Mononuclear Tantalum(IV, d1) Imido Complexes Supported by the

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Mononuclear Tantalum(IV, d1) Imido Complexes Supported by the Monocyclopentadienyl, Amidinate and Guanidinate Ligand Sets As Models to Explore Dinitrogen Fixation by “End-On-Bridged” Dinuclear {[Ta(IV, d1)]}2(μ-η1:η1-N2) Complexes Brendan L. Yonke, Andrew J. Keane, Peter Y. Zavalij, and Lawrence R. Sita* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: Two independent synthetic routes to the monocyclopentadienyl, amidinate (CpAm) and guanidinate (CpGu) mononuclear Ta(IV, d1) terminal imido complexes Cp*Ta[N(iPr)C(X)N(iPr)][N(tBu)] (Cp* = η5-C5Me5) for X = Me (1) and NMe2 (2), respectively, were developed. For 1, synthesis proceeded via the amido, chloride intermediate Cp*Ta[N(iPr)C(Me)N(iPr)](Cl)[NH(tBu)] (4), which was kinetically deprotonated with LiN(iPr)2 to yield the enamido, amido species Cp*Ta[N(iPr)C(CH2)N(iPr)[NH(tBu)] (5). In toluene solution, 5 underwent quantitative tautomerization to the desired CpAm terminal imido 1. For 2, the amido, chloride intermediate Cp*Ta[N(iPr)C(NMe2)N(iPr)](Cl)[NH(tBu)] (8) was first synthesized and then reacted with TEMPO to provide the Ta(V) imido chloride Cp*Ta[N(iPr)C(NMe2)N(iPr)](Cl)[N(tBu)] (9) through oxidative hydrogen-atom abstraction of the amido group. Chemical reduction of 9 with potassium graphite (KC8) then served to provide the desired CpGu terminal imido 2. All new compounds were structurally characterized by single-crystal X-ray analysis. Although 1 and 2 proved to be unreactive toward hydrogenation or hydrosilyation involving the TaN bond, 2 was shown to engage in radical-based chemistry with MeI and PhS-SPh to yield the Ta(V) imido complexes Cp*Ta[N(iPr)C(NMe2)N(iPr)](X)[N(tBu)], where X = Me (10), I (11), and SPh (12). A similar radical-based reaction of {Cp*Ta[N(iPr)C(Me)N(iPr)]}2(μ-η1:η1-N2) (I) with PhS-SPh yielded the product of formal 1,4-addition, {Cp*Ta[N(iPr)C(Me)N(iPr)](SPh)}2(μ-η1:η1-N2) (13).

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dinitrogen complexes of general formula {(η5-C5R5)M[N(R1)C(X)N(R 2 )]} 2 (μ-N 2 ), where X is an alkyl (R) or a dialkylamino (NR′2) substituent, respectively, and M is a metal from group 4, group 5, and group 6.6−8 Importantly, as Chart 1 reveals, both “end-on-bridged” (μ-η1:η1-N2) (A) and “side-on-bridged” (μ-η2:η2-N2) (B) coordination modes have now been established for this family of [LnM]2(μ-N2) complexes. Of these, the open-shell, [Ta(IV, d1), Ta(IV, d1)], CpAm and CpGu end-on-bridged derivatives {Cp*Ta[N(iPr)C(X)N(iPr)]}2(μ-η1:η1-N2) (Cp* = η5-C5Me5), where X = Me (I) and NMe2 (II), respectively, which are shown in Scheme 1, are of particular interest due to an intrinsically low thermal stability of these complexes in solution. More specifically, above 0 °C, both I and II quantitatively convert to the corresponding diamagnetic [Ta(V, d0), Ta(V, d0)] bis(μ-nitrido) products {Cp*Ta[N(iPr)C(X)N(iPr)](μ-N)}2 (III and IV, respectively), in which N−N bonding is now completely absent.7,8 With respect to potential N-atom functionalization reactions, however, it was surprising to find that I does not participate

INTRODUCTION Elucidation of the structural and electronic requirements for metal-mediated activation of dinitrogen that serves to substantially lower energy barriers for subsequent NN bond cleavage and efficient nitrogen-containing product formation through N-atom functionalization reactionsprocesses that collectively constitute dinitrogen f ixationremains an important academic challenge with enormous societal impact if realized.1−4 A successful strategy employed in these efforts has been to utilize low-valent transition metal fragments to formally reduce dinitrogen through bridging coordination within dinuclear complexes of general formula [LnM]2(μ-N2).1 On the other hand, a survey of the magnitudes of crystallographically determined N−N distances in these dinuclear complexes, which range from 1.20 to 1.65 Å, relative to that for free N2 at 1.0971 Å,5 reveals that the extent of dinitrogen activation that can be achieved in this manner is highly dependent upon the nature of both the supporting metal ligands and the mode of bridging dinitrogen coordination. Recently, we have reported that the η5-cyclopentadienyl, η2amidinate (CpAm) and η2-guanidinate (CpGu) ligand sets are valuable for providing access to several series of dinuclear © 2011 American Chemical Society

