Access to tetracyclic aromatics with bridgehead metals via metalla-click reactions

The first π-conjugated tetracyclic-fused aromatics with bridgehead metals have been achieved by metalla-click reactions.

Abstract

The never-ending pursuits for exploring aromatic molecular architectures result in the large libraries of aromatics with fascinating structures, which have greatly broadened the scope of aromaticity. Despite extensive efforts that have been paid to develop aromatic frameworks, the construction of polycyclic aromatics that share a bridgehead atom with more than three rings has never been accomplished. Here, an unprecedented family of aromatics, in which a metal center shared by 4 five-membered aromatic rings, has been achieved by using the metalla-click reactions with excellent yields and remarkable regioselectivity. The distinctive tetracyclic aromatics exhibit a broad absorption in the ultraviolet-visible near-infrared region and excellent thermal stability in air, enabling their potential applications in photoelectric materials and biomedicine. This study now makes it possible to incorporate four aromatic rings with one common sharing metal center by a straightforward strategy that would promote further development of previously unknown polycyclic complex motifs in aromatic chemistry.

INTRODUCTION

Molecular architectures with fascinating aromatic structures have attracted considerable interest across the scientific community, leading to the discovery of enormous aromatic molecules with structural variety. Among those aromatic frameworks, the fused-ring aromatic topology is intriguing as it features structural elegance and distinct properties (1–6). However, a bridgehead carbon atom can only secure up to three aromatic fused rings because of its inherent atomic orbital distribution. To realize creative fused-ring aromatic frameworks, heteroatoms have been incorporated into the aromatic rings (7–12). Transition metal atoms have also been designed as heteroatoms to formally replace at least one carbon unit of the archetypal aromatic hydrocarbons, affording a distinctive class of heterocycles, metalla-aromatics, through synergistic interplay of theoretical and experimental investigations. The participation of the d-atomic orbitals of the transition metal facilitates the construction of fused-ring aromatic frameworks, as exemplified by a number of two– or three–fused-ring metalla-aromatics having one bridgehead atom in common (13–20). Although theoretically feasible, the integration of more than three π-aromatic fused rings sharing one bridgehead atom remains elusive, even in transition metal–involving systems. We speculated that tricyclic metalla-aromatics with high reactivity could be used as excellent building blocks that, upon cycloaddition with unsaturated substrates, would lead to unprecedented polycyclic aromatic motifs. With this hypothesis, a tricyclic complex bearing an M≡C bond was properly designed as starting material to explore straightforward annulation strategy via metalla-click reactions, i.e., the first [3 + 2] cycloadditions of a late transition metal carbyne with azides (Fig. 1). This strategy provides a convenient collection of extraordinary metal-bridgehead tetracyclic aromatics with a high degree of skeletal complexity and functional diversity. Furthermore, these complexes, as new members of the aromatics family, exhibit remarkably broad absorption from the ultraviolet (UV) to the near-infrared (NIR) region, leading to excellent photothermic properties.

Fig. 1. Designed tricyclic source enables the access to tetracyclic aromatics with bridgehead metals by metalla-click reaction.

RESULTS AND DISCUSSION

Synthesis of tetracyclic aromatics via click reaction of cyclic metal carbyne complexes

