Theoretical Studies on the Structure and Bonding of

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Theoretical Studies on the Structure and Bonding of Metallacyclocumulenes, -cyclopentynes, and -cycloallenes Subhendu Roy,† Eluvathingal D. Jemmis,*,†,‡ Martin Ruhmann,§,|| Axel Schulz,*,§,|| Katharina Kaleta,|| Torsten Beweries,|| and Uwe Rosenthal*,|| †

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India Indian Institute of Science Education and Research Thiruvananthapuram, CET campus, Thiruvananthapuram 695016, India § Abteilung Anorganische Chemie, Institut f€ur Chemie, Universit€at Rostock, Albert-Einstein-Strasse 3a, D-18059 Rostock, Germany Leibniz-Institut f€ur Katalyse e.V. an der Universit€at Rostock, Albert-Einstein-Strasse 29a, D-18059 Rostock, Germany

)

‡

bS Supporting Information ABSTRACT: In this paper, we compare the electronic structure of the hafnacycloallene complex Cp2HfC4RR0 2R00 (5Hf), which was previously described by Erker et al., with those of the titanium, zirconium, and hafnium complexes Cp2M(η4-RHC4HR) (3M; i.e. metallacyclopent-2,3,4-trienes, metallacyclocumulenes) and Cp2M(η4-R2C4R2) (4M; i.e. 1-metallacyclopent-3-ynes) using density functional theory (BP86/LANL2DZ) calculations. Moreover, the η3-phenylallenyl zirconocene complex 7Zr, which was synthesized by Wojcicki et al., is included for the comparison. These calculations and extended H€uckel calculations show that the bonding in complex 5Hf is remarkably similar to that of complexes 4M and 7Zr. An analysis of the structural parameters and bonding reveals that the unique interaction of the internal carbon atoms along with the terminal carbon atoms with the bent-metallocene moiety is the reason for the unusual stability of these metallacycles. The molecular orbital analysis further suggests that complex 5Hf can react with another metal fragment to give the bimetallic complexes 9 and 10. The electronic structures of complexes 3M, 4M, 5Hf, and 7Zr have been comparatively studied to get a general understanding of the bonding in these metallacycles.

1. INTRODUCTION Metallacycles (cycloallenes, dienes, alkynes, cumulenes, etc.), as an emerging class of important organometallic compounds, have been the subject of much discussion in recent times due to their unusual geometrical parameters and significant role in multiple catalytic processes. These are unusual because the carbocyclic analogues of the metallacycles, cyclic cumulene 1 or alkyne 2, are very unstable due to the angle strain on incorporation of a linear CdCdCdC or C
CtC
C unit into the small rings (Scheme 1).1,2 However, by replacing the CH2 group with the isolobal Cp2M (M = Ti, Zr, Hf) moiety in these strained molecules, Rosenthal et al. first synthesized the metallacyclocumulene3
5 3Zr and Suzuki et al. synthesized the first metallacyclopentyne 4Zr (Scheme 2).6,7 These complexes are important in stoichiometric and catalytic C
C coupling and cleavage reactions of unsaturated molecules such as alkynes, olefins, acetylides, and vinylides.5d,8,9 Group 4 metallocenes, in particular, have been popular lately, as they form metallacycles containing strongly distorted π-systems. The group 4 bent metallocenes (Cp2M) are 14-electron complexes with a d2 configuration (M2þ). The set of three valence molecular orbitals of Cp2M (i
iii,; cf. Scheme 3) all lie in the plane that bisects the Cp
M
Cp angle.10 Among them, one r 2011 American Chemical Society

molecular orbital (i) contains a lone pair and the other two molecular orbitals (ii and iii) are vacant valence orbitals, making their reactivity comparable to that of carbenes. The metallocenes react with unsaturated ligands in the formal M2þ oxidation state to form complexes with a strong metal
carbon σ-bond and metal-to-ligand back-bonding character.11 The interaction of these frontier molecular orbitals of Cp2M with various C4 ligands and the way these reactive fragments are stabilized will be discussed in the following sections. In spite of the strongly bent geometries at the sp-hybridized central carbon atoms in metallacyclocumulene and metallcyclopentyne, the complexes 3M and 4M are stable at room temperature. However, the corresponding carbocyclic rings would not be stable with this strained geometrical feature.1,2 The remarkable stability of these metallacycles is attributed to the in-plane interaction of the bent-metallocene unit, with the central unsaturated C
C bond involving the vacant acceptor orbitals of the Cp2M moiety and the filled orbitals of the C4 ligands. The mesomeric forms 3M0 and 4M0 (Scheme 2) contribute minimally to the resonance hybrid of these systems. Following these bonding strategies, Erker et al. have recently isolated and Received: January 4, 2011 Published: April 27, 2011 2670

dx.doi.org/10.1021/om2000037 | Organometallics 2011, 30, 2670–2679

Organometallics Scheme 1. Schematic Representation of Cyclocumulene 1 and Cyclopentyne 2

