782
Reactions of Transition Metal-Nitrogen CJ Bonds. 3. Early Transition Metal N ,N - Dimethylcarbamates. Preparation, Properties, and Carbon Dioxide Exchange Reactions Malcolm H. Chisholm* and Michael W. Extine Contribution from the Department of Chemistry, Princeton University, Princeton, New Jersey 08540. Received May 28, 1976
Abstract: Early transition metal dimethylamides M(NMe2),, (where M = Ti, Zr, V and n = 4; M = Nb, Ta and n = 5) and Wl(NMe2)6 react with carbon dioxide to give N,N-dimethylcarbamato compounds ML, and w2L6, where L = 02CNMe2, respectively. W(NMe2)6 reacts with C 0 2 to give only W(NMe2),L3 even in the presence of excess C02. These compounds have been characterized by a number of physicochemical techniques. On the basis of vibrational studies of isotopically labeled compounds we predict that ML4 compounds (M = Ti, Zr) and w2L6 contain only bidentate carbamate ligands. Single-crystal structural determinations show (i) that W(NMe2)3L3 contains afuc-WN303 octahedral moiety with very short W-N bond distances, 1.922 ( 7 ) A, and (ii) that NbL5 contains eight-coordinated niobium. The NbO8 polyhedron (Nb02(O-O),) does not conform to any idealized MX8 geometry. Variable temperature N M R studies reveal interesting dynamical solution behavior for MLs ( M = Nb, Ta) and w2L6. All the carbamato compounds are labile towards CO2 exchange reactions of the type ML* C 0 2 + ML I3CO2, where L* = 02l3CNMe2 and L = 02I2CNMe2. In addition the group 4A metals (Ti, Zr) show M0212CNR2 2 MO2l2CNMe2 + MO2I3CNR2 and ML M’L’ F! ML’ exchange reactions of the type MOz’’CNMe2 M’L, where L = 02CNMe2 and L’ = 02CN(CD3)2.
+
+
+
+
+
The facile formation and rupture of M-H, M-C, C-H, and C-C bonds within the coordination sphere of transition metals form the basis for hydrocarbon catalysis. In principle a n extension to include M-N and M - 0 bonds seems possible and is certainly desirable. With this in mind we set out to study the basic reactions of transition metal complexes containing M-N and M-0 c bonds. Early transition metal dialkylamides, M ( N R z ) ~provide , a suitable starting point for studying the reactions of covalent metal-nitrogen c-bonds. These compounds have already been shown to be reactive toward substitution and insertion reactions.’ A number of considerations led us to examine the reactivity of these compounds towards carbon dioxide: (1) There is currently a general interest in carbon dioxide fixation and the reactions of carbon dioxide with transition metal complexes.s (2) The reaction between carbon dioxide and M ( N R z ) ~ compounds has received little previous attention, although insertion has been established for Ti(NMe2)43band Cr(N-iPr?) 3 . h (3) The coordination properties of M(02CNR2), compounds would be of considerable interest, especially in comparison with the related dithiocarbamates, M(S2CNR2),1. (4)The physical properties of carbon dioxide and its commercial availability in varying isotopically labeled forms make it ideally suited for detailed mechanistic and spectroscopic studies. I n this paper we describe the preparation and properties of M(O?CNMe>),?(where M = Ti, Zr, V and n = 4;Nb, Ta and n = 5 ; W , n = 3 ) and W(NMe2)3(02CNMe2)3. We also note the remarkable lability of these compounds towards COz exchange reactions. I n the following paper we describe studies aimed at delineating the mechanism of COz insertion into the M-N bond and of the carbon dioxide exchange reactions involving these carbamato complexes. Preliminary results, reporting the crystal structures of W(NMe2)3(02CNMe2)3’ and Nb(02CNMe2)5’ and a discussion of the carbon dioxide exchange reaction8 have been published.
Results and Discussion Synthesis. Solutions of early transition metal N,N-dimethylamides M(NMe2), (where M = Ti,3bZr, V and n = 4; Nb, Ta and n = 5) and W2(NMe2)6, react rapidly and Journal of the American Chemical Society
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99:3
quantitatively (via ‘ H NMR) with carbon dioxide (Inequiv) to yield the corresponding dimethylcarbamates M(02CNMe2), as shown in eq 1. M(NMe2),
+ nCO2
-
M(OZCNMe2),
(1)
Although we concentrated on characterizing dimethylcarbamates, other dialkylamides M(NRR’), (R, R’ = Me, Et) of Ti(IV), Zr(IV), and W(II1) were found to react with C02 to yield M(02CNRR’),. W(NMe2)6, however, reacts differently, as is shown in eq 2 . W(NMe2)6 + 3 c o 2
-
W(NMe2)3(02CNMe2)3
(2)
W(NMe2)3(02CNMe2)3 is the only product observed, even in the presence of excess C 0 2 (ca. 10 atm at 25 “C). Most of these reactions were initially carried out on a small scale in NMR tubes and monitored via ‘H N M R spectroscopy. The quantitative nature of eq 1 and 2 was confirmed. In some cases transient species were observed, probably M(NMe2),-,(02CNMe2),. We obtained a qualitative measure of the speed of eq 1 and 2. Equation 1 is complete within ca. 5 min at 25 “C for M(NMe2)4 (M = Ti, Zr, V) and M(NMe2)5 (M = Nb, Ta). W2(NMe2)6 and W(NMe2)6 react somewhat more slowly (complete within ca. 10-20 min). Reactions of M(NMez), with CO2 were usually carried out in toluene or benzene using standard vacuum line techniques to add the C02. M(02CNMe2)4 (M = Ti, Zr, V) precipitated from the benzene or toluene solution. W2(02CNMe2)6, W(NMe2)3(02CNMe2)3,and M(02CNMez)s (M = Nb,Ta) were crystallized by concentrating the solution and adding hexane. Characterization. Dimethylcarbamato compounds are airand moisture-sensitive crystalline solids. ML4, where L = 02CNMe2 and M = Ti, Zr, or V, are stable indefinitely at 25 “ C in the absence of water and oxygen. NbL5, TaL5, and W(NMe2)3L3 decompose slowly over a period of weeks at 25 O C but are stable indefinitely when stored, sealed, in vacuo at -20 OC, in the dark. Satisfactory elemental analyses (Table I) were obtained for all compounds except W(NMe2)3L3 and w2L6. However, all other experimental data, including single-crystal x-ray studies indicate that W(NMe2)3L3 and w2L6 are authentic compounds.