As a classic click reaction, copper-catalyzed azide/alkyne cycloaddition (CuAAC) features the formation of a five-membered aromatic heterocycle, i.e., 1,2,3-triazole, with high reaction efficiency under mild reaction conditions (21, 22). In recent years, Houk and co-workers reported that the high reactivity of strain-promoted Huisgen cycloadditions (in the absence of copper catalysts) is due to the lower energy required to distort the 1,3-dipole and alkyne into the transition-state geometry (23, 24). We thus envisaged that constraining transition metal carbynes into small rings would lead to the activation of the M≡C bond and offer access to the 1,3-dipolar cycloadditions with azides. On the basis of our previous investigation of metallapentalynes (25–29), the synthesis of the activated M≡C bond within the tricyclic ring system was then targeted, in the hope of obtaining suitable starting material to generate desired tetracyclic aromatic motifs through metalla-click reactions. We reacted complex 1 (27) with K₂CO₃ (5 eq) at 40°C for 2 days, which afforded a green solution, from which complex 2 was isolated in 84% yield (Fig. 2A). Complex 2 was characterized by nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS). In the ¹³C NMR spectrum, the signal for the carbyne carbon of complex 2 was observed at 337.2 parts per million (ppm), which is close to the reported value for osmapentalynes (25–28). The solid-state structure of complex 2 was further confirmed by single-crystal x-ray diffraction via counter anion exchange of Cl with BPh₄ (Fig. 2B; see the Supplementary Materials for details). The x-ray diffraction study revealed that 2 has a fused metallapentalyne core structure with good planarity, as reflected by the small mean deviation (0.034 Å) from the least-squares plane through Os1, O1, and C1─C9. The bond distance of the Os1─C1 [1.888(5) Å] and the bond angle at the sp-hybridized carbon atom [129.7(3)°] are comparable to those of previously reported metallapentalynes (25–29). The corresponding bond distances within the metallacycles, together with the planarity of the metallatricycle, indicate electronic delocalization within the metallatricycle in 2. It is noted that 2 is the first cyclic metal carbyne in tricyclic metalla-aromatics with a bridgehead metal. This reaction achieves a rare transformation from a metallapentalene to a metallapentalyne (26, 30) and generates a late transition metal carbyne with extremely high ring strain stemming from the small angle around C_carbyne (129.7°) within the five-membered ring, thus motivating us to further examine the reactivity of 2 with azides.

Fig. 2. Synthesis of polycyclic aromatics 2 and 3.

(A) Designed fused osmacycle containing an M≡C bond enables metalla-click reactions with azides. (B) Molecular structure of the cation of 2′ (drawn with 50% probability). The phenyl moieties in PPh₃ are omitted for clarity. (C) Molecular structure of the cation of 3c (drawn with 50% probability). The phenyl moieties in PPh₃ are omitted for clarity.

As indicated by in situ NMR, stirring a mixture of 2 and benzyl azide in dichloromethane at room temperature for 6 hours resulted in almost quantitative conversion to complex 3a (fig. S1), which was isolated in 92% yield. As illustrated in Fig. 2A, a series of azide substrates bearing different groups were also investigated. No intermediate species were observed even when the reaction was carried out at low temperature (0°C), suggesting that the transformation of 2 and azides to 3 is quite facile. The results demonstrated good compatibility and efficiency of this reaction. A range of diversely functionalized azides readily participated in the reaction to afford the corresponding products 3b to 3e in 85 to 90% yields. These annulation products 3 exhibit good thermal stability both in solution and in the solid state. For example, the solid sample of 3a can persist for several months when exposed to air at room temperature or even when heated at 100°C in air for 3 hours.

A single crystal of 3c suitable for x-ray diffraction studies was obtained by recrystallization in hexane. The x-ray diffraction study established that the osmium-carbon bond is incorporated with the N₃ unit to form a planar, five-membered osmacycle (Fig. 2C). The most remarkable feature is the four-leaf clover motif, in which the osmium center arranges in a seven-coordinated pentagonal bipyramidal geometry and serves as a common vertex for 4 five-membered metallacycles (the bicyclic metallapentalene unit, the metallafuran unit, and the metallatriazole unit). The tetracyclic skeleton containing Os1, O1, C1─C9, and N1─N3 is approximately coplanar, as indicated by the mean deviation from the least-squares plane (0.114 Å) and the sums of the angles of these four rings (540°, 539.7°, 540.0°, and 539.8°, which are close to the ideal value of 540°). All of the Os─C and C─C bond lengths in complex 3c are similar to those reported for osmapentalene derivatives (27, 28, 30–35). The Os1─O1 (2.121 Å) and C9─O1 (1.302 Å) bond lengths are close to those reported for osmafurans (bond length ranges in this article are all based on a search from the Cambridge Structural Database, CSD version 5.39 in November 2017) and metallapentalenofurans (2.082 to 2.234 Å for Os─O and 1.216 to 1.296 Å for C─O, respectively) (27). Both the planarity and the delocalized bond lengths in 3c suggest its delocalized structure. The planar metallatriazole unit in 3c has more delocalized structural parameters than previously reported examples with early transition metal tungsten and molybdenum (36, 37). For instance, the N1─N2 (1.309 Å), N2─N3 (1.310 Å), and C1─N1 (1.368 Å) bond lengths of 3c fall within the range of N─N and C─N single and double bonds (bond length ranges in this article are all based on a search of the Cambridge Structural Database, CSD version 5.39 in November 2017), respectively. Consistent with the solid structure, proton signals of the metallacycle were observed at 8.13 (H5), 8.02 (H8), and 7.62 (H3) ppm. In the ¹³C NMR spectrum, the signals of metal-bonded C1 (233.9 ppm) and C7 (224.7 ppm) are shifted downfield compared with that of C4 (168.4 ppm), suggesting that C1 and C7 show more carbene character than C4.

