New Sandwich and Half-Sandwich Titanium Hydrazido Compounds

ARTICLE pubs.acs.org/Organometallics

New Sandwich and Half-Sandwich Titanium Hydrazido Compounds Jonathan D. Selby,† Marta Feliz,‡ Andrew D. Schwarz,† Eric Clot,*,‡ and Philip Mountford*,† † ‡

Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K. Institut Charles Gerhardt, Universit
e Montpellier 2, CNRS 5253, cc 1501, Place Eugene Bataillon, 34095 Montpellier Cedex 5, France

bS Supporting Information ABSTRACT:

New mono- and bis-cyclopentadienyl terminal titanium hydrazido(2
) compounds were prepared by tert-butyl imide/N,Ndisubstituted hydrazine exchange reactions. Reaction of Cp*Ti(NtBu)Cl(py) (1) with Ph2NNH2 gave the terminal hydrazide Cp*Ti(NNPh2)Cl(py) (4), whereas the corresponding reaction of CpTi(NtBu)Cl(py) gave the dimer Cp2Ti2(μ-η1:η1-NNPh2)(μ-η2:η1-NNPh2)Cl2. Reaction of 1 with Me2NNH2 (1 equiv) also gave a dimer, Cp*2Ti2(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)Cl2 (8), while the reaction with 2 equiv of Me2NNH2 gave Cp*Ti(η2-NHNMe2)2Cl (7) containing two η2-bound hydrazide(1
) ligands. Formation of 7 and 8 proceeds via a common intermediate, Cp*Ti(NHtBu)(η2-NHNMe2)Cl, observed by NMR spectroscopy. Reaction of 4 with LiNHNPh2 gave the mixed hydrazide(2
)/hydrazide(1
) derivative Cp*Ti(NNPh2)(NHNPh2)(py) (10). The corresponding reaction of 1 formed Cp*Ti(NtBu)(NHNPh2)(py), which rearranged to Cp*Ti(NHtBu)(NNPh2)(py). The titanocene derivative Cp2Ti(NNPh2)(py) (14) was prepared by reaction of Cp2Ti(NtBu)(py) (13) with Ph2NNH2, whereas the corresponding reaction with Me2NNH2 gave mixtures including CpTi(NHtBu)(μ-η1:η1-NNMe2) (μ-η2:η1-NNMe2)TiCp(η1-Cp). The electronic structure of 14 was investigated by DFT and compared to that of the imido complex 13. Whereas the HOMO of the formally 20 valence electron compound 13 is a ligand-centered orbital based both on the Cp rings and on the imido N, in 14 this is the HOMO
1 and one of the TidNR π-bonding MOs is the HOMO, destabilized by an NR
Nβ antibonding interaction.

’ INTRODUCTION A wide range of titanium imido compounds (L)TidNR (R = alkyl or aryl) have now been synthesized and studied in detail with regard to their structures, bonding, and reactivity, as summarized in a series of reviews and articles.1,2 In contrast, the apparently related hydrazido complexes (L)TidNNR2 were hardly developed at all until relatively recently, although they have, for example, been postulated as intermediates in hydrohydrazination catalysis.3 The first report of a terminal titanium hydrazide was by Wiberg4 for the synthesis of Cp2Ti{NN(SiMe3)2} from Cp2TiCl2 and Me3SiNNSiMe3, a reagent that decomposes above
35 °C. Subsequently we found that Ti(NtBu)(Me4taa) reacted with Ph2NNH2 to give Ti(NNPh2)(Me4taa) (H2Me4taa = tetramethyl dibenzotetraaza[14]annulene).5 However, the first structurally characterized examples only appeared in 2004 and 2005,3c,6 and to date relatively few classes of titanium hydrazido compound are known. Nonetheless, compounds containing the terminal TidNNR2 group (and their zirconium counterparts7) show a range of exciting reactivity8 involving both saturated and unsaturated substrates. This involves not only addition to the polar and unsaturated TidNR multiple r 2011 American Chemical Society

bond but also insertion into the TidNR and NR
Nβ bonds and/or NR
Nβ bond cleavage.3f,g,5,9 Apart from Wiberg’s initial report of Cp2Ti{NN(SiMe3)2}, no titanocene hydrazide has been reported, although a structurally characterized zirconium example is known.7a Two reports of half-sandwich terminal group 4 hydrazides have appeared containing additional chelating N,N0 -donor groups.9a,l Here we report our recent results in the area of titanium hydrazido chemistry, targeting new sandwich and half-sandwich complexes. We hope that the new compounds reported herein will provide a platform for further future reactivity studies as well as aiding in understanding the differences between hydrazido and imido compounds.

’ RESULTS AND DISCUSSION Half-Sandwich Compounds. We have previously shown that the half-sandwich tert-butyl imido complexes CpRTi(NtBu)Received: January 25, 2011 Published: March 31, 2011 2295

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Scheme 1. Reactions of CpRTi(NtBu)Cl(py) (CpR = Cp* (1) or Cp (2)) with Ph2NNH2

Figure 1. Displacement ellipsoid plot (25% probability) of Cp*Ti(NNPh2)Cl(py) (4). H atoms are omitted.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for Cp*Ti(NNPh2)Cl(py) (4)a Cl(py) (CpR = Cp* (1) or Cp (2)) can be readily prepared from Ti(NtBu)Cl2(py)n (n = 2 or 3) and either LiCp* or NaCp.10 Unfortunately, the corresponding reactions of the known9c Ti(NNPh2)Cl2(py)3 (3) with these metalated reagents gave unknown mixtures. As an alternative approach we used the imido compounds 1 and 2 themselves and Ph2NNH2 in an imide/ hydrazine exchange protocol,5,9a,9c,9g as shown in Scheme 1. Similar approaches have been used to prepare half-sandwich titanium aryl imido complexes.11 Reaction of 1 with Ph2NNH2 in benzene at room temperature gave the monomeric half-sandwich compound Cp*Ti(NNPh2)Cl(py) (4) in 83% isolated yield. When followed by 1H NMR spectroscopy in C6D6, the conversion was quantitative and the expected tBuNH2 side-product was observed. No intermediates (e.g., an amido-hydrazido(1
) species of the type Cp*Ti(NHtBu)(NHNPh2)) were observed. The 1H and 13C NMR spectra of 4 show the expected resonances for Cp*, diphenyl hydrazido, and pyridine ligands. Diffraction-quality crystals were grown from a saturated pentane solution. The solid-state structure is shown in Figure 1, and selected bond distances and angles are listed in Table 1. Figure 1 confirms 4 as a pseudotetrahedral terminal hydrazido(2
) compound. The geometry is very similar to that of 1 itself, and a number of aryl imido analogues Cp*Ti(NAr)Cl(py) (Ar = 2,6-C6H3Br2, 2,6-C6H3iPr2, 2-C6H4tBu, 2-C6H4iPr).11b The Ti
Cp* centroid, Ti
Cl, and Ti
Npy distances are comparable to those of Cp*Ti(NAr)Cl(py) but slightly shorter than in 1 due to the higher labilizing ability of the tert-butyl imido ligand.1e,13 The main point of interest is the terminal TidN
NPh2 ligand, 10 examples of which have now been structurally characterized according the current Cambridge Structural Database.14 The TidNR and NR
Nβ distances of 1.734(2) and 1.368(3) Å, respectively, are equivalent within error to the known mean values for this linkage and, by analogy with recent DFT calculations, indicative of a σ2π4 triple bond and formally dianionic [NNPh2]2
ligands.6,9c,9e The TidNR distance in 4 is within the range of TidNAr distances in Cp*Ti(NAr)Cl(py) (1.730(2)
1.753(2) Å), but considerably longer than the TidNtBu bond in 1 (av 1.697(3) Å). Similar trends in TidN distances for alkyl and

a

Ti(1)
N(1) Ti(1)
Cl(1)

1.734(2) 2.3532(9)

Ti(1)
N(3) Ti(1)
Cpcent

2.179(2) 2.043

N(1)
N(2)

1.368(3)

N(2)
C(1)

1.443(4)

N(2)
C(7)

1.411(4)

Cpcent
Ti(1)
N(1)

121.1

Cpcent
Ti(1)
N(3)

111.5

Cpcent
Ti(1)
Cl(1)

116.6

Ti(1)
N(1)
N(2)

163.5(2)

N(1)
N(2)
C(1)

115.8(2)

N(1)
N(2)
C(7)

119.7(2)

C(1)
N(2)
C(7)

122.0(2)

Cpcent is the computed Cp* ring carbon centroid.

