Angewandte Communications Chemie

International Edition:DOI:10.1002/anie.201706323 Metalloradicals Very Important Paper German Edition:DOI:10.1002/ange.201706323

Ta(CNDipp)6 :AnIsocyanide AnalogueofHexacarbonyltantalum(0)** Khetpakorn Chakarawet, Zachary W. Davis-Gilbert, Stephanie R. Harstad, Victor G. Young,Jr. , Jeffrey R. Long, and John E. Ellis*

In memory of Alan Davison (1936–2015)

Abstract: Hexakis(2,6-diisopropylphenylisocyanide)tantalum Xyl = 2,6-dimethylphenyl, which was far more thermally [1i] is the first isocyanide analogue of the highly unstable Ta(CO)6 stable (m.p.159–1608Cdecomp.) than V(CO)6 (decomp. [8] and represents the only well-defined zerovalent tantalum 60–708C), suggested that Ta(CNXyl)6 and the niobium complex to be prepared by conventional laboratory methods. analog might be isolable.However,despite success in the 0 + Twoprior examples of homoleptic Ta complexes are known, synthesis and isolation of [Ta(CNXyl)7] ,[Ta(CNXyl)6]À,and [11, 12] Ta(benzene)2 and Ta(dmpe)3,dmpe = 1,2-bis(dimethylphos- related complexes, attempts to isolate the corresponding phano)ethane,but these have only been accessed via ligand co- elusive neutral homoleptic tantalum and niobium complexes condensation with tantalum vapor in asophisticated metal- uniformly failed.[13] Many years ago Mann, Gray,and atom reactor.Consistent with its 17-electron nature,Ta- Hammond reported on the zerovalent complex W(CNDipp)6, [14] (CNDipp)6 undergoes facile one-electron oxidation, reduction, Dipp = 2,6-diisopropylphenyl, and it was of interest to or disproportionation reactions.Inthis sense,itqualitatively determine whether this more sterically encumbered isocya- resembles V(CO)6,the only paramagnetic homoleptic metal nide could stabilize the corresponding neutral tantalum carbonyl isolable under ambient conditions. complex. Initial results were promising and provided astruc- tural characterization of the unique ditantalum salt [Ta- [1] [15] Incontrast to vanadium and niobium, isolable homoleptic (CNDipp)7][Ta(CNDipp)6], atype of compound previously complexes of zerovalent tantalum are quite rare,with only unknown for homoleptic metal carbonyls and isocyanides,[16] [2] two examples presently known:Ta(benzene)2 and Ta- and of significance as aformal disproportionation product of [3] (dmpe)3, dmpe = 1,2-bis(dimethylphosphano)ethane.Also, the desired Ta(CNDipp)6 complex. Unfortunately,this salt these species are very poorly accessible because they are only could not be obtained as apure bulk solid and the neutral available via tantalum vapor synthesis methods,which require species was not isolated, so this project was temporarily [4] specialized metal-atom reactors. Although Ta(bipy)3,bipy = abandoned. Now more than adecade later,independent [5] a,a’-bipyridine, and Ta(iPr2-dad)3, iPr2-dad = 1,4-diiso- syntheses of both the cation and anion in the ditantalum salt, [6] [17] propyl-1,4-diazabuta-1,3-diene, were originally suggested as [Ta(CNDipp)7][BF4](1), and K[Ta(CNDipp)6](3), to be zerovalent tantalum complexes,later experimental and respectively,were developed, as well as the synthesis and computational studies indicate that both compounds contain isolation of neutral Ta(CNDipp)6.Ta(CNDipp)6 is the first V [7] high-valent Ta . fully substituted derivative of Ta(CO)6 containing monoden- Perhaps the most interesting comparisons of homoleptic tate ligands,[18a] and is the only one to be obtained by zerovalent complexes of Group 5metals are for the 17- aconventional synthesis.[18b] Theniobium analogue has also [19] electron hexacarbonyls of these species:V(CO)6 is well- been isolated and structurally characterized. known as the only paramagnetic unsubstituted metal carbonyl Thedeep red-brown and very air sensitive precursor [8] [9] isolable under ambient conditions, while Ta(CO)6 and complex TaI(CNDipp)6 (2), was obtained in 90%yield by the [10] [20a] Nb(CO)6 have only been obtained at low temperatures as reaction of I2 with [Et4N][Ta(CO)6] in the presence of six matrix isolated species and may be unstable above 50 K. equivalents of CNDipp in tetrahydrofuran, THF.Asimilar

