Rh(III) Porphyrins as Building Blocks for Porphyrin Coordination Arrays: From Dimers to Heterometallic Undecamers

The coordination chemistry of a Rh(III) porphyrin building block was investigated with a view to the construction
of heterometallic arrays of porphyrins. The Rh(III) porphyrin was found to coordinate methanol in the solid state
and weakly in CDCl3 solution. Crystallization afforded five coordinate π stacked Rh(III) porphyrins. The distribution
of products from reaction of Rh(III) porphyrin with DABCO, 4,4‘-bipyridine, and 4,4‘-bipyrimidine could be
displaced toward dimeric species by silica gel column chromatography or recrystallization which served to remove
excess ligand. Weak coordination to nitriles was observed, although it was sufficiently strong to organize a dimeric
complex of 5,5‘-dicyano-2,2‘-bipyridine in the solid state. Complexes with 4,4‘-bipyrimidine and 5,5‘-dicyano-2,2‘-bipyridine possess uncoordinated chelating nitrogen atoms. Larger heterometallic porphyrin arrays were
assembled using a combination of Sn(IV) and Rh(III) porphyrin coordination chemistry. A Sn(IV) porphyrin
acted as a core around which were coordinated two isonicotinate groups, carboxylic acid functionalized porphyrins,
or porphyrin trimer dendrons. Rh(III) porphyrins were coordinated to pyridyl groups at the periphery of these
entities. In this way an eleven porphyrin array, with four different porphyrin metalation states, was assembled.
The diamagnetic nature of both the Rh(III) and Sn(IV) porphyrins, the slow ligand exchange kinetics on the
NMR time scale, and tight ligand binding permitted the porphyrin arrays to be analyzed by two-dimensional 1H
NMR techniques.

本研究以构建卟啉异金属阵列为目标，对三价铑卟啉（Rh(III) porphyrin）构筑基元的配位化学展开了探究。研究发现该三价铑卟啉可在固态下与甲醇配位，在氘代氯仿（CDCl3）溶液中则表现出弱配位作用。结晶过程得到了五配位π堆积型三价铑卟啉。三价铑卟啉分别与DABCO（1,4-二氮杂双环[2.2.2]辛烷）、4,4'-联吡啶（4,4‘-bipyridine）及4,4'-联嘧啶（4,4‘-bipyrimidine）发生反应，通过硅胶柱层析或重结晶移除过量配体后，可使产物分布向二聚体物种偏移。该卟啉与腈类亦存在弱配位作用，但其配位强度足以在固态下构筑5,5'-二氰基-2,2'-联吡啶（5,5‘-dicyano-2,2‘-bipyridine）的二聚配合物。由4,4'-联嘧啶与5,5'-二氰基-2,2'-联吡啶形成的配合物，仍保留有未配位的螯合氮原子。结合四价锡卟啉（Sn(IV) porphyrin）与三价铑卟啉的配位化学特性，可组装得到更大尺寸的卟啉异金属阵列。以四价锡卟啉作为核心骨架，可在其外围配位两个异烟酸酯基团、羧酸功能化卟啉，或卟啉三聚树枝状分子。三价铑卟啉则可与上述结构外围的吡啶基团配位结合。通过该策略，成功组装出包含四种不同卟啉金属化状态的十一卟啉阵列。三价铑卟啉与四价锡卟啉均具有抗磁性，且配体交换动力学在核磁共振（NMR）时间尺度上较为缓慢，加之配体结合紧密，因此可通过二维¹H核磁共振技术对卟啉阵列进行表征分析。