aryl imido and diphenyl hydrazido homologues have been noted previously.6 The reaction between CpTi(NtBu)Cl(py) (2) and Ph2NNH2 was assessed on the NMR tube scale in C6D6 (Scheme 1). Immediate conversion to the known15 hydrazide-bridged compound Cp2Ti2(μ-η1:η1-NNPh2)(μ-η2:η1-NNPh2)Cl2 (5) and free pyridine was observed. Compound 5 was previously prepared by reaction of CpTiCl3 with Ph2NNH2 and BuLi. Addition of an excess of pyridine to solutions of 5 failed to convert it to a monomeric analogue of 4, in contrast to our previous observations for Cp2Ti2(μ-NtBu)2Cl2, which can be converted to 2 in this way.10 That 2 reacts with Ph2NNH2 to form dimeric 5 whereas Cp*Ti(NtBu)Cl(py) (1) forms monomeric 4 is readily attributed to greater steric protection afforded by the C5Me5 ligand. The reactions of 1 with Me2NNH2 are summarized in Scheme 2. The stoichiometric (1:1) reaction following mixing of the imide and hydrazine in toluene or benzene at room temperature proceeded over two to three days to form the hydrazide-bridged dimer Cp*2Ti2(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)Cl2 (8). Formation of 8 was accompanied by several side-products including the mono(hydrazido(1
)) species Cp*Ti(η2-NHNMe2)Cl2 (9) and the bis(hydrazido(1
)) Cp*Ti(η2-NHNMe2)2Cl (7), indicative of redistribution reactions. Compound 9 was independently prepared from Cp*TiCl3 and LiNHNMe2 and structurally characterized (see the SI for details). The homologue CpTi(η2-NHNMe2)Cl2 has been reported previously.16 Compound 7 is discussed further below. Preliminary NMR tube scale reactions 2296

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Scheme 2. Reactions of Cp*Ti(NtBu)Cl(py) (1) with Me2NNH2

Table 2. Selected Bond Lengths (Å) and Angles (deg) for Cp*2Ti2(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)Cl2 (8)a Ti(1)
N(1) Ti(1)
N(3)

1.903(2) 1.829(2)

Ti(2)
N(1) Ti(2)
N(3)

2.003(2) 2.010(2) 2.217(2)

Ti(1)
Cl(1)

2.3600(5)

Ti(2)
N(4)

Ti(1)
Cpcent(1)

2.077

Ti(2)
Cl(2)

2.3271(5)

N(1)
N(2)

1.393(2)

Ti(2)
Cpcent(2)

2.077

N(3)
N(4)

1.384(2)

Cpcent(1)
Ti(1)
N(1) 126.1

Cpcent(2)
Ti(2)
N(1) 114.8

Cpcent(1)
Ti(1)
N(3) 120.2

Cpcent(2)
Ti(2)
N(3) 132.7

Cpcent(1)
Ti(1)
Cl(1) 111.6 N(1)
N(2)
C(1) 112.8(2)

Cpcent(2)
Ti(2)
N(4) 115.8 Cpcent(2)
Ti(2)
Cl(2) 111.8

N(1)
N(2)
C(2)

113.8(2)

N(3)
N(4)
C(3)

N(3)
N(4)
C(4)

127.65(12)

115.7(2)

a

Figure 2. Displacement ellipsoid plot (30% probability) of Cp*2Ti2(μη1:η1-NNMe2)(μ-η2:η1-NNMe2)Cl2 (8). H atoms are omitted.

between Me2NNH2 and 2 in C6D6 showed the formation of a pyridine-free product assumed to be analogous to 5 and 8. This compound was not pursued on the preparative scale. Carrying out the initial addition of Me2NNH2 to 1 at
78 °C in toluene followed by workup after three days and recrystallization from benzene gave pure 8 in 50% isolated yield. The solidstate structure is shown in Figure 2, and selected bond lengths and angles are listed in Table 2. Figure 2 confirms the pyridinefree, dimeric nature of 8. There are two types of bridging hydrazide ligands: one is bound in a μ-η1:η1-manner, bridging only through NR (i.e., N(1)); the other has μ-η2:η1-coordination with both NR and Nβ (i.e., N(3) and N(4)) bonded to the second titanium. This overall coordination mode is analogous to that in 5 and also Bergman’s Cp4Zr2(μ-η1:η1-NNPh2)(μ-η2:η1-NNPh2).7a The second common arrangement is for both bridging ligands to adopt a μ-η2:η1-NNR2 coordination mode.3c,6 At room temperature the 1 H and 13C NMR spectra show a time-averaged spectrum with equivalent Cp* and β-NMe2 groups. On cooling to
30 °C in toluene-d8, two separate sets of signals for these groups were observed, consistent with the solid-state structure. The fluxional

Cpcent(1) and Cpcent(2) are the computed Cp* ring centroids for Ti(1) and Ti(2), respectively.

process most likely involves a “windscreen wiper” type mechanism17 and an approximately C2h symmetric transition state where both hydrazide ligands are bound in a μ-η1:η1-manner. Formation of 8 presumably proceeds via transiently formed Cp*Ti(NNMe2)Cl, which then dimerizes rather than being trapped by pyridine as in the case of 4 above. Addition of extra pyridine or DMAP (4-dimethylaminopyridine) to the reaction mixture did not lead to a base-stabilized monomer. Likewise, 8 could not be cleaved by the addition of pyridine or DMAP. Terminal TidNNMe2 groups are still relatively uncommon for the group 4 metals because of the more nucleophilic nature of the β-nitrogen when stabilizing phenyl substituents are absent. Nonetheless, a number of terminal dimethyl hydrazido compounds have now been structurally characterized for titanium where polydentate chelating ligands have been employed.3c,g,9a,b,g The most relevant example in this context is Cp*Ti{MeC(NiPr)2}(NNMe2), prepared from the corresponding tert-butyl imido compound and Me2NNH2,9a in which the amidinate ligand prevents dimerization. Monitoring the reaction between 1 and Me2NNH2 in C6D6 showed the intermediate formation of Cp*Ti(NHtBu)(η22297

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Table 3. Selected Bond Lengths (Å) and Angles (deg) for Cp*Ti(η2-NHNMe2)2Cl (7)a

a

Figure 3. Displacement ellipsoid plot (25% probability) of Cp*Ti(η2-NHNMe2)2Cl (7). Carbon-bound H atoms are omitted. Nitrogenbound H atoms are drawn as spheres of arbitrary radius.

NHNMe2)Cl (6) and free pyridine after one hour (Scheme 2). Over two to three days the signals for 6 were replaced by those for 8 and tBuNH2. Attempts to isolate 6 on a preparative scale were unsuccessful and led to mixtures, among which 8 was the major product. The structural assignment of 6 as a mixed amidehydrazide(1
) compound is based on one- and two-dimensional 1 H and 13C NMR spectra. For example, the CMe3 quaternary carbon appears at δ13C = 58.9 ppm, consistent18 with an amido rather than an imido group (for TidNtBu these invariably appear in the range 70 ( 2
3 ppm1b,e) and which correlates with just one of the two NH resonances at δ1H = 5.40 and 5.10 ppm. The η2 coordination of the NHNMe2 ligand is proposed by analogy with the structurally authenticated related compounds CpRTi(η2-NHNMe2)Cl2 (CpR = Cp or Cp* (9))16 and Cp*Ti(η2-NHNMe2)2Cl (7). As mentioned, compound 7 was observed in minor amounts in the 1:1 stoichiometric reaction of Cp*Ti(NtBu)Cl(py) (1) with Me2NNH2. Changing the stoichiometry to 1:2 afforded 7 as a pale yellow solid in 62% isolated yield. The 1H NMR resonances for the NH and NMe2 moieties of 7 are close to those for the η2-NHNMe2 group in 6. When the reaction was performed in C6D6 on the NMR tube scale, 7 was the sole organometallic product, and the expected free pyridine and tBuNH2 side products were also observed. Compound 7 is stable in solution at room temperature for at least several days. However, heating an NMR sample in C6D6 at 70 °C for 16 h gave irreversible conversion to dimeric 8 and free Me2NNH2. Diffraction-quality crystals of 7 were grown by slowly cooling a saturated hexane solution. The molecular structure is shown in Figure 3, and selected distances and angles are listed in Table 3. The solid-state structure of 7 is fully consistent with 1H NMR data assuming fast exchange of the β-NMe2 groups. The Ti
Cp* centroid and Ti
Cl distances are somewhat longer than their counterparts in either 4 or 9, reflecting the higher coordination number of 7. The two η2-hydrazido(1
) ligands are arranged in an approximate “paddlewheel” geometry, with the [Ti(1), N(3), N(4)] plane lying approximately perpendicular to the [Cp* centroid, Ti(1), N(1), N(2)] plane. This differing orientation has an influence on the Ti
NR and Ti
Nβ distances. Thus Ti(1)
N(1) (1.882(2) Å) is shorter than Ti(1)
N(3) (2.005(2) Å), whereas Ti(1)
N(2) (2.304(2) Å) is longer than

Ti(1)
N(1)

1.882(2)

Ti(1)
N(2)

2.304(2)

Ti(1)
N(3)

2.005(2)

Ti(1)
N(4)

2.157(2)

Ti(1)
Cl(1)

2.4437(8)

Ti(1)
Cp*cent

2.094

N(1)
N(2)

1.411(3)

N(3)
N(4)

1.420(3)

N(1)
H(1)

0.86(4)

N(3)
H(2)

0.86(4)

Cpcent
Ti(1)
N(1)

105.5

Cpcent
Ti(1)
N(2)

143.1

Cpcent
Ti(1)
N(3)

110.4

Cpcent
Ti(1)
N(4)

124.4

Cpcent
Ti(1)
Cl(1) Ti(1)
N(1)
N(2)

108.8 87.57(15)

Ti(1)
N(1)
H(1) N(2)
N(1)
H(1)

150(2) 116(2)

Ti(1)
N(3)
H(2)

126(2)

Ti(1)
N(3)
N(4)

75.94(14)

N(4)
N(3)
H(2)

110(3)

Cpcent is the computed Cp* ring carbon centroid.