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Scheme 4. Schematic Representation of Erker’s FiveMembered Metallacycloallene 5M, Its Mesomeric Form 5M0 , and Cycloallene 6

Scheme 2. Schematic Representation of Metallacyclocumulene 3M and Metallacyclopentyne 4M and Their Mesomeric Forms (3M0 and 4M0 , Respectively) Scheme 5. Structural Similarities of Complexes 5M and 7Zr

Scheme 3. Frontier Orbitals of a Bent Metallocene (Cp2M) Unit resonance structures, resulting in a formal four-membered zirconacycle, Suzuki et al. stated that the structure should be comparable to that of the former five-membered metallacycloallene complexes 5M (Scheme 5). In this paper, we have analyzed the geometrical and electronic structures of the recently synthesized complex 5Hf and compared them with those of complex 3M and complex 4M to determine connections among these metallacycles. The electronic structures of complexes 3M and 4M were studied by our group previously and are described here briefly for a better comparison with complex 5Hf. We have also studied the structure and bonding of the η3-phenylallenyl zirconocene complex 7Zr, due to its structural similarity with complex 5Hf.

characterized the first five-membered hafnacycloallene (5Hf; Scheme 4) by spectroscopy, by X-ray diffraction, and also by DFT analysis.12 The carbocyclic analogue 6 of a five-membered metallacycloallene would not also be stable2 due to the angle strain. Similar zirconocene complexes were later prepared by the same group using the same method as for the Hf compound13 as well as by Suzuki and co-workers by reduction of a 1-zirconacyclopent-3-yne precursor with alkali metal followed by alkylation of the dianionic intermediate.14 Erker and co-workers have also studied the bonding of 5Hf and 5Zr by DFT calculations.12,13 Another complex very similar to complexes 5Hf and 5Zr was described before by Wojcicki and co-workers, who reported on a η3-phenylallenyl zirconocene complex (7Zr; Scheme 5).15 Although this complex showed η3-propargyl and η3-allenyl

2. COMPARISON OF UNUSUAL FIVE-MEMBERED ALLCARBON METALLACYCLES The structure and bonding of metallacyclocumulene 3M, metallacyclopentyne 4M, metallacycloallene 5Hf, and phenylallenyl zirconocene complex 7Zr are comparatively discussed below. The analyses of complexes 3M and 4M are described for M = Ti, Zr, Hf. Only the bonding analysis of 3Zr and 4Zr will be described, because the interaction diagrams for complexes 3Ti, 3Zr, and 3Hf as well as for complexes 4Ti, 4Zr, and 4Hf are essentially similar. The first synthesized five-membered metallacycloallene is a hafnium complex; therefore, only the analysis of complex 5Hf will be described here. The theoretical calculations have been carried out at the BP86/LANL2DZ level of theory, which is reasonably reliable to address the main concern of this study rather than to get a rigorous quantitative description of the complexes. For simplicity and reduction of computational cost, we have studied complexes 3M and 4M with H and Ph 2671

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substituents. The theoretical results given below are all with the H substituent unless otherwise stated. 2.1. Metallacyclocumulenes (3M). Many derivatives of metallacyclocumulenes (3M) are known experimentally.3,16 The complex 3M has been reported for Ti,3 Zr,4 and Hf,17 and the structure and bonding of 3Ti and 3Zr have been analyzed at the B3LYP/LANL2DZ level of theory.18 In this study, we have performed a similar analysis at the BP86/LANL2DZ level of theory, as BP86 is well-known for accurately reproducing experimental geometries.19 Calculations show that all M
C bonds are within the bonding range and the internal M
C bonds are slightly longer than the terminal bonds (Table 1). The computed C
C bond lengths are in accord with the cumulenic nature of complex 3M. Apart from this, the computed C
C and M
C bond indices and population analysis (NBO) also suggest the cumulenic nature for complex 3M.18 The metal atom and the four carbon atoms of the MC4 ring of complex 3M are coplanar (Scheme 6). The bonding properties of complex 3M (M = Ti, Zr, Hf) have been analyzed, and there is a strong interaction between the metal and the in-plane C4 system. The bonding of 3Zr is analyzed from a fragment molecular orbital approach.20 The Cp2M fragment has its three frontier orbitals (i
iii, as shown in Scheme 3) located in the MC4 plane. The frontier orbitals of the HCCCCH fragment are formed from the two in-plane p orbitals on the two central carbon atoms (C2 and C3) and the sp2 hybrid orbitals on the metal-bound carbon atoms (C1 and C4). The four orbitals form four linear combinations, which are similar to the π-orbitals of butadiene. The lowest two MOs among the four resulting MOs are filled. The LUMO of the C4H2 fragment, which is bonding between the C2 and C3 carbon atoms, relates to the in-plane equivalent of the LUMO of butadiene π-orbitals. The strongest stabilizing interaction takes place between this LUMO of the C4H2 fragment and the HOMO of the Cp2M fragment (Figures S1b and S1c, Supporting Information), and consequently, it stabilizes the C2
C3 bond (HOMO, Figure S1b). It is worthwhile to note that this is opposite to the effect of metal to π* back-bonding of the known Dewar
Chatt
Duncanson model, which would have