Although the “alkyne-like” metal-carbon triple bond has been rarely used in click reactions, pioneering efforts in the 1990s established that molybdenum and tungsten carbynes can undergo cycloaddition reactions with azides, similar to the azide/alkyne cycloaddition, to form metallatriazole units (36, 37). The inherent nucleophilicity of the sp carbons in early transition metal carbynes might naturally facilitate the addition of azides. However, such reactions have remained largely unexplored for more than 20 years, as demonstrated by these two examples so far, and have never been achieved in the late transition metal carbyne species before.

Theoretical calculations

Note that only 2-substituted products were formed in the metalla-click reactions of complex 2 with azides. To investigate the regioselectivity of this reaction, we calculated the B3LYP transition structures. As shown in Fig. 3, the Gibbs free energies of the transition states and products for the 2-substituted species (16.5 and −35.4 kcal/mol, respectively) were lower than those for the 4-substituted species (25.5 and −19.1 kcal/mol, respectively), suggesting that the 2-substituted product is both thermodynamically and kinetically favored. The bulky phosphonium substituent of 2 likely prevents the formation of the 4-substituted structure, exclusively yielding the sterically preferred 2-substituted product.

Fig. 3. Energy profiles for cycloaddition reactions of 4-anisyl azide and osmapentalyne 2⁺ calculated at the PCM-B3LYP-D3BJ/6–311++G(d,p)-LANL2TZ level.

The Gibbs free energies and selected bond lengths are given in kilocalories per mole and angstroms, respectively. The pathway for the formation of the 4-substituted product (3C⁺) is labeled in red, whereas that for the 2-substituted product (3c⁺) is labeled in blue.

We found that the N─N bond lengths in the 2-substituted model compound 3c⁺ (1.294 and 1.301 Å) are more average than those in the 4-substituted model 3C⁺ (1.224 and 1.445 Å). The experimentally observed delocalized bond lengths, planarity of the tetracyclic framework, downfield chemical shifts of the metallacycle proton, and high stability indicate the aromaticity of 3. Therefore, we calculated the nucleus-independent chemical shift (NICS) values (38) based on the unsubstituted model compounds 3′ and 3″ (the PPh₃ ligands were replaced by PH₃ groups). In general, negative NICS values indicate aromaticity, and positive NICS values indicate antiaromaticity. As shown in Table 1, the calculated NICS(1)_zz value for the metallatriazole unit (ring D) in 3′ is −16.2 ppm, whereas the value in 3″ is −4.8 ppm. The more negative NICS(1)_zz values, together with the more average of the bond lengths in the metallatriazole unit complex 3′, indicate that the metallatriazole unit (ring D) in 3′ might be more stable than 3″ owing to the better aromaticity. We thus infer that the difference in aromaticity may also account for the high regioselectivity of the conversion from 2 to 3. In addition, the NICS(1)_zz values of the other three fused metallacycles (rings A, B, and C) of 3′ are negative, whereas the values in the 4-substituted model 3″ are reduced to some extent. Therefore, the efficient cyclization reactions of late transition metal carbyne complex 2 with azides can be viewed as aromaticity-driven processes.

Table 1. NICS(1) values (in parts per million) of the 4 five-membered rings of the 2-substituted model 3′ and the 4-substituted model 3″ calculated at the B3LYP/6–311++G(d,p) level.

[Os]′ = Os(PH₃)₂.