Ti(1)
N(4) (2.157(2) Å). Five-legged piano stool complexes are relatively unusual. It appears that the compact and chelating nature of NHNMe2 permits the higher coordination number. Compound 7 is the second example of a structurally authenticated bis(η2-hydrazido(1
)) species.3c It is not clear why it and the related 6 do not preferentially exist as the hydrazido (2
)-amino isomeric alternatives Cp*Ti(NNMe2)(H2NR)Cl (R = NMe2 or tBu, respectively), which would be the analogues of 4. In this context we note Winter’s group 5 dialkyl hydrazido(2
)/dialkyl hydrazine complexes [M(NNR2)Cl2(η2-H2NNR2) (TMEDA)]þ (M = Nb or Ta, NR2 = NMe2 or NC5H10).19 A number of imido-amino complexes of the type (L)Ti(NtBu)(H2NtBu) have been structurally characterized.14,20 To gain a better understanding of the structural relationships between group 4 hydrazido(2
) and hydrazido(1
) ligands, the compound Cp*Ti(NNPh2)(NHNPh2)(py) (10) was prepared by the reaction of Cp*Ti(NNPh2)Cl(py) (4) with LiNHNPh2 (eq 1). For comparison, the mixed hydrazido(2
)/tert-butyl amido derivative Cp*Ti(NNPh2)(NHtBu)(py) (11) was synthesized by the same route.

Both compounds have been structurally characterized, and the molecular structures are presented in Figure 4. Selected distances and angles are given in Tables 4 and 5, respectively. The solution NMR data are consistent with the solid-state structures. For 11 the NHCMe3 hydrogen correlates with the CMe3 quaternary carbon which appears at δ13C = 56.3 ppm, consistent with an amido group being preserved in solution (see below for the independently prepared hydrazido(1
)/tert-butyl imido isomer Cp*Ti(NtBu)(NHNPh2)(py) (12)). There are only three structurally authenticated η1-hydrazido(1
) compounds for group 47c,21 and none for titanium. Of these, one is for a η1-NHNPh2 ligand, namely, Zr(N2*Npy)(NHNPh2)Cl (NR
Nβ = 1.427(8) Å; N2*Npy = (2-NC5H4)CMe(CH2NSiMe2tBu)2).7c Two examples of a mixed η1-hydrazido(2
)/η1-hydrazido(1
) complex have been reported, namely, Mo(NNPh2)(NHNPh2)(acac)Cl2 and [Mo(NNPh2)(NHNPh2)Cl4]
.22,23 The ModNR and 2298

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Figure 4. Displacement ellipsoid plot (20% probability) of Cp*Ti(NNPh2)(NHNPh2)(py) (10, left) and Cp*Ti(NNPh2)(NHtBu)(py) (11, right). Carbon-bound H atoms are omitted. H(1) is drawn as a sphere of arbitrary radius.

Table 4. Selected Bond Lengths (Å) and Angles (deg) for Cp*Ti(NNPh2)(NHNPh2)(py) (10)a

a

Table 5. Selected Bond Lengths (Å) and Angles (deg) for Cp*Ti(NNPh2)(NHtBu)(py) (11)a

Ti(1)
N(1)

1.742(2)

N(2)
C(7)

1.388(4)

Ti(1)
N(1)

1.741(3)

N(1)
N(2)

1.379(4)

Ti(1)
N(3)

1.976(2)

N(3)
N(4)

1.415(3)

Ti(1)
N(3)

1.950(3)

N(2)
C(1)

1.433(4)

Ti(1)
N(5)

2.201(2)

N(3)
H(1)

0.94(4)

Ti(1)
N(4)

2.202(3)

N(2)
C(7)

1.380(5)

Ti(1)
Cpcent

2.080

N(4)
C(13)

1.419(4)

Ti(1)
Cpcent

2.075

N(3)
H(1)

0.79(5)

N(1)
N(2)

1.369(3)

N(4)
C(19)

1.388(3)

Cpcent
Ti(1)
N(1)

121.9

Ti(1)
N(3)
C(13)

138.2(3)

N(2)
C(1)

1.435(4)

Cpcent
Ti(1)
N(3)

114.9

N(1)
N(2)
C(1)

115.9(3)

Cpcent
Ti(1)
N(1) Cpcent
Ti(1)
N(3)

122.6 114.1

Ti(1)
N(3)
N(4) Ti(1)
N(3)
H(1)

Cpcent
Ti(1)
N(4) Ti(1)
N(1)
N(2)

112.3 165.5(2)

N(1)
N(2)
C(7) C(1)
N(2)
C(7)

120.8(3) 122.6(3)

Cpcent
Ti(1)
N(5)

112.6

N(4)
N(3)
H(1)

112.(2)

Ti(1)
N(3)
H(1)

115(3)

Ti(1)
N(1)
N(2)

163.4(2)

N(3)
N(4)
C(13)

116.2(2)

a

N(1)
N(2)
C(1)

115.6(2)

N(3)
N(4)
C(19)

119.4(2)

N(1)
N(2)
C(7)

121.5(2)

C(13)
N(4)
C(19)

124.2(2)

C(1)
N(2)
C(7)

121.5(2)

and Ti
NHNPh2 (0.234(3) Å) and NR
Nβ (0.046(4) Å) bond lengths in 10 are comparable to the corresponding differences for Mo(NNPh2)(NHNPh2)(acac)Cl2 and [Mo(NNPh2)(NHNPh2)Cl4]
.22,23 However, the absolute values of the NR
Nβ distances in the ModNNPh2 (av 1.309 Å) and ModNHNPh2 (av 1.358 Å) functional groups are significantly smaller than in 10, consistent with the diminished reducing ability of the later metal, as it approaches its highest formal oxidation state.24

128.23(19) 120.(2)

Cpcent is the computed Cp* ring carbon centroid.

Mo
NR distances differ by ca. 0.20 Å, and the NR
Nβ distances by ca. 0.050 Å, with those for the ModNNPh2 ligand being the shorter in each case. The metric parameters associated with the TidNNPh2 moieties in 10 and 11 are equivalent to those of Cp*Ti(NNPh2)Cl(py) (4) within error, although there is an apparent trend toward longer TidNR and NR
Nβ distances, as would be expected since the NHNPh2 and NHtBu ligands are better σ and π donors than Cl. The Ti
NHNPh2 and Ti
NHtBu distances are comparable and longer than the TidNR counterparts and lie within the expected ranges for titanium amide-type complexes.14 The Ti
NHR bond in 11 is a little shorter than in 10, consistent with the better donor ability of NH tBu. The hydrazido(1
) ligand NR
Nβ distance of 1.415(3) Å in 10 is significantly longer than that for the hydrazido(2
) ligands in 4, 10, or 11 (or for previously characterized TidNNPh2 compounds in general (av 1.360(7) Å)14) and is the same within error as that in Zr(N2*Npy)(NHNPh2)Cl (NR
Nβ = 1.427(8) Å) and also in Ph2NNH2 itself (1.418(2) Å).9b We have discussed the nature of the residual NR
Nβ multiple bond in TidNNR2 systems elsewhere.9c,l These trends in TidN, Ti
N, and NR
Nβ distances are consistent with the NNPh2 and NHNPh2 ligands being highly reduced 2
and 1
ligands as expected. The difference between the TidNNPh2

Cpcent is the computed Cp* ring carbon centroid.