lengthened the C2
C3 bond.21 The shortening of the C2
C3 bond is tempered by two π MOs perpendicular to the MC4 plane. Another stabilizing interaction, observed in HOMO-6 (Figure S1b and S1c), is the donation of electrons from the HOMO-1 (HOMO at B3LYP/LANL2DZ level18) of C4H2 to the LUMOþ1 of the metal atom. 2.2. Metallacyclopentyne (4M). The synthesis of the metallacyclopentyne 4Zr with R = SiMe3, t-Bu by Suzuki et al.7a expanded the scope of study of such metallacycles. The complex 4M has been further reported experimentally for other group 4 metals (Ti and Hf).7d The structure and bonding of 4M (M = Ti, Zr) was studied earlier at the B3LYP/LANL2DZ level of theory by our group.18b We have analyzed complex 4M (M = Ti, Zr, Hf) at the BP86/LANL2DZ level of theory as we did for 3M. The unusually short central C
C bond (1.237 Å, from an X-ray crystal structure)7a supports the alkynic character of the complex 4Zr. The computed terminal M
C1 and M
C4 bonds are longer as compared to the two internal M
C2 and M
C3 bonds (Table 2). The complex can exist in two conformations, cis or trans, with the trans form being 0.2 kcal/mol more stable than the cis form for R = SiH3, CH3.18b The metal atom and the four carbon atoms of the MC4 ring, as in metallacyclocumulene, are also coplanar in this complex (4Zr; Scheme 7). The experimental and computed structural parameters of complex 4M are given in Table 2. Scheme 6. Optimized Structure of Complex 3Zr

Table 1. Computed Bond Lengths (Å) and Angles (deg) of the Metallacyclocumulene Complexes (3M, Scheme 6) at the BP86/ LANL2DZ Level of Theorya bond length

bond angle

C1
C2

C2
C3

C3
C4

M
C1

M
C2

M
C3

M
C4

C1
C2
C3

M
C1
C2

1.319

1.350

1.319

2.236

2.269

2.269

1.329

1.338

1.329

2.238

2.258

2.258

2.236

144.3

74.3

2.238

144.7

(1.277)

(1.338)

(1.277)

(2.252)

(2.210)

(2.210)

(2.252)

(147.6)

73.6

1.329

1.349

1.329

2.349

2.378

2.378

2.349

145.9

74.8

1.340

1.338

1.340

2.353

2.372

2.372

2.353

146.4

74.3

(1.280)

(1.310)

(1.280)

(2.357)

(2.303)

(2.303)

(2.357)

(150.0)

1.335

1.348

1.335

2.307

2.360

2.360

2.307

144.6

1.346

1.337

1.346

2.311

2.354

2.354

2.311

145.0

75.0

(1.288)

(1.334)

(1.298)

(2.281)

(2.294)

(2.297)

(2.308)

(146.3)

(74.2)

Complex 3Ti

Complex 3Zr

Complex 3Hf 75.6

a

For Ti, experimental data of Cp2Ti(η4-t-Bu-C4-t-Bu),3 for Zr, experimental data of Cp2Zr(η4-t-Bu-C4-t-Bu),4 and for Hf, experimental data of Cp2Hf(η4-t-Bu-C4-t-Bu)17 are given in parentheses for comparison. Italicized values are for R = Ph. 2672

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Table 2. Computed Bond Lengths (Å) and Angles (deg) of the Metallacyclopentyne Complexes (4M, Scheme 7) at the BP86/ LANL2DZ Level of Theorya bond length

bond angle

C1
C2

C2
C3

C3
C4

M
C1

M
C2

M
C3

M
C4

C1
C2
C3

M
C1
C2

1.416

1.278

1.416

2.379

2.230

2.230

1.404 (1.393)

1.275 (1.249)

1.404 (1.393)

2.589 (2.353)

2.259 (2.202)

2.259 (2.202)

2.379

151.3

66.4

2.589 (2.353)

160.2 (151.7)

60.6 (66.4)

1.432

1.279

1.432

2.462

2.352

2.352

1.423

1.279

1.423

2.557

2.362

2.362

2.462

151.1

68.5

2.557

155.1

(1.418)

(1.237)

(1.417)

(2.414)

(2.311)

(2.305)

(2.433)

65.8

(150.8)

(68.6)

1.440

1.279

1.440

2.411

2.328

2.328

1.430

1.278

1.430

2.507

2.340

2.340

2.411

149.5

69.2

2.507

153.5

(1.403)

(1.229)

(1.406)

(2.428)

(2.276)

(2.276)

(2.431)

66.5

(153.0)

(66.8)

Complex 4Ti

Complex 4Zr

Complex 4Hf

a Experimental values are given in parentheses. For Ti, experimental data of Cp2Ti(η4-C4H4),7d for Zr, experimental data of Cp*2Zr(η4-C4H4),7a and for Hf, experimental data of Cp2Hf[η4-C4H2(SiMe3)2]7d are given for comparison. Italicized values are for R = Ph.