Received: October 11, 2011 Published: December 19, 2011 345

dx.doi.org/10.1021/om200961r | Organometallics 2012, 31, 345−355

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Chart 1

proceeds through selective Ta−N σ-bond metathesis to provide the “ring-opened” product VII according to Scheme 1.7 As presented in Scheme 2, this last reaction is of interest given that a second σ-bond metathesis reaction of VII with an additional equivalent of H3SiPh would now generate two equivalents of the proposed mononuclear Ta(V) imido VIII in which the origin of the silylated nitrogen atom is from the chemical fixation of molecular N2. Finally, Scheme 2 presents the additional goal of releasing a N-containing product through chemical transformations of the TaN imido bond, such as through exhaustive hydrogenolysis. In order to systematically, and extensively, investigate the chemical reactivity profile of MN double bonds within CpAm and CpGu early transition metal complexes that are relevant to dinitrogen activation and N-atom functionalization, we set out to synthesize and characterize a variety of group 4, group 5, and group 6 mononuclear terminal imido complexes of the general structure {(η5-C5R5)M[N(R1)C(X)N(R2)][N(R3)] (C) in Chart 1, in which the formal oxidation state of the metal remains tetravalent across the series, i.e., M(IV). Following a successful synthesis of the CpAm group 4 Zr(IV, d0) terminal imido Cp*Zr[N(iPr)C(Me)N(iPr)][N(tBu)] (IX),12 we next set sights on preparing analogous group 5 derivatives. In this regard, while the chemistry of first-row, mononuclear V(IV) terminal imido complexes is well established,13 examples of second-row, mononuclear Nb(IV) terminal imido derivatives are extremely rare,14 and, quite surprisingly, to the best of our knowledge, no reports have ever appeared that detail the synthesis, isolation, and structural characterization of a third-row, mononuclear Ta(IV, d1) terminal imido species.15 This dearth of knowledge for secondand third-row mononuclear group 5 metal imidos stands in sharp contrast to the abundance of existing precedent for dimeric M(IV) imido complexes in which two bridging imido fragments (μ-NR) contribute to formation of a four-memberedring M2N2 core and a transannular M−M single bond.16 It is also important to note that a significant wealth of experimental data currently exists for the structures and chemical reactivities of a variety of mononuclear Ta(V, d0) terminal imido complexes.11,17 In this report, we now detail the successful synthesis and structural characterization of the mononuclear CpAm and CpGu Ta(IV) terminal imido derivatives Cp*Ta[N(iPr)C(X)-

Scheme 1

in 1,2-addition of either dihydrogen (H2) or phenylsilane (H3SiPh) across the formal TaN double bonds,1−4 but instead, this complex engages in dinuclear “1,4-addition” of these reagents across the TaN−NTa framework to generate the dinuclear μ-η1:η1-N2 dihydride and silyl, hydride products V and VI, respectively, in which both metal centers are formally oxidized to Ta(V, d0 ) (see Scheme 1). Interestingly, while Fryzuk and co-workers1h have detailed a variety of chemical transformations involving N-atom functionalization within the novel “end-on, side-on-bridged” Ta(IV), Ta(IV) dinuclear dinitrogen complex ([NPN]Ta)2(μ-H)2(μη1:η2-N2) (where [NPN] = (PhNSiMe2CH2)2PPh) and with some of these leading to complete N−N bond cleavage, Schrock and co-workers9 did not report having observed either the 1,2- or 1,4-addition of reagents across the formal TaN− NTa framework of their seminal Ta(V), Ta(V) end-onbridged dinitrogen complex [Ta(CHCMe3)(PMe3)2Cl]2(μη1:η1-N2).10 While sparse precedent does exist for the 1,2-addition of H2 and silanes across the TaN double bond of mononuclear Ta(V) imido complexes,11 in the case of the Ta(V), Ta(V) bis(μ-nitrido) complex III, reaction with H3SiPh alternatively Scheme 2

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Scheme 3

N(iPr)][N(tBu)] where X = Me (1) and NMe2 (2), respectively. These results include the establishment of two independent synthetic routes to 1 and 2 that was necessitated by obvious, and more subtle, differences between the CpAm and CpGu ligand sets. Importantly, the chemical reactivity profiles displayed by 1 and 2 provide new insights that are relevant to understanding the factors that dictate the observed chemical properties of the dinuclear dinitrogen complexes I and II. Finally, with 1 and 2 in hand, a valuable comparison could be made between these group 5 derivatives of C and that of the isostructural CpAm mononuclear group 4 Zr(IV, d0) terminal imido, Cp*Zr[N(iPr)C(Me)N(iPr)][N(tBu)] (IX), that we have previously prepared according to the synthetic route shown in Scheme 3.12

In our previous studies, synthesis of the CpAm Zr(IV) terminal imido IX proceeded through the bis(amido) intermediate Cp*Zr[N(iPr)C(Me)N(iPr)][NH(tBu)]2 (X)12 (see Scheme 3); however, all attempts to prepare a Ta(IV) analogue of X by reaction of 4 with a second equivalent of LiNH(tBu), according to Scheme 4, produced only intractable oils that failed to crystallize, thereby frustrating efforts to obtain unequivocal structural verification of the desired paramagnetic Ta(IV) product. Attempts to either alkylate or deprotonate 4 using a variety of organolithium reagents, LiR (R = Me, Et, and n Bu), likewise yielded a similar end result. On the other hand, as Scheme 5 reveals, the reaction of 4 with slightly less than one equivalent of lithium diisopropylamide, LiN(iPr)2, in tetrahydrofuran (THF) at −30 °C provided a good yield (66%) of the deprotonated product 5 that could be isolated as a dark green crystalline material, for which elemental analysis was consistent with overall loss of HCl from 4. Single-crystal X-ray analysis of 5 served to confirm that deprotonation of 4 occurred on the methyl group of the amidinate fragment to provide the enamido, amido structure Cp*Ta[N(iPr)C(CH2)N(iPr)][NH(tBu)], which is depicted in Scheme 5. The solid-state molecular structure and selected geometric parameters of 5 are provided in Figure 1b and Table 1, respectively. We have previously reported similar deprotonations of the acetamidinate ligand within different classes of group 4 and group 5 metal CpAm derivatives,9,17,19 and in fact, this includes the synthesis and structural characterization of the group 4 Zr(IV) analogue of 5, Cp*Zr[N(iPr)C(CH2)N(iPr)][NH(tBu)] (XI) (see Scheme 3).12 In comparing the structures of 5 and XI, the former compound exhibits a small contraction of the metal−ligand bonding distances relative to the latter due to differences in covalent radii as expected [cf. for 5, Ta(1)− N(1) is 1.9804(13) Å and for XI the corresponding Zr−N bond length is 2.0533(16) Å].20 Both compounds also possess a more reduced M−N−C bond angle associated with the NH(tBu) fragment than that displayed by 4 [cf. Ta(1)−N(1)− C(19), 136.72(10)° for 5 vs 147.82(19)° for 4]. Finally, the Ct(1)−Ta(1)−N(1)−H(1) torsion angle for 5 is 0.2°, and this serves to direct the N−H bond directly toward the Cp* fragment (see Figure 1b). As depicted in Scheme 5, it can be quickly appreciated that compound 5 is the tautomer of the desired CpAm Ta(IV) terminal imido 1. In our previous studies with the corresponding tautomeric group 4 Zr(IV) analogues, IX and XI, no evidence was obtained for a dynamic solution