As mentioned, Cp*Ti(NNPh2)(NHtBu)(py) (11) is stable in solution as the hydrazido(2
)/tert-butyl amido isomer, suggesting that the hydrazido(1
)/tert-butyl imido alternative is thermodynamically unfavored. To probe this further, we prepared 2299

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Organometallics

Figure 5. Optimized geometry of TS-12-11.

the alternative isomer Cp*Ti(NtBu)(NHNPh2)(py) (12) from LiNHNPh2 and Cp*Ti(NtBu)Cl(py) (1) in C6D6 according to eq 2. The quaternary TidNCMe3 resonance of 12 appears at δ13C = 67.1 ppm, consistent with an imido group. Upon formation, compound 12 starts to isomerize to 11, and after several days the conversion is complete (this process can be accelerated by heating to 70 °C). Attempts to isolate 12 on a preparative scale were unsuccessful. The conversion of 12 to 11 is consistent with the reaction of 1 with Ph2NNH2 to form 4 and tBuNH2. Unfortunately the conversion of 12 to 11 was not sufficiently clean at temperatures suitable for practical kinetic measurements. DFT calculations on models of 12 and 11 found that the latter was 5.7 kcal mol
1 more stable in terms of electronic energies. The Gibbs free energy difference, ΔG =
5.0 kcal mol
1, further confirms 11 as the most stable isomer. Qualitatively, the rate of reaction did not appear to be significantly affected by additional pyridine. To probe further the implication that H atom migration takes place without pyridine dissociation, DFT calculations were performed on the two possible pathways (i.e., with or without pyridine dissociation prior to H atom transfer). These found a transition state (TS) for H transfer with pyridine still coordinated to Ti (TS-12-11, see Figure 5) at ΔG = 31.8 kcal mol
1 above 12, consistent with the slow experimental rate of conversion of 12 to 11. The corresponding reaction pathway proceeding via pyridine-free Cp*Ti(NtBu)(NHNPh2) (12-py), which lies at ΔG = 6.5 kcal mol
1 above 12, had a much higher energy rate-limiting TS, lying at 40.7 kcal mol
1 with respect to 12 (i.e., ca. 9 kcal mol
1 higher than TS-12-11). The DFT calculations thus indicate that the preferred mechanism keeps the pyridine coordinated all along the transformation. Sandwich Compounds. As mentioned, attempts to prepare cyclopentadienyl titanium hydrazido complexes by reaction of Ti(NNPh2)Cl2(py)3 (3) with LiCp* or NaCp gave mixtures of unknown products. We therefore again chose a tert-butyl imide/ hydrazine exchange strategy to prepare titanium hydrazido sandwich complexes. The new chemistry is summarized in Scheme 3. Reaction of Cp2Ti(NtBu)(py) (13) with Ph2NNH2 formed the desired titanocene-hydrazido compound Cp2Ti(NNPh2)(py) (14). This reaction is rather sensitive to the conditions, being best carried out in near-frozen (5 °C) benzene with careful addition of Ph2NNH2 and rapid stirring. At the correct concentration, compound 14 separates from solution in 58% yield. Longer reaction times or higher temperatures lead to lower yields and more side-reactions. In the presence of an excess of Ph2NNH2 14

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Figure 6. Displacement ellipsoid plot (25% probability) of one of the four crystallographically independent molecules of Cp2Ti(NNPh2)(py) (14). H atoms are omitted.

Scheme 3. Reactions of Cp2Ti(NtBu)(py) (13) and Cp*CpTi(NtBu)(py) (16) with Ph2NNH2 and Me2NNH2

also undergoes rapid decomposition. Once isolated, 14 is stable in solution for at least several days. Diffraction-quality crystals were grown by slow diffusion of pentane into a saturated toluene solution of 14. The molecular structure is shown in Figure 6, and selected distances and angles are given in Table 6. The solution NMR and other data are consistent with the solid-state structure, which is discussed in detail below along with bonding considerations. 2300

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Table 6. Selected Bond Lengths (Å) and Angles (deg) for Cp2Ti(NNPh2)(py) (14)a

Table 7. Selected Bond Lengths (Å) and Angles (deg) for CpTi(NHtBu)(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)TiCp(η1-Cp) (15)a

Ti(1)
N(1)

1.757(2)

N(1)
N(2)

1.351(3)

Ti(1)
N(3)

2.266(2)

N(2)
C(1)

1.429(3)

Ti(1)
N(1)

1.9094(18) Ti(2)
N(2)

1.8163(16)

Ti(1)
Cpcent(1)

2.21

N(2)
C(7)

1.418(3)

Ti(1)
N(2) Ti(1)
N(3)

2.0362(16) Ti(2)
N(4) 2.1980(17) Ti(2)
C(19)

1.8589(15) 2.293(2) 2.06

Ti(1)
Cpcent(2)

2.15

Cpcent(1)
Ti(1)
N(1)

113.1

N(1)
Ti(1)
N(3)

89.5(1)

Ti(1)
N(4)

2.0395(15) Ti(2)
Cpcent(2)

Cpcent(2)
Ti(1)
N(1)

115.1

Ti(1)
N(1)
N(2)

175.9(2)

Ti(1)
Cpcent(1)

2.08

N(1)
H(1)

0.81(4)

Cpcent(1)
Ti(1)
N(3)

104.8

N(1)
N(2)
C(1)

119.5(2)

N(2)
N(3)

1.375(2)

N(4)
N(5)

1.414(2)

Cpcent(2)
Ti(1)
N(3) Cpcent(1)
Ti(1)
Cpcent(2)

102.8 123.8

N(1)
N(2)
C(7) C(1)
N(2)
C(7)

119.2(2) 121.0(2)

Cpcent(1)
Ti(1)
N(1) 117.3

Cpcent(2)
Ti(2)
N(2) 122.9

Cpcent(1)
Ti(1)
N(2) 129.8

Cpcent(2)
Ti(2)
N(4) 121.7

Cpcent(1)
Ti(1)
N(3) 112.7

Cpcent(2)
Ti(2)
C(19) 117.9

Cpcent(1)
Ti(1)
N(4) 112.8 Ti(1)
N(4)
Ti(2) 93.73(7)

Ti(1)
N(2)
Ti(2)

a

Cpcent(1) and Cpcent(2) are the computed Cp ring carbon centroids for “upper” and “lower” Cp rings, respectively, as depicted in Figure 6.

95.15(7)

a Cpcent(1) and Cpcent(2) are the computed Cp ring carbon centroids for Ti(1) and Ti(2), respectively.

Figure 7. Displacement ellipsoid plot (20% probability) of CpTi(NH tBu)(μ-η1 :η1-NNMe2 )(μ-η2 :η1 -NNMe 2)TiCp(η1 -Cp) (15). Carbon-bound H atoms are omitted. H(1) is drawn as a sphere of arbitrary radius.

Reaction of 13 with Me2NNH2 on the NMR tube scale in C6D6 immediately gave a number of products. Scaling up in benzene at 5 °C gave the binuclear complex CpTi(NHtBu)(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)TiCp(η1-Cp) (15) in 11% yield among other species. Pure 15 decomposes fully over two days in solution to an unknown mixture of compounds. The molecular structure is shown in Figure 7, and selected distances and angles are listed in Table 7. Molecules of 15 feature quite distinct environments at the two titaniums. Ti(1) is bound to a η5-Cp and tert-butyl amido ligand. It is also bonded to the NR atoms of two bridging NNMe2 ligands and additionally to the Nβ atom of one of them. Ti(2) is ligated by both an η5- and an η1-Cp ligand and also to the NR atoms of the bridging NNMe2 ligands. The solution NMR data for 15 are consistent with the solid-state structure, showing resonances for a NHtBu and two equivalent NNMe2 ligands, along with three 5H intensity singlets for the different Cp ligands. Cooling to
70 °C in toluene-d8 did not lead to decoalescence of any of the Cp ligand singlets, showing that the sigmatropic shifts that average the environments for C(19)
C(23) have a low energy barrier. The overall geometry of 15 is reminiscent of that of Cp*2Ti2(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)Cl2 (8), with Cp taking the place of Cp*, and the NHtBu and η1-Cp groups substituting for the Cl ligands. Compound 15 can also be viewed as an adduct between a (ηx-Cp)2Ti(NNMe2) and CpTi(NNMe2)(NHtBu)

moiety. The former is a base-free analogue of 14, whereas the latter is related to Cp*Ti(NNPh2)(NHtBu)(py) (11). The presence of the CpTi(NHtBu) moiety in 15 is an indication that protonolysis by Me2NNH2 can occur both at a Ti
C bond of 13 (or of an intermediate Cp2Ti(NNMe2)-type species) and at the TidNimide bond. This chemistry is probably a manifestation of the “π-loaded”25 nature of compounds of the type Cp2Ti(NR) (R = tBu or NR2), as discussed below, which leads to incomplete donation of ligand electron density to the metal centers and an increase in the Brønsted basicity of the bonded atoms. In an attempt to control the reactions of 13 with hydrazines, we turned to the slightly bulkier Cp*CpTi(NtBu)(py) (16).10 Unfortunately, reaction with Me2NNH2 in C6D6 gave a complex mixture of unknown products. The corresponding reaction with Ph2NNH2 also formed a mixture among which could be identified CpH and Cp*Ti(NNPh2)(NHNPh2)(py) (10), formed by protonolysis of the Cp ligand in 16. Bergman has reported some related observations in the reactions of Cp2TiMe2 with anilines leading to CpTi(NAr)(NHAr)(L).26 The other products formed could not be identified, and a scale-up reaction was not attempted. The reactions of 16 reflect the delicate balance of competing reaction pathways in these π-loaded systems. Molecular Structure and Bonding Considerations for Cp2Ti(NNPh2)(py) (14). As shown in Figure 6, molecules of 14 contain a bent Cp2Ti moiety coordinated to a terminal NNPh2 ligand and a pyridine donor. Compound 14 is the first structurally characterized titanocene hydrazide and is a member of the wider family of metal
ligand multiply bonded group 4 metallocene complexes of the type CpR2M(E)(L) (M = Ti, Zr, Hf; E = NR, PR, O, S, Se, Te; L = Lewis base).10,27 The Cp rings are η5-coordinated to the metal with Ti
C bonds in the range 2.423(3)
2.591(3) Å. The Ti
Cp centroid distances (av 2.18 Å) are comparable to those in 13 (av 2.22 Å) but ca. 0.1 Å longer than those found in general for titanocene complexes of the type CpR2TiX2 (X = Cl, alkyl).14 The TidN
NPh2 linkage is approximately linear (175.9(2) Å), indicating sp hybridization at NR. The environment at Nβ is trigonal planar, as is invariably found for diphenyl hydrazido(2
) ligands due to stabilization of the Nβ-based lone pair by conjugation with the phenyl rings.9c,e,14 The TidNR bond length of 1.757(2) Å is equal within error to that in the π-loaded diamide-amine complex Ti(N2Npy)(NNPh2)(py) (1.759(2) Å, N2Npy = (2-NC5H4)CMe(CH2NSiMe3)2), which is the longest recorded to date.9c It is 2301