/ February 2, 1977
783 of Some Early Transition Metal Dimethylcarbamato Complexes
Table 1. Compound
Color
Carbon Found (calcd)
Hydrogen Found (calcd)
Nitrogen Found (calcd)
Ti(02CN Me2)4 Zr(OrCNMez)4 V(OzCNMed4 N b(O2CNMez)s Ta(02CN Me2)5 W(N Me2)3(02CNMe2)3 W2(02CNMe2)6d
Pale yellow White Green Pale yellow White Orange Orange
35.86 (36.01) 32.78 (32.49) 35.53 (35.74) 33.49 (33.78) 28.75 (28.99) 33.70 (31.04) 36.29 (35.57)e
6.03 (6.04) 5.58 (5.45) 6.08 (6.00) 5.50 (5.67) 4.80 (4.87) 6.82 (6.25) 4.79 (4.85)'
13.78 ( 1 4.00) 12.54 (12.63) 13.67 (13.89) 12.79 (13.13) 10.99 (1 1.27) 14.14 (14.48) 8.65 (7.78)'
Mol wt Found (calcd) 443 f 20 (443) 534 f 15 (533) 858 f 80 (896)
a Elemental analyses performed by Alfred Bernhardt Mikro. Lab., Elbach, W. Germany. Molecular weights determined cryoscopically in benzene. Tungsten, found (calcd) = 30.63 (31.68). Tungsten, found (calcd) = 34.83 (34.04), see e below. e Calcd for Wz(02CNMe2)h(toluene)z. [
Table 11. Solubilities and ' H N M R Data for Some Metal N,N-Dialkylcarbamates
IH NMR data
Solubility Compound
Hexane
C6H6
CHzCl2
Solvent
TiL4'
I
ss
S
Ti(OzCNEt&
I
S
s
ZrL4'
I
ss
S
NbL5'
I
S
S
TaL5'
i
S
S
W(NMQ)~L~)'
s
S
S
W2L6'
I
S
ss
CHzClz C6H6 CHzClz C6H6 CHzCl2 C6H6 CH2Cl2 C6H6 ChDsCD3 CHzClz C6H6 ChDsCD3 C6H6 C6DsCD3 C6H6 CHzClz CHzCIz
Temp ("C)
6 (PPm)"
45 45 45 45 45 d 45 45 45 -80' 45 45 -80' 45 -30 45 45 -70
2.85 2.33 3.27 (9. 2), 1.04 (t, 3) 3.12 (q, 2). 0.88 (t, 3) 2.87 2.39 2.83 2.55 2.50, 2.32 (2:3) 2.85 2.55 2.53, 2.30 (2:3) 4.75, 2.82 ( 1 : l ) f 4.57, 2.78, 2.74 (2:l:l)g 2.58 3.15, 2.90 (1:2) 3.16, 3.03, 2.83, 2.63 (2:2:1:1)
3
J
~
3
(~ H- z~) ~
3.2
3.2 3.2 3.2 3.0 3.0
Spectrum unchanged a Shifts in ppm downfield from hexamethyldisiloxane at 60 MHz. I/ 3J13C.H for (02I3CNR2). L = 02CNMe2. at ca. -120 "C in 1:l CHzClz-Freon 1 1 . T , = -65 OC.f6(NMez) 4.75, 6(L) 2.82. g d(NMe2) 4.57, d(L) 2.74, 2.78, T , = - 1 "C.
' H NMR data and solubility data for ML, and W(NMe2)3L3 are given in Table 11. Molecular weight determinations indicate that ZrL4 and NbL5 are monomeric and w2L6 is dinuclear in benzene (Table I). In general these compounds did not sublime, or yield molecular ions, ML,+, in the mass spectrometer. However, W(NMe2)3L3 and W2(02CNMeEt)6 yielded parent ions corresponding to loss of NMe2 and NMeEt, respectively. For W(NMe2)3L3 this was apparent from a comparison of the mass spectra of W(NMe2)3(I802l2CNMe2)3 and W(NMe2)3('h0212CNMe2)3. In general, the carbamato compounds M(02CNR2), are considerably. less soluble than the parent dialkylamides M ( N R z ) ~but more soluble than the corresponding dithiocarbamate compounds M(S2CNR2),. Infrared Spectra. Carbamato compounds show strong absorptions in the 1560-1685-~m-~region assignable to the O2CN moiety (Table 111). The effect of isotopic substitution (I802CN and O2I3CN) on the positions of these absorptions has been studied (see Table 111). A comparison of A ' with the known coordination of the ligands in W(NMe2)3L3' and NbLsI leads us to the conclusion that a small AI8O (