Model	NICS	A	B	C	D
	NICS(1)	−3.7	−10.9	−9.9	−7.3
	NICS(1)zz	−5.4	−24.8	−21.3	−16.2
	NICS(1)	−3.2	−8.7	−8.7	−3.7
	NICS(1)zz	−5.0	−18.0	−17.2	−4.8

To elucidate the bonding and electronic structure of these unique tetracyclic aromatics, we performed density functional theory (DFT) calculations on simplified unsubstituted models of 3′. The DFT-optimized structural parameters of 3′, especially the bond distances in the metallacycles, agree well with those observed for the crystal structure of 3c. The calculated Wiberg bond indices for Os1─C1, Os1─C4, Os1─C7, Os1─O1, and Os1─N3 in 3′ are 0.85, 0.75, 0.86, 0.54, and 0.67, respectively, indicating strongly covalent bonding between the metal center and the metal-bonded ring atoms. The main characteristic of the orbital interactions in metallacycles of 3′ is the involvement of two filled metal d orbitals of the metal center in the π bonding. The molecular orbitals (MOs) are derived principally from the orbital interactions between the p_π orbitals of the organic fragment and the d orbitals of the Os atom (5d_xz and 5d_yz). The key occupied π-MOs (HOMO, HOMO-1, HOMO-3, HOMO-4, HOMO-6, HOMO-10, HOMO-13, HOMO-16, and HOMO-19) are identified as the valence π-MOs of the tetracyclic ring system (Fig. 4A). However, as also demonstrated in previous examples (25), π orbitals that are not a “perimeter MO” may not contribute to the aromaticity of metallacycles. Because HOMO-3 has the most contribution from the Os d_xz orbital, which is not pointing along the perimeter of the ring, the tetracyclic system is considered as an aromatic molecule with planar 4ne (14c-16e) dπ-pπ π-conjugation.

Fig. 4. Aromaticity evaluation.

(A) Nine key occupied perimeter π molecular orbitals (π-MOs) of the model complex 3′. The eigenvalues of the MOs are given in parentheses. (B) The aromaticity of model complex 3′ evaluated by the ISE method. The energies [in kilocalories per mole, computed with the B3LYP functional and the LanL2DZ basis set for Os and P and the 6 to 311++G(d,p) basis sets for C, O, N, and H] include zero-point energy corrections. (C) AICD plots of 3′ with the contribution from eight π-MOs. The molecular plane is placed perpendicular to the magnetic field vector. Isovalues for AICD isosurface is 0.025 arbitrary unit (a.u.). The diatropic ring currents indicate aromaticity.

The aromaticity of 3 was further confirmed by the isomerization stabilization energy (ISE) values (39) and anisotropy of the induced current density (AICD) analysis (40). As shown in Fig. 4, the calculated ISE results are comparable to the values reported for other metalla-aromatics (Fig. 4B) (25–28, 30–32, 34). The clockwise ring current density of 3′ on the AICD isosurface indicates that 3 is aromatic (Fig. 4C). All calculation results are in good agreement with the experimental results, indicating that tetracyclic complex 3 is aromatic. Therefore, complex 3 represents the first example of a transition metal shared by four π aromatic units simultaneously.

UV-vis-NIR absorption spectra and photothermal properties

With such a unique aromatic framework, will these tetracyclic complexes have novel properties? To address this question, we examined the UV-visible (UV-vis) absorption spectra of these unique π-conjugated complexes. As shown in Fig. 5A, metal-bridgehead tetracyclic complexes 3a to 3e showed remarkably broad absorption from the UV to the visible region and up to 850 nm (NIR region), probably due to the extended conjugation framework in metalla-aromatic 3. For instance, the absorption maximum of tetracyclic complex 3a (λ_max = 747 nm) was red shifted by 119 nm compared with that of tricyclic complex 2 (λ_max = 628 nm). Time-dependent DFT calculations at the B3LYP/6-31G(d) level were used to describe the absorption spectrum of 3a. The absorption bands at 747 and 648 nm can be assigned to the HOMO→LUMO and HOMO→LUMO + 1 electronic transitions (table S1), respectively. The NIR absorption spectra of these metal-bridgehead tetracyclic systems indicated their excellent photothermal properties (Fig. 5B). For example, the solution containing complex 3a (1.00 mg/ml) exhibits a significant temperature increase from 29° to 79°C within 5 min under NIR laser (808 nm, 1 W/cm²) irradiation, while the pure solvent shows a negligible temperature change under similar conditions. These results suggest their potential applications in photoelectric materials or biomedicine (32).

Fig. 5. UV-Vis-NIR absorption spectra and photothermal properties.

(A) The absorption spectra of 2 and 3a to 3e (5.0 × 10⁻⁵ M) measured in dichloromethane solution at room temperature. (B) Temperature elevation of the solvent and solutions of complex 3a at different concentrations (1.00, 0.50, 0.25, and 0.10 mg/ml) in 90% water-ethanol (v/v) solution upon laser irradiation (808 nm, 1 W/cm²).

