The noncarbonylative photochemistry of group 6 fischer carbene complexes

Article abstract of DOI:10.1002/ejic.200800146

The different photochemical reaction mechanisms of group 6 Fischer carbene complexes have been studied by a joint experimental and density functional theory (DFT) computational study. This study unifies the photochemcal behavior of group 6 metal carbene complexes (M = Cr or W), which may occur by three different photoreactions, namely photocarbonylation, stepwise 1,2-dyotropic rearrangement, and α-fragmentation. The metal atom and the substituent on the nitrogen atom attached to the carbene carbon atom influence the topography of the potential energy surface, which leads to two different activated species, diradicals or metallacyclopropanones. Complexes having a hydrogen substituent at the nitrogen atom experience a stepwise dyotropic migration with no significant differences between the chromium- and tungsten-carbene complexes. The excitation of complexes having substituents other than hydrogen at the nitrogen atom leads to two different T1 species, and hence to the formation of mixtures of products arising from α-fragmentation (noncarbonylation) and amino esters (photocarbonylation). The two triplet states are isoenergetic in the Cr derivatives, but the T1 species that leads to α-fragmentation is strongly favored for the W derivatives. Finally, both the chromium-and tungsten-carbene complexes are photoreactive in spite of the longstanding claim that tungsten-carbene complexes were photochemically inert. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800146

FULL PAPER
DOI: 10.1002/ejic.200800146
The Noncarbonylative Photochemistry of Group 6 Fischer Carbene Complexes
Israel Fernández,[a] Miguel A. Sierra,*[a] María José Mancheño,[a] Mar Gómez-Gallego,[a]
and Fernando P. Cossío*[b]
Keywords: Metal carbenes / Photochemistry / Type I dyotropic rearrangements / Excited states / α-Fragmentation
The different photochemical reaction mechanisms of group
6 Fischer carbene complexes have been studied by a joint
experimental and density functional theory (DFT) computa-
tional study. This study unifies the photochemcal behavior of
group 6 metal carbene complexes (M = Cr or W), which may
occur by three different photoreactions, namely photocar-
bonylation, stepwise 1,2-dyotropic rearrangement, and α-
fragmentation. The metal atom and the substituent on the
nitrogen atom attached to the carbene carbon atom influence
the topography of the potential energy surface, which leads
to two different activated species, diradicals or metallacyclo-
propanones. Complexes having a hydrogen substituent at
the nitrogen atom experience a stepwise dyotropic migration
with no significant differences between the chromium- and
tungsten–carbene complexes. The excitation of complexes
having substituents other than hydrogen at the nitrogen
atom leads to two different T1 species, and hence to the for-
mation of mixtures of products arising from α-fragmentation
(noncarbonylation) and amino esters (photocarbonylation).
The two triplet states are isoenergetic in the Cr derivatives,
but the T1 species that leads to α-fragmentation is strongly
favored for the W derivatives. Finally, both the chromium-
and tungsten–carbene complexes are photoreactive in spite
of the longstanding claim that tungsten–carbene complexes
were photochemically inert.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
The irradiation of group 6 (Fischer) metal carbene com-
plexes with visible light in the presence of nucleophiles leads
to the formation of β-lactams,[1] cyclobutanones,[2] amino
acids and peptides,[3] polynuclear hydrocarbons,[4] and β-
lactones (Scheme 1).[5] This rich and synthetically powerful
photoreactivity[6] has no parallel in the photochemistry of
any other class of organometallic compounds. In fact, it is
only comparable with the amazingly diverse thermal chem-
istry of these metal–carbene complexes.[7]
Nevertheless, the synthetically useful photochemistry of
chromium(0)- and molybdenum(0)–carbene complexes [but
not tungsten(0)–carbene complexes] relies exclusively on the
reversible insertion of a CO ligand into the M=C bond to
yield ketene-like species.[8,9] This mechanistic insight was re-
ported by Hegedus in 1988,[10] who systematized the photo-
chemistry of group 6 carbene complexes in the presence of
nucleophiles. Despite the close parallelism of the ketenes
derived from the irradiation of group 6 metal carbene com-
plexes and free ketenes, the different efforts directed toward
[a] Departamento de Química Orgánica, Facultad de Química,
Universidad Complutense,
Scheme 1.
28040 Madrid, Spain
Fax: +34-913944310
E-mail: sierraor@quim.ucm.es
the detection of these elusive intermediates were fruitless.[11]
We have recently shown[8] that the combination of com-
putational and experimental tools is appropriate to study
the mechanism of the photocarbonylation of group 6 metal
carbene complexes. The irradiation of alkoxychromium(0)–
[b] Kimika Fakultatea, Euskal Herriko Unibertsitatea,
P. K. 1072, 20080 San Sebastián-Donostia, Spain
E-mail: fp.cossio@ehu.es
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Noncarbonylative Photochemistry of Group 6 Fischer Carbene Complexes
carbene complexes, either in the region of the LF band fol-
lowed by relaxation to the MLCT band or directly in the
region of the MLCT band, results in the excitation of these
complexes to the S1 state. The S1 decays to T1 by intersys-
tem crossing (ISC) because of spin–orbit coupling to form
metallacyclopropanone species I (Figure 1). These coordi-
natively unsaturated species change their multiplicity to S0
by filling the free coordination site with a molecule of a
coordinating solvent in the apical position to form acyl-
chromate complexes II. These complexes evolve to ketene-
derived products on the S0 hypersurface. The absence of
nucleophiles does not lead to other forms of reactivity and
species I reverts to the starting carbene complex in a
strongly exothermic process (Figure 1).