Scheme 7. Optimized Structure of Complex 4Zr

The bonding of 4Zr has been analyzed from a fragment molecular orbital approach.20 The frontier orbitals of the H2CCCCH2 fragment are formed from the two in-plane p orbitals on the two central carbon atoms (C2 and C3) and the sp3 hybrid orbitals on the end carbon atoms (C1 and C4). These four orbitals form four linear combinations, analogous to the π-orbitals of butadiene. Apart from these, there are two perpendicular p orbitals which generate a localized π-bond on the two central carbon atoms (C2 and C3). The lowest two MOs among these along with the π-orbital are filled. Here also, the LUMO of the C4H4 fragment interacts strongly with the HOMO of the Cp2M fragment to form the HOMO of the complex (Figures S2b and S2c, Supporting Information). The other stabilizing interaction, observed in HOMO-5 (Figures S2b and S2c), is the donation of electrons from the HOMO of C4H4 to the LUMOþ1 of the metal atom. Thus, the nature of HOMO and HOMO-5 of 4Zr is similar to the HOMO and HOMO-6 of 3Zr. However, the perpendicular π-orbital (HOMO-1) is mainly localized on the central carbon atoms (C2 and C3). Hence, these perpendicular π-orbitals in 4Zr are available for further interactions with a second metal. This is confirmed by the existence of Cp2 M(μ-η:ηH2C4H2)M0 L27b,22 (8, Figure S2a) where M = Zr and M0 L2 = ZrCp2, Ni(PR3)2.

Scheme 8. Schematic Diagram of (a) a Hafnacycloallene (5Hf) and (b) Transition State for Conformational Isomerism of 5Hf

2.3. Five-Membered Metallacycloallene (5M). Recently, Erker and co-workers have synthesized the five-membered hafnacycloallene 5Hf (Scheme 8a) with R = SiMe3, t-Bu, another member in the family of these unusual metallacycles.12 The optimized structure of complex 5Hf is shown in Figure 1. The metal atom and the four carbon atoms are nonplanar in 5Hf, unlike the case in 3M or 4M. The substituents at the terminal carbon atoms in an allene are perpendicular to each other. The barrier for internal rotation in an allene is calculated to be 50.1 kcal mol
1.23 Thus, a complete planar arrangement of the metal atom and the four carbon atoms in 5Hf will engender enormous strain in the molecule, which would not be stable enough to be experimentally isolable. The reason for adopting a nonplanar structure of complex 5Hf is explored further in the subsequent section. We have analyzed the geometrical and bonding features of 5Hf with R = H (5Hf-a) and R = SiH3 (5Hf-b) substituents to reduce the computational cost. The structural parameters from the X-ray crystal structure are in good agreement with the calculated values (Table 3). We have also included the calculated values of the structural parameters of complex 5Hf with R = CMe3 substituents in the table from the PBE-D/TZVPP0 level of theory for comparison.12 It can be seen that there are no significant 2673

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Figure 1. Optimized structure of complex 5Hf (R=SiH3, anti). The nonplanarity of the HfC4 fragment is shown beside.

Table 3. Comparison of Computed (at the BP86/LANL2DZ Level of Theory) and Experimental Bond Lengths (Å) and Angles (deg) of Complex 5Hf (anti)a 5Hf-a (calcd)

5Hf-b (calcd)

5Hf (exptl)b

C1
C2

1.480

1.496 (1.461)

1.477

C2
C3

1.383

1.383 (1.361)

1.357

C3
C4

1.321

1.315 (1.300)

1.296

Hf
C1

2.548

2.532 (2.530)

2.536

Hf
C2

2.502

2.529 (2.565)

2.560

Hf
C3 Hf
C4

2.381 2.274

2.359 (2.330) 2.342 (2.310)

2.318 2.295

B
C1

1.511

1.512 (1.505)

1.495

C2
C3
C4

147.2

152.1 (155.2)

155.5

C3
C4
Hf

78.0

74.5

74.6

C1
C2 3 3 3 C4
Hf


45.6


41.0 (
39.1)


39.1

Calculated values of structural parameters at the PBE-D/TZVPP0 level of theory are given in parentheses.12 b Experimental data of Cp2Hf(C4H(CMe3)2B(C6F5)2)12 are given for comparison. a

differences in the values calculated at these two levels of theory. Apart from the obvious use of a nonempirical PBE density functional with corrections for intramolecular dispersion effects (PBE-D) together with large triple-ζ (TZVPP0 ) AO basis sets, the small differences in the values may also be ascribed to the different substituents of two calculated complexes. Therefore, it is evident that the electronic structures of the complexes discussed here are well described by the theoretical level used in this study. The transition state for the conformational isomerism of complex 5Hf-a is found to be the near-planar complex 5Hf-a0 (Scheme 8b) and the barrier for this process is calculated to be 44.6 kcal mol
1 for anti complex to syn complex conversion. The barrier for conformational isomerism of complex 5Hf(anti) to complex 5Hf(syn) with R = SiMe3 is calculated to be 28.2 kcal mol
1, which is comparatively closer to the experimentally measured value of 14.4 ( 0.3 kcal mol
1.12 Calculations including solvent effects also did not reproduce the experimental value of the barrier. It is worthwhile to note that the transition state structure with R = SiMe3 is geometrically more close to the anti or syn complex compared to the transition state structure with R = H (R = H,

