Substituted metal carbonyls. 27.1 Synthesis, structures, and metal-metal bonding of a ferrocenylphosphine exo-bridged cluster with two heterometallic triangles, [AuMn2(CO)8(μ-PPh2)]2(μ-dppf), and a twisted-bowtie cluster, PPN+[Au{Mn2(CO)8(μ-PPh2)} 2]-

Abstract

Redox condensation of PPN[Mn2(CO)8(μ-PPh2)] (1; (PPN = N(PPh3)2) with Au2Cl2(μ-P-P) (P-P = (C5H4PPh2)2Fe (dppf), Ph2PC2H4PPh2 (dppe)) gives two hexanuclear Au-Mn clusters [AuMn2(CO)8(μ-PPh2)]2(μ-P-P) (P-P = dppf, (2), dppe (4)), both of which contain a diphosphine bridging two Mn2Au triangles. Complex 2 is formed via an intermediate, AuCl-(μ-dppf)[AuMn2(CO)8(μ-PPh2)], (3), which was isolated. Bridge cleavage of 2 occurs at thf reflux with PPh3 and room temperature with P(OEt)3 to give the triangular clusters [(PR3)-AuMn2(CO)8(μ-PPh2)] (R = Ph (5), OEt (6)), respectively. The latter exchange of dppf with P(OEt)3 is reversible in solution. Condensation of 1 with AuCl(SMe2) gives an anionic pentanuclear cluster, PPN[Au{Mn2(CO)8(μ-PPh2)}2] (7). Complexes 2 and 7 were structurally characterized by single-crystal X-ray diffractometry. Complex 2, which is centrosymmetric with Fe in dppf at a crystallographic inversion center, consists of a ferrocenylphosphine bridging two heterometallic triangles (Au-Mn = 2.660(1) and 2.776(1) Å; Mn-Mn = 3.049(2) Å). Complex 7 is made up of two planar AuMn2P metallacycles fused at Au at an angle of 85.50(4)°. With crystallographic C2 symmetry, a twisted-bowtie skeleton resulted with gold at its center. Both Au-Mn (mean 2.806(1) Å) and (PPh2-bridged) Mn-Mn (3.105(2) Å) lengths are significantly longer than those in 2. The Mn-Mn bond of 2 is also significantly longer than that of 1. Fenske-Hall MO calculations on 1, 2, and 7 together with Mn2(CO)8(μ-H)(μ-PPh2) (8) and (PPhMe2)AuMn2(CO)8(PPh2) (9) indicate that aside from 1, all the complexes, including 2 and 7, give a negative overlap population in the Mn-Mn interactions. The Mn-Mn distance appears to be determined by the strength of the AuMn2 interaction and/or the size of H compared to Au. The weaker Mn-Mn and Au-Mn interactions in 7 (as compared to those in 2 and 9, respectively) are likely to be caused by the absence of Au orbital reinforcement in the direction of the Mn2 moiety as a consequence of symmetry.