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Aspects of the organometallic nitrosyl chemistry of the group 6 elements.

In recent years, my research group and I have been investigating the characteristic chemistry of organometallic nitrosyl complexes containing the Group 6 elements with a view to developing these compounds as either specific reactants or selective catalysts for various organic or organometallic transformations of practical interest and significance. In particular, two classes of these complexes have kept recurring during our studies, namely those containing Cp M(NO) and Cp'M([NO).sub.2] groups [Cp' = Cp ([Eta.sup.5 - C.sub.5 H.sub.5] Cp'M(NO) and Cp'M(NO)2 [Cp' = Cp (Eta - [C.sub.5 H.sub.5]) or Cp* ([Eta.sup.5]-[C.sub.5 Me.sub.5);M = Cr, Mo, or W]. I intend to provide an overview of our work to date with these species, emphasizing particularly those aspects of our research which have attained the stated objectives.

Both classes of nitrosyl compounds originate with the corresponding hexacarbonyls as summarized in equations 1 to 4, 1,2 :
 M[(CO).sub.6] + NaCp' (in refluxing THF or [n-BU.sub.2 O])
 Na [CP'M[(CO).sub.3] + 3 CO.......... (1)
 Na [CP'M[(CO).sub.3] + Diazald (in THF)
 CP'M[(CO).sub.2](NO).............. (2)
 CP'M[(CO).sub.2](NO) + X2 (in [CH.sub.2 Cl.sub.2])
 Cp'M(NO)[X.sub.2] + 2 CO............ 3)
 CP'M[(CO).sub.2](NO) + CINO (in [CH.sub.2 Cl.sub.2])
 Cp'M[(NO).sub.2] C1 + 2 CO............ 4)

The halo nitrosyl products of reactions 3 and 4 being the synthetic precursors to the species of interest (vide infra). Both classes of compounds typically contain linear M-NO linkages in which the nitrosyl ligand engages in and 7r synergic bonding with the metal centre while functioning as a formal three-electron donor. The [Pi] backbonding component is dominant, however, and results in the nitrosyl ligand being an effective remover of electron density from the metal centre. Consequently, it is not unreasonable to expect that the presence of nitrosyl ligands should impart distinctive chemical characteristics to the complexes containing them. That this is indeed so for the Cp'M(NO)- and Cp'M[(NO).sub.2] -containing compounds can best be illustrated by considering representative examples of each class of complexes in turn.

Cp'M(NO)-Containing Complexes

All the Cp'Mo(NO)[X.sub.2] COMpounds in [CH.sub.2 Cl.sub.2] undergo reversible one-electron reductions, i.e.
 Cp'Mo(NO)[X.sub.2] [forms and is formed from] [Cp'Mo(NO)[X.sub.2]

a cyclic voltammogram of CpMo(NO)[I.sub.2] as a representative example being presented in Figure 1. A second irreversible reduction of this complex occurs at [E.sub.p,c] = - 1.60V vs SCE

bient-temperature cyclic voltammogram of 5 x [10.sup.4] M CpMo(NO)[I.sub.2] in [CH.sub.2 Cl.sub.2], containing 0.1 M [n-Bu.sub.4 N] [PF.sub.6] measured at a platinum-bead electrode at a scan rate of 0.24 V s . (Reproduced with permission from ref 4.) if the scan is extended to the solvent limit, and it is accompanied by the release of I into the solution. This property may be exploited synthetically as summarized in eq 6 where 2L = [2 PR.sub.3] [2 P(OR).sub.3] or an acyclic, con jugated diene,
 Cp'Mo(NO)[I.sub.2] + 2 Na/Hg + 2L (in THF)
 --> Cp'Mo(NO)[L.sub.2] + 2 Nal + Hg................(6)

The latter diene-containing complexes are particularly noteworthy since the Cp'Mo(NO) fragments prefer, for electronic reasons, to bind the dienes in a twisted, transoidal fashion. This feature is clearly evident in the solid-state molecular structure of CpMo(NO)([Eta.sup.4] trans-2, 5-dimethyl2,4-hexadiene), two views of which are presented in Figure 2. This unprecedented type of metal-diene bonding results in the bound dienes exhibiting unusual reactivity patterns, particularly towards nucleophiles such as ketones.

