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. 2026 Sep 14;6(5):575–579. doi: 10.1021/acsorginorgau.6c00082

Light-Enhanced Synthesis of TiSb2 and TiBi2 Metallacycles

Matthias Bollenbeck †, Floria A E Hojer †, Rosa M Gomila ‡, Gregor Schnakenburg †, Antonio Frontera ‡, Alessandro Bismuto †,*
PMCID: PMC13650951  PMID: 42857055

Abstract

The synthesis of small metallacycles of early transition metals and heavy pnictogens has been primarily achieved by reductive coupling of the pnictane moiety with a transition metal fragment. Herein, we report a light-enhanced synthesis of titanocene Sb2 and Bi2 metallacycles starting from the dipnictene (RSb=SbR, RBi=BiR) and a suitable titanocene synthon Cp2Ti­(btmsa) (Cp = C5H5, btmsa = bistrimethylsilylacetylene). We further explored the reactivity of the TiSb2 complex by employing a diazoolefin, leading to the formation of an unusual six membered heterocycle. These findings were supported by thorough theoretical analysis of the electronic features of these complexes and expand upon previous studies of 3d transition metal dipnictene complexes.

Keywords: main group chemistry, heavy dipnictenes, titanium complexes, light-activation, diazoolefins

graphic file with name gg6c00082_0006.webp

graphic file with name gg6c00082_0005.webp

The chemistry of group 4 metallocenes with nitrogen and phosphorus has been widely explored, establishing a foundation for the coordination of group 15 ligands. − Pioneering work by Bergman demonstrated a general strategy to synthesize ZrN2 and HfN2 small cycles using alkali metal hydrazido species as starting materials (Figure A). Employing a similar strategy, Hey-Hawkins and co-workers were able to isolate ZrP2 rings from lithium phosphides shortly after (Figure A). Moving down the group, Hering-Junghans and co-workers recently reported significant advances in phosphorus and arsenic coordination chemistry. By developing a scalable protocol for Dipp-substituted (Dipp = 2,6- i Pr2-C6H3) triphosphiranes and triarsiranes, they used these P3 and As3 rings as synthons for a selective fragmentation upon reaction with Cp2Ti­(btmsa). , This methodology yielded titanocene diphosphene as well as the first structurally characterized titanocene diarsene complexes, demonstrating how bulky aryl substituents can stabilize reactive pnictogen frameworks. Interestingly, they noted that the direct reaction from a bulkier ArMes-diarsene (ArMes = 2,6-[2,4,6-Me3-C6H2]2-C6H3) precursor led to a much slower reaction, establishing the use of small pnictogen rings as a superior synthetic strategy (Figure A).

1.

1

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(A) Selected examples of group 4 metallocene complexes with N, P and As. (B) Previously reported titanocene and zirconocene complexes of Sb and Bi. (C) This work: Light-enhanced synthesis of titanocene Sb2 and Bi2 metallacycles.

Applying similar synthon-based approaches to even heavier pnictogens, however, has not always yielded selective results. , Silvestru and co-workers attempted the synthesis of Ti–Sb complexes using distibene complexes, but the reactions typically resulted in dimers and complicated clusters, which were challenging to isolate preparatively (Figure B). Early milestones in this area were obtained by Robinson and co-workers, with the first structurally characterized organometallic compound containing Bi–Zr bonds, the dibismuthene zirconocene complex Cp2Zr­(BiArMes)2 (Figure B). Shortly after, Rosenthal and co-workers were able to extend this strategy and isolate the first titanium-distibene complex starting from a well-defined ArMesSbH2 precursor (Figure B). A primary challenge in accessing these heavier hydrides is the traditionally challenging synthesis and the high steric stabilization required to prevent decomposition. − Thus, a simpler route from the dipnictene is highly sought after.

Our group has recently used visible light as an efficient means to activate dipnictenes with organic and inorganic substrates. We now wonder whether this may be the key to enhance the chemical space of Ti–Sb/Bi complexes, overcoming existing synthetic hurdles and providing an efficient pathway to these elusive and structurally unique heavy pnictogen metallocycles (Figure C).