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Figure 8. (Left) The four highest occupied MOs of a C2v symmetric dianion [NNR2]2
with the π-donor orbitals highlighted in red. (Middle) Correct orientation of the NNPh2 ligand for optimum match between πv and the frontier MOs of a Cp2Ti(py) fragment. (Right) Out-of-plane πh orbital that must compete with the Cp2 SALCs for a 3dπ acceptor AO at titanium.

significantly longer than in Cp*Ti(NNPh2)Cl(py) (4) and also longer than the TidNtBu bond length of 1.723(6) Å in 13 itself. The phenyl substituents of NNPh2 are oriented “up” and “down” with respect to the equatorial plane that contains Ti, NR, and Npy, as quantified by the Cipso
Nβ 3 3 3 Ti
Npy dihedral angles of 85° and 88o. This orientation of the Nβ substituents in 14 is analogous to that found in Cp2Zr(NNPh2)(DMAP) (17),7a Cp2Nb(NNMe2)Cl (18),28 and Cp2Ta(NNPh2)Cl (19).29 It is also reminiscent of the TadCHPh moiety in Cp2Ta(CHPh)CH2Ph30 and related metallocene- and metallocene-like alkylidene compounds.18,31 The orientation of the alkylidene ligand in these systems is well understood in terms of matching both the σdonor AO and (single) 2pπ bonding AO of CHPh with the frontier MOs of a bent Cp2M fragment.32 These three MOs (two having a1 and one b2 symmetry) lie in the equatorial plane of the C2v bent Cp2M fragment. Thus the alkylidene 2pπ bonding AO is aligned to maximize Ta
C 5dπ
2pπ bonding. However, whereas Cp2Ta(CHPh)CH2Ph and its homologues are formally 18 valence electron compounds, compound 14 and the other metallocene hydrazides 17
19 are formally 20-electron compounds if the NNR2 ligands are capable of acting as four-electron33 donors, as is suggested by the sp hybridization at NR in all four instances. The preferred orientation of the β-NPh2 group in 14 can be explained by considering the frontier MOs of both the Cp2Ti(py) fragment and the hydrazide ligand. Cp2Ti(py) has two Tibased nonbonding MOs for forming the TidNR interaction, and these lie in the equatorial plane.32b,c Figure 8 shows schematically the four highest occupied MOs of planar, dianionic [NNR2]2
.6,34 The 1a1 MO is suitable for MdNR σ bonding, and the 1b2 MO is NR
Nβ π bonding and does not interact to any significant extent with a metal center.6,34a The 1b1 MO (generically labeled πh) is NR
Nβ nonbonding and could in principle form a MdNR π bond. The 2b2 MO (labeled πv) is the HOMO of [NNR2]2
and is significantly higher in energy (g1 eV6,34a) than πh because of its NR
Nβ π* antibonding nature. Of these two π donor MOs, πv has the better energy match with the metal acceptor orbitals. The consistent orientation of the β-NPh2 moiety in 14 and 17
19 is therefore understood in terms of providing the best interaction between the πv MO of NNPh2 and the metal, which preferably takes place in the equatorial plane (see Figure 8, center). The Ti
NR σ interaction is cylindrical and has no orientational preference. The relatively long TidNR and Ti
Cp distances in 14 are explained by the second π interaction formed between titanium

and the NNPh2 πh MO (Figure 8, right). There is no correctly aligned nonbonding acceptor MO on the Cp2Ti(py) fragment, and so formally πh has to donate into a Ti
Cp antibonding orbital. In effect this sets up a competition between πh and the SALCs of Cp2. This leads to incomplete donation from both the Cp2 and NNPh2 moieties and results in a ligand-based nonbonding MO, as discussed below. Although this bonding pattern has not been assessed previously in a metallocene-hydrazido complex, it has been analyzed in detail using theoretical, spectroscopic, and structural methods for a number of group 4
6 complexes of the type CpR2M(E)(L) (E = O or NR; L = two- or one-electron donor or none).32b,35 We have carried out DFT calculations on Cp2Ti(NtBu)(py) (13_Q) and Cp2Ti(NNPh2)(py) (14_Q), models of the real complexes 13 and 14. Details of the geometries, which are in good agreement with experiment, are given in the SI. The HOMO and HOMO
1 in each case are shown in Figure 9. The HOMO of 13_Q lies at
4.4 eV and is an almost entirely ligand-based nonbonding MO, as expected based on previous studies of base-free CpR2M(NR) (M = Ti36 or Mo35b) systems. The HOMO
1 is the in-plane TidN π-bonding MO at
5.8 eV. The hydrazido analogue 14_Q likewise has a ligand-based MO at
4.6 eV, close in energy to that of the HOMO in 13_Q. The πh MO of NNPh2 contributes to this, as can be seen in Figure 9. However, this orbital is the HOMO
1 of 14_Q since the HOMO is now the in-plane TidN π-bonding MO formed using πv of NNPh2 and is at
4.2 eV. The destabilization of this MO compared to its counterpart in 13_Q is due to the π* antibonding interaction between the TidNR π bond and the Nβ lone pair, as has been noted previously.6,9c,34a

A 90o rotation of the NPh2 group about the NR
Nβ bond of 14_Q gives the isomer 14_Q_rot with πv now oriented 2302

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Figure 9. DFT-computed HOMO and HOMO
1 and their energies for Cp2Ti(NtBu)(py) (13_Q, top) and Cp2Ti(NNPh2)(py) (14_Q, bottom). The value for the isodensity surface is 0.04 au.

approximately perpendicular to the equatorial plane (see also Figure S4 in the SI). This isomer is 3.0 kcal mol
1 less stable than 14_Q and has one η5-bound Cp (Ti
C = 2.40
2.49; Ti
Cpcent = 2.13 Å) and one best approximated as an η3-bound Cp (Ti
C = 2.41
2.56; Ti 3 3 3 C = 2.76 and 2.80; Ti
Cpcent = 2.30 Å). The TidNR distance shortens from 1.743 in 14_Q to 1.718 Å in 14_Q_rot, while the NR
Nβ lengthens from 1.337 to 1.357 Å. The shift of one Cp from η5- to η3-coordination is due to more effective donation into one of the Ti
Cp antibonding MOs as πv is brought into alignment. The HOMO (see Figure S4) of 14_Q_rot lies at
3.9 eV and represents the out-of-plane TidNR π-bonding MO formed using πv. The HOMO
1 (
5.4 eV) is again a ligand-based MO but is more localized on the Cp2 fragment because of the reduced average hapticity of the Cp ligands. The destabilization of the HOMO on going from 14_Q to 14_Q_rot is because the metal-based, π-acceptor MO used in the latter is at a higher energy.