Barybinssynthesis of the 17-electron complex V(CNXyl)6, procedure was previously used for the synthesis of TaI- [11] (CNXyl)6. However,the reaction mixture leading to 2 also [*] Z. W. Davis-Gilbert, S. R. Harstad, Dr.V.G.Young Jr., Prof. J. E. Ellis had to be heated at reflux for 20 htoavoid contamination by Department of Chemistry mixed carbonyl-isocyanides.[20b] IR and NMR spectra, as well University of Minnesota as the molecular structure of 2,[21] are very similar to those Minneapolis, MN 55455 (USA) previously reported for the CNXyl analogue.[11] Reduction of E-mail:[email protected] [22] 2 by aminimum of 3equivalents of KC8 in toluene over K. Chakarawet, Prof. J. R. Long aperiod of 5days at 208Cwas necessary for high conversion Department of Chemistry,UniversityofCalifornia, Berkeley to satisfactorily pure dark violet microcrystalline K[Ta- Berkeley,CA94720 (USA) (CNDipp) ], 3,isolated in 70–80%yields.[23] [**] Dipp= 2,6-diisopropylphenyl. Highly Reduced Organometallics, 6 Treatment of cold, 658C, deep red solutions of 3 in part 67. Part 66:W.W.Brennessel, J. E. Ellis,Inorg. Chem. 2012, 51, À 9076. toluene with one equivalent of solid Ph3PAuCl afforded Supporting information and the ORCID identification number(s) for within 2habright green solution of Ph3PAuTa(CNDipp)6 (4) the author(s) of this article can be found under: which was isolated in 79%yield as violet-brown micro- https://doi.org/10.1002/anie.201706323. crystals.Although gold derivatives of metal carbonyls have

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Scheme 1. Synthesis of Ph3PAuTaL6 and TaL 6,L= CNDipp, solvent = to- luene.

reaction was carried out to determine whether the previously [15] isolated ditantalum salt [Ta(CNDipp)7][Ta(CNDipp)6] could be obtained by arational route,but the only products observed were 5 and asmall amount of free CNDipp.For this and other reasons,webelieve the formal disproportionation product of 5 is stable only in the solid state.Figueroasgroup reported very similar comproportionation reactions of [Co- + (CNAr’)4(THF)n] , n = 0, 1, with [Co(CNAr’)4]À to quantita- tively produce the second known 17-electron homoleptic

Figure 1. Molecular structure of 4.Thermal ellipsoids are set at 50% isocyanide metal(0) complex, Co(CNAr’)4,where Ar’ is an probability,with hydrogen atoms omitted for clarity.Selected bond extremely bulky m-terphenyl group,2,6-bis(2,4,6-trimethyl- lengths [Š]and angles [8]: Ta1–Au1 2.7207(2), Au1–P1 2.2737(8), Ta1– [30] phenyl)phenyl. Thesecond reaction with [Et3NH][BPh4] C66 2.111(3), Ta1–C40 2.117(3), Ta1–C53 2.113(3), Ta1–C27 2.137(3), was an attempt to protonate 3 to give the unknown hydride, Ta1–C1 2.148(3), Ta–C14, 2.150(3);avTa-C-N 174(2), av C-N-C 165(8), [20a] Ta-Au-P 176.20(3). HTa(CNDipp)6.But unlike HTa(PF3)6 which is fairly robust, no evidence for the isocyanidehydride was obtained. Isocyanide metal hydrides are rare species and Figueroa and [24] [31a] been known since 1964, 4 appears to be the first gold adduct co-workers recently reported [HFe(CNAr’)4]À and [HCo- [31b] of ahomoleptic metal isocyanide.X-ray structural character- (CNAr’)4]. ization of 4,Figure 1, confirmed the presence of seven Solution IR spectra of 5 are nearly superimposable in coordinate tantalum bound to six discrete CNDipp ligands unreactive solvents,including alkanes,arenes,and THF.In 1 and the gold atom of the Ph3PAugroup.The geometry about THF,one intense broad n(CN) peak is present at 1934 cmÀ , 1 tantalum is best described as amonocapped octahedron, similar to that of V(CNXyl)6,1939 cmÀ ,and those reported 1 where Ph3PAuisthe capping group and the M–L distances fall for tungsten analogs,W(CNAr)6,Ar= Xyl, 1934 cmÀ ;Dipp, 1 [14] 1 13 into the expected 1:3:3pattern. Structural features of 4 are 1944 cmÀ (KBr disk). Hand CNMR spectra of 5 in