The one-electron reduction presented in eq 5 can be effected on a preparative scale by employing cobaltocene as the reducing agent, ie.

Cp'Mo(NO)[X.sub.2] + [CP.sub.2] Co (in THF)

--> CP.sub.2] Co] - [Cp' Mo(NO)[X.sub.2]....(7) the product salts precipitating as air- and moisture-sensitive green to brown solids. Not surprisingly, there is increased backbonding to the nitrosyl ligands in the products as evidenced by the decrease in [[Nu.sub.NO]of approximately [115 cm.sup.-1] accompanying reactions. However, the small isotropic coupling evident in the X-band ESR spectra of the product salts [eg. Figure 3(a) with its absence of [sup.14.N] hyperfine coupling] clearly indicates that the extra electron density extant on the radical anions is not primarily localized in their Mo-NO linkages but is probably in a 7-type orbital instead. Such [Cp'MO(NO)[X.sub.2] anions also result when the neutral precursors are treated with an equimolar amount of a Grignard reagent [Figure 3(b)], thereby indicating the occurrence of at least some single-electron transfer during these latter reactions. The relative stability of the [Cp'Mo(NO)[X.sub.2] anions both in solutions and in the solid state is X = Cl > Br > > I, 4 and we have found that the neutral dichloro precursors are indeed the reagents of choice for effecting most cleanly the halide-for-organic group methatheses summarized in the sequential equations 8 and 9 where M = Mo or W, and R = [CH.sub.2 SiMe.sub.3], [CH.sub.2] CMe.sub.3], [CH.sub.2 CMe.sub.2 Ph, [CH.sub.2 Ph], or [C.sub.6 H.sub.5];

2 Cp'M(NO)[X.sub.2] + 4 RMgX (Et.sub.2 O, 0 degrees C)

[Cp'M(NO)[R.sub.2][sub.2 MgX.sub.2] [Et.sub.2 O] + [3 MgX.sub.2] [(Et.sub.2 O).sub.n])... (8)

Cp'M(NO)[R.sub.2]2 [MgX.sub.2] * [Et.sub.2 O] + 6 [H.sub.2 O] [(Et.sub.2 O, 20 degrees C)

2 Cp'M(NO)[R.sub.2] + [Mg(H.sub.2 O).sup .2+] + 2X + Et.sub.2 O] (9)

The organometallic nitrosyl products of both conversions 8 and 9 are isolable as analytically pure crystals from the final reaction mixtures by fractional crystallization. The unique members of the series of Cp'M(NO)[R.sub.2] complexes resulting from reactions 9 are the bis(benzyl) derivatives of both molybdenum and tungsten which are stereochemically nonrigid molecules whose limiting structures involve an [Eta.sup.1] and an [Eta.sup.2]-benzyl ligand which function as formal one- and three-electron donors, respectively, towards the metal centres." The other Cp'M(NO)[R.sub.2] compounds are remarkable 16-electron, thermally stable species, the solid-state molecular structure of one of these, namely CpW(NO)(CH.sub.2 SiMe.sub.3])2, being shown in Figure 4. The most chemically interesting feature of this structure involves the essentially linear (169.5 degrees) WNO group in which the short W-N (1.757 A) and long N-O (1.226 k) bond lengths indicate considerable W to NO backbonding. Consistently, the very low [Nu. No's] (1590-1625 cm.sup.1]) evident in the IR spectra of all the Cp'M(NO)[R.sub.2] compounds reflect the diminished N-O bond order extant in these species. The thermal stability of these electron-deficient complexes may be rationalized by the fact that their LUMOs are metallocalized, nonbonding molecular orbitals which thus do not result in a loss of metal-ligand binding. 12 Furthermore, the existence of these LUMOs confers Lewis acid properties on the Cp'M(NO)[R.sub.2] complexes, a feature clearly evident in the characteristic chemistry of CpW(NOX)([CH2SiMe.sub.3)2] SUMmarized in Figure 5.9 Hence, in addition to forming isolable 1:1 adducts with small Lewis bases (eg. [PMe.sub.3]), the tungsten dialkyl reacts with other small molecules (eg. CNR', NO, [O.sub.2] etc.) in a manner which reflects initial adduct formation followed by subsequent intramolecular transformations involving the alkyl ligands. One interesting aspect of the chemistry of the Cp'M(NO)[R.sub.2] species not shown in Figure 5 is that some of them [not [CpW(NO)(XCH.sub.2 SiMe.sub.3)2] undergo double insertions of CO into their M-C [sigma]-bonds to produce isolable 18-electron diacyl complexes, a type of transformation that is still relatively rare in transition-metal organometallic chemistry.