We also carried out calculations on base-free model compounds Cp2Ti(NNMe2) with the NMe2 group either oriented perpendicular to the equatorial plane (20_Q) or lying within it (20_Q_rot). As expected, 20_Q is 3.6 kcal mol
1 more stable than 20_Q_rot. The TidNR bond is longer (1.726 vs 1.699 Å) and the NR
Nβ shorter (1.328 vs 1.370 Å) in 20_Q, as expected on the basis of the differences between 14_Q and 14_Q_rot. Similarly, the two Ti
Cp centroid distances are equal in 20_Q (2.12 Å), whereas they are different in 20_Q_rot (2.09 and 2.19 Å) with a tendency toward

η 3 -coordination for one of the Cp ligands. Further details are given in the SI (Figure S5). The hypothetical compounds 20_Q and 20_Q_rot are models of Wiberg’s Cp2Ti{NN(SiMe3)2} (21), for which no crystallographic data were reported.4 On the basis of our DFT calculations, Wiberg’s compound would be expected to have a geometry like that of 20_Q. However, Veith has reported the structurally characterized vanadium analogue Cp2V{NN(SiMe3)2} (22), which has the β-N(SiMe3)2 group lying in the equatorial plane (cf. 20_Q_rot) with a short VdNR bond distance (1.666(6) Å) and Cp rings that are tending toward η3-coordination.37 To probe this apparent discrepancy, we carried out DFT geometry optimizations of Cp2Ti{NN(SiMe3)2} (21_Q, Figure S6 of the SI) and Cp2V{NN(SiMe3)2} (22_Q, Figure S7 of the SI). In the case of 21_Q the optimal geometry corresponds to a structure where the NN(SiMe3)2 ligand is slightly out of the equatorial plane (Cpcent
Ti
Nβ
Si ≈ 62°). The Ti
NR and NR
Nβ bond distances (1.695 and 1.367 Å, respectively) in 21_Q have similar values to those obtained for 20_Q_rot (1.699 and 1.370 Å), and the orientation of the N(SiMe3)2 moiety results from destabilizing steric interactions between the Cp rings and the SiMe3 substituents in the electronically preferred orientation. This tendency is further confirmed by the calculated geometry of Veith’s compound 22, where the NN(SiMe3)2 ligand lies again approximately in the equatorial plane (Cpcent
V
Nβ
Si ≈ 82o), in agreement with the experimental observation. Test calculations failed to locate in each case an extremum geometry (either local minimum or transition state) with the NN(SiMe3)2 ligand perpendicular to the equatorial plane (i.e., analogous to 20_Q). For 21_Q and 22_Q, the HOMO is the orbital associated with the donation from πv into a M
Cp antibonding orbital, hence accounting for the tendency to adopt η3-Cp coordination.

’ CONCLUSIONS Imide/hydrazine exchange routes have allowed access to new sandwich and half-sandwich terminal hydrazido(2
) compounds. 2303

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Organometallics However, these reactions are very sensitive to the precise substitution patterns and bonding situation in the starting materials. Thus Cp*Ti(NtBu)Cl(py) (1) reacted with Ph2NNH2 to give the terminal hydrazide Cp*Ti(NNPh2)Cl(py) (4), whereas reaction of CpTi(NtBu)Cl(py) with Ph2NNH2 or of 1 with Me2NNH2 gave dimeric hydrazido-bridged products. Likewise Cp2Ti(NNPh2)(py) was readily formed from Cp2Ti(NtBu)(py) (13), whereas reaction of 13 with Me2NNH2 or of Cp*CpTi(NtBu)(py) with Ph2NNH2 gave mixtures among which were products of Ti
Cp protonolysis. This latter reactivity is a consequence of the π-loaded nature of bis(cyclopentadienyl) titanium imido and hydrazido complexes. Further studies of the reactions of 1 with Me2NNH2 gave a rare example of a structurally authenticated bis(η2-hydrazido(1
)) compound, namely, 7. Reaction of 4 with LiNHNPh2 gave the first group 4 mixed hydrazido(2
)/hydrazido(1
) compound. DFT studies of Cp2Ti(NNPh2)(py) (14) and associated model complexes showed that the bonding situation in this compound is largely analogous to those reported elsewhere in metallocene oxo and imido compounds. However, the precise orientation of the βNR2 group is important in the hydrazido systems, and the electronically preferred arrangement is that which places the πv donor MO of NNR2 in the Cp2Ti equatorial plane. This electronic preference can be overridden by steric repulsions developing between the Nβ R groups and the Cp ring, as illustrated by the preferred geometries for Cp2Ti{NN(SiMe3)2} and Cp2V{NN(SiMe3)2}.

’ EXPERIMENTAL SECTION General Methods and Instrumentation. All manipulations were carried out under an inert atmosphere of argon or dinitrogen using standard Schlenk-line or drybox procedures. Solvents were predried over activated 4 Å molecular sieves and refluxed over sodium (toluene), sodium/potassium (pentane, diethyl ether), or calcium hydride (dichloromethane) under a dinitrogen atmosphere and collected by distillation. Alternatively, solvents were degassed by sparging with dinitrogen and dried by passing through a column of activated alumina.38 Deuterated solvents were dried over potassium (C6D6), sodium (toluene-d8), or P2O5 (CD2Cl2), distilled under reduced pressure, and stored under dinitrogen in Young’s Teflon valve ampules. NMR samples were prepared under a dinitrogen atmosphere in a drybox, in 5 mm Wilmad NMR tubes equipped with Young’s Teflon valves. 1H and 13C NMR spectra were recorded on a Varian Mercury 300 spectrometer or a Bruker AVII 500 spectrometer with a 13C cryoprobe. 1 H and 13C{1H} spectra were referenced internally to residual protiosolvent (1H) or solvent (13C) resonances and are reported relative to tetramethylsilane (δ = 0 ppm). Chemical shifts are quoted in δ (ppm) and coupling constants in Hz. Where necessary, 1H and 13C assignments were assisted by the use of two-dimensional 1H
1H and 1H
13C correlation experiments. IR spectra were recorded on a Perkin-Elmer 1710 or Nicolet Magna 560 FTIR spectrometer. Samples were prepared in a drybox as Nujol mulls between NaCl plates. IR data are quoted as wavenumbers (cm
1) within the range 4000
400 cm
1. Elemental analyses were carried out by Stephen Boyer at the London Metropolitan University. Starting Materials. The compounds Cp*Ti(NtBu)Cl(py) (1), CpTi(NtBu)Cl(py) (2), Cp2Ti(NtBu)(py) (13), and Cp*CpTi(NtBu)(py) (16) were prepared according to the literature methods.10 Diphenyl hydrazine was obtained from Sigma-Aldrich as the hydrogen chloride salt, from which the free hydrazine was obtained by basification, drying, and removal of residual solvent, followed by distillation under inert atmospheric conditions. Lithiated hydrazines were obtained by reaction

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of the appropriate hydrazine and n-butyllithium. Pyridine was dried over freshly ground CaH2 and distilled before use. All other compounds and reagents were purchased, degassed, and used without further purification. Cp*Ti(NNPh2)Cl(py) (4). To a stirred solution of Cp*Ti(NtBu)Cl(py) (1.02 g, 2.76 mmol) in benzene (20 mL) was added a solution of Ph2NNH2 (0.55 g, 3.00 mmol) in benzene (10 mL). The resultant dark brown solution was stirred for 16 h. The volatiles were removed under reduced pressure to give a light brown powder, which was washed with pentane (3  10 mL) and dried under reduced pressure to give 4 as a brown solid. Yield: 1.10 g (83%). Diffraction-quality crystals were grown from a saturated pentane solution. 1H NMR (C6D6, 299.9 MHz, 293 K): δ 8.36 (2 H, d, 3J = 5.3 Hz, o-NC5H5), 7.49 (4 H, d, 3J = 8.8 Hz, o-C6H5), 7.21 (4 H, app. t, app. 3J = 8.0 Hz, m-C6H5), 6.87 (2 H, t, 3J = 7.4 Hz, p-C6H5), 6.75 (1 H, t, 3J = 7.6 Hz, p-NC5H5), 6.44 (2 H, app. t, app. 3J = 6.8 Hz, m-NC5H5), 1.88 (15 H, s, C5Me5). 13C{1H} NMR (C6D6, 75.4 MHz, 293 K): δ 150.8 (o-NC5H5), 146.7 (i-C6H5), 138.4 (p-NC5H5), 129.5 (m-C6H5), 124.6 (m-NC5H5), 122.9 (p-C6H5), 120.4 (o-NC5H5), 118.5 (C5Me5), 12.0 (C5Me5). IR (NaCl plates, Nujol mull, cm
1): ν 1603 (m), 1593 (s) 1584 (s) 1486 (s) 1444 (s) 1310 (s) 1281 (m) 1239 (s) 1214 (w) 1169 (m) 1152 (m) 1069 (m) 1014 (m) 989 (m) 875 (w) 746 (s) 692 (s) 640 (w) 518 (s). Anal. Found (calcd for C27H31ClN3Ti): C, 67.5 (67.4); H, 6.3 (6.5); N, 8.7 (8.7).

NMR Tube Scale Reaction of CpTi(NtBu)Cl(py) with Ph2NNH2.