Figure 2. (a) Important molecular orbitals of the five-membered metallacycloallene 5Hf. (b) Interaction diagram of complex 5Hf-a(syn).

C1
C2 3 3 3 C4
Hf =
42.9° (anti), 9.5° (TS); R = SiMe3, C1
C2 3 3 3 C4
Hf =
40.1° (anti), 12.5° (TS)). The Hf
C3 and Hf
C4 bond lengths are almost similar (2.359 and 2.342 Å, respectively), which implies that, along with a distinct Hf
C4 bond, there is also a strong interaction between the metal atom and the internal carbon atom C3. Due to the nonplanar nature of the MC4 ring and distortion at the C2 carbon atom of the allenic arrangement, the interaction between the hafnium atom and the carbon atoms C1 and C2 is not as strong as that of Hf
C4 and Hf
C3 bonds. This is why the Hf
C1 and Hf
C2 bonds are comparatively longer than the Hf
C3 and Hf
C4 bonds. The Wiberg bond indices for C2
C3 and C3
C4 bonds are calculated to be 1.49 and 2.05 (Table S1, Supporting Information) for complex 5Hf-b(anti) which is close to that of a C
C double bond. Moreover, the bond order (1.10) and bond length (1.479 Å) of the C1
C2 bond suggest the existence of a C2
C3
C4 allenic arrangement in 5Hf-b(anti). Our NBO analysis is also in good accord with the results obtained at the PBE-D/TZVPP0 level of theory (Table S1).12 2.3.1. Molecular Orbital Analysis of Bonding of Complex 5Hf. We have carried out the molecular orbital analysis of bonding of the metallacycloallene 5Hf with the substituents R = H, SiH3. The interaction diagram for complex 5Hf with both the substituents (R = H, SiH3) as well as for the syn and anti configurations are essentially the same; thus, only the bonding analysis of 5Hfa(syn) will be given. If we consider Cp
as a six-electron donor, we have Hf2þ with two d electrons. The three frontier orbitals of the Cp2Hf fragment are in the MC4 plane (e2g, a1g in Scheme 3). Now, the frontier orbitals of the RCCCRCHB(C6F5)2 fragment are formed from two in-plane p orbitals on the two central carbon 2674

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Table 4. Calculated NICS(x) Values (in ppm) for Complex 5Hf-ba (at the BP86/LANL2DZ Level of Theory) [Cp2Hf(H3SiCCC-

bent-[H3SiHCCC-

(SiH3)C(H)-

(SiH3)C(H)(H)-

(B(C6F5)2))]a

(B(C6F5)2)]b

concave side

convex side

concave side

convex side

NICS(0)


13.2

NICS(0.5)


13.7


17.0


1.8


2.7
4.6

NICS(1.0) NICS(1.5)


13.4
12.4


16.9
10.7


1.4
1.1


4.9
2.9

a

Calculated at the ring center (0) and above the ring center (0.5
1.5 Å). Using the same structure by deleting Cp2Hf and satisfying the two dangling valencies of the bent-[H3SiHCCC(SiH3)C(H)(H)(B(C6F5)2)] by two H atoms. b

Figure 3. Correlation diagram between the planar structure 5Hf-P and the nonplanar structure of complex 5Hf-b. The variation of the HOMO and LUMO of complex 5Hf-b is plotted as a function of the nonplanarity of the complex (on both sides). 5Hf-R refers to the other nonplanar conformer of complex 5Hf-b.