The dioxo alkyl products of the type shown at the top of Figure 5 are also 16-electron 3species and are fascinating entities in their own right.' The remarkable chemical property of the various [CP'M(O).sub.2]R (M = Mo or W) complexes that we have investigated to date is that it is their M = O linkages that are the preferred sites of reactivity, their M-R bonds remaining intact in the presence of reagents (eg. protonic acids) that would normally be expected to cleave them. Consider, for instance, the conversions presented in Figure 6 for which the solid-state molecular structures of representative product complexes have been established by single-crystal X-ray crystallographic analyses. Of the various products shown in this Figure, particularly interesting are to oxo peroxo species indicated on the right-hand side. These [CP'M(O)(Eta.sup.2] - O.sub.2)R complexes are air-stable crystalline solids which effect oxygen-atom transfer to Lewis bases such as phosphines and form 2:1 charge-transfer adducts with TCNE, eg. 16
 2 Cp*W(O)(Eta.sup.2 - [O.sub.2])(CH.sub.2] SiMe.sub.3]) +
[(NC).sub.2] C = [C(CN).sub.2]
 (in Et.sub.2] O at 20 degrees C) --> [CP*W(O)([Eta.sup.2
-O.sub.2]) [(CH.sub.2 SiMe.sub.3)]2]
 [Mu.] [(NC).sub.2] C=C(CN).sub.2] ............(10)

The product complex of reaction 10 is isolable in 80% yield as an orange-brown crystalline solid whose solid-state molecular structure is shown in Figure 7. The TCNE is disordered in a simple manner about the crystallographic centre of symmetry, and the peroxo ligands of the two organometallic complexes are oriented in a skew fashion across the middle of its C =C linkage. Clearly there is sufficient electron density present on the [Eta.sup.2] - [O.sub.2] ligands to permit them to function as donors towards the electrophilic olefin. We are currently endeavouring to ascertain whether this charge-transfer complex can effect intramolecular oxygen-atom transfer to the olefin directly or whether it must first rearrange to a peroxometallacycle of the type conventionally invoked for these processes. One additional aspect of the characteristic chemistry of the 16-electron [Cp'M(NO).sub.2]R complexes not presented in Figure 6 merits mention. That aspect is that treatment of the tungsten dialkyls with dihydrogen under different experimental conditions produces new types of remarkably stable alkyl hydride complexes. Thus, hydrogenation at 80 psig of Cp*W(NO)[(CH.sub.2 SiMe.sub.3)2] in the presence of a Lewis base, L, affords trans-Cp*W(NO)(H)[(CH.sub.2 SiMe.sub.3)] (L) complexes, i.e. eq 11 where L = a phosphine or a phosphite

Cp*W(NO)(CH.sub.2 SiMe.sub.3)2] + L + [H.sub.2] (80 psig)

(in hexanes at 20 degrees C, 3 h) - trans-CP*W(NO)(H) [(CH.sub.2 SiMe.sub.3]) (L) + Me.sub.4 Si....(11)