CONCLUSION

In summary, metalla-click reactions between azides and metal-carbon triple bonds have been used as a new annulation strategy, affording an efficient, precise synthesis of an extraordinary polycyclic species consisting of four fused five-membered aromatic metallacycles sharing with a bridgehead metal center. The unusual topology represents an unprecedented case in a fused-ring aromatic system with a congested center, which breaks through the previous limitation of up to three π-aromatic rings. This approach not only extends the accessible range of classical click reactions but also offers insight into previously inaccessible aromatic skeletons with congested centers, which would ultimately inspire further investigation of discovering challenging and complex aromatic molecular architectures.

MATERIALS AND METHODS

Details of the synthesis and characterization of complexes 2′ and 3b to 3e can be found in the Supplementary Materials. The synthetic procedures for 2 and 3a are described below. The detailed crystal structures of complexes 2′ and 3c, as well as all of the HRMS and NMR spectra discussed in this article can be found in the Supplementary Materials.

Synthesis of complex 2

A mixture of compound 1 (200 mg, 0.15 mmol) and K₂CO₃ (103 mg, 0.75 mmol) in 10 ml of N,N-dimethylformamide was stirred at 40°C for 2 days to give a green solution. The solution was evaporated to dryness under vacuum. Then, the residue was extracted with dichloromethane (5 ml × 3) and filtered. The extractant was concentrated to approximately 3 ml. Subsequent addition of diethyl ether (20 ml) to the extractant gave a green precipitate, which was collected by filtration, washed with diethyl ether (10 ml × 2), and dried under vacuum. Yield: 138 mg, 74%. ¹H NMR plus ¹H-¹³C HSQC (600.1 MHz, CD₂Cl₂): 8.75 (s, 1H, H5), 7.63 (s, 1H, H3, confirmed by ¹H-¹³C HSQC), 7.08 (s, 1H, H8, confirmed by ¹H-¹³C HSQC), 3.82 (s, 3H, COOCH₃), and 7.85 to 7.00 ppm (50H, other aromatic protons). ³¹P NMR (242.9 MHz, CD₂Cl₂): δ = 6.82 (s, OsPPh₃), 5.21 ppm (s, CPPh₃). ¹³C NMR plus DEPT-135, ¹H-¹³C HSQC, and ¹H-¹³C HMBC (150.9 MHz, CD₂Cl₂): δ = 337.2 (td, J_PC = 16.3 Hz, J_PC = 14.4 Hz, C1), 231.1 (t, J_PC = 6.1 Hz, C7), 193.7 (s, C9), 165.1 (s, COOCH₃), 162.5 (s, C5), 159.4 (dt, J_PC = 26.4 Hz, J_PC = 3.1 Hz, C4), 141.4 (d, J_PC = 18.4 Hz, C3), 138.3 (s, C6), 116.2 (s, C8), 115.0 (d, J_PC = 99.1 Hz, C2), 51.4 (s, COOCH₃), and 137.4 to 127.0 ppm (other aromatic carbons). HRMS [electrospray ionization (ESI)]: m/z calcd for [C₇₁H₅₆O₃OsP₃]⁺, 1241.3059; found, 1241.3037. Elemental analysis calcd (%) for C₉₅H₇₆BO₃OsP₃: C 66.84, H 4.42; found: C 66.48, H 4.32.