similar to those of Ph3PAuV(CO)6,where the vanadium also [D8]THF show paramagnetically shifted resonances in posi- [25] [1i] has aslightly distorted monocapped octahedral geometry. tions similar to those reported for V(CNXyl)6. Attempts to TheTa–Audistance,2.7207(2) Š,the first to be reported, is obtain reliable solution magnetic data for 5 by the Evans

longer than the V–Aubond, 2.690(3) Š,inPh3PAuV(CO)6, NMR method were unsuccessful due to its poor solubility in but is quite similar to the Ti–Audistance,2.719(1) Š,in hydrocarbons,ethers and other unreactive solvents.However, [26] [Et3PAuTi(CO)6]À , owing to the nearly identical atomic as measured by SQUID magnetometry,the effective mag- [27] radii of Ti,1.45 Š,and Ta,1.43 Š. netic moment of solid 5 at 300 Kis2.03 mB,and decreases with

Discovery of areagent that would cleanly oxidize K[Ta- decreasing temperature to 1.76 mB at 2K,consistent with an 1 [31c] (CNDipp)6](3)toTa(CNDipp)6 (5)was challenging.With S = =2 spin centered on the Ta atom. Thecyclic voltammo- two exceptions,tobedescribed below,soluble oxidants gram of 5 in 1,2-difluorobenzene displays areversible [Ta- 0/1 À 1 invariably caused overoxidation of 3.For this reason, possible (CNDipp)6] redox couple centered at E =2 = 2.03 Vversus 0/1+ À poorly soluble or insoluble oxidants were examined. Gratify- Cp2Fe .Interestingly,this value is close to the reversible 0/1 ingly,asuspension of oligomeric MoO3 in toluene worked couple observed for [V(CNXy)6] À, 1.97 V, but is about À 0/1 well to selectively oxidize 3 to satisfactorily pure 5 (see 1.5 Vmore negative than that of [V(CO) ] À, 0.53 V, both 6 À Supporting Information). Insoluble compounds of the type measured under the same conditions in this study.[31c] Also,it [28] Kx[MoO3]y,“molybdenum blues,” form,and MoO3 thereby should be noted that crystalline 5 exhibits remarkable thermal functions as an effective “potassium sponge” in this reac- stability and decomposes without melting at 236–2408C, more tion.[29] Scheme 1summarizes steps in the syntheses of 3, 4, than 708Chigher than the decomposition point previously [1i] and 5. reported for V(CNXyl)6. Compound 5 has also been obtained in 80–90%yields by Single crystals of 5 were grown from THF/pentane and

the reactions of equimolar amounts of either [Ta(CNDipp)7]- consisted of dichroic golden-red brown plates of composition  [BF4](1), or [Et3NH][BPh4]with K[Ta(CNDipp)6]. Thefirst Ta(CNDipp)6·2THF and space group R3. In the crystal

10578 www.angewandte.org  2017 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Angew.Chem. Int.Ed. 2017, 56,10577 –10581 Angewandte Communications Chemie structure,the metal atoms lie on inversion centers within the 3 metalloradical nature,[35] but in these reactions 5 often proved axis,resulting in one independent isocyanide per unit cell. The to be much less reactive.For example,whereas V(CO)6 two THF molecules are equivalent and located on athree- undergoes full disproportionation, rapidly in THF[36] and fold axis.The molecular structure of 5 is shown in Figure 2, slowly in toluene at 208C,[37] 5 is stable for at least aweek in these solvents and may even be briefly heated to 608Cfor about 5min without change.This stability is likely due to both the crowded nature of the CNDipp ligand, which protects 5 from nucleophilic attack, and its stronger donor/weaker acceptor character compared to CO.[38] Also,for example,

trimethylphosphane reacts with V(CO)6 in hexane at 208Cto [39] afford the salt [V(CO)3(PMe3)4][V(CO)6], whereas no reaction with 5 occurs under the same conditions.More basic pyridine also causes full and irreversible disproportio- [34,36] nation of V(CO)6, but addition of 5 to neat pyridine at 208Caffords only areversible disproportionation process, from which pure 5 is isolated on removal of solvent.[40] In

contrast, the more basic triphenylstannyl anion, [Ph3Sn]À, reacts with 5 quantitatively within minutes to give high

isolated yields of the disproportionation products Ph3SnTa-

(CNDipp)6 (6)and [K(18-crown-6)][Ta(CNDipp)6](7) according to Eq. (1), L = CNDipp.