We started our investigation by reacting Sb2Tbb2 (Tbb = 2,6-[CH­(SiMe3)2]2-4- t Bu-C6H2) with Cp2Ti­(btmsa). Irradiation of an orange suspension of Sb2Tbb2 in the presence of a small excess of Cp2Ti­(btmsa) with a blue LED (λ = 456 nm) rapidly led to a color change to green and yielded a new product detectable by 1H NMR spectroscopy. Following recrystallization, complex 1 was obtained in very good yield (88%) as deep green crystals (Figure A). Complex 1 was fully characterized by UV/vis (see SI Figure S3) and NMR spectroscopy and its molecular structure was elucidated by single-crystal X-ray diffraction (sc-XRD) (Figure B). Complex 1 consists of a TiSb2 metallocycle, featuring a trans-configuration of the Tbb ligands. The Ti1–Sb1 (2.8355(4) Å) and Ti1–Sb2 (2.8445(4) Å) bond lengths are virtually identical and consistent with Ti–Sb single bonds, and match the literature precedent (2.8402(8) Å and 2.8487(9) Å). The Sb1–Sb2 (2.8502(2) Å) bond length is significantly elongated in comparison to Sb2Tbb2 (2.6677(3) Å) and well comparable to an Sb–Sb single bond (2.835 Å to 2.862 Å). ,− The C1–Sb1–Sb2–C2 (156.94(7)°) torsion angle is noticeably decreased compared to that of Sb2Tbb2 (180°).

2.

2

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(A) Synthesis of complexes 1 and 2. (B) Single crystal X-ray structures of complexes 1 and 2, displacement ellipsoids are drawn at 30% probability level; hydrogen atoms and solvent molecules are omitted for clarity.

Next, we wondered whether the same protocol could be applied to Bi2Tbb2, whose π-bond is known to be harder to activate than that of its lighter congeners, caused among other factors by the relativistic effect. Upon irradiating a purple suspension of Bi2Tbb2 in the presence of an excess of Cp2Ti­(btmsa) with a green LED (λ = 526 nm) the formation of a new complex could be observed by 1H NMR spectroscopy, as well as a color change to deep green (Figure A). Recrystallization of the crude reaction mixture gave complex 2 as deep green crystals in very good yield (84%). Single-crystal X-ray diffraction revealed it to be the corresponding TiBi2 three-membered ring (Figure B), featuring a trans-configuration of the two Tbb moieties. The Bi1–Ti1 (2.8998(7) Å) and Bi2–Ti1 (2.8819(8) Å) bond lengths are almost identical and the Bi1–Bi2 (3.1072(8) Å) bond distance is noticeably elongated compared to Bi2Tbb2 (2.8537(5) Å), consistent with a Bi–Bi single bond (typically between 2.9902 Å to 3.0534 Å). ,− The Bi1–Ti1–Bi2 angle (65.01(3)°) is well comparable with previous ZrBi2 (63.3) and CoBi2 rings (64.91°, 67.36°). ,, Furthermore, the C1–Bi1–Bi2 (114.17(12)°) and C2–Bi2–Bi1 (114.32(13)°) angles are significantly wider than those of Bi2Tbb2 (98.86°), hinting at an incresaed s-character of bonding orbitals. Like in the case of 1, these findings suggest σ-donation along with π-back donation in accordance with the Dewar-Chatt-Duncanson model. The room temperature 1H NMR of both 1 and 2 showed a single set of signals coresponding to Cp ligands as well as a single set of homotopic Tbb ligands, consistent with its C2 symmetric solid state structure. The CTbb–Sb/Bi bond rotation is likely frozen out on the NMR time scale at this temperature, as four sets of singlets for SiMe3 in 1:1:1:1 ratio and two sets of singlets for each disilyl proton (disilyl = CHTMS2) in 1:1 ratio are present.

To investigate the importance of visible-light irradiation to the activation of the heavy dipnictene and the formation of complexes 1 and 2, we perfomed a thermal control experiment. The thermal reaction (T = 60 °C) reached 22% conversion to 1 in 30 min, which increased to 33% after 90 min of heating, while nearly quantitative formation of 1 was achieved already after 30 min under blue light (λ = 456 nm) (see SI Figure S1). A similar result was observed for 2, displaying 93% conversion to 2 after 30 min under green light (λ = 525 nm) and only 42% conversion to 2 under heating (T = 60 °C) after 30 min, increasing to 58% after 90 min (see SI Figure S2). In both cases the photochemical condition led to a faster reaction and an improvement of the yield.

The wavelength-dependent photoactivation observed for each system is rationalized by relativistic effects on the frontier molecular orbitals of the dipnictenes. Sasamori et al. demonstrated that Bi2Bbt2 (Bbt = 2,6-[CH­(SiMe3)2]2-4-C­(SiMe3)3-C6H2) possesses a smaller HOMO–LUMO gap compared to Sb2Bbt2. Subsequently, the dibismuthene system requires lower-energy (longer-wavelength) photons (λ = 525 nm) for efficient activation compared to the distibene system (λ = 456 nm), which possesses a larger π-π* separation.