Synthesis of complex 3a

Benzyl azide (56 μl, 0.45 mmol) was added to a solution of 2 (190 mg, 0.15 mmol) in 10 ml of dichloromethane. The reaction mixture was stirred for 6 hours to give a green solution. Then, the mixture was concentrated to approximately 2 ml under vacuum and washed with diethyl ether (3 × 30 ml) to afford a green solid. Yield: 193 mg, 92%. ¹H NMR plus ¹H-¹³C HSQC (500.2 MHz, CDCl₃): δ = 8.11 (s, 1H, H5), 7.97 (s, 1H, H8), 7.48 (s, 1H, H3, confirmed by ¹H-¹³C HSQC), 5.21(s, 2H, H10), 3.87 (s, 3H, COOCH₃), and 7.78 to 6.81 ppm (55H, other aromatic protons). ³¹P NMR (202.5 MHz, CDCl₃): δ = 11.41 (s, CPPh₃) and −21.46 ppm (s, OsPPh₃). ¹³C NMR plus DEPT-135, ¹H-¹³C HSQC, and ¹H-¹³C HMBC (125.8 MHz, CDCl₃): δ = 234.6 (t, J_PC = 6.9 Hz, C1), 226.5 (t, J_PC = 8.4 Hz, C7), 191.7 (s, C9), 166.5 (d, J_PC = 26.0 Hz, C4), 165.7 (s, C5), 164.3 (s, COOCH₃), 164.2 (d, J_PC = 14.8 Hz, C3), 137.6 (s, C6), 120.9 (s, C8), 114.1 (d, J_PC = 86.8 Hz, C2), 61.5 (s, C10), 51.3 (s, COOCH₃), and 137.4 to 120.7 ppm (other aromatic carbons). HRMS (ESI): m/z calcd for [C₇₈H₆₃N₃O₃OsP₃]⁺, 1374.3700; found, 1374.3723. Elemental analysis calcd (%) for C₇₈H₆₃ClN₃O₃OsP₃: C 66.49, H 4.51, N 2.98; found: C 66.71, H 4.90, N 2.70.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/3/eaay2535/DC1

Supplementary Information Text

Synthetic Procedures

Cartesian coordinate-optimized structures for calculation

Fig. S1. ³¹P{¹H} NMR spectrum of the starting material, complex 2, and the in situ ³¹P NMR spectra for the reaction of 2 with azide at room temperature.

Fig. S2. ACID isosurface of the model 3′ from the contribution of eight π-MOs.

Fig. S3. Positive-ion ESI-MS spectrum of [2]⁺ [C₇₁H₅₆O₃OsP₃]⁺ measured in dichloromethane.

Fig. S4. Positive-ion ESI-MS spectrum of [2′]⁺ [C₇₁H₅₆O₃OsP₃]⁺ measured in dichloromethane.

Fig. S5. Positive-ion ESI-MS spectrum of [3a]⁺ [C₇₈H₆₃N₃O₃OsP₃]⁺ measured in dichloromethane.

Fig. S6. Positive-ion ESI-MS spectrum of [3b]⁺ [C₇₇H₆₁N₃O₃OsP₃]⁺ measured in dichloromethane.

Fig. S7. Positive-ion ESI-MS spectrum of [3c]⁺ [C₇₈H₆₃N₃O₄OsP₃]⁺ measured in dichloromethane.

Fig. S8. Positive-ion ESI-MS spectrum of [3d]⁺ [C₇₈H₆₀F₃N₃O₃OsP₃]⁺ measured in dichloromethane.

Fig. S9. Positive-ion ESI-MS spectrum of [3e]⁺ [C₇₆H₆₇N₃O₅OsP₃]⁺ measured in dichloromethane.

Fig. S10. ¹H NMR (600.1 MHz, CD₂Cl₂) spectrum (inset: partial ¹H-¹³C HSQC spectrum) for complex 2.

Fig. S11. ³¹P{¹H} NMR (242.9 MHz, CD₂Cl₂) spectrum for complex 2.

Fig. S12. ¹³C{¹H} NMR (150.9 MHz, CD₂Cl₂) spectrum for complex 2.

Fig. S13. Two-dimensional ¹H-¹³C HSQC spectrum for complex 2 in CD₂Cl₂.

Fig. S14. Two-dimensional ¹H-¹³C HMBC spectrum for complex 2 in CD₂Cl₂.

Fig. S15. DEPT-135 spectrum (150.9 MHz, CD₂Cl₂) of complex 2.

Fig. S16. ¹H NMR (600.1 MHz, CD₂Cl₂) spectrum (inset: partial ¹H-¹³C HSQC spectrum) for complex 2′.

Fig. S17. ³¹P{¹H} NMR (242.9 MHz, CD₂Cl₂) spectrum for complex 2′.

Fig. S18. ¹³C{¹H} NMR (150.9 MHz, CD₂Cl₂) spectrum for complex 2′.

Fig. S19. Two-dimensional ¹H-¹³C HSQC spectrum for complex 2′ in CD₂Cl₂.

Fig. S20. Two-dimensional ¹H-¹³C HMBC spectrum for complex 2′ in CD₂Cl₂.

Fig. S21. DEPT-135 spectrum (150.9 MHz, CD₂Cl₂) of complex 2′.