Scheme 3.
The different reaction pathways observed for these com-
pounds provides a good opportunity for applying the com-
bination of experimental and computational methods pre-
viously developed by us while studying the photocarbon-
ylation reaction.[8,12] We herein report an extensive compu-
tational and experimental study on the mechanisms of the
different reaction pathways for the processes shown in
Scheme 2 and Scheme 3. Similarities and differences of the
photoreactivity of chromium- and tungsten Fischer carbene
complexes are also explored and discussed.
Figure 1. The carbonylation step in chromium(0)carbene photo-
chemistry.
Concomitant to these studies, we discovered that group
6 aminocarbene complexes having phosphane ligands can
evolve by different noncarbonylative pathways.[12,13] Far
from being limited to the carbonylation reaction, group 6
Fischer carbene complexes have a rich and very complex
photochemistry, which strongly depends on the nature of
the ligands. For example, the stepwise type I 1,2-dyotropic
rearrangement[12] converts chromium(0)–phosphinoami-
nocarbene 1 to syn-N-metalated imine 2 (Scheme 2).
Results and Discussion
The noncarbonylative photochemistry of group 6 metal
carbene complexes presented above clearly shows a strong
dependence of the nature of the reaction products on the
substituent on the nitrogen atom bonded to the carbene
carbon atom.[12,13] This fact clearly differentiates these pho-
tochemical reactions from the photocarbonylation path-
way[2,3,5] that either leads to ketene-derived products or un-
reacted starting materials. Complexes 1 and tungsten(0)–
carbene complexes 3a lead to syn-N-metalated imine com-
pounds 2 and 4 through 1,2-dyotropic processes. This re-
arrangement is limited to complexes such as 1 and 3a that
have a hydrogen atom bonded to the nitrogen atom at-
tached to the carbene carbon atom. N-methylated com-
plexes 3b experience two different processes, namely α-frag-
Scheme 2.
Subsequently, we found that the “photoinert” tung- mentation and photocarbonylation.[13]
sten(0)–carbene complexes were indeed photoreactive, pro-
To study the generality of these processes, chromium(0)–
vided that their structures would be adequately designed.[13] carbene complexes having different tethers that join the car-
Up to three new photoprocesses were discovered for tung- bene nitrogen atom and the phosphane ligand and having
sten(0)–carbene complexes 3: 1,2-dyotropic rearrangement different alkyl substituents at the nitrogen atom were pre-
leading to imino complexes 4, α-fragmentation to form 5, pared. Thus, the chromium–carbene complexes 7a–e were
and more strikingly, photocarbonylation to form the N- obtained in almost quantitative yields by reaction of com-
methylamino ester 6 (Scheme 3). These processes are re- plexes 1a–c with MeI/Cs2CO3 (complexes 7a–c), benzyl bro-
markable since tungsten(0)–carbenes were considered as mide/Cs2CO3 (complex 7d), or allyliodide/Cs2CO3 (complex
photoinert until we reported these findings.
7e) at room temperature in acetone as a solvent (Scheme 4).
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I. Fernández, M. A. Sierra, M. J. Mancheño, M. Gómez-Gallego, F. P. Cossío
FULL PAPER
nucleophilic attack by MeOH.[8b] However, complexes 8
cannot be generated from ketene-like species. Thus, we pro-
pose that N-substituted aminocarbene complexes 3b and 7
experience an α-fragmentation process, which is analogous
to the α-photofragmentation of amides.[16,17] In principle,
the α-fragmentation process should occur in a triplet species
isostructural to those proposed for the 1,2-dyotropic pro-
cess and formed by the initial capture of the photon by the
carbene complex in the S0 state.
We initiate our attempt to generalize the noncarbon-
ylative photochemistry of group 6 metal carbenes by com-
paring the computational results of model tungsten–car-
bene 10 with the results obtained for the analogous chro-
mium(0)–carbene 11. The reaction profile of complex 10
was calculated at the uB3LYP/6-31+G(d)&LANL2DZ level
(Scheme 6). We propose that the type I dyotropic reac-
tion[18] of complex 10 starts with a short-lived triplet state
10(T1), formed after initial photoexcitation of the singlet
ground state S0 to the excited singlet state S1 and subse-
Scheme 4.
Complexes 7 were irradiated (450 W, medium-pressure quent ISC. The triplet state 10(T1) evolves to the carbene
Hg lamp, Pyrex filter and Pyrex well) in MeCN/MeOH species 12(T1) in the triplet hypersurface via transition state
(10:1) (complexes 7a,b) or MeCN/thf/MeOH (5:5:1) (com- 13, which is associated with the 1,2-migration of the penta-
plexes 7c–e). Two different products were obtained in these carbonyltungsten moiety from the carbene carbon atom to
reactions, either as single reaction products (7a, 7b, and 7e) the nitrogen atom. Carbene 12(T1) isomerizes to 14(T1) by
or as mixtures of two products (7c,d). The reaction prod- rotation about the N–C(Me) bond via the saddle point
ucts were identified either as cyclic complexes 8 on the basis 15(T1) transition structure. Finally, the species 14(T1)
of their spectroscopic data and by comparison with pre- yields the final product 16(T1) through transition state 17,
viously reported tungsten analogues[13,14] or as amino esters a saddle point associated with the migration of the hydro-
9
arising from the photocarbonylation reaction gen atom from the nitrogen atom to the carbene moiety.