THF 2TaL6 K 18-crown-6 Ph3Sn 65 to 20  C þ½ ð ފ½ Š À ! 1 Ph3SnTaL6 K 18-crown-6 TaL6 ð Þ ƒƒƒƒƒþ½ƒ ð ފ½ Š Figure 2. Molecular structure of 5.Thermal ellipsoidsare set at the 50%level with tetrahydrofuranofsolvation and hydrogen atoms Product 6 has also been isolated in about 80%yield via omitted for clarity.Selected bond lengths [Š]and angles [8]: Ta1–C1 the more conventional reaction of K[Ta(CNDipp)6]with 2.135(1), C1–N1 1.179(2), N1–C2 1.385(2);Ta1-C1-N1 176.9(1), C1- Ph3SnCl, analogous to the same route used by Davison N1-C2 164.4(1), C1-Ta1-C1A 180.0, C1-Ta1-C1B 86.07(5), C1-Ta1-C1C 45 years ago for the synthesis of Ph3SnTa(CO)6 from [Ta- 93.93(5). [41] (CO)6]À . Interestingly, 6 represents the first isolable tin adduct of ahomoleptic isocyanide of aGroup 5metal,

M(CNR)6,M= V, Nb,Ta. Warnock and Cooper prepared with selected distances and angles.Six discrete isocyanide Ph3SnCo(CNXyl)4 in 1989 as akey derivative in their ligands are present in which the isocyanideterminal carbons characterization of the first homoleptic isocyanidemetalate, [42] form aslightly distorted octahedron about the tantalum. [Co(CNXyl)4]À . [32] [1i] However,unlike V(CO)6 and V(CNXyl)6, which show Because the homoleptic cobalt isocyanides,Co2(CNR)8, small degrees of tetragonal distortion in the M–C distances R = tBu[43] and Xyl,[16] were reported to react with carbon owing to apossible Jahn–Teller effect, as anticipated for low monoxide (R = tBu, rapidly at 208Cunder 1atm;R= Xyl, 5 spin d octahedral complexes,all M–C distances of 5 are slowly at 408Cunder 30 atm) to afford the respective salts identical by symmetry in the crystalline lattice. [Co(CNR)5][Co(CO)4], the reaction of 5 with CO was Theinteratomic parameters of 5 appear to be normal examined. Treatment of deep red-violet solutions of 5 with [14] compared to those of W(CNDipp)6. Thus,the Ta–C CO (1 atm) at 658Cwithin seconds gave adark green À distance of 2.135(1) Š is approximately 0.07 Š longer than solution (of unknown nature). On warming to 208C, the the average W–C distance,2.062(2) Š,consistent with the solution had changed to atransparent deep red color.The larger atomic radius of Ta,1.43 Š,relative to W, 1.37 Š.[27] reaction was monitored by IR spectroscopy and had to be However,the average WC–N distance,1.176(4) Š,isessen- heated to 608Cunder CO for 4days to afford the final tially identical to the value observed for 5,1.179(2) Š,which product, [Ta(CNDipp)7][Ta(CO)5(CNDipp)] (8)isolated as is in agreement with the nearly identical values observed for satisfactorily pure dark red-brown microcrystals in 67%yield, the most intense infrared CN stretching frequencies for both Eq. (2), L = CNDipp. species.Also of interest is that the C-N-C bend angles of 5, [14] THF 164.4(1)8,and of W(CNDipp)6,164(2)8, are statistically 2TaL6 5CO TaL7 Ta CO 5L 4L 2 þ 4days, 60 !C ½ Š½ ð Þ Š þ ð Þ identical, indicative of similar degrees of metal(dp)to p*(CNR) back-bonding in these otherwise analogous 17- Single crystalƒƒƒƒƒX-rayƒ structural characterization of 8 shows + and 18-electron formally zerovalent metal complexes,respec- well defined and discrete [Ta(CNDipp)7] and [Ta(CO)5- [44] tively. (CNDipp)]À ions in the crystalline lattice. Theproduction