Fig. S22. ¹¹B{¹H} NMR (192.5 MHz, CD₂Cl₂) spectrum for complex 2′.

Fig. S23. ¹H NMR (500.2 MHz, CDCl₃) spectrum (inset: partial ¹H-¹³C HSQC spectrum) for complex 3a.

Fig. S24. ³¹P{¹H} NMR (202.5 MHz, CDCl₃) spectrum for complex 3a.

Fig. S25. ¹³C{¹H} NMR (125.8 MHz, CDCl₃) spectrum for complex 3a.

Fig. S26. Two-dimensional ¹H-¹³C HSQC spectrum for complex 3a in CDCl₃.

Fig. S27. Two-dimensional ¹H-¹³C HMBC spectrum for complex 3a in CDCl₃.

Fig. S28. DEPT-135 spectrum (125.8 MHz, CDCl₃) of complex 3a.

Fig. S29. ¹H NMR (500.2 MHz, CD₂Cl₂) spectrum (inset: partial ¹H-¹³C HSQC spectrum) for complex 3b.

Fig. S30. ³¹P{¹H} NMR (202.5 MHz, CD₂Cl₂) spectrum for complex 3b.

Fig. S31. ¹³C{¹H} NMR (125.8 MHz, CD₂Cl₂) spectrum for complex 3b.

Fig. S32. Two-dimensional ¹H-¹³C HSQC spectrum for complex 3b in CD₂Cl₂.

Fig. S33. Two-dimensional ¹H-¹³C HMBC spectrum for complex 3b in CD₂Cl₂.

Fig. S34. DEPT-135 spectrum (125.8 MHz, CD₂Cl₂) of complex 3b.

Fig. S35. ¹H NMR (500.2 MHz, CD₂Cl₂) spectrum (inset: partial ¹H-¹³C HSQC spectrum) for complex 3c.

Fig. S36. ³¹P{¹H} NMR (202.5 MHz, CD₂Cl₂) spectrum for complex 3c.

Fig. S37. ¹³C{¹H} NMR (125.8 MHz, CD₂Cl₂) spectrum for complex 3c.

Fig. S38. Two-dimensional ¹H-¹³C HSQC spectrum for complex 3c in CD₂Cl₂.

Fig. S39. Two-dimensional ¹H-¹³C HMBC spectrum for complex 3c in CD₂Cl₂.

Fig. S40. DEPT-135 spectrum (125.8 MHz) of complex 3c.

Fig. S41. ¹H NMR (500.2 MHz, CD₂Cl₂) spectrum (inset: partial ¹H-¹³C HSQC spectrum) for complex 3d.

Fig. S42. ³¹P{¹H} NMR (202.5 MHz, CD₂Cl₂) spectrum for complex 3d.

Fig. S43. ¹³C{¹H} NMR (125.8 MHz, CD₂Cl₂) spectrum for complex 3d.

Fig. S44. ¹⁹F{¹H} NMR (376.4 MHz, CD₂Cl₂) spectrum for complex 3d.

Fig. S45. Two-dimensional ¹H-¹³C HSQC spectrum for complex 3d in CD₂Cl₂.

Fig. S46. Two-dimensional ¹H-¹³C HMBC spectrum for complex 3d in CD₂Cl₂.

Fig. S47. DEPT-135 spectrum (125.8 MHz, CD₂Cl₂) of complex 3d.

Fig. S48. ¹H NMR (500.2 MHz, CD₂Cl₂) spectrum (inset: partial ¹H-¹³C HSQC spectrum) for complex 3e.

Fig. S49. ³¹P{¹H} NMR (202.5 MHz, CD₂Cl₂) spectrum for complex 3e.

Fig. S50. ¹³C{¹H} NMR (125.8 MHz, CD₂Cl₂) spectrum for complex 3e.

Fig. S51. Two-dimensional ¹H-¹³C HSQC spectrum for complex 3e in CD₂Cl₂.

Fig. S52. Two-dimensional ¹H-¹³C HMBC spectrum for complex 3e in CD₂Cl₂.

Fig. S53. DEPT-135 spectrum (125.8 MHz, CD₂Cl₂) of complex 3e.

Table S1. Calculated absorption spectral data for 3a.

Table S2. Comparison of bond lengths in 3c from the B3LYP functional and experimental data.

Data file S1. CIF files for complexes 2′.

Data file S2. CIF files for complexes 3c.

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