216
J. C. JEFFERY
et al.
species display characteristic resonances for the ligated carbon atoms of thep-tolylmethylidyne groups at 6 279.6 (2) and 282.3 ppm (3). The signal for the C=W nucleus in the 'C-{ ‘H} NMR spectrum of 1 is seen at 6 284.8 ppm.4” The coordination environment of the tungsten atom in complexes 2 and 3 is essentially octahedral with the hydrotris(pyrazol-1-yl)borate group occupying three facial sites. Consequently one of the ligated nitrogen atoms is transoid to the CC6H4 Me-4 moiety, while the other two nitrogen atoms are transoid to CO groups. This arrangement generates a plane of symmetry through the tungsten atom, the alkylidyne-carbon and the pyrazol ring, which is transoid to the latter. The two CO groups and the remaining two pyrazol rings are, respectively, symmetrically related to each other on either side of the symmetry plane. Hence, the number of ‘H and ‘3C-(‘H} resonances seen in the spectra is reduced from the number which would be anticipated if all three C3N2 rings and the two CO ligands were non-equivalent. Consequently, it was possible to make the peak assignments listed in Table 2 on the basis of the observed relative intensities of the various signals. The two compounds [W(&R)(CO),{HB (PZ)~}] (4, R = C6H3Mez-2,6; 5, R = C6H4 OMe-2) were prepared by treating the species w(=CR)Br(CO),] in THF with K[HB(pz),]. Complexes 4 and 5 are closely related to the previously reported compound 1,4aand some of the spectroscopic properties are very similar. Thus all three compounds display two CO stretching bands in their IR spectra as well as a diagnostic resonance for the carbon nuclei of the ligated alkylidyne groups in their ‘3C-{‘H} NMR spectra. For 1 the signal occurs at 284.8 ppm, as mentioned above, while for 4 and 5 the peaks are at 6 288.6 and 280.5 ppm, respectively (Table 2). However, the ‘H and 13C( ‘H} NMR data for 14aindicate that all three pyrazol rings are equivalent, as would be expected if the HB(~z)~ group rotated on the NMR time scale. In contrast, the NMR data for 4 and 5 reveal a pattern of peaks corresponding to two equivalent and one non-equivalent pyrazol rings. This implies that the HB(~z)~ ligand does not undergo dynamic behaviour, presumably because site exchange of the ligated nitrogen atoms is prevented by the presence of the ortho substituents on the phenyl rings of the alkylidyne groups which interact with the sterically demanding HB(pz) 3 moiety (see below). We have previously shown that protonation of the compound [W(=CC,H4Me-4)(CO),(@sH,)] with HBF, - Et20 affords the ditungsten salt [W,
(~-H)(~-C2(CsH4Me-4)2)(C0)4(r-C~H~)~l[BF41.’ Moreover, there is evidence that the latter is formed
in CD,Cl,.
6
“Chemical shifts (6) in ppm, coupling constants in Hz, measurements at room temperature b ‘H decoupled, chemical shifts to high frequency of SiMe, (0.0 ppm).