To gain further insight into the electronic features of compounds 1 and 2, we employed a combination of quantum chemical methods (see SI for computational details). The Quantum Theory of Atoms in Molecules (QTAIM) analysis (Figure A) reveals, for both compounds, three bond critical points (BCPs, red dots) and one ring critical point (RCP, blue dot), connected by three bond paths spanning the Ti and Pn atoms (Pn = Sb, Bi), unambiguously confirming the formation of a three-membered TiPn2 ring rather than a simple η2-dative coordination of the Pn=Pn unit to titanium. This bonding picture is further corroborated by the 2D map of the Laplacian of the electron density (∇2ρ), in which the zero-flux surfaces clearly delineate three distinct atomic basins interconnected by the Ti–Pn and Pn–Pn bond paths. The covalent nature of all three ring bonds is further supported by the QTAIM-derived –G/V ratio at the BCPs. Values of –G/V significantly less than 1 are diagnostic of covalent bonding, whereas values greater than 1 indicate closed-shell interactions. In both 1 and 2, the – G/V values at all three ring BCPs fall well below unity (Figure A, table), confirming the predominantly covalent character of the Ti–Pn and Pn–Pn bonds. In both rings, the delocalization indices (DI) and Wiberg bond indices (WBI) are close to unity for all three bonds (Sb–Ti, Sb–Sb, Bi–Ti and Bi–Bi), consistent with the presence of three discrete single bonds rather than a residual double bond character. The computed Ti–Pn, Pn–Pn distances closely reproduce the experimental values obtained by sc-XRD (Figure A, table).

3.

3

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(A) 2D maps of the Laplacian of the electron density (∇2ρ) for complexes 1 (left) and 2 (right) in the TiPn2 plane (Pn = Sb, Bi). The gradient of the electron density is presented in the background; bond critical points (BCPs) and the ring critical point (RCP) are shown as red and blue dots, respectively, and bond paths as solid lines. QTAIM parameters at the BCPs are provided in the accompanying table (ρ and ∇2ρ, in a.u.) (B) Plots of the LUMO and HOMO of 1 with the corresponding orbital energies. (C) Selected NBOs of 1 showing the Sb–Sb and the two Sb–Ti σ-bonding orbitals, with indicated atomic contributions (60% Sb, 40% Ti). Level of theory: x2c-PW6B95-D4/x2c-QZVPPall//TPSS-D4/def2TZVP.

The frontier molecular orbitals of 1 and 2 (Figure B and SI Figure S6) further support this bonding scenario. The lowest unoccupied molecular orbital (LUMO) is predominantly localized on the titanium center, consistent with its largely metal-based d-orbital character.

The highest occupied molecular orbital (HOMO) is also mainly Ti-centered, but shows a sizable contribution from back-donation of the titanium d-orbitals into the π* orbital of the former SbSb double bond. This back-donation interpretation is corroborated by the significantly elongated Sb–Sb bond in complex 1 (2.8502(2) Å) compared to free Sb2Tbb2 (2.6677(3) Å), which is diagnostic of π-bond weakening characteristic of metal-to-pnictogen back-donation. The analogous orbital features and bonding situation are observed for complex 2 (SI Figure S6).

The three-membered ring nature of 1 and 2 is additionally confirmed by Natural Bond Orbital (NBO) analysis (Figure C), which identifies three localized σ-bonding orbitals describing the Sb–Sb and the two Sb–Ti interactions (NBO130–132) the latter moderately polarized toward antimony (ca. 60% Sb/40% Ti contribution), reflecting the polar covalent nature of these three-membered ring bonds; equivalent localized orbitals were found for the Bi congener 2 (see SI Figure S6). Natural Population Analysis (NPA) charges (Figure A, bottom) reveal comparable, moderate positive charges at both the Ti and the pnictogen centers in 1 and 2, while the ipso-carbon atoms of the Tbb ligands carry a negative charge, consistent with the expected polarization of the Ti–Pn and Pn–C bonds. This characterization aligns with the rigorous structural and electronic definitions of covalent three-membered rings established through comprehensive ring strain energy (RSE) analyses.