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RESULTS AND DISCUSSION A. Synthesis and Characterization of Cp*Ta[N(iPr)C(Me)N(iPr)][N(tBu)] (1). The synthetic route to 1 is presented in Scheme 4. Thus, to begin, reaction of the CpAm Ta(IV) Scheme 4

dichloride Cp*Ta[N(iPr)C(Me)N(iPr)]Cl2 (3)7,18 with one equivalent of lithium tert-butylamide, LiNH(tBu), in diethyl ether (Et2O) at −30 °C provided a good yield (64%) of the paramagnetic Ta(IV) amido chloride Cp*Ta[N(iPr)C(Me)N(iPr)][NH(tBu)]Cl (4), as a dark red, crystalline material for which elemental analysis was fully consistent with the indicated empirical formula. Single crystals of 4 were further subjected to X-ray crystallography, and Figure 1a and Table 1 provide the solid-state molecular structure and selected geometric parameters for this compound. Of special interest with respect to structural comparisons that will be made in this report is the Ta(1)−N(1) bond distance of 1.960(2) Å and the Ta(1)− N(1)−C(19) bond angle of 147.82(19)° for 4. It is also interesting to note that the torsion angle for Ct(1)−Ta(1)− N(1)−H(1) is 88.1°, which orients the N−H bond of the NH(tBu) group “outward” and pointed away from both the Cp* and Cl ligands (see Figure 1a). 347


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Figure 1. Molecular structures (30% thermal ellipsoids) of (a) 4, (b) 5, and (c) 1. Hydrogen atoms have been removed for the sake of clarity, except for selected ones for 4 and 5, which are represented by spheres of arbitrary size.

Table 1. Selected Geometric Parameters for Compounds 1, 2, 4, 5, and 8−12 parameter

1

2

4

Ta(1)−N(1) Ta(1)−N(2) Ta(1)−N(3) C(11)−C(12) C(11)−N(2) C(11)−N(3)

1.790(3) 2.180(2) 2.180(2) 1.499(5) 1.326(3) 1.326(3)

1.785(2) 2.188(2) 2.190(2)

1.960(2) 2.1644(18) 2.2050(17) 1.506(3) 1.341(3) 1.329(3)

Ta(1)−N(1)−C(19) Ta(1)−N(1)−C(20)

177.3(3) 176.2(2)

5

8

Bond Lengths (Å) 1.9804(13) 1.978(2) 2.0432(12) 2.2127(18) 2.0471(13) 2.1607(19) 1.353(2) 1.406(2) 1.330(3) 1.4009(19) 1.332(3) Bond Angles (deg) 147.82(19) 136.72(10) 143.90(17)

9

10

11

12

1.775(2) 2.183(2) 2.220(2)

1.793(2) 2.205(2) 2.219(2)

1.779(2) 2.226(2) 2.178(2)

1.7937(13) 2.2061(13) 2.2132(13)

1.344(4) 1.326(3)

1.328(3) 1.338(3)

1.329(4) 1.348(4)

1.335(2) 1.343(2)

165.0(2)

168.4(2)

169.9(2)

171.34(12)

Scheme 5

equilibrium existing between the two forms in solution.12 In contrast, in the present work, it was determined that, in benzene solution at 25 °C, 5 quantitatively converted after ∼18 h to 1, with the latter being isolated as a bright red, crystalline material (see Scheme 5). These contrasting results for the group 4 and group 5 isostructural tautomeric systems strongly suggest that a lower barrier for tautomerization of the enamido, amido kinetically deprotonated product 5 to the more thermodynamically favored amidinate, imido species 1 exists for the more electron-rich Ta(IV, d1) center, vis-à-vis the Zr(IV, d0) metal center of IX, by virtue of the enamido group in 5 also being more nucleophilic and, hence, more susceptible to tautomerization through bimolecular proton transfer than IX.21 A further important point to make is that, to date, the desired CpAm Ta(IV) terminal imido 1 is accessible only through the two-step kinetic deprotonation/tautomerization pathway of Scheme 5. In practice, a “one-pot” procedure was employed to provide a moderate yield (33%) of 1 that entailed first treating 4 with a slight excess (1.25 equiv) of LiN(iPr)2 in THF at −30 °C, followed by stirring overnight at room temperature, and then isolation of 1 through standard workup. Under these

reaction conditions, it is also possible that the isomerization of 5 to 1 is catalyzed by the base that is present. Although both 1 and 5 possess the same empirical formula, noticeable differences in the paramagnetically shifted 1H NMR resonances for these two compounds could be used to follow the tautomerization process. Unequivocal confirmation of the monomeric terminal imido structure of 1, however, was provided by a single-crystal X-ray analysis, and Figure 1c and Table 1 present the solid-state molecular structure and selected geometric parameters for this compound, respectively. The most notable feature of 1 is that, relative to 5, the Ta(1)−N(1) bond distance is shorter at 1.790(3) Å, while the Ta(1)− N(1)−C(19) bond angle is more linear at 177.3(3)° (see Table 1). Finally, in comparing 1 with the Zr(IV) analogue IX, the two have nearly superimposable structures, but with the former compound once again possessing slightly contracted metal− ligand bond distances [cf. for IX: Zr−N(tBu), 1.839(2) Å].12 B. Synthesis and Characterization of Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)] (2). Although the enamido, amido to amidinate, imido tautomerization route to the CpAm Ta(IV) terminal imido 1 was successful, it could not be adopted for providing access to the corresponding CpGu derivative 2 due 348