atoms (C2 and C3) and the predominantly sp3 and sp2 hybrid orbitals on the terminal carbon atoms (C1 and C4), leading to four linear combinations similar to the π-orbitals of butadiene. Apart from these, there are two perpendicular p orbitals which generate a localized π-bond on the C3 and C4 carbon atoms. The lowest two MOs along with the one π-MO among these are filled. Though the complex 5Hf-a has allenic character, the interaction diagram of this metallacycloallene is strikingly similar to that of metallacyclopentyne 4M (Figure 2). The C1
C2 bond distance of 5Hf-a is 1.480 Å (Table 3), which is close to the C
C single-bond distance (C(sp3)
C(sp2)) of 1.512 Å in cyclopentene.24 The C3
C4 bond distance of 5Hf-a (1.321 Å) is shorter than its C2
C3 bond distance (1.383 Å). In the carbocyclic analogue 6, both the C2
C3 and C3
C4 bond distances are calculated to be 1.390 Å, which are longer compared to that in complex 5Hf-a. The HOMO of the metallacycle (Figure 2) involves donation of electrons from the metal orbital (HOMO) to the vacant ligand orbital (LUMO) which is bonding between the carbon atoms C2 and C3, leading to a shortening of the C2
C3 bond distance. This is in agreement with the relatively lower C2
C3 bond distance in complex 5Hf-a compared to that in 6. The other stabilizing interaction (HOMO-10, Figure 2) involves donation of electrons from the ligand orbital to the vacant metal orbital. It is important to note that the interaction of the central carbon atoms C2 and C3 with the Cp2Hf orbitals, found in HOMO and HOMO-10, is crucial for the unusual stability of complex 5Hf-a. There is a localized perpendicular π-bond on C3 and C4 which further reduces the C3
C4 bond distance (1.321 Å compared to 1.390 Å in 6).

Similar kinds of interactions are found in complex 4M. The bonding of complex 3M is also similar to that in these two complexes, except that there are no localized perpendicular π-orbitals in this complex. The molecular orbital picture supports a cycloallenic electronic structure for complex 5Hf, as is also corroborated by NBO calculations (Table S1). A correlation diagram between the planar structure and nonplanar structure of complex 5Hf-b is given in Figure 3 in order to get an insight as to why the complex adopts a nonplanar structure. The HOMO and LUMO of the planar complex (5Hf-P; Figure 3) are very close in energy (ΔE = 0.3 eV) and, consequently, it undergoes structural distortion, allowing mixing of HOMO and LUMO and thus stabilizing the nonplanar complex 5Hf-b to a greater extent. The structural distortion in complex 5Hf-b takes place due to a secondorder energy change in the HOMO. Therefore, this is a second-order or pseudo-Jahn
Teller distortion.25 This distortion is also observed in the recently synthesized five-membered heterometallacycloallene.26 2.3.2. Description of Complex 5Hf. Complex 5Hf can be described as a coordinated ene-yne complex (5Hf0 , Scheme 4) or a cycloallene complex (5Hf). We have used NICS calculations to look into the issue in addition to MO and NBO analyses. The increase in the negative NICS values of the isolated ligand on coordination with the metal fragment suggests a significant interaction between the Cp2Hf moiety and the allenic ligand moiety (Table 4). Thus, it supports complex 5Hf as the major contributor to the resonance hybrid of the complex. 2.3.3. Reactivity of Complex 5Hf. HOMO, HOMO-1, and HOMO-10 in complex 5Hf-a (Figure 2a) control the reactions with other fragments/ligands. The perpendicular π-MO (HOMO-1) in complex 5Hf-a is mainly localized on carbon atoms C3 and C4 and does not have any significant interaction with the metal atom. Thus, it can be predicted that, as for metallacyclopentyne 4M, these perpendicular π-orbitals in complex 5Hf-a are available for further interaction with a second metal to form a bimetallic complex. Calculations give one such bimetallic complex (9, Scheme 9a), where the ring carbon atoms get rehybridized to interact with the second Cp2Hf fragment nearly on the Hf1
C2
C3
C4 plane rather than interacting perpendicularly. This may be attributed to the nonavailability of matching orbitals of comparable energy in the second Cp2Hf fragment with the perpendicular π-orbitals of complex 5Hf-a. This is in line with the formation of a bimetallic complex of complex 4Zr.18d The second metal fragment can also attach to 2675

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Scheme 9. (a) Schematic Representations of the Optimized Structure of Two Bimetallic Complexes (9 and 10) of Complex 5Hf-a and (b) Carbon Dioxide Inserted Complex 11

Scheme 10. HOMO and HOMO-1 of (a) Complex 9 and (b) Complex 10 (R0 = B(C6F5)2)

Scheme 11. Ring Strain by Partial Hydrogenation Energya

a

All ΔE values are calculated at the BP86/LANL2DZ level of theory.

the two central carbon atoms C2 and C3 of complex 5Hf-a to form another bimetallic complex, 10 (Scheme 9a), because of the availability of the molecular orbitals HOMO and HOMO-10 in the complex (Figure 2a). This bimetallic complex 10 is also found to be a minimum on the potential energy surface. It may be noted that complex 10 is more stable by 3.8 kcal mol
1 compared to complex 9. This can be attributed to the symmetrical positioning of the second “Cp2Hf” fragment in complex 10, which interferes less sterically with the Cp rings of the other

“Cp2Hf” moiety, in comparison to the unsymmetrical attachment in complex 9. The HOMO and HOMO-1 of complexes 9 and 10 are shown in Scheme 10. It is evident that there are effectively no interactions between the hafnium atom Hf (of the five-membered ring) and the central carbon atoms C2 and C3 in the two bimetallic complexes, and this is well-supported by the Hf
C2 (2.619 Å for complex 9 and 2.534 Å for complex 10) and Hf
C3 (2.608 Å for complex 9 and 2.659 Å for complex 10) bond lengths. 2676

