(5) Anderson, S.; Hill, A. F.; Clark, G. R. Organometallics 1992, 11,
1990.
S0276-7333(97)00738-3 CCC: $14.00 © 1997 American Chemical Society

5596 Organometallics, Vol. 16, No. 26, 1997
Communications
Sch em e 2a
a
Reagents: (i) K[EtOCS2]; (ii) Na[S2CNMe2]; (iii) NaC5H5; (iv) dppe; (v) LiNiPr2, I2, PPh3; (vi) K[HB(pz)3]; (vii) LiNiPr2, I2, CNR
(R ) C6H3Me2-2,6); (viii) LiNiPr2, I2, tmeda; (ix) dppf (1,1′-bis(diphenylphosphino)ferrocene); (x) bipy (2,2′-bipyridyl); (ix) LiNiPr2,
I2, bipy.
results. The complexes 2 and 3 serve as stable and
convenient precursors to a wide range of carbamoyl
complexes via ligand exchange reactions: Scheme 2
illustrates the variety of carbamoyl complexes obtained
so far for molybdenum. The tungsten analogue (3) has
not yet been studied in detail but has so far provided
the complexes [W(η2-OCNiPr2)(CO)2{HB(pz)3}] (pz )
pyrazol-1-yl) and [W(η2-OCNiPr2)I(CO)2(dppe)] on treat-
ment with K[HB(pz)3] or dppe, respectively. While
demonstrating the versatility of 2 and 3, in developing
the carbamoyl chemistry of these metals, the transfor-
mations are straightforward and call for little comment.
One point which does, however, emerge from Scheme 2
is the prevalence of bidentate (O,C) carbamoyl coordina-
tion. Two complimentary sequences wherein this is not
the case are noteworthy: Firstly, the complex [Mo{σ-
C(dO)NiPr2)I(CO)3(tmeda)] (4) results from the reaction
of 2 with tmeda or via a direct synthesis involving the
sequential treatment of [Mo(CO)6] with LiNiPr2, I2, and
tmeda. On treating 4 with K[HB(pz)3], the formation
of [Mo(η2-OCNiPr2)(CO)2{HB(pz)3}] (5) is accompanied
by a conversion from monohapto to dihapto carbamoyl
coordination. In contrast, the dihapto carbamoyl com-
plex [Mo(η2-OCNiPr2)I(CO)(CNC6H3Me2-2,6)3] (6) (ob-
tained from sequential treatment of [Mo(CO)6] with
LiNiPr2, I2, and CNC6H3Me2-2,6) reacts with K[HB(pz)3]
to provide a monohapto carbamoyl complex [Mo{σ-
C(dO)NiPr2}(CO)(CNC6H3Me2-2,6)2{HB(pz)3}] (7). These
observations, coupled with the temperature-dependent
(7) Characteristic spectroscopic data for selected complexes (25 °C,
IR (ν(CO), CH2Cl2), NMR (CDCl3) satisfactory microanalytical, and
FAB-MS data obtained): In a typical procedure the following method
for the synthesis of 2 was followed: [Mo(CO)6] (1.13 g) in diethyl ether
(25 cm3) was treated with LiNiPr2 (2.9 cm3, 1.5 mol dm-3, Aldrich) and
cooled (dry ice/propanone). Iodine (1.08 g) was added, the mixture
allowed to warm to 0 °C, PPh3 (2.34 g) added, and the mixture stirred
for 10 h. The orange precipitate which formed was isolated and
recrystallized from dichloromethane/hexane (-20 °C). Yield 1.20 g
(40%, nonoptimized). IR(CH2Cl2): 2027, 1957, 1911 (ν(CO)), 1626 cm-1
(ν(NCO)). 1H NMR (CDCl3, 25 °C): 1.35, 1.38 (d × 2, 12 H, CH3), 3.72,
4.35 (h × 2, 2 H, NCH), 7.21-7.60 (m, 15 H, C6H5) ppm. 13C{1H}
NMR: 210.7, 209.8 (CO), 190.4 (NCO, J (PC) ) 7 Hz), 133.9-130.2