(Scheme 5).[15]
It is very significant that the activation barriers of the
The data above allow for the conclusion that substitution transformation involving tungsten–carbene complexes are
at the nitrogen atom of the aminocarbenes 7 inhibits 1,2- only slightly lower than the activation energies for chro-
dyotropic transformation, which is the exclusive photopro- mium–carbene complexes. The experimental and computa-
cess observed for complexes 1 and 3a. The formation of tional data demonstrate that group 6 aminocarbene com-
complexes 8 together with the carbonylation derivatives 9 plexes having phosphane ligands and an unsubstituted ni-
indicate the co-existence of at least two different activated trogen atom attached to the carbene carbon atom react
species that are formed upon irradiation of metal–carbene through a 1,2-dyotropic stepwise rearrangement.[19] With
complexes 7 and 3b. We have shown that amino esters 9 are the exception of the relative energies of the starting diradi-
formed from the coordinated-ketene pathway followed by cal species with respect to the ground state complexes
Scheme 5.
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Noncarbonylative Photochemistry of Group 6 Fischer Carbene Complexes
Scheme 6. Calculated profiles [uB3LYP/6-31+G(d)&LANL2DZ+∆ZPVE level of theory] for the stepwise 1,2-dyotropic process of model
structures 10 and 11. Numbers below the arrows are the relative energies and, when stated, the activation energies in kcal/mol. Bond
lengths are in Å.
(4.8 kcal/mol higher in tungsten complexes than in their spin densities N = 0.92 and C = 0.80 a.u.) and captures two
chromium analogues), no significant differences were ob- hydrogen atoms from the solvent to form the carbene spe-
served between both metal complexes, which is a very re- cies 26. The evolution of this type of complexes to the iso-
markable and unusual result.
lated metalated amine products similar to 27 has been pre-
The second noncarbonylative pathway, namely the α- viously reported by us and others.[20] Alternatively, the
fragmentation, was studied for both the Cr and W model metallacyclopropane complex 23(T2) evolves to the α-
compounds 23 and 29, respectively. The excited states of amino esters such as 28 by reaction of the ketene-like spe-
complex 23 were computed at the uB3LYP/6-31+G(d)& cies formed from 23(T2) with MeOH.
LanL2DZ level (Scheme 7). We located two coexistent trip-
As discussed above,[13] irradiation of the N-methyl-sub-
let species, namely the diradical 23(T1) and the metallacy- stituted complexes 3b exclusively results in α-fragmentation
clopropane complex 23(T2). This second species (see Fig- reactions leading to complexes 5 (Scheme 3); the exception
ure S1 in Supporting Information) is formed through the is the tungsten complex 3c, which leads to 6 in yields lower
standard carbonylation process reported by us.[8a] Both ac- than 11% (Scheme 3). It is clear that tungsten–carbene
tivated species, 23(T1) and 23(T2), are close in energy complexes 3 have a strong bias toward α-fragmentation, at
(∆ET1–T2 = 0.3 kcal/mol), and therefore, mixtures of ketene- the expense of the photocarbonylation reaction. The reason
derived (amino esters) and α-fragmentation compounds for such a difference in behavior relative to their chromium
would be expected. This is in full agreement with the experi- counterparts could be the higher stabilization of the diradi-
mentally observed mixture of complexed aminophosphanes cal species 29(T1) than that of the tungsten–cyclopro-
8 and carbonylation products 9.
panone species 29(T2), which leads to photocarbonylation
Species 23(T1) evolves to the species 24(T1) in the triplet products. Both triplets originate from the excitation of the
hypersurface via transition state 25, which is associated model tungsten compound 29. The computed energy differ-
with the cleavage of the N–C bond. As stated above, this ence between the activated species 29(T1) and 29(T2) is in-
process is the organometallic equivalent of the α-fragmenta- deed +5.7 kcal/mol (Scheme 8) (relative to 0.4 kcal/mol for
tion of amides. Species 24(T1) is also a diradical (calculated the Cr analogues). This higher difference in energy explains
Eur. J. Inorg. Chem. 2008, 2454–2462
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I. Fernández, M. A. Sierra, M. J. Mancheño, M. Gómez-Gallego, F. P. Cossío
FULL PAPER
Scheme 7. Calculated profiles [uB3LYP/6-31+G(d)&LANL2DZ+∆ZPVE level of theory] for transformations of model structure 23. Num-
bers below the arrows are the relative energies and, when stated, the activation energies in kcal/mol. Bond lengths are in Å.