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specifically, while benzene solutions of the CpAm derivative 5 were indefinitely stable at 25 °C, at a similar solution concentration, the CpGu analogue 8 underwent complete decomposition over the course of 18 h at room temperature. The resulting crude material, however, consisted of only two major degradation products, and one of these could be isolated in analytically pure form as a diamagnetic, white crystalline compound. Subsequent single-crystal X-ray and elemental analyses served to establish its identity as being the mononuclear CpGu Ta(V) imido chloride Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)]Cl (9), which is depicted in Scheme 5. Figure 2b and Table 1 present the solid-state molecular structure and selected geometric parameters, respectively, for 9, and here it can be noted that the Ta(1)−N(1) bond distance of 1.775(2) Å and the more acute Ta(1)−N(1)−C(20) bond angle of 165.0(2)° that are associated with the TaN(tBu) imido moiety are in keeping with the corresponding parameters obtained for other known mononuclear Ta(V) imido species.17d Finally, as with the CpAm derivative 5, attempts to either alkylate or deprotonate 8 by employing a variety of organolithium reagents, RLi, were unsuccessful and only yielded complex mixtures. Formal one-electron-based hydrogen atom abstraction (HAA) from a transition metal amido species, followed by formal one-electron oxidation of the metal, and, alternatively, metal oxidation followed by proton (H+) loss have been established as viable pathways for production of early and late transition metal imido complexes.22−24 Following the lead provided by others,22,24 we sought to develop the 8 → 9 degradative transformation into a synthetically useful procedure. Thus, addition of one equivalent of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to a pentane solution of 8 at room temperature provided an excellent yield (92%) of 9 that could be isolated in simple fashion according to Scheme 6. The desired CpGu Ta(IV) terminal imido 2 was then obtained in likewise excellent yield (85%) through chemical reduction of 9 with potassium graphite (KC8) in THF at −30 °C (see Scheme 6). Elemental and single-crystal X-ray analyses served to confirm the identity of 2, and Figure 2c and Table 1 provide the solid-state molecular structure and selected geometric parameters for this compound, respectively. Interestingly, 2 displays a Ta(1)−N(1) bond length of 1.785(2) Å, which is nearly identical to that of 1, thereby suggesting a minimal, if any, effect on the nature of TaN multiple bonding on going from the

to the lack of a similar enolizable proton on the Gu ligand. Accordingly, it was not known a priori whether the absence of deprotonation pathways would either help or hinder the development of a different synthetic route to 2. As Scheme 6 Scheme 6

reveals, reaction of the CpGu Ta(IV) dichloride Cp*Ta[N(iPr)C(NMe2)N(iPr)]Cl2 (7), obtained from sodium amalgam (NaHg) reduction of the corresponding CpGu Ta(V) trichloride Cp*Ta[N(iPr)C(NMe2)N(iPr)]Cl3 (6), with one equivalent of LiNH(tBu) in Et2O provided an excellent yield (92%) of the corresponding CpGu Ta(IV) amido chloride Cp*Ta[N(iPr)C(NMe2)N(iPr)][NH(tBu)]Cl (8), which was isolated as a dark purple, crystalline material. Given the paramagnetic nature of 8, single-crystal X-ray analysis was once again used to confirm the composition and molecular structure of this compound according to the data presented in Figure 2a and Table 1. In short, only small differences in the Ta(1)− N(1), Ta(1)−N(1)−C(20), and Ct(1)−Ta(1)−N(1)−H(1) bond distance, bond angle, and torsion angle parameters were observed for 8, vis-à-vis the corresponding values already presented for 5 (cf. Table 1). Although any discernible structural differences between the CpAm and CpGu Ta(IV) amido chloride derivatives 5 and 8 are very slight, the two compounds displayed pronounced differences in solution with respect to thermal stability. More

Figure 2. Molecular structures (30% thermal ellipsoids) of (a) 8, (b) 9, and (c) 2. Hydrogen atoms have been removed for the sake of clarity, except for the amido hydrogen atom of 8, which is represented by a sphere of arbitrary size. 349


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paramagnetic resonance (EPR) spectrum presented in Figure 3 reveals that the unpaired electron is strongly coupled [giso =

CpAm to the CpGu ligand set. As a final note, an exploratory study confirmed that the CpAm derivative 5 reacts in similar fashion with TEMPO to provide a corresponding diamagnetic CpAm Ta(V) imido chloride product; however, the complete analysis of this latter compound was not pursued further. C. Chemical Reactivity of Cp*Ta[N(iPr)C(X)N(iPr)][Nt ( Bu)]. Having successfully synthesized the CpAm and CpGu Ta(IV) terminal imido complexes 1 and 2, it was of significant interest to probe the chemistry associated with the TaN double bond of these open-shell compounds, vis-à-vis that was previously established for the [Ta(IV), Ta(IV)](μ-η1:η1-N2) complexes, I and II, and for the isostructural CpAm Zr(IV) terminal imido derivative IX. For these studies, the CpGu derivative 2 was generally preferred over the use of 1 given the greater ease in obtaining larger, analytically pure quantities of the former compound through the synthetic route of Scheme 5. To begin, both 1 and 2 were somewhat surprisingly found to be completely inert toward H2 in toluene solution, even at temperatures of up to 100 °C for extended periods of time (ca. 72 h). Compounds 1 and 2 also proved to be inert in toluene solution toward hydrosilylation employing either H3SiPh or nhexylsilane, H3Si(n-C6H13), nor did any observable reaction occur under CO pressure. A similar lack of reactivity of the CpAm Zr(IV) terminal imido IX toward hydrogenation and hydrosilylation was previously observed.12 Mountford and coworkers25 have previously established a rich cycloaddition chemistry associated with the TiNR moiety of the first-row group 4 congener, Cp*Ti[N(iPr)C(NMe2)N(iPr)][N(tBu)] (XII); however, studies relating to the hydrogenation or hydrosilylation of these compounds have not yet been reported by this group. Finally, it is interesting to note that, in sharp contrast to the apparent general lack of reactivity of group 4 and group 5 CpAm and CpGu derivatives of C (see Chart 1) toward hydrogenation, Chirik and co-workers26 found the opposite to be true for the bis(η5-cyclopentadienyl) titanium imido complex {[η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ]} 2 Ti[N(SiMe 3 )] (XIII), which undergoes rapid 1,2-addition of H2 across the formal TiN bond. It is obvious from the present results that the Ta(IV) CpAm and CpGu imido derivatives 1 and 2, respectively, are not viable models for investigating a mechanism for hydrogenation of I that involves initial 1,2-addition across the formal TaN bonds. Indeed, Lei and Musaev and co-workers27 just very recently published the results of a computational investigation that suggest that the mechanistic pathway for the I → V transformation is far more complicated than either 1,2-addition or simple hydrogen atom abstraction by a metal-centered radical and, more importantly, that this transformation is dependent upon the dinuclear nature of the substrate. More specifically, these authors conclude that the first step involves structural rearrangement of the initial end-on-bridged [Ta]2(μη1:η1-N2) framework of I to an “end-on, side-on” [Ta]2(μ-η1:η2N2) bonding motif that then undergoes 1,1-oxidative addition of H2 at one of the metal centers in a manner that involves a concomitant change in the formal oxidation states of both the N−N bridging group and the other Ta center. Subsequent Hatom migration from the newly formed Ta(V) dihydride center to the other Ta center then proceeds through formation of a bridging hydride group within a dinuclear μ-H intermediate.27 One feature that is common to both the mononuclear Ta(IV, d1) imido complexes 1 and 2 and the dinuclear dinitrogen complexes I and II is that at ambient temperature these are all open-shell paramagnetic species. In the case of 1, the electron