(C6H5), 55.1, 50.1 (NCH), 20.9, 20.4 (CH3) ppm. 31P{1H} NMR: 16.8
ppm. 1: IR (CH2Cl2) 2021, 1953 (ν(CO)), 1610 cm-1 (ν(NCO)). 1H NMR
(CDCl3, 25 °C): 0.55, 1.15, 1.19, 1.52 (d × 4, 12 H, CH3), 3.35, 5.09 (h
× 2, 2 H, NCH), 7.19-7.67 (m, 15 H, C6H5) ppm. 13C{1H} NMR: 220.5
(d, CO, J (PC) ) 25.0 Hz), 212.4 (d, CO, J (PC) ) 20.8 Hz), 197.4 (NCO,
J (PC) ) 19.4 Hz), 134.2-128.3 (C6H5), 55.5, 47.9 (NCH), 21.6, 21.3,
20.4, 19.7 (CH3) ppm. 31P{1H} NMR: 78.5 ppm. 3: IR (CH2Cl2) 2022,
1941, 1900 (ν(CO)), 1613 cm-1 (ν(NCO)). 1H NMR (CDCl3, 25 °C): 1.13,
1.20, 1.35, 1.44 (d × 4, 12 H, CH3), 3.69, 5.31 (h × 2, 2 H, NCH), 7.34-
7.68 (m, 15 H, C6H5) ppm. 13C{1H} NMR: 219.8 (s), 218.5 (d, CO, J (PC)
) 10.7 Hz), 202.9 (d, CO, J (PC) ) 58.6 Hz), 187.6 (NCO, J (PC) ) 5.3
Hz), 134.2-128.2 (C6H5), 55.5, 49.8 (NCH), 20.9, 20.8, 20.5, 20.4 (CH3)
ppm. 31P{1H} NMR: 10.2 (J (PW) ) 227.2 Hz) ppm. 6: IR (CH2Cl2)
2129, 2078 (ν(CN)), 1876 (ν(CO)), 1608 cm-1 (ν(NCO)). 1H NMR (CDCl3,
25 °C): 1.27, 1.45 (d × 2, 12 H, CHCH3), 2.42, 2.46 (s × 2, 12, 6 H,
C6H3CH3), 3.60, 4.72 (h × 2, 2 H, NCH), 7.06 (m, 9 H, C6H3) ppm.
13C{1H} NMR: 242.3 (CO), 197.6 (NCO), 184.1, 174.3 (CN), 134.8-
127.7 (C6H3), 54.0, 48.7 (NCH), 20.9, 20.7 (NCHCH3), 20.9, 19.1
(C6H3CH3) ppm. 7: IR (CH2Cl2) 2094 (ν(CN)), 1774 (ν(CO)), 1618 cm-1
(ν(NCO)). 1H NMR (CDCl3, 25 °C): 1.30 (m, 12 H, CHCH3), 2.50 (s, 12
H, C6H3CH3), 3.30, 4.05 (h × 2, 2 H, NCH), 6.12 (s(br), 3 H, H4(pz)),
6.90-7.83 (m, 12 H, pz + C6H3) ppm. FAB-MS: 729 (M)+, 701 (M -
CO)+, 570 (M - CO, CNR)+, 439 (M - CO, 2CNR)+. Full spectroscopic
data for the new complexes are also available from the authors
(a.hill@ic.ac.uk). The complexes [W(tCNiPr2)I(CO)3(PPh3)], [W(η2-
OCNiPr2)I(CO)3(PPh3)], and [Mo(η2-OCNiPr2)(CO)2(η-C5H5)] have also
been crystallographically characterized.8
1
fluxionality evident in the H NMR spectra of many of
the complexes in Scheme 2, suggest to us that mono-
hapto-dihapto interconversion is a low-energy process
and may, in part, account for the facility of the ligand
exchange reactions described.
Templeton has shown that the reaction of [Mo(η2-
OCMe)(CO)2{HB(pzMe2-3,5)3}] with excess sodium ethox-
ide results in the formation of the ethylidyne complex
[Mo(≡CMe)(CO)2{HB(pzMe2-3,5)3}] in 20% yield.9
A
similar reaction ensues between [W(η2-OCNiPr2)(CO)2-
{HB(pz)3}] and NaOEt, however, after heating in re-
fluxing ethanol for 18 h, only approximately 10%
conversion to [W(tCNiPr2)(CO)2{HB(pz)3}] is observed.
A more unusual carbamoyl/aminomethylidyne conver-
sion occurs when [W(CO)6] is treated with an excess
(1.5-2 equiv) of LiNiPr2 followed by I2 and PPh3. In
addition to the anticipated carbamoyl complex 3, the
aminomethylidyne complex mer-[W(tCNiPr2)I(CO)3-
(8) Slawin, A. M. Z.; White, A. J . P.; Williams, D. J . Unpublished
results.
(9) Brower, D. C.; Stoll, M.; Templeton, J . L. Organometallics 1989,
8, 2786.