Next, we aimed to further explore the chemical space of complexes 1 and 2. Both seem to be labile toward loss of the dipnictene fragment and readily decompose in the presence of CO2, isocyanates, NHCs and nitriles into the dipnictene and unknown titanocene products. Since diazoolefins have been proven to be effective tools for bond activation, − we wondered, whether they might be the key to unlock the reactivity of 1 and 2. While the reaction of 2 with IMe4CN2 (IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) led to an unselective product mixture, addition of an excess of IMe4CN2 to a green solution of 1 produced an immediate color change to dark brown (Figure A). After 5 min, the 1H NMR spectrum displayed two new sets of Cp and Tbb resonances along with the disappearance of resonances of 1. Complex 3 was isolated in 60% yield as a red/brown solid and fully characterized by sc-XRD and NMR spectroscopy. Single-crystal X-ray diffraction confirmed the incorporation of the CN2 moiety into the metallocycle under cleavage of a Ti–Sb bond, leading to a distorted six membered ring (Figure B).

4.

4

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(A) Synthesis of complex 3. (B) Single crystal X-ray structures of complex 3, displacement ellipsoids are drawn at 30% probability level; hydrogen atoms and solvent molecules are omitted for clarity. (C) Plots of the FMOs of complex 3 with the corresponding orbital energies. Level of theory: x2c-PW6B95-D4/x2c-QZVPPall//TPSS-D4/def2TZVP.

The remaining Sb1–Ti1 (2.9134(3) Å) bond length is significantly elongated compared to 1, while the Sb1–Sb2 bond length exhibits only a minor elongation (2.8869(2) Å). The short N1–N2 (1.2170(9) Å) and N2–C2 (1.2930(9) Å) bond lengths, consistent with double bonds, as well as the Ti1–N1 (1.8694(10) Å) bond length, close to a typical double bond, suggest a high degree of electronic delocalization across the Ti1–N1–N2–C1 moiety.

Notably, these findings stand in contrast to previous findings of our group, in which CpCoSb2Tbb2(L) (L = C2H4, CO) was found to react with IMe4CN2 under cleavage of the Sb–Sb bond and loss of N2 to form a [1.1.0]-bicyclobutane analog. In conjunction with stronger Ti–N than Co–N bonds, the observed electronic delocalization across the Ti–N–N–C moiety in 3 might be the reason for the retention of the CN2 fragment. The frontier molecular orbitals (FMOs) of 3 (Figure C) reveal a markedly different electronic picture from that of 1 and 2. The HOMO is essentially a LP localized at Sb1, with only a minor contribution from the Ti center. The LUMO, in contrast, is a π-type orbital corresponding to the π*­(NN) system, delocalized onto the π-system of the IMe4 fragment and extending some contribution onto titanium, reflecting the electronic conjugation across the Ti–N1–N2–C1 unit that likely contributes to the retention of the N2 fragment in 3 (see SI Figure S7 for further information).

In summary, we reported the synthesis of a novel TiSb2 metallocycle as well as the first TiBi2 analog under visible-light irradiation. QTAIM, NBO and FMOs analyses confirm that both complexes feature genuine three-membered TiPn2 rings (Pn = Sb, Bi) held together by three discrete, largely covalent single bonds. Furthermore, the TiSb2 metallocycle 1 was found to undergo a divergent reactivity with IMe4CN2, compared with its CoSb2 analog, forming a distorted six-membered ring under Ti–Sb bond cleavage. Theoretical analysis of this ring-expanded heterocycle 3 reveals pronounced electronic delocalization across the Ti–NN–C fragment, accounting for the unusual retention of the N2 unit and distinguishing its bonding situation from that of the parent three-membered rings.

Supplementary Material

gg6c00082_si_001.pdf (2.1MB, pdf)

Acknowledgments

We thank the NMR and X-ray facilities of the University of Bonn for technical assistance. A. B. thanks the University of Bonn for financial support. We are grateful to the DFG and Prof. Dr. Menche for providing X-ray infrastructure. A. F. thanks MICIU/AEI of Spain (PID2023-14853NB-I00, FEDER funds) for financial support and the CTI (UIB) for computational facilities. M. B. and F. H. thank Mr. Meleschko for fruitful discussions.

The data underlying this study are available in the published article and its Supporting Information.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsorginorgau.6c00082.

  • Experimental procedures, analytical data including UV/vis spectra, NMR spectra and X-ray structures of all complexes, additional theoretical results and Cartesian coordinates of the optimized structures (PDF)

M. B. and A. B. conceived the idea behind the manuscript. M. B. and F. H. performed the experimental work. R. M. G. and A. F. performed theoretical analysis. G. S. performed the crystallographic measurements and solved the structures. The manuscript was written through contributions of all authors. All authors have given their approval to the final version of the manuscript.

The authors declare no competing financial interest.

Published as part of ACS Organic & Inorganic Au special issue “2026 Rising Stars in Organic and Inorganic Chemistry”.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

gg6c00082_si_001.pdf (2.1MB, pdf)

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

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