Figure 3. X-band ESR spectrum (toluene, 25 °C) of compound 1; giso = 1.975, Aiso(181Ta) = 217 G.

1.975, Aiso(181Ta) = 217 G] to the Ta metal center, whereas with I, SQUID magnetic susceptibility data recorded between 4 and 300 K are consistent with a singlet ground state that coexists with a low-lying triplet state that is thermally populated at room temperature.7 Given these similar characteristics, it was of interest to determine whether the mononuclear imido and dinuclear dinitrogen systems might engage in similar oneelectron chemistry. To begin, compound 2 was found to react cleanly with one equivalent of methyl iodide in pentane solution to provide a 1:1 mixture of the CpGu Ta(IV) imido, methyl and imido, iodo complexes Cp*Ta[N( i Pr)C(NMe 2 )N( i Pr)](X)[N( t Bu)], where X = Me (10) and I (11), respectively, according to Scheme 7. This result is supportive of a one-electron halogen abstraction mechanism28 that has been previously reported for 17-electron tantalum29 and chromium30 complexes supported by a closely related η5-C5H5, η2-β-diketiminate [N(Ar)C(Me)]2CH (Ar = 2,6-diisopropylphenyl) ligand set. Efforts to obtain analytically pure samples of 10 and 11 were only partially successful in that multiple fractional recrystallizations provided cocrystals enriched in either 10 (88%) or 11 (84%), as confirmed by both elemental and single-crystal X-ray analyses. Figure 4a and b present the solid-state molecular structures of 10 and 11 as derived from cocrystals in which each is present as the major component (i.e., the structure of 10 in Figure 4a was obtained from a 90:10 cocrystal of 10:11, while that in Figure 4b was obtained from a 10:90 cocrystal ratio). Table 1 provides the values for selected geometric parameters for these two compounds. When a pentane solution of I was treated with MeI in similar fashion to 2, an intractable complex mixture of compounds was obtained that appeared to include products arising from indiscriminate radical abstraction chemistry, e.g., those with (I)[Ta](μ-N2)[Ta](Me), (I)[Ta](μ-N2)[Ta](I), and (Me)[Ta](μ-N2)[Ta](Me) frameworks.31 Unfortunately, all attempts to separate this product mixture into analytically pure samples that could then be subjected to analytical and crystallographic structural analyses were unsuccessful. This impasse, however, motivated efforts to investigate other classes of reagents for radical chemistry that might yield more straightforward results. Thus, as Scheme 8 shows, when a solution of 2 in pentane was treated with one equivalent of diphenyldisulfide, PhS-SPh, at room temperature, near 350


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Scheme 7

Figure 4. Molecular structures (30% thermal ellipsoids) of (a) 10, (b) 11, and (c) 12. Hydrogen atoms have been removed for the sake of clarity.

Scheme 8

quantitative conversion into the Ta(V) imido, sulfido complex 12 occurred after only a few minutes. Single-crystal X-ray analysis of 12 confirmed the solid-state molecular structure of this species that is presented in Figure 4c, and Table 1 presents selected bond lengths and bond angles. Gratifyingly, addition of one equivalent of PhS-SPh to a pentane solution of the dinuclear dinitrogen complex I also resulted in a rapid reaction that led to complete conversion to two new compounds that were easily separated on the basis of their striking difference in solubility in pentane. Elemental analyses and 1H NMR spectra of the pure products strongly suggested that they were most likely diastereomers of the ditantalum dinitrogen disulfide 13, which is depicted in Scheme 9, and indeed, recrystallization of the more soluble product provided single crystals for X-ray analysis that confirmed it to be meso-13 on the basis of the solid-state molecular structure and selected geometric parameters that are presented in Figure

Figure 5. Molecular structure (30% thermal ellipsoids) of meso-13. Selected bond lengths (Å) and bond angles (deg): Ta(1)−N(1) 1.831(2), Ta(1)−S(1) 2.5192(8), Ta(2)−N(2) 1.837(2), Ta(2)−S(2) 2.5235(8), N(1)−N(2) 1.297(3); Ta(1)−N(1)−N(2) 175.9(2), Ta(2)−N(2)−N(1) 169.2(2).