The alkyl hydride products of conversions 11 are capable of effecting the thermal activation of C-H bonds both intra- and inter-molecularly. 17 Finally, hydrogenation of Cp*W(NO)(CH2SiMe3)2 in hexanes at 920 psig of H2 without the added Lewis base affords approximately equimolar amounts of the bimetallic products [Cp * (ON) (H) W] ([Mu]- H)2 [W(R)(NO)Cp*] (R = H or [CH.sub.2 SiMe.sub.3]) which are separable by fractional crystallization. These bimetallic complexes contain unusual W([Mu]-[H).sub.2] W bridging systems held together by delocalized bonding which should result in their exhibiting unprecedented chemical properties.

Cp'M (NO).sub.2]-Containing Complexes

Just as the mononitrosyl complexes considered earlier, neutral 18-electron Cp'M[(NO).sub.2]X complexes in [CH.sub.2 Cl.sub.2] undergo essentially reversible, one-electron reductions, ie. eq 12 where M = Cr, Mo, or W, and X = Me, Et, H, D, or C119
 [Cp'M(NO).sub.2]X [Equivalents of] [Cp'M(NO).sub.2]X] (12)
 - e

a cyclic voltammogram of CpMo(NO).sub.2] Cl as a representative example being shown in Figure 8. These reductions can again be effected on a preparative scale by employing cobaltocene as the reducing agent, ie.

CpM[(NO).sub.2]X + [CP.sub.2 CO] (in Et.sub.2 O)

[Cp.sub.2 Co] + [CpM[(NO).sub.2]X] (13)

but, in these cases, the extra electron density on the radical anions is delocalized over the entire [M(NO).sub.2] grOUPEvidence for this delocalization is provided by both the X-band

ESR spectra of representative anions which exhibit five-line patterns characteristic of coupling of the un aired electron to two equivalent 14 N nuclei (see Figure 9) and the solid-state molecular structure of [CpMo(NO).sub.2]Et] - which displays a large N-Mo-N angle of 100.5 degrees. These radical anions are, however, thermally unstable in solutions at room temperature and generally decompose to non-nitrosyl containing materials.

The three reductions of Cp'M[(NO).sub.2] Cl species that do lead to nitrosyl-containing products are those summarized in eq 14 where for M = Cr, Cp, = [CP.sup.21] or Cp*, and for M = Mo, Cp, = [Cp*,.sup.22]
 Zn/Hg, 20 degrees C
 2 Cp'M[(NO).sub.2] Cl ----------> [CP'M(NO).sub.2]2 ... (14)

The dimeric products of reactions 14 are red to violet solids whose spectroscopic properties are indicative of the chromium complexes possessing both terminal and bridging nitrosyl ligands whereas the molybdenum congener possesses nitrosyl groups bonded in only a terminal fashion to the metal centres. Interestingly, analogous dimers containing tungsten remain unknown despite intense experimental efforts. Interest in dimers of this type stems from the fact that [CpCr(NO).sub.2]2 has a propensity to abstract halogen from various compounds, a feature that may be synthetically exploited for the selective dehalogenation of organic substrates, eg.

The oxidation potentials of the [Cp'M(NO).sub.2 X] species are higher than those exhibited by their carbonyl analogues, another manifestation of the strong 7-acidity of the nitrosyl ligands. Our attempts to perform this oxidation of [CpCr(NO).sub.2] Me on a preparative scale by employing the nitrosonium ion as the oxidant led to our discovery of the first exam le of the insertion of NO' into a metal-carbon a-bond, ie. [Figure 15 has been omitted]

The product formaldoxime complex of conversion 15 is isolable as green, diamagnetic crystals which have been characterized fully by conventional spectroseopic techniques including single-crystal X-ray crystallography (see Figure 10).26 The organometallic cation is best viewed as consisting of the 16-electron CpCr[(NO).sub.2+] cation bearing the formaldoxime ligand functioning as a 2-electron donor towards the metal centre via its nitrogen atom.