Comparisons of the reactivity patterns of 5 and V(CO)6 of salt 8 is very reminiscent to Sattelbergerssynthesis of provided some surprises.The V(CO)6 complex undergoes [Ta(CO)3(PMe3)4][Ta(CO)5PMe3]in85% yield via the car- [33] facile nucleophilic substitution and Lewis-base-induced bonylation of Ta(BH4)(CO)3(PMe3)3 in the presence of [34] [45] disproportionation reactions,consistent with its 17-electron PMe3. It is tempting to suggest that his ditantalum salt

Angew.Chem. Int.Ed. 2017, 56,10577 –10581  2017Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim www.angewandte.org 10579 Angewandte Communications Chemie

arose via the CO promoted disproportionation of an unob- Chem. Commun. 1978,431;d)F.G.N.Cloke,K.A.E.Courtney, 0 A. A. Sameh, A. C. Swain, Polyhedron 1989, 8,1641;e)J.A. served 17-electron Ta complex, perhaps Ta(CO)3(PMe3)3. Only one rather thermally unstable species of this type has Bandy,K.Prout, F. G. N. Cloke,H.C.deLemos,J.M.Wallis, J. been claimed, Ta(CO) (dppe), dppe = 1,2-bis(diphenylphos- Chem. Soc.Dalton Trans. 1988,1475;f)F.G.N.Cloke,P.J.Fyne, 4 M. L. H. Green, M. J. Ledoux, A. Gourdon,K.Prout, J. phano)ethane,but unfortunately this potentially significant Organomet. Chem. 1980, 198,C69;g)C.Elschenbroich, M. material has not been obtained in pure form or structurally Nowotny,B.Metz, W. Massa, J. Graulich, K. Biehler,W.Sauer, authenticated, nor has its reaction with CO or other donor Angew.Chem. Int. Ed. Engl. 1991, 30,547; Angew.Chem. 1991, [46] ligands been reported. Forexample,the well-characterized 103,601;h)M.Pomije,C.J.Kurth, J. E. Ellis,M.V.Barybin,

and presumably less reactive 17-electron complex V(CO)4- Organometallics 1997, 16,3582;i)M.V.Barybin, V. G. (dppe)[47,48] has long been known to undergo disproportiona- Young,Jr.,J.E.Ellis, J. Am. Chem. Soc. 2000, 122,4678; j) B. E. Kucera, R. E. Jilek, W. W. Brennessel, J. E. Ellis, Acta tion with bipy to afford the salt [V(bipy)3][V(CO)4(dppe)]2, whereas it reacts further with dppe to provide the robust 17- Crystallogr.Sect. C 2014, 70,749. [2] a) F. G. N. Cloke,M.L.H.Green, J. Chem. Soc.Dalton Trans. electron species trans-V(CO) (dppe) .[47] In contrast to 5, 2 2 1981,1938;b)F.G.N.Cloke,A.N.Dix, J. C. Green, R. N. V(CNXyl)6 reacts slowly with ambient pressures of CO at Perutz, E. A. Seddon, Organometallics 1983, 2,1150. 608CinTHF over a24-h period to afford quantitative [3] F. G. N. Cloke,P.J.Fyne,V.G.Gibson, M. L. H. Green, M. J. [49] [1i] yields of the previously reported trans-V(CO)2(CNXyl)4. Ledoux, R. N. Perutz, A. Dix, A. Gourdon,K.Prout, J. No evidence for disproportionation was noted in this reaction, Organomet. Chem. 1984, 277,61.

