Rhodium Dimers with 2,2-Dimethyl-1,3-diisocyano and Bis(diphenylphosphino)methane Bridging Ligands

Four rhodium dimers have been synthesized with a bridging diisocyanide ligand, dmb (2,2-dimethyl-1,3-diisocyanopropane):  [Rh2(dmb)4](BPh4)2, [Rh2(dmb)4Cl2]Cl2, [Rh2(dmb)4I2](PF6)2, and [Rh2(dmb)2(dppm)2](BPh4)2 (dppm
= bis(diphenylphosphino)methane). The complexes have been characterized by elemental analysis and mass
spectrometry, as well as UV−visible, IR, and 1H NMR spectroscopies. X-ray crystal structures of the rhodium(I)
complexes, [Rh2(dmb)4](BPh4)2·1.5CH3CN (3.2330(4), 3.2265(4) Å) and [Rh2(dmb)2(dppm)2](BPh4)2·0.5CH3OH·0.2H2O (3.0371(5) Å), confirm the existence of short Rh···Rh interactions. The metal−metal separation for the
rhodium(II) adduct, [Rh2(dmb)4Cl2]Cl2·6CHCl3 (2.8465(6) Å), is consistent with a formal Rh−Rh bond. For the two
luminescent rhodium(I) dimers and six previously investigated diisocyano-bridged dimers with and without dppm
ligands, the intense spin-allowed dσ*→pσ absorption band maximum shifts to longer wavelengths with decreasing
Rh···Rh separation, and there is an approximate correlation between band energy and the inverse of the metal−metal separation cubed. Both [Rh2(dmb)4]2+ and [Rh2(dmb)4(dppm)2]2+ undergo oxidative addition in the presence
of iodine. In the conversion of [Rh2(dmb)4]2+ to [Rh2(dmb)4I2]2+, the observed intermediate is tentatively assigned
to a tetramer composed of two rhodium dimers. In the case of [Rh2(dmb)2(dppm)2]2+, no intermediate was detected.

本研究合成了四种以桥联二异氰配体dmb（2,2-二甲基-1,3-二异氰丙烷）为桥联基团的铑二聚体（rhodium dimers）：[Rh₂(dmb)₄](BPh₄)₂、[Rh₂(dmb)₄Cl₂]Cl₂、[Rh₂(dmb)₄I₂](PF₆)₂及[Rh₂(dmb)₂(dppm)₂](BPh₄)₂，其中dppm为双(二苯基膦)甲烷(bis(diphenylphosphino)methane)。采用元素分析(elemental analysis)、质谱(mass spectrometry)、紫外-可见(UV−visible)光谱、红外(IR)光谱及氢核磁共振(¹H NMR)光谱对上述配合物进行了表征。铑(I)配合物[Rh₂(dmb)₄](BPh₄)₂·1.5CH₃CN（金属-金属间距为3.2330(4)、3.2265(4) Å）与[Rh₂(dmb)₂(dppm)₂](BPh₄)₂·0.5CH₃OH·0.2H₂O（金属-金属间距为3.0371(5) Å）的X射线晶体结构(X-ray crystal structures)证实了其存在短程铑-铑(Rh···Rh)相互作用。铑(II)加合物[Rh₂(dmb)₄Cl₂]Cl₂·6CHCl₃的金属-金属间距为2.8465(6) Å，与形式铑-铑键(formal Rh−Rh bond)相符。针对两种发光铑(I)二聚体及六种此前已研究的带有或不带有dppm配体的二异氰桥联二聚体(diisocyano-bridged dimers)，其自旋允许的dσ*→pσ吸收带(spin-allowed dσ*→pσ absorption band)的最大吸收波长随铑-铑间距减小而红移，且能带能量(band energy)与金属-金属间距的三次方成反比存在近似相关性。[Rh₂(dmb)₄]²+与[Rh₂(dmb)₄(dppm)₂]²+均可在碘单质存在下发生氧化加成(oxidative addition)反应。在[Rh₂(dmb)₄]²+转化为[Rh₂(dmb)₄I₂]²+的过程中，观测到的中间体被初步归属为由两个铑二聚体构成的四聚体(tetramer)；而在[Rh₂(dmb)₂(dppm)₂]²+的转化过程中未检测到中间体。