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Calculations also reveal that complex 5Hf-a can react with one molecule of carbon dioxide (CO2) to form complex 11 (Scheme 9b). Nitrile insertion into a similar five-membered zirconacycloallene complex was observed experimentally; however, in this case insertion into the Zr
C(sp3) single bond takes place.27 The successful experimental realization of complex 5Hf can be understood if the ring strain in the molecule is considered in comparison to that of its carbocyclic analogue (6) (Scheme 11). There is a significant relief in the ring strain on replacing the CH2 group in the five-membered cycloallene by the isolobal Cp2M moiety, as can be observed from the lower ΔE values of these complexes (6 and 5Hf; Scheme 11). We have also calculated the partial hydrogenation energy of complex 3Hf to complex 5Hf along with their carbocyclic counterparts (Scheme 11), and this indicates complex 5Hf to be comparatively less strained than complex 3Hf. It is interesting to note that the first double-bond hydrogenation energy of 1 is lower compared to the second double-bond hydrogenation energy. This is understandable, since both 1 and 6 are much more strained compared to 600 . 2.4. Four-Membered Metallacycloallenic-Type Complex 7Zr. Wojcicki et al. synthesized and characterized a Cp2Zr(CH3)(η3-C(Ph)dCdCH2)15 complex (7Zr; Figure 4). The interaction between Zr and C1, C2 can be described as a Zr
C1 σ-bond or as a part of a Zr and C1
C2 π-interaction. This is why the complex has been termed as a four-membered metallacycloallenic-type complex, not a metallacycloallene complex. The methyl and C1, C2, and C3 atoms of the phenylallenyl ligand are roughly coplanar with the plane that bisects the Cp2Zr fragment. For the phenylallenyl ligand, the Zr
C bond distances increase from C3 to C2 to C1 (Table 5). The calculated structural

parameters are in reasonable agreement with the experimental findings. Wojcicki et al. described the bonding of the CH2CCPh ligand as a combination of η3-propargyl (A) and η3-allenyl (B) resonance structures (Scheme 12). Various experimental data of complex 7Zr imply a greater contribution of resonance structure B than of A in the resonance hybrid.15 This indicates complex 7Zr to behave more like a typical metallacycloallene complex. 2.4.1. Molecular Orbital Analysis of Bonding of Complex 7Zr. The electronic structure of 7Zr is constructed in steps. It is natural to divide 7Zr initially into the metal fragment, Cp2ZrCH3, and the ligand fragment, C3PhH2. The frontier molecular orbitals of Cp2Zr and CH3 group interact to generate the frontier molecular orbitals of the metal fragment, Cp2ZrCH3 (Figure 5b). We see that Zr is now in a formal oxidation state of þ3 with one valence electron, as we have two Cp
and one CH3
group. Addition of the ligand fragment C3PhH2 yields an oxidation state of þ4 for the overall complex 7Zr. The molecular orbitals of C3PhH2 are (i) the allyl type in-plane (xz plane) p orbitals on C3 and C2 and one spx (x = 2, 3) hybridized orbital on C1 and (ii) the C2
C3 π-bond perpendicular to the planes. The HOMO and HOMO-1 of the Cp2ZrCH3 fragment with HOMO-1 in higher coefficient interact strongly with both HOMO and LUMOþ2 of the C3PhH2 fragment with HOMO in higher coefficient and donate electrons to these singly occupied and vacant orbitals of the C3PhH2 fragment (HOMO; Figure 5). The important MOs and the interaction diagram of complex 7Zr are shown in Figure 5. The LUMO and HOMO of Cp2ZrCH3 fragment interact with the singly occupied HOMO of the C3PhH2 fragment to form HOMO-2 of the complex (Figure 5). There is a localized π-bond (HOMO-1; Figure 5) perpendicular to the Zr
C1
C2
C3 plane which does not have any significant interaction with the metal atom. Thus, it can be suggested that these perpendicular π-orbitals are available for further interactions with a second metal and can form bimetallic complexes as for the metallacyclopentyne (4M) and metallacycloallene (5Hf), assuming the Ph Scheme 12. Mesomeric Forms of Complex 7Zr

Figure 4. Optimized structure (left) and schematic representation (right) of complex 7Zr.