similar to the reaction of V(CO)6 with CO.The latter is [4] F. G. N. Cloke,P.L.Arnold in Comprehensive Organometallic known to slowly exchange with CO at 208C,[50] but does not ChemistryIII, Vol. 1 (Eds.: R. H. Crabtree,D.M.P.Mingos), undergo disproportionation in the solid state or in unreactive Elsevier,Amsterdam, 2006,chap.1.08. solvents in the presence or absence of CO.[51] [5] J. Quirk, G. Wilkinson, Polyhedron 1982, 1,209. [6] P. J. Daff,M.Etienne,B.Donnadieu, S. Z. Knottenbelt, J. E. In summary,access of Ta(CNDipp) affords the first 6 McGrady, J. Am. Chem. Soc. 2002, 124,3818. example of an isolable 17-electron and formally zerovalent [7] A. C. Bowman, J. England, S. Sproules,T.Weyhermüller, K. TaL 6 species.Its facile synthesis promises to open up Wieghardt, Inorg.Chem. 2013, 52,2242. adistinctly new area of 5d-block isocyanidechemistry, [8] R. Ercoli, F. Calderazzo,A.Alberola, J. Am. Chem. Soc. 1960, because no prior example of aparamagnetic complex of this 82,2966. type is known. EPR spectral properties of 5 are under [9] R. L. DeKock, Inorg.Chem. 1971, 10,1205. examination and will be presented elsewhere. [10] S. H. Parrish, R. J. Vanzee,W.Weltner,Jr., J. Phys.Chem. A 1999, 103,1025. [11] M. V. Barybin, W. W. Brennessel, B. E. Kucera, M. E. Minyaev, V. J. Sussman, V. G. Young,Jr.,J.E.Ellis, J. Am. Chem. Soc. Acknowledgements 2007, 129,1141. [12] a) M. V. Barybin, Ph.D.Thesis,University of Minnesota, 1999; Financial support was provided by the National Science b) M. V. Barybin, J. J. Myers,Jr.,B.M.Neal in Isocyanide Foundation, Grant No.CHE-146841 to J.R.L.,the Petroleum Chemistry, Applications in Synthesis and Material Science (Ed.: Research Fund, administered by the American Chemical V. Nenajdenko), Wiley-VCH, New York, 2012,chap.14. Society,and Research Gift Support, administered by the [13] Infrared evidence for the production of Ta(CNXyl)6 in solution University of Minnesota Foundation. TheBruker-AXS D8 was obtained on several occasions (with a n(CN) value very Venture diffractometer was purchased through agrant from similar to that of 5 herein), but the species invariably decom- posed on attemptedisolation. M. V. Barybin, A. Romanenkov, NSF/MRI (1224900) and the University of Minnesota. J. E. Ellis,unpublished research, 1998 – 2002. William W. Brennessel, Alexander Romanenkov,Benja- [14] a) K. R. Mann, H. B. Gray,G.S.Hammond, J. Am. Chem. Soc. min E. Kucera, Christopher J. Roberts,and Mikhail E. Min- 1977, 99,306;b)W.Sattler,M.E.Ener,J.D.Blakemore, A. A. yaev are thanked for their initial contributions to this study. Rockford,P.J.Labeaume,J.W.Thackery,J.F.Cameron, J. R. J.E.E. thanks Mollie Dunlap for expert assistance with the Winkler,H.B.Gray, J. Am. Chem. Soc. 2013, 135,10614. manuscript. [15] W. W. Brennessel, A. Romanenkov, J. E. Ellis,unpublished research, 2002.The single-crystal X-ray study shows separate + [Ta(CNDipp)7] and [Ta(CNDipp)6]À units,which have nearly identical structures to those previously reported for [Ta- Conflict of interest [11] (CNXyl)7][BF4]and Cs[Ta(CNXyl)6], respectively. Details of this structure will be reported separately.Crystal data: Theauthors declare no conflict of interest.  C174H233N13Ta 2,triclinic, P1; cell constants a = 14.1843(8), b = 23.048(1), c = 25.273(1) Š, a = 82.674(1), b = 75.610(1), g = Keywords: 17-electron complex ·disproportionation · 89.419(1)8, V= 7935.8(8) Š3, Z = 2, T= 173(2) K; 74364 reflec- isocyanide ligands ·metalloradical ·tantalum tions (26336 for [I > 2s(I)]); R1 = 0.0353; wR2 = 0.0815 (all data).