All the evidence presently available suggests that reaction 15 proceeds via a charge-controlled, intermolecular attack by NO+ at the chromium-carbon a bond in a classical [S.sub.E2] process. In other words, reaction 15 occurs because CpCr[(NO).sub.2] Me is relatively difficult to oxidize and its Cr-[CH.sub.3] bond is prone to nonoxidative attack by electrophiles. Indeed, we have shown subsequently that insertions analogous to that depicted in eq 15 can be effected for a variety of CpCr[(NO).sub.2 R] complexes (R = Me, CH.sub.2 SiMe.sub.3], Or Ph) and several electrophiles NE+ (E = [O, p-O.sub.2 NC.sub.6 H.sub.4 N,] or S). 27 The NE-containing ligands thus formed may be conveniently displaced from the chromium's coordination sphere by the chloride anion. The resulting CpCr[(NO).sub.2 Cl] can be reconverted to CpCr[(NO).sub.2 R] by treatment with the appropriate Grignard or organoaluminum reagent, thereby completing a cycle by regenerating the initial organometallic reactant. The entire sequence of stoichiometric reactions forming the cycle is shown below for the particular case of NO+ insertion, Cr representing the metal centre in the [CpCr(NO).sub. 2] group.

The net organic transformation mediated by the [CpCr(NO).sub.2 R] groups in cycles such as that shown above are thus

NE+ + [R-] --> N(E)R...............(16)

where NE+ is the external nitrogen-containing electrophile and R- is the organic group initially [sigma]-bonded to chromium. The final N(E)R product is formed selectively, the new C-N linkage being generated exclusively at the carbon atom originally attached to the metal centre.

One other aspect of Cp'M[(NO).sub.2] chemistry merits consideration, and that is the synthetically useful electrophilicity of the formally 16-electron cations, Cp'M[(NO).sub. 2+] Green solutions of complexes that react as though they contain these cations may be generated by halide abstraction as shown in eq 17 where Cp' = Cp or Cp*, and M = Cr, Mo, or W.28,29

Cp'M[(NO)sub.2 Cl]+ [AgBF.sub.4] (in [CH.sub.2 Cl.,sub.2)]

--> Cp'M[(NO)sub.2 BF.sub.4] + AgCl................(17)

The electrophilicity of the product complexes is demonstrated by the fact that they abstract Ph - from the [BPh.sub. 4] anion, ie. eq 18 probably via the formation of incipient carbocations. Most interestingly, methyl propiolate and 2,3-dimethyl-2-butene condense in the metals' coordination spheres to form cationic lactone-containing species in the manner depicted in eq 20, ie. 29

Facile O-demethylation occurs upon exposure of the product organometallic cations to I in acetone at room temperature, ie.

The final neutral products are isolable as analytically pure solids which have been fully characterized by conventional spectroscopic techniques including a single-crystal X-ray crystallographic analysis of the CpMo member of this class of complexes. The a-bound lactone ligand in the neutral compounds is removable by treatment with 21 eg. the net result of the transformations summarized in equations 20-22 being the assembly of the iodolactone from its components, the processes being mediated by the [Cp'M(NO).sub.2] groups. The neutral lactone complexes produced via eq 21 are nothing more than fancy [Cp'M(NO).sub.2 R] species, and so the question naturally arises as to whether they can also undergo insertions of NE into their M-C [sigma]-bonds (cf. eq 15). Preliminary experiments indicate that such insertions are feasible.

In closing, I trust I have convinced you that the presence of nitrosyl ligands does indeed impart distinctive chemical characteristics to their complexes and that I have conveyed to you a sense of what may be accomplished by utilizing these remarkable species.


I have been both fortunate and priviliged to have worked with a number of very talented individuals during my years at UBC. I also thank the Aluminum Company of Canada for sponsoring this award and my peers on the selection committee for deeming me to be a worthy recipient of it. Finally, I gratefully acknowledge the support of our investigative efforts over the years by the Natural Sciences and Engineering Research Council of Canada in the form of research grants and fellowships.