the strong preference or even the exclusive formation of cess is 7.9 kcal/mol lower than the corresponding barrier
complexes 5 instead of the corresponding amino esters for for the analogous chromium species 23(T1), which makes
tungsten complexes 3. Similarly to chromium complex this step easier for tungsten than for chromium. The
23(T1), triplet 29(T1) (calculated spin densities W = 0.56 stronger N–C bond in 23(T1) than that in the tungsten spe-
and C = 0.74 a.u.) evolves to diradical 30(T1) (calculated cies 29(T1) accounts for this observation. In fact, the N–C
spin densities N = 0.92 and C = 0.92 a.u.) via the transition NBO Wiberg bond order is higher in 23(T1) (1.40 a.u.) than
state 31, which is again associated with the breaking of the in 29(T1) (1.31 a.u.), and the N–C bond length is therefore
N–C bond. Interestingly, the activation energy of this pro- longer in 29(T1) (1.398 Å) than in 23(T1) (1.382 Å). As a
Scheme 8. Calculated profiles [uB3LYP/6-31+G(d)&LANL2DZ+∆ZPVE level of theory] for transformations of model structure 29. Num-
bers below the arrows are the relative energies and, when stated, the activation energies in kcal/mol. Bond lengths are in Å.
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Noncarbonylative Photochemistry of Group 6 Fischer Carbene Complexes
Computational Details: All the calculations reported in this paper
consequence, the α-fragmentation reaction is kinetically fa-
vored in tungsten–carbene complexes in view of the lower
activation barrier than that of their chromium analogues.
In conclusion, the experimental and computational study
reported in this paper unifies the noncarbonylative photo-
chemical behavior of group 6 metal carbene complexes (M
= Cr or W). These complexes can undergo three different
photoreactions, namely: photocarbonylation, stepwise 1,2-
dyotropic rearrangement, and α-fragmentation. Both the
were obtained with the GAUSSIAN 03 suite of programs.[21] Elec-
tron correlation has been partially taken into account by using the
hybrid functional usually denoted as B3LYP[22] and the standard 6-
31+G(d) basis sets[23] for hydrogen, carbon, oxygen, nitrogen, and
phosphorus, and the Hay–Wadt small-core effective core potential
(ECP) including a double-ξ valence basis set[24] for chromium and
tungsten (LANL2DZ keyword). We denote this basis set as
LANL2DZ&6-31+G(d). Zero-point vibrational energy (ZPVE)
corrections were computed at the same level, and have not been
metal atom and the substitution on the nitrogen atom influ- corrected. Excited states were located at the configuration interac-
tion single (CIS) level,[25] and triplets were optimized at the uB3-
ence the topography of the potential energy surface, which
LYP level. Stationary points were characterized by frequency calcu-
leads to two different activated species, and hence to the
lations[26] and have positive, defined Hessian matrices. Transition
carbonylative or noncarbonylative reaction pathways. Gen-
structures (TSs) show only one negative eigenvalue in their diago-
eration of a diradical species instead of metallacyclopro-
nalized force constant matrices, and their associated eigenvectors
panone leads to two different reaction pathways. Complexes
were confirmed to correspond to the motion along the reaction
having a spherical hydrogen substituent at the nitrogen
coordinate under consideration using the intrinsic reaction coordi-
atom experience a stepwise dyotropic migration with no sig-
nificant differences between the chromium- and tungsten–
carbene complexes. The excitation of complexes having sub-
stituents other than hydrogen at the nitrogen atom gives rise
to two different triplet species and hence to the formation
of mixtures of products arising from α-fragmentation (non-
carbonylation) and amino esters (photocarbonylation). The
two triplet states are practically isoenergetic in the Cr deriv-
atives, but the T1 intermediate that leads to α-fragmenta-
tion is strongly favored for the W derivatives. Additionally,
the α-fragmentation reaction is kinetically facilitated in
tungsten–carbene complexes because of a weaker N–C
bond relative to that in the chromium complexes, which re-
sults in lower barriers for this process in these complexes
relative to their chromium counterparts. The experimental
consequence of these models is the formation of metalloim-
ines for NH complexes, irrespective of the metal, or of mix-
tures of α-fragmentation products or of amino esters for
the NR complexes. Finally, both chromium- and tungsten–
carbene complexes are equally photoreactive in spite of the
longstanding claim that tungsten–carbene complexes were
photochemically inert.
nate (IRC) method.[27] Wiberg bond orders were computed by
using the natural bond order (NBO)[28] method.
General Procedure for the Synthesis of Complexes 7: A solution of
complex 1 in degassed acetone (0.025 ) was treated with 2 equiv.
of the corresponding alkyl halide and 3 equiv. Cs2CO3 and water
(drops) at room temperature and left overnight. The crude product
was filtered through a short pad of Celite, and the solvent was
removed under reduced pressure to yield pure compounds (unless
otherwise specified).
