Methyluronium dimesylamide (1) and 1,1-dimethyluronium dimesylamide (2) have been prepared by proton transfer reactions of the strong nitrogen acid HN(SO2Me)(2) with the corresponding ureas and characterized by low-temperature X-ray diffraction (1: orthorhombic, space group P2(1)2(1)2(1), Z' = 1; 2: orthorhombic, Pna2(1), Z' = 2). Compound 2 appears to be the first crystallographically established 1,1-dimethyluronium salt. As a common feature, the three independent formula units consist of ion pairs held together by an eight-membered ring synthon [N-2 = R-2(2)(8), antidromic] constructed from a syn,syn-sequence H-O-C(sp(2))-N-H of the cation and a V-shaped O-S(sp(3))-N fragment of the anion via homonuclear two-centre hydrogen bonds. The surplus N-H donors form hydrogen bonds to O=S acceptors in adjacent formula units, leading to a three-dimensional network (1) or a pseudo-2(1) symmetric helical catemer of alternatingly inequivalent formula units (2). In both structures, the activated methyl groups give rise to numerous short interionic C-H...O contacts, the more prominent of which are analyzed in terms of weak hydrogen bonding [cut-off criteria: d(H...O) less than or equal to 270 pm, theta(C-H...O) greater than or equal to 130degrees].
