Tripodal Silanolate Ligands Expand [MoX3] Chemistry Beyond Its Traditional Borders

Abstract

Homodimeric complexes [X₃Mo≡MoX₃] are commonplace, but in no case is the corresponding monomeric [MoX₃] species known; conversely, none of the very rare monomeric complexes [MoX₃] has the respective homodimeric analogue. This mutual exclusivity ends with the present study; on top, an entirely unprecedented class of heterodimers of type [X₃Mo≡MoY₃] is reported. Key to success was the use of tripodal silanolates as ancillary ligands; the fence formed by properly chosen peripheral substituents shields the sensitive Mo(+3) center; homodimerization of the resulting [MoX₃] complexes is then kinetically strongly disfavored, though possible. The monomers are able to cleave N₂O and convert gem-dihalides into metal alkylidynes; they exist in different binding modes, in which the basal phenyl ring of the ligand backbone is either completely unengaged with the central metal or tightly bound to it, depending on whether the ligand sphere is complemented by solvent molecules or not. If the latter are sufficiently labile, a surprisingly facile heterodimerization of the d³ electron fragments will ensue; the resulting products [X₃Mo≡MoY₃] incorporate the intact Cummins complex [(tBu)(Ar)N]₃Mo (Ar = 3,5-dimethylphenyl) as one of their constituents, which is famous for not engaging in metal-metal triple bonding otherwise. Heterodimerization was also observed with simple tert-butoxide ligands. The new type of heterodimers features unusually long yet robust Mo≡Mo bonds, which are notably polarized according to DFT. However, there is no direct correlation between the extreme Mo≡Mo bond lengths and the strikingly deshielded ⁹⁵Mo NMR signals, since ligand-based orbitals can also markedly affect the shielding tensor.

Figure 1

Top: the literature knows of either monomeric or homodimeric Mo(+3) complexes but in no case are both types supported by the same ligand set; the present study closes the gap and reports the first full ensemble, including an entirely new type of heterodimeric complexes; middle: known monomeric Mo(+3) complexes; bottom: envisaged use of tripodal ligands for the stabilization of monomeric Mo(+3); these ligands were previously used to craft “canopy catalysts” for alkyne metathesis and analogous Mo(+6) nitrido complexes; R = aryl, alkyl.

Scheme 1. A Dumbbell Dimer with Two Tripodal Silanolate End-Caps

Figure 2

Truncated structure of complex [12·3thf] in the solid state; the 3,5-dimethylphenyl substituents on the silicon linkers and H-atoms are removed for clarity; the full structure is contained in the Supporting Information. Selected bond lengths (Å) and angles (°): Mo1–O1 2.0146(19), Mo1–O2 2.2407(19), Si1–O1–Mo1 163.16(17).

Scheme 2. A New Monomeric Mo(+3) Complex Supported by a Tripodal Silanolate Ligand Scaffold Exists in Three Distinctly Different Bonding Modes

Figure 3

Truncated structure of complex [12·thf] in the solid state; the 3,5-dimethylphenyl substituents on the silicon linkers and H-atoms are removed for clarity; the full structure is contained in the Supporting Information. Selected bond lengths (Å) and angles (°): Mo1A-O1 1.9162(14), Mo1A-O2 1.9134(14), Mo1A-O3A 1.8979(18), Mo1A-O4 2.2524(15), Mo1A-C1A 2.276(2), Mo1A-C2A 2.224(2), C1A-C2A 1.466(3), C2A-3 1.453(3), C3–C4 1.351(3), C4–C5 1.438(3), C5–C6A 1.375(4), C6A-C1A 1.429(4), C1A-C2A–C3–C4 19.7.

Scheme 3. Reactions of [12·thf]

Figure 4

Superposition of the truncated experimental structure (blue) of the THF-free complex 12 in the solid state with the truncated calculated structure (red); the 3,5-dimethylphenyl substituents R on the Si-linkers were removed for clarity; for the full structure and further details, see the Supporting Information.

Figure 5

Truncated structure of the homodimeric complex 13 in the solid state; the 3,5-dimethylphenyl substituents on the silicon linkers and H-atoms are removed for clarity; the full structure is contained in the Supporting Information. Selected bond lengths (Å) and angles (°): Mo1–Mo1′ 2.2873(3), Mo1–O1 1.9107(12), Mo1–O2 1.9043(17), Mo1–O3 1.9148(12), Si1–O1–Mo1 158.67(8), O1–Mo1–Mo1′ 103.23(4).

Scheme 4. Formation of an Overcrowded Homodimer

Scheme 5. A Heterodimeric Complex Comprising an Intact “Cummins Complex” Entity as One of the Constituents

Figure 6

Truncated molecular structure of complex 14 in the solid state; the 3,5-dimethylphenyl substituents on the silicon linkers, disordered parts, and H-atoms are not shown for clarity; the full structure is contained in the Supporting Information. Selected bond lengths (Å) and angles (°): Mo1–Mo2 2.3440(3); Mo1–O1 1.9305(15), Mo1–O2 1.9455(14), Mo1–O3 1.9409(14), Mo2–N1 1.9885(18), Mo2–N2 1.9810(18), Mo2–N3 1.9954(18), O1–Mo1–Mo2 105.10(5), N3–Mo2–Mo1 110.15(5), Si1–O1–Mo1 173.36(11).

Figure 7

Molecular structure of the heterodimeric complex 16 in the solid state; top: side view; bottom: projection along the 3-fold crystallographic Mo–Mo axis; H-atoms, disordered parts, and solute solvent molecules in the unit cell not shown for clarity; the full structure is contained in the Supporting Information. Selected bond lengths (Å) and angles (°): Mo1–Mo2 2.2955(7), Mo1–O1 1.915(7), Mo2–N1 1.998(2), C1–O1–Mo1 144.7(7), O1–Mo1–Mo2 100.44(7), N1–Mo2–Mo1 106.29(8).

Scheme 6. Heterodimers by Alcoholysis of 1

Figure 8

Molecular structure of complex 17 in the solid state; H-atoms and disordered solvent in the unit cell removed for clarity. Selected bond lengths (Å) and angles (°): Mo1–Mo2 2.2944(2), Mo1–N1 1.9893(14), Mo1–N2 1.9910(14), Mo1–N3 2.0010(14), Mo2–O1 1.9147(12), Mo2–O2 1.9122(12), Mo2–O3 1.9175(13), Mo1–Mo2–O1 99.49(4), Mo2–Mo1–N1 106.72(4).

Scheme 7. Cross-Dimerization of Two Molecularly Defined Mo(+3) Complexes

Figure 9

Plot of the Mo–Mo bond distances of complexes comprising an unbridged Mo≡Mo core with a CN = 4 on both metal atoms found in the Cambridge Crystallographic Data Centre (black dots); comparison with the new heterodimeric complexes reported herein (for details including structures and accession codes of the literature-known complexes, see the Supporting Information).

Figure 10

Comparison of the MO schemes of complexes 20, 19, 18 and the new heterodimeric complex 14; for full MO plots and further details, see the SI.