1
7a: 214 mg (99%), yellow solid. H NMR(300 MHz, CDCl3): δ =
2.26 (m, 2 H), 2.67 (s, 3 H), 3.31 (d, J = 1.0 Hz, 3 H), 4.04 (m, 1
H), 4.13 (m,
1
H), 7.26–7.47 (m, 10 H, ArH) ppm. 13C
NMR(75.5 MHz, CDCl3): δ = 25.3 (d, JC,P = 18.9 Hz), 29.2, 40.3
(d, JC,P = 3.5 Hz), 57.8 (d, JC,P = 6.2 Hz), 128.4 (d, JC,P = 8.5 Hz),
129.4 (d, JC,P = 1.5 Hz), 131.5 (d, JC,P = 11.0 Hz), 138.1 (d, JC,P
=
33.3 Hz), 222.9 (d, JC,P = 13.0 Hz), 228.8 (d, JC,P = 1.7 Hz), 231.1
(d, JC,P = 14.6 Hz), 281.1 (d, JC,P = 16.1 Hz) ppm. IR (KBr): ν =
˜
1992, 1900, 1863, 1844 cm–1. C21H20CrNO4P (433.36): calcd. C
58.20, H 4.65, N 3.23; found C 58.45, H 4.78.
1
7b: 221 mg (99%), yellow solid. H NMR (300 MHz, CDCl3): δ =
1.97 (m, 2 H), 2.50 (m, 2 H), 2.72 (s, 3 H), 3.78 (s, 3 H), 3.7.18–
7.71 (m, 10 H, ArH) ppm. 13C NMR (75.5 MHz, CDCl3): δ = 22.1,
25.8 (d, JC,P = 15.6 Hz), 39.6, 40.4, 61.1 (d, JC,P = 10.1 Hz), 128.3–
133.6 (m), 222.7 (d, JC,P = 9.8 Hz), 228.5 (d, JC,P = 1.8 Hz), 230.6
(d, JC,P = 15.0 Hz), 281.9 (d, JC,P = 9.2 Hz) ppm. IR (KBr): ν =
˜
1990, 1901, 1871, 1832 cm–1. C22H22CrNO4P (447.38): calcd. C
59.06, H 4.96, N 3.13; found C 58.87, H 4.84.
Experimental Section
1
7c: 245 mg (99%), yellow solid. H NMR (300 MHz, CDCl3): δ =
General Procedures: 1H NMR, 13C NMR, and 31P NMR spectra
were recorded at 22 °C with Bruker Avance 500 (500.1 and
125.7 MHz), 300 (300.1 and 75.4 MHz), or Bruker 200-AC (200.1
and 50 MHz) spectrometers. Chemical shifts are given in ppm rela-
tive to TMS (1H, 0.0 ppm), CDCl3 (13C, 77.0 ppm), and H3PO4
(31P, 0.0 ppm). IR spectra were recorded on a Perkin–Elmer 781
spectrometer. CH3CN was distilled from calcium hydride and thf
from sodium benzophenone. Flame-dried glassware and standard
Schlenk techniques were used for moisture sensitive reactions.
Merck silica-gel (230–400 mesh) was used as the stationary phase
for purification of crude reaction mixtures by flash column
chromatography. Identification of products was made by TLC (kie-
segel 60F-254). UV light (λ = 254 nm) and 5% phosphomolybdic
acid solution in 95% EtOH were used to develop the plates. All
commercially available compounds were used without further puri-
fication. Aminocarbene complexes 1 were prepared following the
previous reported method.[12]
2.70 (s, 3 H), 3.34 (s, 3 H), 4.23 (m, 1 H), 6.05 (m, 1 H), 6.88 (m,
1 H, ArH), 7.30–7.56 (m, 13 H, ArH) ppm. 13C NMR (75.5 MHz,
CDCl3): δ = 40.2 (d, JC,P = 3.7 Hz), 40.9, 65.9 (d, JC,P = 12.5 Hz),
128.2 (d, JC,P = 8.9 Hz), 129.5, 129.5 (d, JC,P = 5.3 Hz), 129.7,
130.2 (d, JC,P = 1.6 Hz), 133.0 (d, JC,P = 10.9 Hz), 133.6 (d, JC,P
=
18.5 Hz), 133.7, 136.0 (d, JC,P = 34.2 Hz), 140.0 (d, JC,P = 14.0 Hz),
222.2 (d, JC,P = 10.1 Hz), 228.4, 229.7 (d, JC,P = 15.6 Hz), 280.7 (d,
JC,P = 12.0 Hz) ppm. IR (KBr): ν = 1998, 1905, 1877, 1838 cm–1.
˜
C26H22CrNO4P (495.43): calcd. 63.03, H 4.48; found C 63.25, H
4.58.