Communications
Organometallics, Vol. 16, No. 26, 1997 5597
Sch em e 3a
support of this intramolecular mechanism, it should be
noted that the intermolecular alternative, oxidative
cleavage of one carbamoyl ligand as IC(dO)NiPr2, would
produce [W(η2-OCNiPr2)I(CO)4], the precursor for 3. The
possibility that the liberated carbamoyl iodide acts as
an oxide abstracting agent appears unlikely, given that
we have been unable to observe any aminomethylidyne
complex formation on treating either [W{C(OLi)N-
iPr2}(CO)5] or [W{C(OLi)NiPr2}2(CO)4] with N,N-diiso-
propylcarbamoyl chloride. The process by which 8 is
formed from [W{C(OLi)NiPr2}2(CO)4] is, therefore, mecha-
nistically distinct from the unusual conversion of [Fe-
(η2-OCNiPr2)(CF3)(CO)2(PPh3)] by iodine to [Fe(tCN-
iPr2)(CO)3(PPh3)]I, wherein the trifluoromethyl coligand
is essential and intimately involved in the transforma-
tion.
The coupling of carbamoyl and aroyl ligands on a
platinum center has been previously observed11 and
used to provide insight into the mechanism for pal-
ladium catalyzed double-carbonylation reactions which
provide R-ketoamides.12 In a similar manner, bis(al-
koxycarbonyl)complexes of divalent iron have been
shown to eliminate oxalate esters.13 These results all
involve C-C bond formation in the coupling step. The
proposed C-O bond formation in the present example
is, therefore, unusual, although we have previously
shown that C-O bond formation may occur in the coup-
ling of carbamoyl and difluorocarbene ligands.14 The
above results (i) allow convenient and large scale access
to carbamoyl complexes of molybdenum and tungsten
and (ii) provide a new rationale for the origin of the “ate”
fraction in the Fischer-Tropsch synthesis: Thus acyl
ligands may in principle couple and disproportionate to
provide surface alkylidyne and carboxylate groups.
a
Reagents and Conditions: (i) 2 LiNiPr2 (25 °C); (ii) + I2, -
LiI (-78 °C); (iii) -LiO2CNiPr2, + PPh3 (-78 to 25 °C). (iv)
LiNiPr2‚LiI, (CF3CO)2O, PPh3. (*) Presumed intermediate, not
isolated.
(PPh3)] (8) is also obtained. In a similar manner,
replacing iodine with bromine provides a chromato-
graphically separable mixture of fac-[W(η2-OCNiPr2)-
Br(CO)3(PPh3)] (9) and mer-[W(tCNiPr2)Br(CO)3(PPh3)]
(10). The yield of 8 appears to be optimized when 1.6
equiv of LiNiPr2 is used.10 The formation of 5 is sur-
prising, however, the mechanism shown in Scheme 3
seems to be most plausible to us. This involves the
formation of the known bis(carbamoyl)tungstate [W{C-
(OLi)NiPr2}2(CO)4],6a oxidation of which (by iodine or
bromine) initiates the coupling of the two carbamoyl
ligands to provide a carbamato-carbene. Subsequently,
dissociation of the carbamato substituent then results
in formation of the WtC- multiple bond of 8. In
Ack n ow led gm en t. This work was supported by the
EPSRC (U.K.). A.F.H. gratefully acknowledges the
Royal Society and the Leverhulme Trust for the award
of a Senior Research Fellowship.
OM970738Y
(11) Huang, T.-M.; Chen, J .-T.; Lee, G.-H.; Wang, Y. Organometallics
1991, 10, 175.
(12) Ozawa, F.; Yanagihara, H.; Yamamoto, A. J . Org. Chem. 1986,
51, 415; Ozawa, F.; Soyama, H.; Yanagihara, H.; Aoyama, I.; Takino,
H.; Izawa, K.; Yamamoto, A. J . Am. Chem. Soc. 1985, 107, 3235.
(13) Laurent, P.; Salaun, J . Y.; Legall, G.; Sellin, M.; Desabbayes,
H. J . Organomet. Chem. 1994, 466, 175.
(10) Employing the theoretically requisite 2.0 equiv of LiNiPr2 leads
to intractible mixtures. The complex 8 may be alternatively pre-
pared by the sequential treatment of [W(CO)6] in diethyl ether with
LiNiPr2‚LiI, (CF3CO)2O, and PPh3 (40% yield).
(14) Anderson, S.; Hill, A. F.; Slawin, A. M. Z.; Williams, D. J . J .
Chem. Soc., Chem. Commun. 1993, 266.