1 Hoyano, J.K.; Legzdins, P.: Malito, J.T. Inorg. Synth. 18 (1978), p. 126.

2 Legzdins, P.; Martin, D.T.; Nurse, C.R. Inorg. Chem. 19 91980), p. 1560 and references therein.

3 Cotton, F.A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley Interscience: New York (1988), p. 63.

4 Herring, F.G.; Legzdins, P.; Richter-Addo, G.B. Organometallics 8 (1989), p. 1485.

5 Hunter, A.D.; Legzdins, P. Organometallies 5 (1986), P. 1001.

6 Christensen, N.J.; Hunter, A.D.; Legzdins, P. Organometallics 8 (1989), p. 930 and references therein.

7 Hunter, A.D.; Legzdins, P.; Nurse, C.R.; Einstein, F.W.B.; Willis, A.C. J. Am. Chem. Soc. 107 (1985), p. 1791.

8 Christensen, N.J.; Legzdins, P.; Trotter, J.; Yee, V.C. unpublished observations.

9 Legzdins, P.; Rettig, S.J.; Sanchez, L. Organometallics 7 (1988), p. 2394.

10 Dryden, N.H.; Legzdins, P. unpublished observations.

11 Phillips, E.C.; Legzdins, P.; Einstein, F.W.B.; Jones, R.H. unpublished observations.

12 Legzdins, P.; Rettig, S.J.; Sanchez, L.; Bursten, B.E.; Gatter, M.G. J Am. Chem. Soc. 107 (1985), p. 1411.

13 Legzdins, P.; Phillips, E.C.; Sanchez, L. Organometallics 8 (1989), p. 940.

14 Legzdins, P.; Rettig, S.J.; Sanchez, L. Organometalies 4 (1985), p. 1470.

15 Legzdins, P.; Phillips, E.C.; Rettig, S.J.; Sanchez, L.; Trotter, J.; Yee, V.C. Organometallics 7 (1988), p. 1877.

16 Legzdins, P.; Phillips, E.C.; Trotter, J.; Yee, V.C. unpublished observations.

17 Legzdins, P.; Martin, J.T.; Einstein, F.W.B.; Jones, R.H. Organometallics 6 (1987), p. 1826.

18 Legzdins, P.; Martin, J.T.; Einstein, F.W.B.; Willis, A.C. J Am. Chem. Soc. 108 (1986), p. 7971, and references therein.

19 Legzdins, P.; Wassink, B. Organometallics 7 (1988), p. 482.

20 Legzdins, P.; Wassink, B.; Einstein, F.W.B.; Jones, R.H. Organometallics 7 (1988), p. 477.

21 Kolthammer, B.W.S.; Legzdins, P.; Malito, J.T. Inorg. Synth. 19 (1979), p. 208.

22 Chin, T.T.; Legzdins, P. unpublished observations.

23. Hames, B.W.; Legzdins, P. Organometallics 1 (1982), p. 116.

24 Kolthammer, B.W.S.; Legzdins, P. J Chem. Soc., Dalton Trans. (1978), p. 31.

25 Kolthammer, B.W.S.; Legzdins, P.; Martin, D.T. Tetrahedron Lett. 1978), p. 323.

26 Legzdins, P.; Wassink, B.; Einstein, F.W.B.; Willis, A.C. J. Am. Chem. Soc. 108 (1986), p 317.

27 Legzdins, P.; Richter-Addo, G.B.; Wassink, B.; Einstein, F.W.B.; Jones, R.H.; Willis, A.C. J Am. Chem. Soc. 111 (1989), p. 2097.

28 Legzdins, P.; Martin, D.T. Organometallics 2 (1983), p. 1785.

29 Legzdins, P.; Richter-Addo, G.B. submitted for publication.

30 Legzdins, P.; Richter-Addo, G.B.; Einstein, F.W.B.; Jones, R.H. unpublished observations.
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Author:Legzdins, Peter
Publication:Canadian Chemical News
Date:Nov 1, 1990
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