239.4 (C=W), 217.3 (CO), 161.2 [C’(C,H,)], 148.2 (1 C, C3bor5b), 143.8 (2 C, C3aor5a), 139.9 (1 C, C3bor5b), 138.7 (2 C, C3aor5a), 134.3 [C’, C,H,], 133.6, 130.5, 125.8, 111.2 (C,H,), 109.6 (1 C, cb), 109.1 (2 C, C4a), 64.6 (OMe)
3.53 (s, 3 H, OMe), 6.57 (s, 3 H, H4as”d4b), 7.42-7.60 (m, 6 H, CsH4 and H3aor5a), 7.99 (s, 1 H, H3bor5b), 8.08 (s, 2 H, H3aor5a), 8.15 (s, 1 H, H3bor5b), 12.69 [s, 1 H, C(H)C,H,OMe-2, J(WH) 271
5
145.6 (2 C, C3aorSs), 144.1
280.5 (C=W), 225.1 (CO), 160.8 [C’(C,H4)], 145.6 (2 C, C3aar5a), 144.3 (1 C, C3bor5b), 136.1 (1 C, C3bor5b), 135.9 (2 C, C3aor5a), 139.4, 131.6, 129.8, 120.3, 111.2 (C,H,), 106.2 (3 C, C4as”d4b), 55.9 (OMe)
226.8 (CO), 147.7 [C’(C,H,)],
3.90 (s, 3 H, OMe), 6.22 (s, 1 H, H4b), 6.27 (s, 2 H, H”), 6.826.89 (m, 2 H, C,H3, 7.25-7.39 (m, 2 H, CsH4), 7.68-7.71 (m, 4 H, H3b, H5b and H3aor5a), 8.09 (s, 2 H, H3aor5a)
288.6 (C=W),
(1 C, CSborSb), 140.8 (1 C, C3bor5b), 136.0 (2 C, C3aar5a), 143.7, 128.0, 127.6 (C,H,), 106.2 (1 C, C4b), 106.1 (2 C, C^), 20.7 (Me,-2,6)
6.22 (s, 1 H, H4b), 6.27 (s, 2 H, H4”), 6.93 [(AB,),
134.2-126.4 (Ph3, CsH4), 107.5 (2 C, C”), 107.2 (1 C, C4b), 21.7 (Me-4)
Ph3), 7.80 [d, 1 H, H5b, J(H5bH4b) 21, 7.87 [d, 2 H, H5a, J(H5aH4a) 21
2.47 (s, 6 H, Me,-2,6),
282.3 (C=W), 221.5 (CO), 158.5 (1 C, C3b or CSb), 158.2 (2 C, CSaorSa), 146.2 [C’(C,H,)], 137.8 (1 C, C3borSb), 137.1 [3 C, C3aor5a and C4(C,H4)],
2.21 (s, 3 H, Me-4) 6.29 [d, 1 H, H4b, J(H4bHSb) 21, 6.32, 6.78 [(AB),, 4 H, CsH4, J(AB) 81, 6.39 [d, 2 H, Ha, J(H”H’“) 21, 6.9g7.50 (m br, 15 H,
C3aorSa), 148.1 [C’(C,H,)], 145.9 (1 C, C3borSb), 145.3 (2C, C3sorSa), 138.4, 129.4. 129.0 (C,H,), 106.9 (1 C, C4b), 106.6 (2 C, C”), 21.7 (Me-4), 16.5 (2 C, Me3aor5a), 15.3 (1 C, Me3borSb), 12.7 (3 C, Me3aorSa and Me3bor5b)
1681, 152.8 (1 C, C3barSb), 152.4 (2 C,
(s. 3 H, Me3bor5b), 2.49 (s, 6 H, Me3aor5a), 5.83 (s, 1 H, H4b). 5.93 (s. 2 H. H”), 7.11, 7.34 [(AB),, 4 H, C,H,, J(AB) 81
224.2 [CO, J(WC)
“Cb (6) 279.6 (C=W),
3a
2.29 (s, 3 H, Me-4), 2.35 (s, 3 H, Me3bor5b), 2.38 (s, 6 H, Me3aor5a), 2.44
‘H (6)
6:
....7WNc *
N 3a
2 H, CeH3, J(AB) 71, 7.09 [(ABJ, 1 H, ChH3, J(AB) 71, 7.70 (s, lH, H3borSb), 7.71 (s, 1 H, H3bor5b), 7.73 (s, 2 H, H3aor5a), 8.03 (s, 2 H, H3aor5a)
4
3
2
Compound
&
t
Table 2. ‘H and 13C NMR data” for the complexes
-I
N
E
& 9 8 8 3 z
F Y t: ya w P
E E’ 2
Y z
218
J. C. JEFFERY
I_
I
_I
C~H40Me-i? (6)
via the intermediacy of a transient mononuclear alkylidene tungsten species [W{=C(H)C,H,Me4}(CO),(r@,H,)][BF,], in which the tungsten atom with 16 electrons in its valence shell is electronically unsaturated, and therefore reacts further.‘,’ It was, therefore, of interest to protonate the species l-5 which are isolobal with [W(=CC6H,Me-4) (CO)&-C,H,)]. In practice, only complex 5 on treatment with HBF4* Et20 yielded a stable product, which was identified as the alkylidene tungsten salt m{=C(H)C6H40Me-2)(C0)2oB(pz)3)] [BF,] (6), data for which are given in Tables 1 and 2. Protonation of compound 1 or 2 with HBF4* Et*0 afforded salts similar to 6 based on IR and ‘H NMR measurements, but attempts to purify these products by crystallization from Et,0 or by column chromatography were unsuccessful, resulting in deprotonation and formation of the precursor 1 or 2. Although suitable crystals of complex 6 could not be grown for an X-ray diffraction study, the structure is well established by microanalysis