1
7d: 283 mg (99%), yellow solid. H NMR (300 MHz, CDCl3): δ =
2.81 (s, 3 H), 4.21 (br. s, 1 H), 4.62 (br. s, 1 H), 5.20 (br. s, 1 H),
5.91 (br. s, 1 H), 6.89 (m, 1 H, ArH), 7.05–7.35 (m, 18 H, ArH)
ppm. 13C NMR (75.5 MHz, CDCl3): δ = 40.2 (d, JC,P = 3.8 Hz),
55.3, 62.5 (d, JC,P = 12.5 Hz), 126.3, 128.1, 128.2 (d, JC,P = 8.3 Hz),
129.4, 129.6 (d, JC,P = 4.2 Hz), 130.1 (d, JC,P = 6.1 Hz), 130.3,
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133.0 (d, JC,P = 11.2 Hz), 133.5 (d, JC,P = 6.9 Hz), 134.0 (d, JC,P
18.6 Hz), 136.0 (d, JC,P = 34.5 Hz), 140.0 (d, JC,P = 14.1 Hz), 222.4
(d, JC,P = 7.4 Hz), 228.4 (d, JC,P = 1.8 Hz), 229.9 (d, JC,P
=
2.16 (s, 3 H), 2.21 (m, 2 H), 2.50 (m, 2 H), 3.28 (q, J = 7.0 Hz, 1 H),
3.60 (s, 3 H), 7.38–7.71 (m, 10 H, ArH) ppm. 13C NMR (75.5 MHz,
CDCl3): δ = 14.4, 19.7 (d, JC,P = 3.3 Hz), 27.0 (d, JC,P = 73.1 Hz),
37.5, 51.1, 54.2 (d, JC,P = 15.4 Hz), 61.1, 128.5 (d, JC,P = 11.5 Hz),
=
16.4 Hz), 282.9 (d, JC,P = 12.6 Hz) ppm. IR (KBr): ν = 1994, 1913,
˜
1871, 1840 cm–1. C32H26CrNO4P (571.52): calcd. C 67.25, H 4.59,
N 2.45; found C 67.01, H 4.43, N 2.34.
130.7 (d, JC,P = 9.2 Hz), 131.6 (d, JC,P = 2.6 Hz), 133.0 (d, JC,P =
98.3 Hz), 173.6 ppm. 31P NMR (121, MHz, CDCl3): δ = 34.1 ppm.
IR (CCl ): ν = 1936, 1732, 1437, 1215 cm–1. ESI-MS: m/z = 359.
1
˜
4
7e: 258 mg (99%), yellow solid. H NMR (300 MHz, CDCl3): δ =
C20H26NO3P (359.40): calcd. C 66.84, H 7.29, N 3.90; found C
66.67, H 7.05.
2.74 (s, 3 H), 4.20 (br. s, 2 H), 5.11 (br. s, 1 H), 5.25 (d, J = 10.2 Hz,
2 H), 5.72 (m, 1 H), 5.94 (br. s, 1 H), 6.90 (m, 1 H, ArH), 7.26–
7.54 (m, 13 H, ArH) ppm. 13C NMR (75.5 MHz, CDCl3): δ = 39.5
(d, JC,P = 4.1 Hz), 54.4, 62.9 (d, JC,P = 12.1 Hz), 117.8, 128.2 (d,
JC,P = 9.0 Hz), 129.5, 129.6, 130.1 (d, JC,P = 6.3 Hz), 130.3, 132.9
(m), 133.6, 133.8 (d, JC,P = 18.5 Hz), 136.1 (d, JC,P = 34.2 Hz),
140.1 (d, JC,P = 14.2 Hz), 222.4 (d, JC,P = 10.6 Hz), 228.4 (d, JC,P
= 1.7 Hz), 229.8 (d, JC,P = 16.4 Hz), 282.0 (d, JC,P = 12.8 Hz) ppm.
9c: 80 mg (41%), yellow solid. 1H NMR (300 MHz, CDCl3): δ =
0.84 (d, J = 7.1 Hz, 3 H), 2.00 (s, 3 H), 3.32 (q, J = 7.1 Hz, 1 H),
3.59 (s, 3 H), 3.83 (qd, J1 = 6.0, J2 = 1.7 Hz, 2 H), 6.86 (m, 1 H,
ArH), 7.10–7.37 (m, 13 H, ArH) ppm. 13C NMR (75.5 MHz,
CDCl3): δ = 14.4, 35.5, 51.0, 58.2 (d, JC,P = 18.2 Hz), 59.7, 127.2,
128.1, 128.3 (d, JC,P = 10.3 Hz), 128.3 (d, JC,P = 6.9 Hz), 128.6,
IR (KBr): ν = 1996, 1894, 1875, 1846 cm–1. C H CrNO P
˜
28 24
4
129.3 (d, JC,P = 5.7 Hz), 133.4 (d, JC,P = 19.3 Hz), 133.7 (d, JC,P
22.3 Hz), 134.4, 136.7 (d, JC,P = 15.5 Hz), 137.8 (d, JC,P = 10.3 Hz),
137.9 (d, JC,P = 10.0 Hz), 144.2 (d, JC,P = 23.6 Hz), 173.9 ppm. 31
=
(521.46): calcd. C 64.49, H 4.64, N 2.69; found C 64.61, H 4.73.
P
General Procedure for the Photochemical Reactions of Complexes 7:
Photochemical reactions were conducted by using a 450 W me-
dium-pressure Hg lamp, through a Pyrex filter in dry degassed
CH3CN or CH3CN/thf containing CH3OH in a sealed Pyrex tube
filled with argon. In a typical experiment, after irradiation for 30 h,
the solution was filtered through a short pad of Celite, the solvent
was removed under reduced pressure, and the crude product was
submitted to flash chromatography to give pure complexes 8 and
9.
NMR (121, MHz, CDCl ): δ = –12.6 ppm. IR (CCl ): ν = 1942,
˜
3
4
1736, 1433 cm–1. C24H26NO2P (391.44): calcd. C 73.64, H 6.69, N
3.58; found C 73.89, H 6.48.