Howtocite: Angew.Chem. Int. Ed. 2017, 56,10577–10581 [16] Mixed carbonyl-isocyanide salts,such as [Co(CNR)5][Co(CO)4], Angew.Chem. 2017, 129,10713–10717 are well known, e.g.,Y.Yamamoto,H.Yamazaki, Inorg.Chem. 1978, 17,3111, but to our knowledge salts containing homoleptic

[MLn+1][MLn]units for the identical metal and ligand, L = [1] a) F. Calderazzo,G.Pampaloni, J. Organomet. Chem. 1992, 423, CO,CNR, or other acceptor groups,have not been previously 307;b)J.Chatt, H. R. Watson, J. Chem. Soc. 1962,2545; reported. c) F. G. N. Cloke,M.L.H.Green, D. H. Price, J. Chem. Soc.

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[17] Compound 1 was obtained by the analogous procedure used in MoO3 with 3 is continuedfor 12 hat208C, significant [11]  the synthesisof[Ta(CNXyl)7][BF4]. See Supporting Informa- oxidationof5 is observed. tion. [30] G. W. Margulieux, N. Weidemann, D. C. Lucy, C. E. Moore, [3] [18] a) Similarly,Ta(dmpe)3 may be regarded as the only homo- A. L. Rheingold,J.S.Figueroa, J. Am. Chem. Soc. 2010, 132,

leptic derivative of Ta(CO)6 containing bidentate ligands; 5033. b) D. A. Vicic,G.D.Jones in Comprehensive Organometallic [31] a) C. C. Mokhtarzadeh, G. R. Margulieux, A. E. Carpenter,N. ChemistryIII, Vol. 1 (Eds.: R. H. Crabtree,D.M.P.Mingos), Weidemann, C. E. Moore,A.L.Rheingold, J. S. Figueroa, Inorg. Elsevier,Amsterdam, 2006,chap.1.07. Chem. 2015, 54,5579;b)A.E.Carpenter, A. L. Rheingold,J.S.

[19] Satisfactorily pure Nb(CNDipp)6 has been isolated in 84%yield Figueroa, Organometallics 2016, 35,2309;c)See Supporting from the reaction of bis(mesitylene)niobium(0) with 6equiva- Information for more details. lents of CNDipp in THF.Details of this study will be presented [32] S. Bellard, K. A. Rubinson, G. M. Sheldrick, Acta Crystallogr. elsewhere.C.J.Roberts, V. G. Young,Jr.,J.E.Ellis,unpublished Sect. B 1979, 35,271.  research(2013 –2015). Crystal data:C78H102N6Nb,trigonal, R3; [33] a) J. E. Ellis,R.A.Faltynek, G. L. Rochfort, R. E. Stevens,G.E. cell constants a = 15.258(1), b = 15.258(1), c = 26.731(2) Š, a = Zank, Inorg.Chem. 1980, 19,1082;b)Q.-Z. Shi, T. G. Rich- 90, b = 90, g = 1208, V = 5389.6(6) Š3, Z = 3, T= 173(2) K;21272 mond, W. C. Trogler,F.Basolo, J. Am. Chem. Soc. 1984, 106,71. reflections (2606 for [I > 2s(I)]); R1 = 0.0393; wR2 = 0.0986 (all [34] T. G. Richmond,Q.-Z. Shi, W. C. Trogler,F.Basolo, J. Am. data). Chem. Soc. 1984, 106,76. [20] a) J. E. Ellis,G.F.Warnock, M. V. Barybin, M. K. Pomije, Chem. [35] J. E. Ellis, J. Organomet. Chem. 1975, 86,1. Eur.J.1995, 1,521;b)The bulkier CNDipp ligand requires more [36] T. G. Richmond, Q. Z. Shi, W. C. Trogler,F.Basolo, J. Chem. forcing conditionsthan CNXyl for complete substitution of all Soc.Chem. Commun. 1983,650. CO groups in this reaction. [37] F. Calderazzo, Inorg.Chem. 1964, 3,1207. [21] Details of the structure of 2 will be reported separately.Crystal [38] An especially cogent comparison of the donor/acceptor charac-

data:C78H102IN6Ta,monoclinic, P21/n;cell constants a = 12.192- ter of the closely related CNXyl ligand versus CO has been [1i] (3), b = 43.57(1), c = 13.981(4) Š, a = 90, b = 100.592(6), g = 908, described for trans-V(CO)2(CNXyl)4. V= 7300(3) Š3, Z = 4, T= 173(2) K; 54060 reflections (10006 [39] J.-P.Charland,E.J.Gabe,J.M.McCall, J. R. Morton, K. F. for [I > 2s(I)]); R1 = 0.0403, wR2 = 0.0825 (all data). Preston, Acta Crystallogr.Sect. C 1987, 43,48.