Scheme 9

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Cp*Ta[N(iPr)C(Me)N(iPr)][NH(tBu)]Cl (4). A solution of LiNH( Bu) (74 mg, 0.911 mmol) in 10 mL of Et2O was cooled to −30 °C and added dropwise to a solution of 3 (471 g, 0.891 mmol) in 50 mL of Et2O, precooled to −30 °C, over a period of 5 min. The reaction mixture was allowed to warm to room temperature and stirred for 3 h to produce a red-purple-colored solution. Volatiles were removed in vacuo to yield a red-purple solid. The crude product was then taken up in pentane and filtered through a short pad of Celite on a glass frit. The collected dark red filtrate was concentrated and cooled to −30 °C to provide dark red crystals of 4 (324 mg, 64% yield). Anal. Calcd for C22H42N3ClTa: C, 46.75; H, 7.50; N, 7.44. Found: C, 46.94; H, 7.39; N, 7.22. Cp*Ta[N(iPr)C(CH2)N(iPr)][NH(tBu)] (5). A −30 °C solution of LiN(iPr)2 (43 mg, 0.402 mmol) in 5 mL of THF was added dropwise to a −30 °C solution of 4 (226 mg, 0.403 mmol) in 15 mL of THF. The reaction mixture was warmed to room temperature and stirred at this temperature for 1 h to produce a dark-green-colored solution. Volatiles were removed in vacuo to yield a dark green solid. The crude product was dissolved in minimal pentane and filtered through a short pad of Celite on a glass frit. The collected filtrate was concentrated and cooled to −30 °C to give dark green crystals of 5 (140 mg, 66%, yield). Anal. Calcd for C22H41N3Ta: C, 49.99; H, 7.82; N, 7.95. Found: C, 50.21; H, 7.67; N, 7.93. Cp*Ta[N(iPr)C(Me)N(iPr)][N(tBu)] (1). A solution of LiN(iPr)2 (53 mg, 0.496 mmol) in 10 mL of Et2O, precooled to −30 °C, was added dropwise to a solution of 4 (224 mg, 0.396 mmol) in 20 mL of Et2O, precooled to −30 °C, over a period of 5 min. The reaction mixture was allowed to warm to room temperature and stirred for 2 h to produce a green-brown-colored solution. Volatiles were removed in vacuo to give a brown oil. The oil was dissolved in pentane and filtered through Celite. The collected dark brown filtrate was dried in vacuo to give a brown solid, which was then dissolved in toluene and left at room temperature for 16 h to produce a dark-red-colored solution, and then the solvent was removed in vacuo to give a bright red oil. The oil was dissolved in a minimal amount of pentane and cooled to −30 °C for 1 week to give bright red crystals of 1 (76 mg, 33% yield). Anal. Calcd for C22H41N3Ta: C, 49.97; H, 7.82; N, 7.95. Found: C, 49.76; H, 7.51; N, 7.86. Cp*Ta[N(iPr)C(NMe2)N(iPr)]Cl3 (6). A solution of N′,N′-diisopropylcarbodiimide (746 mg, 4.60 mmol) in 10 mL of THF, precooled to −30 °C, was added dropwise to a solution of LiNMe2 (235 mg, 4.60 mmol) in 10 mL of THF, precooled to −30 °C. The reaction mixture was allowed to warm to room temperature and stirred for 1 h. The resulting pale yellow solution was then cooled to −30 °C and added dropwise to a solution of Cp*TaCl4 (2.01 g, 4.38 mmol) in 80 mL of THF, precooled to −30 °C, over a period of 5 min. The reaction mixture was allowed to warm to room temperature and stirred for 16 h to produce a red-colored solution, after which time, volatiles were removed in vacuo to give a red solid. The red solid was dissolved in toluene and filtered through a short pad of Celite on a glass frit. The filtrate was concentrated, pentane was layered on top, and then the mixture was cooled to −30 °C to provide red crystals of 6 (2.18 g, 84% yield). Anal. Calcd for C19H35Cl3N3Ta: C, 38.50; H, 5.95; N, 7.09. Found: C, 38.81; H, 5.68; N, 7.22. 1H NMR (400 MHz, benzene-d6): 1.17 (6H, d, CH(CH3), J = 6.7 Hz), 1.88 (6H, d, CH(CH3)2, J = 6.9 Hz), 2.22 (15H, s, C5(CH3)5), 2.32 (6H, s, N(CH3)2), 4.26 (1H, sep, CH(CH3)2, J = 6.7 Hz), 4.57 (1H, sep, CH(CH3)2, J = 6.9 Hz). Cp*Ta[N(iPr)C(NMe2)N(iPr)]Cl2 (7). Sodium amalgam, 0.5% (w/ w) Na/Hg (16.7 g, 3.64 mmol), was added dropwise to a solution of 6 (1.99 g, 3.36 mmol) in 100 mL of Et2O, precooled to −30 °C. The solution was allowed to warm to room temperature and stirred for 16 h to give a dark-orange-colored solution, at which time the volatiles were removed in vacuo to give a brown-orange solid. The solid was dissolved in toluene and filtered through a short pad of Celite on a glass frit. The dark red filtrate was concentrated and cooled to −30 °C to give dark orange crystals of 7 (1.59 g, 85% yield). Anal. Calcd for C19H35N3Cl2Ta: C, 40.94; H, 6.33; N, 7.54. Found: C, 41.11; H, 6.20; N, 7.43. Cp*Ta[N(iPr)C(NMe2)N(iPr)][NH(tBu)]Cl (8). A solution of LiNH(tBu) (123 mg, 1.55 mmol) in 20 mL of Et2O, precooled to

5. Unfortunately, efforts to obtain single crystals of the other isomeric product for a similar analysis have so far been unsuccessful. The above results confirm that each of the metal centers in I can be viewed as having a formal Ta(IV, d1) electronic configuration and that the reaction of I with PhS-SPh likely proceeds via a radical abstraction pathway that leads to formal (nonconcerted) 1,4-addition across the TaN−NTa framework. In the disulfide product, the metal centers in meso-13 are now formally Ta(V, d0) and the N−N bond distance of the bridging group is 1.297(3) Å, which is similar to the values obtained for other CpAm [Ta(V, d0), Ta(V, d0)] dinitrogen derivatives, such as V, N−N 1.307(6) Å; VI, N−N 1.284(4) Å; and {Cp*Ta(Cl)[N(iPr)C(Me)N(iPr)]}2(μ-η1:η1-N2), N−N 1.288(10) Å.7

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CONCLUSION The present report demonstrates the ability to utilize the CpAm and CpGu ligand sets to access, apparently for the first time, mononuclear Ta(IV, d1) terminal imido complexes. Although these complexes were found to be inaccessible using traditional routes of α-abstraction or direct base deprotonation of an amido group, novel routes involving an enamide, amido to amidinate, imido tautomerization in the case of the CpAm derivative 1, and an amido group oxidative hydrogen atom abstraction followed by chemical reduction in the case of the CpGu derivative 2, have now been established. Unfortunately, the lack of reactivity of 1 and 2 toward hydrogenation or hydrosilyation of the TaN bond limits the utility of these compounds as models for similar reactions of group 5 dinuclear dinitrogen complexes. On the other hand, the radical-based chemistry of CpAm and CpGu Ta(IV, d1) imido complexes appears rich and currently unexplored. Importantly, making direct parallels of this chemistry to that of formal [Ta(IV, d1), Ta(IV, d1)](μ-η1:η1-N2) complexes looks promising, and this avenue of investigation may ultimately provide transformations that can aid the objective of developing new classes of catalysts for dinitrogen fixation. Efforts along these lines are currently in progress.