and by the spectroscopic data. As expected, in the IR spectrum of 6 the two CO stretching bands at 2032 and 1954 cm-’ occur at appreciably higher frequency than the corresponding bands (1976 and 1886 cm-‘) in the spectrum of the neutral complex 5. In the ‘H NMR spectrum of 6 there is a diagnostic resonance for the proton attached to the alkylidene-carbon at 6 12.69 and this signal displays ‘*‘W satellite peaks [J(WH) 27 Hz]. In the “C-(‘H} NMR spectrum of the salt 6 the resonance for the ligated carbon of the alkylidene group C(H)&H,OMe-2 is seen at 6 239.4 ppm. These ‘H and “C-{ ‘H} NMR chemical shifts are comparable with those previously reported for the compound [W(=C(H)C,H,Me4}I(CO),(+Z,H,)] [‘H, 6 13.08 for C(H)C,H,Me4; “C-{ ‘H}, 6 267.4 ppm for C(H)&H,Me-41, the structure of which was established by X-ray diffraction.’ Our success in isolating a stable alkylidene tungsten complex by protonating compound 5 and our failure to obtain stable products by protonating the species l&4 is probably related to the nature of the substituent groups attached to the alkylidynecarbon atoms. In the alkylidene tungsten species produced by protonation, the metal atom has a 16
et al.
valence electron shell, a situation not conducive to stability. However, the presence of the OMe-2 group on the phenyl ring in 6, with a lone pair of electrons on the oxygen atom, may exert a stabilizing influence. All the products of protonating compounds l-5 would have six coordinate metal atoms. Nevertheless, tungsten can increase its coordination number beyond six. Hence, there is the possibility that the lone pair on the OMe-2 group in complex 6 interacts with the tungsten atom so as to give the latter a filled 18-electron valence shell. This could inhibit decomposition observed with the other species obtained by protonating l-4. It is interesting to contrast the protonation of compounds l-5 with HBF4 * Et,0 with the formation of the ditungsten salt [W,(p-H){p-C, (C6H4Me-4)&C0)4(~-C5H5)Z][BF4] on protonating ~(=CC6H4Me-4)(CO)2(n-CSH5)], mentioned above. Formation of the ditungsten species requires a combination of an initially formed alkylidene complex [W{=C(H)C,H4Me-4}(CO)2(r-C5HS)1+ with a second molecule of [W(=C&,H,Me4)(CO),(q-C,H,)], followed by C-C bond formation and migration of a hydrogen atom. 5 In the case of compounds l-5 the presence of the hydrotris(pyrazol-1-yl)borate ligands would inhibit further reaction of an initially formed alkylidene species because of their steric bulk. The cone angles for the groups HB(3,5-Me,(pz),}, HB(pz), and n-C,H, are 225, 180 and loo”, respectively.‘,’ EXPERIMENTAL
All experiments were carried out under nitrogen using Schlenk tube techniques. Solvents were rigorously dried before use. The salts K[HB(pz),], K[HB(3,5_Me,pz),] and K[HB(3-Phpz)3]7,8 and the compounds [W(=CR)Br(CO),] (R = C,H,Me-4, C,H,OMe-2 and C6H3Me2-2,6)9 were prepared by methods described previously. The reagent HBF,. Et,0 consisted of a 54% solution of HBF4 in Et,O. The NMR spectra were recorded with JEOL JNM GX270 and GX400 spectrometers, and the IR spectra were measured with a Perkin-Elmer FT1600 spectrophotometer. Analytical and physical data for the new compounds are given in Table 1. Preparation
qf the
alkylidyne
tungsten compounds