1
9d: 140 mg (60%), pale yellow oil. H NMR (300 MHz, CDCl3): δ
= 1.14 (d, J = 7.1 Hz, 3 H), 3.38 (q, J = 7.1 Hz, 1 H), 3.56 (s, 3
H), 3.58 (q, J = 14.1 Hz, 2 H), 3.87 (qd, J1 = 17.8, J2 = 3.3 Hz, 2
H), 6.73 (m, 1 H, ArH), 7.04–7.29 (m, 17 H, ArH), 7.68 (m, 1 H,
ArH) ppm. 13C NMR (75.5 MHz, CDCl3): δ = 14.3, 51.2, 52.1 (d,
JC,P = 24.6 Hz), 54.4, 56.5, 126.8 (d, JC,P = 9.2 Hz), 128.1, 128.4,
128.5 (d, JC,P = 3.5 Hz), 128.6 (d, JC,P = 6.6 Hz), 128.7, 128.8 (d,
1
8a: 199 mg (98%), yellow solid. H NMR (300 MHz, CDCl3): δ =
2.03 (m, 1 H), 2.22 (m, 1 H), 2.58 (d, J = 4.1 Hz, 3 H), 2.69 (m, 1
H), 2.69 (m, 1 H), 2.96 (m, 1 H), 7.31–7.60 (m, 10 H, ArH) ppm.
13C NMR (75.5 MHz, CDCl3): δ = 28.1 (d, JC,P = 16.1 Hz), 46.1,
JC,P = 11.5 Hz), 133.1, 133.7 (d, JC,P = 3.5 Hz), 134.0 (d, JC,P
=
3.2 Hz), 136.0 (d, JC,P = 14.5 Hz), 136.5 (d, JC,P = 10.6 Hz), 136.8
(d, JC,P = 10.5 Hz), 139.7, 144.0 (d, JC,P = 22.2 Hz), 174.2 ppm.
54.0 (d, JC,P = 11.8 Hz), 128.7 (d, JC,P = 9.3 Hz), 128.8 (d, JC,P
=
31P NMR (121, MHz, CDCl ): δ = –14.8 ppm. IR (CCl ): ν = 1942,
9.1 Hz), 129.9 (d, JC,P = 1.6 Hz), 130.0 (d, JC,P = 1.7 Hz), 131.1 (d,
JC,P = 11.7 Hz), 131.6 (d, JC,P = 11.8 Hz), 135.7 (d, JC,P = 17.6 Hz),
136.2 (d, JC,P = 14.6 Hz), 218.1 (d, JC,P = 13.7 Hz), 218.4 (d, JC,P
= 14.1 Hz), 226.6 (d, JC,P = 2.6 Hz), 228.0 (d, JC,P = 12.8 Hz) ppm.
˜
3
4
1736, 1433 cm–1. C30H30NO2P (467.54): calcd. C 77.07, H 6.47, N
3.00; found C 76.89, H 6.66.
1
9e: 115 mg (55%), pale yellow oil. H NMR (300 MHz, CDCl3): δ
IR (CCl ): ν = 2012, 1954, 1888 cm–1. C H CrNO P (407.32):
˜
4
19 18
4
= 1.02 (d, J = 7.1 Hz, 3 H), 3.03 (qd, J1 = 13.6, J2 = 5.6 Hz, 2 H),
3.38 (q, J = 7.1 Hz, 1 H), 3.54 (s, 3 H), 3.88 (qd, J1 = 15.9, J2 =
2.2 Hz, 2 H), 4.93 (d, J = 10.1 Hz, 1 H), 5.02 (dd, J1 = 17.2, J2 =
1.5 Hz, 1 H), 5.60 (m, 1 H), 6.78 (m, 1 H, ArH), 7.06–7.27 (m, 12
H, ArH), 7.55 (m, 1 H, ArH) ppm. 13C NMR (75.5 MHz, CDCl3):
δ = 14.5, 51.1, 52.8 (d, JC,P = 22.4 Hz), 53.4, 57.0, 116.8, 126.9,
128.3, 128.4, 128.5 (d, JC,P = 3.2 Hz), 128.9 (d, JC,P = 5.4 Hz),
133.6, 133.7 (d, JC,P = 10.9 Hz), 133.9 (d, JC,P = 9.8 Hz), 136.0 (d,
calcd. C 56.03, H 4.45, N 3.44; found C 56.28, H 4.59.
8c: 82 mg (35%), yellow solid. 1H NMR (300 MHz, CDCl3): δ =
2.41 (d, J = 6.3 Hz, 3 H), 2.98 (br. s, 1 H), 3.43 (m, 2 H), 6.74 (m,
1 H, ArH), 7.28–7.42 (m, 13 H, ArH) ppm. 13C NMR (75.5 MHz,
CDCl3): δ = 46.1, 60.7 (d, JC,P = 12.7 Hz), 128.5 (d, JC,P = 1.8 Hz),
128.7 (d, JC,P = 2.0 Hz), 129.9, 130.2 (d, JC,P = 13.6 Hz), 131.1 (d,
JC,P = 22.6 Hz), 131.5, 132.0 (d, JC,P = 7.4 Hz), 132.9 (d, JC,P
=
11.5 Hz), 134.1 (d, JC,P = 34.8 Hz), 138.7 (d, JC,P = 17.1 Hz), 218.2
(d, JC,P = 13.1 Hz), 218.8 (d, JC,P = 13.1 Hz), 225.1, 226.2 (d, JC,P
JC,P = 14.7 Hz), 136.6, 136.9 (d, JC,P = 10.2 Hz), 137.3 (d, JC,P
=
10.9 Hz), 144.3 (d, JC,P = 22.8 Hz), 174.3 ppm. 31P NMR (121,
= 13.6 Hz) ppm. IR (KBr): ν = 3288, 2004, 1878, 1838 cm–1.