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EXPERIMENTAL SECTION

General Considerations. All manipulations with air- and moisture-sensitive compounds were carried out under an inert atmosphere of N2 or Ar using standard Schlenk-line or glovebox techniques. All solvents were dried (Na for toluene and Na/ benzophenone for pentane, Et2O, and THF) and distilled under N2 prior to use. Benzene-d6 was dried over Na/K alloy and isolated by vacuum transfer. Celite was oven-dried (150 °C for several days), and all amines were dried over CaH2 and distilled by short-path distillation prior to use. Cooling for reactions was achieved using the internal freezer of a glovebox maintained at −30 °C. TEMPO, N,Ndiisopropylcarbodiimide, and LiNMe2 were purchased from SigmaAldrich, and Cp*TaCl4 was purchased from Strem Chemicals. All chemicals were used as received. Cp*Ta[N(iPr)C(Me)N(iPr)]Cl2 (3) was prepared according to previously reported procedures.17 All 1H NMR spectra were recorded at 400 MHz. Solution EPR spectra were recorded at Georgetown University on a JEOL JES-FA200 continuous wave spectrometer equipped with an X-band Gunn oscillator bridge and a cylindrical mode cavity employing a modulation frequency of 100 kHz. All spectra were obtained from freshly prepared toluene solutions (5 mM) in a nitrogen-filled glovebox. Aiso(181Ta) values in gauss were determined by the separation of the middle two lines of the eight-line pattern. Elemental analyses (C, H, and N) were performed by Midwest Microlab, LLC. 352


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−30 °C, was added dropwise to a solution of 7 (823 mg, 1.48 mmol) in 35 mL of Et2O, precooled to −30 °C, over a period of 5 min. The reaction mixture was allowed to warm to room temperature and stirred for 4 h to give a purple-colored solution, at which time the volatiles were removed in vacuo to yield a purple solid. The solid was dissolved in pentane and filtered through a short pad of Celite on a glass frit. The dark purple filtrate was concentrated and cooled to −30 °C to give dark purple crystals of 8 (805 mg, 92% yield). Anal. Calcd for C23H45N4ClTa: C, 46.48; H, 7.64; N, 9.43. Found: C, 46.41; H, 7.39; N, 9.32. Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)]Cl (9). TEMPO (192 mg, 1.23 mmol) was added to a solution of 10 (688 mg, 1.16 mmol) in 30 mL of pentane at room temperature and stirred. The solution turned a transparent orange color, and the volatiles were removed in vacuo to give an orange-white solid, which was then dissolved in a minimal amount of toluene. A small amount of pentane was added, and the mixture cooled to −30 °C to provide white crystals of 9 (632 mg, 92% yield). Anal. Calcd for C23H44N4ClTa: C, 46.58; H, 7.48; N, 9.45. Found: C, 46.77; H, 7.28; N, 9.67. 1H NMR (400 MHz, benzene-d6): 1.21 (3H, d, CH(CH3)2, J = 6.6 Hz), 1.28 (3H, d, CH(CH3)2, J = 6.6 Hz), 1.33 (3H, d, CH(CH3)2, J = 6.6 Hz), 1.41 (3H, d, CH(CH3)2, J = 6.7 Hz), 1.44 (9H, s, C(CH3)3), 2.08 (15H, s, C5(CH3)5), 2.35 (6H, s, N(CH3)2), 3.68 (1H, sep, CH(CH3)2, J = 6.6 Hz), 3.80 (1H, sep, CH(CH3)2, J = 6.6 Hz). Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)] (2). Potassium graphite (KC8) (90 mg, 0.666 mmol) was transferred over a period of 5 min to a solution of 9 (247 mg, 0. 416 mmol) in 15 mL of THF, precooled to −30 °C, by slurrying with 15 mL of THF that had been precooled to −30 °C. The stirred suspension was allowed to warm to room temperature and stirred for an additional 3 h to provide a red-colored solution containing a black suspension. The volatiles were removed in vacuo, and the crude material was taken up in pentane and filtered through a short pad of Celite on a glass frit. The red filtrate was concentrated and cooled to −30 °C overnight, and any unreacted 9 that crystallized out was removed by decanting. The red mother liquor was then concentrated and cooled to −30 °C for three days to provide red crystals of 2 (197 mg, 85% yield). Anal. Calcd for C23H44N4Ta: C, 49.55; H, 7.95; N, 10.05. Found: C, 49.62; H, 7.71; N, 10.02. Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)](Me) (10) and Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)](I) (11). Methyl iodide (6.0 μL, 0.096 mmol) was added to a solution of 2 (54 mg, 0.096 mmol) in 3 mL of pentane at room temperature to provide a clear solution. The volatiles were removed in vacuo to give a colorless solid, and then this crude product was fractionally crystallized in pentane at −30 °C to provide opaque light yellow cocrystals that were enriched with 11 in a 10:90 10:11 ratio (26 mg, 44% yield). The mother liquor was then allowed to slowly evaporate at −30 °C, whereupon white cocrystals enriched with 10 in a 90:10 10:11 ratio were obtained (15 mg, 30% yield). For enriched-10 containing 12 mol % of 11: Anal. Calcd for C23.9H46.6N4I0.1Ta: C, 49.01; H, 8.03; N, 9.57. Found: C, 49.14; H, 7.96; N, 9.50. 1H NMR (400 MHz, benzene-d6): 0.54 (3H, s, TaCH3), 1.12 (6H, d, CH(CH3)2, J = 6.8 Hz), 1.25 (3H, d, CH(CH3)2, J = 6.6 Hz), 1.41 (3H, d, CH(CH3)2, J = 6.6 Hz), 1.42 (9H, s, C(CH3)3), 2.00 (15H, s, C5(CH3)5), 2.35 (6H, s, N(CH3)2), 3.59 (1H, sep, CH(CH3)2, J = 6.8 Hz), 3.78 (1H, sep, CH(CH3)2, J = 6.6 Hz). For enriched-11 containing 12 mol % of 10: Anal. Calcd for C23.1H44.4N4I0.9Ta: C, 41.56; H, 6.69; N, 8.38. Found: C, 41.61; H, 6.50; N, 8.40. 1H NMR (400 MHz, benzene-d6): 1.22 (3H, d, CH(CH3)2, J = 6.7 Hz), 1.24 (3H, d, CH(CH3)2, J = 6.8 Hz), 1.27 (3H, d, CH(CH3)2, J = 6.7 Hz), 1.34 (3H, d, CH(CH3)2, J = 6.8 Hz), 1.52 (9H, s, C(CH3)3), 2.12 (15H, s, C5(CH3)5), 2.29 (6H, s, N(CH3)2), 3.70 (1H, sep, CH(CH3)2, J = 6.8 Hz), 3.86 (1H, sep, CH(CH3)2, J = 6.7 Hz). Cp*Ta[N(iPr)C(NMe2)N(iPr)][N(tBu)](SPh) (12). Diphenyl disulfide (19.9 mg, 0.091 mmol) was added to a stirred solution of 2 (102 mg, 0.182 mmol) in 5 mL of pentane at room temperature. After stirring for 2 min, the colorless solution was concentrated in vacuo and then cooled to −30 °C to provide white crystals of 12 (115 mg, 94% yield). Anal. Calcd for C29H49N4STa: C, 52.24; H, 7.41; N, 8.40. Found: C, 52.32; H, 7.68; N, 8.54. 1H NMR (400 MHz, benzene-d6):