Table 5. Comparison of Computed (at the BP86/LANL2DZ Level of Theory) and Experimental Structural Bond Lengths (Å) and Angles (deg) of Complex 7Zr bond length

7Zr (calcd)

7Zr (exptl)

Zr
C1

2.707

2.658

Zr
C2

2.487

2.438

Zr
C3

2.382

Zr
C(CH3) Zr
C(Cp)

2.371 2.284

bond angle

7Zr (calcd)

7Zr (exptl)

C1
C2
C3

153.9

155.4

C1
Zr
C3

61.2

60.6

2.361

C1
Zr
C(CH3)

68.0

68.0

2.364 2.236

Zr
C3
C(Ph) Zr
C1
H1

143.1 109.2

144.4 106.0

C1
C2

1.370

1.344

Zr
C1
H2

109.1

107.0

C2
C3

1.305

1.259

H1
C1
H2

115.7

124.0

C3
C(Ph)

1.464

1.462 2677

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capable of forming bimetallic complexes (9 and 10) with another Cp2Hf fragment, and it can also react with one molecule of carbon dioxide (CO2) to form complex 11, which could be worth investigating experimentally. These reactions support our proposed bonding picture for complex 5Hf. The similarity of bonding of the five-membered metallacycloallene 5Hf to that of the metallacyclocumulene 3M and metallacyclopentyne 4M reveals the remarkable tendency of the bent metallocene (Cp2M) moiety to internally stabilize strained organic π systems, such as bent cumulenes, alkynes, and allenes. The synthesis of the fivemembered zirconacycloallene13,14 has reconfirmed this fact.

4. COMPUTATIONAL DETAILS

Figure 5. (a) Important molecular orbitals of complex 7Zr. (b) Interaction diagram of complex 7Zr.

substituent will not interfere in this case. It is also interesting to note that the bonding of complex 7Zr is quite similar to that of the five-membered metallacyclopentyne (4M) and metallacycloallene (5Hf). The Wiberg bond indices for the C1
C2 and C2
C3 bonds are calculated to be 1.59 and 2.00, respectively, which are close to a C
C double bond. This indirectly corroborates the experimental observation of complex 7Zr being allenic in nature. The Wiberg bond indices for Zr
C1, Zr
C2, and Zr
C3 are calculated to be 0.31, 0.18, and 0.51, respectively. The smaller value for Zr
C2 bond is expected, since C2 is not strongly involved in the bonding interaction with the metal atom, as the coefficient on C2 of the allyl type nonbonding MO is very small.

3. CONCLUSIONS Theoretical investigations on all-carbon metallacycles revealed similarities in the electronic structures of the metallacycloallene complex 5Hf with metallacyclopentynes 4M and phenylallenyl zirconocene 7Zr. These can be attributed to similar kinds of in-plane and out-of-plane molecular orbitals of the acetylene and allene moieties. The strong interaction of the two central carbon atoms C2 and C3 along with the terminal carbon atoms C1 and C4 with the bent Cp2Hf moiety (HOMO and HOMO-10) is the reason for the unusual stability of this metallacycloallene 5Hf. This type of unique interaction is also the main driving force for the experimental realization of metallacyclocumulene 3M and metallacyclopentyne 4M. Like metallacyclopentyne 4M, metallacyclocumulene 3M, and complex 7Zr, the metallacycloallene 5Hf is theoretically predicted to be

All complexes have been optimized at the BP86/LANL2DZ level of theory using the Gaussian 03 program package.28 BP86 includes the exchange function of Becke29,30a,30b and the correlation functional of Perdew.31 The LANL2DZ basis set uses the effective core potentials (ECP) of Hay and Wadt.32 The natures of the stationary points of complexes are characterized by vibrational frequency calculations. These complexes are found to be minima on the potential energy surface. In order to get a better understanding of the formation of these unusual metallacycles, fragment molecular orbital (FMO) analysis has been done at the BP86/TZ2P level of theory using the ADF2007.01 package.33 Here the basis sets for all the atoms are of triple-ζ quality, having two sets of polarization functions for all atoms. The optimized structures obtained at the BP86/LANL2DZ level are used for single-point fragment analyses in the ADF calculations. The interaction diagrams are constructed on the basis of the energy values obtained in the ADF calculations. Further, the nature of bonding has been studied using natural bond orbital (NBO) analysis.34 Calculations using the PCM35 solvent model with toluene (ε = 2.379) as the solvent have been carried out at the same level of theory using the Gaussian 09 program package.36 The correlation diagram in Figure 3 is drawn from single-point calculations of the planar and intermediate structures, since optimization of the structures leads to the nonplanar geometry (5Hf). The whole discussion is based on the results obtained at the BP86/LANL2DZ level of theory, unless otherwise specified.

’ ASSOCIATED CONTENT Supporting Information. Text, tables, and figures giving the results of NBO calculations of complex 5Hf-b, important molecular orbitals as well as interaction diagrams of complexes 3Zr and 4Zr, computed structural coordinates and energies for all the structures, and the complete ref 28. This material is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*E.D.J.: e-mail, [email protected]; tel, þ91 471 259 7421; fax, þ91 471 259 7442. A.S.: e-mail, [email protected]; tel, þ49 381 498 6400; fax, þ49 381 498 6382. U.R.: e-mail, uwe. [email protected]; tel, þ49 381 1281 176; fax, þ49 381 1281 51176.

’ ACKNOWLEDGMENT S.R. and E.D.J. thank the SERC of IISc, the HPCF, and the CMSD of the University of Hyderabad for computational facilities. We thank the Department of Science and Technology, New Delhi, India, for funding this research (J C Bose Fellowship). 2678

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