To a stirred solution of CpTi(NtBu)Cl(py) (0.30 g, 1.00 mmol) in benzene (10 mL) was added a solution of Ph2NNH2 (0.22 g, 1.20 mmol) in benzene (10 mL). The resultant dark red solution was stirred for 16 h. The volatiles were removed under reduced pressure to give a dark red powder, which was washed with pentane (3  10 mL) and dried under reduced pressure to give 5 as a red solid. Yield: 0.12 g (36%). Analysis by 1H NMR indicated that a reaction had occurred immediately and quantitatively to give the previously reported Cp2Ti2(μ2-η1:η1-NNPh2)(μ2-η2:η1-NNPh2)Cl2 (5) and free pyridine.15

NMR Tube Scale Synthesis of Cp*Ti(NHtBu)(η2-NHNMe2)Cl (6). To a solution of Cp*Ti(NtBu)Cl(py) (0.020 g, 51.7 μmol) in C6D6

(0.4 mL) was added H2NNMe2 (3.9 μL, 51.7 μmol). An immediate color change to pale yellow was observed, and after 1 h the 1H NMR spectrum showed quantitative conversion to 6 and free pyridine. 1H NMR (C6D6, 299.9 MHz, 293 K): δ 5.40 (1 H, s, NHCMe3), 5.10 (1 H, s, NHNMe2), 2.41 (6 H, NHNMe2), 1.39 (C5Me5), 1.24 (NHCMe3). 13C{1H} NMR (C6D6, 75.4 MHz, 293 K): δ 121.1 (C5Me5), 58.9 (NHCMe3) 50.6 (NHNMe2), 33.1 (NHCMe3), 11.8 (C5Me5). Cp*Ti(η2-NHNMe2)2Cl (7). To a stirred solution of Cp*Ti(NtBu)Cl(py) (0.50 g, 1.35 mmol) in toluene (20 mL) at
78 °C was added Me2NNH2 (20 μL, 2.70 mmol) in toluene (5 mL) over 10 min. The resultant brown solution was allowed to warm to RT and stirred for 16 h. The volatiles were removed under reduced pressure to give a tan powder, which was washed with hexanes (3  10 mL) and dried under reduced pressure to give 7 as a pale yellow solid. Yield: 0.28 g (62%). Diffraction-quality crystals were grown by slow cooling of a saturated hexanes solution. 1H NMR (C6D6, 299.9 MHz, 293 K): δ 5.05 (2 H, s, NHNMe2), 2.45 (12 H, s, NHNMe2), 1.88 (15 H, s, C5Me5). 13 C{1H} NMR (C6D6, 75.4 MHz, 293 K): δ 120.3 (C5Me5), 51.32 (NHNMe2), 12.4 (C5Me5). IR (NaCl plates, Nujol mull, cm
1): ν 3241 (s), 1456 (s), 1398 (w), 1378 (s), 1292 (w), 1261 (w), 1201 (w), 1088 (w), 1073 (w), 1023 (s), 821 (w), 805 (w), 773 (w), 737 (m), 665 (m). Anal. Found (calcd for C14H27ClN4Ti): C, 49.8 (50.2); H, 8.1 (8.1); N, 16.7 (16.7). Cp*2Ti2(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)Cl2 (8). To a stirred solution of Cp*Ti(NtBu)Cl(py) (0.40 g, 1.08 mmol) in toluene (20 mL) at
78 °C was added Me2NNH2 (8.10 μL, 1.08 mmol) in toluene (5 mL) over 10 min. The resultant brown solution was allowed to warm slowly to RT and stirred for 72 h. The volatiles were removed under reduced pressure to give a gray powder, which recrystallized from 2304

dx.doi.org/10.1021/om200068k |Organometallics 2011, 30, 2295–2307

Organometallics benzene, washed with hexanes (3  10 mL), and dried under reduced pressure to give 8 as a pale green powder. Yield: 0.15 g (50%). Diffraction-quality crystals were grown from slow cooling of a hot benzene solution. 1H NMR (C6D6, 299.9 MHz, 293 K): δ 2.82 (12 H, s, NNMe2), 2.07 (30 H, s, C5Me5). 1H NMR (toluene-d8, 299.9 MHz, 243 K): δ 2.86 (6 H, s, br, NNMe2), 2.75 (6 H, s, br, NNMe2), 2.13 (15 H, s, br, C5Me5), 2.01 (15 H, s, br, C5Me5).13C{1H} NMR (C6D6, 75.4 MHz, 293 K): δ 120.8 (C5Me5), 52.9 (NNMe2), 13.5 (C5Me5). IR (NaCl plates, Nujol mull, cm
1): ν 1304 (w), 1259 (m), 1156 (w), 1121 (m), 1086 (m), 1053 (s), 1022 (s), 988 (w) 869 (w), 796 (m), 774 (w), 665 (m), 644 (w), 580 (m). Anal. Found (calcd for C24H42Cl2N4Ti2): C, 51.9 (52.1); H, 7.8 (7.7); N, 9.9 (10.1). Cp*Ti(NNPh2)(NHNPh2)(py) (10). To a stirred solution of Cp*Ti(NNPh2)Cl(py) (0.20 g, 0.42 mmol) in benzene (20 mL) was added a solution of LiNHNPh2 (0.08 g, 0.45 mmol) in benzene (10 mL). The solution darkened and was stirred for 4 h. The volatiles were removed under reduced pressure, and the residue was extracted into Et2O (20 mL) and filtered to yield a green solution. Et2O was removed under reduced pressure, and the resultant solid was washed with hexanes (3  10 mL) and dried under reduced pressure to give 10 as a brown solid. Yield: 0.11 g (45%). Diffraction-quality crystals were grown by slow cooling of a saturated hexanes solution. 1H NMR (C6D6, 299.9 MHz, 293 K): δ 7.85 (2 H, d, 3J = 6.5 Hz, o-NC5H5), 7.34 (4 H, d, 3J = 8.8 Hz, o-NNC6H5), 7.23 (4 H, d, 3J = 8.8 Hz, o-NHNC6H5), 7.08 (4 H, app. t, app. 3J = 8.0 Hz, m-NNC6H5), 7.02 (4 H, app. t, app. 3J = 8.0 Hz, m-NHNC6H5), 6.96 (1 H, s, NHNPh2), 6.84 (2 H, t, 3J = 7.1 Hz, p-NNC6H5), 6.74 (2 H, t, 3J = 7.5 Hz, p-NHNC6H5), 6.66 (1 H, t, 3J = 7.7 Hz, p-NC5H5), 6.24 (2 H, dd, 3J = 7.7 and 6.5 Hz, m-NC5H5), 1.88 (15 H, s, C5Me5). 13C{1H} NMR (C6D6, 75.4 MHz, 293 K): δ 152.1 (o-NC5H5), 150.9 (i-NHNC6H5), 147.1 (i-NNC6H5), 137.4 (pNC5H5), 129.2 (m-NNC6H5), 128.9 m-NHNC6H5, 123.6 (m-NC5H5), 122.0 (p-NNC6H5), 121.0 (p-NHNC6H5), 120.3 (o-NC5H5), 116.2 (C5Me5), 11.9 (C5Me5). IR (NaCl plates, Nujol mull, cm
1): ν 3259 (m, NHN(C6H5)2) 1585 (s), 1490 (s), 1459 (m), 1444 (m), 1377 (w), 1328 (m), 1311 (w), 1260 (w), 1213 (w), 1167 (w), 1069 (w), 1044 (w), 1024 (w), 989 (w), 838 (w), 792 (m), 744 (s), 696 (s), 665 (m), 640 (s). Anal. Found (calcd for C39H41N5Ti): C 74.5 (74.6), H 6.8 (6.6), N 11.1 (11.2). Cp*Ti(NNPh2)(NHtBu)(py) (11). To a stirred solution of Cp*Ti(NNPh2)Cl(py) (0.30 g, 0.63 mmol) in benzene (20 mL) was added a solution of LiNHtBu (0.05 g, 0.63 mmol) in benzene (10 mL), and the solution darkened and was stirred for 4 h. The volatiles were removed under reduced pressure, and the residue was extracted into Et2O (20 mL) and filtered to yield a green solution. Et2O was removed under reduced pressure, and the resultant solid was washed with hexanes (3  10 mL) and dried under reduced pressure to give 11 as a brown solid. Yield: 0.10 g (31%). 1H NMR (C6D6, 299.9 MHz, 293 K): δ 8.14 (2 H, d, 3J = 6.6 Hz, o-NC5H5), 7.39 (4 H, d, 3J = 8.8 Hz, o-C6H5), 7.21 (4 H, app. t, app. 3J = 8.4 Hz, m-C6H5), 6.89 (2 H, t, 3J = 7.1 Hz, p-C6H5), 6.70 (1 H, t, 3J = 7.6 Hz, p-NC5H5), 6.34 (2 H, app. t, app. 3J = 7.0 Hz, mNC5H5), 6.16 (1 H, s, NHCMe3), 1.90 (15 H, s, C5Me5), 1.50 (9 H, s, CMe3). 13C{1H} NMR (C6D6, 75.4 MHz, 293 K): δ 152.3 (o-NC5H5), 147.2 (i-C6H5), 137.5 (p-NC5H5), 129.3 (m-C6H5), 123.6 (m-NC5H5), 121.6 (p-C6H5), 120.3 (o-NC5H5), 115.6 (C5Me5), 56.3 (CMe3), 35.4 (CMe3), 11.9 (C5Me5). IR (NaCl plates, Nujol mull, cm
1): ν 3258 (m, br, NH), 1600 (m), 1592 (s), 1585 (s), 1489 (s), 1462 (m), 1444 (w), 1376 (m), 1359 (w), 1349 (w), 1325 (s), 1302 (w), 1259 (w), 1214 (m), 1207 (m), 1168 (w), 1152 (w), 1068 (w), 1042 (w), 1022 (m), 987 (w), 972 (w), 840 (m), 793 (m), 752 (m), 740 (s), 697 (s), 666 (w). Anal. Found (calcd for C31H40N4Ti): C 71.6 (72.1); H 7.7 (7.8); N 11.1 (10.9).