[22] Stoichiometric amounts of KC8 are often employed in reduc- [40] See Supporting Informationfor discussionofthe interaction of 5

tions,but as the potassium reacts the “Stage 1,”KC8,changes to with pyridine.Reactions of 5 with other solvents and neutral

“Stage 2,”KC24,which is undoubtedly aweaker reducing agent donors will be reported elsewhere. (i.e.lower K-C ratio) and may be kinetically more inert towards [41] A. Davison,J.E.Ellis, J. Organomet. Chem. 1972, 36,113. release of additionalpotassium. See:N.N.Greenwood, A. [42] G. F. Warnock, N. J. Cooper, Organometallics 1989, 8,1826. Earnshaw, Chemistry of the Elements,2nd ed.,Butterworth- [43] W. E. Carroll, M. Green, A. M. R. Galas,M.Murray,T.W. Heinemann, Oxford, 1997,pp. 293 –295. Turney,A.J.Welch, P. Woodward, J. Chem. Soc.Dalton Trans.

[23] Interestingly,K[Ta(CNXyl)6]could only be isolated as the 1980,80. [K(cryptand 2.2.2)] salt. Theunsolvated potassium salt, unlike [44] See Supporting Information for more details.CCDC 1551212(4),

Cs[Ta(CNXyl)6], was exceedingly unstable above 08C, rapidly 1551214 (5), and 1551213 (8)contain the supplementary decomposing to uncharacterized oligomeric CNXyl species. crystallographic data for this paper.These data can be obtained

Also,solutions of Na[Ta(CNDipp)6]are quite stable in THF at free of charge from TheCambridgeCrystallographic Data

208Cunder anaerobic conditions,unlike “Na[Ta(CNXyl)6],” Centre. which rapidly decomposes in solution above 408C, in the [45] M. L. Luetkens,Jr.,J.C.Huffman, A. P. Sattelberger, J. Am. À absence of crown ethers or cryptands.[11] Chem. Soc. 1985, 107,3361. [24] C. E. Coffey,J.Lewis,R.S.Nyholm, J. Chem. Soc. 1964,1741. [46] a) M. A. Koeslag,M.C.Baird, S. Lovelace,W.E.Geiger, [25] M. G. B. Drew, Acta Crystallogr.Sect. B 1982, 38,254. Organometallics 1996, 15,3289;b)T.F.Miller III, D. L. Strout, [26] P. J. Fischer,V.G.Young,Jr.,J.E.Ellis, Chem. Commun. 1997, M. B. Hall, Organometallics 1998, 17,4164. 1249. [47] H. Behrens,K.Lutz, Z. Anorg.Allg.Chem. 1968, 356,225. [27] J. Emsley, The Elements,3rd ed.,Oxford Press,Oxford, 1998. [48] A. Davison,J.E.Ellis, J. Organomet. Chem. 1972, 36,131. [28] a) N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, [49] J. E. Ellis,unpublished results, 2016. 2nd ed.,Butterworth-Heinemann, Oxford, 1997,pp. 1016 –1017; [50] F. Basolo,R.G.Pearson, Mechanisms of InorganicReactions:A b) I. Nakamura, H. N. Miras,A.Fujiwara, M. Fujibayashi, Y. F. Study of Metal Complexes in Solution,2nd ed.,Wiley,New York, Song,L.Cronin, R. Tsunashima, J. Am. Chem. Soc. 2015, 137, 1967,p.541, Table 7.5. 6524. [51] G. F. Holland, M. C. Manning,D.E.Ellis,W.C.Trogler, J. Am.

[29] We are unaware of the prior use of MoO3 as aselective oxidant Chem. Soc. 1983, 105,2308. in inorganic/organometallic synthesis,other than in the prepa- ration of “molybdenum blues.”[28] It can be anticipatedthat Manuscript received:June 22, 2017

MoO3 will also be useful in the removal of alkali metals from Acceptedmanuscript online: July 11, 2017 other strongly reducing metallates. Note also if the reaction of Version of record online: July 26, 2017

Angew.Chem. Int.Ed. 2017, 56,10577 –10581  2017Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim www.angewandte.org 10581