(i) The complex [W(=CCgH4Me-4)Br(CO),] was first prepared by treating a hexane (80 cm’) solution of ~{=C(OMe)C6H4Me-4}(C0)5] (3.03 g, 6.61 mmol) at -20°C with BBr, until an IR spectrum showed that none of the alkylidene tungsten reagent remained. The mixture was cooled to -30°C
219
Synthesis of alkylidyne tungsten complexes
solvent was removed with a syringe, and the residue was washed with cold (ca -78°C) hexane (3 x 50 cm3) to obtain [W(=CCbH4Me-4) Br(CO),] (3.16 g, 6.61 mmol). The latter was dissolved in THF (100 cm’) at - 20°C and treated with the salt K[HB(3,5-Me2p&] (2.34 g, 6.96 mmol). Following this procedure, the mixture was stirred at ca - 10°C for 24 h. Filtration through a Celite pad (ca 1 cm) at -20°C followed by removal of solvent in vacua gave brown microcrystals of [W(=CC6H4Me-4)(C0)2{HB(3,5-Me2pZ>3)] (2) (3.56 g) after crystallization from CH,C12-Et20 (1:4). (ii) The complex [W(=CC6H4Me-4)(C0)2{HB (3-Phpz),}] (3) was similarly obtained, by stirring a mixture of [W(rCC6H4Me-4)Br(CO)J (prepared in situ from [W{=C(OMe)C,H,Me-4}(CO),] (0.47 g, 1.02 mmol) and BBr,) and K[HB(3-Phpz),] (0.48 g, 1.OOmmol) in THF for 4 days at ca - 20°C. The mixture was filtered through a Celite pad (ca 1 cm) and solvent was removed in vacua. The residue was dissolved in CH,Cl,-hexane (10 cm3, 1 : 4) and chromatographed on silica gel (2 x 10 cm). Elution with the same solvent mixture, removal of solvent in vacua, and recrystallization of the residue from CHzC12-Et10 (1 : 5) gave brown microcrysta1.v of compound 3 (0.13 g). (iii) The alkylidene tungsten complex [w{=C (OMe)C6H3Me2-2,6}(CO)51 (3.10 g, 6.57 mmol) in hexane-CH,Cl, (100 cm3, 1: 2) was converted to [W(=CC6H3Me,-2,6)Br(CO),] using BBr,. After removal of solvent with a syringe, the residue was washed with cold (ca -5O’C) hexane (3 x 50 cm’) and dried in VUCUO. The lJV(=CC6H3Me2-2,6) Br(CO),] obtained in this manner was dissolved in THF (100 cm’) and treated with K[HB(pz),] (1.71 g, 6.79 mmol). After stirring the mixture for 24 h at room temperature, it was filtered through a Celite pad (ca 1 cm). Solvent was removed in vacua, and the residue was crystallized from CH,Cl,Et,O (1: 5) to afford brown microcrystals of [W(=CC,H, Me,-2,6)(Co),{HB(pz),)l(4)(2.60 8). (iv) The complex [W(=CCgH40Me-2)(C0)2 CfW~zhIl (5) was obtained by converting [W{=C(OMe)C,H,OMe-2}(CO),] (3.01 g, 6.35 mmol) to [W(=CCgH40Me-2)Br(C0)4] and treating the latter with K[HB(pz),] (1.71 g, 6.79 mmol) in THF (80 cm’). The crude product was dried in vacua, and then dissolved in CH2C12hexane (10 cm’, 1 : 2) and chromatographed at - 10°C on alumina (Brockman activity III, 2 x 10 cm column), eluting with CH,Cl,-hexane (1.2). After removal of solvent in vacua from a pink fraction, complex 5 was obtained as dark pink microcrystals (0.45 g) after crystallization from CH,Cl,-hexane (1: 4) at - 78°C.
Protonation
studies
A CH2C12 (20 cm3) solution of complex 5 (0.10 g, 0.17 mmol) was treated with drops of HBF4 * Et,0 solution (ca 0.1 cm3), until the IR spectrum of the mixture showed that all of the starting compound had been consumed. Solvent was removed in vacua, and the resulting residue was crystallized from CH,Cl,-Et,0 (1 : 3) to give deep red crystals of
[W{=C(H)C6H,0Me-2}(CO),(HB(pz)3}I[BF,I (6) (0.094 g). Acknowledgements-We
thank BP Chemicals, Grangemouth Division, for the award of a CASE SERC Research Studentship to G.K.W.
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