NMR Tube Scale Synthesis of Cp*Ti(NtBu)(NHNPh2)(py) (12). A solution of LiNHNPh2 (0.0052 mg, 27.1 μmol) in C6D6

(0.4 mL) was added to Cp*Ti(NtBu)Cl(py) (0.01 mg, 27.1 μmol).

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After 4 h the 1H NMR spectrum showed quantitative conversion to 12. H NMR (C6D6, 299.9 MHz, 293 K): δ 8.25 (2 H, d, 3J = 6.4 Hz, o-NC5H5), 7.42 (4 H, d, 3J = 8.8 Hz, o-C6H5), 7.13 (4 H, app. t, app. 3J = 8.0 Hz, m-C6H5), 6.79 (2 H, t, 3J = 7.1 Hz, p-C6H5), 6.72 (1 H, t, 3J = 7.5 Hz, p-NC5H5), 6.64 (1 H, s, NHNPh2), 6.34 (2 H, app. t, app. 3J = 6.8 Hz, m-NC5H5), 1.98 (15 H, s, C5Me5), 1.17 (9 H, s, CMe3). 13 C{1H} NMR (C6D6, 75.4 MHz, 293 K): δ 152.1 (o-NC5H5), 151.0 (iC6H5), 137.6 (p-NC5H5), 128.9 (m-C6H5), 123.7 (m-NC5H5), 121.1 (p-C6H5), 120.1 (o-NC5H5), 116.0 (C5Me5), 67.1 (CMe3), 33.5 (CMe3), 12.3 (C5Me5). Cp2Ti(NNPh2)(py) (14). To a vigorously stirred solution of Cp2Ti(NtBu)(py) (1.50 g, 4.60 mmol) in benzene (10 mL) cooled to 5 °C was added dropwise a solution of Ph2NNH2 (0.84 g, 0.91 mmol) in benzene (10 mL) over 10 min. The resultant brown solution was allowed to warm to RT and stirred for 4 h, in which time a light brown solid formed. The solid was filtered and washed with pentane (3  20 mL) and dried under reduced pressure to give 14 as a brown powder. Yield: 1.16 g (58%). Diffraction-quality crystals were grown from slow diffusion of pentane into a concentrated toluene solution. 1H NMR (C6D6, 299.9 MHz, 293 K): 8.39 (2 H, d, 3J = 6.5 Hz, o-NC5H5), 7.25 (4 H, d, 3J = 8.8, o-C6H5), 7.15 (4 H, app. t, app. 3J = 8.2 Hz, m-C6H5), 6.88 (2 H, t, J = 7.7 Hz, pC6H5), 6.66 (1 H, t, 3J = 7.6 Hz, p-NC5H5), 6.23 (2H, app. t, app. 3J = 7.0 Hz, m-NC5H5), 5.88 (10 H, s, C5H5). 13C{1H} NMR (C6D6, 75.5 MHz, 293 K): δ 155.0 (o-NC5H5), 146.5 (i-C6H5), 137.0 (p-NC5H5), 129.9 (m-C6H5), 123.8 (m-NC5H5), 123.3 (p-C6H5), 120.8 (o-C6H5), 110.0 (C5H5). IR (NaCl plates, Nujol mull, cm
1): ν 1592 (m), 1584 (m), 1440 (s), 1291 (w), 1261 (s), 1214 (w), 1151 (m), 1075 (s, br), 795 (s), 782 (s), 760 (m), 628 (m). Anal. Found (calcd) for C27H25N3Ti: C, 73.8 (73.8); H, 5.6 (5.7); N, 9.5 (9.6). 1

CpTi(NHtBu)(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)TiCp(η1-Cp) (15). To a stirred solution of Cp2Ti(NtBu)(py) (0.30 g, 0.91 mmol)

in hexanes (30 mL) cooled to 5 °C was added Me2NNH2 (68 μL, 0.91 mmol). The resultant dark red solution was allowed to warm to RT and stirred for 2 h, during which time dark brown crystals crystallized. The solid was filtered and washed with hexanes (3  20 mL) and dried under reduced pressure to give 15 as brown crystals. Yield: 0.05 g (11%). Diffraction-quality crystals were grown from a concentrated hexanes solution. 1H NMR (toluene-d8, 299.9 MHz, 243 K): δ 6.29 (5 H, s, η5-CpTi), 5.99 (5 H, s, η5-Cp(NHCMe3)Ti), 5.72 (5 H, s, η1-C5H5), 5.17 (1 H, s, NHCMe3), 2.42 (12 H, s, NNMe2), 0.89 (9 H, s, NHCMe3). 13 C{1H} NMR (toluene-d8, 75.5 MHz, 243 K): δ 117.2 (C5H5), 116.2 (Cp(NHCMe3)Ti), 113.6 (η1-Cp), 62.0 (NHCMe3), 56.8 (NNMe2), 38.4 (NHCMe3). IR (NaCl plates, Nujol mull, cm
1): ν 2955 (NHCMe3), 1432 (s), 1382 (s), 1362 (m), 1260 (s), 1245 (m), 1198 (w), 1023 (s), 1012 (m), 983 (w), 946 (w), 872 (w), 813 (s), 726 (m), 690 (w), 602 (w). Anal. Found (calcd) for C23H37N5Ti2: C, 57.8 (57.6); H, 7.6 (7.8); N, 14.6 (14.6).

Crystal Structure Determinations of Cp*Ti(NNPh2)Cl(py) (4), Cp*Ti(η2-NHNMe2)2Cl (7), Cp*2Ti2(μ-η1:η1-NNMe2)(μη2:η1-NNMe2)2Cl2 (8), Cp*Ti(η2-NHNMe2)Cl2 (9), Cp*Ti (NNPh2)(NHNPh2)(py) (10), Cp*Ti(NNPh2)(NHtBu)(py) (11), Cp2Ti(NNPh2)(py) (14), and CpTi(NHtBu)(μ-η1:η1-NNMe2)(μ-η2:η1-NNMe2)TiCp(η1-Cp) (15). Crystal data collection and processing parameters are given in Table S3 of the SI. Crystals were mounted on glass fibers using perfluoropolyether oil and cooled rapidly in a stream of cold N2 using an Oxford Cryosystems Cryostream unit. Diffraction data were measured using an Enraf-Nonius KappaCCD diffractometer. As appropriate, absorption and decay corrections were applied to the data, and equivalent reflections were merged.39 The structures were solved by direct methods (SIR9240), and further refinements and all other crystallographic calculations were performed using the CRYSTALS program suite.41 N-Bound H atoms were located from Fourier difference syntheses and positionally refined isotropically. C-Bound H atoms were placed geometrically and refined 2305

dx.doi.org/10.1021/om200068k |Organometallics 2011, 30, 2295–2307

Organometallics in a riding model. Details of the structure solution and refinements are given in the Supporting Information (CIF data). A full listing of atomic coordinates, bond lengths and angles, and displacement parameters for all the structures have been deposited at the Cambridge Crystallographic Data Centre. Copies of the data can be obtained, free of charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44-(0)1223-336033 or e-mail: [email protected]). Computational Details. All the calculations have been performed with the Gaussian03 package42 at the B3PW91 level.43 The titanium, vanadium, and silicon atoms were represented by relativistic effective core potentials from the Stuttgart group and the associated basis sets,44 augmented by an f (Ti, V) or d (Si) polarization function (R = 1.506, Ti; R = 1.751, V; R = 0.284, Si).45 The remaining atoms (C, H, N) were represented by a 6-31G(d,p) basis set.46 Full optimizations of geometry without any constraint were performed, followed by analytical computation of the Hessian matrix to confirm the nature of the located extrema as minima on the potential energy surface.

’ ASSOCIATED CONTENT

bS

Supporting Information. Data collection and processing information and X-ray crystallographic data in CIF format for the structure determinations of Cp*Ti(NNPh2)Cl(py) (4), Cp*Ti(η2-NHNMe2)Cl (7), Cp*2Ti2(μ-η1:η1-NNMe2)(μ-η2: η1-NNMe2)Cl2 (8), Cp*Ti(η2-NHNMe2)Cl2 (9), Cp*Ti(NNPh2)(NHNPh2)(py) (10), Cp*Ti(NNPh2)(NHtBu)(py) (11), Cp2Ti(NNPh2)(py) (14), and CpTi(NHtBu)(μ-η1:η1NNMe2)(μ-η2:η1-NNMe2)TiCp(η1-Cp) (15). Details of the synthesis, diagram of the molecular structure, and selected distances and angles for 9. Additional information regarding the DFT calculations. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT We thank the EPSRC, British Council, MESR, and the Spanish Ministerio de Educaci
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