According to another embodiment at least one R is OSi(OR¹)₃. R¹ is selected independently at each occurrence from C₁ to C₁₀ alkyl, C₂ to C₁₀ alkenyl, and C₂ to C₁₀ alkynyl, wherein alkyl, alkenyl, and alkynyl may be substituted by one or more substituents selected from F and CN, preferably R¹ is selected independently at each occurrence from C₁ to C₁₀ alkyl, which may be substituted by one or more substituents selected from F and CN, more preferred R¹ is selected independently at each occurrence from C₁ to C₆ alkyl, which may be substituted by one or more substituents selected from F and CN, e.g. R¹ is selected independently at each occurrence from methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, n-pentyl, i-pentyl, and n-hexyl. Examples of R selected from OSi(OR¹)₃ are trimethyl silyl and triethyl silyl.

n is an integer selected from 1, 2, 3, 4, and 5, preferably n is 1 or 2, more preferred n is 1.

Examples of pyridine-SO₃ complexes of formula (I) are 2-vinyl pyridine sulfur trioxide, 3-vinyl pyridine sulfur trioxide, 4-vinyl pyridine sulfur trioxide, 2-trifluoromethyl pyridine sulfur trioxide, 3-trifluoromethyl pyridine sulfur trioxide, 4-trifluoromethyl pyridine sulfur trioxide, 2-fluoro pyridine sulfur trioxide, 3-fluoro pyridine sulfur trioxide, 4-fluoro pyridine sulfur trioxide, 2-propargyl pyridine sulfur trioxide, 3-propargyl pyridine sulfur trioxide, 4-propargyl pyridine sulfur trioxide, 2-trimethylsilyl pyridine sulfur trioxide, 3-trimethylsilyl pyridine sulfur trioxide, and 4-trimethylsilyl pyridine sulfur trioxide.