The structure of tetramethyl μ-monothiopyrophosphate

The compound tetramethyl μ-monothiopyrophosphate (C₄H₁₂O₆P₂S) crystallizes in the monoclinic space group C 2/c, with (at -130°C) a = 10.322 Å, b = 8.229 Å, c = 12.062 Å, β = 98.44°, and D_calc = 1.639 g/mL³ and Z = 4. The crystal structure has been determined by single crystal X-ray diffraction to give a final R value of 0.0329 for 614 independent observed reflections F_˚ > 2.5σ(F_˚)]. The sulfur atom resides on a crystallographic two-fold axis. The P S P bond angle is 105.4° and the P S bond lengths are 2.093 Å. The bond angles around phosphorus range from 99.1° to 118.2°. The terminal PO bond is 1.465 Å, and the methoxyl P O bond is about 1.556 Å. The H₃C O P bond angle is about 119.5°. Many structural features are interpreted in terms of π-bonding to phosphorus. Comparisons with the structures of pyrophosphate and related compounds indicate that the combined effects of increased acuteness of the P S P bond and the increased length of the P—S bonds lead to an increase of about 0.4 Å in the separation of phosphorus atoms in the sulfur-bridging compound. These facts, together with the weakness of the P S bond, must be taken into account in the interpretation of kinetic data for enzymatic reactions of phosphorothiolates as substrates in place of phosphates.