Polynuclear Rhodium Complexes with Dinucleating PNNP Ligand: Dynamic and Diverse M···M Interactions in [(μ-X)Rh₂(PNNP)(CO)₂]<i>ⁿ</i>⁺ and [(μ₄-X)Rh₄(PNNP)₂(CO)₄]<i>ⁿ</i>⁺ [X = H, O, C⋮C−R, R−C⋮C−R, C⋮C, CHCH₂, SMe₂; <i>n</i> = 0, 1: PNNP = 3,5-bis(diphenylphosphinomethyl)pyrazolato]

The dirhodium-tetracarbonyl complex with the PNNP ligand, [Rh2(PNNP)(CO)4]BF4, 1·BF4 [PNNP = 3,5-bis(diphenylphosphinomethyl)pyrazolato], serves as a precursor for the active species [Rh2(PNNP)(CO)2]+, A, with cis-divacant coordination sites, which can accommodate a donor species with up to four donor electrons. Interaction of 1·BF4 with Li−C⋮C−R, Li−CHCH2, PPN[M(CO)n], CN−Cy, SMe2, and Et−C⋮C−Et leads to the formation of the corresponding dinuclear adducts [(X)Rh2(PNNP)(CO)2]n+(BF4)n [X/n = μ-η1:η2-C⋮C−R/0 (2), μ-η1:η2-CHCH2/0 (3), μ-Co(CO)4/0 (4a), μ-Mn(CO)5/0 (4b), (η1-C⋮N−Cy)2/1 (5·BF4), μ-η1:η1-SMe2/1 (6·BF4), μ-η2:η2-Et−C⋮C−Et/1 (7·BF4)], respectively. Selective replacement of the inner CO ligands trans to the P atoms is verified by spectroscopic and crystallographic characterizations of the adducts. The μ-acetylide (2) and μ-vinyl complexes (3) show dynamic behavior via the conventional windshield wiper motion of the unsaturated hydrocarbyl ligand. Systematic structural analysis of a series of the μ-acetylide complexes 2 (R = H, SiMe3, n-Bu, Ph, p-tol) reveals three typical conformations for the Rh2(PNNP)(CO)2 backbone: Cs- (type I), C2- (type II), and C2v-symmetrical ones (type III). On the other hand, interaction of 1·BF4 with 1-alkyne, hydrosilanes, and hydroxo anion produces tetranuclear adducts resulting from 1 (donor):2 [Rh2(PNNP)(CO)2] coupling reactions. The μ4-η1(Cα):η2(Cα⋮Cβ)-acetylide complexes, [(μ4-C⋮C−R)Rh4(PNNP)2(CO)4]BF4, 8·BF4, consisting of an acyclic, folded Z-shaped Rh4 linkage are fluxional via a combination of windshield wiper-like motions between the wingtip Rh centers and between the wingtip and hinge Rh centers, which involve reversible M−M bond scission and recombination processes. The ethynyl complex 8a·BF4 (R = H) is readily deprotonated by the action of a base to give the μ4-dicarbide complex (μ-η1:η1:η2:η2-C⋮C)Rh4(PNNP)(CO)4, 9, via cleavage of three Rh−Rh bonds, and this process is reversed upon protonation of 9 with HBF4. The reaction of 1·BF4 with hydrosilanes and hydroxo anion furnishes the isostructural tetranuclear complexes [(μ4-X)Rh4(PNNP)(CO)4]n+(BF4)n [X/n = H/1 (10·BF4), O/0 (11)], in which the four Rh atoms arranged in a tetrahedral array are connected by the μ4-bridging ligand (X), as characterized by X-ray crystallography. The tetranuclear structures are retained mainly by Rh−X interactions even in the case of the electron-deficient hydride complex 10·BF4 with 58 valence electrons (cf. 64e for a coordinatively saturated species), where the filled, spherically distributed 1s orbital of the hydride ligand interacts with the four Rh centers arranged in a tetrahedral array. Depending on the size of the bridging ligand X, the structure of the tetranuclear complexes varies from the encapsulated structure (10·BF4 and 11) to the one with a folded Z-shaped metal linkage (8·BF4). The present study reveals (i) the high reactivity (electrophilicity) of the active species A with cis-divacant coordination sites, (ii) M−M bonds being not always essential for a polynuclear system, (iii) dynamic behavior via reversible M−M bond cleavage and recombination processes, and (iv) flexible coordination properties of the Rh2(PNNP) system.