˜
MHz, CDCl3): δ = –13.4 ppm. IR (CCl ): ν = 1942, 1736,
˜
4
1433 cm–1. C26H28NO2P (417.48): calcd. C 78.40, H 6.76, N 3.36;
C24H20CrNO4P (469.39): calcd. C 61.41, H 4.29, N 2.98; found C
61.25, H 4.46.
found C 78.55, H 6.58, N 3.55.
1
8d: 87 mg (32%), yellow solid. H NMR (300 MHz, CDCl3): δ =
Supporting Information (see footnote on the first page of this arti-
cle): Cartesian coordinates (in Å), and total energies (in a.u., non-
corrected zero-point vibrational energies included) of all the sta-
tionary points discussed in the text.
3.22–3.45 (m, 4 H), 4.08 (d, J = 13.2 Hz, 1 H), 6.79–6.88 (m, 2 H),
7.11–7.45 (m, 17 H, ArH) ppm. 13C NMR (75.5 MHz, CDCl3): δ
= 54.4 (d, JC,P = 12.0 Hz), 60.8, 128.2, 128.4, 128.5 (d, JC,P
=
5.3 Hz), 128.6 (d, JC,P = 5.5 Hz), 129.1, 129.9 (d, JC,P = 2.7 Hz),
129.9, 130.1, 130.1 (d, JC,P = 5.5 Hz), 131.6, 131.7, 131.9, 132.2 (d,
JC,P = 7.5 Hz), 132.5, 132.8 (d, JC,P = 11.5 Hz), 133.2, 133.4 (d,
JC,P = 36.8 Hz), 133.7 (d, JC,P = 11.6 Hz), 134.2 (d, JC,P = 34.9 Hz),
136.7, 138.5 (d, JC,P = 16.5 Hz), 218.2 (d, JC,P = 12.9 Hz), 219.0
(d, JC,P = 12.9 Hz), 224.8, 226.4 (d, JC,P = 13.4 Hz) ppm. IR (KBr):
Acknowledgments
Support for this work by the Ministerio de Ciencia y Tecnología
(Spain) and the Euskal Herriko Unibertsitatea (9/UPV 00040.215-
13548/2001) (to F. P. C) under the grants CTQ2004-06250-C02-01/
BQU, CTQ2007-67730-C02-01/BQU, CAM-UCM-910762 (to
M. A. S), and CTQ2004-0681/BQU (to F. P. C) are gratefully ac-
ν = 3288, 2004, 1878, 1838 cm–1. C H CrNO P (545.48): calcd.
˜
30 24
4
C 66.06, H 4.43, N 2.57; found C 65.87, H 4.66.
9b: 180 mg (quantitative yield), pale yellow oil. 1H NMR
(300 MHz, CDCl3): δ = 1.17 (d, J = 7.0 Hz, 3 H), 1.17 (m, 2 H),
2460
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2454–2462

Noncarbonylative Photochemistry of Group 6 Fischer Carbene Complexes
[12]
M. A. Sierra, I. Fernández, M. J. Mancheño, M. Gómez-Gal-
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Complexes 8 lack the methyl group and the carbene carbon of
the starting complex 7 and present a set of four signals as dou-
blets (as a result of P–C coupling) corresponding to the
[Cr(CO)4] fragment in their 13C NMR spectra in the range δ =
218.1–228.0 ppm, while the N-Me moiety appears at δ =
46.1 ppm for compounds 8a,c and δ = 60.8 ppm for the N-
CH2Ph in compound 8d.
Compounds 9 also lack the pentacarbonylchromium(0) moiety
and show the typical amino ester structure derived from a pho-
tocarbonylation process. The 13C NMR spectrum of these
compounds show a signal in the range δ = 173.6–174.30 ppm
attributable to the ester carbonyl group. 31P NMR spec-
troscopy ascertained the oxidation of the phosphorus in the
latter compounds. While ester 9b presents a 31P signal at δ =
34.1 ppm, compounds 9c–e present a 31P signal in the range
–12.6 to 14.8 ppm (referenced to internal H3PO4). These data
indicate that oxidation of the phosphane moiety occurs in com-
pound 9b. Additional ESI-MS confirms the presence of the
phosphane oxide moiety in compound 9b (m/z = 359).
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knowledged. The SGI/IZO-SGIker UPV/EHU (supported by the
National Program for the Promotion of Human Resources within
the National Plan of Scientific Research, Development and Innova-
tion-Fondo Social Europeo, MCyT and Basque Government) is
gratefully acknowledged for the generous allocation of computa-
[13]
tional resources. I. F. was a MCyT-FPU fellow.
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2462
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Eur. J. Inorg. Chem. 2008, 2454–2462