0.91 (3H, d, CH(CH3)2, J = 7.0 Hz), 0.95 (3H, d, CH(CH3)2, J = 6.7 Hz), 1.27 (9H, s, C(CH3)3), 1.30 (3H, d, CH(CH3)2, J = 6.7 Hz), 1.33 (3H, d, CH(CH3)2, J = 7.0 Hz), 2.07 (15H, s, C5(CH3)5), 2.33 (6H, s, N(CH3)2), 4.04 (1H, sep, CH(CH3)2, J = 7.0 Hz), 4.12 (1H, sep, CH(CH3)2, J = 6.7 Hz), 6.96 (1H, m, S(C6H5)), 7.20 (2H, m, S(C6H5)), 8.03 (2H, m, S(C6H5)). {Cp*Ta[N(iPr)C(Me)N(iPr)](SPh)}2(μ-η1:η1-N2) (13). Diphenyl disulfide (24 mg, 0.111 mmol) was added to a solution of {Cp*Ta[N(iPr)C(Me)N(iPr)]}2(μ-η1:η1-N2) (I) (104 mg, 0.111 mmol) in 5 mL of pentane that was precooled to −30 °C. After warming to room temperature and stirring for 5 min, the volatiles were removed in vacuo to provide a mixture of a dark orange oil and a light orange powder (isomer). The mixture was taken up in 5 mL of Et2O and cooled to −30 °C, whereupon the solids were isolated by filtration through a glass-frit. The dark orange filtrate was concentrated in vacuo and then cooled to −30 °C to provide dark orange crystals of meso-13 (33 mg, 25% yield). The light orange solids were collected and washed with first pentane and then Et2O. The solids were then dissolved in a minimal amount of toluene and allowed to precipitate from solution at −30 °C to provide an analytically pure orange powder that is an isomer of 13 (44 mg, 44% yield). For meso-13: Anal. Calcd for C48H74N6S2Ta2: C, 49.65; H, 6.42; N, 7.24. Found: C, 49.15; H, 6.47; N, 7.08. 1H NMR (400 MHz, benzene-d6): 0.67 (6H, d, CH(CH3)2, J = 7.2 Hz), 1.00 (6H, d, CH(CH3)2, J = 7.2 Hz), 1.34 (6H, d, CH(CH3)2, J = 7.0 Hz), 1.43 (6H, d, CH(CH3)2, J = 7.0 Hz), 1.69 (6H, s, N(iPr)C(CH3)N(iPr)), 2.13 (30H, s, C5(CH3)5), 3.86 (2H, sep, CH(CH3)2, J = 7.0 Hz), 4.48 (2H, sep, CH(CH3)2, J = 7.2 Hz), 6.95 (2H, m, S(C6H5)), 7.21 (4H, m, S(C6H5)), 8.07 (4H, m, S(C6H5)). Isomer of 13: Anal. Calcd for C48H74N6S2Ta2: C, 49.65; H, 6.42; N, 7.24. Found: C, 50.14; H, 6.65; N, 7.45. 1H NMR (400 MHz, benzene-d6): 0.67 (6H, d, CH(CH3)2, J = 6.6 Hz), 0.94 (6H, d, CH(CH3)2, J = 6.8 Hz), 1.37 (6H, d, CH(CH3)2, J = 6.6 Hz), 1.64 (6H, br, CH(CH3)2), 1.69 (6H, s, N(iPr)C(CH3)N(iPr)), 2.05 (30H, s, C5(CH3)5), 4.03 (2H, br, CH(CH3)2), 4.41 (2H, br, CH(CH3)2), 7.00 (2H, m, S(C6H5)), 7.14 (4H, m, S(C6H5)), 8.00 (4H, d, S(C6H5), J = 7.6).

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details and X-ray crystallographic reports, including tables of bond lengths, angles, and anisotropic displacement parameters, for complexes 1, 2, 4, 5, 8−12, and meso-13. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS Funding for this work was provided by the DOE (BES-SIS-GR Grant DE-SC0002217), for which we are grateful. We thank Kamille D. Williams and Prof. Timothy H. Warren for recording EPR spectra of compounds 1 and 2 at Georgetown University; they are grateful to the NSF for an award (CHE0840453) to purchase the EPR spectrometer.

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REFERENCES

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Mononuclear Tantalum(IV, d1) Imido Complexes Supported by the

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