and other ring sizes are challenging to access due to low yield steps. In this communication, we report the serendipitous finding that the dibridgehead diarsines 1 are available by routes that involve an earth abundant metal, iron, and avoid low yield steps. These diarsines can, per their parentage, serve as cage like trans spanning ligands for a variety of metal fragments, and readily undergo other types of derivatization. Some of these processes implicate unusual dynamic phenomena involving out,out and in,in isomers (see I, II in Scheme 1), and crystallographic reference points for both stereochemical limits are described.

As sketched in Scheme 2, the cationic iron dicarbonyl nitrosyl complexes trans-[Fe(CO) 2 (NO)(As((CH 2 ) n ) 3 As)] + BF 4 À (3; n = a, 10; b, 12; c, 14) were prepared in three steps from (BDA)Fe(CO) 3 and the readily available arsines As((CH 2 ) m CHQCH 2 ) 3 (m = 4, 5, 6) as previously reported. 14 The key step involves a threefold intramolecular (and interligand) olefin metathesis, which despite several obvious potential side reactions proceeds in reasonable yields, aided by conformational factors detailed earlier. 15,16 The dibridgehead diarsine ligands feature thirteen-to seventeen-membered macrocycles, and there is no intrinsic upper limit on the ring size, as long as it is odd. The salts 3a-c were treated with Bu 4 N + Cl À in CH 2 Cl 2 . The analogous diphosphine complexes undergo carbonyl ligand Scheme 1 out,out (I) and in,in (II) isomers of macrocyclic dibridgehead diarsines or Lewis acid adducts thereof. Structure III illustrates the homeomorphic isomerization of II (pull the distal blue chain through the ring defined by the other two) to I.

substitutions to give neutral Fe(CO)(NO)(Cl) adducts. 17 Surprisingly, workups gave the free dibridgehead diarsines 1a-c in 77-34% yields as air stable white solids. The salt with the smallest cage, 3a, required longer times and heating. Conditions and nucleophiles were varied in attempts to secure any type of Fe(CO)(NO)(X) species, as these would be valuable candidates for molecular gyroscopes. 16 However, the weaker metal-arsenic versus metal-phosphorus bonds 18 apparently lead to orthogonal reactivity channels. Dark green reaction solutions always formed, but IR spectra did not reveal any n CO or n NO bands and the iron byproduct remains unidentified.

The diarsines 1a-c were characterized by NMR ( 1 H, 13 C) and microanalyses. Consideration was first given to the disposition of the lone pairs. Pyramidal inversion at arsenic normally requires temperatures of B200 1C, 19 and the lone pairs in the precursors 3a-c are directed in towards the central iron atom. Thus, one might expect that 1a-c would be generated as in,in isomers (II, Scheme 1). However, suitable macrobicyclic compounds can turn themselves ''inside out'', a topological process termed homeomorphic isomerization. 9 This would convert II to an out,out isomer I, for the same net effect as a two-fold pyramidal inversion. A key stage in the homeomorphic isomerization is illustrated in III.

Compounds 1a-c lack NMR ''handles'' that facilitate the investigation of such processes, which are very rapid with the phosphorus analog of 1c. 11a,b Thus, the accessibility of out,out isomers was probed in several ways. First, crystals of 1b were obtained, and the X-ray structure (Fig. 1, top) showed an out,out conformation. Second, 1c was treated with H 2 O 2 as depicted in Scheme 2. Workup gave the expected dibridgehead diarsine dioxide 1cÁ2O as a hygroscopic white solid that exhibited a characteristic IR n AsQO band at 881 cm À1 . 20 As shown in Fig. 1 (middle), the crystal structure also featured an out,out conformation. However, these data by no means require that out,out isomers dominate in solution.

Reactions of 1b,c and various types of Lewis acids were studied. The addition of 1b and Me 2 SÁBH 3 afforded the oily bis(borane) adduct 1bÁ2BH 3 in 94% yield (Scheme 2). 1,21 As shown in Scheme 3, the reaction of 1c and PtCl 2 afforded the gyroscope like complex trans-Pt(Cl) 2 (As((CH 2 ) 14 ) 3 As) (4c, 86%), which is formally an adduct of in,in-1c. Combining 1c and common NiCl 2 sources gave the nickel analog 5c (80-68%). Diphosphine analogs of 4c and 5c have been previously reported. 11b,22 Reactions of 1c with [Rh(Cl)(COD)] 2 /CO and Fe(CO) 3 (BDA) were also investigated. As shown in Scheme 3, the former afforded trans-Rh(CO)(Cl)(As((CH 2 ) 14 ) 3 As) (6c; 86%). The latter gave 2c (51%), which serves as a precursor to 1c in Scheme 2. Thus, such diarsines can be viewed as ''container molecules'' from which metal fragments can be reversibly incorporated and extruded. A speculation that ties these phenomena together is presented as IV in Scheme 3. Perhaps the mechanism by which the arsines are extruded from 3a-c involves the cleavage of one iron-arsenic bond, homeomorphic isomerization to IV, and displacement of the second arsenic atom by a chloride nucleophile. For the reactions in Scheme 3, the appropriate metal fragment would bind to one arsenic atom of out,out-1c, and then homeomorphic isomerizations coupled with metal-arsenic bond formation would yield the cage like products.

The crystal structure of the platinum complex 4c could also be determined (Fig. 1, bottom), and sets up a number of comparisons. The arsenic-arsenic distance expands from 4.76 Å in trans-chelated 4c to 16.77 Å in 1cÁ2O, both of which feature (CH 2 ) 14 bridges. The distance in (CH 2 ) 12 -bridged 1b is intermediate (11.15 Å). The degree of bridgehead pyramidalization, as reflected by the sums of the three CH 2 -As-CH 2 bond angles, increases in the order 1cÁ2O (331. 4-328.11) o 4c (318.3-314.71) o 1b (288.3-287.91). The average angle in 1b (96.01) reflects the high degree of p character in the orbitals used for bonding in trivalent organoarsines, 23 whereas that in 1cÁ2O (109.91) indicates a nearly tetrahedral geometry. None of the compounds exhibit any crystallographic symmetry, but there is an approximate C 3 axis that passes through both arsenic atoms in 1b. The As-As-X angles in 1b and 1cÁ2O (X = lone pair, O), which would be 1801 in idealized out,out isomers, contract to 172.41/172.81 and 150.91/167.11, respectively. Arsine oxides are known to be good hydrogen bond acceptors, 24 and the hydrogen atoms of some of the water molecules associated with 1cÁ2O clearly interact with the AsQO moiety.

As noted above, there is little precedent for the chemistry in Schemes 2 and 3. However, Johnson has prepared novel diarsines that feature sulfur substituted AsS 3 bridgeheads, 7 a few of which are exemplified in Fig. 2. These have only been accessed as in,in isomers that feature rigid arylene bridge components such that homeomorphic isomerization would be challenging. No coordination chemistry has yet been reported, although interesting possibilities exist.

In summary, this study has established the ready availability of a series of structurally flexible macrocyclic dibridgehead diorganoarsines that hint at fascinating dynamic properties and represent launching pads for a variety of unprecedented molecular architectures. These themes will be developed in subsequent reports.

The authors thank the US National Science Foundation (CHE-1566601 and CHE-1900549) for support. This paper is dedicated to a valued colleague and pioneer in organoarsenic chemistry, Prof. Ralph Zingaro, 20,25 on the occasion of his 96th birthday (October 27, 2022).